Arxiu de la categoria: OTHER INVERTEBRATES

4 marine animals that maybe you don’t know

There are many groups of marine animals, like cnidarians, ctenophores, echinoderms or amphioxus, that are well-known, mainly because they are taught in school or because divers can enjoy their presence for its macro size. In this post, I am talking about 4 groups of marine animals that probably, unless you are a biologist or similar, you don’t know. 


Placozoa is a group of marine animals that were discovered in 1883 in an aquarium of Austria. This group has been considered for a long time a phyllum with just one species, Trichoplax adherens, but a research from 2010 found 7 new possible species, with different geographical distributions.

The species is a flat (thickness between 0.01 to 0.015 mm) and small (3 mm) animal, without neither a particular shape nor symmetry. Its body present just 4 types of cells, but doesn’t include neither nervous, sensory or muscular cells. Trichoplax lives in the bottom of tropical and subtropical seas. To feed, it can withdraw its body surrounding the feeding particles in order to constitute a cavity, in which digestive enzymes (external digestion) are poured and then the digested particles penetrate into the body. Placozoa can reproduce both sexually by fission, fragmentantion or budding and sexually.

Placozou Trichoplax adherens (Foto: Taringa).
Placozoa Trichoplax adherens (Picture: Taringa).


Gnathostomulida is a group of 100 species of small size (between 0.5 and 1 mm), with a worm-like shape and that lives in the sand of the bottom of the oceans poor in oxygen. Some curiosities are the absence of anus or it is temporary, the epithelium has just one layer of cells, normally have one penis, two testis and one ovary (hermaphrodite animals) and they produce one egg per lay. Despite its simplicity, Gnathostomulida has a nervous system with sensory cells, musculature and protonephridia to excrete liquids, but circulation and breathing take place for diffusion through the epithelium. They feed on bacteria and fungi.

Gnatostomúlid Problognathia minima (Foto: Xtec).
Gnathostomulida Problognathia minima (Picture: Xtec).


Kinorhyncha is a group of 150 species with a size less than 1 mm. The body is covered by 13 plates of chitin (like insects and other arthropods), which are articulated between them. They can eat thanks to the presence of a trunk with spines, which is also involved in its movement. They present all the anatomical systems, but circulation and breathing is for diffusion. Their body is full of sensory cells: pigment pits to detect light and mechanoreceptor spines. Sexes are separated. They feed on detritus and diatoms. They live in the intersticial water of the sea bottom.

Quinorrinc (Foto: O Cays Doze)
Kinorhyncha (Picture: O Cays Doze)


Until recently, Cycliophora was considered to include just one species: Symbion pandora. This animal lives on the mouth pieces of Norway lobster (Nephrops narvegicus). Males are tiny, so tiny that are sperm sacks, and live joined to females. Females have digestive system, a cilium crown and an adhesive disc in the base to join in the oral pieces. Reproduction is both asexual, forming a larvae known as pandora, and sexual, with a larvae known as chordoid, which always gives females. Other species are Symbion americanus, which lives on the American lobster (Homarus americanus); but it could be more species (still not described and named), that would live on other crustaceans.

Cicliòfor Symbion pandora (Foto: )
Cicliòfor Symbion pandora (Foto: Peter Funch, University of Copenhagen)


  • Nots of the subject Invertebrates from the Degree in Biology (University of Barcelona).
  • Brusca RC & Brusca GJ (2005). Invertebrados. Ed. McGraw Hill (2 ed).
  • Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Principios integrales de Zoología. Ed. McGraw Hill (13 ed).


Summer is the perfect season of the year to go and enjoy the sea and you probably are one of these people who practise snorkel or who dive. In this case, I want to explain which are the main features of starfishes, with which animals can be confused and some examples of the Mediterranean sea. 


Starfishes (Asteroidea) are included in the phylum of Echinoderms, together with sea urchins (Echinoidea), sea cucumbers (Holothuroidea), crinoids (Crinoidea) and brittle stars or ophiuroids (Ophiuroidea).

Equinoderms: (A) Asteroïdeus, (B) Ofiuroïdeus, (C) Holoturoïdeus, (D) Crinoïdeus i (E) Equinoïdeus. (Fotos de Fresno.pntic)
Echinoderms: (A) Asteroidea, (B) Ophiuroidea, (C) Holothuroidea, (D) Crinoidea y (E) Echinoidea. (Pictures: Fresno.pntic)

Echinoderms are all marine animals, which present the following main features:

  • Body with pentamerous radial symmetry in the adult phase, but with bilateral symmetry in the larval phase (there is just one symmetry axis).
  • They are all mobile, except some sessile species (affixed on the sea-floor) of crinoids.
  • Endoskeleton consisting of ossicles.
  • Water vascular system: system projected from the body wall with some expansions similar to tentacles called podium, which can be spread out due to de liquid pressure. Normally, they present an opening called madreporite.

It is important to remember that they are marine animals and, if you want to observe them, you must not extract them from the water because they begin to die in just 10 seconds. 


There is about 1,500 species of starfishes, which are all included in the Asteroidea class. Starfishes live in sandy, muddy, rocky and coral reefs seafloors, depending on the species. They can measure from some centimetres to one metre.

Externally, starfishes have a central disc from which the arms are originated. From the mouth, which is placed in the lower part (or oral part), and throughout the arms there is the ambulacral ridge, from which the podiums are originated. The upper part (or aboral part) is usually coarse and with spines. In the base of this spines there is structures called pedicellarie, which function is to remove the particles that remove debris from the body surface and in some cases are used to capture small fishes. Gas exchange takes place through papulae, which are thin-walled bulges on the aboral surface of the disc and arms. Anus and madreporite are placed in the aboral surface.

External anatomy of a starfish (Picture: JKL Bahweting Middle School).
External anatomy of a starfish (Picture: JKL Bahweting Middle School).

An important feature of echinoderms is the water vascular system. In the case of starfishes, it plays an important role in locomotion, in food capture, in excretion and in breathing.

Many starfishes are carnivorous and feed on molluscs, crustaceans, worms, echinoderms and other invertebrates, sometime on small fishes too. Some starfishes can feed on small particles of plankton or other organic particles.

WATCH OUT! Starfishes can be confused with ophiuroids, but ophiuroids have thinner and more mobile arms than starfishes, in addition to the absence of anus and the fact that they do not use podium to get around, so they move the arms. 



This group includes 6 Mediterranean species, which live in sandy and muddy seafloors and with 5 arms covered by scales and spikes. The most common is the red comb-star (Astropecten aranciacus), which present two lines of big and sharp spines, and with a red – orange colouration.

Estrella de sorra (Astropecten aranciacus) (Foto: Natura Mediterráneo).
Red comb-star (Astropecten aranciacus) (Picture: NaturaMediterráneo).

BLUE SPINY STARFISH (Coscinasterias tenuispina)

This starfish measures between 7 and 10 cm and usually present between 7 and 9 arms of different sizes, which are covered by small spines. It is bluish white and brown. The most common reproduction system of this species is fission, and for this reason a large section of the coast can be occupied by the same genetic individual. The blue spiny starfish usually lives under the stones.

Estrella blavosa (Coscinasterias tenuispina) (Foto: Animales Marinos).
Blue spiny starfish (Coscinasterias tenuispina) (Picture: Animales Marinos).

SPINY-STARFISH (Marthasterias glacialis)

The spiny-starfish, which can measure 80 cm of diameter, always presents 5 arms, which are covered by hard spikes. Its colouration is greenish or brownish, with light spots when they live deeper. It can live in rocky or sandy seafloors, until 180 metres deep.

Marthasterias (Foto: Tato Grasso, Creative Commons).
Spiny-starfish (Marthasterias glacialis) (Picture: Tato Grasso, Creative Commons).

RED STARFISH (Echinaster sepositus)

Red starfish, which can measure 30 cm, has a red to orange body, with 5 long and cylindrical arms, covered by small spines. It can be found utill 1000 metres deep, always in rocky bottoms.

Estrella vermella (Echinaster sepositus) (Foto: , Creative Commons).
Red starfish (Echinaster sepositus) (Picture: Tato Grasso, Creative Commons).

HACELIA (Hacelia attenuata)

This starfish, with also 5 cylindrical arms, has a red to orange body. It can be confused with the red starfish, but this has the papulae in longitudinal lines, while in Echinaster their distribution is irregular.

Hacelia (Hacelia attenuata) (Foto: European Marine Life).
Hacelia (Hacelia attenuata) (Picture: European Marine Life).

STARLET (Asterina gibbosa)

The arms of this little star (from 2 to 4 cm) are not many differentiated from the central disc. It can present different colours, from greenish grey to red. It can be observed on rocks, sand or behind sea-grass.

Estrelleta (Asterina gibbosa) (Foto: Glaucus).
Starlet (Asterina gibbosa) (Picture: Glaucus).


Homosexuality is so animal

Fortunately for LGTB collective, greater and greater countries and societies understood that homosexuality is something natural and that it is not an illness. Anyway, despite this is true, it is also true that it is necessary to work hard to achieve equality on lesbian, gay, transexual and bisexual rights and to eradicate the false belief that homosexuality is unnatural. In the next weeks, in cities all over the world like Barcelona and Madrid will take place LGTB Pride parties. For this reason, this article hope to show clear examples that homosexuality is not exclusive of human, but present in many animals. So, there is no reason to continue believing in the argument that homosexuality is unnatural! 



Homosexuality is a fact present in many animal species. In fact, it has been documented in 1,500 of the more than 1 million described animal species (Bagemihl, 1999). Without going any further, a study of the California University demonstrated that in all the analysed species there were some individuals with homosexual traits or behaviours, including worms, flies, birds, dolphins and chimpanzees, among others.

In the animal kingdom, the concept “homosexuality” refers to any sexual behaviour between same sex animals, like copulation, flirt, mating, genital stimulation and young breeding. In the case of humans, it is more complex than this because there is much more factors and feelings are involved in this.

From the biological point of view, it is supposed that the goal of any species is its perpetuation. So, which is the function of homosexuality? There are many theories about it and they are not particular because for each species there is one explanation or another. Let’s explain three of them! Marelen Zuk, professor in biology at the University of California, propose that not producing their own offspring, homosexuals could help to breed and take care of their relatives, what also contributes to genetic pool. According to the biologist and phsycologist Janet Mann from the Georgetown University, it is a way of creating links and alliances between individuals. Finally, in the case of fruit fly and other insects, the evolutionary biologist Nathan Bailey suggest that the reason of their homosexuality is the lack of the gene that let them to distinguish between both sex. There is also the possibility that homosexuality doesn’t have any function. At any rate, homosexual behaviour may have evolutionary consequences, but it is still being studied.


On February 2004, New York Times published that Roy and Silo, two male chinstrap penguins (Pygoscelis antarctica) from the Central Park Zoo, coiled their necks, vocalized one to other and had sex. When they were exposed to females, they rejected them. Moreover, zookeepers gave them a fertile egg in order they incubate them and when the little penguin was born they feed her until she was able to live by herself. This is not an isolated case because it have happened more in this and other zoos, like in Bremerhaven Zoo (Germany), Faunia (Spain) and Dingle Ocean World (Ireland).

But this is not exclusive of captive animals. A research done on Adélie penguin (Pygoscelis adeliae) found homosexual behaviours in some of their young individuals. Another research was carried on king penguin (Aptenodytes patagonicus), in which it was observed that 28.3% of males flirted with other males. The reason in this case seems to be an excess of males or high testosterone levels. Anyway, it was found two partners (male-male and female-female) in which one knew the vocalization of the other.

Los pingüinos son un claro ejemplo de aves con comportamientos homosexuales (Foto de Listverse).
Penguins are a good example of birds with homosexual behaviours (Picture from Listverse).


The bonobo (Pan paniscus), apes very close to humans, are a good example of homosexual behaviours. They are so sexual. It has been observed that, in captivity or free, half of their sexual relationships are with same sex animals. In addition, females have sex with other females almost every hour. The main function of this is to strengthen links between animals. In the case of males, in order to reduce the stress after a fight, a penis fight takes place, that consists on rubbing their genitals together.

En los bonobos, las relaciones con seres del mismo sexo podrían servir para hacer los vínculos más fuertes (Foto de BBC).
In bonobos, same sex relationships may be done to strengthen social links (Picture from BBC).


Homosexual interactions between male killer whales (Orcinus orca) are an important part of their social life. When resident groups join together during summer and autumn to feed, males show flirting, affectionate and sexual behaviours between them. Normally, interactions take place one to one and lasts for an hour, but it can be longer. In this interactions, they caress, chase and carefully push one to the other. Another amazing behaviour is the beak – genital orientation, but it also take place between males and females. Just under the water surface, one male swims in an upside down position, touching the genital zone of the beak. Then, they dive together in a double helix spiral. This happens several times, but they interchange their positions. It is not strange to see them with the erected penis during this interaction. Despite it happens in all ages, it is specially abundant in young animals.

Las orcas (Orcinus orca) son cetáceos con comportamientos homosexuales habituales (Foto de WorldPolicy)
Killer whales (Orcinus orca) are cetaceans with homosexual behaviours (Picture from WorldPolicy)


A research made on guppies (Poecilia reticulata) demonstrated that the lack of females in the environment during a long period of time produce that males prefer other males even when there are females in the environment. Not only this. When males that had been with females during a long period of time are deprived from females for a short time (two weeks) they prefer males instead of females.

Los machos de guppy preferían otros machos cuando no había hembras en su ambiente durante largos períodos de tiempo (Foto de GuppyFish).
Male guppies prefer other males when there is no females in the environment during a long period of time (Picture from GuppyFish).


Some studies lay bare that there is a high rate of mating between same sex individuals in dragonflies. The reasons could be the lack of individuals of the other sex or that female tricks to avoid sexual advances of males could produce that males look for same sex individuals. One specific example is blue-tailed damselfly (Ischnura elegans), in which 17% of males of wild populations prefer male partners.

Los machos del cola azul (Ischnura elegans) prefieren a otros machos cuando son alojados en ausencia de hembras (Foto: L. B. Tettenborn, Creative Commons).
17% of male blue-tailed damselfly (Ischnura elegans) prefer other males (Picture: L. B. Tettenborn, Creative Commons).


  • Studies on wild occidental gull (Larus occidentalis) show that between 10 and 15% of females are homosexual. It has been seen that they show flirting rituals between them and that they set nets together. They only copulate with males to produce fertile eggs, but then go with their initial partner.
  • On domestic sheep, 8% of males from a flock prefer other males despite the presence of females. But this could benefit other males because they can present the same genes and pass to next generation. But this also benefits females by doing them more fertile.
  • The king of savannah, the lion, also have homosexual behaviours. It has been observed wild male and female lions with this behaviour, include mating.

  • In some species of seahorse, homosexual behaviours between females are frequent, more than heterosexual.


Homosexual behaviours are no only in humans, but they are more complex in people. The reason that lead to the development of these behaviours in animals are several: lack of females, to stablish harder links… but there are some examples in which the behaviour is permanent. Moreover, it has been seen that this behaviours are not artificial due to the captivity of animals, like humans in prison, but they haven in wild animals too. So, homosexuality happens in many animals and cannot be considered unnatural. In addition, if it is the result of natural forces it cannot be considered immoral. 




What do insects tell us about the health of our rivers?

Nowadays, concern about the health of inland waters (rivers, lakes, etc.) is growing, mainly due to increased use (and abuse) of these for human consumption. A few years ago, an expansion of the use of biotic indices took place, which allow us to determine the health of aquatic ecosystems; these indices usually use data such as presence, absence or/and abundance of different organisms known as ‘bioindicators’, that is, species that can be used to monitor the health of an environment or ecosystem. Among these organisms, there are a lot of arthropods.

Along this article, I will briefly explain what bioindicators are, the main role of arthropods as bioindicators and also introduce some of the most used bioindication indices to monitor the quality of riverine ecosystems in the Iberian Peninsula.

What is a bioindicator?

The term ‘bioindicator’ is used to refer to those biological processes, species or/and communities of organisms which can be used to assess the quality of an ecosystem and also how this ecosystem evolves over time, which is especially useful when changes take place due to anthropogenic disturbances, such as pollution.

Thus, in accordance with the above, a bioindicator can be:

  • A particular species, whose presence/absence or abundance rate informs us of the state of health of a studied ecosystem, or
  • A population or a community composed of various organisms which varies functionally or structurally according to the conditions of  environment.

Example: Lecanora conizaeoides lichen is highly resistant to pollution. Its presence on the studied ecosystem, coupled with the disappearance of another lichens, is indicative of high air pollution.

Lecanora conizaeoides (Picture by James Lindsey).

What do we consider a ‘good bioindicator’?

Do all the organisms have the necessary traits to become bioindicator subjects? The answer is no. Even though there is not a bioindicator prototype (because all depends on the studied ecosystem), we can resume here some of the traits that scientists take into account to select good bioindicator organisms:

  • They have to respond to disturbances that take place on their ecosystem to a greater or lesser degree. This response should be comparable to that emitted by the rest of the organisms of the same species, and this response also has to be well correlated with the studied environment disturbances.
  • Their response have to be representative of all the community or population.
  • They must be native of the studied ecosystem and also be ubiquitous (that is, to be present in almost all ecosystems of the same or similar characteristics).
  • They have to be abundant (rare species aren’t optimum subjects).
  • They must be relatively stable to moderate climate changes (i.e. a storm or a natural temperature change does not affect them more than normal).
  • They should be easy to detect and, as possible, they have to be sedentary.
  • They have to be well studied, both from an ecological point of view as taxonomic (to know, therefore, their tolerance to environmental disturbances).
  • Finally, they should be easy to manipulate and monitor in the laboratory.

The use of bioindicators will be optimized if we use entire communities or populations instead of using a single or a couple species, because this allows us to cover a wide interval of environmental tolerances: from organisms with a narrow tolerance range (that is, stenotopic) and sensitive to pollution, to very tolerant organisms that can survive in very polluted environments.

Thus, we will be able to know if an ecosystem is highly altered if we find only a very tolerant species and none of the considered sensitive species.

Bioindicator animals from inland waters

Nowadays, scientists use a lot of animals as bioindicators: from microorganism and microinvertebrates to terrestrial and aquatic vertebrates (micromammals, birds, fishes, etc.). In inland waters, and especially in the context of studies of riverine water quality, scientists mostly use aquatic macroinvertebrates to assess the quality of these ecosystems. Next, let’s see what a macroinvertebrate is.

What are macroinvertebrates?

The term ‘macroinvertebrate’ does not correspond to any taxonomic classification, but with an artificial concept that includes different aquatic invertebrate organisms.

Generally, is said that macroinvertebrates are organisms that can be trapped by a net with holes about 250μm.

Collecting macroinvertebrates by using a kick seine (Picture by USFWS/Southeast , Creative Commons).

Macroinvertebrates are mainly benthic, that is, animals that inhabit the substrate of aquatic ecosystems at least during some stage of their life cycle (although there are some that swim freely in water column or on its surface).

We can find a lot of macroinvertebrate groups in rivers and lakes, which can be classified in two main groups:

Picture sources: (1) Luis Silva Margareto ©, (2) DPDx Image Library, (3) Oakley Originals, Creative Commons, (4) Ryan Hodnett, Creative Commons, (5) Will Thomas, Creative Commons, (6) Duncan Hull, Creative Commons.

Among these groups, there are both tolerant organisms to environment distrubances (i.e. leeches) and sensitive organisms (i.e. a lot of larvae insects).

Most inland aquatic macroinvertebrates (≃80%) are arthropods (of which I will discuss in the next section), among which there are many insects and, especially, their larvae (which are generally benthic), whose study and observation play an essential role on calculating indices of water quality.

Importance of insects in bioindication

As I’ve said above, about 80% of macroinvertebrates of inland waters are arthropods and, mostly, different orders of insects in its larval or nymphal form. Let’s see some of the most common groups we can find in rivers and lakes:

Trichoptera (or caddisflies)

They are insects closely related to the Lepidoptera order (butterflies and moths). Their aquatic nymphs can build a shelter around their bodies made of substrate materials. We can distinguish them from other aquatic insect larvae because they have a couple of anal filaments provided with strong hoofs. They usually inhabit clear and clean waters with a lot of currents.

Trichoptera nymph (inside its shelter, left) and adult (right). Picture of the nymph by Matt Reinbold (Creative Commons) and picture of the adult by Donald Hobern (Creative Commons).

Ephemeroptera (or mayflies)

One of the most ancient orders of flying insects. Their aquatic nymphs, which usually inhabit rivers, are characterized for having three long anal filaments. Adults, which fly over the water surface, are very fragile and have a short life cycle in comparison with nymphs (the name Ephemeroptera is derived from Greek ‘ephemera’ meaning sort-lived, and ‘ptera’ meaning wings).

Ephemeroptera nymph (left) and adult (right). Picture of the nymph by Keisotyo (Creative Commons) and picture of the adult by Mick Talbot (Creative Commons).

Plecoptera (or stoneflies)

Flying insects very similar to Ephemeroptera order. Like these, they have anal filaments, but they differentiate from them because they have two apical hooks in each leg. They usually inhabit lakes and streams.

Plecoptera nymph (left) and adult (right). Picture of the nymph by Böhringer (Creative Commons) and picture of the adult by gailhampshire (Creative Commons).

Other groups of insects with aquatic larvae or nymphs

Among the most common insects inhabiting rivers and lakes we can also find species of Odonata order (dragonflies and damselflies), Coleoptera (beetles), Diptera (mosquitoes and flies), etc.

Among all the organisms mentioned above, there are very tolerant species to pollution (i.e. some Diptera larvae; this is the case of some species of Chironomidae family, which are very tolerant to organic and inorganic pollution due to the presence of heavy metals in their environment) and also very sensitive species (i.e. some species of Trichoptera order).

Depending on their tolerance to environment disturbances, scientists group these organisms (plus the rest of macroinvertebrates) into different categories that are assigned a value. This values, at the end, allow us to calculate water quality indices.

Biotic indices for riverine waters

The different pollution tolerance degrees among macroinvertebrates of a community allow us to classify them and to assign them a qualitative value (the bigger the number is, more sensitive are organisms to pollution). Thanks to these values, we can calculate different biotic indices, which are no more than qualitative values assigned to a community in order to classify it according to its quality: the greater the value is, better is the water quality.

One of the most used indices on the assessment of ecological state of rivers from the Iberian Peninsula is the IBMWP index (Iberian Bio-Monitoring Working Party), an adaptation by Alba Tercedor (1988) of the British index BMWP. In rough outlines, the greater the value is, better is the water quality. On this website you will find more details about this index, and also the pre-established values assigned to each macroinvertebrate (available in Spanish only).

In additions, there is also used the IASPT index, a complementary index which is the result of divide the IBMWP value by the number of identified taxa. This index give us information about the dominant community in the studied location. You can see more details on this website (available in Spanish only).

.      .      .

As you probably have seen while reading of this article, macroinvertebrates, and insects especially, play an important role in the study of inland water quality. Furthermore, their presence or absence is extremely important for the rest of the organisms of their ecosystem, because of what we must become aware of the problems deriving from the reduction of their number or diversity.


Head photography by U.S. Fish and Wildlife Service Southeast Region.


Amphioxus: animals which wanted to be vertebrates

Amphioxus is the goal of this article, animals that are included in the Cephalochordata group, inside the Chordata phyllum. Cephalochordata is a group of marine animals placed between invertebrates and vertebrates. Here, we are going to explain the importance of this animals in Zoology and its biology. 


Amphioxus, placed in the Cephalochordata subphyllum, is a marine animal in the Chordata group. Chordata includes, in addition to this group, Urochordata (among which there is Pyrosomida), hagfishes and vertebrates (fishes, amphibians, reptiles, birds and mammals). Despite they represent just a 4% of the amount of organisms in the planet (that correspond to 55,000 species), Chordata has had a very important evolutionary success.

The importance in Zoology of amphioxus is that present all the features of Chordata visible, so other chordata has lost them later or has modified them. These are the features:

  • Notochord: dorsal bar placed under the nervous system with a skeletal function.
  • Epineuria: dorsal position of nerve cord.
  • Endostyle: ventral groove in the pharynx that produce mucus to catch food and also produce iodized compounds. This gives thyroid.
  • Caudal fin: locomotive appendix.
Basic features of Chordata in a Cephalochordata (Picture obteined from here).
Basic features of Chordata in a Cephalochordata (Picture obteined from here).


Cephalochordata, known as amphioxus, is a group of 25 species of marine animals with a thin body, laterally compressed and transparent, that measures between 5 and 7 cm.

Brachiostoma lanceolatum (Foto: Hans Hillewaert, Creative Commons)
Brachiostoma lanceolatum (Picture: Hans Hillewaert, Creative Commons)


The skin of cephalochordata consists on one layer of prismatic cells with mucus glands that produce mucus, followed by the basal connective lamina and the dermis.

The most characteristic is notochord, which is composed by cells surrounded by a conjunctive case of actin and paramyosin. These cells have neurons that come from the nerve cord, allowing their contraction in diameter.

General anatomy of a cephalochordate. 1. brain-like blister 2. notochord 3. dorsal nerve cord 4. post-anal tail 5. anus 6. food canal 7. blood system 8. abdominal porus 9. overpharynx lacuna 10. gill's slit 11. pharynx 12. mouth lacuna 13. mimosa 14. mouth gap 15. gonads (ovary/testicle) 16. light sensor 17. nerves 18. abdominal ply 19. hepatic caecum 20. swim bladder 21. lateral line (Imatge: Piotr Michał Jaworski, Creative Commons)
General anatomy of a cephalochordate. 1. brain-like blister 2. notochord 3. dorsal nerve cord 4. post-anal tail 5. anus 6. food canal 7. blood system 8. abdominal porus 9. overpharynx lacuna 10. gill’s slit 11. pharynx 12. mouth lacuna 13. mimosa 14. mouth gap 15. gonads (ovary/testicle) 16. light sensor 17. nerves 18. abdominal ply 19. hepatic caecum 20. swim bladder 21. lateral line (Imatge: Piotr Michał Jaworski, Creative Commons)

They are swimming animals, with several fins: they have a dorsal fin, with vesicles placed one after another; a caudal fin and an anal fin, that extends from caudal fin till atriopore, opening from where water leaves the body. This anal fin bifurcates in two sheets and give place two folds to slightly stabilize them, which are known as metapleural folds.

They have a series of muscular fascicles called myomeres, which are in a shape of V with the apex in a forward position.

Oral region has an oral hood cirri to distinguish the entering particles, the Wheel organ (produce water movements) and a diaphragm to regulate the water entrance into the body. Pharynx is perforated for 80 fissures wit the endostyle in the basis, that produce mucus and it is pick into a dorsal lamina, where there are a small bars and then goes to oesophagus.


In order to feed, water with particles gets in through the mouth, it is propelled by the oral hood cirri and then cross the gill’s fissures, where food gets stuck thanks to mucus produced by endostyle, and finally goes to intestines. Here, food particles go to an hepatic cecum and phagocytosis process takes place. Then, water goes to the inner cavity of the body (called atrium) and leaves the body through a pore (atriopore). Digestive system is composed by the oral system, the pharynx with endostyle, the oesophagus and a digestive tube without muscles; which is composed at the same time by the intestine, the hepatic cecum (produce enzymes and absorb nutrients) and the anus, placed in the left side of the body. Its movement is due to a cilium ring.

Circulatory system doesn’t have heart and consists on two circuits: the ventral circuit goes from caudal fin to head and the dorsal, the other way around. The circulatory liquid goes to pharynx fissures to become oxygenated and has amebocytes, but it has not respiratory pigments, so breathing takes places by diffusion.

Excretory system is formed by solenocytes, cells that filter the blood from arteries, placed in the nefritic crest, that connects the atrium with a channel, so that allows that excretory products are expelled with the water in the atrium.

Nervous system consists on a simple nerve cord with a vesicle in the anterior part. This cord, in each metamere, emits two dorsal mixed nerves (with sensitive and motor nerves), which are branched off in two branches: a sensitive dorsal branch and a mixed ventral branch. This ventral branch goes to viscera, tegument and muscles. Sensitive system is constituted by a pigment spot (sensitive to light) and chemoreceptors.

About reproduction, each animal has just one sex (dioic animals), but its anatomy is very similar. They present between 25 and 38 gonads and to do the lay, the body wall is broken.


Amphioxus lives buried in sand seafloor of the shallow and coastal waters and in estuaries all over the world.

branchistoma lanceolatum
Common amphioxus (Branchiostoma lanceolatum) (Picture from UniProt)


  • Notes of the Chordata subject of the Degree in Biology of the University of Barcelona
  • Brusca & Brusca (2005). Invertebrates. Ed. Mc Graw Hill (2 ed)
  • Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Integrated principles of Zoology. Ed. Mc Graw Hill (13 ed)
  • Cover picture: Ricardo R. Fernandez

If you enjoyed this article, please share it on social networks to spread it. The aim of the blog, after all, is to spread science and reach as many people as possible. Your comments are welcome. 

This publication is under a Creative Commons License:

Llicència Creative Commons

Jellyfishes: what are they and how can we identify them?

Jellyfishes generally are marine animals that, like anemones, gorgonians and corals, like the red coral of the Mediterranean, are part of the cnidarians. In this article, we will see what is a jellyfish and how can we identify the most common ones. Moreover, we will know its danger. If you arrive at the end of the post, you will find a little surprise. 


Cnidarians are one of the most ancient animals that inhabit in the Earth, as they appeared 600 million years ago. They are characterized by the presence of a cells called cnidocytes, which have urticating organelles. It is thought that nowadays there are more than 9,000 species, classified in four classes: Anthozoa, Scyphozoa, Cubozoa and Hydrozoa. Despite they have a simple structure and functionality, they inhabit almost all the aquatic environments, mainly marine. There are two basic forms in the life cycle of a cnidarian: polyp, in which the animal is sedentary, with a tubular body and that reproduces asexually; and medusa, which can freely swim, with a bell-like shape and that reproduces sexually. There are organisms that only are one of this two stages, while others are polyp first and then medusa.

cnidarian body plan
Cnidarian body plan (Picture from College of DuPage BIO1151)


Like we have seen, jellyfishes are a morphological type of cnidarians and they don’t constitute a taxonomical group by themselves. Despite its shape is a little bit variable, they are much less variable than polyps because all of them live in a similar way. Almost all of them have a free life, but there are some cases in which they are retained in the polyp’s colony, acting as reproductive structures.

Its shape is bell-like, plate-like or umbrella-like, with a thick jellied layer. The external surface (exumbrella) is convex and the internal (subumbrella) is concave. Hydrozoa’s jellyfishes have the mouth in the central, lower part of the umbrella, in the end of a tubular extension called manubrium, while in the Scyphozoa this is very reduced. Tentacles hang from the umbrella and they are full of cnidocytes. Jellyfishes never form colonies, but they can live in shoals. Many people confuse jellyfishes with ctenophora, but with this features you can’t confuse them.


Anthozoa never produce jellyfishes.

Scyphozoa constitute the group of the biggest jellyfishes. The fact that they don’t have velum is what allows to differentiate Scyphozoa’s jellyfishes from Hydrozoa’s jellyfishes. The margins of the mouth form oral arms, which can be very long.

Many hydrozoa produce jellyfishes. These are almost transparent and small. Different from Scyphozoa’s jellyfishes, they present velum in the margin of the umbrella, which is a withdrawal of the tissues. 

General morphology of an Hydrozoan's jellyfish (Picture obtained from Systematic Biology).
General morphology of an Hydrozoa’s jellyfish (Picture obtained from Systematic Biology).

Jellyfishes of the Cubozoa class have cube-like shape, with one or more tentacles in each edge. They are usually very poisonous.


Despite the main species of the Mediterranean Sea can be easily identified by their looks, here we are exposing a simplified dichotomous keys  in order to recognise the 8 most common species.  

  1. Jellyfishes with velum (Hydrozoa’s jellyfish)
    • Pelagic colony in which the individuals are specialized on doing different functions (Subclass Siphonofora)
      • The central part of the colony is a flatten disc with a jellied consistency (Order Anthomedusae): Velella velella
      • Centre without a disc-like shape (Order Siphonophora): Physalia physalis
    • Jellyfish with a little contractile umbrella and a very mobile velum:
      • Entire margin of the umbrella: Olindas phosphorica
      • Margin of the umbrella with vertical lines that are divided into lobes: Solmissus albescens
  2. Jellyfishes without velum (Scyphozoa’s jellyfishes)
    • Jellyfishes with just one oral opening:
      • Short tentacles: Aurelia aurita
      • Long tentacles: Pelagia noctiluca
    • Jellyfishes with the mouth plugged by tentacles:
      • Long tentacles: Rhizostoma pulmo
      • Short tentacles: Cothylorhiza tuberculata


By-the-wind-sailor (Velella velella)

By-the-wind-sailors (Velella velella) are organisms with a diameter of the disc between 1 and 8 cm. This disc is circular or oval, blue and has an small sail. In the periphery, there is a ring of polyps with a tentacle-like shape. It means that they actually aren’t jellyfishes, but they are colonies with appearance of a jellyfish. This jellyfish is frequently seen in our coasts, whose danger is law, almost non-existent.

velella velella
By-the-wind-sailor (Velella velella) (Picture: Denis Riek)

Portuguese man of war (Physalia physalis)

The Portuguese man of war (Physalia physalis) present a buoyant part that measure 30 cm long and 10 cm wide, which is purple and transparent. In the submerged part, there are the tentacles, which are thin and long, so long that can measure 20-30 m (yes, metres!). Despite it is a rare species, it is highly dangerous due to neurotoxic, citotoxic and cardiotoxic toxins. Their bites are very painful and in some cases can produce death. Like the previous one, it is a colony of polyps, so it is neither a jellyfish.

physalia physalis
Portuguese man of war (Physalia physalis) (Picture from Madrimasd)

Olindias phosphorica

Olindias phosphorica is a jellyfish with a yellow and pink-blue umbrella, that present several channels towards the centre. Gonads are very patent and have a dun and reddish colouration. It has a high danger because its bite is painful, similar to a wasp.

Olindias phosphorica (Foto de Apena Team Torino)
Olindias phosphorica (Picture of Jordi Regàs)

Solmissus albescens

Solmissus albescens is characterised by the presence of 12-16 white tentacles and many quadrangular lobes. The umbrella is transparent and it looks like crystal. Measure between 2,3 – 3 cm of diameter.

Solmissus albescens (Foto de Jordi Regàs).
Solmissus albescens (Picture of Jordi Regàs).

Common jellyfish (Aurelia aurita)

Common jellyfish (Aurelia aurita) is an animal with an umbrella similar to a plate, with 25 cm of diameter, transparent but spotted in blue. It has 4 oral and long tentacles and other shorter in the margin. The four reproductive organs are purple-violet and has a shape similar to horseshoe. It is frequently seen, with a low danger because its poison is little toxic.

Borm (Aurelia aurita) (Foto de Alexander Vasenin, Creative Commons)
Common jellyfish (Aurelia aurita (Picture of Alexander Vasenin, Creative Commons)

Pink jellyfish (Pelagia noctiluca)

Pink jellyfish (Pelagia noctiluca) is the most frequent jellyfish in the Mediterranean. It can be recognized for the presence of a pink to red umbrella of 5-10 cm, from which hangs 4 oral tentacles and 16 marginal tentacles that can measure 2 m long. The surface of the umbrella has brown spots. Its danger is high because its poison is powerful, although it is not lethal. Curiosity: it is luminescent during the night.

pelagia noctiluca
Pink jellyfish (Pelagia noctiluca) (Picture of Jordi Regàs).

Compass jellyfish (Chrysaora hysoscella)

Compass jellyfish (Chrysaora hysoscella) is an animal with flatten umbrella that can achieve 30 cm of diameter, which is reddish white and with 16 brown strips. It has 4 oral and long tentacles and 24 marginal ones. Its danger is high, similar to pink jellyfish, despite is much less frequent than this.

Chrysaora hysoscella
Compass jellyfish (Chrysaora hysoscella) (Picture: Antoni López-Arenas i Cama)

Shiff arms jellyfish (Rhizostoma pulmo)

Shiff arms jellyfishes (Rhizostoma pulmo) are animals with an umbrella that measures between 10 and 40 cm of diameter, bell-like shaped, blue white and with a violet margin. They just have 8 oral, joined, not branched and blue white tentacles. It is frequently seen and its danger is high because it causes irritation and burning.

Rhizostoma pulmo
Shiff arms jellyfish (Rhizostoma pulmo) (Picture of Jordi Regàs)

Cotylorhiza tuberculata

Cothylorhiza tuberculata has a look similar to fried egg. The umbrella is flatten, measures between 20 and 35 cm of diameter, yellowish brown and with an orange protuberance in the middle. They have 8 oral tentacles covered with button-like appendixes in the end that are blue or white. Its danger is low and it is one of the most common.

Cotylorhiza tuberculata
Cotylorhiza tuberculata (Picture of Jordi Regàs)


The Institute of Marine Sciences (CSIC) is doing a research about jellyfishes, which is at the same time a citizen science project in the context of Seawatchers. If you want to collaborate, here there is the information.


If you enjoyed this article, please share it on social networks to spread it. The aim of the blog, after all, is to spread science and reach as many people as possible. Feel free to give your comments. 

This publication is licensed under a Creative Commons:

Llicència Creative Commons

Marine unicorns: Pyrosomida

This week, it has been spread through Social Media a video of a marine animal that is hardly seen. A group of divers, while they was diving in the Philippines, had the opportunity to observe a marine unicorn or Pyrosomida. Or maybe not! Some professionals said that this is a Thysanoteuthis squid egg case. In fact, it’s easy to distinguish a Thysanoteuthis squid egg case from a Pyrosomida: while in the egg case the balls (eggs) are distributed forming a spiral, in the Pyrosomida is an homogeneous mesh of organisms.  In this article we will talk about Pyrosomida. 


Pyrosomida are an order of marine animals included in the Chordata (a group that also includes vertebrate animals). In concrete, they are inside the Thaliacea class in the Urochordata or Tunicata group. There are 2,000 species widely distributed in all oceans, from close to shore to big depths. The reason of their name, Tunicata, is that they present a tough tunic that covers and protects the animal and that contains cellulose. Urochordata or Tunicata can be classified into three classes: Ascidians (or sea squirts), Larvacea and Thaliacea.

OLYMPUS DIGITAL CAMERAUrochordata classes. (A) Ascidian (Picture: Gronk, Creative Commons), (B) Larvacea (Picture: Rocco Mussat Sartor, Università degli Studi di Torino); and (C) Thaliacea (Picture: Mingorance Rodríguez, Creative Commons)


The Thaliacea are pelagic organisms similar to a lemon or a barrel, with a transparent and gelatinous body; and for this reason is difficult to see them when they are in the sea surface. Each individual is constituted for a belt of circular musculature and an inhalant and exhalent siphon in opposed poles. They can live in two ways: while some of them are solitaries, that is that each individual live independently one from another; other form colonies that can measure some metres.

They get around with body contractions, so they bomb water through the body and propel themselves with a water stream. This also allow them to breath and to feed on the particles of the water.

Most of them are luminescent and produce bright light during the night.


As we said, Pyrosomida are a group of marine pelagic animals included in the Thaliacea, which have been explained in the previous section.

The individuals live grouped in colonies, that measure between 20 and 30 cm in the Mediterranean, despite in tropical seas they can be 4 meters long and, in extreme cases, more than 10 meters. Each individual is called blastozooid and measures few millimetres, the body present an oral siphon inside the colony and a cloacal outside. Despite each organism maintains its individuality, they live together under the same tunic. Colonies have an inner cavity, which communicates with the exterior through an opening.

They are filtering animals, so water pass through faringial fissures with feeding particles and, thanks to an organ that produces mucus (called endostyle) and Listers tongues, they form a feeding cord.

Concerning to reproduction, each individual release gametes inside the colony and then these are expeled outside. After fecundation, it is formed the oozooid, called ciatozooid, which produce four individuals by budding, and these constitute the tip of the new colony. These four organisms, by budding, produce the entire colony.

Pyrosomida are the most bioluminescent organisms of the zooplanckton because they produce a blue light that can be seen easily some metres under the water.

800px-CombjellyYoung colony of Pyrosomida. These animal measured about 1 cm long. (Picture: Nick Hobgood, Creative Commons). 
800px-Pyrosoma_atlanticumPyrosoma atlanctium (Picture: Show_ryu, Creative Commons)


  • Notes of the subject Chordata of the Degree in Biology of the University of Barcelona
  • Hickman, Roberts, Larson, Anson & Eisenhour (2006). Integreted principles of Zoology. Mc Graw Hill (13 ed).
  • Storer, TI; Usinger, RL; Stebbins, RC & Nybakken JW (1975). Zoología general. Ed. Omega (6 ed)

If you enjoyed this article, please share it in the Social Media to spread it, so the goal of the blog is to disseminate science. 

This post is under a Creative Commons licence: 

Llicència Creative Commons

Licencia Creative Commons Atribución-NoComercial-CompartirIgual 4.0 Internacional.

Comb jellies: Ctenophora

Today we are talking about Ctenophora, commonly known as comb jellies, marine animals that for many years were considered jellyfishes due to its apparent similarity. Here, we will give what features can we use to distinguish them from cnidarians and, in addition, we will present examples of the Mediterranean. 


Ctenophora is a group of about 100 species of marine bioluminescent invertebrates that lives in all oceans, mainly in warm waters. The word ctenophora comes from Greek and means literally “comb carriers” and the reason is that they have eight comb rows, which are used for locomotion. Ctenophora are commonly known as comb jellies. It has been confused with cnidarians (anemones, jellyfishes, corals…) for years due to both groups present bilateral symmetry and for other features.

Figure1.1This ctenophore (Mnemiopsis leidyi) is an invasive species in the Mediterranean (William Browne, University of Miami).

There are some features to distinguish Ctenophora from cnidarians:

  1. They don’t have nematocysts, poisonous cells that are shot when something touch them. The species Haeckelia rubra is an exception because it takes these cells from cnidarians that feeds on.
  2. They have adhesive cells (colloblasts), which are used to catch preys.
  3. Generally they have pharynx.
  4. They never make up colonies; it means that every individual live independently.
  5. They have anal openings.
  6. They have 8 comb rows.


Ctenophora are hermaphroditic animals with a gelatinous consistency that can measure from 5 cm to 2 meters length. The feature that best defines them is the presence of eight equidistant comb rows in their surface, which are distributed from the aboral pole (the place of the anal openings) to the oral pole (the place where the mouth is). Each row is constituted by transversal sheets of joined ciliums. Their beat is produced simultaneously and allow their movement, taking place from the aboral to oral pole, what causes a movement with the mouth ahead. They can achieve a speed of 5 cm for second, it is about 2 km/h.

In the case of having tentacles (Tentaculata class), these are long, massive and very extensive (till 15 cm). These can be kept inside a tentacular sheaths. In the surface, they have colloblasts or adhesive cells, which produce a sticky substance that let them capture and retain small animals. In the case of not having tentacles, they present lobules, an expansions each side that help them to capture their preys.

Dryodora glandiformis Alexander SemenovCtenophore Dryodora glandiformis (Alexander Semenov): (1) comb rows y (2) tentacles.


Ctenophora are swimming animals, except some sessile species. They usually live in surface waters, but some species cn live at 3000 meters depth. They constitute gelatinous plankton. They are predators and carnivore. It means that they feed on larvae fishes, small fishes, crustaceans, cnidarians and other ctenophora.


In the Mediterranean sea, we can find different species of ctenophora.

Beroe ovata_5Ctenophora Beroe ovata (Foto: Lyubomir Klissurov). This animal has a mitt-like shape with a milky colour, pink or reddish.
Mn 07Ctenophora Mnemiopsis leidyi (Foto: Lyubomir Klissurov). This animal has a oval transparent body. Its rows produce blue-green colour when is disturbed (bioluminescence). In addition, it has iridisence (the property of certain surfaces that appear to change colour as the angle of view or the angle of illumination changes). It have several feeding lobes. It lives in the sea surface, in opened seas. It's an invasive species in the Mediterranean.
ktenofora3Ctenophora Pleurobranchia pileus (Foto: Lyubomir Klissurov). It has a pear-like shape, with long tentacles and its body is generally transparent, with some milky parts.
Callianira_bialata00_1Ctenophora Callianira bialata (Foto: Jordi Regàs). Animal with clear pink lobes. In the lower part, adults have two horns, with a similar length of the body. They have two tentacles. This animal lives in the Mediterranean and North Atlantic.
IH2-079 Venus' girdle, Cestum veneris 1Ctenophora Cestum veneris (Foto: Rokus Groeneveld). This cenophore has a belt-like shape of 1,5 meters lenght and 4 cm width. It's transparent. They live from sea surface to 200 meters depth.


If you have enjoyed reading this article you can feel free of share it. The goal of this blog is the dissemination of science. Thank you!

This publication is under a Creative Commons license:

Llicència Creative Commons Licencia Creative Commons Atribución-NoComercial-CompartirIgual 4.0 Internacional.

Red coral: the Mediterranean gold

This week we are talking about red coral (Corallium rubrum), an endemic animal in the Mediterranean that is endangered for the captures, either legals or illegals, in the last years. I could write a lot about this topic because my Final Project of the Master was about this species, but today I’m focussing in the biology and distribution of this animal. 


Red coral (Corallium rubrum) is an animal included in the phyllum Cnidaria (which includes jellyfishes and anemones), in the class Antozoa, which is characterized by the presence of a red skeleton of calcium carbonate. Its high value in the jewellery explain its conservation status. Red coral is a colonial animal, in which every individual is called polyp.

Las joyerías... ¿Futuro hábitat del coral rojo mediterráneoRed coral (Corallium rubrum). The red part is the skeleton of calcium carbonate, while every individual is white.


Red coral inhabits in rockery substrate between 5 and 300 meters depth, mainly in dark zones where the competence with fast growing species is little. It is in this places where red coral grows forming small and dispersed populations, while in deeper places it grows densely due to the lack of competence. It has an arborescent structure, achieving 50 cm length in the best cases.

Corallium_rubrumRed coral grows in places with low light in shallow waters, specially in coves.


Red coral is a species that reproduces more than once along its life (iteroparous species), with inner fertilization and incubates larvae. These larvae live just 30 days, what mean that they can’t travel very far and the persistence of the populations depends on the local recruitment. There are two separated sexes (dioic species), what mean that there are female and male individuals. The liberation of the eggs happens simultaneously during the summer, taking place sooner in shallower waters.

An important thing about its reproduction is that the size of the colony determines its reproductive potential: the larger individuals, despite are a small fraction of the population, are responsible of 50 to 98% of the production of gametes.


Red coral has an extremely slow growth, so its basal diameter grows 1 mm every 4 years approximately. In addition, its recruitment rate, that is how many individuals appear every year, is very low too, about 1.8 individuals for square meter every year.

Due to the intensive fishing of this spices to use it in jewellery, their populations are becoming smaller, with smaller  individuals, which has the lowest capacity in reproduction.


Red coral is an endemic species in the Mediterranean and East Atlantic, what means that this is only present in this region.

m1_clip_image002_0002Distribution map of red coral


Nowadays, red coral is not protected by any law, but its extraction is regulated by law. Any extraction without a licence is considered illegal.

 This post is under a Creative Commons licence:
Llicència Creative Commons Licencia Creative Commons Atribución-NoComercial-CompartirIgual 4.0 Internacional.

Voyage to the bottom of the deep sea (II): Biodiversity in the deep sea

This week we are continuing our voyage to the bottom of the deep sea. While last week we focused on the adaptations that fishes have suffered, this week we are focussing on the biodiversity. In concrete, we are explaining crustaceans, squids, cnidarians (corals, jellyfishes and anemones), fishes and worms. 


In 1840, the scientist Edward Forbes concluded that there wasn’t life under 550 meters depth. Nowadays, it is known that this is not true because recently it has been found a fish at 8,100 meters. It has been determined that the relative abundance of animals depends on depth. In fact, in general terms, the abundance decreases with depth, but this don’t exclude that there are a lot of species.




Amphipods are by far the most abundant crustaceans in the deep sea. They are small animals with the body compressed laterally and without a carapace, which feeds on carrion and live inside cavities made by themselves in the sea floor. These small animals are transparent, except for them eyes, which are red due to a pigment in the retina.

amphipode-abysseDeep sea amphipod. They are characterized by the presence of a transparent body with red eyes. (Picture from

Other deep sea crustaceans are stone crabs, with a carapace of 7.5 cm length and legs of about 15 cm; the armoured shrimp, one of the species that lives at 6,000 meters and has a length of 7 to 10 cm; and more.


In spite of the general thinking that deep sea squids are all large, like the giant squid, which can achieve a length of 18 meters; the truth is that this is an exemption because there are some spices of just 4 cm. They hunt with the suckers in the tentacles and driving the prey to the mouth. Most of these squids are bioluminescent and can regulate the colour, the intensity and the angular distribution of the light.

The Humboldt or jumbo squid (Dosidicus gigas) lives in the western coasts of Central and South Amercia and can achieve a length of 4 meters, which feeds on fishes and practise cannibalism.

Dosidicus_gigasHumboldt or jumbo squid (Dosidicus gigas). They have bad reputation because they attack divers.


Differences between shallower cnidarians and deep ones are due to differences in the food distribution. In the deep sea, anemones and corals don’t have directly phytoplankton and zooplankton, and they depend on the nutrient rain from the shallower waters of the ocean. On the other hand, jellyfishes have a slow metabolism to survive in hard conditions. It supposes slower growth, but a longer life.

To give an example, this crown jellyfish inhabits between 200 and 2000 meters depth and can measure until 15 cm. It feeds on small crustaceans and organic matter. Its red colour let them be camouflaged in the environment. In addition, they are bioluminescent animals.

Atolla wyvillei[3]Crown jellyfish. Its red colour let them be camouflaged in the environment.

Deep-sea jellyfishes are voracious predators, but also can be a prey for some fishes. They produce light discharges to attract small animals. To dissuade predators, they expel a brilliant particles stream.

An habitual feature of deep-sea jellyfishes, but also present in other groups, is gigantism. It means they are bigger than their equivalents in the shallow ocean. The possible explanation to this could be that bigger animals are more efficient than smaller to get food when the environmental conditions are almost constant during long periods of time.


Gonostomatidae fishes are the most abundant vertebrates in the Earth and live in the mesopelagic zone. Together with the lantern fishes, they represent a 90% of the captures in the pelagic trawling fishery. Deep-sea fishes usually have a length between 2,5 – 10 cm and a thin and soft body, but there are exceptions.

There are some examples here:

  • Anglerfish: These fishes inhabit in the deepest parts of the oceans and present the optimal colouration to absorb the few light that arrive and, in this way, to be camouflaged. They present a light in the end of the antenna, which let them to capture preys.
  • Spiny lantern fish: Because of its silvery body, this fish is not much vulnerable since its contour can’t be seen clearly. In addition, spiny lantern fish presents a bag in the eye with bioluminescent bacteria.
Pez linterna espinoso
Spiny lantern fish
  • Pelican eel: This animal can measure 2 meters long. Its enormous mouth are connected directly to the stomach.
Pelican eel
Pelican eel
  • Tripodfish: Tripodfish has long prolongations in its pelvic and caudal fins, which let them put on the sea floor, while it is waiting for its prey.
  • Black swallower: This small fish has the ability to dilate a lot its stomach and, in this way, it can swallow preys bigger than itself.
Black swallower
Black swallower



Deep-sea worms can be from microscopic to measure 2 meters long and are one of the most abundant and different invertebrates. They can be of different groups: polychaetes, tubular worms, sipunculids and equiurids. They live partly or totally buried in the sediments.

Tubular worms usually live in big groups near to thermal springs and present red bright gills as a consequence of a high level in hemoglobin to absorb oxygen. In addition, they can retain sulfurs, which will be used for symbiotic bacteria.

Riftia_fish_EPR_Kristof_Lutz-pTubular worms. They use the sulphur produce in the thermal springs thanks to symbiotic bacteria.

 This post is under a Creative Commons licence:
Llicència Creative Commons Licencia Creative Commons Atribución-NoComercial-CompartirIgual 4.0 Internacional.