Arxiu d'etiquetes: 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).

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

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Maratus sp.: The spider that wants to be a peacock

If I told you that there exists a 5 millimeters Australian peacock, would you believe me? Although we can find a large number of incredible animals in this country, scientists haven’t yet discovered such a small bird. However, we can find a small peacock-like animals: the peacock spiders (Maratus sp. Salticidae Family, also known as jumping spiders), whose ‘abdomen’ or opisthosoma (the posterior part of the body in some arthropods, including arachnids) have a flap-like extensions that they can unfold to the sides of its body as real peacocks do.

The last month we introduce you these organisms at our different websites (Facebook and Twitter). Through this article, you will learn its most relevant characteristics and you’ll find out the hidden function of its drop down opisthosoma.


Peacock spiders are a part of Salticidae family, whose members are also known as ‘jumping spiders’. This family has up to 5000 species (probably, they form the largest and diverse group of spiders known nowadays), and they’re located all over the world (they can be found even at the top of Mount Everest; this is the case of Euophrys omnisuperstes). Even so, most of them inhabit tropical forests.



Usually, spiders from Salticidae family get to be a size of few millimeters as adults (normally they don’t exceed 10mm long). From an anatomical point of view, the members of this group are characterized by its two big, simple front eyes flanked by two smaller ones, plus four eyes more located over them. The size and the position of these eyes give them an excellent vision in comparison with other spiders, and even compared to other group of arthropods its vision is extraordinary.

Look at these big eyes! Can you resist them?

Specimen of Paraphidippus auranticus (Picture by Thomas Shahan (c)).

Besides its excellent vision, these organisms have the ability to cover a distance of 50 times its length in one jump, because of what they received the nickname ’jumping’. Thus, their ability to travel long distances in just one leap and their extraordinary vision are the main traits that make these spiders being excellent predators: they hunt by stalking their prey without making spider webs or silk traps. Moreover, some of their legs tend to be longer than the others, letting them to catch preys way better.

Jumping spider predating a specimen of Diaea evanida or pink flower spider (Picture by James Niland on Flickr, Creative Commons).

Spiders of this family usually present a noticeable sexual dimorphism (that is, remarkable physiognomic differences between males and females). Jumping spider males have bigger oral appendixes (or pedipalps) than females, which they use during mating dance and copulation as much for attracting the attention of females as for giving females their spermatophore (capsule or mass containing spermatozoa) during mating.

Sitticus fasciger male (with dark big pedipalps) (Picture by sankax on Flickr, Creative Commons).
Sitticus fasciger female (Picture by sankax on Fickr, Creative Commons).

In addition to these developed pedipalps, males of some species of jumping spiders have a colorful, and even iridescent, opisthosoma (the posterior part of the body in some arthropods, including arachnids). Some of them even have an opisthosoma that can reflect UV radiations which are detected by females thanks to their extraordinary vision (as some studies suggest). In contrast, females use to be more cryptic or darker colored than males (but not always).



The secret life of bees

If we talk about bees, the first thing that comes to mind might be the picture of a well-structured colony of insects flying around a honeycomb made of perfectly constructed wax cells full of honey.

But the truth is that not all bees known nowadays live in hierarchical communities and make honey. Actually, most species of bees develop into a solitary life-form unlike the classical and well-known honey bees (which are so appreciated in beekeeping).

Through this article, I’ll try to sum up the different life-forms of bees in order to shed light on this issue.


Bees are a large diverse group of insects in Hymenoptera order, which also includes wasps and ants. To date, there are up to 20,000 species of bees known worldwide, although there could be more unidentified species. They can be found in most habitats with flowering plants located in every continent of the world (except for the Antarctica).

Bees pick up pollen and nectar from flowers to feed themselves and their larvae. Thanks to this, they contribute on boosting the pollination of plants. Thus, these insects have an enormous ecological interest because they contribute to maintain and even to enhance flowering plant biodiversity on their habitats.

Specimen of Apis mellifera or honey bee (Picture by Leo Oses on Flickr)

However, even though the way they feed and the sources of food they share could be similar, there exist different life-forms among bees which are interesting to focus on.



Most species of bees worldwide, contrary to the common knowledge, develop into a solitary life-form: they born and grow alone, they mate once when groups of male and female bees meet each other and, finally, they die alone too. Some solitary bees live in groups, but they never cooperate with each other.

Female of solitary life-form bees build a nest without the help of other bees. Normally, this kind of nest is composed by one or more cells, which are usually separated by partition walls made of different materials (clay, chewed vegetal material, cut leaves…). Then, they provide these cells with pollen and nectar (the perfect food for larvae) and, finally, they lay their eggs inside each cell (normally one per cell). Contrary to hives, these nests are often difficult to find and to identify with naked eyes because of its discreetness.

The place where solitary bees build their nest is highly variable: underground, inside twisted leaves, inside empty snail shells or even inside pre-established cavities made by human or left behind by other animals.

These bees don’t make hives nor honey, so these are probably the main reasons because of what they are less popular than honey bees (Apis mellifera). Although solitary bees are the major contributors on pollination due to their abundance and diversity (some of them are even exclusive pollinators of a unique plant species, which reveals a close relation between both organisms), most of the studies related with bees are focused on honey bees, because of what studies and protection of these solitary life-forms still remain in the background.

There exists a large diversity of solitary bees with different morphology:

3799308298_ff9fbb1bcc_n7869021238_a811f13aa4_n1) Specimen of Andrena sp. (Picture by kliton hysa on Flickr). 
2) Specimen of Xylocopa violacea or violet carpenter bee (Picture by Nora Caracci fotomie2009 on Flickr).
3) Specimen of Anthidium sp. (Picture by Rosa Gambóias on Flickr).

There are also parasite life-forms among solitary bees, that is, organisms that benefit at the expense of another organism, the host; as a result, the host is damaged in some way. Parasitic bees take advantage of other insects’ resources and even resources from other bees causing them some kind of damage. This is the case of Nomada sp. genus, whose species lay their eggs inside other bee nests (that is, their hosts), so when they hatch, parasite larvae will eat the host’s resources (usually pollen and nectar) leaving them without food. Scientists named this kind of parasitism as cleptoparasitism (literally, parasitism by theft) because parasitic larvae steal food resources from the host larvae.


From now on, we are going to stop talking about solitary bees and begin to introduce the pseudosocial life-forms, that is, bees that live in relatively organized and hierarchical groups which are less complex than truly social life-forms, also known as eusocial life-forms (which is the case of Apis mellifera).

Probably, the most famous example is the bumblebee (Bombus sp.). These bees live in colonies in which the queen or queens (also known as fertilized females) are the ones who survive through the winter. Thus, the rest of the colony dies due to cold. So is thanks to the queen (or queens) that the colony can arise again the next spring.

5979114946_9d491afd84_nSpecimen of Bombus terrestris or buff-tailed bumblebee(Picture by Le pot-ager "Je suis Charlie" on Flickr).


Finally, the most evolved bees known nowadays in terms of social structure complexity are eusocial bees or truly social bees. Scientist have identified only one case of eusocial bee: the honey bee or Apis mellifera.

Since the objective of this article was to refute the “all bees live in colonies, build hives and make honey” myth, I will not explain further than the fact these organisms form complex and hierarchical societies (this constitutes a strange phenomenon which has also been observed in thermites and ants) normally led by a single queen, build large hives formed of honeycombs made of wax, and make honey, a very energetic substance highly appreciated by humans.

Specimens of Apis mellifera on a honeycomb full of honey (Picture by Nicolas Vereecken on Flickr).

As we have been seeing, solitary bees play an important role in terms of pollination, because of what they must be more protected than they currently are. However, honeybees, and not solitary bees, still remain being on the spotlight of most scientists and a great part of society because of the direct resources they provide to humans.


  • Notes taken during my college practices at CREAF (Centre de Recerca Ecològica i d’Aplicacions Forestals – Ecological Research and Forest Applications Centre). Environmental Biology degree, UAB (Universitat Autònoma de Barcelona).
  • O’toole, C. & Raw A. (1999) Bees of the world. Ed Blandford
  • Pfiffner L., Müller A. (2014) Wild bees and pollination. Research Institute of Organic Agriculture FiBL (Switzerland).
  • Solitary Bees (Hymenoptera). Royal Entomological Society:
  • Stevens, A. (2010) Predation, Herbivory, and Parasitism. Nature Education Knowledge 3(10):36

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