Arxiu de la categoria: FISHES

Animal genitalia: birds and fish

The function of the reproductive system is apparently simple: to transmit the genes to the next generation. Why does it has so different and curious shapes in all kinds of animals? Would not  be enough with a simple sperm-emitting channel and a simple receiver? Find out in this post different shapes and reproductive strategies of birds and fish. If you want to learn about amphibia, reptile and mammal genitalia, click here.

ANIMALS’ SEX LIFE

Animals have different breeding strategies. In elementary school we learned that the fertilization can be:

  • External (outside the female’s body)
  • Internal (inside the female’s body)

And according to where the embryo develops the species are:

  • Oviparous: in an egg hatching outside the mother’s body (most fish, amphibians and reptiles)
  • Ovoviviparous: in an egg that hatch inside the mother’s body (sharks, vipers, boas…)
  • Viviparous: in a womb inside the body of the mother.

In high school we learned that there are strategists species:

  • r-strategists: they do not care for the offspring, which suffers a high mortality at birth. To compensate, they lay lots of eggs. They are usually short-live animals that quickly reach sexual maturity (invertebrates, fish, amphibians …)
  • K-strategists: they dedicate more energy to the care of the breeding, reason why they assure their survival and therefore the number of offspring in each laying or delivery is lower. Usually they are animals of longer life (dog, elephant, human …).

But if we analyze the reproduction in detail, it is not as simple as this. The sexual life of invertebrates is full of unlikely strategies to ensure fertilization, but we will talk about them another time and now we will focus on birds and fish.

THE EVOLUTION OF GENITALIA

The reproductive organs seem to have been the most diverse and most rapidly changed throughout evolution, giving rise to structures of almost every shape and size imaginable. If we believe that the only function is to give sperm, to receive it and to transport it to the ovule, we may be surprised by such diversity. In fact, the reproductive apparatus does much more than that and that is why the anatomy is so different between different animal groups.

Some insects, for example, use their penis for courtship, others use it to make sound and transmit vibrations to the female during mating. If the female likes the music, she will allow the male to take care of her offspring. Females also have adaptations to administer the sperm, like some flies, that can keep the sperm of different males in different receptacles of his reproductive apparatus.

Penis with bristles of the beetle Acanthoscelides obtectus. Source

The use of the penis in courtship and mating by the male and the administration of sperm in females would be two reasons that would explain the complexity of animal genitalia. The competition to ensure that a male’s sperm is actually the one that fertilizes all the female eggs would be another, with strategies as radical as plugging the ducts of the female once inseminated so that no other male can access it.

We will focus on this article on the genitals of fish and birds, do not miss the following post on amphibians, reptiles and mammals.

FISH GENITALIA

Although in most fish the fertilization is external, some have structures or pseudopenises to put the sperm inside the female.

SHARKS AND RAYS

They have the pelvic fins modified into two appendages called pterigopods or claspers, with which they introduce the sperm into the female.

Male shark (left) and female shark (right). Source

During copulation only one is used, which is filled with water thanks to a structure called siphon to expel it under pressure mixed with the sperm. According to the species, the young can be born from the mother’s body or from an egg.

Birth of a shark and fertilized egg. Source

POECIILIDAE FISH

Fish of the family Poeciliidae (guppys, mollys, platys, xhipos…), well known in aquarism, have the anal fin modified in a copulatory structure called gonopodium. They do not lay eggs, but the offspring are born directly from the mother’s body.

Male and female guppy. Source

PRIAPIUM FISH

It is a family of fish (Phallostethidae) that present the copulatory organ under the head. They mate face to face with the female, a thing almost unique in animals that live underwater. With the priapium they are anchored to the female and fertilize the eggs internally for a longer time than usual in other species.

Phallostethus cuulong male, discovered in Vietnam.. Source

Other curiosities in fish are possession of both sexes (hermaphroditism) or sex change, as in clown fish.

LOPHIIFORMES FISH

The best known representative of the Lophiiformes is the monkfish. In this order of fish, males are much smaller than females and they latch to females with their teeth, because of the difficulty of finding a mate in the abyssal bottoms. As time passes by, the male is physically fused to the female. He loses its eyes and its internal organs, except the testicles. A female may have six or more males (pairs of testicles) fused in her body.

Lophiiforme with a fused male. Source

BIRD GENITALIA

Most bird species (97%) do not have a penis and fertilization is performed with cloaca-cloaca contact (cloacal kiss, or cloacal apposition), a conduit that is used both as a reproductive and excretory system.

Left: urogenital apparatus of the male: F) testis, B) vas deferens, A) kidney, E) ureter, C) urodeum of the sewer. Right: urogenital apparatus of the female: A) ovary with mature follicle, F) infundibulum, E) oviduct, B) kidney, C) ureter, D) cloacal urodeum.Source

There are different hypotheses by which birds are believed to have lost their penis during evolution (since their reptilian ancestors did have it): to lighten the weight during flight, to avoid infections, by chance during the evolutionary process or to the females had better control over whom to reproduce. It seems that the latter would be the most accepted, since for example ducks fly long distances and have large and heavy penises.

But some birds do have a penis, which unlike mammals and reptiles, goes into erection by filling it with lymph, not blood.

WATERFOWL GENITALIA

Ducks, geese and swans are among the few birds that have a penis. The vagina of the ducks is shaped like a spiral clockwise, so when the male penetrates the female with his penis also spiral counterclockwise, if she is not interested flexes his vaginal muscles and penis leaves her body.

Vagina (left) and penis (right) of Mallard (Anas platyrhynchos).Source

The vertebrate with the longest penis in proportion to its body is precisely a duck, the Argentine diving duck (Oxyura vittata). He keeps it rolled up inside, but in erection he can be twice as long (42.5 cm) as his body (20 cm).

Oxyura vittata male with penis out. Unknown author

Duck penises, in addition to their different sizes and curvatures, can be smooth or have spines or furrows. This variability is due to the competitive pressure to overcome the females’ vagina. Both genitals are a clear example of coevolution. If you want to know more about coevolution visit this post.

Vaginas with blind ducts, penises with spines to extract sperm from previous copulations… ducks have a real “war” for reproductive control. In monogamous species such as geese and swans, the reproductive apparatus is not so complex, but in more promiscuous species, such as ducks, they are more complex and with longer penises so that it can be guaranteed that the male who has fertilized the eggs is the one who will take care of the chicks.

RED-BILLED BUFFALO WEAVER GENITALIA

This African passerine (Bubalornis niger) has a pseudopenis 1.5 cm long. It does not have blood vessels nor spermatozoa, reason why apparently its function is to give pleasure to the female and to favor the attraction of the male. Males in colonies have longer pseudopenises than those living alone, so the evolution of this appendix could also be explained by male-male competition.

Red-billed buffalo weaver. Photo by Reg Tee

OSTRICHES AND RELATIVES

African ostriches (Sthrutio camelus) are from the Ratites family, which also includes kiwis, rheas (American ostriches), tinamous, emus and cassowaries. All of them have a penis, and except for tinamu, they are running birds.

Ostriches about to mate. Source

The genitalia of the cassowary are really very peculiar. We have already discovered in this post how exceptional this animal is. Both sexes have a phallic appendage, but it is not connected to any reproductive organ. In the case of males, it is invaginated in a sort of “vaginal cavity”. At the moment of the copulation, it comes out (as if we turned out the finger of a glove), but the sperm leaves the cloaca, that is, from the base of this pseudopenis, not from the tip. In the case of females, the phallic appendage (sometimes referred to as clitoris) is a little smaller than in males.

Cassowaries mating. You can watch the video here

These male-female characteristics have given rise to rituals and beliefs in the folklore of New Guinea. They consider the cassowary an androgynous creature of mixed genders, therefore powerful because they have the attributes of both sexes. The remote Bimin-Kuskusmin tribe (Central New Guinea) celebrates rituals where intersexual people are considered representatives of these animals, so they are revered and powerful. On the other hand, Mianmin people tells stories about a human woman with a penis that became a cassowary.

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REFERENCES

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Vision in fish: the world from the eyes of a fish

Fishes, like other vertebrates and many invertebrates, have developed mechanisms to perceive light, which quickly disappears with depth. Let’s see vision in fishes.

VISION IN FISH: THE WORLD FROM THE EYES OF A FISH

Vision consist on the perception of the light in the environment around us. Because of fishes live in the aquatic environment, the light is quickly extinguished. In addition, because they live in very different habitats, the system to perceive the light varies considerably between species. For details of the vision in general, you can read how animals see the world.

LIGHT UNDER THE WATER

Before beginning to speak of vision in fish, it is important to understand the light pattern with the increasing depth.

As mentioned, light rapidly  disappears with depth, but not all colours do the same: red light is absorbed in the first 10 meters; orange and yellow, in the 30 m; green in the first 50 m, and blue at 200 m. For this reason, when we dive we see starfish black!

The amount of light in the water column that led oceanographers  to distinguish two zones: the zone in which there is light is called photic and where light lacks is known as aphotic (from 1,000 meters). The photic zone can be subdivided into:

  • Euphotic zone: is the most superficial layer and where photosynthetic organisms can carry out photosynthesis. Although it can vary, it is usually considered that reaches 200 meters.
  • Oligophotic zone: the area that receives enough sunlight to permit the organisms to see, but not enough to carry out photosynthesis (between 200 and 1,000 m).

THE EYES OF FISHES

The organisation of eyes in fishes is similar to mammals, but they have their singularities.

ojo pez, vision peces
Despite the eyes of fishes are similar to the rest of vertebrates, they have some singularities (Picture: Macroscopic Solutions, Creative Commons).

The lenses of bony fishes are spherical, while elasmobranchs have slightly flattened lenses, and have a high refractive power because the cornea is in direct contact with water. In addition, to focus the images do not change the shape of the lens, but move them forward or backward. This mechanism is also carried out by snakes.

Another curiosity of the optical system is that, in many fish, the iris cannot be contracted, so that the pupil cannot close if the light intensity increases. To avoid overexposure, the rods and cones (photoreceptor cells, the first detect colours and second not) change their shape and melanosomes (pigment organelles) are arranged so that make “shadow”. The opposite process happens when light is scarce.

Fish can have up to four different types of cones, one of which detects ultraviolet light. Ultraviolet cones are used to detect plankton, although not all have. Some species only have them when they are larvae and others only during certain stages of adulthood. For example, the rainbow trout (Oncorhynchus mykiss) have ultraviolet cons only when living in the freshwater.

vision peces, trucha arcoiris, Oncorhynchus mykiss
Rainbow trout (Oncorhynchus mykiss) have ultraviolet cons only when living in the freshwater  (Picture: Eric Engbretson, Creative Commons).

On the other hand, some fishes, such us elasmobranchs and deep fishes, only have rods. Thus, they cannot see colours.

Another notable difference is that in the bony fishes, eyes grow throughout life and, therefore, so does the retina. In addition, the retina has the ability to regenerate in case of getting hurt.

Finally, some nocturnal fish and sharks, among others, present tapetum lucidum behind the retina, the function of which is to return light rays that have escaped from the retina to the retina to improve vision. It is also present in some mammals, such as cats.

CHANGES IN EYES IN MIGRATORY SPECIES

The adaptive capacity of fish is so great that some migratory species change the eye’s anatomy according to the environment. The lampreys, for example, are fishes that migrate from rivers to the sea. They have a different pigment for each environment: in freshwater is porphiropsin (red) and in the sea is rhodopsin (blue).

vision peces, ojos lampreas
The lampreys change the pigments of their eyes according to the habitat they are (Picture: Aquarium Finisterrae, Creative Commons).

Eels also change the habitat and also change their eyes. When they are ready to begin the migration to the sea, the diameter of the eye doubles, the lens size increases and the number of cones increases significantly (only 3% of the photoreceptors before the migration), among others changes.

VISION IN DEEP-SEA FISHES

Deep-sea fishes have a set of adaptations to live in the deep ocean. Vision also have its adaptations.

The mesopelagic fishes (living in the oligophotic zone) are characterised by big eyes, with wide pupils and big lenses. Some species, such as the telescopefish (Gigantura) also have tubular eyes.

ojos peces, gigantura chuni
Some fishes, such as the telescopefish (Gigantura), have tubular eyes (Picture: Hadal~commonswiki, Creative Commons).

The barreleye (Macropinna Microstoma) also presents tubular eyes, which are usually directed upwards to identify the silhouettes of fishes. Unlike other fish with such eyes, it can rotate them.

Macropinna microstoma, ojos tubulares
The barreleye (Macropinna microstoma) have tubular eyes, which can rotate (Picture: MBARI).

Batipelagic fishes (living below 1,000 meters) usually have either small eyes or degenerate. In this case, the eye lenses are very large compared to the rest of the eye, which does not allow them to create clear images and, moreover, they can only detect objects close to them.

ADAPTATION OF VISION TO DARKNESS

When a fish moves from a lit area to another dark, adaptation to the second condition is done in two phases: the first phase is mainly due to the sensitivity of cones, while in the second phase dominate rods.

In the zebrafish (Danio rerio), for example, the first phase lasts 6 minutes and the sensitivity is mainly due to cones. After this time, the sensitivity is mainly for rods. However, rods need 20 minutes in the dark to work at their maximum efficiency.

vision peces pez cebra
In zebrafish (Danio rerio), the first stage of adaptation in the darkness lasts for 6 minutes (Picture: Thierry Marysael, Creative Commons).

OTHER CURIOUS ADAPTATIONS IN THE EYES OF FISHES

Some fish species have strikingly curious adaptations in their eyes. We are giving you some examples.

Limnichthys fasciatus is a small fish that lives in shallow, well-lit water, which buries in the sand and only leave the eyes outside the sand. The retina is very thick, but at one point presents a sharp narrowing of the retina, which magnifies images on this point. In other words, this fish has telescopic vision.

El pez Limnichthytes fasciatus tiene visión telescópica, gracias a la presencia de una fóvea en la retina (Foto: Izuzuki, Creative Commons).
Limnichthytes fasciatus have telescopic vision thanks to a fovea in the retina (Picture: Izuzuki, Creative Commons).

Most fishes have adapted eyes to underwater vision. However, some species, such as the Atlantic flying fish (Cypselurus heterurus) has also adapted to the air. To get a good view out of the water, the cornea, rather than being spherical, has a triangular shape with three flattened zones.

A fish with an extreme adaptation to the aquatic and aerial view is the four-eyed fish (Anableps anableps). This freshwater species swims with the upper half of each eye out of the water and the other half in the water. Both lenses and the whole eye are extremely asymmetric, so that they can perfectly see inside and outside of the water. If you want to see this fish swimming with its eyes half-submerged in the water, you can watch this video:

As you have seen, the vision in fish is a great deal more complex than it seems, since the water largely determines the anatomy of the eye and its adaptations. Do you know some other curious case of vision in fish? Leave your comments in the article!

REFERENCES

  • El mar a fondo: La luz en el mar
  • Farrell, A (2011). Encyclopedia of Fish Physiology: From Genome to Environment. Volume 1: The senses, supporting tissues, reproduction and behaviour. Academic Press. 2266 p.
  • Hara, T. & Zielinski, B. (2006). Fish Physiology: Sensory Systems Neuroscience. Academic Press. 536 p.
  • Hill, Wyse & Anderson. (2006). Fisiología Animal. Editorial Médica Panamericana. 916 p.
  • Rhodes University: Fish Sensory System
  • Cover picture: Forum Acvarist

Difusió-anglès

The courtship in fishes

One of the things that really grab our attention in the documentaries are the different forms of courtship in the animal kingdom: the amazing dances of the birds of paradise, the pirouettes of the dolphins… But, how is the courtship in fishes?

INTRODUCTION

With almost 33,500 species of fish living around the world, it is easy to imagine that among the ichthyofauna there are a multitude of different ways of courting the opposite sex. It is for this reason that, in this article, we are going to show some of the most amazing courtships. Let’s begin!

THE CURIOUS CASE OF THE PUFFER FISH

The Japanese puffer fish creates underwater artwork to attract the attention of a female. Their creations, which were discovered just 10 years ago, consist of a pattern of circles on the seabed, similar to crop circles. It is known that these circles are part of the courtship ritual of puffer fish.

This amazing pattern is drawn by the males with their fins, with which they move the sand. The final result is obtained after a week of intense work. The pattern, perfectly geometric, measures about 2 meters in diameter and can have different shapes: from a basic model to complex patterns composed of lines, valleys and ridges, passing through mounds in the form of a crater. Some artists even look for other elements, like shells of molluscs and sediments of different colours, to beautify their work of art.

El pez globo utiliza sus aletas para dibujar el patrón de círculos (Foto: Quo).
The puffer fish use its fins to draw the circle pattern (Picture: Quo).

The most curious of all is the fact that the construction is made so that the water currents attract the finest sediments towards the centre. In addition, the radial lines that leave the centre cause that the speed of the water is reduced a 25% in the middle.

Watch this video so that you marvel at the art they have:

Females have the last word. The selection of the male is performed according to the circles: a female will put the eggs in the centre of the circular shape that she thinks is the best. Then the male externally fertilises them and then the female foes away, leaving them in charge of the male, which protects them for 6 days.

Anyway, although it is not confirmed, everything seems to indicate that what really interests the female is the fine sediment of the centre, but if so, it is not known the reason why.

Puffer fishes are not the only ones that perform this type of courtship. Another example is the featherfin cichlid (Cyathopharynx furcifer), that lives in Tanganyika Lake (Africa). In this case, they create a mound of sand, as can be seen in the photo.

Nido del cíclido de la especie Cyathopharynx furcifer (Foto: Arkive).
Nest of the species Cyathopharynx furcifer (Picture: Arkive).

THE ROMANTICISM OF SEAHORSES

The courtship in seahorses has nothing to envy to the puffer fish.

Los caballitos de mar son unos auténticos románticos (Foto: Alex Griffioen).
Seahorses are really romantic (Picture: Alex Griffioen).

It is known that seahorses are monogamous species, which means that a male mates with a single female, being reciprocal also for the female.

The courtship in these curious fish includes caresses, hugs and changes of colour. Even an authentic dance that can last up to 8 hours, in the style of Dirty Dancing. In fact, the males and females of these fish dance every morning to strengthen their bond. Such dances are also useful to know if the other partner is ready to reproduce.

I leave another video for you to see how they dance:

As you have observed, the dance consists of a set of synchronised movements, in which the two animals swim side by side as if one were the reflection of the other.

A curious fact about seahorses is that the female lays the eggs (approximately 1,500) in a male’s bag, so that when the eggs are fertilised and the embryos develop, it is the male that gives birth the juveniles at the end of 45 days.

MARIACHI’S STYLE

Okay, maybe I have exaggerated by saying that there is a fish that wins over its partner in the mariachi style, as if using a band of musicians to get its “beloved”.

Anyway, the truth is that the drummer fish (Aplodinotus grunniens) produces grunting and rumbling sounds to seduce the female.

La corvina de agua dulce produce diferentes tipos de sonido para atraer a la hembra (Foto: Dr. Hans Schneider).
The drummer fish produces different types of sounds to grab the female’s attention (Picture: Dr. Hans Schneider).

If its “music” grab the attention of the female, the couple meets on the surface of the water, where the female releases thousands of eggs into the water.

COURTSHIP WITH ROLE SEX CHANGE

We have already spoken in an earlier article about the fact that sex change is present in some animal species. What we did not comment is that some simultaneous hermaphrodite species (male and female at the same time) change roles several times during courtship.

This is the case of the fish of the genus Hypoplectrus, typical of coral reefs of the Caribbean Sea and the Gulf of Mexico.

Los peces del género Hypoplectrus cambian de sexo durante el cortejo (Foto: Laszlo Ilyes).
Fishes included in the genus Hypoplectrus change the sex role during the courtship (Picture: Laszlo Ilyes).

In this fish, courtship ensures that the two members of the partner are going to produce eggs fairly, in what has been called “egg trading”.

In this behaviour, the couple alternates the role of the sexes up to four times throughout a single mating. As males, they fertilise their partner’s eggs. To do this, the male is curved around the body of the female. I leave another video for you to see it better:

On the other hand, as females, they produce the eggs in batches, in order to ensure that they are not all generated by the same individual and, therefore, that the egg production effort is distributed.

CONCLUSION

In this article we have shown only 4 different types of courtship in fishes. As we have said, there are 33,500 species of fish recorded in the world, so that dealing with the subject in depth would require an entire encyclopedia.

Do you know any species of fish that has a curious form of courtship? You can leave your contribution in the comments of this article.

REFERENCES

Difusió-anglès

Sex change is also in animals

Sex change is not only present in humans (known as transexuality), but there are some examples of animals that change sex, that is, they are born male or female, and throughout his life, species change to the opposite sex. Do you want to know some of these species? Remember that you can also read a post in this blog about Homosexuality in animals.

TRANSEXUALITY IN ANIMALS

The animal sex change is a fact not very widespread, but is present especially among fish and some mollusks, jellyfishes, crustaceans, echinoderms and worms.

However, in the case of animals, the term used is not transsexuality. The change of sex in the animal kingdom is a particular type of hermaphroditism: sequential hermaphroditism.

This change of sex is usually genetically programmed and  it is influenced by the environment in which it develops. However, at birth they have already both sexes, so that sex determination is not given by the genes.

There are different types of sequential hermaphroditism:

  • Protandry: when an organisms is born male and changes to female, such as the clownfish (Amphip
  • Protogyny: when an organisms is born female and changes to male, such as wrasses.
  • Bidirectional sex change: when an organism has both full female and male sexual organs, but act as a male or female during different stages of its life, such as the  fish Lythrypnus dalli. 

It is clear that this strategy supposes an important benefit in front of other species: in front of extreme conditions, the organisms have the capacity of assuring the future generations by changing their sex.

THE CLOWNFISH

The clownfish is one of the best known examples of sex change in the animal kingdom. Our friend Nemo, throughout its life will become a female. Clownfishes are all born males, but after a certain age they change sex. They can also change sex in case the female dies, so although Nemo’s mother died, found his mother in his father.

pez payazo cambio de sexo
Couple of clownfishes, with the female bigger than the male (Picture: Georggete Douwma, Arkive).

The form of reproduction of these very colourful and known fishes is most curious: in each anemone, cnidarian animals with which they live in symbiosis, lives a harem, consisting of one female (bigger in size than the male), a reproductive male and several non-breeding males.

Ciclo de vida del pez payaso (Foto: The fisheries blog).
Cycle of clownfish changing sex (Picture: The fisheries blog).

Si la hembra muere, el macho reproductor se transforma en hembra y el macho no reproductor de mayor tamaño madura sexualmente.

JANTHINIDAE SEA-SNAILS

Janthinidae is a group of sea-snails with a unique feature: they use their mucus to produce a bubble raft to float in the ocean. Some of them can produce a bubble per minute.

janthina janthina
Violet sea-snail (Janthina janthina) (Picture: Roboastra).

Well, this family of gastropods is made up of individuals who may change sex. Like the clownfish, organisms are born male and then change to female.

BLUESTREAK CLEANER WRASSE

The bluestreak cleaner wrasse (Labroides dimidiatus) is a fish in which the sex change is triggered by some behavioural patterns.

Tordo limpiador (Labroides dimidiatus) (Foto: Darwin Books Cats).
Bluestreak cleaner wrasse (Labroides dimidiatus) (Picture: Darwin Books Cats).

There is usually a dominant male that keeps a harem of females, but if he dies, the dominant female will assume the position of the male in a few hours, courting other females although the change of sex can be extended for two weeks.

THE COMMON SLIPPER SHELL: WHEN SIZE MATTERS

The common slipper shell (Crepidula fornicata) is a marine snail in which the sex change is produced by the size of the animal. This molluscs are born male but, at a certain size, they become females.

They are very curious snails: they live stacked on the top of other animal, with larger organisms at the bottom. This means that the individual of the base is a female and males are above. Thus, when the female dies, the larger male becomes the female of the group.

crepidula fornicata
Common slipper shell (Crepidula fornicata) (Picture: Dr. Keith Hiscock).

It is an exotic species in Spain, which could be living in the whole Galician coast. Anyway, its natural distribution area is North America.

THIS ALSO HAPPENS IN THE MEDITERRANEAN

Until now, we have explained species that live far from the place we life, but the truth is that this behaviour also happens in some Mediterranean species. Some examples are the starlet cushion star (Asterina gibbosa) and the ornate wrasse (Thalassoma pavo).

The ornate wrasse is one of the most colourful fishes in the Mediterranean sea. In that case, they are born females, but according to the sex ratio, they can change to males.

pez verde thalassoma pavo
Ornate wrasse (Thalassoma pavo) (Picture: Matthieu Sontag, Creative Commons).

REFLECTION

If you are one of those who thing that sex change in human beings is unnatural, you have read some examples of animals that change their sex.

All you need is Biology is a LGTB-friendly blog and we love everbody equally. More love and respect, and less hate!

REFERENCES

Difusió-anglès

Large sharks that eat plankton

Baleen whales are known to be the biggest animals on Earth that feed on plankton, but they are not the only big animals that eat these tiny organisms. In this post, you will discover three species of shark that consume plankton. 

WHAT IS PLANKTON?

Plankton refers to those tiny organisms that drift in the water with the currents. They can be classified as phytoplankton, which include the planktonic algae and other autotrophs or producers that may be the most important producers in many marine ecosystems, or as zooplankton, which include the heterotrophic plankton (the primary consumers). There are so many groups of organisms that spend all their life in the plankton, but other may be present just in some phases.

Organisms included in the zooplankton (Picture: Sci-news).
Organisms included in the zooplankton (Picture: Sci-news).

So many marine groups of animals feed on plankton, but baleen whales are known to be among the biggest animals that eat these small creatures. Some sharks are known to eat these little organisms: the famous Whale shark (Rhincodon typus), the awesome Basking shark (Cetorhinus maximus) and the stunning Megamouth shark (Megachasma pelagios). Did you know about these three species of planktophagous sharks? 

THE BIGGEST FISH IN THE WORLD

The biggest fish in the world usually is 12 meters long (but may achieve a length of 15 meters), weights 22 tonnes and its mouth is so wide that could swallow a car. We are referring to the famous whale shark (Rhincodon typus).

Whale sharks are true sharks, so they breathe using gills and are cold-blooded fishes. The reason of their name is the fact that they feed on plankton in a similar way of whales: they swim slowly (1,5-5 km per hour) with the mouth opened (which has dense filter screens) and swallow the small organisms present in the water, like coral and teleost spawn, krill, copepods, jellyfishes, small cephalopods and schooling fishes. They are also reported to feed almost vertically in the water. They can be identified so easily for their colossal size and for their dark blue colouration with white spots all over the body.

The whale shark is the biggest fish in the world (Picture: Mauricio Handler).
The whale shark (Rhincodon typus) is the biggest fish in the world (Picture: Mauricio Handler).

Little is known about the biology of these big animals. They live usually alone in coastal, pelagic and oceanic waters of the tropics and warm temperate zones, except the Mediterranean sea. They migrate extremely large distances. Every spring, they migrate to the continental shelf of the central west coast of Australia.

Due to direct and indirect fishing, their populations have reduced and the IUCN classifies them as a vulnerable species. Nowadays, their fishing is widely forbidden. Do you know that swimming with whale sharks has a negative impact on their populations? 

THE BASKING SHARK

The world’s second largest fish is also a filter feeder and is also a shark: it is the basking shark (Cetorhinus maximus). This shark lives in coastal and pelagic of temperate and boreal waters, but is a migratory species.

They can be distinguished from other sharks for their big mouth, small teeth and long gill slits. They are solitary animals, but sometimes they can form a small group of animals. Basking sharks consume small fish, fish eggs and zooplankton. They capture them on their gill rakers with the help of mucus secreted in the pharynx. In average, they swim at 3,7 km per hour. So, how many tonnes of water filter per hour? 

The basking shark (Cetorhinus maximus) is the second biggest fish in the world (Picture: FLMNH).
The basking shark (Cetorhinus maximus) is the second biggest fish in the world (Picture: FLMNH).

Their conservation status is vulnerable, but is considered to be endangered in the North Pacific and the Northeast Atlantic subpopulations. The fact that their fins are among the most valuable in international trade explains their conservation status. Moreover, accidental fishing is a threat to consider.   In some regions, such us European Union, they are protected by law.

THE MEGAMOUTH SHARK

The megamouth shark (Megachasma pelagios) is a large oceanic shark (at least 5 metres long) with long pectoral fins, but sometimes is littoral on the continental shelves. They live in tropical and subtropical waters. Little is known about this species because it was discovered in 1976.

The megamouth shark (Megachasma pelagios) is a strange planktophagous shark (Picture: ).
The megamouth shark (Megachasma pelagios) is a strange planktophagous shark (Picture: Theethogram).
Its low-flow filter apparatus suggest it is less active than the whale and basking sharks. Megamouth might swim slowly with its jaws wide open to capture their preys by suction. A surprising feature of this shark is its bioluminescent mouth, which acts to attract prey. Its conservation status is unknown, but accidental fishing might be a threat.

REFERENCES

  • Camhi, MD; Pikitch, EK & Babcock, EA (2008). Sharks of the Open Ocean: Biology, Fisheries & Conservation. Blackwell Publishing.
  • Carrier, JC; Musick, JA & Heithaus, MR (2010). Sharks and their relatives II: Biodiversity, adaptive physiology and conservation. CRC Press.
  • Castro, P & Huber ME (2003). Marine biology. The McGraw-Hill (4 ed).
  • Fundación Squalus (2011). Guía para la identificación de especies del Programa de avistamiento de tiburones y rayas de la Reserva de Biosfera SEAFLOWER.
  • IUCN: Cetorhinus maximus 
  • IUCN: Megachasma pelagios
  • IUCN: Rhincodon typus
  • National Geographic: Whale sharks 
  • Oceana (2008). Guía de los Elasmobranquios de Europa.

Difusió-anglès

How do fish breathe?

It’s probable that you know that most of the fishes that inhabit in the Earth breathe due to the presence of gills. However, this is not the only respiratory system present in fishes. In this post, we will review different types of breathing in fishes. 

INTRODUCTION

The respiratory system of fishes have to be adapted to two important limitations of underwater life. On the one hand, the amount of dissolved oxygen is smaller in the water than in the air: at 23ºC, air has 210 ml of oxygen per litre of air, while in freshwater is about 6,6 ml/l and in salt water is 5,3 ml/l. On the other hand, water is much more dense and viscous than air. These limitations explain the adaptations in the breathing of this group of animals.

BREATHING WITH GILLS

The oral cavity of teleostei fishes (modern ray-finned fishes) is communicated with the exterior through the mouth and pharyngeal pouches, lateral openings present in the pharynx in which the gills develop. Thanks to the opercle (or gill cover), a hard structure placed in each side of the head, gills are protected.

The structure of the gills is complex. From branchial arches, curved structures that pierce through pharyngeal pouches in each side of the head, two gill filaments grow forming a V. These filaments produce the gill lamellas, folds of the wall’s filaments with a perpendicular disposition. In each side of the filament, we may find between 10 to 40 lamellas per mm. So, it is in these lamellas where the gas exchange happens because they are a very thin wall of tissue and are well supplied with blood.

Structure of the fishes' gills (Picture: AS Biology Ms Timms).
Structure of the fishes’ gills (Picture: AS Biology Ms Timms).

So, the oxygenated water that passes through the mouth cross the gills and finally abandon the oral cavity through the opercle, while the blood flows in the opposite direction across lamellas to catch the oxygen.

The larva of many fishes have external gills in each side of the head. In the rest of phases, gills become internal. Fishes with a respiration with gills are hagfishes, lampreys, elasmobranchii and bony fishes.

Les mixines són peixos amb respiració branquial (Foto: Natureduca).
The hagfish is a species with a breathing with gills (Picture: Natureduca).

BREATHING WITH LUNGS

About 400 bony fish species are known to have the ability of breathing from air, most of them living in freshwater ecosystems. Anyway, most of them have both gills and lungs. These species with the two mechanisms usually use the air in certain occasions:

  • When the oxygen level in the water goes down.
  • When the temperature increases, so the higher the temperature is, the higher are the oxygen necessities.

Lungfishes (Dipnoi) are among the species with the most advanced system. Their lungs have crests and septums similar to those in the lungs of amphibians. The Australian lungfish (Neoceratodus) can breathe with both gills and a lung. African lungfishes (Protopterus) and the South American lungfish (Lepidosiren) breathe with a complex lung and single gills. These fishes need to compulsorily breathe air, as in the contrary they die.

Peixos pulmonats: Peix pulmonat australià (Neoceratodus forsteri), africà (Protepterus annectens) i sud-americà (Lepidosiren paradoxa) (Foto: Encyclopaedia Britannica).
Lungfishes: Australian lungfish (Neoceratodus forsteri), African lungfish (Protepterus annectens), South American lungfish (Lepidosiren paradoxa) and Devonian lungfish (Dipterus) (Picture: Encyclopaedia Britannica).

OTHER BREATHING MECHANISMS IN FISHES

Many fishes have the capacity of breathing through the skin, specially when they are born because they are so small that they do not have specialised organs. As the animal is growing, gills or/and lungs are developing because the diffusion through the skin is not enough. Anyway, skin may be responsible of a 20% or more of gas exchange in some adult individuals. Others can do it through the mouth, the pharynx, the oesophagus, the intestine or the rectum, as is the case of Hoplosternum.

El peix Hoplosternum té la capacitat de respirar a través del tub digestiu (Foto: Free Pet Wallpapers).
Hoplosternum has the ability of breathing through the digestive tube (Picture: Free Pet Wallpapers).

Some species have developed cavities beyond the gills, the suprabranchial chambers, which can be filled of air. In other, complex organs developed from a very irrigated branchial arch can be formed and act as a lung. This is the case of the catfish and Electrophorus .

Some fishes have the ability to breathe air without a specific adaptation. This is the case of the American eel (Anguilla rostrata), that cover the 60% of the oxygen requests through the skin and the 40% swallowing air from the atmosphere.

REFERENCES

  • Notes of the subject Chordates of the Degree in Biology (University of Barcelona).
  • Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Principios integrales de Zoología. Ed. McGraw Hill (13 ed)
  • Hill, Wyse & Anderson (2006). Fisiología animal. Ed. Medica Panamericana

Difusió-anglès

Warning: Blowfish is already in the Mediterranean!

Since some time ago, newspapers are talking and warning about the arrival of the blowfish in the Mediterranean. This post expects to describe these famous animals and explain which the danger is for humans. 

HOW IS  THE BLOWFISH OF THE MEDITERRANEAN?

Blowfishes are a group present in the Mediterranean because they have accessed through the Suez Canal, so blowfishes are an exotic species. The species in the Mediterranean is the silver-cheeked toadfish (Lagocephalus sceleratus). Quickly, they established in the Eastern Mediterranean, but recently they have been found in the Adriatic sea (east of Italy). In this article published in the Mediterranean Marine Science, you may find a map with the concrete places (we cannot publish here for copyright). The silver-cheeked toadfish is native to tropical waters of the Pacific and Indian oceans, including Red Sea, where lives in muddy and sandy sea bottoms close to coral reefs between 10 and 180 m deep. They feed on a wide variety of animals and plants thanks to its powerful beak.

Their colouration is greenish with dark spots above, silver band from mouth to caudal fin and white belly. They can be distinguished from other blowfishes of similar colouration for their symmetric tail. The spines of the fins, moreover, are all soft. Their body is not covered by scales, but some small spines in the belly and in the dorsal part of the body. With two big teeth in each jaw, they constitute some kind of beak. When they feel in danger, they absorb water in a lateral cavity in the stomach to swell the body. The biggest animals can measure more than one metre long, but in average, they measure 40 cm.

Lagocephalus sceleratus (Foto: Il Giornale dei Marinai).
Lagocephalus sceleratus (Picture: Il Giornale dei Marinai).

WHICH IS THE DANGER?

Like other fish with a powerful jaws and big enough, the blowfish can produce a deep injury. Some species of blowfish can be specially aggressive. Most of the accidents are produced when manipulating alive animals.

However, this fish is poisonous and, in fact, it has caused some human deaths due to the presence of the toxin tetrodotoxin, which is 1,200 times more poisonous than cyanide. This toxin can paralyse the muscles, so it stops breathing and produce death by suffocation. It is known that with 0.009 mg for each kg of body weight is lethal. To give an example, a person with 70 kg who ingest 0.63 mg of the toxin may die (yes, in mg!). Most curious is the fact that the toxin is not produced by theirselves, but bacteria that ingest through food.

REFERENCES

  • Bergbauer, Myers & Kirschner (2009). Guía de animales marinos peligrosos. Ed. Omega
  • FishBase: Lagocephalus sceleratus
  • IUCN: Lagocephalus sceleratus
  • Nader M., Indary S., Boustany L., 2012. FAO EastMed The Puffer Fish Lagocephalus sceleratus (Gmelin, 1789) in the Eastern Mediterranean. GCP/INT/041/EC – GRE – ITA/TD-10