Echinoderms 2BIbDF09 10/11

What is an Echinoderm?

Echinoderms are deuterostomes and belong to the phylum of marine animals. They are called "echinoderms" because of their spiny surface ("echin" means spiny in greek). Despite the fact that adult echinoderms have a radial symmetry they are grouped in the bilaterial category because during their larval development, they have a bilateral symmetry but when they grow, their symmetry become radial. However, ancestors of echinoderms had a bilateral symmetry and their coelum had three pair of space. Echinoderms'body is made of hard, calcium-based plates that are often spiny and covered by a thin skin. While most echinoderms are either stationary or slow-moving methodical animals, they are nevertheless prominent members of the marine environment. Echinoderms are constituted with a set of plumbing tubes. This kind of hands are really important because they take part in most essential things like locomotory, feeding and respiratory activities. Although they generally act with only one environment, they tend to move quite well in all directions.

• See appendix Echinoderms 2

In the following paragraphs we will focus on different echinoderms' anatomies.

There are different classes in this phylum: Crinoidea, like crinoids Ophiuroidea, like brittle stars Asteroida, like sea stars Echinoidea, like sea urchins Holothuroidea, like sea cucumbers

*See appendix Echinoderms 6

Crinoids anatomy and features

Also called "sea lilies" or "feather stars", crinoids are marine organisms. Their anatomy is composed of a cup shaped-head part, called the calyx and which hold the five arms like a root-system. which permit them to stand up in the sediment. Their symmetry is pentaradial as the sea stars. In the crinoids the mouth is situated next to the anus. The skeleton is made up calcerous plates.

Sea star anatomy and features

Most of sea stars have a radial symmetry, called precisely pentaradial symmetry because of the five folds. But this number depends on the species: for example some Helicoilaster posse up 50 arms and some species of crinoids like Comanthina schlegelii posse up 200 arms! The body of the sea stars is generally divided in: the pharynx, the stomach, the intestine and the anus(cloaca). The mouth and the anus are close together on the underside.The upper surface is often very colorful. Like the sea cucumber, the sea star can lose its body to escape. Indeed it is well known for its power of regeneration: a new organism can quickly regenerate from a small fragment, an arm for example.

Brittle star anatomy and physiology The brittle star has a radial symmetry with a central body and five snakelike arms which are linked to it. They're close to sea stars. Their arms are also flexible but compared to sea stars, brittle stars have a smaller central disc and no anus. So wastes are eliminated trough the mouth. We can say their body is "breakable" that means that the arms of the brittle stars are rather liable to break: this allow them to escape. The arms can quickly regenerate or even an entire organism can be regenerate. The reporduction of brittle stars is very particular because they are asexual so they reproduce by self division.

*See Appendix Echinoderms 7

Sea urchin anatomy and physiology More than 700 species of sea urchin are counted. They belong to marine invertebrates.Some are camouflated with debris like rubble or pieces of seagrass and carry them on their back. The tube-feet and the movement of the spines on the underside of their body make enable the locomotion activity.There is two sexes in sea urchins , female and male and the young are formed indirectly by the fusion of sperms and eggs release into the water.

Sea cucumbers anatomy and physiology We can count more than 1250 species of sea cucumber and ocean inhabitans. Like the other echinoderms, sea cucumber have a pentaraddial symmetry.They can be described as tiny and elongate animals without arm. Their skin is covered with spines. Their body is really important to protect themselves of hunters,they can contract their muscles and "eject" internal organs out of their anus.

*See appendix Echinoderms

Why are Echinoderms considered as deuterostome animals?

Deuterostomes are grouped together because they all undergo the same early embryonic development. Unlike protostomes, in which the mouth develops before the anus and blocks of mesoderm dig to form the coelom, deuterostomes develop first the anus and then the mouth. Echinoderms are closely related to chordates because they have the same larval evolution, they are both echinoderms. However, in adulthood, echinoderms acquire a radial symmetry with five-part symmetry and no central brain while chordates keep their bilateral symmetry.

See appendix Echinoderms 1 and 4

Classification

Domain : Eukarya

Kingdom : Animals

Phylum : Echinodermata

Classes : Crinoidea, Ophiuroidea, Asteroida, Echinoidea, Holothuroidea

How do they reproduce ?

I. Sexual reproduction

Sexual maturity of echinoderms is obtained between the age of two and three years, depending on the species and the environmental conditions. Most echinoderms have a sexual reproduction with an emission of eggs and sperm cells in the water where fertilization takes place. After the egg fertilization, the development of echinoderms occurs indirectly and thus presents a larval stage. The larva that hatches is named gastrula. This larva swims and turns into another larvae called dipleurula. It will give proper larvae for each class of echinoderms: echinopluteus in echinoids(sea urchins), bipinnaria brachiolaria in starfish, ophiopluteus vitellaria in ophiuroids (brittle stars), doliolaria for crinoids, auricularia doliolaria and pentactula in Holothuroidea (sea cucumber). These larvae whose way of life is generally planktonic, have a bilateral symmetry and metamorphose into organisms with a pentaradial symmetry.

II. Asexual reproduction

However, some echinoderms have an asexual mode of reproduction done by a splitting of the body followed by a regeneration of the missing part. For example, some sea stars are able to regenerate lost arms. In some cases, lost arms have been observed to regenerate a second complete sea star.

Where do they live ?

Echinoderms are found at every ocean depth.

More than 2000 species of sea stars live in all world's oceans.In these oceans we can include the Atlantic,Pacific,The indian Ocean and we can also find them in the Arctic and southern oceans reagions.But sea stars may occur in the intertidal and abyssal zone.Contrarily to them,Sea urchin inhabit all oceans.

How do echinoderms nourish themselves?

Echinoderms feed and breathe with a unique water-vascular system ending in a tube feet. They use different modes of feeding and developp particular and adapted kinds of muscles, limbs, organs or digestive juice. For expemple :

- Sea cucumbers developp tentacles to capture food from the sea floor. They create like a net in which smaller organisms are trapped. They also suck vast quantities of water where we find plankton and decaying organic matter.

- Crinoids also developp large net-like structures to filter water and to adsorb particles of matter. Once a particle touches the arms of the creature, it acts to lead it to the central mouth of the crinoid, where it is ingested. Nutrients are retained and the rest is released through the anus.

- Sea stars are hunters, they seize fish with their legs and are able to eat them alive thanks to their gastric juices.