Starfish and Member of Echinoderm

Starfish and Member of Echinoderm

Starfish are members of a large group of marine invertebrates called echinoderms. This group also includes brittle stars, feather stars, sea cucumbers, sea lilies and sea urchins. The word echinoderm comes from the Greek words ekhinos, which means ‘hedgehog’ and derma, meaning ‘skin’. This term aptly describes the bumpy or spiny bodies (called tests) of these marine invertebrates. Most echinoderms are found in shallow, coastal waters and generally live on the ocean floor, although a few species are free- swimming and live in the open ocean. Echinoderms also have a hard internal skeleton made up of calcium compounds. All echinoderms have the same basic body plan with a body divided into five sections, each bearing between one and four arms. Echinoderms have large, fluid filled body cavities called coeloms. Tiny hair like structures, called cilia line the wall of the coelom. The cilia waft body fluids around, which helps transport nutrients and dissolved oxygen inside the body. The hair like cilia, also line the tube feet on the outside of the body. The gentle wave like movement of the cilia helps echinoderms move around and draw food into their stomachs. Brittle stars are some of the most mobile of all the echinoderms, while starfish and sea cucumbers are very slow and creep across the ocean floor. Sea lilies do not move at all and remain rooted to the spot.

Scientists think that there are at least 7,000 different species of echinoderms that are divided into six main groups. Starfish form one large group which range in size from a few centimetres to more than 1m (3ft.). All starfish have flat, star-shaped bodies with several arms spreading out from a central disk. Most starfish have five arms, but some species have many more. Brittle stars and basket stars are some of the largest echinoderms. They have long, slender arms that can break off but re-grow again. Sea urchins belong to a group of their own. These unusual sea creatures generally live on the sea floor and use their long tentacles to catch food in water currents. Sea lilies and feather stars are another group of echinoderms. These creatures have star- shaped bodies and can be found in the water column or on the sea floor. Sea cucumbers have fleshy, cucumber like bodies, ranging in size from a few centimetres to more than 1m (3ft.). The final group are called sea daisies. These echinoderms are found within a limited range off the coast of New Zealand, in the Bahamas and most recently, the Northern central Pacific.

Most echinoderms have separate males and females, which reproduce by casting the sperm and egg cells into the water. To ensure success, the males and females produce vast quantities of sex cells. Fertilized eggs develop into embryos that go through several stages of development before they become the adult echinoderms. Other echinoderms are hermaphroditic, which means they have both male and female sex organs and can reproduce without the need for a mate.

Echinoderms show a wide range of feeding behaviours. Some starfish are scavengers and feed on almost any kind of dead animal matter. Others are deadly predators and feed on other marine animals, such as corals and sponges. The crown of thorns starfish is such a voracious predator of coral that it is partly responsible for the destruction of coral reefs in the Indian and Pacific Oceans. Predatory Starfish can push their stomachs through their mouths to engulf their prey. The starfish then secretes enzymes onto the body of its victim and sucks up the digested juices. Other echinoderms feed in different ways, for example by trapping particles in the water current using long, feathery tentacles.

Reference

1. Britannica Reference Encyclopaedia

2. Knowledge Encyclopedia Animal!: The Animal Kingdom as you've Never Seen it Before

3. Images are taken from Zoo-Que and Google.

4. In Defense of Plants: An Exploration Into the Wonder of Plants

5. A Naturalist’s Guide to the Birds of India

6. Earth: A Sustainable Planet

Also See

• Migration & Navigation of Birds

• Archaeopteryx – The First Bird`s Fossil

• Parrots

• Bats