What is an echinoderm?
The echinoderms are a group (actually, a phylum) of animals. Their name comes from the Greek words meaning “spiny skin”. Baby echinoderms swim freely with the help of tiny hairs, called “cilia”. As they grow, they become beautifully symmetric – this is most obvious amongst the starfish – and develop tough plates or spines just under their skin. Echinoderms use water instead of blood to move food around their body, and to breathe. There are two contenders for the world’s largest echinoderm, and both are starfish.
What kinds of echinoderms are there?
The phylum of the echinoderms, called Echinodermata, is divided into five groups, called classes. These are :
- The Starfish, or Sea Stars (Asteroida). There are about 1800 species of these, spread throughout all the world’s oceans. They hunt other animals for their food.
- Feather Stars and Sea Lilies (Crinoidea). They normally attach themselves to rocks, and have many arms with which they catch their food. There are about 600 species of Crinoid.
- Urchins and Sand Dollars (Echinoidea). These ball-shaped animals can often have many movable pointy spines sticking out from a hard shell. There are about 1000 species
- Sea Cucumbers (Holothuroidea) look a bit like slugs, and have a leathery skin. There are about 1000 species of sea cucumber, and some are regarded as delicacies in some Asian countries!
- The Brittle Stars (Ophiuroidea) are not starfish, but look a lot like them, so are often called starfish. There are about 1500 species of brittle star.
In addition to these, there are a number of other types of echinoderm that are only known from fossils.
What is the world’s largest echinoderm?
There are two contenders for the title of World’s Largest Echinoderm, both starfish. There’s one heavyweight champ, the Catala’s Starfish (Thromidia catalai), otherwise known as the Fat Star, weighing in at 6 kilograms (13 pounds). However, its fat arms are only just over 1 feet (30 cm) long. Midgardia Xandaros has arms that can reach up to twice that length!