Starfish and Sea Urchins: Echinoderm Structure and How Different Groups Evolved

There are over 7,000 species of echinoderm alive today, and they are all marine. Most of them do not really have a front or back end, but have what is called ‘radial symmetry’ – being either roughly circular or having a number of ‘arms’ (usually five) spread out around a central body part. They all have tough armour which fossilizes readily, so there is a good fossil record for the phylum.

Five Classes of Living Echinoderms

There are many extinct forms in the phylum Echinodermata, but only five classes survive.

  • Starfish (Asteroidea) are familiar to most people who visit the seaside, since there are many shallow-water species and they are often brightly coloured. Most have five ‘arms’, with rows of ‘tube feet’ below and armour above. Some are active predators of bivalve molluscs – they pull them slightly open with their ‘arms’ and then turn their stomach inside-out and slip it in between the two valves of the shell to digest the animal. (The recently discovered ‘Sea Daisies’ also seem to belong to this class.)
  • Brittle Stars (Ophiuroidea), as their name suggests, break apart very easily and are usually roughly star shaped with five very mobile ‘arms’. Most scrape food off rocks or filter feed.
  • Sea Urchins and Sand Dollars (Echinoidea) have roughly circular outlines, but close examination reveals that they have the same five-fold symmetry as the starfish and brittle stars. Urchins usually graze on algae, while the sand dollars burrow in the sediment and filter feed.
  • Sea Cucumbers (Holothuroidea) seem to be the exception to the ‘radial symmetry’ rule for this phylum – being long sausage-like creatures with a front and back end – but they have the remains of the five arms around the mouth and have tube-feet (unique to the phylum Echinodermata).
  • Feather Stars and other Crinoids (Pelmatazoa) often look more like plants than animals. Most are permanently attached to the seabed by a long stalk at the end of which is a feathery flower-like structure where the animal filter feeds. The feather stars are similar at the ‘flower’ end, but have no stalk and can swim when they need to.

Molecular Studies and Echinoderm Relationships

Although the echinoderms have a very good fossil record it is was not possible to decide how the modern forms are related to one-another until the advent of molecular research.

Mitochondrial DNA studies suggest that the starfish and brittle stars are close, as are the sea cucumbers and urchins, while the crinoids seem to be quite far away from either group.


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