The most familiar members of the phylum Echinodermata are the sea stars (aka starfish). Also contained within this phylum are diverse members such as sea cucumbers, crinoids, brittle stars, and sea urchins. Echinoderms possess an interesting body shape known as pentaradial, or five-sided, symmetry. For some the members of the phylum like sea cucumbers this symmetry is difficult to see but for the sea stars it tends to be an easily observed and well-known feature.

Granular Sea Star, Choriaster granulatus, and Watson’s Sea Star, Gomophia watsoni, showing the distinctive pentaradial symmetry shared by members of the phylum Echinodermata.

In addition to this novel body shape, sea stars have another remarkable trait—regeneration. Many species are capable of losing an arm or two in a process called autotomy (Greek for “self-severing”). Shedding an appendage to elude a predator’s grasp or distract the predator may allow an animal to escape. Interestingly not only will the animal regenerate the lost limb but a limb may also regenerate an entirely new animal.

Sea stars in the genus Linckia are commonly called ‘comet stars’ as shed arms regrowing stubs of new arms make the animal look similar to how comets were once depicted in artwork. It is not uncommon for sea stars that once had a complement of five arms to end up with more arms after limbs have been shed and regrown.

Halley’s Comet in 1066 as depicted in the Bayeux Tapestry and a “comet shaped” arm regenerating a new central disc and arms.

Stages of regeneration as these single arm fragments regrow a central disc and five new arms resulting in six-legged sea stars.

An odd two-armed fragment which will regrow a central disc and full complement of arms and a sea star with nine arms instead of the normal five after shedding and regrowing its limbs.

Here are some other images of sea stars we found.
(Click-thru on each image for more detail.)

Photos by Ken Marks except Bayeux Tapestry image from the Public Domain: http://en.wikipedia.org/wiki/Halley’s_Comet#mediaviewer/File:Bayeux_Tapestry_scene32_Halley_comet.jpg

If you’d like to see more sea stars, check out our Sea Star Gallery: