Seeing stars in the sea

by B. N. Sullivan

Awhile back we had some fun with our 'Every Seashell Has a Story' series, in which we presented photos and tales of marine molluscs that produce and live in shells.  Another interesting and often very pretty class of marine animals are Sea Stars -- also known colloquially as 'Starfish' (although they really are not fishes, of course).

Sea stars are marine invertebrates that belong to the phylum Echinodermata, which also includes brittlestars, urchins, and sea cucumbers.  Sea stars make up the class Asteroidea.

Although the sea stars we have come to know best live in tropical reef environments, sea stars can be found in all the world's oceans and seas.  There are about 1,800 sea star species worldwide.

The pretty sea star pictured on this page has quite a few common names: Mesh Sea Star, Tiled Starfish, Necklace Sea Star, and Peppermint Starfish.   (Choose which one you prefer!).

Its scientific name is Fromia monilis, and it is a member of the Ophidiasteridae family. This is an Indo-Pacific species widely distributed in the Western Pacific and also common in the waters around Indonesia. The ones here were photographed at Indonesia's Bunaken National Marine Park.

The classic, prototypical sea star has a central disc shaped body from which five rays (i.e., arms) radiate. While that may be the idealized form that comes to mind when we think of sea stars, in fact there are many species that have more than five arms.  Some sea star species have a body that looks less like a disc than a sort of lump where the rays appear to converge; others have a large body disc and rather stubby arms.

The rays have rows of little tube feet on their undersides.  (We'll show you some macro images of those in a forthcoming post.).  A sea star's mouth also is on its underside, in the center of the body, while its anus is on the top (aboral) side of the body disc. The external covering, or skin, of sea stars can be leathery, or fuzzy, or warty, or prickly.

Did you know that sea stars operate hydraulically?  On the top side of the disc is a structure called the  madreporite, which includes an opening -- a pore -- through which the sea star draws in water to its hydraulic vascular system. The sea star's respiration, feeding and movement all are powered by its hydraulic vascular system. If you look closely at the second photo on this page, you can see this sea star's madreporite. It is that little bump that looks a bit like a pimple, to the right of center on the red disc. [Click on either photo to enlarge.]