If you watch people around an ocean tidal pool or those looking in a saltwater reef aquarium, you'll notice that one of the most popular creatures is the starfish. Even the most blandly colored specimens draw attention. There's a strange reason for this; a five-armed starfish, when viewed with a single arm pointing straight up, vaguely resembles the head-body-arms-legs arrangement of the human form. Most pictures of starfish are taken in this position, and we feel a gut-level identification with them.

But beyond this resemblance, starfish can be fascinating for many other reasons. They come in a brilliant range of colors and patterns, and some starfish have as many as 40 arms! They can move in ways that surprise; seemingly stationary starfish can appear anywhere in the tank at any viewing. Some species can seem to "glide" across the bottom of a tank.

THE SEA STAR

There is a campaign to change the name from "starfish" to "sea star", and for an acceptable reason; it is not a fish. Technically, the sea star is an Echinoderm, and they are most closely related to sand dollars, sea urchins, and sea cucumbers.

Sea stars typically have the five-arm arrangement we are familiar with. Each arm can move independently using rows of "suckers" (for lack of a better term) on its underside, the suckers alternately clinging to and releasing from the surface below to move the sea star. The suckers are also used for grasping shellfish and pulling the shells open

And then things get really weird. The sea star, rather than reaching into the shell and extracting the yummy bits, instead pushes out an exterior stomach (called the "cardiac" stomach) and wraps it around the soft parts. It then partially digests the shellfish, and draws the broken-down remains into its body to be further digested by the inner (or "pyloric") stomach.

Sea stars are also believed to do some filter-feeding (like shellfish), where they pump seawater through their system to extract nutrients.

There are over 1800 known varieties of sea star, ranging from drab sand-colored specimens to brilliant blue and red varieties. There are deep-sea varieties and those that exist only on coral reefs or shallow water. Almost all have a bony skin that protects them from most predators, and all are marine creatures; they cannot survive in fresh water.

Sea stars have most of their vital organs in their arms, so if they lose an arm or two they can usually regenerate the missing parts, and some sea stars

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