Friday, April 26, 2013

Creature Feature: Starfish

Reticulated starfish

Comet starfish (not looking very comet-like)
Starfish are cool. How many other critters can you cut up into multiple pieces, only to have each piece regenerate a whole body? Not that we’ve ever done this, of course (more about this later). Also called sea stars, starfish are not really fish, but invertebrates called echinoderms, related to sea urchins and sea cucumbers.  The species we’ve seen the most in the Bahamas and Caribbean is the reticulated starfish, so called because of the network of ridges across its body. It’s also called a cushion sea star, because it’s so puffy.  Puffy it might look, but it’s solid and big. Once in the Bahamas, we were entering an anchorage and, from the bow, I called out every reticulated starfish I saw: “There’s one. There’s one. There’s another one…” I probably kept this up for a full minute before Chris finally told me to stop it, the point being that this species can be VERY common and occur at high frequencies (and also that I can be very annoying).  We usually see them on sand flats or in seagrass. If you come across a starfish, pick it up and look at the bottom. You’ll see deep ridges along each leg, and if you look long enough, you might see the little hydraulically operated tube feet emerge. If you hold the starfish on your hand, it will eventually start to walk, the tube feet sucking onto your hand, then releasing, as it pulls itself across. It feels neat! Starfish walk along the bottom slooowly on their little tube feet, feeding on benthic invertebrates such as clams, snails, or anything that moves more slowly than they do. So, back to the regeneration. Some species can regenerate an entire body from just one arm and a piece of the central disk. Nice trick! This ability is particularly conspicuous for the comet sea star. Below is a picture of Chris holding an intact comet star, but take a look at the picture here, which shows an individual regenerating from a single leg (the large leg). It does look like a comet, doesn’t it! The name of the comet sea star is particularly apropos, since true starfish are in the class Asteroidea.  Asteroids, comets, stars—this sounds more like astronomy than marine biology.

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