ANiMAL MUNDi: Stars of the Deep by TheFrogBag

Stars of the Deep

written by Corinna of TheFrogBag

Photos courtesy of So. California Sea Life Center

Humans have always had a hard time naming things that live in the sea. It’s only been in the last hundred years or so that we’ve been able to get a good look at many of them, and this has led to some unhelpful overlap of nomenclature between land animals and sea creatures. 

We have sea horses, sea cows, dog fish and parrotfish, none of which are closely related to any of their namesakes. And then there are starfish, which sometimes superficially resemble stars but are certainly not fish. Brittle stars are at least a little brittle, although they resemble a nest of snakes more than anything celestial.

At least, like starfish, brittle stars are classified as echinoderms so the common name is helpful in that respect. Unlike starfish, they have a small central disk and very thin arms. They also lack an anus, so waste is expelled through the same opening that nutrients enter. But don’t let that fact fool you into thinking that these beasts are primitive. They in fact have five jaw segments and 10 infolds within their “blind” stomachs to aid in digestion. Different types of brittle stars have evolved to eat different things in this way, from detritus to plankton. One species is even specialized to slurp up the mucus of their host coral. 

Who would have thought that such an elegantly named creature could have such disgusting habits?

As highly specialized as brittle stars are, the list of organs that they do without is impressive. They lack brains, eyes, a sense of smell, a sense of taste, and a complex digestive tract. Instead they have a ring of nerves around their central disk with radial offshoots that run to the end of each limb. These limbs are covered in an epidermis that is highly sensitive to touch, chemicals in the water, and even light. This light sensitivity makes it possible for brittle stars to spend most of the day hiding in crevices, emerging only after sunset to feed unobtrusively. 

Despite their cryptic habits predators frequently target them. That’s where the “brittle” part of their nature becomes important, since they can easily lose a limb (much like a lizard can lose a tail) without undue stress. The regenerated “leg” may be shorter than the other limbs, but it will work just as well. 

Limbs aren’t the only body part that these animals can regenerate. Even though most species have separate sexes and reproduce by shedding gametes into the water, a few actually give birth to live young via special brood pouches. But others create new stars by literally splitting in half, a process that seems to belong more to the plant kingdom than the animal kingdom. The result is two less-symmetrical echinoderms (half the body will have short “legs”) where formerly there had been one, but each will have a fully intact set of internal organs and nerves. 

In some species this process may produce 15 new animals in one year!

So, to summarize, brittle stars have no eyes, no brains, and no anus. They’re kind of squishy, but they do break easily. And some of them have unsettling ways of reproducing. 

Maybe we don’t want a more descriptive name for them after all...