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We’ve discussed sea stars in this series before, but so far I haven’t touched on the group of Asteroids known as brittle stars. They also go by "serpent stars", for reasons which will be made clear below, as well as by the name Ophiuroids, after the scientific name for these animals’ Class, which is Ophiuroidea.

There are about 1500 species of ophiuroids, found all over the world from the tropics to polar regions and from the intertidal zone down to 2,000 feet (where they are most common). Here in Southern New England we only have one species, the short-spined brittle star (Ophioderma brevispina), and it is not very common at all.

Like most regular sea stars, brittle stars have five arms, are covered with small spines and have suction cups known as tube feet. See the diary How Tube Feet Work for more info on these structures. But brittle stars use these appendages in a unique way. In most sea stars the spines are mainly protective while the tube feet are locomotive. Brittle stars have only a few tube feet and these are concentrated around the mouth on the underside of the circular central disc. The animals use these to help guide food into the mouth.

The spines are found on the sides of the thin arms, sticking out horizontally. They are used to dig into the sand or algae and give the animal the friction it needs to pull itself along the bottom. On the species I’ll be mainly talking about these spines are rather short, hence the name. Most other types of ophiuroids have longer spines, and look like this:

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That image is by NOAA. All others, including the first movie below, are of my own specimens and were taken this morning by me.

Brittle stars are masters at regeneration, or growing back lost body parts. Athough the phylum Echinodermata as a whole are pretty good at this in general, few groups can match the regenerative abilities of Ophiuroids.

Unlike other starfish, which needs to have the arms forcibly damaged or pulled off, the brittle stars can drop their arms at will. This is a defensive behavior that works kind of like the tail of a gecko. If a predator grabs the brittle star by an arm the animal escapes by dropping that arm off. It doesn’t even need to lose the whole arm. Notice the arm is divided into dozens of segments. The arm can be broken off at any segment depending on where on the arm the predator is grasping.

Like the tail of a lizard, the arm will continue to move after it falls off. This increases the chances of the rest of the animal escaping by distracting the predator. I’ve had my brittle stars drop arms off and the arm will continue to crawl around the aquarium for up to two weeks before finally coming to a stop.

The regeneration process is very fast in these animals. A lost arm will be completely grown back in about two months. Most other groups of starfish require a year or more for this to happen. Below is one of my stars that lost an entire arm about four weeks ago. In another few weeks you won’t even be able to tell which arm it was that it lost.

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I have several dozen of these animals, yet I haven’t seen a single wild one since the summer of 1991. That year I came across twenty adults, tangled in a small mass of writhing arms and embedded among the holdfast of some kelp. Seventeen years later I still have all of them. Here they are in a video I took about ten years ago, and this gives you an idea why they are sometimes known as "serpent stars". The two pictured above are in there somewhere.

Five years ago I noticed several babies about a quarter inch across. This was the only time before or since that my stock group reproduced, but the young are now nearly as large as the adults. You can tell the younger ones by the more delicate looking arms and the lighter color, like the one on the right in the image below.

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I’ve searched around and most sources give the life span of brittle stars at five to ten years. I know this is wrong, and based on the growth rate of the babies I have, the ones I caught seventeen years ago were at least five years old when I found them, and probably older.

The behaviors of the young and the adults are nearly identical, being equally nocturnal and predatory at all stages of their lives, although I do notice that the older ones tire more quickly (yes, I’m serious). I can tell this by forcing them to right themselves. "Righting" is simply a way that an animal has of turning itself over if flipped upside down. This may seem kind of elementary, but considering the wide variety of body plans exhibited by animals, this isn’t always an easy endeavour. Just ask a tortoise.

Righting in brittle stars is accomplished by spreading two of the arms (they don’t have a front or a back, so it doesn’t matter which two), twisting them under the body and then reaching up and over with the most distal arm. Below are some of my brittle stars and I flipped them over so you can watch the righting behavior in action.

You’ll notice how quickly righting happens as I can barely pull my hand away before they are rightside up again. Do this a half dozen times and the older ones will start having trouble turning over while the younger ones right themselves just as fast as they did the first time.

Shallow water ophiuroids are mainly nocturnal, but as mentioned up top, brittle stars are most numerous in the light-challenged deep sea. Not only in number of species present in this habitat, but also in total number of individuals. Some benthic areas of the Atlantic Ocean, such as the deep water seamounts along the mid-Atlantic ridge, contain millions of these echinoderms living side by side. Here is some footage of one of these seamounts in an area known as "brittle star city".

Other diaries in this series can be found here.

Originally posted to Mark H on Fri Nov 28, 2008 at 05:59 PM PST.


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