It is hard to imagine that you have not heard of its close relative, the sea star. Sea stars are one of the exceptions; these particular invertebrates have not escaped public notice. Even small children can recognize the sea star (sometimes called "starfish"). You can spot them everywhere from children's books to jewelry, and a visit to just about any public aquarium will reveal a touch tank with one or more of these animals. Of course, many species of sea stars can be found in the intertidal zone along our shores, and that proximity also raises their profile.
Ophiosamus, perhaps a couple hundred million years old!
Brittle stars, on the other hand, are not seen clinging to the surfaces of wave-swept rocks. The brittle star is a close genealogical relative of the sea star, yet it is an animal that is rarely seen at all by the average person.
Brittle stars are members of the phylum Echinodermata ("spiny-skinned") which includes the sea star, sea urchin, sand dollar, sea cucumbers, crinoids and their kin. More than 2,000 species of brittle stars have been described to date, though specialists who study this group almost certainly would tell you than many species remain undiscovered. That's not surprising, given the ecology of these animals. The group is represented in the fossil record dating back into the Early Ordovician, about 480 million years ago.
External anatomy of the brittlestar (by Mathias Oddo)
The mouth of a brittle star.
Brittle stars are exclusively marine, benthic organisms. That is to say, they are found only in the saltwater environments, and, at least as adults, they are associated with the seafloor. Of course, as the oceans cover 71% of our planet, that's still quite a bit of territory. Brittle stars can be found from the high intertidal zone to depths exceeding 8000 meters! In some areas, they occur in incredible abundance, reaching hundreds to thousands of individuals per square meter (see Hughes, 1998).
The general body plan of a brittle star is not terribly different from that of sea stars. Most (but not all) species in the group possess a well defined central disk from which five arms radiate. The animal possesses an endoskeleton that consists of bony elements called ossicles. In most species, the arms break relatively easily, perhaps as a defensive mechanism. Like other echinoderms, the brittle stars possess a network of canals, collectively called the water vascular system, that extend from the central disk through the arms. The system is used for locomotion as well as the transport of nutrients and wastes. While the water vascular system does permit them to shuffle water into and out of their tube feet, these animals are able to use their arms to move rather quickly along the substrate, whether toward a food item or to shelter (see video). The digestive system includes a central mouth that is surrounded by an interesting feeding structure composed of five jaws. The gut of the brittle star is incomplete, meaning they lack an anus and therefore the mouth is a two-way street. (Be thankful that our guts are complete.) Brittle stars appear to have an incredibly ability to respond to stimuli such as light and chemicals in their environment. Not everything is understood about their sensory systems, yet some interesting finds have been made in recent years. For instance, close examination of the endoskeletal features in the species Ophiocoma wendtii show that its upper (aboral) surface consists of many tiny, calcitic lenses that might collectively function like a compound eye (Aizenberg et al., 2001). In other words, the whole surface of the animal could be functioning as a single eye.
Brittle stars feed in a variety of ways. Some species are predators, but most species are either scavengers or suspension feeders, capturing organic matter that they encounter on the seafloor (e.g., see Loo et al., 1996 and the video here).
Larva of a brittle star.
Species of brittle stars produce both asexually and sexually. When it comes to sexual reproduction, these animals spawn, releasing their gametes into the water column where fertilization can then occur. Their life cycle includes a pelagic larval stage, the general term for which is "pluteus larva." Naturally, variations exist among species in everything from larval forms to the finer details of the life cycles. The common thread is that a larva persists in the water column for some time before settling to the substrate and continuing its development into an adult. For an example of a study of their reproductive strategies, see McGovern (2002).
Brittle stars on the seafloor at depth.
If you have ever walked in a rocky intertidal zone or you have gone diving on a reef, you undoubtedly have been within reach of the brittle star, perhaps without ever knowing it. Of course, some species are more conspicuous, particularly those that feed by extending their thin arms into the water column in order to capture their meals.
Dense aggregation of brittle stars.
With half of the planet's surface covered by expansive deep-sea plains, it is probably safe to assume that brittle stars are among the more common macroscopic organisms on Earth. They're just largely out of reach, out of sight, and therefore out of mind.
That's too bad, because they are fascinating organisms that hold physiological and behavioral secrets we have yet to uncover. The brief notes here barely scratch the surface, so I encourage you to explore the group further!
Several brittle stars on a sea star in California waters.
Diver above brittle-star-covered rocks in California (from CA Diver Magazine).
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