Echinoderms Regenerate Lost Limbs
March 15, 2012 by staff
Echinoderms Regenerate Lost Limbs, Taxononmy
The phylum Echinodermata (echinos, spiny; derma, skin) is composed of sea stars, sea urchin, sea cucumbers, and brittle stars (Hyman 1955). All echinoderms have pentamerous, radial symmetry and possess a water vascular system, which plays a role in locomotion and feeding. Echinoderms also possess the ability to survive predation by regenerating lost limbs (Hendler et al. 1995). In fact, the ophiuroids (brittle stars) have the ability voluntarily shed an arm or disc (known as autotomy) to escape a predator (Hendler et al. 1995). Echinoderms lack a head, true eyes, a heart, and even a brain (Hyman 1955). Ophiuroids can be identified by a small flattened disk, which is usually rounded or pentagonal, distinctly separated, symmetrical, long arms, that are either smooth or spiny (Hyman 1955).
Ophionereis reticulata can be found throughout the Caribbean from as far north as Bermuda and South Carolina to as far south as Brazil. They are usually found under rocks and coral, or may be buried in sand (Sterrer 1986). O. reticulata is mostly seen in shallow water, but has been recorded at depths up to 220 m (Hendler et al. 1995).
While the disk varies between species in size, shape, texture, and patterns, Ophionereis reticulata has a small flattened, pentagon shaped disk that can grow to as large as 15 mm (Hendler et al. 1995, Hyman 1955). The disk is made up of several fine scales with a pale gray color and a reticulating network of brown to reddish-brown lines covering the aboral surface, hence the name: reticulated brittle star (Hendler et al. 1995). The five arms, which are attached at each point in the pentagon are the same pale gray color and banded with dark brown stripes every fourth joint, can grow up to 120 mm long (Hendler et al. 1995). The coloration and patterns allow O. reticulata the ability to camouflage against the background of rocks and san grains in intertidal zones (Hyman 1955).
The name brittle star comes from the fact that all ophiuroids possess extremely fragile arms that can easily break when disturbed or handled (MacGinitie and MacGinitie 1949). All echinoderms have the ability to regenerate lost body parts, which is known as autotomy, but ophiuroids are believed to purposely throw off their arms and sometimes even the aboral sides of their disk. They are able to do this because of the mutable collagen connective tissue that is found in all echinoderms (Ruppert et al. 2004). It has been concluded in some studies that this is done as a way of escaping predation (Sides 1987; Skold and Rosenberg 1996). Since most species of brittle stars, including O. reticulata, capture food by spreading out there arms and filter feeding, their arms can easily become food for squirrelfish, triggerfish, crabs, and even predatory asteroids (Sides 1987; Hendler et al. 1995). Having the ability to throw off an arm that is being attacked allows the brittle star a chance to escape from a predator.
Ophiuroids are described as being the most lively and active of all echinoderms (Hyman 1955). No arm preference has ever been observed and ophiuroids are capable of moving in any direction (Hyman 1955; Ruppert et al. 2004). One method is placing one arm out front as a feeler and the adjacent two push the body along, while the rear two legs do nothing. Another method is to perform alternating rotating actions using two pairs of arms while the fifth arm drags behind (Hyman 1955). However, one behavior observed in all ophiuroid movement is that the disk is held up above the substrate (Hyman 1955; MacGinitie and MacGinitie 1949; Ruppert et al. 2004). In most ophiuroids, the tube feet, or podia, are not essential in movement, but the O. reticulata makes important use of the podia by resting on them and not touching the substrate otherwise (Hyman 1955). By extending the tube feet, bending them at the tips, then straightening, the reticulate brittle star is able to travel across the substrate at about 50 cm per minute (May 1925).
The diet of ophiuroids ranges from bottom detritus to suspended particles and plankton, depending on the species (Reese 1966). Due to this large variation in food preference, different species have adapted different methods of prey capture. Some of the methods for feeding on small particles is arm waving and creating mucus nests between arms to capture phytoplankton or other suspended particles (Reese 1966). Some species also make use of the podia to capture small prey and push it towards the mouth (Reese 1966). It has been recorded that Ophionereis reticulata feed exclusively on plant material, such as algae and diatoms (May 1925). Since O. reticulata spends its time under rocks, it retrieves its food by waving its arms out in the open and capturing plant particles from the sediment surface (May 1925; Hyman 1955). Ophiuroids, unlike other echinoderms, do not have an a**s, so all food matter that is not digested is excreted out of the mouth (Ruppert et al. 2004).
The majority of all ophiuroids are dioecious, meaning that each individual is either a male or female and only a few species are hermaphroditic (MacGinitie and MacGinitie 1949). Little is known about the specific reproductive strategy of Ophionereis reticulata or any other ophiuroid species. However, most brittle stars are believed to be broadcast spawners that release their eggs and sperm through genital bursae into the water column and become fertilized (Hottenrott 2002). Sexual dimorphism is uncommon in brittle stars but in a few species it can be seen that the male is significantly smaller than the female (Hottenrott 2002). Some species also reproduce asexually by splitting into two halves and each half will regenerate the missing side (Hyman 1955).
The most recent research being conducted on ophiuroids does not concern Ophionereis reticulata, but another species found in Bermuda, Ophiocoma wendtii. All species of brittle stars are negatively phototaxic, which means they have an aversion to light and seek the closest refuge possible when placed in a well lit area (Hottenrott 2002). It has been documented that brittle stars can quickly detect shadows of predators and safely escape, yet it was never understood how this was possible since no type of photosensory organ has even been documented (Aizenberg et al. 2001). However, Aizenberg et al. (2001) discovered that there were crystal clear bumps on top of each of the plates on the dorsal side of the disk and arms. If one lens was at the end of each arm, the brittle star could distinguish light from dark but would not be able establish much more than that (Aizenberg et al. 2001). However, since the entire dorsal surface is covered with these spherical lenses, all collecting light from different angles, they form a compound “eye” (Aizenberg et al. 2001). These lenses have interested computer companies because the brittle star is able to make these lenses smaller and more precise than man made lenses. If a way is found to recreate this matrix of lenses then it will push forward the goal of building a completely optical computer (Summer 2004).
While some people have brittle stars in their home aquariums, most do not due to the fact that brittle stars spend the majority of time hiding under rocks. They also are not desirable food for humans, though some fish that are commercially important do often feed on brittle stars.
No Bermudian laws were found concerning Ophionereis reticulata or any other species of brittle star.
I first became interested in brittle stars during our first field trip to Bailey’s Bay. Even though the intention of the trip was to collect crustaceans, it seemed like almost every rock had a brittle star underneath it. The way they seem to move so gracefully and effortlessly over the surface of the sediment caught my attention. Once I learned of how fragile they are I was curious at what the benefits would be of breaking so easily, which caused me to have more interest in them. As I began to do more research on them I came to realize that there is not an over abundance of research conducted on brittle stars, especially in Bermuda. Though this has become a little frustrating at times, the fact that there is not a lot known about brittle stars, specifically Ophionereis reticulata, it makes me more curious about them. Only three years ago, a discovery was made about the compound “eye”, imagine what other important things are still left to be discovered about brittle stars. However, I was surprised to find how little commercial importance they have, though since they exist in almost every type of marine environment, they place a very important role in reef ecology, which is why I feel they should be further studied.
Aizenberg J., Tkachenko A., Weiner S., Addadi L., Hendler G. 2001. Calcitic microlenses as part of the photoreceptor system in brittlestars. Nature. 412:819-822.
Hendler G., Miller J.E., Pawson D.L., Kier P.M. 1995. Sea Stars, Sea Urchins, and Allies. Smithsonian Institution. p. 90-195.
Hottenrott S. The Ophiuroidea website. Accessed November 14, 2004. http://home.att.net/~ophiuroid/html/main.html.
Hyman L.H. 1955. The Invertebrates. Vol 4. McGraw-Hill Book Company. p. 589-689.
MacGinitie G.E. and MacGinitie N. 1949. Natural History of Marine Animals. McGraw-Hill Book Company. p. 241-245.
May R.M. 1925. Les reactions sensorielles d’une ophiure. Bull. Biol. France Belgique 59.
Reese E.S. 1966. The complex behavior of Echinoderms. In: Boolootian R.A. (Ed.) Physiology in Echinodermata. John Wiley & Sons, Inc. p. 157-218.
Ruppert E.E., Fox R.S., and Barnes R.D. 2004. Invertebrate Zoology. Brooks/Cole, Thomson Learning, Inc. p. 890-896.
Sides E.M. 1987. An experimental study of the use of arm regeneration in estimating rates of sublethal injury on brittle-stars. J. Exp. Mar. Biol. Ecol. 106:1-16.
Skold M. and Rosenberg R. 1996. Arm regeneration frequency in eight species of ophiuroidea from european sea areas. J. Sea Res. 35(4): 353-362.
Sterrer W. 1986. Marine Fauna and Flora of Bermuda: A Systematic Guide to the Identification of Marie Organisms. John Wiley & Sons. p. 527-531.
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