Marine Invertebrates of Bermuda Blunt-Spined Brittle Star Ophiocoma echinata By Katherine Degnan Dr. James B. Wood and Kim Zeeh - Editors Abstract  Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links

Abstract


Ophiocoma echinata is the blunt-spined brittle star found in Bermuda. This species is found in coral reef habitats and is common on sandy bottoms under rocks in shallow waters (Sterrer 1986). Unlike common sea stars, the central disk of brittle stars contains the entire digestive system, gonads, and all other organ systems (Sterrer 1986). O. echinata has two defined spines on each side of the first arm segment and two tentacle scales (Sterrer 1986). The spines of O. echinata function for feeding and establishment of burrows in the sand beneath rocks. The unique morphology of the brittle-star arms is due to the ambulacral ossicles in the dermal endoskeleton fusing to develop vertebrae (Lawrence 1986). O. echinata posesses the characteristic echinoderm ability to regenerate autotomized arms. Echinoderms have the ability to not only autotomize limbs but also eviscerate their gut and undergo fission for asexual reproduction (Mladenov et al. 1999). The energy expenditure of O. echinata the effects on somatic and gonadal tissues during regeneration of arms and also the relationship between food level and the amount of arm regeneration were investigated (Pomory and Lawrence 1999). Despite these unique charactersitics, O. echinata are not protected in Bermudian waters by any specific legislation.

Taxononmy


Phylum: Echinodermata
  Class: Ophiuroidea
    Order: Ophiurida
      Family: Ophiocomidae


Habitat


Ophiocoma echinata are common in the Atlantic Ocean; populating reefs off the coast of Florida, the Bahamas and the Caribbean (Hunnan 1992). It has been recorded that in some areas populations of Ophiuroidea can reach densities of thousands of individuals per square meter (Sterrer 1986). There have been twenty-one species of Ophiuroidea brittle stars recorded off the coast of Bermuda (Wood and Jackson 2005). O. echinata inhabit coral reefs and areas of coral rubble (Chartock 1983). This species of brittle star is common on sand substratum under rocks in shallow waters (Sterrer 1986).

O. echinata are often found hiding in crevices of reef cavities or rocks (Lewis and Bray 1993). Ophiuroidea are often found living in association with other animals, living in large numbers in the cavities of sponges or crawling on the branches of sessile coelenterates (Sterrer 1986). Individuals of O. echinata are larger and more active in shallow, back-reef areas than in deeper fore-reef habitats (Lewis and Bray 1993).

Ecology


Ophiuroidea are commonly known as brittle stars due to their stiff, brittle consistency. Species within the class Ophiuroidea are star-shaped with a central circular disc that has five long slender arms radiating from it (Sterrer 1986). The major coelomic cavity in ophiuroids is isolated to the central disc (Pomory and Lawrence 1999). The digestive system, gonads and other organ systems are all located in the disc and cannot extend into arms (Sides 1987). Most ophiuroids have a stout and rigid calcium carbonate covering on the central disk (Dobson 1985). Ophiuroids only have ciliated epithelial surfaces along parts of the arm and the ventral surface which creates surface water currents around the bursal slits for reproductive and respiratory purposes (Lawrence 1987).

Unlike common sea stars, in class Asteroidea, brittle stars have the madreporite located on the oral surface of the disc and no anus. Ophiuroidea are also unique due to the heavily calcified arms with ossicles forming lateral arm shields and the radial water canal has been enveloped by the ossicles (Sides 1987). Species within the family Ophiocomidae have stout arms and long erect arm spines (Sterrer 1986).

The blunt-spined brittle star, Ophiocoma echinata, are black or dark brown with two spines on each side of the first arm segments and two tentacle scales (Sterrer 1986). The arms of O. echinata are unable to coil and are only capable of lateral movements (Humann 1992). The spines of O. echinata function for feeding and establishment of burrows in the sand beneath rocks. Unlike other echinoderms, ophiuroids do not have spines that have been modified into pedicellariae. The lack of pedicellaria may be explained due to the amount of time ophiuroids spend burrowing (Chartock 1983). Exposed ophiuroids use their arms and appendages as prehensile organs to attach to substrata and to feed. The arms are indicative that O. echinata are carnivores and microphagous suspension feeders (Chartock 1983). O. echinata hold their arms vertically into the water column and the particles settle out on the spines and are transferred to the oral disk due to the current of the bursal slits and tube feet (Lawrence 1987).

The location of the gut within the disc may restrict digestion and energy acquisition (Sides 1987). Ophiuroids intermittently feed allowing periods of ingestion, digestion, and egestion but the uptake of energy may be limited due to the space conflict within the central disk (Lawrence 1987). Energy allocation for brittle stars and most animals is divided into maintenance of existing somatic tissue, growth of new somatic tissue, reproductive output, and losses due to excretion (Pomory and Lawrence 1999). It has been determined that the stomach is the main source of energy storage and reallocation to maintenance, reproduction and regeneration in O. echinata (Pomory and Lawrence 1999).

Locomotion of ophiuroids requires movement of the entire arm. The ambulacral ossicles have fused in ophiuroids to develop vertebrae (Sides 1987). Opiurids have vertebrae with ball-and-socket joints allow twisting (Lawrence 1986). The central skeletal articulating vertebrae have opposed pairs of contracting and relaxing muscles that cause the arms to flex and move. During movement the central disc is elevated and the tube feet help grip the substrate by friction or adhesion since they do not have suckers (Lawrence 1987). Observation of brittle star locomotion demonstrates that individuals move with one or two arms leading, the next pair moving synchronously in a rowing motion, and the remaining arms trailing (Pomory and Lawrence 1999). From 46-100% of ophiuroid species examined had at least one arm regenerating and frequently more than 40% of the total number of arms showed signs of damage (Sides 1987).

Echinoderms have the ability to autotomize, eviscerate, or undergo fission (Mladenov et al. 1999). Autotomization allows marine organisms a chance for survival. The immediate advantage of being able to lose a structure is to escape lethal predation or combat. Autotomy in echinoderms involves disintegration of the mutable collagenous tissue between the autotomized structure and the remainder of the organism (Dobson 1985). Arm autotomy is facilitated by the rapid loss of tensile strength and the collagen fibrils separate at the arm base and at the place of insertion into the abulacral ossicles (Dobson 1985). Autotomy results from the sudden release of longitudinal muscles and connective tissues linking the ossicles, and by the softening and tearing of the tissues of the aboral body wall in the plane of breakage (Mladenov et al. 1985). Once autotomy occurs the animal must begin to heal the wound and regenerate the lost limb. Regeneration represents an additional energy expenditure that echinoderms such as O. echinata often experience (Pomoroy and Lawrence 1999).

Recent Research


Studies regarding Ophiocoma echinata energy allocation and their relative feeding levels have been conducted (Pomory and Lawrence 1999). Energy content of the disk, intact arms, gonads, and the stomach were examined after two months of regeneration (Pomory and Lawrence 1999). Energy allocated for regeneration cannot be used for any other life process, so the changes in energy requirement were thought to cause changes in other metabolic processes. Pomoroy and Lawrence (1999) investigated the affect on somatic and gonadal tissues during regeneration of arms and also the relationship between food level and the amount of arm regeneration. They concluded that regereration represents an additional energy expenditure that normally has not been considered for most groups of animals, despite the occurrence of regeneration in most phyla (Pomory and Lawrence 1999).

Commercial Importance


Ophiocoma echinata has no particular commercial importance in Bermuda.

Bermuda Laws


This species does not have any special Bermudian Law protecting it. However, Bermuda has 29 marine-protected areas, which have a radius of between 300 and 1000 m, which fall under the Fisheries Act, 1972, Fisheries (Protected Areas) Order 2000 (Wood and Jackson 2005). Within the Bermudian marine-protected areas, all marine life including Ophiocoma echinata are not allowed to be collected.

Personal Interest


I have always been facinated with echinoderms and their specialized dermal endoskeleton and use of a coelomic water vascular cavity. Ophiocoma echinata is unique due to the isolation of the coelomic water vascular cavity in the central disk and locomotion is achieved mainly through muscle contractions. The vertebrae of brittle star arms are specialized to break off at certain points when it is threatened with predation. O. echinata is interesting since it is a relatively large brittle star with spines that are not necessarily for protection but aid in feeding and locomotion.

References


Chartock, M. A. 1983. Habitat and feeding observations on species of Ophiocoma (Ophiocomidae) at Enewetak. Micronesica 19:131-149

Dobson, W. 1985. A Pharmacological Study of Neural Mediation of Disc Autotomy in Ophiophragmus filograneus (Lyman) (Echinodermata: Ophiuroidea). J. Exp. Mar. Biol. Ecol. 94:223.

Humann, P., ed. Deloach, N. Reef Creature Identification: Florida, Caribbean Bahamas. New World Publications, Inc., 1992.

Lawrence, J. A Functional Biology of Echinoderms. The Johns Hopkins University Press, Baltimore. 1987.

Lewis, J. B. and R. D. Bray. 1983. Community Structure of Ophiuroids (Echinodermata) from Three Different Habitats on a Coral Reef in Barbados, West Indies. Marine Biology. 73:171-176.

Mladenov, P.V., Igdoura, S., Asotra, S., and Burke, R.D. 1989. Purification and Partial Characterization of an Autotomy-Promoting Factor from the Sea Star Pycnopodia helianthoides. Biol. Bull. 176:169.

Pomory, C. and J.M. Lawrence. 1999. Energy content of Ophiocoma echinata (Echinodermata: Ophiuroiea) maintained at different feeding levels during arm regneration. J. Exp. Mar. Biol. Ecol. 238: 139-150.

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.

Sterrer, Wolfgang., ed. Marine Fauna and Flora of Bermuda: A Systematic Guide to the Identification of Marine Organisms. New York: John Wiley and Sons, 1986.

Wood, J.B. and K.J. Jackson. 2005. Bermuda In: Caribbean Marine Biodiversity: The Known and the Unknown. Miloslavich, P. and E. Klein eds. DEStech Publications, Inc. 19-36.

Links

Monterey Bay Aquarium
Oceanic Research Group
Bermuda Aquarium Museum and Zoo
NOAA's National Ocean Service