Marine Invertebrates of Bermuda Little Brittle star (Ophiactis savignyi) Kara McKeton and James B. Wood (Ed) Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links

Abrstract: Ophiactis savignyi is as a small, fissiparous brittle star (Sterrer 1986), found throughout the tropics and subtropics (McGovern 2002). O. savignyi inhabits coral reef zones, is found under rocks and can occur in high densities in different sponges (Boffi 1972). O. savignyi is a deposit feeder, using the podia on the oral surface of the arms to collect food particles from the benthos (Ruppert and Barnes 1994). O. savignyi is capable of reproducing sexually and asexually through broadcast spawning and fission (Mladenov and Emson 1988). It appears that larger individuals (disc size over 4mm), possessing 5 arms, undergo sexual reproduction, while smaller individuals (disc size less than 3mm), possessing 6 arms, undergo fission (Mladenov and Emson 1988, Sterrer 1992, McGovern 2002). Those individuals experiencing asexual reproduction will generate new individuals of the same sex, creating clonal aggregations. Studies have shown that O. savignyi populations are dominated by males, suggesting that males undergo asexual reproduction more frequently than females (Mladenov and Emson 1988). Whether O. savignyi reproduces sexually or asexually, there are still energetic costs for either process of reproduction. The most recent research for O. savignyi focuses on the reproductive cycles of this species.

Taxononmy

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

O. savignyi is identified as a small, fissiparous brittle star, with a disc 5 mm across (Sterrer 1986). O. savignyi can be multicolored green and white, with scales and small spines on the central disc (Sterrer 1986). Young specimens of O. savignyi are almost always found with 6 slender arms (Sterrer 1986), but specimens of O. savignyi can be found with any number of arms up to seven. O. savignyi is found throughout the world, mainly in the tropics and subtropics (McGovern 2002). Studies have focused on populations of O. savignyi off the coasts of Brazil, southern Taiwan, Jamaica, Belize, Florida and Bermuda, specifically in Harrington Sound, Bermuda (Boffi 1972, McGovern 2002, Mladenov and Emson 1988). It is believed that O. savignyi is ‘the most common brittle star in the world’ (Clark 1946).

Habitat

Ophiactis savignyi is found living among coral reefs (Chao and Tsai 1995), inside crevices, under rocks, in association with bryozoans (Schizoporella errata) and within coralline algae and sponges (Mladenov and Emson 1988, Morgado and Tanaka 2001). Populations of O. savignyi have been found living in the exhalant passages of different sponges, such as Fire sponge (Tedania ignis) in Bermuda and the Florida Keys (McGovern 2002, Sterrer 1972) and Haliclona sp., in southern Tawain (Chao and Tsai 1995). O. savignyi can be found in extremely high densities in sponge, reaching densities of 1,892 individuals per 100g of sponge (Boffi 1972). O. savignyi is unique for its occurrence in high densities in sponges. O. savignyi may choose to live inside sponges because of the presence of water canals within the sponge structure; these canals provide currents for circulation and feeding for the brittle stars (Ruppert and Barnes 1994).

Ecology

Locomotion: The Ophiuroids are the most mobile Echinoderms, using their arms for locomotion across the benthos. It appears that Ophiuroids show no arm preference and can move in any direction (Ruppert and Barnes 1994). The arms of O. savignyi can measure up to 25 mm in length and can move in a snake-like manner (Sterrer 1992). Brittle stars move rapidly by pushing and pulling with their flexible arms, with their spines acting as traction (Ruppert and Barnes 1994). Some Ophiuroids begin locomotion with their disk held above the substrate, with one or two arms stretching forward in the desired direction; the two lateral arms perform a rowing motion against the substratum which pushes the animal forward in leaps or jerks, while the remaining arms trail behind (Ruppert and Barnes 1994). There is a lack of information on the method of locomotion of O. savignyi.

Feeding: Ophiuroids demonstrate a variety of feeding methods, including predation, deposit feeding, suspension feeding and scavenging (Brusca 2003). Some species of brittle stars are able to feed using multiple methods (Brusca 2003). Most feeding methods involve the arms and tube feet of brittle stars (Ruppert and Barnes 1994). The brittle star, O. savignyi, is considered a deposit feeder and a detritophagous, which refers to feeding on detritus (Boffi 1972). O. savignyi keeps the oral surface of their disks toward the substrate, while raising the tips of their arms to selective deposit feed (Boffi 1972). Deposit feeding is accomplished by the podia (tube feet), which collect the food particles from the substratum and form them into a compact food ball, which is transported along the arm by other podia toward the mouth, where it is digested (Ruppert and Barnes 1994). In a study, the stomach contents of O. savignyi consisted of large quantities of detritus, silica grains, bryozoans, foraminiferans and small remains of gastropods, suggesting that O. savignyi feeds on these particles (Boffi 1972). There is a lack of information on the feeding method of O. savignyi living inside of a sponge.

Reproduction: O. savignyi is dioecious and is capable of reproducing asexually and sexually (Mladenov and Emson 1988). Asexual reproduction occurs by fission, which is the voluntary splitting of the central disc of the brittle star and each piece regenerating the missing parts of the disc and arms, forming whole brittle stars (McGovern 2002, Sterrer 1992). Sexually reproducing O. savignyi will spawn by releasing their gonads into the water column (Chao and Tsai). Sexually mature female O. savignyi have enlarged brown gonads (up to 10,000 eggs), while mature male O. savignyi have enlarged white gonads, which are stored in the bursal slits (Chao and Tsai 1995, McGovern 2002). The bursal slits in Ophiuroids are unique and are located at the base of the arms, on the oral surface (Pechenik 2005). Studies show that larger individuals (disc size over 4mm), possessing 5 arms, undergo sexual reproduction, while smaller individuals (disc size less than 3mm), possessing 6 arms, undergo fission (Mladenov and Emson 1988, Sterrer 1992, McGovern 2002). As a result of fission, a female breaking into several pieces will only generate females, while a male will only produce other males (Sterrer 1992). In Harrington Sound, Bermuda, clonal aggregations of male O. savignyi predominated in sponges, showing no sign of female O. savignyi (Mladenov and Emson 1988). Perhaps male O. savignyi undergo fission more frequently than females, because females are more likely to lose their sexual reproductive capacities following division, resulting in lower female population densities (McGovern 2002). It appears that O. savignyi reproduces asexually more frequently than sexually (Mladenov and Emson 1988, McGovern 2002). Sexual and asexual reproduction of O. savignyi may occur simultaneously because recently split individuals may still contain mature gonads (Chao and Tsai 1995). Fission and sexual reproduction occur most frequently during the warmer months, suggesting that temperature plays a role in spawning time (Chao and Tsai 1995). Both sexual and asexual reproduction requires energy to accomplish (McGovern 2002).

Regeneration: Ophiuroids, along with all other Echinoderms, possess the capability of regeneration (Pechenik 2005). Ophiuroids are known to regenerate tissue following autotomy, which is the shedding of an appendage or other body part due to stress (Pechenik 2005). Ophiuroids are able to autotomize due to the presence of mutable connective tissue, which is controlled by nerves and allows the rapid changes of stiffness in the connective tissue to occur (Pechenik 2005). Regeneration occurs in O. savignyi after these invertebrates undergo fission, which is a form of asexual reproduction. Regeneration of both the arms and discs of O. savignyi can occur. Usually, when you see O. savignyi with three long arms and three short arms, the short arms are in the process of regenerating after fission or predation.

Recent Research

Chao and Tsai (1995) examined the reproductive cycles in O. savignyi populations for one year, in southern Taiwan. Chao and Tsai (1995) discovered that sexual recruits were found throughout the year, but were at highest frequencies in June, and recently split individuals occurred throughout the year, with highest densities in July. These results suggest that the peak of sexual reproduction of O. savignyi is during July and the peak of asexual reproduction (fission) of O. savignyi is during June (Chao and Tsai 1995). Interestingly, the occurrence of fission was highest after spawning. Chao and Tsai (1995) also found a small portion of recently split individuals with mature gonads, suggesting that sexual and asexual reproduction may be simultaneous. However, most mature gonads were found in intact individuals (Chao and Tsai 1995).

McGovern (2002) performed a similar study to Chao and Tsai (1995); however, this study was accomplished over three years. McGovern (2002) examined the associations of sexual and asexual reproduction with body size and sex. McGovern (2002) found that individual O. savignyi need to reach a size threshold in order to attain sexual mortality, and male O. savignyi have a lower size threshold for sexual maturity than do female O. savignyi. Potential explanations for these results is unclear, however, gonad mass alone and prey availability to males verses females may be factors controlling the difference in size thresholds (McGovern 2002). McGovern (2002) also found seasonal patterns to both sexual and asexual reproduction. The highest densities of mature O. savignyi was observed in the late summer through fall, and the incidence of asexual reproduction increased in the late fall though late winter (McGovern 2002). Just like Chao and Tsai (1995), McGovern (2002) found that division occurred following sexual reproduction time period. The time periods of sexual and asexual reproduction were different in Florida compared to Taiwan, suggesting that O. savignyi undergo reproduction at different times in different locations.

Within the same year, McGovern (2002) published another scientific paper on a study concerning the sex-ratio bias and clonal reproduction in O. savignyi. She demonstrated that the males of O. savignyi were more likely to divide than the females (McGovern 2002). McGovern (2002) suggests that the genders may differ in their rates of cloning if they differ in the costs associated with cloning. McGovern (2002) concluded that the females were more likely to lose sexual reproductive capabilities following division, and this greater cost connected with division may explain why females are less likely to divide than males.

Commercial Importance

The brittle star, O. savignyi, has not been reported for any commercial importance.

Bermuda Laws

There are no Bermuda laws directed toward the brittle star, O. savignyi. There are Marine protected areas (MPA’s) in Bermuda in which you may find aggregations of O. savignyi under protection of the MPA.

Personal Interest

O. savignyi fist caught my eye when we went on a class field trip to Harrington Sound, determined to collect sponges. When I was cutting off a small piece of a certain type of sponge (for lab analysis), all of these tiny brittle stars popped out of the sponge opening and I thought it was the coolest thing. I did not know much about brittle stars before this past summer, but now I love them. Brittle stars are so unique from all the other echinoderms, especially with the snake-like movement of their arms and the ease at which they shed off an arm. In the future, I would enjoy heading back to Harrington sound and performing a study on O. savignyi and their habitat preference.

References

Boffi, E. 1972. Ecological aspects of ophiuroids from the phytal of S.W. Atlantic Ocean warm waters. Mar. Biol. 15:316-328.

Brusca R.C., and Brusca, G.J. 2003. Invertebrates. 2nd ed. Sinauer Associates, Inc. Publishers. Sunderland, Massachusetts. P:804-805,812.

Chao, S.M., Tsai, C.C. 1995. Reproduction and population dynamics of the fissiparous brittle star Ophiactis savignyi (echinodermata: Ophiuroidea). Mar. Biol. 124:77-83.

Clark, S.M. 1946. The echinoderm fauna of Australia; it composition and origin. Carnegie Inst Washington Pub. 566:1-567.

McGovern, T.M. 2002. Patterns of sexual and asexual reproduction in the brittle star, Ophiactis savignyi in the Florida Keys. Mar. Ecol. Prog. Ser. 230:119-126.

McGovern, T.M. 2002. Sex-ratio bias and clonal reproduction in the brittle star, Ophiactis savignyi. Evolution. 56(3):511-517.

Mlandenov, P.V. and Emson, R.H. 1988. Density, size structure and reproductive characteristics of fissiparous brittle stars in algae and sponges: evidence for interpopulational variation in levels of sexual and asexual reproduction. Mar. Ecol. Prog. Ser. 42:181-194.

Morgado, E.H. and Tonaka, M.O. 2001. The macro fauna associated with the bryozoan Schizoporella errata (Walters) in southeastern Brazil. Sci. Mar. 65(3):173-181.

Pechenik, J.A. 2005. Biology of the invertebrates. 5th ed. McGraw-Hill companies, New York, NY. P:485-510.

Sterrer, W. 1992. Bermuda’s marine life. Bermuda Natural History Museum and Bermuda Zoological Society. Flatts, Bermuda. P:181.

Sterrer, W. 1986. Marine fauna and flora of Bermuda: a systematic guide to the identification of marine organisms. John Wiley & Sons Inc. Canada. P:529-531.

Ruppert, E.E., Barnes, R.D. 1994. Invertebrate Zoology. 6th ed. Saunders College Publishing. New York. P:941-950.

Links

Deep sea benthos
brittle stars
ophiactis savignyi