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A new distribution record and notes on the biology of the brittle star Ophiactis simplex (Echinodermata: Ophiuroidea) in Texas.

Brittle stars (Echinodermata: Ophiuroidea) are a common component of marine communities and often make up a significant portion of the biomass. Identification, however, can be problematic, particularly in the small fissiparous species. Fissiparity, asexual reproduction in which an individual divides in two and regenerates missing parts, occurs in 34 of the 2,000 species of brittle star (Emson & Wilkie 1980). One of these is Ophiactis simplex, an eastern Pacific species, with distribution from the Channel Islands to Panama and the Galapagos Islands (Neilsen 1932; Lonhart & Tupen 2001). Like other fissiparous brittle stars most specimens have six arms and are asymmetrical, with three long arms and three shorter arms. However, individuals with five and seven arms are not uncommon; the author has collected one with nine arms. One of the distinguishing characteristics of this species is the red tube feet. The red color is due to the presence of hemoglobin containing coelomocytes (RBCs) present in the water vascular system (Christensen 1999).

In late May 2001, five specimens of O. simplex were collected in a tide trap located on the research pier at the University of Texas Marine Science Institute, in Port Aransas, Texas. The specimens were found on algae caught in the net and were very small (disc diameter < 2 mm). Later that week approximately 200 specimens were collected from algae and other fouling material scraped from the rocks of the south jetty at Port Aransas. This represents a first report of this species along the Texas coast. Official counts were not made at this time. Voucher specimens were sent to Dr. Gordon Hendler at Museum of Natural History of Los Angeles for positive identification. Several subsequent collections have been made from the south jetty to determine habitat preference and population structure.

In January 2002, various species of algae, sponge, hydroid and tunicate colonies were scraped from the south jetty during an extremely low tide. Brittle stars were removed from the substrate, counted, and the volume of the substrate was estimated by measuring displacement volume. The brittle stars were sorted by disk diameter (large > 3 mm; medium 2-3 mm; small < 2 mm), regeneration state (recently split [2 or more arms < 2 mm], regenerating [2 or more arms of unequal length] and whole [all arms of equal length]) and redness of tube feet (bright red, medium red and colorless). The redness of the tube feet is a crude measure of the hematocrit (proportion of RBCs to water vascular system fluid). It is noted that individual hematocrit is variable in the Texas population: individuals possessing bright red tube feet have large numbers of RBCs in the water vascular system while others have colorless tube feet due to the scarcity of RBCs in the water vascular system. Actual hematocrits were not measured but were inferred from microscopic examination of several dissected individuals.

Collections were made again in June 2002, January and July 2003, primarily from colonies of the tunicate Eudistoma carolinense.

The densest aggregations of Ophiactis simplex were found in colonies of the sandy lobed tunicate, Eudistoma carolinense (75 individuals per 100 mL) (Table 1). Other substrates in which O. simplex were found included fire sponge (Tedania ignis), eroded sponge (Haliclona loosanoffi) and brown ribbed algae (Dictyopteris sp.) (Table 1). In January 2002, a total of 537 individuals was collected. Medium size individuals (2-3 mm disc diameter) were dominant (67%) and 58% of the individuals were nearly full or fully regenerated (Table 2). In contrast, the June 2002 collection yielded 414 individuals, 70.8% belonging to the small size class (< 2 mm disc diameter) and 82.6% of the individuals were in some stage of regeneration (Table 2). These animals were not sorted by tube feet color as significant mortality occurred before sorting. In July 2003, 229 individuals were collected, 88.2% belonging to the small size category and 83.4% were in some stage of regeneration.

Fission appears to be an important means of reproduction in the small and medium size classes, as most collected were in some stage of regeneration. Only two of the 27 large individuals collected were regenerating. The large size class also appears to be fairly uncommon; the largest individual collected had a disc diameter of 4.8 mm. Sexual reproduction also plays a role in this population of O. simplex. In the June 2002 collection, a large proportion (186 individuals) of the small size class was < 1 mm. The high number of small individuals indicates larval recruitment into the area (Mladenov & Emson 1984). Although nothing has been reported on the reproductive periodicity of O. simplex, the appearance of so many extremely small individuals in the summer suggests an early spring spawn period. The July 2003 sample also yielded many very small animals but the exact numbers were not quantified.

There does not appear to be any relationship between regeneration state and color of tube feet. However, there does appear to be a weak relationship between size and color. There was only one large individual (disc diameter 3 mm) with colorless tube feet; all other large individuals possessed either medium or bright red tube feet. The larger individuals may be dependent upon hemoglobin for oxygen transport due to their reduced surface area to volume ratio whereas smaller individuals are likely small enough to obtain sufficient oxygen needed for aerobic metabolism by simple diffusion. Differences in the numbers of RBCs among individuals of the same size class may be due to oxygen availability in the microhabitat: those with bright red tube feet may inhabit areas with a lower oxygen tension than those with colorless tube feet. This possibility will be investigated further.

[FIGURE 1 OMITTED]

It is not known if this population of O. simplex is a recent introduction (e.g., through ballast water or drift algae) or if it has been present, but misidentified. A closely related species, Ophiactis savignyii, appears on collection lists for the area. Both are small and fissiparous, but O. savignyii does not possess hemoglobin. As mentioned earlier, not all specimens of the Texas population possess large amounts of hemoglobin and the red color disappears upon preservation with alcohol or formalin. Even with the small size, the two species are morphologically different. The radial shields (two at the base of each arm) of O. savignyii are very large; the length often exceeds half the disc radius, while those of O. simplex are much smaller (Hendler et al. 1995) (Figure 1). The arm spines are also markedly different: 4-5 long thin spines in O. simplex and 5-6 shorter, stubby spines in O. savignyi.
Table 1. List of substrates and densities from which Ophiactis simplex
was collected. The different numbers associated with Eudistoma
carolinense represent different colonies of the tunicate.

Species Density

Tedania ignis 15/100 mL
Haliclona loosanoffi 17/100 mL
Dictyopteris sp. 8/100 mL
Eudistoma #1 35/100 mL
Eudistoma #2 41/100 mL
Eudistoma #3 79/100 mL
Eudistoma #4 28/100 mL
Eudistoma #5 61/100 mL
Eudistoma #6 25/100 mL

Table 2. Results of sorting the collections on the basis of size (small:
disc diameter <2 mm; medium: disc diameter 2-3 mm; and large: disc
diameter > 3 mm); regeneration state (recently split: half disc and 2 or
more arms < 2 mm; regenerating: 2 or more arms of unequal length; and
whole: all arms of equal length), and color of tube feet (indication of
hematocrit).

 January, 2002 June, 2002 July, 2003

Size
Small 157 293 202
Medium 359 117 25
Large 21 4 2

Regeneration state
Recently split 34 67 31
Regenerating 191 275 160
Whole 312 72 38

Color of tube feet
Bright red 199 * 94
Medium red 196 * 128
Colorless 142 * 7

* June, 2002, sample not sorted for color of tube feet due to
significant mortality before sorting. The red color fades with death.


ACKNOWLEDGMENTS

I would like to thank the following: Dr. Gordon Hendler for the morphological identification; Dr. David Hicks for his aid in collections; Denise Dean for assistance in counting and sorting brittle stars; and Jay Carroll at Tenera Environmental for collection of California O. simplex for comparisons; Drs. Richard Harrel and Andy Kasner for their comments on the manuscript.

LITERATURE CITED

Christensen, A. B. 1998. The properties of the hemoglobins of Ophiactis simplex (Echinodermata, Ophiuroidea). Am. Zool., 38:12.

Emson, R. H. & I. C. Wilkie. 1980. Fission and autotomy in echinoderms. Oceanogr. Mar. Biol. Ann. Rev., 18: 155-250.

Hendler, G., J. E. Miller, D. L. Pawson & P. M. Kier. 1995. Sea stars, sea urchins, and allies: Echinoderms of Florida and the Caribbean. Smithsonian Institution Press, Washington. 390 pp.

Lonhart, S. I. & J. W. Tupen. 2001. New range records of 12 marine invertebrates: The role of El Nino. Bull. Southern California Acad. Sci., 100:238-248.

Mladenov, P. V. & R. H. Emson. 1984. Divide and broadcast: sexual reproduction in the West Indian brittle star Ophiocomella ophiactoides and its relationship to fissiaprity. Mar. Biol., 81:273-282.

Nielsen, E. 1932. Ophiurans from the Gulf of Panama, California, and the Strait of Georgia. Vidensk. Medd. fra Dansk naturh. Foren., 91:241-346 [pp.257-60].

Ana Beardsley Christensen

Department of Biology, PO Box 10037

Lamar University, Beaumont, Texas 77710

ABC at: christenab@hal.lamar.edu
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Author:Christensen, Ana Beardsley
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:May 1, 2004
Words:1523
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