About the Cover
Cover
Most echinoderms are dioecious, but the sexes are usually indistinguishable on the basis of external structure. A very few species of brittle stars are, however, strikingly dimorphic, with a dwarf male clinging to a much larger female, usually mouth-to-mouth. Although this romantic posture suggests a role in reproduction, spawning has not previously been observed in these species. However, in this issue of The Biological Bulletin (p. 25) Hideyuki Tominaga and his colleagues describe the reproduction and development-from spawning to metamorphosis-of a dimorphic brittle star, Ophiodaphne formata. And they report further that these brittle stars, both couples and singles, have adopted a close living arrangement with sand dollars.
The upper portion of the cover shows an enlarged image of two brittle stars, coupled, and with their oral surfaces appressed. Her central disk is about 5 mm in diameter, his only 1 mm; and his arms are interdigitated with hers. At a much lower magnification (bottom left panel), we see this couple reclining on a host sand dollar, close to one of its five lunules. The female's aboral surface is fixed to the spiny oral surface of the sand dollar, and two of her arms are hooked over the edge of the lunule. A specimen of the host sand dollar from this study (Astriclypeus manni; diameter, about 14 cm) is shown in the lower right corner of the cover. But O. formata is not host-specific; two other species of sand dollars have been reported to accommodate this brittle star. Notice also that the figured specimen of A. manni bears two unpaired female brittle stars on its oral surface-near the lunules at 10 and 12 o'clock; thus, all O. formata are found on sand dollars, but only about half of the brittle stars are paired.
The sand dollars live in relatively shallow waters, partially buried in the sandy bottom. Thus, they provide the brittle stars with a ready, stable site of attachment on a shifting substrate; and the lunule protects against abrasion by the sand, which is the source of food for both organisms. At spawning, the gametes of brittle stars are released from bursal slits on the oral surface of the disk. The mouth-to-mouth posture brings those openings of the coupled male and female close together, maximizing the efficiency of fertilization. This is critical, because the population of brittle stars is sparse (only one of ten sand dollars is inhabited), and because these small animals produce a limited number of eggs. These relatively small eggs develop indirectly; but the time in development to metamorphosis is rapid, which minimizes transport of the brittle star larvae away from the habitat of the host sand dollars. Thus, Tominaga and colleagues suggest that the sexual dimorphism, the coupling behavior, and the characteristics of development all seem to be adaptations enabling a sparse population of brittle stars to survive on the sandy bottom of shallow seas.
The photographs on the cover were taken by Hideyuki Tominaga (Toyama University), and the cover was designed by Beth Liles (Marine Biological Laboratory, Woods Hole, Massachusetts).
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Copyright © 2004 by the Marine Biological Laboratory.
