Section 8
Chapter 7,773

Seasonal breeding aggregations in low density populations of the bathyal echinoid stylocidaris lineata

Young, C.M.; Tyler, P.A.; Cameron, J.L.; Rumrill, S.G.

Marine Biology (Berlin) 113(4): 603-612


ISSN/ISBN: 0025-3162
Accession: 007772254

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Reproduction of bathyal cidaroid sea urchins was studied between 1985 and 1991 with the aid of a manned submersible in the northern Bahamas [West Indies]. During May 1988 and February 1990, discrete, single-species aggregations of Stylocidaris lineata containing 2 to 6 individuals were observed between 510 and 640 m depths. Reproductive conditions of isolated and aggregated individuals were determined histologically. In February, all individuals, regardless of their spatial distribution, contained developing gametes. During the May spawning season, most isolated individuals contained spent gonads with only relict gametes, whereas individuals in discrete aggregations mostly contained ripe gonads with mature or nearly-mature gametes. Aggregations were not observed during the autumn months, when gonads were spent or immature. These data suggest that bathyal cidaroids aggregate for production. An in situ survey during May indicated that pairs of S. lineata are more common than larger clumps, but no more so than predicted by chance. Homosexual and heterosexual pairs occur at the relative frequencies predicted on the basis of sex ratio. We evaluated the need for reproductive aggregation by applying Denny's (1988) model that predicts downstream sperm concentration in a turbulent boundary layer. Flow parameters were measured with dye injected from the submersible, sperm-release rate was estimated in the laboratory, and the relationship between fertilization success and sperm concentration was determined in a laboratory dilution-experiment. The model predicted that, on smooth sandy bottoms at bathyal depths sperm concentration should remain high enough to fertilize at least some eggs several meters directly downstream from a spawning male. Aggregation may facilitate spawning synchrony, increase gamete encounter probabilites, or cause gametes to be retained at high concentrations near the adults long enough for fertilization to occur.

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