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In this paper we describe results of a study on the reproductive biology of the hydrother- mal vent crab Bythograea thermydron. At vent sites along the East Pacific Rise, histological analyses of gonads from adult female crabs collected in November 1997, May 1999 and May 2000 provide strong evidence that B. thermydron undergoes a seasonal cycle of reproductive activity, with egg hatching peaking in April-May. Females undergo a synchronous pattern of oocyte development, whereby one cohort of oocytes undergoes vitellogenesis, while a second cohort is present and remains immature until the initial cohort reaches full maturity and is extruded onto the pleopods. When mean oocyte-size-frequency distributions were compared across months, the 2 May samples were similar to each other, with a dominance of previtellogenic oocytes and a second, smaller group of vitellogenic oocytes also present. In contrast, the November sample exhibited a wide variety of oocyte sizes, with no one size class predominating. In addition, May samples from the vent periphery contained a high proportion of females that showed evidence of recent egg hatching, while females collected from the region near the vent orifice exhibited a wide range of the intermediate stages of gonadal development. This set of data suggests an annual cycle whereby a given cohort of oocytes is in a previtellogenic state in early spring (April-May), but has begun vitellogenesis by late autumn (November) and has developed to maximum maturity by the time the eggs are extruded the follow- ing spring. While the current clutch of eggs is being brooded, a second cohort of previtellogenic oocytes begins to develop in the ovary and the cycle is repeated. Superimposed on this annual cycle is a pattern of behavior whereby females with mature gonads migrate to the vent periphery where fertilized eggs are extruded, brooded, and hatched.

Spatial and temporal patterns in development of eggs in the vent crab Bythograea thermydron