Reference: Bio/. Bull. 180: 34-55. (February. 1991) Gastropod Egg Capsules and Their Contents From Deep-Sea Hydrothermal Vent Environments R. G. GUSTAFSON, D. T. J. LITTLEWOOD, AND R. A. LUTZ Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08903 Abstract. Egg capsules from three different prosobranch gastropods were retrieved from the Galapagos Rift and Juan de Fuca Ridge deep-sea hydrothermal vent fields. The morphology of these capsules and their excapsulated embryos and larvae are described and illustrated. Based on their capsule type and the protoconch morphology of their contained larvae, 29 lenticular capsules from the Galapagos Rift could be attributed to a provisionally de-scribed neogastropod turrid, Phymorhynchus sp. But 3 inflated, triangular capsules from the Galapagos Rift, and 56 different egg capsules from the Juan de Fuca Ridge, each shaped like an inflated pouch, could not be unam-biguously assigned to a member of the known vent gas-tropod fauna. The mode of development and potential for dispersal is inferred from egg capsule type, the number of embryos per capsule, and protoconch characters com-parable to those of confamilial shallow-water gastropods for which the type of development is known. These criteria and a comparison to the known juvenile shell morphology of Phymorhynchus sp., suggest that, after encapsulation, this species develops planktotrophically and is capable of long-range dispersal. Similar evidence suggests that the larvae contained in the inflated triangular capsules from the Galapagos Rift may also develop planktotrophically after hatching; but the larvae in the pouch-like egg capsules from the Juan de Fuca Ridge probably develop non-planktotrophically without a dispersal stage. These de-velopmental patterns are characteristic of shallow-water members of the systematic groups to which these species belong, indicating, as previous studies have shown, that vent gastropods can persist in these patchy, ephemeral environments in the absence of unique adaptations al-lowing dispersal between active hydrothermal sites. Received 14 August 1990; accepted 30 November 1990. Introduction Active hydrothermal vent systems accompanied by dense benthic fauna occur at several widely separated sites along active oceanic ridges in the eastern Pacific, from 48N along the Juan de Fuca Ridge, to 22S along the East Pacific Rise. Known hydrothermal fields on the Juan de Fuca Ridge are separated by as much as 100 km, whereas separation along transform faults on the East Pa-cific Rise indicates that vent fields are at least 100 km apart in this region (Crane, 1985; Grassle, 1986). Local vent habitats appear to be transient, with populations being susceptible to intermittent establishment and ex-tinction (Lulzeial., 1985; J. F. Grassle. 1985; Lutz, 1988). Despite their apparent geographic isolation and ephemeral nature, vent areas are characterized by the remarkable similarity of their faunal assemblages (Lutz, 1988). Fun-damental biological questions remain regarding both the manner in which these ephemeral habitats are colonized, and the mechanisms of organism dispersal and rates of gene flow between discrete areas of hydrothermal activity associated with contiguous and non-contiguous oceanic ridge systems. Because laboratory culture of deep-sea organisms is dif-ficult (Turner et ui, 1985), many of our perceptions about the development and larval dispersal of vent biota have been, by necessity, inferred from analyses of egg size, fe-cundity, and morphology of larval structures retained on juvenile and adult specimens. Gastropod mollusks have been widely used for such studies, because a record of the larval developmental pattern can be inferred from the morphology of the initial shell, comprising the Protoconch I in non-planktotrophic species and, also, the Protoconch II shell stages, in planktotrophic species (Powell, 1942; Thorson, 1950;Shuto, 1974; Robertson, 1976: Jablonski and Lutz, 1980). The mode of larval development in recent (Rodriguez Babio and Thiriot-Quievreux, 1974; Bandel, 1975a, b, c, 34
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