Abstract

Calyptogena gallardoi is a vesicomyid bivalve inhabiting a methane seep area located at a depth of 740870m off the Bay of Concepcin, Chile. Vesicomyids host chemoautotrophic sulphide-oxidizing endosymbiont bacteria and are always found associated to reducing environments. In this study, the gonadal structure and the gametes produced by C. gallardoi are described. Light microscopy is used to examine serial histological sections of the gonads, and scanning electron microscopy is used to visualize the external morphology of gametes. The gonads of both males and females are organized in ramified tubular acini. In males, mature sperm are stored near genital openings in acini lined with a secretor epithelium that resembles a seminal receptacle. Spermatozoids have bullet-like heads with an average length of 30.32.6m (mean1 SD). In females, the mature oocytes are driven toward the genital opening through evacuator conduits lined by paddle cilia. The average diameter of oogonias is 11.62.5m and that of mature oocytes is 273.823.1m, making the size of the mature oocyte among the largest reported for bivalves. In addition, C. gallardoi is shown to have external sexual dimorphism. Shells of males are significantly smaller and more elongated with sloping postero-dorsal margin compared with shells of females. The data are discussed in the context of available information on reproductive biology of vesicomyids. © 2009 Copyright Marine Biological Association of the United Kingdom.