Reproductive characteristics ofCalyptogena gallardoi(Bivalvia: Vesicomyidae) from a methane seep area off Concepción, Chile

Abstract

Calyptogena gallardoiis a vesicomyid bivalve inhabiting a methane seep area located at a depth of 740–870 m off the Bay of Concepción, 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 byC. gallardoiare 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 30.3±2.6 μm (mean±1 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 11.6±2.5 μm and that of mature oocytes is 273.8±23.1 μm, making the size of the mature oocyte among the largest reported for bivalves. In addition,C. gallardoiis 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.