* Corresponding author. Invertebrate Reproduction and Development, 50:2 (2007) 75–84 75 Balaban, Philadelphia/Rehovot 0168-8170/07/$05.00 © 2007 Balaban Reproductive cycle of the southern geoduck clam, Panopea abbreviata (Bivalvia: Hiatellidae), in north Patagonia, Argentina SILVINA VAN DER MOLEN 1* , MARINA KROECK 2 and NÉSTOR CIOCCO 1 1 Centro Nacional Patagónico–CONICET, Boulevard Brown 2850 (U9120ACV), Puerto Madryn, Chubut, Argentina Tel. +54 (2965) 451024; Fax: +54 (2965) 451543; email: svandermolen@cenpat.edu.ar 2 Instituto de Biología Marina y Pesquera “Almirante Storni”, Costanera s/n (8520), San Antonio Oeste, Rio Negro, Argentina Received 18 October 2006; Accepted 29 May 2007 Summary On the basis of histological examination, we present the first description of gametogenesis in Panopea abbreviata. Six stages of gonad development were identified using morphological and histochemical criteria. The gonads are diffuse and cannot be sexed with the naked eye. The analyses of gonad tissues showed that the gametogenic cycle is continuous with no resting period. Ripe individuals of both sexes were found for most of the year, and the spawning season seems to be protracted. The application of histological techniques to individuals collected monthly revealed a bias in the sex ratio depending on the body length. Even when the sex ratio was balanced in large clams, for small individuals, the sex ratio was biased in favour of the males. Although no hermaphroditic individuals were found in our samples, the possibility of protandry should be examined further. Key words: Southern geoduck, P. abbreviata, gametogenesis, sex ratio, reproductive cycle, North Patagonia Introduction The clam Panopea abbreviata (Valenciennes, 1839), is endemic from the south-western Atlantic. This species is a large and long-lived (maximum age recorded in Patagonia: 40 years) burrowing bivalve which occurs between 23º S and 48º S in the SWA (Morsán and Ciocco, 2004). It is found deeply buried, up to 70 cm in the sediment, showing only the tips of the siphons, from the low intertidal to at least 75 m (Ciocco, 2000). The clams pump water from the sediment-water interface through a long pair of fused siphons. The common name in Argentina has not been standardized, and P. abbreviata is usually called “Almeja Panopea” (Spanish version of “Panopea clam”) or “southern geoduck”. P. abbreviata is a significant recent addition to the non-traditional fishery resources of north Patagonia. The fishery of P. abbreviata from north Patagonia was launched in 1999 under an experimental status. Three to ten metric tons of living clams have been caught annually using artisanal fishing methods. During the exploratory phase of the fishery, some high density areas were located, yielding a catch per unit of effort (CPUE) of up to 50 kg. diver !1 .hour !1 (Ciocco, 2000). A different species of the same genus, Panopea abrupta (Conrad, 1849), sustains one of the most significant invertebrate fisheries of the north-