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The edible volutid snail Adelomelon ancilla occurs on the Argentine coast and may have the capacity to serve as source of support for local Wsheries. However, knowledge of its reproduction is lacking. Over 3 years (from September 2000 to July 2004) we studied the oviposition at Golfo Nuevo, Patagonia by diving. Monthly collections were performed to examine gonadal stages histologically during 2002-2003. Gamete development stages corresponded with the expected period for oviposition occurring from July to November, a time of increasing day length. Oviposition also took place in March when water temperature attained a maximum of 18°C. Following the spawning period, remaining gametes were resorpted in both sexes. Males contained sperm throughout the year, parasperm and eusperm forms were found within the same acinus. Oogonia/oocytes ranged from 20 to 240 m in diameter during oogenesis. Females commenced laying egg capsules from a shell length of 114 mm. Reproduction is compared with other Wshable volutid snails from the Argentine shelf, and suggests the need to apply certain Wshing restrictions if the resource begins to be utilized commercially. The imposex condition was found in snails from some shipping areas. The frequency of this condition declined with distance from the port.Communicated by M. Byrne.

The ultrastructure of the euspermatozoa and the paraspermatozoa is investigated in Adelomelon ancilla, through histological section observed by transmission electron microscopy. Euspermatozoa of A. ancilla consists of: (1) a conical acrosomal vesicle (with a short basal invagination, constricted anteriorly) which is Xattened at the apex and associated with an axial rod, a centrally perforated basal plate and a short accessory membrane, (2) a rodshaped, solid and highly electron-dense nucleus (with a short basal fossa containing a centriolar complex and a initial portion of a 9 + 2 axoneme), (3) an elongate midpiece consisting of the axoneme sheathed by 5-6 helical mitochondrial elements each exhibiting a dense U-shaped outer layer, (4) an elongate glycogen piece (where the axoneme is sheathed by nine tracts of glycogen granules), (5) a dense annulus at the junction of the midpiece and glycogen piece, and (6) a short free tail region (where the axoneme is surrounded only by plasma membrane). We observed a parasperm in A. ancilla. This is vermiform in shape and is composed of multiple axonemes and extensive cytoplasm with numerous vesicles, and mitochondria are scattered inside the axonemes. Sperm of A. ancilla is characterized by the euspermatozoa type 2 and the paraspermatozoa morphology belongs to type 5. The U shaped electrodense mitochondrial element in the midpiece of the eusperm and the constriction in the acrosomal vesicle present in A. ancilla are exclusive. We suggest that these characteristics could have taxonomic importance, because these was observed in other volutids and have not been observed in the rest of caenogastropods studies. We consider that the morphology of paraspermatozoa in A. ancilla corresponds to the "lancet" type.

The carnivorous snail Adelomelon ancilla usually carries, attached to its shell, the anemone Antholoba achates, which also lives attached to hard substrates in the same area. Interaction between both species was studied by stable isotopes analyses (SIA), direct observations, and analysis of gut contents. Results did not show evidences of dietary overlap between anemones and snails. A. ancilla consumed mainly bivalves and secondarily gastropods. The diet of A. achates involved sea urchins and echiurids as main prey. The trophic niches of anemones and snails did not overlap; the species had similar d 15 N values but differences in their d 13 C values. Sessile and epibiotic anemones also showed differences in their d 13 C means, revealing access to different resources. Whereas SIA provided information on trophic relationships and structure, observation of feeding events provided details of prey utilization patterns. Combining direct and indirect approaches helps to overtake method weaknesses and greatly improve dietary descriptions.

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We studied the reproduction of Ophioplocus januarii from Playa Villarino (42°24' S, 64°17' W), based on monthly samples collected between April 2005 and April 2007. Gonad indices were generally synchronous for males and females with little seasonal pattern. The changes of the gonad index were not significantly correlated with day length or water temperature. Histological analyses showed no evidence of seasonal differences in the maturation stages of the gonads, but that ovaries contained oocytes at different stages of development during the year. Thus, variations in the gonad index resulted from differences in the number of gametes in the gonads each month, suggesting that there are some periods when spawning intensity is higher than in others. Oocyte size-frequency analysis revealed only slight accumulation of large oocytes, with the largest oocytes over 400 µm in diameter. This, together with the absence of oocytes being resorbed, suggests that gametes are being released continuously without a defined resting phase for the ovary. O. januarii breeds continuously, differing from the seasonal pattern of many other invertebrates living in shallow waters off the Patagonian coast as well as in other temperate seas, providing helpful information towards a better understanding of reproduction in Ophiuroidea.

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