JUVENILE MATING AND SPERM STORAGE IN THE TROPICAL CORALLIVOROUS NUDIBRANCH PHESTILLA-SIBOGAE

Nudibranch molluscs typically are semelparous, undergoing a single per iod of repeated spawning, and these opisthobranchs generally are categ orized as obligately cross-fertilizing simultaneous hermaphrodites. Th e tropical corallivorous aeolid nudibranch Phestilla sibogae has a sub -annual life cycle, with individuals cycling from egg to egg in simila r to 40 d and adults commonly attaining 30-40 mm in body length (simil ar to 800-1700 mg live weight). Spawning (egg laying) may commence at 15-25 mm length (similar to 90-480 mg). In a preliminary experiment, 2 2 individuals from batch cultures were paired at sizes expected to be pre-reproductive (5-7 mm; 3-8 mg) and the isolated couples further rea red to first spawning. Larvae from each pair's first egg mass were cul tured and juveniles reared to sizes at which they also could be typed electrophoretically. Comparisons between the parental and offspring ge notypes for up to three allozyme loci revealed that, for 6 of the 11 p airings, certain offspring did not conform to parental genotypes: in t otal, 18% of all offspring typed were non-conforming, ranging up to 75 % in one pair. Evidently P. sibogae can become inseminated at very sma ll sizes, and may store sperm, despite the opportunity for continued a nd repeated copulation with a large adult partner. The first received allosperm may display high levels of fertilization success. Three furt her experiments involved the batch culture of post-larval juveniles an d the subsequent isolation of individuals at varying sizes and ages. T hese latter experiments confirmed that P. sibogae can inseminate, and be inseminated, at extremely small sizes (similar to 5-8 mg) and well in advance of their ability to lay fertile eggs. The relevance of sper m competition models to these data is discussed in the context of appa rent first-partner precedence.