'Juvenile' mating and sperm storage in the tropical corallivorous nudibranch, Phestilla sibogae (Bergh).

Nudibranch molluscs typically are semelparous, undergoing a single period of repeated spawning, and these opisthobranchs generally are categorized as obligately cross-fertilizing simultaneous hermaphrodites. The tropical corallivorous aeolid nudibranch Phestilla sibogae has a sub-annual life cycle, with individuals cycling from egg to egg in similar to 40 d and adults commonly attaining 30-40 mm in body length (similar 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, 22 individuals from batch cultures were paired at sizes expected to be pre-reproductive (5-7 mm; 3-8 mg) and the isolated couples further reared to first spawning. Larvae from each pair's first egg mass were cultured and juveniles reared to sizes at which they also could be typed electrophoretically. Comparisons between the parental and offspring genotypes for up to three allozyme loci revealed that, for 6 of the 11 pairings, certain offspring did not conform to parental genotypes: in total, 18% of all offspring typed were non-conforming, ranging up to 75% in one pair. Evidently P. sibogae can become inseminated at very small sizes, and may store sperm, despite the opportunity for continued and repeated copulation with a large adult partner. The first received allosperm may display high levels of fertilization success. Three further experiments involved the batch culture of post-larval juveniles and the subsequent isolation of individuals at varying sizes and ages. These 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 sperm competition models to these data is discussed in the context of apparent first-partner precedence.