Sperm competition and male mating tactics in the bitterling fishes

Bitterling are a group of freshwater fishes that lay their eggs on the gills of living freshwater mussels, using the mussel as a protective environment for embryo development and utilising the mussels own respiration to ensure fertilisation. This unusual spawning mechanism, using a spawning site that can be easily manipulated, makes bitterling ideal of reinvestigating sperm competition and making system evolution.;Here, using a range of bitterling species, a series of aquarium experiments were conducted, as well as morphological and histological studies of the sperm and testes. It was shown that males were highly sensitive to sperm competition, ejaculating at a higher frequency and subsequently becoming more sperm depleted where sperm competition was high. There were few differences between mating tactics except in relative testis size, where larger males had proportionally larger reproductive apparatus, but ejaculates were of a similar size. The timing of ejaculates was found to be crucial, with a peak in sperm concentration within the mussel mantle cavity 30 seconds after ejaculation. The spatial clustering of fertilisation opportunities and OSR were found to affect ejaculate frequency, ejaculate distribution among mussels, the dominance of gender males and subsequently the opportunity of subordinate males to sneak fertilisations. Significant differences in the spermatogenic strategy and the structure of the reproductive apparatus among species were identified, as well as significant differences between species in the morphology of spermatozoa.