Sperm and milt characteristics and male versus female investment in the Caribbean reef fish, Thalassoma bifasciatum

Individual sperm cells produced by two male morphs of the bluehead wrasse Thalassoma bifasciatum did not differ in size (i.e. cell volume). Initial phase (IP) males (high sperm competition) had a 60% higher sperm concentration in their milt than did terminal phase (TP) males (low sperm competition), which may reflect differences in how accurately the two male morphs need to allocate sperm to their spawns. The energy density of milt was about 16% lower than that of eggs. Estimates of gametic energy investment based on (a) the difference in testis weights between the beginning and the end of the spawning period and (b) the number of sperm released in natural spawns (determined in other studies), suggested that, on a daily basis, IP males invest about 65% of that of females. Estimates based on stripping milt from IP males at the beginning and the end of the spawning period, however, indicated that their daily energy investment in gamete production is about 10% of that of females. Gametic investment by TP males is lower than that by both IP males and females.