Sperm and milt characteristics and male v. female gametic investment in the Caribbean reef fish, Thalassoma bifasciatum

Individual sperm cells produced by two male morphs of the bluehead wrasse T halassoma bifasciatum did not differ in size (i.e. cell volume). Initial ph ase (IP) males (high sperm competition) had a 60% higher sperm concentratio n in their milt than did terminal phase (TP) males (low sperm competition), which may reflect differences in how accurately the two male morphs need t o 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 (de termined in other studies), suggested that, on a daily basis, IP males inve st 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, indicat ed 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. (C) 1999 The Fisheries Society of the British Isles.