Egg size in relation to fertilization dynamics in free-spawning tropical reef fishes

Abstract

In marine invertebrates that spawn by simply releasing their gametes into the water (free-spawning), fertilization success likely is often limited by low sperm concentrations, due to dispersion of mates and dispersal of gametes by water movements. Production of large, low density eggs might be advantageous when sperm concentrations consistently are low, because large target size might increase egg/sperm encounters, and more low than high density eggs could be produced per clutch. Although average fertilization success in the labrid Thalassoma bifasciatum is apprx 95% in both group spawns (in which multiple males compete for fertilizations by producing large quantities of sperm) and pair (mono-male) spawns, it is slightly lower in pair spawns, due to low level sperm limitation that arises because pair-spawning males release near the minimum number of sperm necessary for maximum fertilization. I examined whether variation in egg size and content in T. bifasciatum and other free-spawning fishes is related to variation in spawning mode, to assess whether compensatory production of large, low-density eggs might be contributing to high fertilization success in pair spawns. I found no difference between the volume or density of eggs of (1) pair- and group-spawning females of T. bifasciatum, or (2) pair and group-spawning congeneric species of labrids, scarids, and serranids, or (3) labrids and scarids with vigorous, rapid spawning movements (which could turbulently diffuse gamete clouds) and those with slow movements. Further, egg density does not decline with increasing egg volume among those fishes. Assuming that egg size can affect fertilization success, then sperm limitation seems unlikely to represent a significant problem for pair-spawning T. bifasciatum, probably because mates place their vents close together during gamete release. The situation regarding sperm limitation in other fishes, and effects of environmentally generated water turbulence on it, are less clear. Interspecific variation in the size and content of these fishes' eggs may relate to provisioning of offspring for different larval life-histories.