METHODOLOGICAL ANALYSIS OF FERTILIZATION RATE IN THE BLUEHEAD WRASSE THALASSOMA-BIFASCIATUM - PAIR VERSUS GROUP SPAWNS

Accurate measurement of fertilization rate (FR) is essential for repro ductive studies of pelagically spawning marine organisms. We compared the reliability of 3 methods of collecting eggs for estimates of FR in the coral reef fish Thalassoma bifasciatum: collecting in hand-held n ets 30 s after spawning or in plastic bags 3 to 4 s ('early bag') or 3 0 s ('late bag') after spawning. FRs were significantly and consistent ly lower in eggs collected by net than in eggs collected in early or l ate bags. FRs were consistently highest in late bags. In field experim ents, netted samples contained higher proportions of negatively buoyan t, unfertilized eggs and the negative effects of the net declined with increasing time from initial exposure of eggs to seawater and sperm. Thus, the net appeared to interfere mechanically with the fertilizatio n process. FRs were not higher in smaller than in larger bags, as woul d have been expected if bags artificially prolonged high average sperm concentration around eggs. The average concentration of sperm in a ba g containing twice the largest number of sperm ever collected in any s pawn was less than the minimum sperm concentration needed to initiate fertilization. Thus, the difference in FR between netted and bagged sa mples was not the result of prolonged maintenance of an artificially h igh average sperm concentration in the bag. Using bag collection techn iques, we compared FRs for pair and group spawns. Mean FRs ranged from 86.6 to 99.5%, with analyzable variance. Contrary to the results of a previous study using netted samples, group spawns had significantly h igher FRs than pair spawns using early bag samples. Female mating tact ics ought to be influenced evolutionarily by this difference between p air and group spawns.