DOES MASS SPAWNING ENHANCE FERTILIZATION IN CORAL-REEF FISH - A CASE-STUDY OF THE BROWN SURGEONFISH

'Mass spawning' refers to the simultaneous and apparently synchronous spawning of the majority of a mating aggregation. In this study we com pare the fertilization rates (FRs) attained by the brown surgeonfish A canthurus nigrofuscus, an externally fertilizing coral reef fish, when spawning in small groups (4 to 15 individuals) and as part of a mass spawning aggregation (500 to 2000 individuals). Our objective is to te st an hypothesized fertilization advantage to mass spawning and, there by, to mating aggregations. Specifically, we ask whether mass spawning enhances FRs beyond those achieved during group spawning. Results fro m artificial fertilization experiments demonstrated that egg viability greatly exceeds that of sperm, and suggested one means by which enhan ced fertilization may be achieved. Namely, eggs not fertilized by a fe male's spawning partners may be fertilized by fresh sperm released in subsequent and nearby matings within the aggregation. Using egg sample s collected in the field, we show that mass spawning makes no signific ant contribution to the already high FRs attained by group spawning (( x) over bar = 98.5%). We further demonstrate that FRs saturate well wi thin the average life-span of sperm, with over 60% of spawned eggs fer tilized within 5 s of gamete release.