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 compare the fertilization rates (FRs) attained by the brown surgeonfish Acanthurus 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 test an hypothesized fertilization advantage to mass spawning and, thereby, to mating aggregations. Specifically, we ask whether mass spawning enhances FRs beyond those achieved during group spawning. Results from artificial fertilization experiments demonstrated that egg viability greatly exceeds that of sperm, and suggested one means by which enhanced fertilization may be achieved. Namely, eggs not fertilized by a female's spawning partners may be fertilized by fresh sperm released in subsequent and nearby matings within the aggregation. Using egg samples collected in the field, we show that mass spawning makes no significant contribution to the already high FRs attained by group spawning (x̄ = 98.5%). We further demonstrate that FRs saturate well within the average lifespan of sperm, with over 60% of spawned eggs fertilized within 5 s of gamete release.