Reproductive traits and relative gonad expenditure of the sexes of the free spawning Chiton articulatus (Mollusca: Polyplacophora)

Reproductive studies of an intertidal free-spawning population of Chiton articulatus (Mollusca: Polyplacophora) from Puerto Angel, Oaxaca, Mexico were undertaken during 2011. We used gonad histology and gonadal indices to assess the relative gonad expenditure of the sexes (RGES) and other reproductive traits, accounting for individual and seasonal variation within this population. At this location, C. articulatus is gonochoric, without sexual dimorphism, except internally by gonad colour (testis is ‘salmon’ coloured and ovary olive green). Annual and monthly sex ratios (m/f) do not differ significantly from 1:1. Highest population-level gonadosomatic index (GSI) corresponded to maximum (peak) ripe stage (i.e. maximum gonad investment), with a first peak in May with a high value (8.4 ± 0.5) and a second peak during August-September with a lower value (4.7 ± 0.3). GSI fluctuated throughout year implying that gonad expenditure may be seasonally constrained, but with overall synchrony between sexes of ripe and spawning stages. July to December was the main reproductive season with some facultative spawning occurring off-season. Ripe and spawning RGES did not differ between sexes, suggesting that either sperm competition is intense and/or that sperm limitation is high. Early spawning individuals may quickly replenish their gametes for a second phase of gamete release later.