A Reproductive Histological Analysis of Rangia cuneata (Venerida: Mactridae): Effects of Abiotic Factors

Understanding the relative times of gametogenesis and spawning of gametes in bivalves provides crucial insight on how a species may be influenced by environmental factors, in addition to the potential impacts a species of bivalve has on the ecosystem as a whole. The focus of this study was on gametogenesis and times of spawning in Rangia cuneata (G. B. Sowerby I, 1831), an infaunal estuarine bivalve inhabiting Johnson Bayou, Pass Christian, MS, and how salinity gradients may influence spawning. Specimens were collected monthly in 2016 from three sites along an established salinity gradient (0–16 ppt) within Johnson Bayou. Using standard histological methods and a quantitative method to estimate both the abundance of gametes and spawning times, this study showed that over the course of one year the rate of gametogenesis and times of spawning differed among sites. Clams of both sexes in the site closest to the opening of the bayou (lower site) produced gametes and spawned earlier (June) than those collected from the middle and upper sites (October–November). Interestingly, two separate spawning events are likely to have occurred by clams collected from the lower and middle sites. There was a positive correlation between increasing water temperature and mean percent gonadal tissue in all sites with a significant correlation at the middle and upper sites. In contrast, no correlation was found between salinity levels and mean percent gonadal tissue in clams from the lower site, while a weak positive but non-significant correlation was seen with clams from the middle and upper sites. There was a weak negative correlation between salinity levels and mean percentage of ripe gametes from the lower and middle sites, but a weak positive correlation from the upper site. The results indicate that the same species can vary the amount of gametes and spawning times in the same estuarine system, presumably due to the effects of abiotic factors, namely water temperature and salinity.