Seasonal Dimorphism in Gastropod Hatchling Size: What Can We Learn from Egg Mass Morphology and Deposition Site Conditions?

Egg size is one of the most important features of marine invertebrate reproduction because it provides insight into developmental patterns, offspring size, and adult investment per offspring. Intraspecific variation in egg size and the resulting hatchling size may depend on many factors including environmental and maternal conditions and geographic location. Therefore, both the season in which eggs are laid and the spatial heterogeneity of deposition sites may influence egg size and hatchling size. Natica chemnitzii deposits egg masses on sandy beaches along the coast of the Bay of Panama. During the wet (non-upwelling) season hatchling size is unimodal. However, monthly sampling in the same transect of beach shows that in the dry (upwelling) season hatchling size is bimodal with a peak at 136.5 microns, the typical hatchling size during the wet season, and an additional peak at 152.9 microns. To determine if this bimodal pattern in hatchling size is due to plasticity in one species or the concurrent seasonal reproduction of two species, we performed a more extensive survey of egg mass morphology and deposition site conditions at four sites around the Bay of Panama, during the dry season. Small hatchlings (\u3c 145 microns) that appear in both the wet and dry seasons are produced from small to medium sized masses (30 to 70 mm diameter) and are abundant at all four sites. Large hatchlings (\u3e 145 microns) which appear only in the dry season are produced from two clusters of egg mass sizes (20-35 mm diameter, and 60-100 mm diameter), and only commonly occur at one site. Additionally, sediment data suggests that large hatchlings are more abundant at deposition sites comprised of small sediment (63-250 microns) and small hatchlings are more abundant in large sediment (\u3e500 microns). The highest frequency of all large hatchlings occur at one site in the month of January, the beginning of the dry season. The two clusters of egg masses with large hatchlings suggest that there are two species occuring at this site during the dry season: one species that produces a bimodal hatching size, and another species that produces large hatchlings from small egg masses