EGG SIZE EVOLUTION IN RESPONSE TO HISTORICAL REGIME SHIFTS IN THE GULF OF MEXICO

Egg size is a critical life history trait which affects fecundity, juvenile survivorship and population growth. Paleontological studies have documented changes in egg size over millions of years, but relatively little is known about egg size evolution over much shorter timescales. Life history theory predicts that when nutrients are limited, individuals that produce fewer, larger eggs will leave behind more offspring, whereas under higher nutrient conditions, smaller offspring are as likely to survive and consequently individuals that produce more numerous, small eggs will leave behind more offspring. The size of the prodissoconch (PI), the earliest larval shell of developing marine bivalve mollusks, is highly correlated with egg size and can be preserved on adult shells. We hypothesize that enhanced nutrient availability and increased primary production in coastal waters of the Gulf of Mexico as a result of human activities have led to a decrease in the size of bivalve eggs over time. We test this hypothesis by comparing the PI sizes of live and dead individuals of the bivalve species Nuculana acuta that we collected from surficial sediments on the continental shelf (-20m depth) in the northern Gulf of Mexico (Dauphin Island, Alabama). The live-dead comparison reveals that present-day mean PI size (134.6 microns) is significantly smaller than the mean PI size of the death assemblage (137.5 microns) (two sample t-test, p = 0.015), a pattern that is consistent with our hypothesis. The frequency distributions of live and dead PI sizes are also significantly different (K-S test, p = 0.022). We use these preliminary results in conjunction with published data on the ages of dead shells in surficial shelf sediments to model the evolution of egg size under varying nutrient regimes. Expanding the scope of our analysis in the future, we will consider additional species that exhibit a diversity of life history strategies (e.g., planktotrophs as well as lecithotrophs), and additional localities distributed along a geographic gradient in net primary production in the Gulf of Mexico.