Does female shell disease status influence mating success in American lobsters?

Abstract

The increased prevalence of epizootic shell disease in American lobsters (Homarus americanus) over the past two decades has coincided with the collapse of the lobster stock in Southern New England. The degree to which shell disease can be implicated as a cause for the population decline remains unclear, but both direct increases in mortality rates and sublethal effects on reproductive success are possible factors. In this project we conducted both laboratory experiments and field studies to test the hypothesis that male lobsters either do not mate with diseased females, or they allocate less sperm, which could both lead to an overall decrease in reproductive output. In the laboratory, when a single pre-molt female was paired with a single male (no-choice mating trial), they mated successfully in 94% of the trials (60/64). However, healthy and shell disease-scarred males passed a significantly smaller volume of sperm to severely diseased females than to other females. When one male was in a mesocosm with two females of different disease states (mate choice trials, n = 13), they showed no preference for mating with one or the other female and there was no differential allocation of sperm. Instead, the first female to molt was the first to mate. Collections of females in the wild showed that, regardless of disease state, >95% of mature females had successfully mated, as indicated by the presence of a sperm plug in their seminal receptacle. Furthermore, analysis of their seminal receptacles revealed that there was no difference between healthy and diseased females in the amount of sperm they had received, although female size and disease severity may interact to influence male sperm delivery. Taken together, these data indicate that, while severe shell disease could impact American lobster reproductive output, both in terms of mating behavior and with regard to sperm allocation by males, shell disease is likely not the primary cause of low reproductive output by American lobsters in SNE.