Marine Reserves and Spiny Lobster: Effects on Male Size and Reproductive Success

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Changes in the abundance, size structure and sex ratio of populations can have large and consequential effects on mating systems, and thus on reproductive output. Yet, we know relatively little about these effects on the large, mobile marine species commonly exploited by humans. These species are generally difficult to observe and their populations are not generally amenable to manipulation at the requisite scale. There is the additional problem that in most cases the unexploited population was poorly, or never, studied and there are no unexploited populations with which to make contemporary contrasts. The advent of marine sanctuaries over the last 30 years and the recovery of exploited populations within some of them, offers opportunities to contrast the effects of changes in population structure on reproductive dynamics at appropriately large scales. Spiny lobsters (Palinuridae) are excellent models for such studies. Spiny lobsters are widely distributed and heavily exploited, and although mobile, their movements are limited enough that their populations respond to the protection afforded by many marine sanctuaries. Male spiny lobsters grow substantially larger than females, and although both male and female size is reduced by exploitation (fishing), male size declines more precipitously. The reproductive output (clutch size) of females is exponentially related to female size, so a shift toward smaller females in exploited areas has clear implications for reproductive potential. Less clear is the effect of reduced male size on lobster reproductive dynamics and this unfamiliarity extends to ecology as a whole. We have evidence from laboratory experiments on two species of spiny lobsters (Caribbean spiny lobster and New Zealand Rock Lobster) that the availability of large males has a large, direct effect on female fecundity. In both species, clutches fertilized by small males are significantly smaller than clutches fertilized by large males. There also is strong evidence for gamete limitation and it applies to two species in separate genera that dwell in very different habitats on opposite comers of the world. Male size effects on mating systems are known in only a few systems, primarily insects, but the population-level consequences are poorly understood. We propose a series of laboratory experiments to describe more fully the mating system of the Caribbean spiny lobster, with special reference to the effect of male size on reproductive success, and then to utilize field studies of exploited and unexploited lobster populations and simulation modeling to investigate the population-level consequences of reduced male size on reproductive dynamics.