Agonistic Behavior and Costs of Aggression in Decapod Crustaceans

Animals that engage in aggressive behavior necessarily incur some costs (e.g., energy expenditure) and risk much greater costs (e.g., injury), both of which are presumably weighed against the potential benefits (e.g., refuge, food, or mates). Such costs and benefits directly impact their distribution, life history, and fitness, so an understanding of the role aggression plays is crucial to a complete understanding of a species’ ecological niche. The research presented here evaluates the degree to which aggression has ecological and life history consequences, using several species of co-occurring crabs as models in a combination of laboratory and field experiments. Specifically, we investigated: 1) how conspecific aggressive interactions compare across several ecologically overlapping crab species, 2) the impact exposure to aggression during the juvenile life phase has on the growth, maturity, injury, and survival rates of decapod crustaceans, 3) whether both intra- and interspecific aggression and competition among co-occurring crab species alter habitat selection and play a role in the success of invasive species, and 4) if injury patterns, spatial distribution, and habitat selection across the intertidal zone can be correlated to aggression. In Chapter One, we present a comparative analysis of the degree to which different species of crabs engage in aggressive behavior. We focused on six species of co-occurring New England crabs, including the invasive Hemigrapsus sanguineus and Carcinus maenas, and the native Cancer irroratus, Dyspanopeus sayi, Libinia emarginata, and Ovalipes ocellatus. We found significant differences in the occurrence of agonistic behavior among species: O. ocellatus engaged in prolonged fights, and L. emarginata showed very little aggressive behavior, while the other four species were intermediate in the instances of, and the amount of time engaged in, aggressive behavior. Our findings shed light on the ecological implications of agonistic behavior in the context of varying life history and ecological strategies, and set a useful benchmark for understanding the role of aggression in the following chapters. We then studied the costs of aggressive behavior in Chapter Two, focusing particularly on the impact agonism has on growth, maturity, injury, and survival. We selected juvenile Hemigrapsus sanguineus, Carcinus maenas, and Cancer irroratus crabs, three species that directly co-occur and display comparable levels of aggression. Treatments were comprised of conspecific individuals paired together for different lengths of time each day, to simulate encounter rates from post-settlement to the onset of maturity. All three species experienced substantial decreases in survival, significant limb loss, and a decrease in final carapace size. Only time to maturity was not substantially affected, though this may have been partly due to the dramatic mortality rates leading to relatively few crabs surviving to adulthood. Hemigrapsus sanguineus, one of the species used in all four experiments presented in this dissertation, is a highly successful invasive species whose success stems partly from its ability to exclude established crab species from preferred rocky and cobble intertidal habitat. In Chapter Three, we assessed preference and competition for habitat types (cobble vs. sand) for H. sanguineus and two competitor species; the previously established invasive green crab, Carcinus maenas, and the native rock crab, Cancer irroratus, in New England. We established different groupings of similarly sized heterospecifics and conspecifics from each species in order to test intra- and inter-specific competition at different densities. While all three species preferred the shelter of cobble substrate in individual trials, H. sanguineus invariably displaced the other species, and retained the cobble substrate even when exposed to superior heterospecific numbers. Additionally, multiple H. sanguineus would cohabitate in cobble, whereas C. maenas and C. irroratus individuals each excluded conspecifics from cobble. These patterns illustrate a clear mechanism for overcoming invasion resistance and the exclusion of other intertidal crab species. In addition, despite the aggression seen in Chapter One, this study demonstrates that much of this species’ invasion success may stem from its relative lack of intraspecific aggression, when costs and benefits are both present. One outcome of aggression is often injury, and these intertidal crab species exhibit high occurrences of both claw and leg loss. However, while it is difficult to ascertain the cause of such injuries, the impact on the distribution and demography of injured individuals across habitat types is relatively easy to assess. We found little evidence for differences in the distribution of injured H. sanguineus (the sole common species found in surveys) across intertidal zones, with half of all crabs exhibiting loss of at least one limb. Injury also correlated with age and reproductive status which, in turn, did effect distribution. The results of this series of studies underscore the impact of aggression (and related competition) on shaping many aspects of a species’ ecology. Though the outcome of aggressive behavior is not always easy to ascertain, the overall results of our research serve to further illuminate our understanding of community ecology, invasion biology, life history and fitness, and ethology in general