Population Dynamics and Size-Scaling Interactions of Carcinus maenas and Littorina obtusata in Northern Maine
The European green crab, Carcinus maenas, invaded the mid-Atlantic coastline in the 1800s and has since expanded its range northwards into northern Maine and the Bay of Fundy (Scattergood 1952). C. maenas feeds on the native smooth periwinkle, Littorina obtusata. C. maenas and L. obtusata exhibit different growth strategies and absolute size of both species can affect the prey vulnerability. The critical size of vulnerability is the upper size at which a prey is vulnerable to a given predator size (Palmer 1990). The relationship of the critical size of L. obtusata vulnerability and C. maenas size indicates if there is isometric, positive-, or negative-allometric scaling between the two organisms (Palmer 1990). In this project, spatial and temporal variation in the scaling relationship and critical size of vulnerability of L. obtusata fed to C. maenas was tested in a series of laboratory trials. Both species were collected from two coves (Carrying Place Cove and Haycock Harbor) in close proximity to each other and at two time points (June and July). C. maenas from both sites were fed L. obtusata from their home sites as well as from the other site. Offering L. obtusata to C. maenas from the opposite site allowed me to test if the scaling advantage relied on nuanced differences in the predator-prey interaction. It was found that in June, for 3 of 4 treatment combinations L. obtusata had an advantage over C. maenas as both increased in size. In July, the scaling relationship for C. maenas eating L. obtusata from the foreign site remained similar. Both of the scaling relationships for C. maenas eating L. obtusata from their home sites experienced a shift from allometry favoring L. obtusata towards isometry, and in the case of Haycock Harbor organisms, reaching allometry favoring C. maenas. The change in the scaling relationship for C. maenas and L. obtusata from the same site from June to July indicated a greater advantage for C. maenas in the second month than at the beginning. Based on size frequency distributions for C. maenas, populations at both sites contained more large individuals in July, suggesting C. maenas individuals molted and increased in size proportionately more than L. obtusata could, as they grow linearly. Moreover, the upper limit of the critical size of vulnerability for the largest C. maenas specimen found during the population survey, or more simply put as the size refuge threshold, increased at both sites from June to July, and the portion of the L. obtusata population that was vulnerable varied over space and time. I found differences in size scaling interactions as well as in size frequency distributions and relative vulnerabilities over narrow temporal and spatial scales. The outcome of predator-prey interactions between C. maenas and L. obtusata in northern Maine relies on a subtle relationship between the two species.