Cannibalism, post-settlement growth rate and size refuge in a recruitment-limited population of the shore crab Carcinus maenas.

Previous studies on intra-specific interactions among green shore crabs have been undertaken in very large populations, in which individuals concentrate at high densities in physically complex substrates. Under such conditions, cannibalism and interference competition often override initial density patterns delivered by larval supply. In this study, we focused on a Carcinus maenas population receiving a low supply of settlers. We reported crab abundance in habitats of different physical structure, and measured cannibalistic interactions, in different substrates, at a prey density expected only after a major recruitment event. Different predator and prey sizes were used to verify if juveniles froma critical range attain a size refugewhen coexistingwith subadults, which are still commonly found in the nursery habitat. Virtually no juvenile shore crabs were found on sandy habitats, and differences between sparsely and densely vegetated cover were only detected shortly after a settlement pulse, indicating that nursery habitats are under their carrying capacity most of the time. Even under extreme high densities, cannibalism on juveniles of 10 mm carapace width (CW) remained undetected, and predation on crabs half this sizewas only significant in lowand mediumZostera cover,when larger predators (20–25 mmCW)were included in experimental plots. The allometricmodel predicted the non-linear decrease of predation rate as a function of relative prey size, with a nearly asymptotic value of one prey consumed each 5 d by one predator in a square metre, when relative prey size attains 0.6. Such a relationship is expected for ‘cruising’ predators which rely on encounter rate. Growth estimates obtained using time series of the catch of distinct juvenile stages in artificial collectors indicated that crabs surviving 50 d after settlement have attained a size refuge from predation by larger conspecific

Shelf and estuarine transport mechanisms affecting the supply of competent larvae in a suite of brachyuran crabs with different life histories.

Supply of competent larvae to the benthic habitat is a major determinant of population dynamics in coastal and estuarine invertebrates with an indirect life cycle. Larval delivery may depend not only on physical transport mechanisms, but also on larval behavior and physiological progress to the competent stage. Yet, the combined analysis of such factors has seldom been attempted. We used time-series analyses to examine tide- and wind-driven mechanisms responsible for the supply of crab megalopae to an estuarine river under a major marine influence in SW Spain, and monitored the vertical distribution of upstream moving megalopae, their net flux and competent state. The species Panopeus africanus (estuarine), Brachynotus sexdentatus (euryhaline) and Nepinnotheres pinnotheres (coastal) comprised 80% of the whole sample, and responded in a similar way to tide and wind forcing. Tidal range was positively correlated to supply, with maxima 0 to 1 d after spring tides, suggesting selective tidal stream transport. Despite being extensively subjected to upwelling, downwind drift under the effect of westerlies, not Ekman transport, explained residual supply variation at our sampling area. Once in the estuary, net flux and competence state matched the expected trends. Net upstream flux increased from B. sexdentatus to P. africanus, favoring transport to a sheltered coastal habitat (N. pinnotheres), or to the upper estuary (P. africanus). Competence state was highest in N. pinnotheres, intermediate in B. sexdentatus and lowest in P. africanus, as expected if larvae respond to cues from adequate benthic habitat. P. africanus megalopae were found close to the bottom, not above, rendering slower upstream transport than anticipated