Biogeographic differences in claw size and performance in an introduced crab predator Carcinus maenas

Abstract

Introduced predators must forage effectively to persist in new areas, and effective resource use is correlated with a predator's trophic (feeding) structures. The extent to which trophic morphology responds to spatial variation in prey resistance, and the time scale of that response, however, are not well understood. The introduced European green crab Carcinus maenas, which has expanded its range in the northwest Atlantic Ocean over the last century and encountered a latitudinal cline in prey armor, offers an ideal opportunity to test for such post-invasion changes. In rocky intertidal habitats in the Gulf of Maine, southern populations of the snail Littorina obtusata are better defended than northern populations. To test for correlated differences in claw size and prey defense, green crabs and common molluscan prey were collected at southern and northern sites in the Gulf of Maine. C. maenas are heterochelous, and analyses revealed that crusher claws of crabs collected in the south were significantly larger than those of crabs from the north after adjusting for carapace width. The size of the cutter claw, however, did not differ between regions. Shell weights for similarsized snails differed among sites for 3 of the 4 species examined, but did not differ by region. To test crushing performance, southern and northern crabs were presented with increasing sizes of southern or northern L. obtusata in the laboratory. For similar-sized individuals, southern crabs were able to crush significantly larger snails than northern crabs regardless of the geographic origin of the snails. Thus, patterns in claw size and performance strongly suggest trophic responses to geographic differences in prey armor. Rapid changes in trophic morphology could facilitate the spread of this invasive species and initiate an ecological arms race if both predator and prey respond ecophenotypically to each other.

Invasive species cause large-scale loss of native California oyster habitat by disrupting trophic cascades

“…And for density-mediated cascades, it is reasonable that the top predator's evolutionary history with common prey (and its own predators) selected a dietary preference, hunting mode, and/or morphological features that facilitate hunting and eYciently consuming a widely available intermediate consumer (Vermeij 1982b). Although the behavior underlying trophic cascades could be learned by individual top predators and intermediate consumers, each animal's capacity for learning would be inXuenced by the presence/absence of innate morphological or behavioral traits that would increase Wtness and ultimately be selected for within a population over time (Berger et al 2001;Cox and Lima 2006;Smith 2004). We therefore suspect that a common feature of functioning cascades is that the top predator and intermediate consumer species share an evolutionary history and/or have historical exposure with similar types of predator and prey species so that their predator and anti-predator strategies are well matched.…”

Temperature constraints on phenotypic plasticity explain biogeographic patterns in predator trophic morphology

“…Juanes & Hartwick 1990, Behrens Yamada & Boulding 1998. Second, the use-induced changes generated in our experiment were comparable in magnitude to differences in relative crusher claw height (approximately 9%) between southern and northern populations of Carcinus maenas in the Gulf of Maine (Smith 2004), and these geographic differences affect foraging performance. By offering crabs Littorina obtusata of increasing size, Smith (2004) demonstrated that the larger-clawed southern crabs crushed both thick-and thin-shelled snails up to a greater size than did the smaller-clawed northern crabs of similar carapace width (see also Rochette et al 2007).…”

“…thicker shelled, more resistant snails are found in the south rather than in the north (Trussell 2000). In contrast, cutter claws showed no geographic differences in relative size (Smith 2004). The correlation between crusher claw size and prey armor suggests an adaptive trophic response by the predator to its prey, but the causal mechanism (diet-induced plasticity, rapid selection on claw size) has not been established.…”

“…We used claw height as a proxy of claw size because it is an accurate predictor of claw strength in Carcinus maenas (Lee 1993, Smith 2004. To assess change in claw height at the molt, we conducted a series of 2-way ANCOVA models with the difference in claw height between successive exoskeletons as the response variable and temperature and diet as fixed factors.…”

“…In rocky intertidal habitats in the Gulf of Maine, sizes of C. maenas crusher claws have diverged in the last century such that green crabs in the southern Gulf of Maine have relatively larger crusher claws than those in the north (Smith 2004). This pattern mirrors a cline in shell thickness and breakage resistance in a native species of snail prey, Littorina obtusata (L.); i.e.…”