Variation in safety factors of claws within and among six species of Cancer crabs (Decapoda: Brachyura)

Abstract

To better understand how safety factors of biological structures evolve, we examined the frequency of claw failure, and the intra-and interspecific patterns of variation in maximum biting force and breaking strength in the claws of six species of Cancer (Linnaeus) crabs that live in sympatry along the coast of the northeastern Pacific: C. antennarius, C. branneri, C. gracilis, C. magister, C. oregonensis and C. productus. Although the breakage frequencies in natural populations were similar among species (≈ 6%), they were higher than predicted based on failure probabilities calculated from laboratory measurements of biting force and breaking strength for healthy pristine claws. The incidence of claw damage was correlated with the degree of wear, suggesting that claws later in the intermolt interval were more likely to fail. Within species, safety factors increased from 3.1 to 4.6 with increasing instar number due primarily to a decline in muscle stress (force per unit area of apodeme). Surprisingly, the lower maximum muscle stress generated by later instars appeared to be due to behavioral restraint, since it was not accompanied by relatively lower muscle mass. In addition, among individuals of the same claw size, lower breaking forces were correlated with lower maximum biting force, and both were correlated with lighter cuticle and closer muscle mass, suggesting a coupling that maintains a more stable safety factor over the moult cycle. In some species, size-adjusted maximum biting forces were higher for males than females, but this paralleled differences in breaking strength, so safety factors did not differ between the sexes. Among the six Cancer species, one exhibited an unusually high safety factor (C. oregonensis, 7.4) and another an unusually low one (C. magister, 2.6). The remaining four species were similar to each other and exhibited an intermediate safety factor (3.6). From a phylogenetic perspective, the species with more extreme safety factors appeared to be derived from a common ancestor with an intermediate safety factor. From an ecological perspective, species more closely associated with rocky substrata, and presumably a higher incidence of hard-shelled prey, exhibited higher safety factors. But safety factors were also correlated with relative claw size, and sexual dimorphism in claw size. Although we cannot say whether habitat, diet or sexual selection are primarily responsible for the differences in safety factors observed among species, the cost of producing a relatively larger claw seems an unlikely explanation because safety factors did not differ between males and females in any of the sexually dimorphic species.

Functional mechanics of beetle mandibles: Honest signaling in a sexually selected system

“…This suggests that individuals with even the lowest safety factors can still withstand approximately twice the force they can produce or that they are likely to experience in combat with another size-matched male. These safety factors are comparable to rhinoceros beetle horns (3.5-10.3) (McCullough, 2014), and crab claws (2.6-7.4) (Taylor et al, 2000). When breaking the mandibles in our experiments, force was applied so that the mandibles experienced uniaxial cantilever bending.…”

Maximum force production: why are crabs so strong?

“…Six north-eastern Paci¢c Cancer species (Cancer antennarius, Cancer branneri, Cancer gracilis, Cancer magister, Cancer oregonensis and Cancer productus) were collected from various shallow-water sites in the vicinity of the Bam¢eld Marine Station, Bam¢eld, British Columbia, Canada. In order to measure their claw biting forces, several mid-intermoult crabs of each species were selected based on their estimated claw wear (claw index 2 as described in Taylor et al (2000)). The crabs were housed individually in plastic mesh containers (200 mm £140 mm £ 90 mm) which were submerged in large ¢breglass aquaria supplied with running seawater (salinity 32% at 10^12 8C).…”

Is there a better chela to use for geometric morphometric differentiation in brachyuran crabs? A case study usingPachygrapsus marmoratusandCarcinus maenas

“…To test these shape differences, several body parts can be analysed, such as the carapace (Clark et al , 2001; Rufino et al , 2006a), chelipeds (Rosenberg, 1997) or mouthparts (Skilleter & Anderson, 1986). In fact, chelate appendages are amongst the most conspicuous and characteristic anatomical features of decapod crustaceans, and several aspects have already been studied: biomechanics of chelipeds in some decapod crustaceans, such as cancrid crabs (Taylor et al , 2000) and portunid, grapsid, xanthid and ocypodid crabs (Brown et al , 1979; Lee, 1995); development of asymmetry and claw shape variation in several species of ocypodids (Ahmed, 1978; Weissburg, 1991; Levinton et al , 1995; Rosenberg, 2002) and heterochely and handedness in portunids (Abby-Kalio & Warner, 1989; Smith, 2004) and ocypodids (Barnwell, 1982); feeding behaviour and manipulation of diet (Smith & Palmer, 1994; Smith, 2004); and agonistic interactions (Keiser et al , 1990; Lee, 1995).…”