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Aquatic versus terrestrial crab skeletal support: morphology, mechanics, molting and scaling

Crab image by Facundo Nehuén López under CC licensing
TitleAquatic versus terrestrial crab skeletal support: morphology, mechanics, molting and scaling
Publication TypeJournal Article
Year of Publication2018
AuthorsTaylor J.RA
JournalJournal of Experimental Biology
Date Published2018/11
Type of ArticleArticle
ISBN Number0022-0949
Accession NumberWOS:000449824800015
Keywordsblue-crab; Callinectes sapidus; callinectes-sapidus; Crustacea; differential; earthworm lumbricus-terrestris; Exoskeleton; Gecarcinus lateralis; ghost-crab; growth; hydrostatic skeleton; hydrostatic skeletons; jumping performance; land; Life Sciences & Biomedicine - Other Topics; Mechanical properties; ocypode-ceratophthalma; relative growth

The transition from aquatic to terrestrial environments places significant mechanical challenges on skeletal support systems. Crabs have made this transition multiple times and are the largest arthropods to inhabit both environments. Furthermore, they alternate between rigid and hydrostatic skeletons, making them an interesting system to examine mechanical adaptations in skeletal support systems. I hypothesized that terrestrial crabs have modified morphology to enhance mechanical stiffness and that rigid and hydrostatic skeletons scale differently from each other, with stronger allometric relationships on land. Using the aquatic blue crab, Callinectes sapidus, and the terrestrial blackback land crab, Gecarcinus lateralis, I measured and compared body mass, merus morphology (dimensions, cuticle thickness and the second moment of area I) and mechanics (flexural stiffness ElI, elastic modulus E, critical stress and hydrostatic pressure) of rigid and hydrostatic stage crabs encompassing a range of sizes (C. sapidus: 1.5-133 g, N <= 24; G. lateralis: 22-70 g, N <= 15). The results revealed that rigid G. lateralis has similar morphology (limb length to diameter LID and cuticle thickness to limb diameter TID ratio) to C. sapidus, and the mechanics and most scaling relationships are the same. Hydrostatic land crabs differ from aquatic crabs by having different morphology (thinner cuticle), mechanics (greater intemal pressures) and scaling relationship (cuticle thickness). These results suggest that the rigid crab body plan is inherently overbuilt and sufficient to deal with the greater gravitational loading that occurs on land, while mechanical adaptations are important for hydrostatically supported crabs. Compared with other arthropods and hydrostatic animals, crabs possess distinct strategies for adapting mechanically to life on land.

Short TitleJ. Exp. Biol.

Crabs have many adaptations to life on land that reflect the physical, chemical and biological challenges of the terrestrial environment (Bliss and Mantel, 1968), yet they do not appear to require biomechanical adaptations of exoskeletal support. Comparison of the morphology and mechanics of walking leg meropodites from a highly aquatic species and a highly terrestrial species of crabs revealed no significant differences between them. In contrast, the hydrostatic skeleton used during molting differs between the two species, with higher internal pressure, higher stress and a lower cuticle thickness scaling exponent for land crabs. In the hydrostatic phase, both species experience significant decreases in locomotor performance as they grow to larger sizes, with scale effects being more dramatic in the terrestrial crabs.

The alternating use of two different skeletal support mechanisms presents additional complications for understanding how body size relates to skeletal morphology and function. Rigid and hydrostatic skeletons function by different mechanisms and are affected by scale in different ways. Thus, the two skeletons influence crab growth independently. In order to grow to a larger size, crabs must first successfully undergo ecdysis. If the hydrostatic animal cannot support its own weight and shape, severe deformations may occur that will compromise the hardened skeleton. Thus, the hydrostatic stage may present the more limiting stage, as suggested by Kennedy (1927) for why insects are limited to small size. Whether or not this is the case, the hydrostatic skeleton used by crabs during molting should be recognized as a biomechanical feature of significant importance in the growth to maximum size of crabs and their successful transition to land.

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