Abstract
A fundamental question in biology is whether phenotypes can be predicted by ecological or genomic rules. At least five cases of convergent evolution of the crab-like body plan (with a wide and flattened shape, and a bent abdomen) are known in decapod crustaceans, and have, for over 140 years, been known as "carcinization." The repeated loss of this body plan has been identified as "decarcinization." In reviewing the field, we offer phylogenetic strategies to include poorly known groups, and direct evidence from fossils, that will resolve the history of crab evolution and the degree of phenotypic variation within crabs. Proposed ecological advantages of the crab body are summarized into a hypothesis of phenotypic integration suggesting correlated evolution of the carapace shape and abdomen. Our premise provides fertile ground for future studies of the genomic and developmental basis, and the predictability, of the crab-like body form.
Highlights
Biologists strive to explain the evolution of form, and the drivers of biodiversity across related groups
We focus on the success of the crab body plan within the economically and ecologically significant decapod crustaceans, as a system to address these fundamental questions
Some examples include: the coconut crab Birgus latro and other hermit crabs that have lost or reduced their domiciles; the porcelain crab Allopetrolisthes spinifrons (a ‘hypercarcinized’ anomuran with a sexually dimorphic pleon, strongly resembling brachyurans); the homolodromiid and homoloid brachyurans; the thumbnail crab Thia scutellata; and the gallforming cryptochirid crabs (Boyko 2002; Hiller et al 2010; Anker and Paulay 2013; Vehof et al 2016; Keiler et al 2017)
Summary
A fundamental question in biology is whether phenotypes can be predicted by ecological or genomic rules. For over 140 years, convergent evolution of the crab-like body plan (with a wide and flattened shape, and a bent abdomen) at least five times in decapod crustaceans has been known as ‘carcinization’. The repeated loss of this body plan has been identified as ‘decarcinization’. We offer phylogenetic strategies to include poorly known groups, and direct evidence from fossils, that will resolve the pattern of crab evolution and the degree of phenotypic variation within crabs. Proposed ecological advantages of the crab body are summarized into a hypothesis of phenotypic integration suggesting correlated evolution of the carapace shape and abdomen. Our premise provides fertile ground for future studies of the genomic and developmental basis, and the predictability, of the crab-like body form
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