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)

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Summary

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

Introduction
Carcinization has been gained and lost throughout decapod evolution
Progress in resolving crab relationships
Novel body plans appear to have evolved in singleton species
It is unclear whether the earliest crabs looked like crabs
The ecological advantages of becoming a crab are complex
The crab body plan may aid in protection and locomotion
Escalation of predation cannot explain early crab success
The crab body plan as a case of phenotypic integration
Towards predicting the evolution of crabs
Conclusions
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