One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea)

Archaeochiapasidae n. fam., a new early Cenomanian brachyuran family from Chiapas, Mexico, new hypothesis on Lecythocaridae Schweitzer & Feldmann, 2009, and phylogenetic implications (Crustacea, Decapoda, Brachyura, Eubrachyura)

“…We agree with Keiler et al (2017) that carcinisation is an evolutionary process as a whole, but the actual objects of transformation are individual characters (sensu Hennig 1966). It remains however logical to consider the widening of the thoracic sternum is a part of the pro-cess during which the gonopores, first the female ones and then the male ones, abandoned their appendicular location (likewise in other decapods and podotremes) as a consequence a brachyuran carcinisation.…”

A phylogenomic framework, evolutionary timeline and genomic resources for comparative studies of decapod crustaceans

“…Several synapomorphies have been proposed, such as a short asymmetric flagella on the antennule, bent exopods on the maxillipeds and fusion of ganglia [6]. Carcinization, the overall crab-like body plan including a flattened carapace with lateral margins, fused sternites and strongly bent pleon [56] has been suggested as a developmentally co-opted trait of Meiura with a 'tendency' to evolve repeatedly [57]. Our topology suggests at least three separately carcinized clades in Anomura, and one in Brachyura; however, increased taxon sampling will complicate character distribution [13,56] and introduce secondary losses [58,59].…”

How to Become a Crab: Phenotypic Constraints on a Recurring Body Plan

“…Basic anatomy illustrated in Figure 1. Specific features common to most carcinized groups (Keiler et al 2017) include:…”

“…-A flattened and widened carapace (at least slightly wider than long), often with lateral margins (raised edges of the dorsal carapace) -Sternites (sclerotized ventral segments) fused to some degree into a single wide plate called the thoracic sternum or plastron -A flattened and bent 'abdomen' or pleon, hidden from dorsal view, partially or completely covering the thoracic sternum -Loss or significant reduction of the uropods (appendages of the sixth pleonal somite, usually forming a tail fan in other decapods) -Fusion of pleonal ganglia, reduction of pleonal muscles (documented for representative anomurans and two species of eubrachyuran: (Keiler et al 2017;Castejón et al 2018;Spitzner et al 2018) Decarcinization, or the secondary loss of the crab-like body form, has occurred multiple times in both Brachyura and Anomura. This form is more cylindrical, but has evolved from a wide oval shaped ancestor (Scholtz 2014;Luque et al 2019b), as opposed to the ancestrally uncarcinized forms (that never evolved a crab-like form in the time since their common ancestor with mud shrimp).…”

“…-An elongated, narrow carapace -A pleon that is not strongly flattened and/or bent, and is sometimes visible in dorsal view or even elongated -Legs with modified distal segments There are varying degrees of carcinization and decarcinization (Scholtz 2014;Luque et al 2019b), so not all species can be easily labeled as 'carcinized', 'uncarcinized', or 'decarcinized'. Some examples include: the coconut crab Birgus latro (a semi-carcinized anomuran with a bent pleon but incompletely fused sternites and no lateral margins) 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 (which have characteristically carcinized pleons but lack wide carapaces and lateral margins; Figure 2B); the thumbnail crab Thia scutellata (a somewhat decarcinized eubrachyuran); and the gallforming cryptochirid crabs (decarcinized brachyurans, but with the female pleon modified as a large brood pouch) (Boyko 2002;Hiller et al 2010;Anker and Paulay 2013;Vehof et al 2016;Keiler et al 2017).…”