Evolutionary morphology of the organ systems in squat lobsters and porcelain crabs (Crustacea: Decapoda: Anomala): An insight into carcinization

Abstract

Porcelain crabs (Porcellanidae) are one of three taxa within anomuran crustaceans (Anomala) which possess a crab-like body form. Curiously, these three lineages evolved this shape independently from true crabs (Brachyura) in the course of the evolutionary process termed carcinization. The entire pleon in porcelain crabs is flexed under the cephalothorax and the carapace is approximately as broad as long. Despite their crab-like habitus, porcelain crabs are phylogenetically nested within squat lobsters (Munidopsidae, Munididae, Galatheidae). With a pleon which is only partly flexed under the cephalothorax and a cephalothorax which is longer than it is broad, squat lobsters represent morphologically intermediate forms between lobster-like and crab-like body shapes. Carcinization has so far mostly been studied with respect to outer morphology; however, it is evident that internal anatomical features are influenced through this change of body shape too. In this paper, the situation in Galatheoidea is elucidated by adding more taxa to existing descriptions of the hemolymph vascular systems and associated structures and organs. Micro-computer tomography and 3D reconstruction provide new insights. Autapomorphic states of various internal anatomical characters are present in nearly all the studied species, also reflecting some degree of anatomical disparity found within Galatheoidea. The ventral vessel system of porcelain crabs differs distinctly from that of squat lobsters. The differences in question are coherent (i.e. structural dependent) with morphological transformations in the integument, such as the shortening of the sternal plastron, which evolved in the course of carcinization. Shifts in the gonads and the pleonal neuromeres are coherent with the loss of the caridoid escape reaction, which in turn is a consequence of carcinization. The arterial transformations, however, are minor compared to other instances of carcinization in anomuran crustaceans since the last common ancestor of squat lobsters and porcelain crabs was already "half carcinized".

Evolutionary morphology of the hemolymph vascular system of basal araneomorph spiders (Araneae: Araneomorphae)

“…changing origins of arteries, has also been reported in crustaceans (Keiler et al, 2015(Keiler et al, , 2013. Keiler et al (2013) correlate this variability with changes in the morphology of the endophragmal skeleton.…”

An “ancient” complexity? Evolutionary morphology of the circulatory system in Xiphosura

“…The identity of these structures as circulatory organs, however, is doubtful for two reasons: (i) The identification was based on exclusion, in comparison to structures thought to be the digestive tract and nervous system (Ma et al, 2012(Ma et al, , 2014 rather than on convincing structural details. (ii) The reconstruction of this structure as a whole does not resemble the vasculature in any studied Recent arthropod group (Klußmann-Fricke et al, 2012;Huckstorf et al, 2013;Keiler et al, 2013Keiler et al, , 2015Wirkner et al, 2013;this study). Nevertheless, complex circulatory systems in groups of stem lineage arthropods seem likely.…”

“…The occurrence of complex circulatory systems in a broad range of arthropod taxa (Klußmann-Fricke et al, 2012;Huckstorf et al, 2013;Keiler et al, 2013Keiler et al, , 2015Wirkner et al, 2013) suggests that this complexity might be plesiomorphic and a feature of the arthropod ground pattern. As yet there is no definite answer to this question, but recent work on the fossil Fuxianhuia protensa Hou, 1987 claims to have found a complex circulatory system in this Cambrian stem lineage arthropod (Ma et al, 2014).…”

The word is not enough: on morphemes, characters and ontological concepts

“…While identical statements are generalized on the species level, intraspecific variability is pointed out explicitly (e.g. Huckstorf et al., ; Göpel and Wirkner, ; Keiler et al., ). But because populations/species represent things , just as single organisms do, morphemes can also be generalized to the population/species level and the generalized morpheme, referring to a generalized semaphoront, still represents a thing (Fig.…”