Contributions to Zoology, 85 (4) – 2016Jonas Keiler; Stefan Richter; Christian S Wirkner: Revealing their innermost secrets: an evolutionary perspective on the disparity of the organ systems in anomuran crabs (Crustacea: Decapoda: Anomura)

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Morphological transformations in the pleon – pleonal muscle reduction in Lomis is a result of carcinization

Besides the shape of the ventral vessel system, Lomis possesses several other internal anatomical features whose morphology obviously changed in coherence with external morphological transformations in the course of carcinization. The most obvious of these changes are associated with the pleon. While the ground pattern of Australopoda exhibits a pleon similar to that in Aegla or galatheoid squat lobsters, i.e. incompletely bent and featuring strong muscular bundles (Fig. 3D), Lomis evolved a flattened, more strongly bent pleon whose muscles are largely reduced (Fig. 3F), implying that it sees less active movement than in the ground pattern of Australopoda. The pleon in Aegla still permits a caridoid escape reaction (‘tail flipping’; our own observations; Martin and Abele, 1988) similar to that found in macruran decapods. In Lomis, however, the escape reaction has been lost completely in accordance with the species’ lifestyle hidden under stones in the intertidal zone (our own observations). A similar evolutionary scenario has been described for porcelain crabs (Keiler et al., 2015a), though the pleonal muscles in porcelain crabs are less drastically reduced than in Lomis and some porcelain crab species seem to have retained the ability to rapidly flip their pleon (Števčić, 1971; Hiller et al., 2010). The reduction of the pleonal muscles in Lomis in turn provided more space for the ovaries, a similar phenomenon to that which occurred in the evolution of porcelain crabs (Keiler et al., 2015a).