Mushroom bodies and reniform bodies coexisting in crabs cannot both be homologs of the insect mushroom body

Abstract

In one species of shore crab (Brachyura, Varunidae), a center that supports long‐term visual habituation and that matches the reniform body's morphology has been claimed as a homolog of the insect mushroom body despite lacking traits that define it as such. The discovery in a related species of shore crab of a mushroom body possessing those defining traits renders that interpretation unsound. Two phenotypically distinct, coexisting centers cannot both be homologs of the insect mushroom body. The present commentary outlines the history of research leading to misidentification of the reniform body as a mushroom body. One conclusion is that if both centers support learning and memory, this would be viewed as a novel and fascinating attribute of the pancrustacean brain.

Contextual memory reactivation modulates Ca2+-activity network state in a mushroom body-like center of the crab N. granulata

“…The center initially proposed by us as the structure that might resemble the reniform body in N. granulata 18 , was then described as a region of a crab’s mushroom body homologue that presents unique transformations from the ancestral ground pattern 27 . In other words, the MB-ls and the “reniform” body centers described in N. granulata 17 , 18 were described in other true crabs using swapped identities 27 , 29 , 74 . Certainly, the characterization of the complex net of neuropils in the lateral protocerebrum, where the mushroom bodies are situated, of Brachyura continues to be a challenge 75 .…”

“…Our recent new neuroanatomical and immunohistochemical data 18 show that the MB-ls from N. granulata (and possibly all true crabs) exhibit an ensemble of several traits that comprise many characteristics proposed for centers of integration like mushroom bodies 4 , 13 , 76 . Added to the previous functional evidence 17 they show that the MB-ls (a) is multimodal, (b) the output regions exhibit stimulus-specific activity 18 , and that (c) neuronal activity reflect context-associative components of memory processes in the short term 17 , we considered that the debate regarding the identity of these true crab's centers is open (discussed in 18 , 29 ). The present study adds new functional evidence of the expected MBs' cognitive functions.…”

“…Recent functional and morphological evidence support that the N. granulata high-order brain center (hemiellipsoid body) 19 , 20 , 23 – 26 can be interpreted as an equivalent structure to the insect MBs, i.e., a mushroom body-like structure (MB-ls) 17 . A counterview was introduced by other authors 27 – 29 , who have recently proposed that true crabs possess novel, inverted, and highly modified MBs with an expansive system of gyri 27 . The above-mentioned neurophysiological evidence 17 , showing that the associative context attribute of an aversive memory trace was present in the crabs’ MB-ls, is restricted to the short-term.…”

The velvet worm brain unveils homologies and evolutionary novelties across panarthropods

“…For example, the calyx is lacking in myriapods [147], paleopteran and some neopteran insects [118], and partially also in stomatopod crustaceans [152]. Moreover, a structure called the hemiellipsoid body-a potential homologue of the mushroom body calyces (Bellonci 1982 apud [153,154]) or the mushroom body itself [155,156]-has so far been identified in only three crustacean clades: Malacostraca [152,[157][158][159][160], and Remipedia [161], which both lack the mushroom body pedunculi and lobes, as well as Cephalocarida [162]. This suggests either an independent origin in these clades or an evolutionary loss in other crustaceans.…”

Immunocytochemical Localization of Enzymes Involved in Dopamine, Serotonin, and Acetylcholine Synthesis in the Optic Neuropils and Neuroendocrine System of Eyestalks of Paralithodes camtschaticus

“…As observed in insects, optic neuropils in crustaceans are connected via two chiasmata: one connects the lamina to the medulla, and the other connects the medulla to the lobula. The optic lobe also comprises other regions that form the lateral protocerebrum (Harzsch and Hansson, 2008;Maza et al, 2021;Strausfeld, 2021). To date, the neural organization and cellular morphologies of the crustacean optic neuropils have been studied in few taxa, including entomostracans and malacostracans, in the mysid Neomysis integer (Strausfeld and Nässel, 1981), euphausiacean Meganyctiphanes norvegica (Strausfeld and Nässel, 1981), isopod Ligia occidentalis (Sinakevitch et al, 2003), stomatopods (Strausfeld and Nässel, 1981;Thoen et al, 2017), and various decapods (Elofsson and Dahl, 1970;Nässel, 1977;Stowe et al, 1977;Sinakevitch et al, 2003;Sztarker et al, 2005Sztarker et al, , 2009Sztarker and Tomsic, 2014).…”