The reniform body: An integrative lateral protocerebral neuropil complex of Eumalacostraca identified in Stomatopoda and Brachyura.

Mantis shrimps (Stomatopoda) possess in common with other crustaceans, and with Hexapoda, specific neuroanatomical attributes of the protocerebrum, the most anterior part of the arthropod brain. These attributes include assemblages of interconnected centers called the central body complex and in the lateral protocerebra, situated in the eyestalks, paired mushroom bodies. The phenotypic homologues of these centers across Panarthropoda support the view that ancestral integrative circuits crucial to action selection and memory have persisted since the early Cambrian or late Ediacaran. However, the discovery of another prominent integrative neuropil in the stomatopod lateral protocerebrum raises the question whether it is unique to Stomatopoda or at least most developed in this lineage, which may have originated in the upper Ordovician or early Devonian. Here, we describe the neuroanatomical structure of this center, called the reniform body. Using confocal microscopy and classical silver staining, we demonstrate that the reniform body receives inputs from multiple sources, including the optic lobe's lobula. Although the mushroom body also receives projections from the lobula, it is entirely distinct from the reniform body, albeit connected to it by discrete tracts. We discuss the implications of their coexistence in Stomatopoda, the occurrence of the reniform body in another eumalacostracan lineage and what this may mean for our understanding of brain functionality in Pancrustacea.

An insect-like mushroom body in a crustacean brain.

Mushroom bodies are the iconic learning and memory centers of insects. No previously described crustacean possesses a mushroom body as defined by strict morphological criteria although crustacean centers called hemiellipsoid bodies, which serve functions in sensory integration, have been viewed as evolutionarily convergent with mushroom bodies. Here, using key identifiers to characterize neural arrangements, we demonstrate insect-like mushroom bodies in stomatopod crustaceans (mantis shrimps). More than any other crustacean taxon, mantis shrimps display sophisticated behaviors relating to predation, spatial memory, and visual recognition comparable to those of insects. However, neuroanatomy-based cladistics suggesting close phylogenetic proximity of insects and stomatopod crustaceans conflicts with genomic evidence showing hexapods closely related to simple crustaceans called remipedes. We discuss whether corresponding anatomical phenotypes described here reflect the cerebral morphology of a common ancestor of Pancrustacea or an extraordinary example of convergent evolution.