Comparative Neuroanatomy of Pediveliger Larvae of Various Bivalves from the Sea of Japan

SIMPLE SUMMARY: Here, we described and compared the morphology of the nervous system of the late larval stage of bivalve mollusks, pediveligers, using confocal microscopy. Pediveligers from four ecological groups were studied: burrowing (Callista brevisiphonata, Mactromeris polynyma), cementing (Pododesmus macrochisma, Crassostrea gigas), attaching to byssus (Kellia japonica, Crenomytilus grayanus) and mobile forms (Mizuhopecten yessoensis, Azumapecten farreri). We tried to identify possible similarities and/or differences in a number of structures of the nervous system in pediveligers of different species and correlate them with the lifestyle of a particular ecological group. We have not found a direct connection between the presence or absence of a number of peripheral neurostructures with an ecological group, but we have identified a possible evolutionary connection. A stomatogastric neuron communicating with the apical organ was found in species from the Heterodonta clade. The enteric nervous system and unrelated to the apical organ neurons present in Pectinida were also detected. ABSTRACT: Here, we describe the nervous system structures from pediveligers of eight bivalve species (Callista brevisiphonata, Mactromeris polynyma, Crenomytilus grayanus, Kellia japonica, Mizuhopecten yessoensis, and Azumapecten farreri) with different modes of life in their adult stages, corresponding to the ecological niches that they occupy (burrowing, cemented, byssally attached, and mobile forms). We have identified neuromorphological features of the central and peripheral nervous systems in larval bivalves. We show that the unpaired sensory apical organ is still present in pediveligers along with the developing paired cerebral ganglia characteristic of an adult mollusk. Pediveligers have the pleural ganglia connected to the pedal ganglia via the pedal nerve cords and to the visceral ganglia via the lateral nerve cords. We have found a number of structures of the peripheral nervous system whose presence varies between pediveligers of different species. Mactromeris, Callista, and Pododesmus have 5-HT-immunopositive stomatogastric neurons, whereas the Yesso and Farrer’s scallops have an FMRFamide-immunopositive enteric nervous system. The innervation of the anterior part of the velum is connected to a system of the apical organ and cerebral ganglia, and the innervation of the posterior part is connected to the visceral ganglia. Most differences in the structure of the peripheral elements of the nervous system are species-specific and weakly depend on the ecological niche that pediveligers occupy.