GABA is an inhibitory neurotransmitter in the neural circuit regulating metamorphosis in a marine snail

The marine mud snail, Tritia (=Ilyanassa) obsoleta, displays a biphasic life cycle. During the initial phase of early development, embryos hatch from benthic egg capsules to become weakly swimming veliger larvae. In the second phase, adult T. obsoleta are facultative carnivores and major agents of community disturbance. Metamorphosis is the irreversible developmental event that links these two life history stages. When physiologically competent, larvae can respond to appropriate environmental cues by settling onto their mudflat habitat and transforming themselves into miniature adult snails. Two neurotransmitters—serotonin and nitric oxide—have opposing effects on the metamorphic process in this species. In multiple other species of gastropod and bivalve molluscs, a third neurotransmitter, the classically inhibitory compound γ‐aminobutyric acid (GABA), can induce settlement or metamorphosis upon external application to competent larvae. In this situation, GABA is presumed to mimic the action of ligands from the juvenile environment that bind to larval chemosensory receptors and activate the metamorphic pathway. Results of our experiments contradict this commonly reported action of GABA on molluscan larvae. External application of GABA to competent larvae of T. obsoleta elicited no response, but instead attenuated the action of serotonin (5‐HT), a metamorphic inducer. Our investigations into the responses of larval T. obsoleta to multiple GABAergic reagents support our hypothesis that GABA functions internally as a neurotransmitter in the pathway that controls the initiation of metamorphosis. Our results also suggest that GABA acts directly on or downstream from serotonergic neurons to regulate the metamorphosis‐inducing effects of this neurotransmitter. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 736–753, 2018     

Ectaquasperm-like parasperm in an internally fertilizing gastropod

Abstract. Pomacea canaliculata is an internally fertilizing gastropod that produces, besides fertilizing sperm (=eusperm), a large number of unfertile sperm (=parasperm) that have no chromatin, are fusiform, and have three to five flagella. Here, we report that this snail also produces another type of parasperm, which results from a peculiar spermiogenesis including an anterior cytoplasmic migration. The mature oligopyrene parasperm has: (1) a rounded head including a partly lysed nucleus, (2) a conical mid-piece with eight large mitochondrial structures, and (3) a single flagellum (∼20 μm). These characteristics, although not found in any other gastropod parasperm, are shared with the externally fertilizing "ectaquasperm" and with the early spermiogenic stages of internally fertilizing "introsperm" found among the Annelida and basal Mollusca. There are indications that this sperm type may be produced by a truncation of euspermiogenesis, as proposed by Buckland-Nicks & Scheltema for the expression of ectaquasperm in bilaterian evolution.