Accidental associates are not symbionts: the absence of a non-parasitic endosymbiotic community inside the common periwinkle Littorina littorea (Mollusca: Gastropoda)

摘要

One of the most thoroughly studied marine snails in the world is the North Atlantic periwinkle Littorina littorea. In 2013, Buckland-Nicks et al. reported upon an "extensive community of organisms" living "inside" L. littorea in Nova Scotia, Canada. Conflicting with this report is a vast body of research on this snail since the mid-nineteenth century that has never identified the presence of such a taxonomically diverse endosymbiotic community, with the exception of a well-known assemblage of parasitic digenean trematodes that utilize the snail as a first-intermediate host. The absence of prior reports of a non-parasitic endosymbiotic community, and our own long-term observations, inspired us to study the biological community associated with L. littorea in two locations (Nova Scotia and Connecticut). We were unable to verify the endosymbiotic nature of any of the species (other than trematodes) noted by Buckland-Nicks et al. (Can J Zool 91:293-301, 2013) associated with L. littorea. Instead, we identified three likely sources of associated taxa reported by Buckland-Nicks et al. (Can J Zool 91:293-301, 2013) and by us: species were (1) ingested and defecated alive; (2) attached to the mucous-covered foot and drawn into the mantle cavity when the snail closed its operculum; and/or (3) associated with shell surface cracks and crevices, including within encrusting algae. We could not find any evidence that L. littorea hosts a diverse community of endosymbionts. Instead, we argue that the organisms interpreted as members of an endosymbiotic community by Buckland-Nicks et al. are instead free-living taxa that are transiently associated with the snails.