Exploring the diversity of mesopsammic gastropods: How to collect, identify, and delimitate small and elusive sea slugs?

摘要

Sediment-covered ocean floors constitute one of the largest and at the same time least explored habitats on Earth, still hiding an unknown level of species diversity. Coastal areas of this marine mesopsammic habitat harbor a variety of heterobranch snails and slugs. These gastropods long were puzzling due to their unclear phylogenetic positions, their aberrant morphologies and the lack of knowledge regarding their biology and diversity. Herein, we briefly review the advances of interstitial gastropod exploration, emphasizing that molecular approaches on formerly enigmatic mesopsammic groups like rhodopemorphs or acochlidians contributed to a drastic reconsideration of heterpbranch systematics and evolution. We give an overview of the known diversity of mesopsammic heterobranchs and a list of type localities. In order to enhance surveys on the biodiversity of yet unexplored coasts, we then provide a suitable method to take samples of mesopsammic heterobranchs, and to extract and document slugs and snails from sands. A key based largely on externally-visible features allows for initial identification of already known taxa. Most mesopsammic gastropods show a "meiofaunal syndrome", i.e., their morphology is constrained by the spatially-restricted interstitial environment, favoring rather uniform, worm-like body shapes and simple internal organization, which causes problems in conventional taxonomic approaches. Here, we present and discuss an integrative taxonomic workflow for delimiting potentially cryptic and elusive mesopsammic species, that also may be of use for other rare(ly) sampled invertebrates.