Within-host speciation events in yoyo clams, obligate commensals with mantis shrimps, including one that involves a change in microhabitat and a loss of specialized traits

Compared to host shifts, the importance of within-host cladogenesis in the diversification of symbionts remains less well understood in marine systems. Yoyo clams (Galeommatidae: Vasconiellinae) are a clade of marine bivalves that live commensally with burrowing mantis shrimp. Almost all yoyo clams byssally-attach to the host burrow wall via a specialized hanging foot structure bearing a thread-like posterior extension. In contrast, Parabornia squillina (Vasconiellinae) byssally-attaches directly to the host shrimp and lacks a hanging foot structure. In this study, we examine phylogenetic relationships among vasconiellines by performing molecular analyses based on five genes (28S and 16S rRNA, H3, COI and ANT). We found evidence for two within-host speciation events among Floridian vasconiellines commensal with the same mantis shrimp host, Lysiosquilla scabricauda. One involved a cryptic sister species pair of burrow-wall commensals. The other involved the ectocommensal P. squillina and its somewhat unexpected sister taxon, the burrow-wall commensal Divariscintilla octotentaculata. This latter result suggests that a habitat shift from host burrow wall to host body surface occurred while retaining the same host species and led to the loss of the specialized hanging foot structure. Our findings suggest that ostensibly modest within-host ecological shifts can lead to major morphological changes in these clams.