The protobranch Aequiyoldia eightsii (Jay, 1839) (Nuculanida: Sareptidae) reveals uncommon siphonal anatomy among bivalves

Siphons represent a remarkable example of evolutionary convergence in different bivalve lineages. In Protobranchia, the siphons, when present, are used exclusively to channel the water currents used for respiration, waste removal and gamete release. Their emergence is thought to be associated with the taxonomic and morphological diversification within the Nuculanida. While siphons have been extensively studied in other bivalve groups, particularly the Heterodonta, gaps remain in the knowledge of protobranch siphons, especially on their morphology, including the musculature and innervation, and their functioning. The few data on Protobranchia siphons are restricted to some Nuculanoidea, and information is still missing for other nuculanid lineages, such as the Sareptoidea. The present study provides a description of the siphonal morphology of Aequiyoldia eightsii, a sareptid bivalve. Histological data were obtained to investigate their organization and infer their functioning. All traits observed were compared with those of heterodont siphons. In A. eightsii, the siphons are fused with each other and distinguished by their reduced longitudinal musculature in bundles that do not form layers, by the presence of a blood lacunal system that fills most of the wall of the siphon and by the innervation that is likely diffuse. This suggests that the siphons are hydrostatic organs with a hydraulic mechanism whose expansion occurs when the lacunar system is filled with blood. Our results provide the first morphological and functional details of a protobranch siphon and thus contribute to knowledge on the comparative anatomy of Bivalvia.