A symbiotic oyster in a shrimp burrow: phylogenetic position of Anomiostrea within the Ostreidae (Bivalvia)

The ostreid genus Anomiostrea Habe & Kosuge, 1966, is monotypic for A. coralliophila Habe, 1975, which is known as a symbiont inhabiting the burrow of the ghost shrimp Neocallichirus jousseaumei (Nobili, 1904), but despite this unusual habit among oysters its phylogenetic position within the Ostreidae remained unknown. Using specimens collected from two distant localities of the Indo-Pacific, Oman and Japan, we compared shell morphology of these specimens with the holotype and assessed their phylogenetic relationships based on DNA sequence data from two mitochondrial and two nuclear genes. The genetic distance in cytochrome c oxidase subunit I between the specimens from Japan and Oman was 5.5%, indicating substantial geographic differentiation. Our molecular phylogenetic results suggest that A. coralliophila is sister to Ostrea algoensis G. B. Sowerby II, 1871, an oyster from rocky shores in South Africa, and both are closely related to other Ostrea species. This confirms assignment of Anomiostrea to Ostreinae and suggests that this burrow-wall symbiont evolved from typical rocky-shore oysters. Moreover, A. coralliophila was not monophyletic with another symbiotic ostreid Ostrea permollis G. B. Sowerby II, 1871, nor with other symbiotic oysters, indicating that the symbiotic habit evolved multiple times in the Ostreinae.