Predation scar frequencies in chemosymbiotic bivalves at an Oligocene seep deposit and their potential relation to inferred sulfide tolerances

• Three species of Oligocene methane-seep bivalves show repair scars.
• Scar frequency is inversely correlated with their inferred sulfide tolerance.
• Scar frequency may be used to infer sulfide tolerance in extinct seep-inhabitants.

Three species of chemosymbiotic bivalves with different inferred sulfide tolerances and life habits from a lower Oligocene seep deposit in eastern Hokkaido, Japan, were investigated for drill holes and scars of durophagous predation. The thyasirid Conchocele bisecta had the lowest inferred sulfide tolerance and showed the highest frequencies of repair (0.4) and drilling predation (0.1), and specimens with drill holes were on average smaller than non-drilled specimens, indicating that large size is a refuge from drilling predation in this species. The bathymodiolin Bathymodiolus inouei and the vesicomyid Hubertschenckia ezoensis are inferred to have lived at higher sulfide concentrations than C. bisecta based on comparisons to their living relatives, and show lower repair frequencies (0.24 in B. inouei and 0.17 in H. ezoensis). Based on the assumption that increased sulfide concentrations are increasingly toxic to predators, we conclude that the frequency of shell injuries in the three investigated species is inversely related to the sulfide concentrations that these species are able to tolerate. Thus when applied carefully, the frequency of shell injuries among shelled invertebrates at fossil seeps may be used to infer their relative tolerances toward sulfide, and may thus represent a tool to assess the life habits of extinct invertebrates that inhabited methane seeps in the geologic past.