Turrids whelk, Phymorhynchus buccinoides feeds on Bathymodiolus mussels at a seep site in Sagami Bay, Japan

The Turrid whelk Phymorhynchus buccinoides aggregates Okutani, Fujikura & Sasaki on only four outcrops in a grey and black sediment area (25 m ²) at a depth of 1180m at the Off Hatsushima Island seep site, Sagami Bay, Japan. To determine the food web structure of the deep-sea chemosynthesis-based community, we conducted the dietary food habits of the whelk by a combination of in situ behavioral observations, stable isotope compositions, symbiotic bacteria in their soft part body parts, in situ bait trap experiments, and anatomic examinations. The whelks had a δ¹³C value similar to that of the mytilid mussel Bathymodiolus platifrons Hashimoto & Okutani, no symbiotic bacteria, and very small radula (probably useless). In bait trap experiments, the whelk swarmed toward crushed B. platifrons. These results strongly suggest that P. buccinoides feeds on B. platifrons. The ecological niche of the whelk is that of a carnivore or scavenger. B. platifrons was distributed on other outcrops at this seep site, but P. buccinoides did not occur elsewhere. Thus, the distribution pattern of this whelk is not determined solely by its dietary habits.