A Scavenging Behavior of Cyclammina cancellata (Foraminifera, Rhizaria) on a Crustacean Carcass: Amino Acid Nitrogen Isotope and Microbiome Analyses

Cyclammina cancellata is a common benthic foraminifer found at bathyal depths across the world's oceans. Despite its important role in deep-sea food webs and biogeochemical cycles, the feeding habits of this species remain poorly understood. In this study, we document an aggregation of C. cancellata on a crustacean carcass at a depth of 750 m in the upper bathyal zone of Sagami Bay, Japan. Over 40 specimens of C. cancellata were observed on a crustacean carcass approximately 3 cm in length, with few other foraminiferal species, such as Lobatula wuellestorfi and Globobulimina affinis, present. To investigate whether the carcass served as a food source, we employed two recently developed techniques: individual amino acid nitrogen isotope analysis and metabarcoding. The nitrogen isotopic compositions of phenylalanine, which is an indicator of the primary producer in the food source, were similar across C. cancellata individuals and the carcass, suggesting that C. cancellata derives its nutrition from the carcass. Based on microbiome analyses of the crustacean carcass and the foraminifers, some potential endobiotic bacteria present in the foraminifer may be responsible for the degradation of the carcass. These findings suggest that C. cancellata may scavenge animal carcasses that fall to the deep-sea floor, while also acting as a deposit feeder when carcasses are unavailable. Given its wide geographic distribution and high individual and population biomass, C. cancellata likely plays a significant role in the degradation of both photosynthetic and non-photosynthetic organic matter that reaches the deep-sea floor.