Feeding selectivity and rapid particle processing by deep-sea megafaunal deposit feeders: A <SUP>234</SUP>Th tracer approach

Abstract

Deposit-feeding megafauna occur in virtually all deep-sea environments, yet their feeding selectivity and particle processing rates are poorly known. Excess 234 Th activity is commonly used as a geochemical tracer for recently settled (<100-d old) particles in the quiescent deep sea, but it has rarely been applied to the study of deposit feeders. To explore the selectivity and rates of megafaunal deposit feeding, we compared excess 234 Th activities in the gut contents of deposit feeders from Santa Catalina Basin (SCB) (∼1200 m depth) and the Hawaiian slope (∼1680 m) to the activity of surface sediments and, in SCB, to material from sediment traps moored ∼150 m above the seafloor. We also measured concentrations of chlorophyll a and phaeopigments in animal guts and surface sediments to evaluate feeding selectivity. In the SCB, excess 234 Th ( 234 Th xs ) activities in the guts of four species of surface-deposit feeders were 14-17 fold greater than those of the top 5 mm of sediment. Pannychia moseleyi and Scotoplanes globosa, two highly mobile, surface-deposit-feeding elasipodid holothurians, were the most enriched in gut 234 Th xs activity, suggesting that these species fed very selectively on particles settled to the seafloor within the previous ∼20 d. Pannychia moseleyi guts also exhibited 500-fold enrichment of chlorophyll a relative to surface sediments indicating highly selective ingestion of phytodetritus. Chiridota sp., a burrowing, surface-deposit-feeding, chiridotid holothurian, and Bathybembix bairdii, a surface-deposit-feeding trochid gastropod, were less enriched in gut 234 Th xs activity, reflecting lower mobility and/or less selectivity at time of particle pickup. A subsurface-deposit-feeding, molpadiid holothurian was not enriched in gut 234 Th xs activity compared to surface sediments, but was greatly enriched compared to average activities at its presumed feeding depth of 6-7 cm. On the Hawaiian slope, gut contents of two surface-deposit feeders, the synallactid holothurians Mesothuria carnosa and Paleopatides retifer, were not enriched in 234 Th xs activity; however, M. carnosa and Phryssocystis sp. (a surface-deposit-feeding echinoid) were enriched in chlorophyll a, suggesting that the Hawaiian slope species are also selective feeders. Presumably, frequent sediment resuspension makes 234 Th xs activity a poor tracer for recently settled, food-rich particles on the Hawaiian slope. Based on a newly developed 234 Th-flux model, we calculate that the three dominant megafaunal, surface-deposit feeders in SCB consumed on average 39-52% (s.e. 13-27%) of the daily flux of 234 Th xs activity to the SCB floor. By chemically altering (e.g., digesting) and redistributing recently settled particulate organic matter, these megafauna are likely to substantially influence carbon diagenesis and food-web structure in this bathyal habitat.