Utilization of Shallow-Water Seagrass Detritus by Carribbean Deep-Sea Macrofauna: δ13C Evidence

Abstract Three dives were made using the DSRV Alvin in the deep-sea basin north of St. Croix, Virgin Islands. Detrital seagrasses and macrofaunal distributions at 2455 to 3950 m depth were assessed quantitatively. Counts of the manatee grass Syringodium filiforme ( ca. 5 to 100 blades m −2 ) contrasted sharply with those of the turtle grass Thalassia testudinum ( ca. 0.1 to 2.0 blades m −2 ), reflecting an abundance proportional to previously reported export rates of the same species from Tague Bay, a nearby shallow source lagoon. Of the macrofaunal consumers that could potentially utilize this detrital nutrient source, three species of holothurians ( Mesothuria verrilli, Psychropotes semperiana , and Benthodytes linqua ) and two species of sea urchins ( Hygrosoma petersi and Salencidaris profundi ) were collected and/or observed. Gut content analyses revealed that all three holothurians deposit-feed on sediment and at least one species of sea urchin ( H. petersi ) feeds almost exclusively on Syringodium . Carbon: nitrogen analyses of naturally occurring abyssal Thalassia detritus showed very low nitrogen content (0.21% N) and a high C:N ratio (214.8), thus yielding a loo nutritional value. Fresh Thalassia blades held in a litter bag experiment (by R. D. Turner) at 3950 m changed little in nitrogen content and C:N ratio after four years. A comparison was made of the stable carbon isotope ratios of 13 C: 12 C for abyssal seagrass detritus and other potential carbon sources with those for tissues from the holothurian and urchin consumers. The results indicate that a significant proportion of the nutrition of both groups is derived from detrital seagrasses either by direct consumption (sea urchins) or indirectly by deposit-feeding on sediments enriched by decomposed seagrasses (holothurians).