FEEDING ECOLOGY AND TROPHIC ROLE OF SEA-URCHINS IN A TROPICAL SEAGRASS COMMUNITY

The grazing impact of urchins on seagrass and algal resources, and the relative importance of this to the lower-level trophic flux of a tropical seagrass community were investigated. Thalassia hemprichii (Ehrenb.) Aschers. accounted for 80-93% of seagrass frond biomass at Bolinao in the Philippines. Growth rate of seagrass was 6.6 mm per shoot day-1, or 2.3 mg AFDW per shoot day-1. Production of seagrass fronds per unit area of seagrass bed varied with location from 870 to 1850 mg AFDW m-2 day-1. Urchin density ranged from 0.9 to 4.2 m-2, with Tripneustes gratilla (L.) and Salmacis sphaeroides (L.) being the most common species. Tripneustes gratilla fed mostly on attached seagrass fronds (77-89% of diet), especially Thalassia hemprichii, whereas S. sphaeroides was a generalist, consuming Thalassia hemprichii fronds (13-65%), detached seagrass debris (5-39%), the red alga Amphiroa fragilissima (L.) Lamour. (0-30%), algal-coated sediment and rubble (0-51%) in proportions that varied with the availability of preferred food types. Live Thalassia hemprichii fronds were clearly preferred over macroalgae or dead seagrass fronds by Tripneustes gratilla, but S. sphaeroides consumed all three food types without preference. Both urchins avoided the common brown alga, Sargassum crassifolium J. Agardh. Urchins absorbed 73-76% of organic matter in seagrass fronds with epiphytes (75% of DW), and 55% of that in epiphyte-free fronds. Seagrass debris and the macroalgae A. fragilissima were of lower food quality as they were lower in organic matter, and this matter was absorbed less efficiently by urchins. Rates of ingestion (IR in g WW per urchin day-1) were proportional to body weight (W in g WW) according to the functions: IR=0.56W0.34 (T. gratilla) and IR=0.17W0.53 (Salmacis sphaeroides). Predicted grazing impact of urchins on seagrass resources varied spatially and temporally. Estimated annual grazing rate at the main study site was 158 g AFDW m-2, equivalent to 24% of annual seagrass production, but owing to large changes in urchin population structure and density, grazing impact is expected to vary from < 5% to > 100% at different times of year. A synthesis of knowledge on the lower-level trophic pathways in this system indicates that seagrass-urchin and periphyton-epifauna grazing interactions are both important in their contribution to overall trophic flux.