Surveys of seagrass communities and sea urchins were undertaken at 5 sites on the reef flat around Santiago Island, Bolinao, Luzon, Philippines, between October and December 1991. The sites chosen were all in subtidal mixed species seagrass beds, which varied in sediment type and exposure to waves and currents.
Seagrass abundance was determined by identifying and counting seagrass shoots within 20 randomly placed quadrats (0.5 m x 0.5 m with 10 cm subdivisions) within an area of 1000 m² at each site. The relative abundance of dead seagrass fronds was estimated visually. Common species of macroalgae, of potential importance in the diet of urchins were collected to determine dry weight (DW). At each site, 50 plants from each seagrass species were collected to calculate the average weight of fronds, which was used to calculate biomass/unit area.
A sample of 50 Thalassia hemprichii shoots was collected in October 1991 from the Malilnep reef flat site. After removing epiphytes, the length of each frond was measured and the fronds dried to a constant weight. Ten dried fronds were ashed in a muffle furnace for 5 hours at 450°C to calculate ash free dry weight (AFDW) as a percentage of dry weight. These measurements were used in conjunction with shoot linear growth rates from another study to calculate growth rates.
Ten belt transects (10 x 1 m) were laid randomly at each site and sea urchins within the transects were identified and counted. During random swims at each site (except Malilnep nearshore), searching a field of approximately 0.3 m width, the horizontal diameter (mm) of the first 100-150 of the most common urchins (Tripneustes gratilla and Salmacis sphaeroides) were measured to determine size distributions. Forty specimens of each species were collected to determine relationships between test diameter and wet and dry weight. The feeding habits of 100 specimens of each species, at each site were observed and data on whether urchins were feeding and the type of food being ingested was collected. Six specimens of each species of urchin were collected, dissected and the composition of the oesophagus-foregut quantified to compare with field observations.
Ingestion rates for Tripneustes gratilla and Salmacis sphaeroides feeding on Thalassia hemprichii, were investigated at the Lucero site. Pre-weighed bunches of live Thalassia hemprichii fronds were attached at the frond base to 30 cm x 1 cm perspex strips and inserted into the sediment bordering the seagrass bed. The strings were arranged in five separate patches, each patch approximating natural seagrass density, and covered by plastic-mesh cages (0.8 m x 1.6 m, 0.4 m high, 1 cm mesh). Three similar-sized urchins of one of the species, collected from the adjacent seagrass bed, were introduced to each of four of the cages, with one cage left as a control. After 48 h the seagrass and urchins were collected from the cages and the experiment was repeated with the second species of urchin. Seagrass material collected from the cages was immediately weighed. Test diameter and wet weight (WW) of urchins were measured. The mean daily ingestion rate per urchin was calculated and corrections were made to account for the slight change in the controls (3-6% of the weight change in test cages).
Food preferences of the urchins Tripneustes gratilla and Salmacis sphaeroides, collected from Lucero were investigated in the laboratory using four types of plant material: live fronds of Thalassia hemprichii, dead seagrass fronds, the red alga Amphiroa fragilissima and the brown alga Sargassum crassifolium. Urchins (average test diameter 50± 5 mm) were starved for 2 days in four aquaria (500 l; 5 urchins per aquarium) supplied with aeration and flowing seawater and shaded using 50% shade cloth. Five weighted bunches of each of the four food types in approximately equal quantities were placed randomly in each of the four experimental aquaria. Food supplies were replenished regularly throughout each experiment to ensure food types were all in excess of demand, and the quantity (as WW) of each type consumed was determined after 24 h. The experiment was repeated for each species of urchin.
The relationship between urchin size and ingestion rate was established using urchins fed live fronds of Thalassia hemprichii. Urchins collected from Lucero were separated into size categories and placed into aquaria containing similar known weights of seagrass fronds (supplied in excess). Urchins were allowed to feed for 24 h, then the seagrass and urchins in each of the four aquaria were removed for WW and DW determination, and the calculation of the weight of seagrass consumed per urchin. Altogether, seven size-groups of Tripneustes gratilla (11-253 g WW) and eight groups of Salmacis sphaeroides (25-176 g WW) were tested in random order of size.
The efficiency of absorption of organic matter from different foods (live and dead fronds from Thalassia hemprichii, with and without epiphytes, and the algae Amphiroa fragilissima and Sargassum crassifolium) by the urchins Tripneustes gratilla and Salmacis sphaeroides was determined. Groups of urchins (average size 50 mm), freshly collected from Lucero and maintained in the above aquarium system, were allowed to graze upon one food type for 3 days and were subsequently dissected to extract faeces from the final 2 cm of the intestine. Faeces (n = 8) and subsamples of the corresponding food material (n = 8) were dried and ashed to derive AFDW as a proportion of DW. For comparative purposes, additional samples of food and faeces were analysed for total organic carbon and total nitrogen (CHN analyser).
