Separation of recently settled manila clams, Tapes philippinarum (A. Adams and Reeve, 1850), from three sediment types using sucrose density solution

摘要

Thorough study of the life history of important fishery and aquaculture shellfish species is essential for the most appropriate management and development of the resource. The early juvenile life stages of wild intertidal clam populations are difficult to study because of their existence in sediments of similar size and color to the animal. Here, we test the efficacy of methods for separation of recently settled Manila clams (Tapes philippinarum) from different types of intertidal sediments using density. Three aquaria were set up with sterilized sediments of each of the three sediment types-cobble/sand/shell, cobble/mud, mud/sand-resulting in nine aquaria total for the experiment. Each tank was filled with filtered, sterilized sea water; the water was heated to 20degreesC and aerated. Competent T. philippinarum larvae were added to each tank at a known density and were allowed to metamorphose and settle. Aquaria were then drained and four sediment samples were taken from each aquarium for evaluation of number of juvenile clams present. Samples were wet-sieved to isolate the size fraction from 125 to 500 mum, and then this size fraction was settled through high-density (1.9 g/mL) sucrose solution to separate stained clams from sediments. The average numbers of clams counted per sample for each sediment type were 57.9 (SD = 37.1) for mud/sand, 60.0 (SD = 36.1) for cobble/sand/shell, and 53.1 (SD = 39.6) for cobble/mud. The average number of clams expected per sample based on the number of larvae added to each tank was 58.8 for all three sediment types. Statistical analysis revealed no significant difference between the mean number of clams per sample and the expected value of 58.8 for all three sediment types. Therefore, these results indicate that the methods used here can be used to extract rapidly recently settled manila clams front all sediment types with a high degree of accuracy.