Separation of recently settled manila clams, Tapes philippinarum , from three sediment types using sucrose density solution

Journal of Shellfisheries Research, April, 2004 by Daphne M. Munroe, Doug Bright, Scott Mckinley

ABSTRACT 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 20[degrees]C 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 tam, and then this size faction 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 from all sediment types with a high degree of accuracy.

KEY WORDS: density separation, juvenile clams, sampling, sucrose, Types philippinarum

INTRODUCTION

The Manila clam (Tapes philippinarum. A. Adams and Reeve, 1850) is of major importance to both the wild fishery and aquaculture industry worldwide. In British Columbia, Canada, this species is non-native and is thought to have been introduced along with oyster seed from Japan in the 1930s (Bourne 1982). Since its introduction, T. philippinarum has become a very important species economically and is the basis of the current clam culture industry (Jones et al. 1993). The conditions in British Columbia are favorable for the Manila clam, and it has become well established throughout the southern coastline (Quayle 1974).

In the general bivalve life cycle, a pelagic larval stage is followed by metamorphosis, during which the swimming organ or velum is lost, and the bivalve transforms into the benthic or epibenthic juvenile form, depending on the species. The postmetamorphic juvenile stage of the Manila clam is found in the upper layers of the sediment; however, sampling juveniles front these sediments presents many problems because of the similar coloration and size of the clams and fine sediments. Consequently, very few studies have focused on this life stage. When sampling has been carried out and curly juveniles in the sediment are counted (Glock 1978, Jones 1974, Williams 1980), the work is extremely time-consuming and prone to error. Rarely, however, is reference made in published studies of bivalve settlement and early life-stage ecology to the accuracy of the sorting and counting methods involved. A simple, consistent, and effective method for separation of postsettlement bivalves from the sediments would allow for more studies to be carried out and more insight into the recruitment patterns of the Manila clam and other valuable clam species to be gained.

The density of juvenile postsettlement clams and cockles was estimated to be 1.036 1.076 g/mL in a study by de Montaudouin (1997) and 1.1 g/mL in a study by Jonsson et al. (1991), whereas minerals are more dense, typically with a specific weight of 2.5 g/mL and higher (Denny 1993). Density gradients of silica sols have been used in many studies to separate lighter meiofauna from higher density sediment fractions (Burgess 2001, Nichols 1979, Schwinghamer 1981). It has been shown that high-density sucrose solutions can also be used to separate meiofauna from muddy organic sediment (Heip et al. 1974). The technique explored herein involves wet seiving to isolate the size fraction of the sediment containing the bivalves, then allowing that size fraction to settle through a high-density sucrose solution (1.9 g/mL) to isolate the meifauna and allow for easier counting of the bivalves. This technique will be tested using three different sediment types to determine the efficacy of the method with various sediments.

The clam juveniles would be expected to float in a solution with a density of 1.9 g/mL. However, the high concentration of sucrose increases the osmotic pressure on the animal cells causing them to dehydrate, thereby increasing the density of the animals and causing them to sink (Bowen et al. 1972, Price et al. 1978). Although the juveniles become more dense and sink, they sink slowly in relation to the higher density mineral components of the sample and are found in the top layer of the sediment once it has settled out. This process of isopycnic sedimentation at one solute density to separate particles of different densities is called "rho spectrometry" by Price et al. (1977).