Effects of physical tags and chemical markers on the shell growth of tridacnids

Shell growth tridacnids was studied using selected external physical tags and internal (chemical) shell markers. The effectiveness of these tags/markers to permanently mark Tridacna gigas and Hippopus hippopus juveniles was determined from short-and long-term studies on survival and growth in tagged juvenile clams. Three kinds of physical tags were compared: Floy tag, monofilament-dymo tag, and cable tie tag. The clam shell was drilled near the shell edge (above the shell's pallial line), the tag was inserted through the hole, and either the hole was plugged with dental curing mixture or left alone to be sealed with shell deposition. Clam survival and growth were monitored up to 34 months. Shell-staining experiments were conducted with alizarin red-S and tetracyline, to assess the chemicals as a permanent shell marker and to study the conditions that may vary their effectivity as a permanent marker. Tridacna gigas juveniles were exposed to the chemical marker at particular concentrations for up to 4 weeks. The clams were further reared in just seawater in three different set-ups (aquarium, raceway, ocean nursery) under ambient light and under shading for a similar period of time. All stained clams (both alizarin-and tetracyline-stained) and their controls were monitored for survival and shell growth. Groups of clams were subsampled temporally and dissected for shell and tissue analysis. The amount of alizarin in stained shells was determined by colorimetry, while tetracyline assimilation was observed under long wave ultraviolet light. The clam kidneys were excised from the clam tissues preserved in formalin, and sectioned using microtome. The physical tags used in this study were found useful as a permanent tag, each tag with varying influences on shellgrowth. Alizarin was found to be a useful chemical marker, and may be more effective in determining shell growth. The use of tetracyline on tridacnids required further study in terms of relative toxicity. The different tags and markers were evaluated according to their permanency, easy application, easy detection, and toxicity to the clam