Comparative growth of triploid and diploid juvenile hard clams Mercenaria mercenaria notata under controlled laboratory conditions

Induced triploidy has been used in oyster culture to improve growth, but has not been fully explored for the hard clam Mercenaria mercenaria notata. Therefore, growth was examined in approximately 14 week-old (Exp I) and 15–18 week-old (Exp II) triploid juvenile hard clams in two 3-week experiments. Triploidy was induced chemically (cytochalasin B, 1.0 mg/l) by inhibiting polar body I (PBI) or polar body II (PBII). Growth, as a percentage change in live weight (LW), of triploids was significantly (P < 0.001) less compared to diploids in both experiments. In Experiment I, LW increased 250% and 269% for PBI and PBII triploids (initial average LW 93.6 ± 19.0 and 59.5 ± 11.7 mg/clam), respectively, and 341% for diploids (initial average LW 72.0 ± 16.7). Additionally, diploids within triploid groups of Experiment I had lower LW increase (218–296%) as compared to untreated control diploids (341%). In Experiment II, LW increased 422% for PBII triploids (initial avg. LW 11.8 ± 1.6 mg/clam) and 549% for diploids (initial average LW 11.7 ± 1.9 mg/clam). Juvenile triploid clams did not exhibit better growth than diploids in these laboratory trials, but triploid clams may have a growth advantage during stressful conditions or as adults during reproduction as triploids are virtually sterile, which would allow for somatic growth during a time when diploids are spawning and losing mass. Additionally, the use of untreated control diploids is recommended for ploidy experiments rather than diploids found within triploid groups.