NONSELECTIVE SETTLEMENT OF MERCENARIA-MERCENARIA (L) LARVAE IN SHORT-TERM, STILL-WATER, LABORATORY EXPERIMENTS

Sediment selection by settling larvae of the hard clam Mercenaria mercenaria (L.) was determined in 4-h, still-water, laboratory experiments where larvae were given a choice between two highly contrasting sediment treatments: a natural, organic-rich mud, and an abiotic, glass-bead mixture with a grain-size distribution similar to the mud. The experiments were designed to evaluate the effects of intrinsic (larval age) and extrinsic (sea water temperature) factors on sediment selectivity. These specific effects were tested because results of initial experiments designed to replicate those of Butman et al., where M. mercenaria larvae had selected beads over mud in still water but not in flow, showed no significant selection. In the present study, only five of the 23 experiments conducted showed significant selection and, in all cases, for mud over beads. Larval age and water temperature had no significant effect on the outcome. In the earliest of these experiments, we discovered a problem with larval preservation in mud samples (dissolution of the larval shell due to low pH) that may have resulted in underestimates of the number of larvae settled in mud in Butman et al., thus confounding interpretation of those results. The conclusions of Butman et al. are therefore modified based on results of the experiments presented here: settling M. mercenaria larvae do not select between two extreme sediment treatments in 4-h, still-water, laboratory experiments. In addition, competency tests used here failed to establish a consistent, predictable, precipitous rise in competency for a given batch of M. mercenaria larvae. This may be due to natural, large variation in physiological development within a given larval pool such that a small proportion of competent larvae are available each day over an extended period. Furthermore, the between-experiment variability in selectivity and some results of the competency tests suggest that the duration of these experiments may be too short to document definitive, initial larval settlement. Occasional selection would then reflect continual redistribution of larvae prior to final settlement and metamorphosis.