Effect of enhanced shell structure on functional burrowing traits and dislodgement force in Potamilus alatus

Background/Question/Methods: Movement is an important trait that is critical for the survival of many animals.  Because of their sedentary lifestyle, studies examining the movement capabilities of freshwater mussels are relatively limited.  However, the ability to move and burrow into the substrate and, conversely, the capacity to resist dislodgement despite hydrologic forces and attempts by predators to remove them are likely crucial components of their behavior.  Performance of such behaviors is likely to be directly linked to survival and fitness.  Likewise, performance of these tasks is likely to be affected by the presence and nature of shell ornamentation which can vary widely within and among species.  Studies of marine bivalves suggest that shell ornamentation introduces a tradeoff between their ability to burrow effectively and to anchor themselves when an extrinsic force attempts to dislodge them from the substrate once buried.  We examined the effect of the presence and size of shell ornamentation on the burrowing performance and force required to dislodge buried mussels by artificially creating shell ornamentation on a relatively smooth-shelled species (Potamilus alatus).  Mussels were videotaped in laboratory aquaria to quantify burrowing sequences.  Dislodgement force was also measured in these aquaria.

Results/Conclusions: We found that enhanced artificial shell ornamentation had no significant effect on any of the burrowing performance measures, including latency to and duration of burrowing.  However, we found significant relationships between morphological characteristics, such as shell length and width, and several measures of burrowing performance, including latency to upright, latency to burrowing and burrowing duration.  The presence of artificial ridges increased the force required to dislodge buried mussels.  This suggests that varying amounts of shell ornamentation present in mussels help individuals resist dislodgement from the substrate by abiotic or biotic forces without substantial inhibition of burrowing abilities.  These results contradict previous hypotheses that a trade-off exists between burrowing abilities and dislodgement force.  This begs the question, why do some mussels possess such ornamentation and others do not?  Perhaps smooth mussels live in more variable habitats and must dislodge themselves more often.  Our on-going work seeks to test such hypotheses.