ANALYZING REPAIRABILITY OF SHELL BREAKAGE IN BIVALVES

Repair scars on bivalves can represent unsuccessful attacks by durophagous predators or damage induced by abiotic factors, such as wave-borne clasts and wear and self-induced injury during burrowing. Most studies attempt to differentiate between these causes, but proposed criteria are ambiguous, especially in their application to the fossil record. Different processes may produce similar scarring patterns, while a single agent can also result in different patterns. All scars, however, affect survivorship during subsequent encounters with enemies and the environment. It may be beneficial then to focus on the frequency of scars, their severity, and the relative distribution frequencies of length at death and length at the incidence of scarring as measures of repairability, or adaptedness to mortality-risk from shell breakage. Repairability, regardless of whether it is initially driven by biotic or abiotic selective agents, may contribute to long-term arms races in evolution.

Here, we examine these parameters for a population of Mactra violacea, a modern bivalve from India. It has a repair frequency of 0.5 scar per individual at one of the study sites, which is within the range of repair frequencies of shallow burrowers. Only 20% of scars are located on the posterior shell region, which may be exposed above the sediment surface to predators. Most scars (~70%), however, are along the ventral margin, affect both valves, and include reattachment of broken chips near repair scars suggesting damage due to non-predatory origin. There is also a marked difference between frequency distribution of length at death (skewness = -0.023) and of length at incidence of scarring (skewness = 1.177). This shows that although individuals suffered maximum damage at early ontogeny, they are generally capable of repairing these and surviving. As many as 65% scars are found to be of the lowest severity grade i.e. length and width of scar is negligible with respect to body dimensions, with no tissue damage. Only 17% of scars were severe or life-threatening, i.e., included either soft tissue damage or very large scars. Combined, these indices reflect a species’ general ability to resist breakage, whether induced by predators or other sources, and could be used in fossil studies to test the primary predictions of escalation theory.