On the recovery of malformed horseshoe crabs across multiple moulting stages

Malformed horseshoe crabs have been documented for over a century. However, most of these records are anecdotal observations of often striking morphologies recorded in isolation. There is therefore little understanding of how malformations are manifested and how they can develop in the group. Here we consider the moult sequences of three extant Limulus polyphemus individuals to explore different patterns of malformation development. One specimen with an injured telson demonstrates a gradual recovery of the telson section over three moulting events. The second individual demonstrates a fused thoracetron-telson articulation with a hole for the telson. This individual shows consistent growth of a reduced telson across moults. The third individual shows a thoracetronic injury incurred during at least moult-stage 7 that shows no evidence of recovery over five moulting stages. These records illustrate that horseshoe crab malformation recovery is far more complicated than previously thought. This also suggests that unless an exoskeletal section has functional morphological importance (i.e., the telson), the region is unlikely to recover from an older malformation. From a conservation standpoint, the ability or inability to fully recover from injury affects a horseshoe crabs ability to survive and/or reproduce in the wild particularly if the injury affects the telson. Given the global decline in horseshoe crab populations and conservation efforts underway, the extent of injuries in extant populations of horseshoe crabs may affect population recovery and should be considered.