Allometric Changes Across Horseshoe Crab Moults Evidence Developmentally Controlled Ecological Shifts and Possible Exoskeletal Modularity

Developmental changes in animals reflect important behavioral, biological, and ecological shifts. Allometric adjustments in arthropods, specifically, are associated with changes in sexual maturity or alterations in life mode. Examining post-embryological allometry of the American horseshoe crab—Limulus polyphemus—here evidences early shifts in prosomal development, later changes in thoracetronic size, and possible modularity across exoskeletal sections. Modifications in prosomal allometry reflect transitions from living above the substrate to primarily burrowing. This change occurs at the 3–4 moult stage and is associated with a 70% mortality rate in both natural settings and under aquaculture conditions. Thoracetron allometry changes record the fusion of opisthosomal tergites into a plate, where tergal development drives shifts in thoracetron morphology. Allometric changes between main body sections present possible evidence for modularity within the horseshoe crab exoskeleton that manifest across moulting events. These allometric shifts reflect the complex evolutionary history of the group, especially changes from surface dwelling and enrollment to burrowing, likely in response to increased predation pressures.