Molts Reveal Life-History Patterns for Juvenile American Horseshoe Crabs in Fringe Habitats

In many regions of the world, horseshoe crab populations are sparse or difficult to study. We investigated the utility of molted shells, which may be more accessible than live animals, to provide essential data on population structure for juvenile horseshoe crabs. We collected and compared horseshoe crab (Limulus polyphemus) molts from two representative habitats near the edge of horseshoe crabs’ known range in the U.S., Taunton Bay, Maine (2005, 2006) and Petit Bois Island, Mississippi (2008, 2009, 2011). Molts were collected and measured every 2 weeks and characterized in terms of abundance, prosomal width, and sex. The greatest number of molts was found during full moon periods, corresponding to known spawning periods and following seasonal temperature increases at both sites. Molt prosomal width ranged from 3 to 169 mm at Petit Bois Island and 19–165 mm at Taunton Bay. Molt size frequency distributions and estimated growth increments were similar between sites and to size classes known for living juvenile L. polyphemus found elsewhere. Male to female sex ratios of molts ranged from 1.0 to 2.3, similar to sex ratios measured among live juveniles elsewhere and among live subadult and adult horseshoe crabs previously measured at the study sites. Molt numbers and size distributions on Petit Bois Island following the Deepwater Horizon oil spill confirmed survival of young crabs through the oil spill period. The overall similarity in molt frequency patterns and size distributions between distinct habitat types suggest some endogenous plasticity in horseshoe crab molting and subsequent growth patterns, despite regional differences in environmental attributes. Hence, data from molted shells have potential as an alternative to live animal studies by at least providing data on timing and rate of growth under different environmental conditions. These data will be particularly beneficial in habitats where live horseshoe crab populations are difficult to study or there are few background data to support live animal studies.