SIZE FREQUENCY DISTRIBUTION BIASES IN HORSESHOE CRAB MOLT ASSEMBLAGES: IMPLICATIONS FOR THE FOSSIL RECORD

The Atlantic Horseshoe Crab, Limulus polyphemus, is common on the eastern coast of the United States. Horseshoe crabs come to shore for mating and molting with molting starting at very frequent rates in immature individuals and slowing to about once per year as they reach maturity. Apart from this annual molting and mating event, horseshoe crabs live offshore on the continental shelf, and the depth of habitat increases with age and size. Both full bodied and disarticulated sclerites were collected from 102 individuals (24 carcasses and 78 molts) between 2000 and 2015 from a tidal inlet, Miss Annie’s Creek, Shelter Island, New York. Photographs of 12 and 58 live individuals were taken in August of 2014 and 2015, respectively. Size frequency distribution of both the live individuals and the molt assemblage reveals similar patterns indicating distinct instars. However, the earliest instar recognizable in the live population is 20mm in total length, estimated to be the fifth or sixth instar, corresponding to about one year based on growth patterns noted by Carmichael et al, 2003 while the 36mm is the smallest identifiable in the molt assemblage. Furthermore, the latest instar present in the molt assemblage is much greater than that identified in live populations.

This study reflects preservation biases in the arthropod fossil record, primarily in trilobites and eurypterids. Horseshoe crabs are commonly used as a modern analogue for trilobite ecology, including molting behavior. Like horseshoe crabs, trilobites frequently congregate for molting, and instar patterns similar to what is seen here in the horseshoe crabs have been described in trilobite molt assemblages. Like the horseshoe crabs, the size frequency distributions of preserved exuviae in arthropod beds likely represent populations with larger variation for which the smaller sizes are missing from the fossil record.