Temperature and Salinity Preferences of Adult American Horseshoe Crabs (Limulus polyphemus) in the Great Bay Estuary, New Hampshire U.S.A

Each spring and early summer horseshoe crabs (Limulus polyphemus) approach beaches at high tide and spawn. While we have some understanding of the factors that influence which beaches they choose and how they find their mates, the environmental cues they use to guide their large-scale movements to the vicinity of these spawning areas remain to be elucidated. In the Great Bay Estuary, New Hampshire, when water temperatures exceed ~11 °C in the spring, horseshoe crabs move into the upper regions of the estuary that are generally characterized by lower salinities and warmer water. Therefore, these abiotic factors could serve as cues to guide them to their preferred spawning beaches. The first goal of this study was to determine if they would prefer certain temperatures or salinities in a Y-maze. When given a choice between ambient temperature water and water that was 2–3 °C warmer, horseshoe crabs spent significantly more time on the warm water side of the Y-maze. However, when given a choice between ambient water and water that had a salinity that was 2–3 psu lower, they did not express a preference. The second goal of the study was to determine if horseshoe crabs could detect small changes in water temperature, using a cardiac assay. When 17 horseshoe crabs were exposed to water that was at least 2.6 °C warmer than ambient water, they all expressed a “startle response”, characterized by a brief slowing of their heart rate followed by a prolonged increase of their heart rate. These results suggest that water temperature, but not salinity, is likely to be one of the most important cues used by horseshoe crabs to guide their large-scale movements and migrations to the vicinity of known spawning beaches each spring.