Climate migrations are the new norm. Our warming climate facilitates poleward shifts in the distribution of diverse species, thereby disrupting local ecosystems. The range of marine invertebrates is expanding poleward at an average rate of 3,000 km/decade1. For example, mangrove tree crabs (Aratus pisonii) now reside in salt marshes that are hundreds of kilometers northward of their historic mangrove habitats. However, adult crabs cannot swim or even crawl significant distances. The mechanism for poleward expansion of A. pisonii is unknown.
Long-range movement of crabs relies on dispersal during planktonic larval stages. For the first few weeks of life, these pen-tip-sized larvae drift with currents in the water column before settling in adult habitat. Larvae of some coastal crabs swim upwards during outgoing tides to “ride” tidal currents offshore and develop in waters with fewer predators before later migrating back to adult habitat. We predict that larval A. pisonii employ similar depth-regulation behaviors, which expedite their dispersal offshore and thereby increase the probability of drifting northward along oceanic currents.
We are examining behaviors and predictive factors that affect larval depth and thereby dispersal, including endogenous tidal rhythms, geotaxis, phototaxis, visual function, and sinking rate.
1 Richardson, E. L., Pringle, J. M., & Byers, J. E. (2025). Increasing global expansion speeds of marine invaders. Diversity and Distributions, 31(12).