Northern range expansion of the Atlantic mangrove fiddler crab <i>Leptuca thayeri</i> Rathbun, 1900 (Decapoda: Brachyura: Ocypodidae)

Abstract Tropicalization is a global phenomenon where tropical or sub-tropical species expand their ranges poleward into temperate areas. We document the northern range expansion of a tropical species, the Atlantic mangrove fiddler crab, Leptuca thayeri, on the east coast of the United States beyond its historical northern range (Jacksonville, Florida). We used iNaturalist observations, personal observations, and surveys to establish its expanded range. Based on 31 iNaturalist and 17 personal observations, we found that its current northernmost limit occurs in Beaufort County, South Carolina, 307 km north of its historical limit. Leptuca thayeri expanded its range beyond mangroves and into salt marshes, where it was found among the saltmarsh plants Spartina alterniflora and Juncus roemerianus. This result suggests that L. thayeri is a habitat generalist. Sea surface temperatures along the coast of Georgia have warmed 1.15°C over 22 years (1998–2022) or 0.5°C per decade, on average. These rising ocean temperatures suggest that climate change is driving the range expansion of L. thayeri making it a climate migrant. Future work could examine the impacts of this burrowing crab on salt marshes.