Distribution mechanism of Ocypodoidea in intertidal zone of north shore of Hangzhou Bay (Crustacea: Decapoda: Brachyura)

The protection of estuarine intertidal zones is a global hotspot issue. Ocypodoidea crabs are key components of intertidal ecosystems. Understanding the driving mechanisms behind the distribution of Ocypodoidea crabs is of great significance for intertidal conservation. The study, which combined an intertidal zone survey with laboratory salinity selection experiments, investigated the distribution of Ocypodoidea and the environmental factors driving their distribution along the north shore of Hangzhou Bay. The results indicate that we collected a total of 6 Ocypodoidea species, among which Uca arcuata , Macrophthalmus abbreviatus , Macrophthalmus banzai , and Ilyoplax deschampsi were dominant. Under natural conditions, from the bay head to the bay mouth, the four Ocypodoidea crab species transition from freshwater-adapted species ( I. deschampsi ) to saltwater-adapted species ( U. arcuata , M. abbreviatus , M. banzai ) along the increasing salinity gradient. Laboratory experiments demonstrated that salinity had an indirect influence on the distribution of I. deschampsi possibly mediated by biological competition. These crabs prefer distinct substrate types (divided into vegetated salt marsh, bare muddy flats and bare sandy flats). Macrophthalmus banzai and M. abbreviatus inhabit bare muddy and sandy flats. Ilyoplax deschampsi adapts to both. Uca arcuata inhabits vegetated salt marsh together with those two flats. Differences in crab substrate preferences and the presence or absence of these substrate types across different sections further drive compositional differences in the Ocypodoidea community. Human activities have altered the distribution patterns of salinity and the aforementioned substrate types along the north shore of Hangzhou Bay, likely driving shifts in the spatial distribution of Ocypodoidea crabs. • Species adaptation to salinity: Dominant Ocypodoidea species exhibit a bay-type distribution, transitioning from freshwater- to saltwater-adapted species, demonstrating species-specific adaptations. • Salinity's role in distribution: Laboratory studies show that salinity directly impacts most Ocypodoidea distributions, while its effect on Ilyoplax deschampsi is mediated by competition. • Diverse habitats: Three distinct habitats (vegetated salt marsh, bare muddy flats, bare sandy flats) were identified, showcasing habitat diversity. • Habitat-specific residence: Macrophthalmus species occupy muddy/sandy flats, Ilyoplax deschampsi adapts to both, and Uca arcuata resides in vegetated marsh and flats, illustrating habitat-specific patterns. • Human impact on crustaceans: Observed changes in Ocypodoidea distributions highlight the need for sustainable management to mitigate human impacts on coastal ecosystems.