Supporting data for Ecology of Semi-terrestrial Crabs Found in Back Mangroves and Adjacent Woodlands in the Monsoonal Tropics

The colonization of land from the sea, known as ‘terrestrialization’, occurred relatively recently among Brachyuran crabs (Decapoda, Crustacea). Many crab species invaded the supralittoral and terrestrial environment from the sea via intertidal habitats, which involves crabs spending most of their time in air and facing the challenges of temporal variations in temperature, water salinity and availability, and changes in food sources. This thesis studies the various adaptations to semi-terrestrial life in eight species of semi-terrestrial crabs common in the back mangroves and adjacent riparian woodland in Hong Kong (Chiromantes haematocheir , Orisarma patshuni, Orisarma intermedium, Orisarma dehaani, Neosarmatium indicum, Parasesarma continentale, Parasesarma pictum and Chasmagnathus convexus) to identify their behaviours, life histories and gut microbiomes, as well as the potential resource partitioning that allows their co-existence.

The distribution of the semi-terrestrial crabs was defined by the salinity gradient, with only two species (C. haematocheir and O. patshuni) expanding into the riparian woodlands, while the remaining species were confined to the back mangroves and other lower shore habitats. Within the riparian woodlands, C. haematocheir was more abundant than O. patshuni at higher elevations, suggesting it was better adapted to the low salinity and variable water conditions in upper stream areas. These two riparian woodland species had slower growth rates and longer lifespans compared with species that live in intertidal habitats.

To cope with variable environmental conditions, crabs showed seasonal activity patterns, with higher activity in the wet-and-hot summer than in the cold-and-dry winter. Temperature was a key factor, as the abundances of C. haematocheir and O. patshuni were positively correlated to mean daily temperatures. Burrows were refuges for N. indicum and C. convexus, which they remained in throughout winter and when the soil surface was hotter than the temperature in the burrows in summer.

In terms of diet, the crabs showed subtle partitioning of food sources. Apart from P. pictum, which consumed more animal tissues, the other species ingested large amounts of plant materials. Differences in the freshness of consumed leaves were found: whilst most crabs consumed leaf litter, O. intermedium consumed more green leaves . The crabs also relied on soil microalgae and animal tissues to supplement their diets. Competition for food might induce caching behaviour in summer as N. indicum and C. convexus brought potential food sources into burrows more frequently in areas with higher burrow density. This caching of leaf litter improved leaf palatability as it promoted the leaching of phenolic compounds, especially in winter. The gut microbiomes of the crabs reflected their phylogeny, habitat and food sources and may improve their nutrition by providing essential amino acids and vitamins.

In summary, this thesis highlights the variety of potential adaptations of semi-terrestrial crabs to survive in the challenging supralittoral habitats, which may be used to inform their conservation, given the threats to their habitats now and in the future.