Wikipedia
Mangrove crabs are crabs that live among mangroves, and may belong to many different species and even families. They have been shown to be ecologically significant in many ways. They keep much of the energy within the forest by burying and consuming leaf litter. Along with burrowing in the ground, these crustaceans can climb trees to protect themselves. The hermit crab and the mangrove crab are the only crustaceans that can climb trees as a defense mechanism. Furthermore, their feces may form the basis of a coprophagous food chain contributing to mangrove secondary production.
Mangrove crab larvae are the major source of food for juvenile fish inhabiting the adjacent waterways, indicating that crabs also help nearshore fisheries. The adult crabs are food for threatened species such as the crab plover.
Their burrows alter the topography and sediment grain size of the mangrove, and help aerate the sediment. Removing crabs from an area causes significant increases in sulfides and ammonium concentrations, which in turn affects the productivity and reproductive output of the vegetation, supporting the hypothesis that mangrove crabs are a keystone species.