Species information
Scientific name: Geograpsus grayi
Japanese name: カクレイワガニ
Chinese name: 格雷陸方蟹、葛氏陸方蟹
English name: Purple-backed shore crab
Origin: Tropical and subtropical regions from the Indian Ocean to the Pacific
Ecology and description
Within the family Grapsidae, the genus Geograpsus is a group with a high degree of adaptation to land. Land crabs store water in their bodies and breathe the oxygen in that water through their gills. Sometimes they blow bubbles of water mixed with mucus, which exposes the water to the air so they can take in oxygen. The gills of the purple-backed shore crab exchange gases very efficiently, so it can travel several hundred meters from the shore and go deep into forests, where there are fewer predators.
The carapace width reaches about 5cm at most, and the whole body is a vivid deep purple. After sunset around the full moon from May to August (especially in June and July), females go to the sea and release their larvae where the waves wash onto the shore. Exhausted females are sometimes found on the shore.
Field photographs taken during our tours, with observation dates
