Spider Crab

SCUBA diving is one of the most productive and rewarding ways to observe spider crabs in their natural setting, as these slow-moving crustaceans are easily approached underwater. Because they are primarily active at night and tend to hide in rocky vents, holes, or along sandy ocean floors, night dives and careful searches around rocky recesses yield the highest chance of encounters. Divers should always maintain a respectful distance and avoid harassing or handling the crabs. Although they exhibit a calm and gentle disposition, their powerful claws can exert considerable pressure if defensive, and disrupting their attached decorative sponges or anemones can strip them of vital camouflage. When illuminating spider crabs with dive lights during night excursions, avoid shining bright beams directly into their short eye stalks for extended periods. In regions where diving harvest is permitted, divers must rigorously check local marine regulations regarding minimum size limits, bag limits, and protected seasonal closures before entering the water.

The optimal time of year to encounter spider crabs in diveable waters depends largely on their depth distribution and seasonal migration patterns. For the Japanese giant spider crab (*Macrocheira kaempferi*), adults typically inhabit deep offshore waters ranging from 160 to nearly 600 metres for most of the year. However, during their spring breeding season, which lasts from January through April, these crabs migrate into shallower areas around 50 metres depth to mate and spawn. This winter-to-spring window provides the best opportunity for technical or deep-certified recreational divers to observe them, as females gather in shallower zones to carry their eggs before hatching. In Japan, strict commercial fishing prohibitions are enforced during this spring breeding period to protect the reproducing population from overexploitation. Along the Eastern Pacific coast, where species like the sheep crab (*Loxorhynchus grandis*) reside, underwater encounters on rocky and sandy substrate occur throughout the year, with nearshore sightings proving most consistent during warmer months when crabs move into shallower coastal shelf zones.

Spider crabs inhabit temperate marine environments ranging from nearshore coastal waters down to the abyssal slopes of the deep sea. The Japanese giant spider crab is primarily distributed off the southern coasts of Honshū island, from Tokyo Bay down to Kagoshima Prefecture, with outlying populations recorded in Iwate Prefecture and near Su-ao in Taiwan. Adult crabs are predominantly found at depths between 50 and 600 metres (160 to 1,970 feet), where they prefer to shelter within deep ocean vents, holes, and rocky ocean floors. They thrive in chilled waters around 10 to 13 °C (50 to 55 °F), though public aquarium studies indicate they can tolerate temperatures between 6 and 16 °C. In contrast, Eastern Pacific spider crab species, such as the sheep crab (*Loxorhynchus grandis*), range from Alaska south to Baja California, inhabiting both rocky reefs and soft sandy substrate from nearshore shallows to offshore bottom environments.

Spider crabs are opportunistic omnivores and scavengers that feed primarily along the ocean floor. Their diet consists of a mixture of plant material and live animal prey, as well as decaying organic matter. They frequently act as scavengers by consuming dead or decaying fish. In addition to scavenging, spider crabs actively scrape the substrate for living macroalgae and plants, such as kelp. They also prey upon small marine invertebrates and mollusks, utilizing their slender chelipeds and claws to pry open thick shells. Because spider crabs are nocturnally active, the majority of their foraging and feeding behavior occurs under the cover of darkness. During the night, they move slowly across sandy flats and rocky reef slopes, searching for food debris or picking up invertebrates from the seabed. Their slow movement pattern reflects an energy-conserving foraging strategy, relying on steady searching rather than high-speed pursuit of prey.