Invertebrate atlas

Geographical Origin and Habitat: Pachygrapsus crassipes is native to the temperate and subtropical rocky shores of the Northeastern Pacific Ocean, ranging from British Columbia, Canada, to Baja California, Mexico. It is also found in Japan and Korea, where it is considered an introduced or disjunct species. It primarily inhabits the high and middle intertidal zones, living in rocky crevices, tide pools, and breakwaters, often spending hours out of water foraging on damp rocks.

Taxonomy: Belongs to the phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, family Grapsidae, genus Pachygrapsus. This species is easily identified by the square shape of its carapace, which is slightly wider than it is long, and the fine transverse lines that run across its back. The chelipeds (claws) are large, symmetrical, and particularly robust in adult males.

Social Behavior: Highly active, alert, and defensive, Pachygrapsus crassipes is a semi-terrestrial crab that moves with great speed across vertical rock faces. When startled, they quickly retreat into narrow rock fissures or dive into the nearest tide pool. They are territorial and aggressive toward conspecifics; disputes over hiding spots are frequent and involve threat displays with open claws.

Coloration and Sexual Dimorphism: The carapace color ranges from dark olive green to reddish-purple, brown, or black, marked with distinct, fine, light-colored transverse lines. The claws are often reddish-pink or purple with white tips. Sexual dimorphism is visible on the ventral side: males have a narrow, T-shaped abdomen, while females have a broad, rounded abdomen used to carry eggs.

Tank Setup: Requires a specialized paludarium (half land, half water) with a secure land area composed of stable, non-calcareous rocks arranged to create crevices and climbing paths. The water section should contain marine water with moderate current. A tight-fitting lid is absolutely mandatory, as these crabs are excellent climbers and can escape through very small gaps.

Diet and Feeding: A generalist omnivore and opportunistic scavenger with a voracious appetite. In the wild, they feed on microalgae scraped from rocks, small snails, mussels, worms, and organic detritus. In captivity, they accept commercial crustacean pellets, fresh vegetables (crushed peas, blanched spinach), nori seaweed, and pieces of shrimp or fish. Provide calcium supplements.

Water Quality: The aquatic section must meet standard marine parameters, preferably on the cooler side: temperature 15-22 °C, pH 8.0-8.4, and salinity 1.021-1.025. While they are hardy and can tolerate temporary water quality fluctuations, efficient filtration and adequate aeration are necessary to prevent ammonia accumulation and maintain clean conditions.

Compatibility and Tankmates: Not suitable for a standard reef aquarium. Their predatory and aggressive nature makes them a hazard to small, bottom-dwelling fish (like gobies and blennies) and slow invertebrates like snails or small hermit crabs, which they will actively hunt and eat. Keep them in a species-specific tank or with fast, surface-swimming fish.

Aquarium Breeding: Breeding in home aquariums is extremely difficult due to the complex pelagic larval stage. After mating, the female carries the fertilized egg mass under her abdomen until hatching, releasing zoeae larvae into the water. Raising these larvae requires specific planktonic food, delicate water flow, and isolated rearing tanks.

Risks and Diseases: The primary risk is escaping the tank, leading to rapid dehydration and death. Molting problems can occur if the water is deficient in calcium or iodine; regular iodine supplementation is recommended to ensure a clean break from the old shell. Any medications containing copper are highly toxic and must never be used.