Freshwater Crab

Austrothelphusa agassizi is endemic to north Queensland, Australia, distributed from Ravenshoe on the Atherton Tablelands north to Coen on Cape York Peninsula. Its range spans both eastward-flowing catchments draining to the Coral Sea coast and westward-flowing catchments draining toward the Gulf of Carpentaria — an unusually broad catchment distribution suggesting historic landscape connectivity across the base of Cape York.

Within this range it inhabits the waterline zones of creeks, rivers, gullies, lagoons and swamps, sheltering in burrows excavated into creek banks and under rocks near the water's edge. Like other Austrothelphusa species it is semi-terrestrial and amphibious, regularly emerging to forage on land. During the dry season it aestivates in sealed burrows, tolerating drought conditions that would eliminate most other aquatic animals from the same habitat.

In the aquarium trade, A. agassizi is sold primarily as 'Cherry Claw Spider Crab' or 'Freshwater Crab (Large)' and is occasionally misidentified as Holthuisana sp. It is regarded as one of the most visually appealing native freshwater crabs available in the hobby, and is specifically noted as highly suitable for aqua-terrarium keeping. As a native Australian species, licensing requirements may apply and vary between states; confirm current requirements with your state wildlife authority before purchasing.

A. agassizi requires an amphibious setup with both a water section and accessible land. A purpose-built aqua-terrarium or modified aquarium (minimum 60×40×40 cm; 100 litres recommended for pairs) with a divider or raised land platform works well. Provide a land section of sand, fine gravel or sand-coco peat mix at least 8–12 cm deep with cork bark, smooth rocks and hiding structures. A gentle slope or ramp connects water to land.

Water section depth: 8–15 cm, allowing the crab to stand with its carapace at or above the surface. Temperature 20–28°C; pH 7.0–7.5; general hardness 100–200 ppm. A low-flow sponge or internal filter maintains water quality without strong currents. Atmospheric air access is essential — never fully flood the enclosure.

A completely escape-proof lid is non-negotiable. A. agassizi is an active climber capable of scaling textured surfaces, tubing, filter equipment and plants reaching the water surface. Any gap is an escape route. House individually unless the enclosure is large with multiple well-separated territories; individuals are territorial and will fight.

A. agassizi is an opportunistic omnivore and scavenger, consuming plant material, detritus, invertebrates and carrion in the wild. In captivity, a quality sinking crustacean or shrimp pellet should form the dietary foundation. Supplement with blanched vegetables (zucchini, carrot, sweet potato, spinach), small earthworms, dried shrimp, and small pieces of prawn or fish. Offer food every one to two days; remove uneaten food from the water section within 24 hours to maintain water quality.

Calcium is critical for moulting and claw integrity. Provide crushed cuttlebone in the enclosure permanently, and include calcium-rich foods regularly. Leave shed exoskeletons in the enclosure for 24–48 hours after moulting — the crab will consume them for the calcium content.

Never use copper-based products or medications anywhere in the enclosure; copper is rapidly lethal to all freshwater crustaceans. Use only a copper-neutralising water conditioner for all water changes.

Primarily crepuscular and nocturnal but readily becomes active during the day in settled enclosures. Territorial toward other crabs; will fight and potentially injure or kill a conspecific in a restricted space.

A. agassizi can be gently picked up from behind but should be handled only when necessary for health assessment or enclosure maintenance. The claws of adults can deliver a painful, skin-breaking pinch — approach from behind, grasping firmly above the rear legs with thumb and forefinger, keeping all fingers clear of the front claws. Always handle close to a surface.

A recently moulted crab must not be handled under any circumstances — the soft new exoskeleton is extremely fragile and even careful handling can cause permanent damage. Wash hands thoroughly after handling the crab or any tank water. These are primarily observational animals best appreciated through the enclosure wall.

The most important enrichment is a well-structured aqua-terrarium that allows natural expression of foraging, climbing and sheltering behaviours. Varying the substrate surface with rocks, cork bark and root sections at different levels creates environmental complexity. The water section can include java moss, hornwort or other hardy aquatic plants for additional surface area and cover.

Varying the diet in rotation — offering live earthworms occasionally, different vegetable types on different days — stimulates natural foraging behaviour. Introducing a new piece of cork bark or a smooth rock during cleaning days provides novelty and encourages investigation.

Moulting complications: The most common serious health risk. Most often caused by calcium deficiency, overly soft or acidic water, or disturbance during the moult. Ensure calcium is permanently available, water hardness is appropriate, and a moulting crab is never disturbed.

Copper toxicity: Any copper in the water section is lethal to freshwater crabs. Use copper-neutralising water conditioner for all water changes and never use copper-based medications or algae treatments.

Aggression injuries: A. agassizi will fight conspecifics in confined spaces. House individually or in large enclosures with well-separated territories and abundant hiding places.

Escape and desiccation: An escaped crab found dry outside the enclosure may still recover if returned promptly to the water section. Check the lid at every access. Ensure the lid has no gaps; a heavy, clip-fastened lid is strongly recommended.

Ammonia and nitrite toxicity: Maintain a cycled, filtered water section and perform regular water changes.