The term “reflex crab” is a colloquial descriptor, not a formal scientific classification. It refers to crabs in the genus Lybia, known for their rapid defensive reflex. These small crustaceans are more widely known as Boxer Crabs or Pom-Pom Crabs from their practice of carrying a living sea anemone in each claw for protection.
The Defensive Reflex
When a Boxer Crab perceives a threat, it instinctively engages in a distinct defensive display. The crab holds its specialized claws, or chelipeds, forward and waves the anemones clutched within them, resembling a boxer with gloves up. This waving motion is a direct response to potential danger and serves as a visual warning to would-be attackers.
The anemones provide a potent deterrent. Each anemone is armed with powerful stinging cells called nematocysts, which can deliver a painful sting to fend off fish and other predators. The crab uses the anemones as living, stinging weapons, relying on their biological properties to ensure its own safety.
A Unique Symbiotic Partnership
The relationship between the Boxer Crab and its anemones is a mutualistic symbiosis, where both organisms benefit. The crab gains a formidable defense system from the anemones’ stinging capabilities. The crab also uses the anemones like mops to soak up food particles from the substrate, which it then eats. This relationship is so ingrained that the crab’s claws have evolved specifically to hold anemones and are not used for other functions.
In return for providing protection, the anemone receives mobility. By being carried around, the anemone is exposed to a wider range of feeding opportunities than it would have if it remained stationary. It also consumes leftover food scraps from the crab’s meals. If a crab loses one of its anemones, it will carefully tear the remaining one in half. Each fragment then regenerates into a new, complete anemone, ensuring the crab is never left defenseless.
Habitat and Characteristics
Boxer Crabs are native to the shallow tropical waters of the Indo-Pacific Ocean, from the Red Sea to Hawaii and Japan. They inhabit coral reefs, where they find shelter in crevices, but they are also found on sandy and rubble-strewn seabeds.
These crabs are quite small, with a carapace or shell that rarely exceeds one inch (about 2.5 cm) in width. Their shells often feature intricate, mosaic-like patterns in shades of cream, brown, and red, which provides excellent camouflage against the complex surfaces of the reef. Their walking legs are much larger and more robust than their delicate chelipeds.
Keeping Reflex Crabs in an Aquarium
The unique behavior of the Boxer Crab makes it a popular choice for marine aquarists. Due to their small size and shy nature, they are best suited for smaller, well-established saltwater aquariums, often called nano tanks. These tanks should contain plenty of live rock, which creates the small caves and crevices the crabs need for hiding and foraging.
Boxer Crabs are peaceful scavengers, and their diet is easy to manage in captivity as they will consume most standard aquarium foods. Their timid disposition means they should be housed with other small, peaceful tank mates that will not bully them or attempt to steal their anemones. Careful selection of companions is necessary for the crab to thrive.