News / Species Spotlight / Japanese Spider Crab (Naxioides robillardi) (12/05/18)

Japanese Spider Crab (Naxioides robillardi)

Species Spotlight - Japanese Spider Crab (Naxioides robillardi)
Spider crabs are a highly diverse group with somewhere around 800 known species; however, few aquarists pay them much attention. This may partly be due to a lack of awareness. There are many different common names that these crabs often go by. When we speak of spider crabs, were referring to anything from the superfamily Majoidea, and its within this group that we find the familiar Emerald Crab (Mithraculus scultpus).

This group also includes a number of species referred to as decorator crabs, so named for their tendency to camouflage themselves with bits of coral and algae. Camposcia retusa is one example available in the aquarium trade. A more commonly seen example is the Arrow Crab (Stenorhynchus seticornis). On coral reefs youll find many species which form intimate relationships with corals, such as those from the genus Hoplophrys and Xenocarcinus, often perfectly matching the color and pattern of their host.

But the most famous of spider crabs is the enormous Macrocheira kaempferi, which can reach 18 feet across its legs, making it the largest arthropod in existence today. This is a popular species to exhibit at public aquariums, though, for obvious reasons, not something practical for the home aquarist. But for those interested in keeping a more manageably sized relative, an alternative is Naxioides robillardi, capable of reaching a respectable 3 inches in carapace length and more than twice this when measured across its legs.

Naxioides belongs to a different family among the majoids, Epialtidae, relatively few of which are ever seen in captivity. The morphological traits used to separate these two groups (along with a third common family, the Inachidae) relate to minor differences in how the carapace surrounds and protects the eyes. One epialtid seen on occasion is the West Atlantics Furcate Spider Crab (Stenocionops furcata), a bulky species that looks like a spikier version of Naxioides, often found adorned with bits of sponge.

Another close relative commonly found on coral reefs (and one which could easily cause confusion) is Hyastenus. Both genera possess dramatically enlarged rostral spines extending forward above the head, but Naxioides uniquely possesses a pair of short accessory spines that project off dorsally just before the tip.

Naxioides is a small genus, with just 9 species recognized. Naxioides robillardi is reported to be widespread in the Indo-Pacific and is apparently a common find in deeper muddy or sand/rubble habitats. The stated depth range varies from 30 meters down to an impressive 260 meters (or 850+ feet), though Japanese divers have documented specimens in water as shallow as 10 meters. This preference for deeper water and cooler, high-latitudes has some ramifications for the recommended husbandry, as a warm, tropical reef aquarium is potentially detrimental to the long-term health of this crab.

Spider crabs are omnivores and prone to pestering benthic invertebrates, which also has to be taken into consideration when housing them. The ideal set-up for Naxioides would be a chilled, subtropical fish-only tank, devoid of any fishes that might consider crab to be on the menu (e.g. pufferfishes, triggerfishes, eels), but also devoid of anything a crab might wish to pick at. In such an aquarium, this beautiful spindly beast would make for a fascinating and rarely seen centerpiece though arachnophobes might think otherwise.