A new species of spider crab in northern Australia

Andrew Hosie's blog | Created 8 years ago

Spider crabs are members of the superfamily Majoidea. Not your average-looking crab, many of them possess long thin legs (hence their name) and weird body shapes. Majoids are also known as decorator crabs because they camouflage themselves by attaching other organisms, such as seaweeds and sponges, to the spines and hairs on their legs and carapace. One of the largest species of spider crabs found in Australia is from the genus Paranaxia. Until now there was only one recognised species: Paranaxia serpulifera, found in the north of the country from WA to Queensland. This species has a body length of up to 18 cm, with claws and legs that can reach almost 30 cm.

Several specimens of Paranaxia were collected during recent dredging surveys in the north-west of WA and while identifying these specimens we noticed that some of them had much longer and thinner legs than others. Looking in more detail there were differences in the placement and size of spines on the carapace and claws as well as differences in the sculpture on the underside of the carapace. Genetic data also showed that there were clearly two species. However, these small specimens were only juveniles. We hunted through the crustacean collections here at the WA Museum as well as at the Queensland Museum looking for additional specimens of both species to complete the description and to get a complete range of sizes for both males and females, as well as map their distribution and depth range. Our search revealed that many specimens of the new species had been collected and were simply waiting for someone to take notice and give them a name. We described the new species and named it Paranaxia keesingi, after Dr John Keesing from CSIRO, in recognition of his contribution and commitment to the knowledge of Western Australian biodiversity.

Paranaxia keesingi in full camouflage

A juvenile male Paranaxia keesingi in full camouflage. CSIRO photo taken during the Pilbara Marine Conservation Partnership study.

Paranaxia serpulifera is an Australian endemic and found as far south as Perth in WA, while Paranaxia keesingi can be found as far south as the Houtman Abrolhos Islands and north into Indonesian waters off New Guinea as well as in northern Queensland. Paranaxia serpulifera lives in shallow waters and can even be seen stranded in rock pools at low tide, but isn’t found much deeper than 20 m. Paranaxia keesingi, on the other hand, is recorded at depths of up to 175 m.

Most marine crabs will normally spend a period of time as planktonic larvae, before living on the seafloor as a juvenile crab. Direct development of embryos to juveniles is common in terrestrial and freshwater crabs, but the species of Paranaxia are two of only 11 species of marine crabs that do this (oddly, 10 of these are found in Australia and New Zealand). The juveniles are held under the abdomen of the mother for at least one moult cycle before they disperse. This makes female Paranaxia very good mothers, by crab standards at least.

Large males of Paranaxia serpulifera (left) and P. keesingi (right)

Large males of Paranaxia serpulifera (left) and P. keesingi (right) with representative juveniles found under the abdomens of females.
Image copyright WA Museum

The full paper describing this species can be found here: http://www.mapress.com/j/zt/article/view/zootaxa.4127.1.6/6467