Ovalipes catharus

At first glance, Ovalipes catharus appear to be fairly similar to many other crabs. Paddle crabs have a hard shell, or carapace and ten legs, five to each side. The first set of legs of the crab is tipped with large claws. The claws of the paddle crab are fairly long and narrow and are curled in a sickle shape.[5] They are decapods which means they have 10 legs and are situated symmetrically with five legs on each side. The claws are also serrated with large points on the insides.[6] The carapace, or shell, of the crab can range from pale orange to light gray, and can be spotted with brown and red.[6] The carapace is a rough oval shape, and the front end has five teeth to either side of the eyes.[6] Paddle crabs can reach sizes ranging from 100–140 mm across the shell upon reaching maturity.[7] Unlike some other crab species, the last pair of legs of this crab is flattened into a paddle, hence their name. These paddles are also colored blue, and sometimes have a purple tinge, making them distinctive in color from the rest of the body.[6] Unlike prawns and lobsters which have narrow bodies with long tails, paddle crabs have also developed a wide body (carapace) and very short tail (abdomen), which is usually tucked underneath to its body making it compact and wide.

Paddle crab (O. catharus) found to be quite a unique species to which is native to New Zealand and Australia.[8] Paddle crabs are often active swimmers, but if they are not active, they normally bury themselves underneath the sand. There are hundreds of species worldwide, showing that they are common throughout New Zealand, and species found in different areas have turned up in New Zealand, mainly in warmer waters to the North, and have made themselves at home when they invade either naturally or accidentally by ship.[8] Ships need to pump water to keep them balanced, however many marine species like paddle crabs have tiny larvae in their life cycle, where they might get sucked into ballast tanks and get pumped out again when the ship reaches a destination. If they survive, the larvae would grow to become an adult in its new home. According to all records in 2003 from the ‘Aquatic Sciences and Fisheries’ Ovalipes catharus were locally found on open sandy beaches, in surf areas and open coast. Adults are relatively distributed throughout mainland New Zealand, Stewart Island and Chatham Island from intertidal to more than 10 meters[8] with size maturity varies geographically, i.e., those from the Tasman Bay maturing smaller with carapace 50mm width, compared to those from the Pegasus Bay, 65mm.[9] However, fishery reports of this species from Stewart Island were thought to be misidentified as Nectocarcinus spp.[10] and so, research bottom trawl records (trawl database) and some other electronic databases, recRamp, CELR, AllSeaBio database were created to record all crabs of this type (i.e., O.catharus) with at least 55mm carapace width taken to be the minimum adult size.[11]

Ovalipes catharus has also been observed to be highly migratory in an experiment that tagged a number of crabs. This means that populations in New Zealand are not likely to be genetically diverse.[7]

Ovalipes catharus reach sexual maturity within the first year of benthic life and females produce clutches of eggs from early spring to late summer.[14][15] It is not known how many clutches can be fertilised from one insemination, but females have been observed to produce up to four or five without re-mating.[15][16] In one clutch the female crabs produce between 82,000 and 683,000 eggs but like in other crabs a very large proportion of these are lost to disease or predation.[16] Fecundity is also heavily influenced by carapace width and body mass, with larger and heavier crabs having higher fecundity.[16] Larvae develop synchronously and are released in large numbers by vigorous waving of the female's body, which disturbs their egg cases and causes them to break out.[16] Females mostly release their larvae at night and when releasing they stand on their dactyli before extending themselves as far as possible. They then angle themselves up and begin flexing their abdomen to release large clouds of larvae.[16] Females have also been observed to preen the egg mass during development. Higher temperatures have been observed to prolong the breeding season, increase growth rates, and cause them to obtain sexual maturity sooner.[15] This causes slight differences in mating times between populations of Ovalipes catharus living at different locations in the wild.

The population of paddle crabs in New Zealand is believed to be increasing since the late 20th Century, due to their predator numbers decrease due to over-fishing.[12][10] Formal studies investigating these changes have yet to be undertaken.[7]

Ovalipes catharus is a common motif in Māori art, with designs being incorporated into weaving patterns, tā moko and the designs of wharenui and whare wānanga (houses of learning). The crabs are known to be a traditional food source, but researchers in the early Colonial period did not record much about harvesting traditions.[17]

The crabs are known for their aggression on beaches, often pinching swimmers in New Zealand.[17][6] It can be fairly easy to come in contact with the crabs, since their preferences for sandy beaches put them into a similar area as many vacationers.

Paddle crabs have caught in large numbers and sold commercially since the 20th Century. With much of the catch occurring off the course of the North Island, paddle crab has become increasing important economically.[10][7] However, since the exact population is not known, there is the potential for overfishing to occur.[7] Since much of the paddle crab habitat is close to shore, they are fairly easy for recreational fishermen to catch. The paddle crab is known for having meat with both good flavor and texture, which contributes to a large amount of its market success.[10]