Ovalipes catharus

Ovalipes catharus, commonly known as paddle crab and pāpaka, is a species of crab of the family Portunidae. It is found around the coasts of New Zealand, the Chatham Islands, and in south-eastern parts of Australia. Individuals from shallow waters, 0.1 - 0.5 m deep, have a carapace width of only 10 - 15 mm, while those from 5 - 15 m are 100 - 140 mm wide.

Identification
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. 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. The carapace, or shell, of the crab can range from pale orange to light gray, and can be spotted with brown and red. The carapace is a rough oval shape, and the front end has five teeth to either side of the eyes. Paddle crabs can reach sizes ranging from 100–140 mm across the shell upon reaching maturity. 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. 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.

Geographic distribution and habitat
Paddle crab (O. catharus) found to be quite a unique species to which is native to New Zealand and Australia. 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. 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 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. However, fishery reports of this species from Stewart Island were thought to be misidentified as Nectocarcinus spp. 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.

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.

Diet and foraging
Paddle crabs consume a wide variety of organisms. Much of their diet is made up of mollusks and crustaceans, but they have also been observed catching and eating fish as well. Crabs have been found to consume vegetative tissues that they find while they are foraging in the sediment. Cannibalism of smaller crabs occurs as well, especially in the winter when other food sources are more limited. Some larger crabs are also cannibalized as well, especially if they have recently moulted, and are lacking the hard outer shell. Paddle crabs hunt in several ways. The crabs spend much of their time scavenging for food in the sediment layers, but will also actively hunt fish, shrimp and other creatures. The flattened hind legs of the crab allow them to swim rapidly, which aid them in catching faster prey. These paddles also allow the crabs swim in water up to 10 meters deep, giving them a larger hunting range than other crab species that can be limited to shallow water. The crab’s claws are also extremely important for their success in hunting. Not only can they be used to catch fish, but the claws are extremely important for accessing other prey as well. These crabs are very good at harvesting mussels and other bivalves, In order to get to the soft flesh inside, the paddle crab can either pry the shell of the mussels open with its claws, or in the case of smaller mussels, the crab’s claws are strong enough to crush the shell. With all of these tools at its disposal, the paddle crab is one of the more dominant predators of the intertidal zone.

Predators, parasites, and diseases
Large fish such as dogfish and snapper make up the majority of the predators that usually prey upon Ovalipes catharus. Along with these predatory fish, larger crabs can also cannibalize younger paddle crabs. When it comes to parasites, nematodes and bryozoans are the main groups that parasitize these crabs. These parasites reach the highest density on mature crabs that no longer moult. However, the type and intensity of parasitism can differ depending on the setting that the crab is living. In a study looking at the crossing over of parasites between native and invasive crabs, it appears that Ovalipes catharus has not been affected by new parasites brought by invasive species. Since paddle crabs come in contact with an invasive species of crab, Charybdis japonica, they risk being exposed to new parasites brought by the invasive species that they have little to no resistance to. However, in the study when comparing crabs caught in the same site, there were no similar parasites between the species.

Mating and Reproduction
Ovalipes catharus reach sexual maturity within the first year of benthic life and females produce clutches of eggs from early spring to late summer. 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. 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. Fecundity is also heavily influenced by carapace width and body mass, with larger and heavier crabs having higher fecundity. 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. 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. 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. This causes slight differences in mating times between populations of Ovalipes catharus living at different locations in the wild.

Mating Behaviors
Males and females meet in shallow sheltered bays during winter, which is July to August in New Zealand. The time is also variable between populations with some mating as early as May and some as late as November. Males often attract mates by creating a rasping sound, by rubbing their legs together vigorously.

In these shallow bays they mate when the females molt. Male crabs can only mate with soft post-molt females, so they begin to carry female crabs under their bodies up to 8 days prior to the females molting. Once the female has molted, she depends entirely on the male for protection, and he will even assist with the molting process by using his legs to remove parts of the female's old carapace. Once molting is complete they will mate for between 12 and 36 hours and even up to four days. After mating the female is released and the females move onto spawning grounds deeper in the water, of which the location is not known, and the males move to foraging grounds.

Sexual cannibalism
Sexual cannibalism has been frequently observed in Ovalipes catharus, and it is directed by males towards females. This sexual cannibalism occurs when the female is soft-shelled after molting during the mating process. Male crabs protect the females they mate with from being cannibalised but unreceptive females or those that have mated with other crabs are in danger of being cannibalised. This is most likely done to maximize the number of eggs fertilised by his sperm. Some examples of males cannibalising females they have mated with have also been observed. This sexual cannibalism is not about mate choice or adaptation and is instead more opportunistic. In one example male crabs that had not cannibalised female crabs all accepted frozen squid afterwards, while very few males that had cannibalised female crabs accepted frozen squid afterwards. This indicates that hunger could be a part of the cause behind the cannibalism, and male crabs just take the opportunity to feed while the females are vulnerable in their soft-shelled state. Unlike in other examples of sexual cannibalism, size was not shown to have an effect on whether cannibalism occurred. Sexual cannibalism in Ovalipes catharus is very prevalent with studies on the diet of the crab detecting it in almost every location tested. Part of the prevalence of Ovalipes catharus in its own diet could be due to males consuming each other during the breeding season, as the smaller males guarding females do not have protection themselves from cannibalism and can sometimes be mistaken for females.

Population
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. Formal studies investigating these changes have yet to be undertaken.

In a human context
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.

The crabs are known for their aggression on beaches, often pinching swimmers in New Zealand. 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. However, since the exact population is not known, there is the potential for overfishing to occur. 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.