Ovalipes catharus

Ovalipes catharus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Malacostraca
Order:	Decapoda
Suborder:	Pleocyemata
Infraorder:	Brachyura
Family:	Ovalipidae
Genus:	Ovalipes
Species:	O. catharus
Binomial name
Ovalipes catharus (White in White and Doubleday, 1843)
Synonyms[1]
Portunus catharus White in White and Doubleday, 1843

Ovalipes catharus, commonly known as the paddle crab, swimming crab, or Māori: pāpaka,^[2] is a species of crab in the family Ovalipidae.^[1][3] They are found in shallow, sandy-bottomed waters around the coasts of New Zealand, the Chatham Islands, and uncommonly in southern Australia.^[4][5][6] They are opportunistic and versatile feeders active mostly at night, preying predominantly on molluscs and crustaceans.^[7][8] They are also highly prone to cannibalism both as part of their regular diet and as part of their mating behaviour.^[7][9] Their paddle-shaped rear legs and streamlined carapace allow them to swim rapidly to capture prey and to burrow in the sand in order to escape predation.^[10] Their mating season is in winter; the male carries the female until she moults, after which the two mate and the female moves into deeper waters to disperse her larvae.^[11]

The paddle crab has been targeted by commercial fisheries since the 1970s, although the amount caught has dramatically decreased since a peak in the late 1990s.^[8] Their population is expected to be increasing,^[8] although ecologists have raised concerns that the paddle crab could become outcompeted by Charybdis japonica, an invasive species with a similar size, diet, and habitat.^[12] O. catharus is present in Māori culture, both as an artistic motif and as a traditional source of food.^[13]

Ovalipes catharus has an oval-shaped, streamlined, and slightly grainy carapace with five teeth to either side of the eyes and four teeth at the front.^[4][10][15] It is overall sandy grey with orange-red highlights and dotted with small, brown spots.^[4][15] Its carapace has two large, maroon eye-spots at the rear and two smaller ones near the front.^[4][16] Its underside is white, and its rear legs – which are flattened and function as swimming paddles^[14] – have a purplish tinge.^[4] Unlike about half of known Ovalipes species, O. catharus exhibits no iridescence as a form of signalling.^[17]

Mature paddle crabs can reach carapace widths ranging from >100 mm (3.9 in) up to a reported 150 mm (5.9 in).^[8] Individuals from shallow waters, 0.1–0.5 metres (4–20 in) deep, have a carapace width of only 10–15 millimetres (0.4–0.6 in), while those from 5–15 m (16–49 ft) are 100–140 mm (3.9–5.5 in) wide.^[18] Its left chela is smaller than its right.^[19]

It has a long period of larval development with eight zoeal stages. The zoea is transparent or blackish, later develops red chromatophores, and then turns black when it moults into a megalopa.^[20] The zoea features a prominent dorsal spine and similarly prominent rostral spine; neither are present in its megalopal form, whose carapace is entirely smooth.^[20]

Ovalipes catharus was described in 1843 by zoologist Adam White, originally placed into the genus Portunus.^[15] They are colloquially known as the paddle crab, the swimming crab, or Māori: pāpaka.^[2] The following cladogram shows the relationship between O. catharus and the other extant species of Ovalipes:^[17][a]

Ovalipes catharus is native to New Zealand, where it can be found from Stewart Island to Northland and in the Chatham Islands.^[21][14] They are also uncommon on the southern coast of Australia, where they're known as far west as the state of South Australia and as far east as Port Phillip Bay in Victoria.^[4][5][20] They live along sandy-bottomed coastal waters, generally at depths of <10 m (33 ft) in estuaries and the subtidal zone,^[21][6][22] and they move into the intertidal zone during the evening or the night in order to feed.^[8] Although they generally stick to shallow waters, they can be found at depths of up to 100 m (330 ft),^[21] and their larvae can be found in deeper waters, up to at least 700 m (2,300 ft).^[8] They have been observed to be highly migratory.^[8] Anecdotal information suggests a dramatic population increase since the 1970s.^[8]

The diet of paddle crabs consists predominantly of molluscs (especially of genus Paphies) and crustaceans, but they also feed on fish, bristle worms, and occasionally algae.^[7][8][14] Large Ovalipes catharus tend to feed less frequently but on larger animals such as decapods and teleosts, while smaller ones prey frequently on smaller, softer crustaceans such as amphipods, isopods, opossum shrimp, and hooded shrimp.^[14] A major portion of their diet is cannibalizing other members of O. catharus which are either smaller than them or have lost their shell during the winter moulting season.^[7][14]

The flattened hind legs and streamlined body shape of the crab allow them to swim rapidly and catch faster prey,^[14][10] and they have relatively small chelae which are well-suited to handling small molluscs.^[23] Its chelae are dimorphic; the left is used for cutting while the right is used for crushing.^[19]

Predators of the paddle crab include spiny dogfish,^[24] snapper (Pagrus auratus), rig (Mustelus lenticulatus),^[25], groper (Polyprion oxygeneiosis), Hector's dolphin,^[26] and the invasive crab species Charybdis japonica.^[12] Younger individuals are prone to being cannibalized,^[7][14] and all paddle crabs are vulnerable to cannibalism during moulting.^[14] Commercial fisheries additionally target the paddle crab.^[8] In order to escape predation, Ovalipes catharus creates temporary burrows in soft sand using their paddles, taking only several seconds to completely submerge itself.^[10]

O. catharus do not appear to be typically parasitized by nematodes or barnacles.^[22] Instead, the overwhelming majority of them^[b] are hosts to the ctenosome bryozoan Triticella capsularis, which forms a fur of up to almost 10 mm (0.4 in) thick on their underside after their final moult.^[27][22] It is only found on O. catharus,^[27] and it is speculated to be an obligate symbiont of the crab.^[22] O. catharus does not appear to be affected by parasites present in the invasive C. japonica, which tends to be sympatric with the paddle crab and is heavily parasitized by serpulids.^[22]

Ovalipes catharus reach sexual maturity within the first year of benthic life, and females produce clutches of eggs from early spring to late summer.^[28][14] 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.^[14][11] 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.^[11] Fecundity is also heavily influenced by carapace width and body mass, with larger and heavier crabs having higher fecundity.^[11] 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.^[11] 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.^[11] 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.^[14] This causes slight differences in mating times between populations of O. catharus living at different locations in the wild.^[14]

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.^[13][11]

In these shallow bays, they mate when the females moult. Male crabs can only mate with soft post-moult females, so they begin to carry female crabs under their bodies up to 8 days prior to the females moulting.^[11] Once the female has moulted, she depends entirely on the male for protection, and he will even assist with the moulting process by using his legs to remove parts of the female's old carapace. Once moulting is complete they will mate for between 12 and 36 hours and even up to four days.^[11] 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.^[14]

Sexual cannibalism has been frequently observed in O. catharus, and it is directed by males towards females. This sexual cannibalism occurs when the female is soft-shelled after moulting during the mating process.^[9] 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.^[9] This is most likely done to maximize the number of eggs fertilised by his sperm.^[29] Some examples of males cannibalising females they have mated with have also been observed.^[9] This sexual cannibalism is not about mate choice or adaptation and is instead more opportunistic.^[30][31][32] 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.^[9][30] Sexual cannibalism in O. catharus is very prevalent, with a study on the diet of the crab detecting it in almost every location tested.^[7] Part of the prevalence of O. 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.^[9]

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.^[13]

Commercial fisheries have targeted targeted paddle crabs since the late 1970s, mostly to the east of the North Island and the north of the South Island.^[14][8] Catch is sold both locally in New Zealand and overseas to Japan.^[33] The paddle crab is known for having meat with both good flavor and texture.^[14] The amount of paddle crabs landed generally increased until the late 1990s, reaching a peak at 519 t (1,144,000 lb) in 1998–99, at which point it began generally decreasing for the next two decades, reaching an average of 16.6 t (37,000 lb) annually from the five-year period of 2017–2022.^[8]

They are known for their aggression on beaches, often pinching swimmers in New Zealand.^[13][4]

• Media related to Ovalipes catharus at Wikimedia Commons
• Data related to Ovalipes catharus at Wikispecies