Ovalipes catharus, commonly known as the paddle crab, swimming crab, or, is a species of crab in the family Ovalipidae. They are found in shallow, sandy-bottomed waters around the coasts of New Zealand, the Chatham Islands, and uncommonly in southern Australia. They are opportunistic and versatile feeders active mostly at night, preying predominantly on molluscs and crustaceans. They are also highly prone to cannibalism both as part of their regular diet and as part of their mating behaviour. 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. 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. Commercial fisheries have harvested paddle crabs since the 1970s, with catches declining considerably from a peak in the late 1990s. Their population is expected to be increasing, 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. O. catharus is present in Māori culture, both as an artistic motif and as a traditional source of food.

Description

Ovalipes catharus has an oval-shaped, streamlined, and slightly grainy carapace with five large teeth to either side of the eyes and four teeth at the front. It is overall sandy grey with orange-red highlights and dotted with small, brown spots. Its carapace has two large, maroon eye-spots at the rear, two smaller eye spots near the front, and cervical grooves which form a butterfly-shaped mark near the centre. Its underside is white, and its rear legs – which are flattened and function as swimming paddles – have a purplish tinge. The area above its mouth near the base of the antennae is somewhat hairy, and a line of setae runs from the base of its deep orbits out to the area underneath the carapace teeth. Like other Ovalipes, O. catharus has well-developed, relatively large eyes. Unlike about half of known Ovalipes species, however, it exhibits no iridescence as a form of signalling. Ovalipes catharus' relatively short chelipeds feature spines and tubercles on the wrists and setae on the posterior border of the arms. The left (minor) chela is smaller than its right (major), and both dactyli are slender and tapered. The minor cheliped grows in direct proportion to the carapace width in females, but it may exhibit negative allometry in males. The minor chela, used for cutting, is lined with small, conical teeth on both fingers, while the major chela also features a large proximal tooth used for crushing. It has three pairs of walking legs, which are somewhat granular and relatively flat. The flattened rear paddles are fringed with setae. Mature male paddle crabs can reach carapace widths ranging from >100 mm up to a reported 150 mm. Individuals from shallow waters, 0.1 - 0.5 m deep, have a carapace width of only 10 - 15 mm, while males from 5 - 15 m are 100 - 140 mm wide. The largest males weigh around 600–700 g. Sexually mature females typically have a carapace width of >70 mm and are known to be as wide as about 100 mm. Abdomen growth in males and juvenile females is isometric with carapace width, but after the pubertal moult, the female's abdomen exhibits positive allometry. Relative length diminishes compared to the width as the crab grows. On average, the carapace is about 1.35x as broad as it is long. 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. The zoea features a prominent dorsal spine and similarly prominent rostral spine; neither are present in its megalopal form, whose carapace is entirely smooth. Paddle crabs have 13 instars after their megalopal form and reach their maximum size at 3–4 years old.

Internal anatomy

Ovalipes catharus is either an osmoconformer or a weak osmoregulator. It can reverse its ventilatory flow by adjusting the sizes of apertures located at the bases of its legs. These apertures lead into the branchial chamber and are covered in dense setae for filtration. Unlike in most decapods, this period of reversed flow can be sustained, and it is commonly seen when the crab is buried or at rest. Its heart is a single ventricle which ejects hemolymph to seven arteries. Five arteries, including the anterior aorta, leave the heart anteriorly and supply organs such as the cerebral ganglion, eyes, antennae, hepatopancreas, and various digestive organs. One, which leaves the heart ventrally, is called the sternal artery and accounts for nearly 70% of flow; this branches into vessels which supply its five pairs of legs, the largest of which are those supplying its rear paddles. Finally, a relatively small posterior aorta runs down the middle of the crab's abdomen. Ovalipes catharus is a stenotherm. An increase in water temperature of just a few degrees substantially accelerates their growth. At summer temperatures of about 20 C, O. catharus' heart rate is approximately 50 bpm. Above this temperature, its heartbeats begin to shorten. Its heart rate is more than doubled to 125 bpm at 25 C, and temperatures around 30 C are fatal. Phosphorylation of ADP during respiration also decreases at temperatures over 20 C, indicating reduced ability of the mitochondria to produce ATP. At temperatures around 10 C – near the lower end of what it experiences in the wild – O. catharus needs to be actively encouraged to eat, juveniles eat less, and it takes over three times as long to digest its food as it does at 20 C. Ovalipes catharus hears underwater by using a small canal system located under its first antenna called a statocyst. The statocyst contains an agglomerate of sand particles called the statolith and functions similarly to the otolith in vertebrates. They are known to be able to hear sounds between at least 40–2000 Hz, but they are especially sensitive to the range between 100–200 Hz. O. catharus uses a yet-unknown internal mechanism to create a broad-frequency, multi-pulse "rasp" sound which is hypothesised to communicate food availability to other members of the species. Males additionally use a yet-unknown internal mechanism to produce a sub-bass sound used in their mating behaviour.

Taxonomy

Ovalipes catharus is colloquially known as the paddle crab, the common swimming crab, or. They were described in 1843 by zoologist Adam White, originally placed into the genus Portunus. In 1968, marine biologists William Stephenson and May Rees identified, on the basis of its colour patterns, that it actually belonged in Ovalipes. Having been synonymised with O. punctatus alongside three other species prior to 1968, O. catharus is part of a distinct subgroup of Ovalipes which also includes O. australiensis, O. elongatus, O. georgei, O. punctatus, and O. trimaculatus. O. catharus additionally closely resembles (and is likely conspecific with) a fossilised cheliped fragment from New Zealand's Upper Pleistocene. Three aspects taken together reliably distinguish O. catharus from other members of Ovalipes: fine granules on the raised ridges of the top side of its hands, moderately fine stripes on the underside of its hands, and a notably broad carapace (~1.35x broader than long). The following cladogram shows the relationship between O. catharus and the other extant species of Ovalipes:

Distribution and habitat

Ovalipes catharus is native to New Zealand, where it can be found from Stewart Island to Northland and in the Chatham Islands. They are also uncommon on the southern coast of Australia, where they are known as far west as the state of South Australia and as far east as Port Phillip Bay in Victoria. They live along sandy-bottomed coastal waters, generally at depths of <10 m in estuaries and the subtidal zone, and they move into the intertidal zone during the evening or the night in order to feed. They are typically buried under the sediment during the day. Although they generally stick to shallow waters, they can be found at depths of up to 100 m, and their larvae can be found in deeper waters, up to at least 700 m. They have been observed to be highly migratory, moving from sheltered bays during the winter breeding season into large, open beaches in the spring. Anecdotal information suggests a dramatic population increase since the 1970s.

Diet

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. 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. They frequently cannibalise smaller conspecifics and those that have recently molted during winter. They tend to eat more during the summer than during the winter. The flattened hind legs and streamlined body shape of the crab allow them to swim rapidly – over 1 m/s – and catch faster prey, and they have relatively small chelae which are well-suited to handling small molluscs. Their chelae are dimorphic; the left is used for cutting while the right is used for crushing. The paddles also allow the crab to stabilise itself and balance on its third pair of walking legs when digging bivalve prey out of the sand.

Predators and other interactions

Predators of the paddle crab include spiny dogfish, snapper (Pagrus auratus), rig (Mustelus lenticulatus), groper (Polyprion oxygeneiosis), Hector's dolphin, and the invasive crab species Charybdis japonica. Younger individuals are prone to being cannibalised, and all paddle crabs are vulnerable to cannibalism during moulting. Commercial fisheries additionally target the paddle crab. In order to escape predation, Ovalipes catharus creates temporary burrows in soft sand using their paddles, taking only several seconds on average to completely submerge itself. It rests horizontally under about 10–20 mm of sand, sometimes leaving its eyestalks poking out. Ovalipes catharus does not appear to be typically parasitised by nematodes or barnacles. Instead, the overwhelming majority of them are hosts to the ctenosome bryozoan Triticella capsularis, which forms a fur of up to almost 10 mm thick on their underside after their final moult. It is only found on O. catharus, and it is speculated to be an obligate symbiont of the crab. 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 parasitised by serpulids. Ecologists have raised concerns that, as C. japonica continues to expand its range, it could outcompete O. catharus with its similar size, diet, and habitat as well as – due to global warming – its better thermal tolerance.

Mating and reproduction

Ovalipes catharus undergo a pubertal moult at a carapace width of about 40 mm in males and about 30–40 mm in females. They 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 maintain and clean their egg masses throughout development. Warmer temperatures extend the breeding season, accelerate growth, and lead to earlier sexual maturity. This causes slight differences in mating times between populations of O. catharus living at different locations in the wild.

Mating behaviours

Males and females meet in shallow sheltered bays during winter – July to August in New Zealand – when the water temperature is ~15 C. The time is also variable between populations with some mating as early as May and some as late as November. In response to male competition near a receptive female, males become aggressive and communicate using sounds, although it is unknown if these are directed toward the female, the competing males, or both. It alternates between two sounds: a multi-pulse, low-mid frequency "zip" sound – created by rubbing a plectrum-like joint on its first walking legs against ridges on the underside of its chelae; and a sub-bass vibration – accompanied by periodic swaying but produced by a yet-unknown internal mechanism. 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. 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. 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. The females of a group may spawn at the same time or asynchronously throughout the season depending on their location.

Sexual cannibalism

Males of O. catharus commonly practice sexual cannibalism toward females. This sexual cannibalism occurs when the female is soft-shelled after moulting 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 females readily accepted frozen squid, while those that had engaged in cannibalism rarely did. 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 O. catharus is very prevalent, with a study on the diet of the crab detecting it in almost every location tested. 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.

Relation to humans

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. Commercial fisheries have targeted paddle crabs since the late 1970s, mostly to the east of the North Island and the north of the South Island. Catch is sold both locally in New Zealand and overseas to Japan. The paddle crab is known for having meat with both good flavour and texture. The amount of paddle crabs landed generally increased until the late 1990s, reaching a peak at 519 t in 1998–1999, at which point it began generally decreasing for the next two decades, reaching an average of 16.6 t annually from the five-year period of 2017–2022. Whereas the majority of catch in the 1990s and 2000s came from the east coast of North Island and the west coast of South Island, this declined steeply in the 2010s, and catch in the 2020s has so far come almost exclusively from the east coast of South Island. They are known for their aggression on beaches, often pinching swimmers in New Zealand.