Ovalipes catharus

[4][9] It is found in shallow, sandy-bottomed waters around the coasts of New Zealand, the Chatham Islands, and uncommonly in southern Australia.

[1][2][3] O. catharus is an opportunistic, aggressive, and versatile feeder active mostly at night, preying predominantly on molluscs and crustaceans.

[13] The crab's paddle-shaped rear legs and streamlined carapace allow it to capture prey by swimming rapidly and to escape predation by burrowing in the sand.

[19] Ovalipes catharus has an oval-shaped, streamlined, and slightly grainy carapace with five large, sawtooth-like projections to either side of the eyes and four smaller ones at the front.

[1][23] The crab's underside is white, and its rear legs – which are flattened and function as swimming paddles – have a purplish tinge.

[26] Ovalipes catharus' relatively short front legs – the chelipeds – feature spines and granules on the wrists and setae on the posterior border of the arms.

[41] After its megalopal form, the paddle crab has 13 distinct developmental stages, called instars, and reaches its maximum size at 3–4 years old.

[45] Members of the isolated population of O. catharus from the Chatham Islands tend to be larger and take longer to mature than those in mainland New Zealand.

[61][62] Ovalipes catharus hears underwater by using a small canal system located under its first antenna called a statocyst.

[65] It 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.

[69][f] O. catharus additionally closely resembles (and is likely conspecific with) a fossilised cheliped fragment from New Zealand's Upper Pleistocene.

[76] Afterward, males move into large, open beaches in spring, while females migrate to yet-unknown areas – speculated to be deeper spawning grounds for egg incubation.

[11] The diet of Ovalipes catharus consists predominantly of molluscs (especially of genus Paphies), crustaceans, fishes, bristle worms, and algae.

[10] The flattened hind legs and streamlined body shape of the crab allow it to swim at speeds up to 1 m/s (2.2 mph) and catch fast prey.

[27] 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.

[32] Ecologists have raised concerns that the invasive Asian paddle crab Charybdis japonica, as it expands its range in New Zealand, could outcompete O. catharus with its similar size and diet, some overlap in habitat, high aggression, ability to best O. catharus in one-on-one competition for food, and – due to global warming – its better thermal tolerance.

[97] The overwhelming majority of Ovalipes catharus[h] are instead 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.

[99] Ovalipes catharus undergoes a pubertal moult at a carapace width of about 50–60 mm (2.0–2.4 in), reaching sexual maturity within the first year of benthic life.

[38][100][e] Warmer temperatures extend the breeding season, accelerate growth, and lead to earlier sexual maturity, causing variation in mating times between populations.

[105] The zip is accompanied by what may be a courtship display whereby the crab "walks forward and flicks both swimming paddles in a twisting motion.

[113][114] Number of eggs per batch is strongly correlated with carapace width and body mass, with larger and heavier crabs producing more.

[116][106] They are released in large numbers through vigorous waving of the female's body, which disturbs their egg cases and causes them to break out.

[118] Ovalipes catharus is known for its aggression on beaches, often pinching swimmers in New Zealand,[19][1] and paddle crab shells are frequently found washed ashore by beachgoers.

[119][120] The paddle crab is known for having meat with both good flavour and texture,[121][122] and catch is sold both locally in New Zealand and overseas to Japan.

Frontal view of Ovalipes catharus walking on the seabed or potentially resting on a dark-coloured rock. The chelipeds, mouthparts, eyestalks, antennae, walking legs, and some of the dorsal carapace are visible. Fine, white hairs line the area directly underneath the lateral teeth. Fine hairs in the orbits and above the mouth are a sandy brown colour. The bottoms of the eyes are covered in sand.
The front of O. catharus directly under the dorsal carapace is lined with setae . [ 20 ]
Rear view of Ovalipes catharus swimming with its rear, paddle-shaped legs raised and its walking legs spread out laterally. Set against a teal, watery background with no visible seabed.
O. catharus uses its paddle-shaped rear legs to swim. [ 24 ]
Dorsal view of a preserved Ovalipes catharus specimen with the anterior facing the bottom of the frame. A note at the top reads "Norman Douglas collection. Beneath that, another reads: "Ovalipes bipustulatus. Swimming crab. Maketu, Bay of Plenty, 5 April, 1977. Ashore, dead, in a wash up of seaweed on the western shore of the peninsula. N. Douglas." A note to the right of these two, another reads: "AK 78855. Auckland Museum, N.Z. Porcellanidae. Ovalipes catharus. L3523: N.Z., Bay of Plenty, Maketu, on the western shore of the peninsula Washed up, dead in seaweed. c. 37 44.8 S, 176 27.8 E. Coll: Douglas, N., 05 APR 1977. CMROVA.CAT." To the crab's left is an OpCard 201 colour correction chart. A ruler in centimetres is at the bottom of the frame, revealing a carapace width of approximately 100 millimetres. The crab's carapace and chelipeds are a light-yellowish colour with maroon spots. The walking legs and paddles are a dark, sandy brown.
A preserved O. catharus specimen
The off-white shell of a lone specimen of Paphies australis sitting on a background of grey pebbles
Bivalves such as Paphies australis are a significant portion of the paddle crab's diet. [ 78 ]
A video of Ovalipes catharus retreating backward into loose, pebbly sand on the seabed, fully covering itself from back to front over a period of 6.5 seconds. The video is shot from the front at an angle of about 45 degrees to the right relative to where the crab is facing. Brownish seaweed is visible in the foreground of the shot.
O. catharus burrow in the seabed to escape predation. [ 14 ]
A perspective looking out from a sandy, brownish-grey beach (covered in to the dark pebbles to the left and smooth to the right) toward the somewhat-distant mouth of the bay, which is flanked on either side by peninsulas. The left is steep and lightly forested, while the right is shallow and bald except for grass. Very weak waves wash up onto the shore.
During winter and spring, O. catharus meet in sheltered bays such as Little Akaloa for breeding. [ 76 ]