New Zealand has a popular paddle crab (scientific name Ovalipes catharus) found at sandy beaches all round the country and often in fish shops, sometimes alive. I talked about paddle crabs and why they’re important during December’s episode of Science Live, Coastal Creatures. 

Paddle crabs or swimming crabs are named for their back pair of legs which are paddle-shaped rather than pointed like its other legs. They use these paddles to swim and to burrow in sand.

There are hundreds of paddle crab species world wide. New Zealand has about 12 although you are unlikely to spot most of them because they are small or live offshore.

But over the years paddle crabs that normally live in other places have turned up in New Zealand, mainly in the warmer water up north. Paddle crabs are interesting examples of marine species that invade our coastal water naturally, or accidentally by ship.

Paddle crabs are often beautiful, definitely good to eat and important members of the crab fauna all over the world.

In the year 2000 an Asian paddle crab (scientific name Charybdis japonica) was discovered in fishing nets in Auckland Harbour. Its numbers have grown and it has made a home there and in other northern harbours. It seems to be here to stay. The Asian paddle crab was not recorded in New Zealand before 2000 so must have been introduced, probably by ship. There are different ways this can happen.

If the bottom of a ship or smaller boat is covered with marine growth, crabs and other animals may cling to it and travel to other places.

Ships also need to pump in water for ballast to keep them balanced whether full of cargo or people, half-full or empty. They have ballast tanks inside and pump sea water in and out of them as required. Many marine species like paddle crabs, have tiny larvae in their life cycles that live in the plankton. They may get sucked into ballast tanks somewhere else and get pumped out again when the ship reaches its destination. If they survive this, larvae may grow to become adults in their new home.

And there’s another way paddle crabs use this water pumping system for free travel; sea-chests. Water isn’t simply pumped into ships from the outside. There are tank-like recesses in the hulls of ships, open to the outside through grills, which provide a calmer reservoir for the ship’s water needs. Water for ballast, engine cooling or fire-fighting is pumped to and from the sea-chests, through sieves that keep out anything larger than plankton. Sea-chests are also ideal places for paddle crabs to crawl into and live while the ship steams from one port to another.

A specimen of the small paddle crab Charybdis hellerii, was found in a sea chest on a vessel in Nelson. The crab was alive and carrying eggs. Fortunately, this is a tropical species picked up in the tropics so very unlikely to survive in New Zealand. But this discovery made it quite obvious that crabs and other animals can live and travel far in sea chests. And female specimens with eggs must have a much greater potential to invade when the eggs hatch.

There are a couple more, large overseas paddle crab species that are seldom found, but do turn up occasionally; the mud or mangrove crab Scylla serrata and the blue swimmer or blue manna crab Portunus pelagicus. Home for both species is East Africa, Australia, Japan and French Polynesia. How these two crabs get to New Zealand is uncertain. Whether they actually live here or get re-introduced every so often is also unknown.

The mud crab specimen photographed is the latest find, caught by a fisherman in an estuary near Leigh north of AucklandCity. This crab, caught in December 2013, is still living happily in an aquarium at the public Information Centre at AucklandUniversity’s Leigh Marine Laboratory, at Leigh.

Perhaps mud crabs live and reproduce in the north but they seem hardly common enough for that. Perhaps they grow to be adults that don’t reproduce after drifting here from Australia or Lord Howe Island as larvae.

The same may be so for the blue swimmer (Portunus pelagicus) whose larvae have actually been found off eastern Northland. New Zealand sightings of adult crabs of this species are extremely rare.

If you find a paddle crab that doesn’t look like the local species, you can send questions about its identity, photographs or drawings to RickW@tepapa.govt.nz. This would be much appreciated as we are keen to build up as much information about invasive crabs as possible.