Boaties are being warned to check their hulls for "weird, hitch-hiking" skeleton shrimp invading New Zealand waters.
The marine amphipod crustacean, caprella mutica, are poor swimmers, and are spreading rapidly around the country by attaching themselves to boat hulls or drifting algae to move around.
"They readily colonise artificial structures, at times occurring in huge densities on anchored buoys, fish cages, wharves and vessel hulls," National Institute of Water and Atmospheric Research (Niwa) biosecurity scientist Chris Woods said.
"We have observed densities up to 180,000 caprellids per square metre. Boat owners are saying to us, 'what are these waving things all over the hulls of our boats?' when they slip their craft and discover the hull alive with movement."
The creatures, originally from northeast Asia, have spread along coastlines throughout the northern hemisphere in the last 40 years.
They were first detected by Niwa in the Port of Timaru in 2002, and so far, New Zealand seems to be the only southern hemisphere country it has invaded.
Woods said Niwa has detected the caprellids in the Lyttelton Harbour, Port Levy and Pelorus Sound in the Marlborough Sounds.
New populations in the Ports of Dunedin and Bluff have also been discovered, Woods said.
"It will likely spread to most areas of marine human activity throughout New Zealand in the near future, so please take care this summer when transporting your boat between different areas and think about what uninvited guests you may be taking along for the ride.
''Maintaining a clean and antifouled boat hull is one of the best defences we have against the spread of marine invaders and pests."
Woods said it was not known what impact the invader would have on New Zealand's marine biodiversity, but overseas studies showed that it could displace native caprellids and affect food supply to filter-feeding organisms.
The caprellids have long, thing, segmented bodies and short abdomens, so that their legs appear clustered towards their posterior.
The creatures grow to 50mm in length, have two pairs of antennae on their head and the body has multiple segments.
They hold their enlarged claws in a mantis-like pose, and use these for feeding, grasping and fighting.
In the water, they appear to wave as they stand erect, but they are actually trying to catch passing food.
Caprellids graze on algae and other small invertebrates and eat each other when food is scarce.
They gradually change colour based on what they have been eating to match their background.
"The males often have big fights with each other, it's like seeing swinging handbags at dawn,” Woods said.
"Seahorses like to eat them. Caprellids are also an excellent food for many other marine fish because they contain relatively high levels of beneficial polyunsaturated fatty acids."
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