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A population of an invasive marine crustacean that "hitch-hikes" on the hulls of boats is living quietly in Otago Harbour.
Caprella mutica or skeleton shrimps are spreading rapidly around New Zealand, relying on boat users or drifting algae for transport as they do not swim well, Niwa biosecurity scientist Dr Chris Woods says.
"They readily colonise artificial structures, at times occurring in huge densities on anchored buoys, fish cages, wharves and vessel hulls. We have observed densities up to 180,000 caprellids per sq m."
In Otago Harbour, a small population was found in October 2010 clinging to the underside of the pontoon off Custom House Quay during the harbour's general six-monthly biosecurity check.
The population had not increased in size since its discovery but could be more prevalent as its favourite habitats were not regularly included in the biosecurity checks, he said.
They had previously been detected in Lyttelton Harbour, Port Levy and Pelorus Sound in the Marlborough Sounds after first being found in the Port of Timaru in 2002.
"It will likely spread to most areas of marine human activity throughout New Zealand in the near future."
Boat owners transporting their vessels between different areas needed to think about what uninvited guests they might be taking along for the ride, he said.
"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."
Maintaining a clean and antifouled boat hull was one of the best defences there was against the spread of marine invaders and pests.
It was not known what impact the "invader" would have on New Zealand's marine biodiversity, but overseas studies had shown that it could displace native caprellids and potentially affect food supply to filter-feeding organisms, Dr Woods said.
Originally from northeast Asia, it had spread in the past 40 years along coastlines throughout the northern hemisphere.
So far, New Zealand seemed to be the only southern hemisphere country it had invaded, he said.
The "praying mantis of the sea", the unusual invertebrates had long, thin, segmented bodies and short abdomens, so their legs appeared clustered towards their posterior.
They hade two pairs of antennae on their head and the body had multiple segments.
They hold their enlarged claws in a mantis-like pose, and use these for feeding, grasping and fighting.
In the water, Caprellids appeared to "wave", as they stood erect, but they were trying to catch passing food.
They had very small mouths but were omnivorous feeders so could filter-feed with their antennae, graze on algae, and scavenge and prey on other small invertebrates.
They were abundant in high current or wave-exposed places and often in large groups, they attached to substrates using their small posterior legs.
"It's a short-lived, but reproductively quick species," Dr Woods said.
Reproduction could occur year-round, but was typically greatest in spring and summer, when populations could boom with warmer temperatures.
"The males often have big fights with each other. It's like seeing swinging handbags at dawn," Dr Woods said.
"Sea horses 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.
Species fact file: alien caprellid
• Scientific name: Caprella mutica.
• Common names: The Japanese skeleton shrimp, spiny red caprellid amphipod, skeleton shrimp.
• Type: Invertebrate.
• Family: Caprellidae.
• Size: 50mm.
• Life span: 1-2 years.
• Diet: They are opportunistic feeders, consuming everything from suspended particles of decaying plants and animals to diatoms, macroalgae, other • crustaceans, and farmed salmon food.
• Reproduction: This species can reproduce within a month of hatching, and large females can produce over 300 eggs in a single brood.
• Things you need to know: This alien species is now well established outside its original range. With its wide environmental tolerances, rapid growth, early reproduction and high population densities, variable feeding habits, and penchant for settling on artificial structures and vessels, it is likely to become widespread in New Zealand.
• Something strange: They can gradually change colour, depending on what they are feeding upon.