The National Institute of Water and Atmospheric Research is warning boaties to check the hull of their vessels for a skeletal hitchhiker.

Caprella mutica is a marine amphipod crustacean which is spreading rapidly through the country's waterways.

Caprellids on a boat's hull. Photo: Chris Woods

NIWA says the skeleton shrimp relies on boaties or drifting algae for transport because they are poor swimmers.

They also attach themselves to aquatic farms and other structures.

Biosecurity scientist Dr Chris Woods says they recently invaded New Zealand and quickly established themselves in southern waters.

He says it's not known what impact this invader will have on New Zealand's marine biodiversity, but overseas studies have shown that it can displace native caprellids and potentially affect food supply to filter-feeding organisms.

Originally from northeast Asia, in the last 40 years it has spread along coastlines throughout the northern hemisphere.

So far, New Zealand seems to be the only southern hemisphere country it has invaded, says Chris.

The shrimp are described as the praying mantis of the sea, with long, thin, segmented bodies and short abdomens, so their legs appear clustered towards their posterior. They grow to 50mm in length.

They 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.

Chris says they readily colonize 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 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.”

Chris says it will likely spread to most areas of marine human activity throughout New Zealand in the near future.

'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.”

Species Fact File: Alien Caprellid

Common names
The Japanese skeleton shrimp, Spiny red Caprellid amphipod, skeleton shrimp

Scientific name
Caprella mutica

Type

Invertebrate

Family

Caprellidae

Size

50mm

Lifespan

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.