Summer Series 7: Boaties beware of the hitch-hiking, waving ‘alien’ invader

Boaties, beware this summer of a weird hitch-hiker waving at you in the water, as a peculiar marine amphipod crustacean, Caprella mutica, may be freeloading on your boat hull.

These skeleton shrimps are spreading rapidly around New Zealand, relying on boaties or drifting algae for transport because they are poor swimmers, and attaching themselves to aquaculture farms and other structures.

They recently invaded New Zealand and quickly established themselves in our southern waters. It is 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. It was first detected by NIWA in the Port of Timaru in 2002, during baseline biota surveys of major ports and marinas for MAF Biosecurity New Zealand.

The praying mantis of the sea, these unusual invertebrates have long, thin, segmented bodies and short abdomens, so that their legs appear clustered towards their posterior. They grow to 50 mm 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.

In the water, Caprellids appear to 'wave', as they stand erect, but they are actually trying to catch passing food. They have very small mouths but are omnivorous feeders so can filter-feed with their antennae, graze on algae, and scavenge and prey on other small invertebrates. They can also eat each other when normal food is scarce! Interestingly, they can gradually change colour, based on what they have been eating, to match their background.

They are abundant in high current or wave-exposed places. Often in large groups, they attach to substrates using their small posterior legs.

NIWA biosecurity scientist Dr Chris Woods 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"

"The males often have big fights with each other, it's like seeing swinging handbags at dawn," says Dr Woods.

"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," says Dr Woods.

"We have detected this invasive species in the Lyttelton Harbour, Port Levy and Pelorus Sound in the Marlborough Sounds. Most recently, we have observed new populations in the Ports of Dunedin and Bluff," says Dr Woods.

"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 defenses 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: 50 mm

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.

"It's a short-lived, but reproductively quick species," says Dr Chris woods.

Reproduction can occur year-round, but is typically greatest in spring and summer, when populations can boom with warmer temperatures.

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.