The skeleton crustacean hitch-hiking its way around New Zealand on the hulls of boats. Photo by Chris Woods.
A population of an invasive marine crustacean that
"hitch-hikes" on the hulls of boats is living quietly in
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
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
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
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
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
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
• Reproduction: This species can reproduce within a month of
hatching, and large females can produce over 300 eggs in a
• 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.