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Farming green-lipped mussels – introduction

Green-lipped mussels (kūtai, Perna canaliculus) are endemic to New Zealand. Discover how these mussels are farmed, and how a tiny parasitic crab affects the mussel industry.

New Zealand’s unique green-lipped mussels

Green-lipped mussels thrive in New Zealand’s clean, nutrient-rich waters. They grow abundantly in the intertidal zone and in shallow coastal waters. The article Life of a green-lipped mussel details their transition from a free-swimming form to the immobile adult form that is a familiar sight on rocks and wharf piles.

Pile of freshly harvested green-lipped mussels.

Green-lipped mussels

Freshly harvested green-lipped mussels.

Rights: Tom Coates

Mussel farming: a very Kiwi industry

New Zealand’s mussel farming industry began in the late 1960s, after overfishing caused the collapse of wild mussel harvesting. Originally, individuals set up mussel farms on a small scale, sharing ideas and experimenting with new technologies. The industry has developed rapidly – now, green-lipped mussels (exported under the trade name GreenshellTM) are New Zealand’s most important farmed marine organism. They account for about three-quarters of New Zealand’s total earnings from aquaculture exports (with king salmon and Pacific oysters accounting for most of the remainder). Learn more about mussel farming in New Zealand’s green-lipped mussel industry.

The mussel and the crab: an ancient relationship

Ever opened a green-lipped mussel to find a tiny crab nestled inside? That’s the New Zealand pea crab. It lives within mussels and other crustaceans. In some mussel populations, well over half the mussels contain a pea crab.

A female pea crab inside an opened male mussel.

Pea crab inside mussel

A female pea crab inside an opened male mussel. Pea crabs live alone within mussels throughout their adult lives. Female pea crabs never leave their mussel hosts, while males leave only to fertilise the eggs of a female within another mussel.

Rights: The University of Waikato Te Whare Wānanga o Waikato

New Zealand pea crabs are parasites – they have a negative impact on their host mussels. Pea crabs steal phytoplankton that mussels have collected for food, damage the mussel’s gills and cause nodules on the mussel shell.

Until recently, it wasn’t known how pea crabs affected the mussel industry, but now, Oliver Trottier, a scientist at Leigh Marine Laboratory (University of Auckland), has shown that pea crab infestation makes farmed mussels up to a third lighter. This affects the price that mussel farmers can command for their product and ultimately decreases the industry’s profits. Read about Oliver's research in Testing how pea crabs affect mussel farming.

Getting to grips with pea crabs

Oliver and his colleague Jessica Feickert (also a scientist at Leigh Marine Laboratory) have been investigating the biology of the pea crab. They are looking for ways to decrease the level of pea crab infestation on mussel farms.

Oliver and Jessica have shown that male pea crabs may use pheromones to locate female crabs at mating time. They have discovered previously unseen stages in the pea crab’s life cycle, and in the process, they’ve learned a lot about the surprising habits of the pea crab – such as how they seem to ‘tickle’ a mussel for hours on end to get it to open its shell!

The articles Life of a pea crab and Getting into and out of mussels add insight into these interesting creatures.

Testing the pheromone hypothesis

Oliver Trottier (Leigh Marine Laboratory) set out to test whether pea crabs use pheromones at mating time. In this video, Oliver talks us through his experimental set-up. He demonstrates how male and female pea crabs (each in their host mussels) were placed close to each other in flowing seawater and shows how he used a trap to test whether male crabs were leaving their hosts in response to a signal from female crabs.

Teaching point:
Try showing this clip without sound. You could ask students to write about, draw a sketch of or explain Oliver’s experimental design. What are the strengths of Oliver’s experimental set-up?

Point of interest:
Leigh Marine Laboratory is situated right beside the sea, within the Cape Rodney-Ōkakari (Goat Island) Marine Reserve. This site means that the laboratory has access to piped seawater from the ocean. It is this seawater that is flowing through the pheromone tubes in Oliver’s experiment.

Rights: © Copyright 2013. University of Waikato. All rights reserved.

Testing the pheromone hypothesis

Oliver Trottier (Leigh Marine Laboratory) set out to test whether pea crabs use pheromones at mating time. In this video, Oliver talks us through his experimental set-up. He demonstrates how male and female pea crabs (each in their host mussels) were placed close to each other in flowing seawater and shows how he used a trap to test whether male crabs were leaving their hosts in response to a signal from female crabs.

Teaching point:
Try showing this clip without sound. You could ask students to write about, draw a sketch of or explain Oliver’s experimental design. What are the strengths of Oliver’s experimental set-up?

Point of interest:
Leigh Marine Laboratory is situated right beside the sea, within the Cape Rodney-Ōkakari (Goat Island) Marine Reserve. This site means that the laboratory has access to piped seawater from the ocean. It is this seawater that is flowing through the pheromone tubes in Oliver’s experiment.

Rights: © Copyright 2013. University of Waikato. All rights reserved.

Could biocontrol keep pea crabs out of mussels?

Biocontrol of the New Zealand pea crab explains how Oliver and Jessica hope to use their findings to develop a biocontrol approach that limits pea crab numbers on mussel farms. They are focusing on developing a pheromone trap that could confuse male pea crabs and stop them from finding females to mate with. They are also looking at ways to stop larval pea crabs from settling in farmed mussels.

A pheromone trap for pea crab biocontrol

Scientists at Leigh Marine Laboratory, including Oliver Trottier, are investigating biocontrol approaches that could limit pea crab infestation on green-lipped mussel farms. In this video, Oliver describes how one biocontrol approach – a pheromone trap – might stop male pea crabs from finding females that are ready to mate. He also describes some of the advantages to using biocontrol, rather than other control methods, to limit pea crab numbers.

Focus question:
What are the advantages of biocontrol of pea crabs over other control methods?

Teaching point:
Students could design a pheromone trap for trapping male pea crabs on mussel farms. Some points to consider:

  • Where on the farm could the trap be placed?

  • How could crabs be prevented from leaving the trap?

  • How could the pheromone be released?

  • How could any distress to the male crabs be minimised?

Rights: © Copyright 2013. University of Waikato. All rights reserved.

Take up the challenge

The activities Investigating how pea crabs affect supermarket mussels, Similarities and differences: wild and farmed green-lipped mussels and Finding a female give students the opportunity to learn more about mussels, pea crabs and their impact on the aquaculture industry.

Published:25 June 2013