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Rights: © Copyright 2013. University of Waikato. All rights reserved.
Published 20 June 2013 Referencing Hub media
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To work out how pea crabs affect mussel growth, Oliver Trottier (Leigh Marine Laboratory) collected thousands of green-lipped mussels from a single farm off Great Barrier Island. In this video, Oliver describes the results of his experiment. He explains the evidence that New Zealand pea crabs are parasites of (and therefore have a negative impact on) their green-lipped mussel hosts.

Jargon alert:
A commensal organism is one that benefits from its interaction with another organism but does not harm the other organism. Commensalism differs from parasitism, in which one organism benefits (in this case, the New Zealand pea crab) whereas the other is harmed (the green-lipped mussel).

Teaching points:
What is a parasite? Students could research the features of a parasite-host relationship and how it differs from commensalism.
How does the New Zealand pea crab affect its host mussel? While watching the video, students could list the effects that Oliver describes.
Students could watch the video Mussels are filter feeders after watching this video. Why might gill damage limit mussel growth?

Point of interest:
It wasn’t always recognised that pea crabs can harm their hosts. In fact, Aristotle called the pea crab the “guardian of the pinna” [pinna = a type of mussel] because he thought that pea crabs were important for the survival of the mussels they lived in.

Transcript

Oliver Trottier (Leigh Marine Laboratory)
My research has shown that the presence of the pea crab within the mussels in an aquaculture setting reduces the size of the mussel by about 30%. I found that traditionally people think that they are commensals, that they’re not parasitic, and it’s taken a certain body of literature to convince people that they are.

We actually went out into a mussel farm, took a very, very large sample – it was about 7000 green-shells – brought that back, ran some pretty rigorous statistics on it and came up with this value of approximately a 30% reduction in shell size and height.

Some of the impacts that the crabs have on the hosts, they are a true parasite. What they’re doing is they’re stealing food that the mussel is filtering out of the water column, so they’re stealing food and resources from the mussel, which is likely what causes it to be smaller, but they also cause gill damage, and this can reduce the rate that the mussel can respire.

Acknowledgements:
Oliver Trottier – Leigh Marine Laboratory, Auckland University.

Impact of the parasitic pea crab Pinnotheres novaezelandiae on aquacultured New Zealand green-lipped mussels, Perna canaliculus.
Oliver Trottier, Dion Walker, Andrew G Jeffs.
Elsevier, Aquaculture, Volumes 344–349, 21 May 2012, pages 23–28.
http://dx.doi.org/10.1016/j.aquaculture.2012.02.031