Testing how pea crabs affect mussel farming
New Zealand pea crabs are known to damage the green-lipped mussels they live in, but what impact do they have on mussel farming in New Zealand?
Oliver Trottier (a PhD student at Leigh Marine Laboratory) wanted to answer this question. He carried out a study of a green-lipped mussel farm and found that mussels that contained pea crabs were about a third smaller than their crab-free neighbours. Based on his results, he estimated that pea crab infestation costs the mussel industry over $2 million a year in lost product.
Mussel farm
A green-lipped mussel farm near Great Barrier Island. Oliver Trottier collected mussels from this farm to determine the impact of pea crabs on mussel aquaculture.
Finding a farm
Oliver was interested in how farmed mussels – rather than wild ones – are affected by pea crabs. Because farmed mussels grow rapidly throughout their lives, Oliver thought the effect of pea crabs on mussel growth might be more pronounced than in wild mussels. To test his ideas, he identified a small farm on Great Barrier Island to study.
Defining the questions
Long before collecting any mussels, Oliver thought carefully about the questions he wanted to answer in his study. He decided on three key questions:
What proportion of mussels on the farm contain a pea crab?
How does the weight of mussels with and without a pea crab differ?
What effects do the pea crabs have on mussel size and shape?
Careful planning and consultation
Next, Oliver started to plan exactly how to carry out his experiment. In particular, he considered:
how many mussels to take in total from the farm
the age at which to harvest the mussels
how many different sites on the farm to take mussels from
how many to take from each site
what information to take from the collected mussels.
It was crucial that he took enough mussels from enough different sites on the farm to allow him to answer his three key questions confidently. After consulting with statisticians and other biologists, he decided to collect a total of 7000 mussels from a total of 225 sites spread evenly across the farm, including at three different depths. He chose to collect the mussels when they were approximately 10 months old.
Oliver also had to plan the logistics of the experiment carefully. He worked out what boat would be used to access the farm, asked his friends (fellow postgraduate students at Leigh Marine Laboratory) to help him dive for mussels on the day and planned food, drink, equipment and dive times for everyone involved. He discussed all his plans in detail with Peter Vitasovich, the mussel farmer.
Pilot study: “heaps of zeros”
On the day before the 7000 mussels were collected, Oliver and his colleagues carried out a pilot study – a small-scale collection of mussels to double check that the experimental set-up would give useful data. This was particularly important because of the cost of running his experiment – hiring the boat alone cost $1200 a day.
Running a pilot study
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, but first, he and his colleagues carried out a pilot study. This small-scale version of the larger experiment gave Oliver an indication of what results he could expect. In this video clip, Oliver describes how and why he did his pilot study, and how the results led him to change his experimental approach.
Focus questions:
What is a pilot study?
Why did Oliver choose to do a pilot study before his main experiment?
How did the results from the pilot study affect how Oliver carried out his main experiment?
From the pilot study, Oliver discovered that pea crab infection levels on the farm were far lower than he had expected – only about 1 in 20 mussels contained a pea crab (the infection rate on New Zealand farms is typically 1 in 3 or more). He had to think on his feet: he decided to collect more mussels from each site on the farm and to collect from fewer sites.
Collection and analysis
The next day, Oliver and his friends carried out the big mussel collection. They spent all day diving for mussels, and the 7000 mussels were frozen for later analysis
Collecting and analysing mussels
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 how he and other divers collected the mussels from different points in the farm. He also explains the detailed analysis that was carried out on the collected mussels – measuring the size and weight of the mussels and noting whether a pea crab was present.
Teaching points:
Students could watch the video without sound at first, noting how the mussels were collected and the way in which they were analysed. They could discuss their interpretation with a partner then view the video with sound.
Students could read the article Testing how pea crabs affect mussel farming after watching this video. The article provides further information about how Oliver planned and carried out his experiment as well as his conclusions about the effect of pea crabs on green-lipped mussels.
The activity Investigating how pea crabs affect supermarket mussels could be done in conjunction with watching this video.
Oliver’s analysis of the mussels was a time-consuming task. He measured the height and width of the shell, the weight of the flesh and whether it contained a pea crab. He then recorded the sex, life cycle stage and number of eggs for every crab he found. He estimates that processing each mussel took upwards of 5 minutes. That’s well over a month of full-time work to collect all his data!
How pea crabs affect farmed mussels
The effect of pea crab infection on mussel size. This image shows some of the key findings from Oliver Trottier’s large-scale study of pea crab infection on a mussel farm in Great Barrier Island, New Zealand.
Image of mussel measured by calliper courtesy of Elsevier, Copyright © 2012.
Impact of the parasitic pea crab Pinnotheres novaezelandiae on aquacultured New Zealand green-lipped mussels, Perna canaliculus. Trottier O, Walker D, Jeffs AG. Aquaculture 2012;344–346, 23–28.
A decrease in mussel size – and industry profits
As the mussels were processed, Oliver began to notice a trend. The mussels with a pea crab were, on average, a third lighter than crab-free mussels. That’s a big difference – it’s proportionally the same as the size difference between an average 12-year-old boy and an average 17-year-old boy in New Zealand.
Pea crabs harm farmed mussels
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.
By using these results, Oliver was able to calculate how pea crab infestation might affect New Zealand’s mussel industry as a whole. Assuming an infection level of 5% across all mussel farms, he worked out that pea crabs are likely to be responsible for an annual loss of over $2 million in mussel production. Because infection rates on farms are often much higher than 5%, the true loss to the industry is likely to be considerably higher.
