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.
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 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.
Oliver Trottier (
I had to come up with a methodology. I mean, the farm is quite large – it’s about 1 square kilometre – and obviously under water, and you can’t just walk up to it and pick the mussels off.
They were collected by taking a polypropylene rope which we would attach to the top of the mussel farm, and that rope was suspended from the farm and at three different depths you’d have – well basically, it was an onion bag – and you’d go in there and randomly pick 40 or 60 mussels from each specific location, and then the next day, we’d go along and collect those bags.
Back in the lab, Oliver analysed the mussels he’d collected. With each mussel, he first checked whether it contained a pea crab. He noted whether the crab was male or female and whether it was a juvenile or an adult. Next, he recorded the width and height of the mussel shell and the weight of the mussel flesh. This meant that he could compare mussels with and without pea crabs to see what impact the pea crab was having.