Dr Dave Taylor of the Cawthron Institute in Nelson explains what the Asian date mussel has to do with the grey side-gilled sea slug and the dogs dying on the Auckland beaches. Asian date mussels are an introduced species that thrive in the Auckland Harbour. It appears that Asian date mussels provide the perfect habitat for our toxic grey side-gilled sea slugs.
DR DAVE TAYLOR
We were asked to come up to Auckland and try and find out why dogs were dying on the beaches. So we sent a scientist, Andy Selwood, up to the beach, and he walked along the beach at high tide and looked for anything that might be able to be tested for toxins. He picked up a number of samples, and one of those happened to be a sea slug that turned out to be highly toxic
We then came out and did dive surveys off boats to try and determine how many slugs there were out there and why there were so many. In doing so, we came across huge populations of these sea slugs on Asian date mussel beds, and the Asian date mussel is an invasive mussel that came into Auckland Harbour in the 1970s, probably from ships’ ballast water, which is the water that ships use to keep themselves upright.
The sea slugs are attracted to the Asian date mussels probably because they provide food and habitat for them to live in amongst, so somewhere to lay their eggs and live.
When we swam over those Asian date mussel beds off Narrow Neck and Cheltenham Beaches, we found about one slug per metre squared, and given that there are about a kilometre long beds of Asian date mussel that were at least a couple of hundred metres wide, we were dealing with thousands and thousands of slugs that were only a short distance from the beach, and as those slugs died off or during a storm, the slugs were getting washed onto the beach, and that was why we think the dogs were dying.
Those beds hang around for about 2 years, and when those beds go, anything that was living or on the seabeds tends to go as well, and it turns out that the sea slugs were living on the Asian date mussel beds, and once those Asian date mussel beds died off, then the population of slugs here decreased rapidly.