Electronic tagging of marine animals is common, but tagging marine invertebrates is very unusual! For example, in 1998, an estimated 11,800 marine animals were tagged electronically, and only 35 of these were invertebrates.

Dr Miles Lamare and postgraduate research student Tracey Channon from the University of Otago, along with colleagues from the Cawthron Institute in Nelson, recently tested a new tagging technique to investigate the behaviour of Coscinasterias muricata, the New Zealand sea star.

Sea stars – a keystone species

Sea stars are invertebrate animals that belong to the phylum Echinoderm. They are important marine predators and play a major role in keeping the numbers of other organisms down. As a result, they are often referred to as a keystone species, as their impact on the community is greater than you would expect in relation to their abundance. The removal of a keystone species from an ecosystem results in a major change, in the same way that removing a keystone from an arch or bridge could cause the structure to collapse.

Observing sea stars in their natural habitat

A lot of the previous research into New Zealand sea stars relied on observations by scuba divers. This meant that it was really only possible to get an idea of what was happening over a short period of time. Sea stars don’t move very quickly and so this type of observation gave the impression that they are fairly static animals.

By tagging individuals and monitoring their behaviour, the scientists hoped to gather information about how individual sea stars behave, what they feed on and how often they feed and get a better understanding of the role they play in structuring marine communities.

The challenge of attaching the electronic tags

Miles and his colleagues talked to a company in Iceland who make tags for large oceanic fish. This company had some very small tags that record variables such as depth, temperature and salinity, but the challenge was how to attach them.

Sea stars can shed their arms, and with the tags costing $500 each, the group didn’t want to attach a tag only to find it had dropped off with the arm somewhere on the seabed, so they tried out a few different techniques in the lab. One idea involved making a little backpack for the sea star with 11 straps for all its arms, but the sea star in the lab managed to get out of the backpack overnight, like a little Houdini.

As a last desperate measure, the group decided to try piercing one of the arms with stainless steel wire and attaching the tag this way. They were convinced that the sea star would just shed the arm with the tag – but luckily, it didn’t!

Testing whether the tag affected behaviour

One of the aims of this research was to find out more about the behaviour of individual sea star, so the next important stage of the project was to test whether the tag affected the sea star’s behaviour. There would be no point in tagging an animal if its behaviour was totally different to an untagged animal.

The scientists found that tagging had no effect on feeding rate, health or the way the sea star moved so they were confident that tagged animals could be used to accurately represent normal sea star behaviour.

Nature of science

Scientists often trial new methods in the laboratory before using them in the field. This allows them to test the methods under known conditions so that they can be more certain that effects seen in the field are real and not a result of the method.

Tagging sea stars in the field

The site for the research was Doubtful Sound in Fiordland. At any one time, the group tagged about 5 individuals in the population and left the tags on the animals for about 3 weeks. It was pretty easy to find the tagged individuals, as the water in the fiord is really clear and the sea stars didn’t move huge distances horizontally. A number of students helped in this aspect of the work.

The tags used in this research are called ‘archival tags’ (different to ‘passive tags’ commonly used to identify an animal), which record and archive useful information over a set period of time, for example, these tags recorded water temperature and depth every 5 minutes for up to 2 weeks.

After each tag was removed, it was placed into a communication box plugged into a computer. Using special software, the scientists downloaded the data from the tag and analysed it.

The results of the study were really interesting and provided information about different sea stars’ behaviour, the amount of vertical movement and the influence of the physical environment (for example, salinity layers). Miles and his colleagues were really surprised to find out how dynamic sea star behaviour is. They do sometimes sit still for a couple of days but then suddenly they will quickly move to the freshwater layer, grab a mussel and move back down.

Testing out the archival tags was the trial part of this research project. The next research question Miles and Tracey want to investigate in more detail is the impact of increased levels of freshwater, due to a nearby power plant, on the sea stars in the fiord.

    Published 8 October 2009