Nudibranchs come in a range of dazzling colours and unusual shapes, and in an annual census they will again indicate the health of our environment.
- The annual Sea Slug Census is held in a range of Australian and overseas locations
- The census monitors numbers which helps track environmental conditions
- Changes in sea slug distribution in recent years has seen some species moving south
The colourful sea slugs are closely monitored by scientists who say they are more than just dazzling, they are key indicators of our changing environment.
The annual Sea Slug Census runs at a range of Australian locations from the Gold Coast to Melbourne and offshore on Lord Howe Island.
Southern Cross University (SCU) marine scientist Professor Steve Smith helped establish the first census in 2013 at Nelson Bay in New South Wales.
Since then, the program has recorded 631 species, with around 200 having been photographed only once.
Professor Smith said nudibranchs were proving to be useful indicators of environmental health.
"Sea slugs have a very short life span, usually less than a year, so it means they are going to respond very quickly to changing environmental conditions — particularly if their food disappears," he said.
"We think they are fantastic group to target for citizen science-based monitoring of marine health."
Indicators of climate change
Professor Smith said the changing distribution of sea slugs provided good evidence of how the climate is changing.
"We've already found at least 60 species that have been recorded well south of their previously known distribution limit," he said.
"Some of these are 2,500 kilometres further south than they've been recorded before."
He said there were a number of possible causes.
"The East Australian Current is a great mechanism for transporting organisms in their larval stages," he said.
"One of the things forecast to happen under climate change is the increasing strength and frequency of the East Australian Current.
"So it means potentially we have this conveyer belt of tropical species which is going to come into our coastal waters more frequently.
"It could be partly that … but at the same time we are getting increased temperature, which means species that might have a physiological tolerance for warmer temperatures can exist in this area for longer."
Citizen scientists brave rough conditions
The most recent Sea Slug Census has just been held on the NSW Coffs Coast with support from the Solitary Islands Underwater Research Group.
Professor Smith said conditions were challenging and numbers were still being tallied, but so far the Coffs census was on track to set a new record of more than 100 species.
"We had fairly big seas which meant we couldn't get to the offshore islands which are normally great for sea slugs," he said.
"But we had an incredibly great group of people who enthusiastically went to the local rocky shores instead and braved some considerable waves, and also some fairly poor visibility."
Professor Smith said four species were recorded that had not previously been documented during a census.
Marine research volunteer Nicola Fraser takes part in the annual Coffs census and said there was much to learn about the tiny molluscs.
"Sea slugs are part of the whole food web. Some have very specific diets, so they might only eat one particular type of sponge, and often they will look like that sponge colour. Often even look like they have the same texture," she said.
"They can be environmental sentinels. We know if we don't find a particular type of sea slug or sea sponge maybe there's a problem."
Ms Fraser said many nudibranchs were only tiny, but extremely eye-catching.
"Some of them are really, really spectacular," she said.
"I remember one I found down at Port Stephens. I thought it was actually a child's toy. It was this incredibly vivid, yellow colour with spots on it.
"I picked it up because I thought it was a piece of rubbish and it moved so I immediately put it back down again.
"They are all hermaphrodites, which means they have male and female sexual parts … which means for animals that are somewhat rare you can mate with any species you come across, so that's handy."
Census set to expand
Professor Smith said a number of sea slug censuses were coming up with plans to expand to new areas.
He said they had run an event in Indonesia, and the first Vanuatu sea slug census was happening soon.
"March is the single most popular month for censuses this year with events at Lord Howe Island, Melbourne, Nelson Bay, the Sunshine Coast and Vanuatu," he said.
"My vision is that we will have events all around Australia, and at overseas locations, as a vehicle to educate the public about our amazing marine biodiversity and to promote community stewardship of the marine environment.
"You never know, just maybe sea slugs can be a primary catalyst to help foster conservation of marine biodiversity in a changing world."