The breeding aggregation of giant Australian cuttlefish (Sepia apama) that occurs in the upper Spencer Gulf of South Australia every winter is the largest of its kind on the planet. It has become a mecca for scientists and eco-tourists, but recently the number gathering to spawn has declined from around 180,000 to as few as 19,000 between 1999 and 2012*. As a result, catching cuttlefish has been banned in this region until at least 2014 or until the reason for the decline in numbers has been determined.
But what’s the big deal? Surely this unique spawning event will recover by a simple process of migration of more giant Australian cuttlefish into the region, from the many other waters where they are found in Southern Australia.
Research conducted by Professor Steve Donnellan, Head of the Museum’s Evolutionary Biology Unit (EBU), and colleagues suggests this might not actually be the case. The team has found evidence that the giant Australian cuttlefish in the upper Spencer Gulf are a separate population from those found in other waters in South Australia, including the lower Spencer Gulf and Gulf St Vincent.
Analysis of DNA shows there are clear gene differences between the different cuttlefish populations. Studies of the mineral composition of ear structures (known as statoliths) showed that upper Spencer Gulf S. apama have a restricted environmental range compared to other giant Australian cuttlefish.
Also, S. apama in the upper Spencer Gulf have beaks that are consistently and clearly different in shape to more southern populations. S. apama may also be better equipped to survive in the saltier, higher-temperature waters of the upper Spencer Gulf. They are the only giant Australian cuttlefish population to aggregate and perform the elaborate spawning rituals observed in past years. The upper Spencer Gulf S. apama are acting as a separate population in terms of their behaviour and perhaps their environmental preferences.
This first stage of the research was conducted as part of a joint project with Professor Bronwyn Gillanders from the University of Adelaide as well as the Department of Primary Industries and Resources South Australia (PIRSA), the Department of Environment, Water and Natural Resources (DEWNR) and the Nature Foundation.
Steve now has funding for further research to investigate whether these cuttlefish are actually a different species or if they are seeing a population in the early stages of speciation. The next phase of the research will involve more careful analysis of S. apama genes. The team will also examine the diets of the different populations to see if the differences in beak shape translate to differences in feeding preferences between the populations.
The first stage of the research was funded by the Fisheries Research and Development Corporation, the South Australian Research and Development Institute, PIRSA, DEWNR, the University of Adelaide and the South Australian Museum.
*A paper released in 2013 by the Department of Primary Industries and Resources of South Australia and South Australian Fisheries.