Sunfish: Dissecting and preserving an ocean giant

When a sunfish, the world’s heaviest bony fish, washed up on a beach Dr Tom Trnski from Auckland Museum rushed down to pick up the rare specimen.

At 2.1 metres long and weighing 243 kilograms – Tom knew that Te Papa would be the only fish facility with large enough tanks.

Scientists think there are five species of sunfish, but they don’t know much about these mysterious animals. They are so large and difficult to store that it’s hard to get enough specimens to compare with one another. The fish at Te Papa – a sharp-tail sunfish (Masterus lanceolutus) – is the rarest in the family.

Photograph by Jean-Claude Stahl, Te Papa

Our scientists were excited, it’s rare to find a sunfish specimen in such good condition. Generally, sunfish specimens washed up on beaches have been dead for some time, so all that remains is their tough, leathery skin. The scientists start their research by taking measurements and looking for external parasites.

Photograph by Jean-Claude Stahl, Te Papa

The sunfish specimen donated to Te Papa had been dead for only a few hours before it was frozen. It appeared to be young and uninjured, so how did it die? How deep do they live? What sex is it? What has it been eating? To answer these questions, our scientists had to look inside…

Photographe by Jean-Claude Stahl, Te Papa

The stomach of the sunfish was filled with long jellyfish tentacles, suggesting that it had eaten jellyfish shortly before it’s death. There were even some jellyfish tentacles still in its mouth.

Scientists still aren’t sure what makes up the bulk of a sunfish’s diet. Sunfish eat jelly-like small gelatinous animals (including jellyfish) but are known to also snack on algae, crustaceans, and small fishes if the opportunity presents itself.

One of the big problems for sunfishes is mistaking plastic in the water for their preferred food. To them, a plastic bag would look like a jellyfish.

Photograph by Jean-Claude Stahl, Te Papa

Scientists examining parasitic worms found inside the liver. Unluckily for sunfish, they are thought to house over 40 different parasites at one time.

Photographe by Jean-Claude Stahl. Te Papa

Dissecting the sunfish enables the scientists to get a look inside so they can try to determine the fish's sex and examine its stomach contents. As a bonus, the dissection allows the formalin, a preservative, to get into the internal organs. Without dissection, the tough skin would act as a barrier, and the guts would rot before the formalin managed to get inside.

Photograph by Ruth Hendry. Te Papa

Sunfish don’t have a tail. Some people call them a ‘gigantic swimming head’. Instead of a tail their dorsal and anal fins are fused together into a rudder-like structure called a clavus. The sunfish swims by sculling its pectoral fins. To travel at speed it flaps its dorsal and anal fins side to side synchronously, like oars. While it isn’t exactly fast off the block, it can get up to quite a good speed.

Photograph by Jean-Claude Stahl. Te Papa

A sunfish scientist’s tool kit. Toothbrushes are the best tool for cleaning out sand from the sunfish’s gills and beaked mouth.

Photograph by Jean-Claude Stahl. Te Papa

How many scientists does it take to lift a sunfish? At least four when the sunfish weighs nearly a quarter of a tonne. Here it is being lowered into a specially built tank to be fixed in formalin. Because of the need for so many people to be ‘hands-on’ the sunfish first goes into a pre-determined volume of water.

Photograph by Jean-Claude Stahl. Te Papa

Scientists have to suit up to add formalin to the tank. This is a very poisonous chemical, but necessary to kill the bacteria and fix the sunfish's tissues. To do this safely, scientists need to wear all this protective gear.

Photograph by Jean-Claude Stahl. Te Papa

Any air pockets are emptied so the sunfish is completely immersed in the 10% solution of formalin. The sunfish soaks in the solution for at least three months to make sure it was properly fixed, before being housed in its tank of alcohol. Done properly, this specimen will still be available for examination in 200 year’s time.

Photograph by Jean-Claude Stahl. Te Papa