Parasite larvae exposed

21 May 2008

It seems incredible, but even after more than 100 years of research into the fauna of the oceans small larvae are still swimming around without researchers having any idea how they might look or behave as adults. Now two researchers from the Department of Biology in collaboration with Japanese colleagues have lifted the veil on the riddle of the ‘Y-larvae’ of the oceans. These are larvae which emerge as adult parasites and are an important factor in the biological balance of the ocean.

The Y-larvae

The Y-larvae are of great significance to the ecological balance in coral reefs, among other places, and they can be found in all waters from the poles to the tropics. Through biological experiments researchers Jens Høeg and Henrik Glenner have managed to get the Y-larvae to ‘emerge as adult parasites’.

“Firstly, we collected over 40 different unknown species of Y-larvae on the coral reef off the Japanese island of Okinawa. We then exposed the larvae to a hormone which in the natural world causes crustaceans to change their skin. The result was that the free-swimming larvae shed their shells and out came a very simple, worm-like pulsating clump of cells which lacked all resemblance to normal crustaceans which was believed to be a close relative to the Y-larvae. Our discovery leaves no doubt that the worm-like creature must be an internal parasite. But what the host animals are, we don’t know at this time. That will be our challenge when we return later to the coral reef at Okinawa,” says Associate Professor Jens Høeg.

Previously, parasites have been regarded as harmful and undesirable, but this point of view is fast losing ground in the world of biological research. Research Associate Professor Henrik Glenner, also from the Department of Biology, says that they are of great value to the ecological balance in coral reefs and other natural environments.

“There are numerous examples known to science of parasites which increase the ecological quality or which help maintain the normal standard. For instance, in the American salt marshes worm-like parasites (the so-called flukes) change the behaviour of the local fish in such a way that the birds in the region find it easier to catch the fish. Quite simply, the parasites mean there is more food for the birds and, therefore, a lot more birds,” Henrik Glenner stresses.

Parasites sterilise crabs

Glenner and Høeg have previously worked on using the parasitic barnacles called Rhizocephala in the biological fight against our domestic shore crab. This has undesirably spread to other parts of the world, including the sensitive fauna of Australia and Tasmania, where it has sparked an ecological catastrophe. The parasites sterilise their host, the shore crab. However, the crab continues to live on unconcerned, in the belief that the parasite is its own young. And, what is even more curious, male shore crabs that are infected with Rhizocephala actually start to resemble females and believe they are carrying eggs. However, they, too, become sterilised by the cunning parasite, the two Danish researchers explain.

“The adult Y-organisms probably have just as great a controlling effect in nature as we know is the case with the Rhizocephala. But when you don’t fully know the biology of these forms, which are rich in species, it’s almost like ‘taking a leap in the dark’ when you are studying the ecology of coral reefs. We hope, however, that further work can make us wiser about parasites and their importance to the coral reefs, which unfortunately are in severe decline,” say Høeg and Glenner.

Funding

The work of the two researchers and their Japanese research colleagues is now being published in the acknowledged scientific journal, BMC Biology. The project is financially supported by the Carlsberg Foundation in Denmark and the Lake Biwa Museum in Japan.