Science - June 17, 2004

Anaemic fish sinks to the bottom of the ocean in search of food

The icefish is an evolutionary exception. It has no haemoglobin in its blood, the substance used by vertebrates for oxygen transport. This gives the fish its pale and weak appearance. Hauke Flores of Alterra discovered that the fish has a clever strategy for making sure its food is presented on a dish, thus saving itself energy.

The icefish’s strategy was uncovered by examining the contents of its stomach. Flores did this for a large number of Blackfin icefish (Chaenocephalus aceratus) that were caught around the Southern Shetland Islands and Elephant Island in the Antarctic. Other large fish and also whales and penguins in the Antarctic Ocean are big consumers of krill, the plankton that swims on the surface of the sea and is sometimes phosphorescent. But adult icefish apparently eat fish as the main part of their diet, and according to Flores this is unusual. It also tells us something about how this species conserves its valuable energy. “My hypothesis is that the older the icefish gets the deeper it goes for its food. It lies on the seabed waiting for smaller fish to swim by, thereby expending minimal energy on catching its meals.”

Flores, whose findings were published in Polar Biology earlier this year, is pleased that his research has shed some light on the ecology of this fascinating icefish. Biologists have long been puzzled about how the icefish can survive without haemoglobin. It was already known that the fish has a relatively large heart for its body weight, which improves its circulation.

Flores also discovered that another icefish, the Mackerel icefish (Champsocephalus gunnari), does not employ the same eating strategy as the Blackfin icefish. The Mackerel icefish eats almost only krill. It is not yet known whether the Mackerel icefish also uses a similar method to limit its energy expenditure to the Blackfin icefish.

Flores research’ was partly fundamental, but was also carried out in relation to the Convention on the Conservation of Antarctic Marine Living Resources (1982). One of the main aspects of the convention is monitoring of the krill reserves in Antarctic waters, in particular for the whale population. Flores has estimated how much krill is consumed each month by the icefish around the South Shetlands and Elephant Island, and puts the figure at around a thousand tons. According to Flores this amount does not pose a threat to other krill eaters. The whale population is probably more threatened by Russian and American krill fishing activities.

Hugo Bouter

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