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Unusual eggs reveal fishy relatives

03 July 2006

An unusual surface on the eggs of the mysterious pike-head fish helps to resolve its position in the fish family tree.

DNA data and a strange spiral structure on the eggs of two species of fish, the pike-head and the chocolate gourami, have shown them to be each others closest relative.

Controversial relationships
Black and white image of a 3 millimetre long pike-head fish egg © Dr Ralf Britz

Black and white image of a 3 millimetre long pike-head fish egg © Dr Ralf Britz

Because of its pike-like appearance the pike-head Luciocephalus pulcher , was assigned to the pike family Esocidae in 1831. Later, scientists thought it was more closely related to the labyrinth fishes - the popular highly-coloured aquarium fishes such as the Siamese fighting fish and the pearl gourami - but no strong scientific evidence could be found to prove this. The exact relationship remained controversial and scientists proposed many different ideas.

Strange spiralling ridges
The pike-head looks similar to a pike but is closest to the chocolate gourami © Andreas Hartl

The pike-head looks similar to a pike but is most closely related to the chocolate gourami species © Andreas Hartl

Dr Ralf Britz, fish expert at the Natural History Museum, had studied the pike-head eggs and found the surface had a complex structure highly unusual among bony fishes. The surface is covered in many spiralling ridges, most likely there to guide sperm to the opening of the egg, so they can fertilise it.

This strange surface structure occurs only in one other group of fish - the chocolate gourami and relatives that were thought to belong to a different family of fish.

Mouth-brooders

The chocolate gourami feed on tiny zooplankton and unlike many other labyrinth fishes, are mouth-brooders - the male incubates the eggs in its mouth.

The pike-head is a fish-eating predator that grows up to 15 centimetres long. It lives in acidic black water streams in Penninsular Malaysia, Sumatra, and Borneo. It is also a mouth brooder - the male broods up to 150 eggs in his mouth for about four weeks, going without food for that time.

Relationships resolved
Pear-shaped egg of the pike-head species Luciocephalus Eier © Andreas Hartl

Pear-shaped egg of the pike-head species Luciocephalus Eier © Andreas Hartl

Dr Lukas Rüber also a fish researcher at the Museum and Dr Ralf Britz, tested previous ideas about the systematic position of Luciocephalus with molecular data.

'When we analysed our DNA sequence data we were amazed,' said Dr Rüber. 'The elusive pike-head, a fish predator with hugely protrusible jaws (jaws that can be extended from the mouth), is the closest relative of the chocolate gourami, a fish with a tiny mouth that feeds on small zooplankton'.

'This agrees nicely with the previous idea based on the unusual spiralling egg surface.'

This research is published in the June edition of Systematic Biology .