University of Salford and Irish research team reconstructs the origin of a unique species of fish

Tuesday 27 August 2013
Fish
A twaite shad shortly before it is returned to the water.

A team of researchers including a University of Salford professor has reconstructed the origin of an endangered species of fish which can only be found in a lake in the south west of Ireland.

Professor Stefano Mariani, Chair in Conservation Genetics at the University’s School of Environment & Life Sciences, is amongst a team of researchers which has provided genetic evidence for the timing of colonisation of Lough Leane by a migratory fish, after the last glaciations ended.

The existing population of Killarney shad is now genetically isolated from its recent ancestor, the twaite shad, which instead migrates between rivers and the sea. Although it has historically been found along the north east coasts of the Atlantic Ocean, it is now one of the rarest kinds of fish breeding in Ireland and is protected by the EU Habitats Directive.

Under certain conditions, the link between the rivers and sea can become blocked by natural processes or human intervention (such as the building of locks or weirs along a water course). The fish consequently may become trapped or ‘landlocked’ in freshwater lakes. There are several examples of landlocked fish populations which have since adapted to living permanently in freshwater environments, but the evolutionary processes that lead to their origin have rarely been reconstructed.

The only example of landlocked shad to have survived in north western Europe is the Killarney shad, found exclusively in Lough Leane, a lake situated in Ireland’s Killarney National Park. Listed as critically endangered on the IUCN Red List, its survival continues to be threatened by invasive species and eutrophication.

This distinct ‘dwarf’ variant of the shad is characterised by its smaller body size and a lower number of rakers along its gill arches. By examining DNA from fish caught in Lough Leane and shad from other areas, researchers were able to show that after the end of the last ice age, some twaite shad were trapped in the lake on two separate occasions. One coincided with the retreat of the glacial ice sheet from the south west of Ireland some 16,000 years ago; and the other occurred around 7,000 years ago. The descendants of those colonisers interbred, giving rise to the Killarney shad.

Currently there are no natural or manmade barriers in the river connecting Lough Leane to the sea – but the Killarney shad has become so adapted to its new habitat that no migration to the sea is needed for the completion of its life cycle.

Professor Mariani said: “Our DNA results, together with the differing adaptive traits between Killarney shad and twaite shad, show that we should regard the landlocked population as a distinct species. Its protection is paramount, as if we were to lose those fish in Lough Leane, that piece of biodiversity would be gone for good, never to be replenished.”

The research was funded by the Irish Research Council, with support from Inland Fisheries Ireland. It formed part of Dr Ilaria Coscia’s PhD studies at University College Dublin.

 The full research paper can be downloaded here: http://dx.doi.org/10.1016/j.ympev.2013.07.029

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