Killarney National Park situated in South West Ireland is famous for containing some unusual species that are not found elsewhere in the country including the Kerry spotted slug (Geomalacus maculosus), a small snail that is distributed in Ireland and Iberia, but is intriguingly absent from Britain. It also contains some of the oldest natural woodlands in Ireland, and the yew and oak woods of Killarney are of international significance.
The park is surrounded by Ireland’s highest mountain peaks, and the Killarney Lakes are found in the centre of the park and attract thousands of visitors annually. The three lakes while interconnected are surprisingly ecologically independent of one another, with each lake containing its own ecosystem. The largest of these lakes, Lough Leane contains a fish species called the Killarney shad (Alosa fallax killarnensis), a subspecies of twaite shad that is not found elsewhere in the park, or even in the rest of Ireland.
Ireland has three species of shad in total, with two others the twaite shad (Alosa fallax) and the allis shad (Alosa alosa) also present. Shads are known as anadromous species as they spend most of their lives in the sea and then migrate to freshwater sites to spawn. The best known example of an anadromous species is the salmon. Of the shads, the only landlocked example is the Killarney shad, and that is currently listed as critically endangered according to the IUCN Red List. Shads occur throughout North East Atlantic coasts and can be found from Morocco to Iceland, but they are among some of the rarest breeding fish in Ireland.
How the shad in Killarney became landlocked is quite a mystery as there are currently no barriers preventing its movement to the sea. Earlier this year, a seal even made its way into the Killarney lakes from the sea. There is also some debate on the taxonomic status of the Killarney shad species, and if it is indeed a subspecies. Those are questions that Ilaria Coscia and a team of co-authors from University College Dublin, Ireland; University of Leuven, Belguim; Inland Fisheries Ireland and the University of Salford, UK, have attempted to address in a new study just published in the journal of Molecular Phylogenetics and Evolution.
Coscia collected 27 samples of the Killarney shad, 34 twaite shad from the River Barrow (South East Ireland) and 32 allis shad from various locations in the south of Ireland. Both nuclear and mitochondrial DNA markers were used to assess the genetic differentiation and to build a picture of the genetic history of the species in the region.
In the mitochondrial DNA (inherited down through the maternal line), out of only 41 obtained sequences, a total of 21 different DNA haplotypes were identified. An investigation into the history of these DNA sequences revealed that there was a major split between allis and twaite shad, and it showed that the Killarney shad had diverged from the twaite shad. The Killarney shad exhibited two very different DNA haplotypes indicating that at least two colonisation events occurred during the retreat of the ice sheet, the first was estimated to have occurred about 16,000 years ago after the Last Glacial Maximum, and again during the Younger Dryas glaciation about 7000 years ago. The two haplotypes are likely to have originated from two different glacial refugia, a pattern that has also been seen in Atlantic salmon colonisation.
Coscia also examined the nuclear DNA (composed of material inherited from both parents) which gives a more contemporary view of the genetics of the species. The results gave additional support for the separation of the Killarney shad from the twaite shad, a separation which was statistically similar to the separation of allis from twaite (both recognised species). Interestingly, the data also showed that once the two twaite lineages established in the lake, an event that was likely many thousands of years apart, the population became isolated in Killarney and the two lineages began to breed and formed a single population. Coscia also showed that the two divergent lineages in Lough Leane have successfully admixed over the years so that no contemporary signature of genetic subdivision remains evident in the species.
Cosica conservatively points to various characteristics such as size, behaviour, morphological and physiological changes that the Killarney shad has undergone due to thousands of years of isolation from its closest related ancestor, the twaite shad, and believes that this combined with the genetic evidence could support the subspecies status for the Killarney shad. However, Coscia warns that the species is critically endangered, and the closest extant relative, the twaite shad (also quite rare) would not be suitable to supplement the population through translocations due to differences such as life history traits. In order to conserve this unique species, Coscia recommends that the species and the surrounding environment needs to be closely monitored to prevent euthropication and the establishment of invasive species. Coscia says that such measures should be of the paramount importance, given the uniqueness of species shown in this study.
This study has added to the ecological importance of Killarney National Park and has shown that migrations into the region happened as long as 16,000 years ago, a time when much of Ireland and Europe was still covered in ice. The study has not shown us how these fish moved into the area, but if freshwater fish species were present here at that time, other aspects of the ecosystem were also likely beginning to become established.
Coscia I, McDevitt AD, King JJ, Roche WK, McLoughlin C, & Mariani S (2013). A species-to-be? The genetic status and colonization history of the critically endangered Killarney shad. Molecular phylogenetics and evolution PMID: 23933070