Rapid Communication The Ponto-Caspian quagga mussel, Dreissena rostriformis bugensis Andrusov, 1897, is established in Ireland

The quagga mussel Dreissena rostriformis bugensis was identified in a horizon scan of species likely to arrive in Ireland. We report on the first records of this dreissenid from two lakes, a reservoir and interconnecting navigable river sections on Ireland’s largest river, the Shannon, during July 2021. The abundance in Lough Ree was greater than in the larger downstream lake of Lough Derg. Specimens occurred to depths of 24 m and 32 m in each of these lakes respectively. Highest densities were in the southern region of Lough Ree attached to piles supporting floating pontoons and quay walls. Here D. r. bugensis outnumbered the D. polymorpha present. It was present at lower densities downstream to Lough Derg and the Parteen Reservoir. This species has a wide range of colour morphs from a pale yellow to dark concentric or longitudinal banding.


Introduction
The Ponto-Caspian bivalve Dreissena rostriformis bugensis Andrusov, 1897 is an aggressive freshwater bivalve.Once established it can result in significant changes to the benthos and to water quality (Reynolds and Aldridge 2021).By possessing a byssus it can attach in large numbers to each other and to firm structures, resulting in reduced flows in abstraction pipework and screens as well as the fouling of boat hulls and other firm surfaces which can lead to high economic costs (Connolly et al. 2007).Its filtration ability can result in clearer water (Cross et al. 2010), thereby altering phytoplankton composition and causing a possible decline in some planktonic crustaceans as was noted in Lake Erie in North America (Zhang et al. 2011).In Ireland, the closely related Dreissena polymorpha (Pallas, 1771) arrived in 1993/4, modifying the ecological component of lakes and causing heavy fouling of craft and blockages of municipal water pipes (Minchin et al. 2002).It is expected that D. rostriformis bugensis will behave in a similar way.Both species are readily capable of spreading through interconnected waterways to lakes and reservoirs.This occurs by downstream dispersal of their larvae and with leisure craft movements upstream and to other river catchments through interlinking canals and possibly trailered craft.
There is evidence to suggest D. rostriformis bugensis was confined to a small area to the north-west of the Black Sea in the Dnieper River Delta and Southern Bug River where it is considered to be native (Son 2007).From there, it spread within the Ponto-Caspian to the freshwater region of the Azov Sea, appearing in the northern Caspian Sea region in the mid-1990s and advancing up the Volga River in Russia (Orlova et al. 2004).It subsequently spread to the Romanian region of the Danube River by 2004 (Popa and Popa 2006) and by 2006 it may have spread through the Danube River, the Main-Danube Canal to the Rhine Delta in The Netherlands (Molloy et al. 2007).Although this is possible, Bij de Vaate (2010) provides a case for its arrival in the Rhine with ships' ballast water which is also considered to be the vector for its arrival in the Great Lakes of North America (Grigorovich et al. 2003).In 2007 it was found in the Main River in Germany (van der Velde and Platvoet 2007).It also spread further west to the river Moselle in France where it was recorded in 2011 (Bij de Vaate and Beisel 2011).Within a decade, it appeared in a stream, in part of the Thames Catchment, Britain (Aldridge et al. 2014).According to Matthews et al. (2014) it has spread more rapidly than the zebra mussel, Dreissena polymorpha, and since has fouled a number of reservoirs (Reynolds and Aldridge 2021).The Quagga mussel has also been spreading in North and Central America since it was first recorded in 1989 in Lake Erie in the Great Lakes (Mills et al. 1993).Since then, it has spread to the Upper Mississipi in the 1990s (Grigorovich et al. 2008) and to Lake Mead in 2007 (Wong et al. 2010).In 2014 it was found further south in Mexico (Wakida-Kusunoki et al. 2015).
In a recent horizon scan of impacting species likely to arrive in Ireland, D. rostriformis bugensis was considered a high-risk invader (Lucy et al. 2020).This follows a horizon scan for Britain (Roy et al. 2014) which also considered this species to be of high risk.We report the first occurrence of D. rostriformis bugensis in Ireland, appearing in two lakes and a reservoir and interlinking river sections in the Shannon catchment in early July 2021.

Materials and methods
Sites in Lough Derg were visited by road and in Lough Ree by boat, using a rapid assessment approach during 2 to 14 July 2021.This followed the initial finding of three specimens on 28 June 2021.Sampling involved the use of a scraper mounted on an extendable pole to which a mesh pocket was attached.The blade of the scraper used was either 10 cm or 15 cm to detach mussels from vertical piles supporting marina pontoons or quay walls.All the detached mussels collected within the meshed pocket were sorted, identified and weighted.Estimates of density were calculated from the scraper blade width, the length of the vertical draw and the number of successive scrapes undertaken.This provided estimates of numbers and biomass per m 2 .Twenty-three separate sites were visited throughout the Shannon River.
Fifty-seven dredge samples deployed from 5 m to 36 m in Lough Ree and Lough Derg provided qualitative data.The dredge used had previously been used to acquire the clam Corbicula fluminea (Müller, 1774) and is described in Minchin and Boelens (2018).
Our specimens conformed to the illustrations of original material of D. bugensis described by Andrusov (1897) illustrated in Rosenberg and Ludyanskiy (1994).We also submitted photographs of the internal and external shell features to Dr Mikhail Son who stated that these corresponded to the typical D. bugensis/D.rostriformis bugensis in Ukraine.
The difference in outline shapes and colour patterns of D. polymorpha and D. rostriformis bugensis (Pathy and Mackie 1993;van der Velde et al. 2010) were utilised to distinguish between the species.Small individuals up to 2 mm shell length required microscope examination at x 40.Shell length was measured from the anterior to posterior shell exterior.

Results
There were clear differences in abundance between the two Shannon lakes with greater levels of abundance in Lough Ree within the small lakes cojoined to the south-east of Lough Ree (Table 1).Lower numbers were found in Lough Derg, with the species absent in samples at some sites (Figure 1).
Three year-class modes were present in Lough Ree, with specimens attaining a shell length of 34 mm.While both D. rostriformis bugensis and D. polymorpha were present at the same sites, D. rostriformis bugensis dominated, representing up to 72% of all dreissenids present.Shells had a wide range of colour morphs.Shell patterns were not always symmetrically displaced on each valve.Pale specimens were frequent (Figure 2).
Dredging found quagga mussels over a depth range of < 5 m to depths of 24 m in Lough Ree, down to 32.5 m in Lough Derg.In our current findings, specimens were more frequent between depths of 2 to 10 m.

Discussion
The population in Lough Ree is firmly established, is likely to have been present for a number of years and is locally the dominant fouling species.The likely scenario is for the quagga mussel to expand its population rapidly (Strayer et al. 2019) as Irish climatic conditions are suitable for such expansion.D. rostriformis bugensis is capable of tolerating water temperatures from 0 °C, by surviving under ice (Spidle et al. 1995) and has an upper thermal tolerance limit of 28 °C (Spidle et al. 1995) which is unlikely to be reached in Ireland.It will almost certainly spread through navigable routes in lakes, rivers and canals as well as over land by boats, as happened during the expansion of the zebra mussel (Minchin et al. 2006).Overland spread could also facilitate the colonisation of D. rostriformis bugensis in the upper reaches of estuaries where salinities do not exceed between 1 and 5 (Spidle et al. 1994).More recently, it has been shown wet surfaces may be capable of transferring the veliger stage (Choi et al. 2013), this could result in additional vectors being involved in further spread such as angling equipment (Mueting et al. 2011) including waders (Aldridge et al. 2014).
It is presently unclear how the species arrived in Ireland; but in the past, species arriving in Britain often subsequently appear in Ireland (Minchin 2007).In the case of D. polymorpha there is evidence that its 1993/4 arrival in Ireland was through the transportation of fouled leisure craft by trailer and ferry from the midlands of Britain (Pollux et al. 2003).This may have been the seminal site of inoculation with downstream dispersal of veligers to Lough Derg based on the greater abundance of the D. r. bugensis, recovered from Lough Ree.
Both dreissenid species result in changes to biomass (Heiler et al. 2013), biodiversity and water quality (Reynolds and Aldridge 2021) and will result in economic costs, particularly where abstraction has not considered their fouling impacts.The full impact of these effects will depend upon the extent of their populations.In the Great lakes of North America D. rostriformis  bugensis appears to become dominant over D. polymorpha within the cooler deeper water, extending down to depths in Lake Erie of 130 m (Mills et al. 1996).Furthermore, D. rostriformis bugensis has a greater capability than D. polymorpha to form druses over softer sediments which can result in increased biomass at depth.Consequently, the biomass in the hypolimnion of Irish lakes is likely to increase.This could lead to possible consequences of depleting oxygen especially during prolonged high atmospheric pressure events of windless days and high temperatures.This species, in association with the biomass from the earlier invaders of D. polymorpha and Corbicula fluminea, within this zone make such an event more likely to take place.

Figure 1 .
Figure 1.Relative abundance of the quagga mussel in the Shannon.

Figure 2 .
Figure 2. Selection of colour morphs of D. rostriformis bugensis, from the river section between loughs Ree and Derg.Photograph by Dan Minchin.

Table 1 .
Relative abundance of the two dressenid species in the Shannon.