Introduction
The fairy shrimp Branchinecta gaini Daday is the largest freshwater invertebrate in Antarctica. Despite growing evidence of faunal endemism, it is believed to represent a mid- to late Holocene immigrant (Gibson & Bayly Reference Gibson and Bayly2007, Hawes Reference Hawes2009, Reference Hawes2015). To complete its life cycle, a significant period with liquid water is needed and subzero temperatures represent a real physiological constraint on adult animals. Its success in the extreme environment of Antarctica is associated with a ‘ruderal’ strategy that consists of metabolic eurythermy and formation of highly resistant cysts suitable for passive dispersal (Hawes et al. Reference Hawes, Worland and Bale2008, Hawes Reference Hawes2009).
The distribution range of B. gaini includes Patagonia, South Georgia, the South Orkney and South Shetland islands, and half way along the western Antarctic Peninsula (Hawes Reference Hawes2009). Palaeolimnological data confirm the presence of B. gaini on James Ross Island for a significant period of the Holocene. However, it is currently considered to be extinct in this area (Björck et al. Reference Björck, Olsson, Ellis-Evans, Håkansson, Humlum and de Lirio1996).
James Ross Island is situated in the transitory zone between Maritime and Continental Antarctica. Only the northernmost part of the island, Ulu Peninsula, is significantly deglaciated, representing one of the largest ice-free areas in the north-eastern Antarctic Peninsula. Recently, six lake types were described on the Ulu Peninsula, classified according to their geomorphological position, physical and chemical characteristics, and biota (Nedbalová et al. Reference Nedbalová, Nývlt, Kopáček, Šobr and Elster2013). We report the common occurrence of B. gaini on James Ross Island and the nearby Vega Island observed during limnological surveys in 2008–16.
Field surveys
Abundant populations of B. gaini were recorded in the Ulu Peninsula lakes in January/February 2008 and 2009. The fairy shrimp occupied shallow lakes located near the coast as well as inland lakes at higher altitudes (up to 230 m a.s.l.). Limnological characteristics of the lakes with Branchinecta (Black, Blue–green, Cyanobacterial, Dan, Green 1–2, Katia 1–2, Lachman 1–2, Monolith, Muddy, Phormidium, Red, Vondra 1–4 and White), as well as the sampling dates, can be found in Nedbalová et al. (Reference Nedbalová, Nývlt, Kopáček, Šobr and Elster2013). In the Lachman lakes, the continuous presence of B. gaini was documented from early January to late February. No Branchinecta were found in deep cirque and kettle lakes.
The Clearwater Mesa on the south-eastern side of Croft Bay, James Ross Island, is characterized by the presence of tens of shallow lakes and ponds at an altitude of 170–250 m a.s.l. In February 2009, B. gaini was observed in all of the lakes. Thick layers of dead animals accumulated by wind cover the shores of some lakes, indicating high population density at these sites. The occurrence of B. gaini in this area was confirmed in 2015 and 2016.
In January 2013, a field campaign in a small coastal area of Devil Bay (c. 8 km2) provided the first record of B. gaini on Vega Island. It was found in low altitude shallow lakes but not in lakes situated on mesas at higher altitudes. In total, B. gaini was recorded in 14 of the 45 surveyed lakes. In 2014, the shrimp was observed at several sites on Cape Lamb, south-west Vega Island.
Figure 1 presents the areas where B. gaini has been observed. Lakes inhabited by B. gaini are characterized by rich photoautotrophic mats and by the presence of the calanoid copepod Boeckella poppei Mrázek. Birds (especially the Antarctic tern Sterna vittata Gmelin) were observed feeding on B. gaini in the coastal lakes on James Ross Island.
Areas where B. gaini was observed in 2008–16. Blue crosses indicate sites where B. gaini cysts were recorded in sediment cores by Björck et al. (Reference Björck, Olsson, Ellis-Evans, Håkansson, Humlum and de Lirio1996).
Discussion
In contrast to Björck et al. (Reference Björck, Olsson, Ellis-Evans, Håkansson, Humlum and de Lirio1996), who state that ‘this crustacean does not occur today anywhere on James Ross Island or the surrounding area’, our field observations confirm that B. gaini is widely distributed in the ice-free areas on James Ross and Vega islands. The lakes in which Branchinecta populations have been found belong to three types: stable shallow lakes on higher lying levelled surfaces, shallow coastal lakes and stable lakes in old moraines (Nedbalová et al. Reference Nedbalová, Nývlt, Kopáček, Šobr and Elster2013). These lakes are characterized by massive photoautotrophic mats that serve as a food source for B. gaini. The absence of such mats probably limits its distribution in kettle and cirque lakes.
The current climate on James Ross Island shows strong continental features with higher temperature variability and lower annual mean temperatures in comparison with the western Antarctic Peninsula (Hrbáček et al. Reference Hrbáček, Oliva, Láska, Ruiz-Fernández, de Pablo, Vieira, Ramos and Nývlt2016). The requirement of liquid water for at least c. 2.5 months is considered to be the main physiological constraint on the distribution of B. gaini (Hawes et al. Reference Hawes, Worland and Bale2008). Since the temporal window available for non-cyst life stages can reach three months in coastal areas (Váczi et al. Reference Váczi, Barták, Nedbalová and Elster2011), the sites on James Ross Island are at present suitable for B. gaini development. However, populations at higher altitudes probably survive at their physiological limits.
Although Björck et al. (Reference Björck, Olsson, Ellis-Evans, Håkansson, Humlum and de Lirio1996) were not able to date the disappearance of B. gaini from Boulder (=Monolith) Lake exactly, it was attributed to the arrival of a harsher climate that restricted the availability of liquid water. It was also hypothesized that the increased deposition of minerogenic matter impaired the growth of benthic mats (Björck et al. Reference Björck, Olsson, Ellis-Evans, Håkansson, Humlum and de Lirio1996). We cannot ascertain if B. gaini did disappear from James Ross Island for a certain period. Valuable data could be gained from further analysis of sediment cores from both islands. Molecular phylogeography could also elucidate the dispersal history of B. gaini within its current range.
The north-eastern Antarctic Peninsula is experiencing a period of rapid warming that began c. 600 years ago (Mulvaney et al. Reference Mulvaney, Abram, Hindmarsh, Arrowsmith, Fleet, Triest, Sime, Alemany and Foord2012). Generally, the biota, including B. gaini, is likely to benefit from these changes, which could be reflected in its abundance in this region at present. However, accelerated desiccation of shallow ponds (Váczi et al. Reference Váczi, Barták, Nedbalová and Elster2011) due to warming might have adverse effects on the long-term persistence of this species (Hawes et al. Reference Hawes, Worland and Bale2008).
Acknowledgements
The research was supported by the projects LM2015078, PICTO 2010–0096 and RVO67985939. The authors would like to thank to the J.G. Mendel Czech Antarctic Station, the logistical support of Instituto Antártico Argentino and the EU research network IMCONet funded by the Marie Curie Action IRSES (No. 319718). The authors acknowledge the comments received from an anonymous reviewer.
Author contribution
All authors performed the field surveys, LN wrote the manuscript, JML and JE co-ordinated projects focused on lake research in the region.
