Freshwater invertebrates of subantarctic South Georgia

Twenty‐one lakes, ten pools, seven coastal pools, three streams, two seal wallows, a penguin wallow, and three flooded moss carpets on South Georgia were sampled for aquatic invertebrates. More than 100 invertebrate species were found comprising 59 species of Rotifera, 29 Arthropoda (including five Anomopoda, three Calanoida, two Harpacticoida, three Ostracoda, 12 Acarina and four Insecta) and at least 22 other invertebrate species (including four Platyhelminthes, three Gastrotricha, six Tardigrada, six Nematoda, and two species of Annelida). The fauna of South Georgia, although similar to that of the other Scotia Arc Islands, particularly Signy Island, is much richer by virtue of its lower latitude and milder climate.


Introduction
This paper describes the results of a three-month (3 November 1992 to 23 January 1993) limnological survey of South Georgia. The work was prompted by an earlier, and clearly incomplete, opportunistic study that yielded 27 species (Dartnall and Heywood 1980). At least four additional species of aquatic arthropods were then known to inhabit the island, and while Signy Island, which lies 6u further south, supports 38 species of rotifer (Dartnall and Hollowday 1985), only 15 species of rotifer had been found at South Georgia. As 11 of the rotifers were recorded from both locations, Dartnall and Heywood (1980) speculated that subsequent surveys at South Georgia would find many more rotifer species common to both locations, as well as many ''new'' ones. The objective of this paper was to produce a definitive list of freshwater invertebrates for comparison with other subantarctic and Antarctic locations.

South Georgia and the collection sites
South Georgia, a crescent-shaped island (Figure 1), 170 km long and from 3 to 40 km wide, lies within the Antarctic Convergence, some 2000 km east of Cape Horn. The island is extremely rugged with a central spine of snow-capped mountains, 13 of which exceed 2000 m. More than half of the island is covered with permanent snow and ice, with some glaciers reaching down to the sea. The climate is harsh, deemed subantarctic, with vegetation restricted to altitudes below 300 m. The flora is dominated by Festuca contracta grasslands with localized areas of tussock grass (Parodiochloa flabellata) and moss banks (Greene 1964;Lewis-Smith and Walton 1975). These vegetated coastal lowlands harbour hundreds of lakes and pools.

Materials and methods
Four different habitat-specific sampling methods were utilized. In large lakes plankton nets (53 mm mesh) were thrown into deep water and allowed to sink before being slowly retrieved. Benthic samples were obtained using rigid plastic tubes and a bilge pump  (Dartnall and MacAlister 1999), while streams and shallow-water sites were sampled with Platyhelminthes Four species of freshwater flatworms were observed: (1) a common, eyed vermiform acoel with a slightly grooved ventral surface; (2) a large-eyed species with a broad chisel-shaped head, which was restricted to the Karrakatta pool; (3) a common eyeless species with a mid-ventral  sucker observed feeding on dead Boeckella poppei nauplii and copepodite larvae; and (4) a rare eyeless species only found in Lakes 2, 4 and 6 on the Tønsberg Peninsula.

Gastrotricha
The presence of solitary specimens in most samples suggests that gastrotrichs are probably ubiquitous. Observed taxa include ''long-haired'' and ''short-haired'' Chaetonotus spp., along with a scaly Leptodermella sp.

Tardigrada
Six tardigrade species including Dactylobiotus sp., Echiniscus jenningsi Dastych, E. macronyx Richters, two species of Macrobiotus, and Mopsechiscus imberis (Richters) have been identified (S. McInnes, personal communication). They were not differentiated in the field, but are ubiquitous and, with the possible exception of Gulbrandsen Lake, probably occur in all of the freshwater lakes and pools.

Rotifera
Fifty-nine species of rotifer (47 Monogononta and 12 Bdelloidea) were identified. The majority are common cosmopolitan species that require little or no comment. Keratella heywoodi is new and is described elsewhere (Dartnall 2005), while the two unidentified Cephalodella spp. and the unidentified Testudinella are possibly ''new'' and currently under investigation. Insufficient specimens were obtained to comment on the status of the other unidentified Monogononta. Most of the unidentified bdelloid rotifers are familiar, and were noted on Signy Island (Dartnall and Hollowday 1985).

Annelida
At least two species of enchytraeid worm are now known to be present, Lumbricillus antarcticus Stephenson (K. Dozsa-Farkas, personal communication) and a recently described tubificid Ainudrilus dartnalli (Erséus and Grimm 2002). They were not differentiated in the field so details of their distributions are unavailable.

Arthropoda
Twenty-seven arthropod species were recognized including 13 Crustacea, four Insecta and 10 Acarina.

Freshwater invertebrates of South Georgia
was present in most of the pools and lakes. The chydorids Alona weinecki, Camtocercus aloniceps, and Chydorus sphaericus clamp the valves of their shells on, and browse their way along, algal filaments. Chydorus sphaericus outnumbered the other ''chydorids'', making up 95% of the specimens collected in most of the pools and all the lakes while Camptocercus aloniceps, which was initially confused with Alona weinecki, was rare.
Copepoda. Dwarf specimens of Boeckella poppei occurred in the lakes and pools on the Husdal plain and in the tiny pools/ponds on the point of Tønsberg Peninsula while normal-sized specimens occurred in most lakes (see also Hessen et al. 1989). Both Boeckella michaelseni and the predatory Parabroteus sarsi were restricted to lakes.
Harpacticoida. Two species of Canthocamptidae were present in most of the freshwater lakes and pools sampled and probably represent universal distributions. Five common inshore marine species (D. Hamond, personal communication) were found in the brackish pools (Table III).
Ostracoda. The distributions of Eucypris fontana (Graf), together with Tanycypris sp. and Candona sp., both of which are new records for South Georgia (K. Martins, personal communication), were not determined.

Hexapoda
Diptera. Two species of midge (chironomid larvae)-Parochlus steinenii (Gerke) and an unidentified species of Limnophyes-were collected from rivers and streams (Brundin 1970). Parochlus steinenii has also been reported from the South Shetlands Islands (Edwards and Usher 1985). A related species, Eretmoptera murphi Schaeffer, was found at Signy Island (Block et al. 1984) where it has survived since being introduced in transplants from South Georgia and/or the Falkland Islands in 1967.
Coleoptera. The beetle Angusticolis claussi (Mü ller) was present in all Tønsberg Peninsula lakes and from a number of other locations. This powerful swimmer is able to avoid the plankton nets, and probably has a much wider distribution on South Georgia (Arnold and Convey 1998) than described here.
Collembola. The springtail Cryptopygus antarcticus Willem formed rafts up to 10 cm across comprising hundreds of individuals which were observed floating on several of the coastal pools near seal and penguin colonies while isolated individuals were occasionally observed on the surface of other pools.
(Antarcticola georgiae Wallwork (Cryptostigmata) and Hydadesia halophila Fain (Astigmata), which were collected in brackish water, have not been included in the analysis of the freshwater fauna.)

Discussion
The occurrences of the various aquatic invertebrate taxa found at South Georgia are given in Tables I-V. The Grytviken data (Table I) appear poor when compared with similar  habitats at Husvik (Tables III, IV). This can be explained by a reduced sampling effort at an early stage of the season when the lakes were ice-covered, and sampling was restricted to shallow outflows where melting had occurred. All of the species collected from Grytviken were subsequently found at Husvik. The marine pools (Table II) yielded unique species including marine Crustacea and mites as well as the cosmopolitan brackish water rotifers-Colurella colurus compressa and Encentrum spathetium-both of which have been reported from a number of other Antarctic and subantarctic locations (Dartnall and Hollowday 1985;Dartnall 1993Dartnall , 1995Dartnall , 2000. The pool fauna, with the possible exception of the artificial and overgrown Karrakatta pool (Table III), is essentially similar to that of the lakes (Table IV). The Karrakatta pool is enigmatic. It is devoid of copepods and ostracods, supports only one species of cladoceran instead of three, and is the only habitat where the rotifers Notholca salina and Lecane latissima were collected.
Block, Parochlus and the Tønsberg Peninsula lakes (Table IV) have very similar faunas. The paucity of species for Gulbrandsen Lake is undoubtedly true as this lake is very isolated at the bottom of an ''amphitheatre'' formed by several mountains, and is dammed by the Neumayer Glacier. Prevailing west-east winds across the glacier offer limited colonization opportunities. The lake is ice-covered for much of the year and its size and depth fluctuate dramatically, sometimes by many metres a day, as the glacier moves, causing the lake to periodically fill and drain. No trace of benthic vegetation has been found in this lake. The two Boeckella species probably feed on microphytes that grow around the edge of the lake, where sun-warmed stones thaw out the ice cover in still air.
The relatively low number of species recorded for the outlying stations (Table V) is undoubtedly a consequence of the small sample volume of the plunge-pots (125 ml) compared with the tens of litres sampled by a plankton net trawl. Consequently, no significance is attached to the paucity of species from Stromness Lake where the flora and fauna is undoubtedly as rich as the Tønsberg Peninsula lakes. Plunge-pots are normally used to augment other sampling methods rather than as a primary sampling method. Nevertheless, the rotifer Cephalodella catellina, which is specific to seal and penguin wallows in the Antarctic, and Notholca labis, Lindia torulosa and the tardigrade Echiniscus jenningsi, were only recovered using this method.
Three species not recorded in the present survey must also be considered as freshwater inhabitants of South Georgia. They are (1) a green hydra reported by Headland (1984)the first and only record of a freshwater cnidarian from anywhere in the Antarctic region; (2) the anostracan Branchinecta gaini (Daday) (Dartnall and Heywood 1980), noted at Stromness in the 1993/94 season (J. C. Ellis-Evans, personal communication) and is believed to be absent from water bodies populated by the predatory diving beetle Angusticolis claussi; and (3) the ostracod Notiocrypridopsis frigogena (Graf) reported from a freshwater pond fed by a spring above Grytviken and re-described from there (De Deckker 1981).
The anomopodan Pleuroxus truncata (Mü ller), previously reported from South Georgia (Pesta 1928), has not been included as it is now believed to have died out. According to Frey (1993): ''no species of the subgenus Pleuroxus or related genera occur on the islands of the Scotia Arc except for the aberrant specimen of Peracantha (5Pleuroxus) truncata reported from South Georgia (Pesta 1928) which was obviously introduced in connection with whaling operations''. The unknown amphipod reported by Weller (1975) has also not been included as it has not been reported subsequently and is now considered doubtful.
The 110 invertebrate species reported here from the freshwater lakes and pools of South Georgia (Table VI), although nearly four times greater than that reported by Dartnall and Heywood (1980), contain no surprise taxa. Although this survey only covered a small portion of South Georgia the rest of the island is not expected to support a radically different fauna.
The South Georgia fauna does not have an endemic fauna, nor does it include Antarctic endemics, such as the rotifers Philodina gregaria Murray and Adineta grandis, Murray (Dartnall 1983). South Georgia, which shares 28 species of rotifer and six species of crustacea with Signy Island, is part of a continuum within the Scotia Arc (Table VI) that links South America with the Antarctic and illustrates a clear decrease in the number of species with increasing latitude.