Biology, affinity and description of an unusual aquatic new genus and species of Isotomidae (Collembola) from high altitude lakes in Tasmania

A new species of Isotomidae (Collembola) was collected from submerged stones on the edge of nine lakes on Tasmania’s Central Highland Plateau. Because it did not comply fully with the characters of any existing genus, a new genus, Chionobora gen. n. is erected for it here. An Antarctic species, Desoria klovstadi (Carpenter), has characters which conform with the new genus so is formally transferred to the new genus here. The Antarctic Continent and Tasmania were last in proximity 60 million years b.p. so it is suggested both species are relicts persisting in probable ice-free refugia during glacial cycles. Gut contents of specimens of the new species exclusively contained diatoms in various stages of digestion and the species appears to graze on aquatic macrophytes, a feeding habit not recorded before for Collembola. We note the high numbers of endemic invertebrate taxa of restricted distributions in cold habitats of southern regions compared to warmer regions and stress their conservation values and threats to their populations.


INTRODUCTION
In 1988, numerous individuals of a previously unknown species of Collembola were collected by Tasmanian aquatic biologists during a survey of the littoral fauna of highland lakes on the Great Lakes Plateau Superficially the species was similar in habitus and some characters to Hy droisotoma schaefferi (Krausbauer, 1898) from a similar habitat in North Hemisphere.Characters in common were cylindrical dens, claw and mucro as well as habitus, all of which are adaptations to an aquatic habitat (Deharveng & Bedos, 2004) however it lacked bothriotricha which are a diagnostic character for Hydroisotoma.
In order to demonstrate the phylogenetic and biogeographical affinities of the new species, a new genus is erected for it, is diagnosed and its affinities discussed.It was found that Desoria klovstadi complied with the characters of the new genus rather than with Desoria sensu stricto so it is transferred formally to the new genus, Chionobora, here.
We also here discuss the paleogeographic reasons why high generic and high taxon diversity in Collembola and other invertebrates is found in severe habitats in the extreme south including Tasmania.

METHODS
The specimens were collected by removing submerged pebbles from the edge of the lake, turning them over and washing any organism adhering to the surface through a sieve of mesh size 500 µm aperture, into a vessel containing 10% formalin and later preserved in 75% alcohol.A minimum of 20 rocks were sampled at each site.chaetae (as) always present on Abd IV and V, often absent on Abd IV.Accp-s chaetae located in p-row (not anterior to it), very rarely with one or two s chaetae in a segment slightly displaced to a more anterior position .Laterally positioned accp s chaetae thicker and shorter than others.Micro s chaetae 1,1/1,1,1 from thorax to abdomen III inserted anteriorly on all segments (Fig. 10).
Thorax.Tibiotarsus broad and flattened, shortest on leg 1 and 2 (Fig. 1).Claw long and thin, apparently without in-ner tooth although minute lateral teeth sometimes visible.Tenent hairs not clavate (Figs 6-8).Empodial appendage with lobe at base, broad, with 3 lamellae and no teeth.Nomenclature of chaetae in tibiotarsal distal row of tibiotarsi uncertain as masked by polychaetosis (basal set is 7 Achaetae and 4 T-chaetae, but some chaetae can be displaced to distal row from the main part of tibiotarsi).Distal row with from 8 to 11 (maximum primitive number 11) with at least T1 and T2 chaeta absent in some specimens (Fig. 8).
Etymology.Anagram of Alima (an Arabic word meaning water sprite or sea maiden).
Distribution.Only known from lakes on the Central Plateau of Tasmania.
Ecology.Chionobora amila sp.n. was collected by removing submerged or partially submerged stones from the edges of lakes and washing the surfaces of the rocks.The habitat is illustrated in Figs 20 and 21.
The species is living in an aquatic medium which is confirmed by examination of gut contents of some individuals, each of which contained numerous diatoms belonging to the genera Gomphonema, Navicola and Frustulia (Fig. 22).Gomphonema species are epiphytic and Frustulia are acidophilic.The C. amila sp.n. specimens collected appeared to graze exclusively on diatoms growing on epiphytes and, as the species has a grinding molar plate, is probably able to penetrate the silicon coating of the diatoms.
Remarks.Chionobora amila sp.n. cannot be placed into any existing genus of Isotomidae but it shares characters with several genera of Isotominae.The genus to which the new species is most closely approximated is the Northern Hemisphere Desoria Nicolet, 1841, although it differs in several diagnostic morphological characters for this genus.The only southern representatives of Desoria sensu stricto currently known from Australia are likely to be introduced.The exception is Desoria klovstadi (Carpenter, 1902) from the Antarctic, which was redescribed by Stevens et al. (2006) and noted as an aberrant member of the genus; it is herein transferred to the new genus Chionobora (see below).Significant characters can be grouped into three types as follows: 1. Characters that are adaptations to aquatic habitat.Subcylindrical tuberculated dens, massive lamellate mucro, slender claw, large hypognathous head, covering of short and numerous chaetae, flattened tibiotarsi and polychaetosis of labium (Deharveng & Bedos 2002).The new species is morphologically similar to Hydroisotoma schaefferi only in characters that are adaptations to an aquatic habitat.Characters in common are: lamellate mucro, reduced number of sublobal hairs (absent in H. schaef feri), increased number of chaetae on labial palp and form of dens.The new genus differs from H. schaefferi in the absence of bothriotricha, absence of mucronal chaetae, absence of spines on female anal valves and tubercles on dens.The four species of another Holarctic genus, Grani sotoma Stach, 1947, live in contact with cold water, have long, cylindrical, tuberculated dens and lamellate mucro, and manubrium in some species with numerous chaetae on anterior side (G.danilevskyi Martynova, 1968, G. sadoana Yosii, 1965) similar to the new genus.Granisotoma species, however, have chaetae on the mucro and labial baseis oligochaetotic.This last character has only been observed in the type species of the genus, G. rainieri (Folsom, 1937) but other species of Granisotoma are not well known so the position of the genus within Isotomidae is unclear.Two other semi-aquatic genera Agrenia Börner, 1906 (dens is always tuberculated) and Isotomurus Börner, 1903 (particularly species with tuberculated dens) have polychaetotic labia but both have other diagnostic differences, for example, a long subapical chaeta on dens and bothriotricha on body, respectively.Azoritoma possesses adaptations to an euedaphic way of life with loss of ocelli and pigment, as well as development of specialised s chaetae on Ant I and Abd V. Table 1 lists other characters that differ between these two genera.The unusual modification of mouthparts of Azoritoma is likely to be related to the specialised habitat in which it is found.The new genus also shows some morphological similarities with Tibiolatra Salmon, 1941, a genus living in a similar habitat in New Zealand, as both posses flattened tibiotarsi.However Tibiolatra has antennal segments III and IV subsegmented, is furnished with serrated chaetae on body and a dens with serrated spines.

Characters showing affinities with
3. Similarity to groups of Northern Hemisphere.The new taxon shows a few affinities with three, largely Northern Hemisphere, genera, Isotoma, Desoria and Isotomurus but it did not comply sufficiently to place the new species in any of the three.The relationships of these genera have been discussed in several papers (Deharveng, 1981;Potapov, 2002;Stevens et al., 2006).To graphically illustrate the currently known affinities of genera of Isotominae lacking dental spines, a phenetic scatter diagram in given in Fig. 19 using the characters listed in Table 1.The programme PAST (Hammer et al., 2001) was used to perform non-metric multidimensional scaling basing on Euclidian distances matrix.One hypothesis, based on a selection of characters, is that Chionobora gen.n. and Isotomurus an tennalis group are possibly sister groups (Bagnall, 1940) (the group which has no bothriotricha).

Chionobora klovstadi (Carpenter, 1902) comb. n.
Isotoma klovstadi Carpenter, 1902: 222. Desoria klovstadi: Stevens in Stevens et al., 2006: 822.Desoria klovstadi, from East Antarctica, is the most closely related species to C. amila and complies with the diagnosis of the new genus.Both species combine two rare characters: absence of anterior chaetae on ventral tube and strong polychaetosis of labium.Other characters in common are: position of accp-s chaetae in p-row, form of mucro, short and numerous smooth chaetae on body, general shape of maxillary head rectangular and loss of some of Tchaetae on tibiotarsus.The Antarctic species however does not possess characters that are adaptations for species living in contact with water as does C. amila.The morphological characters of Desoria klovstadi placed the species in an intermediate position between the Northern Hemisphere Isotoma s. str.and Desoria s. str.Nicolet, 1841 (Stevens et al., 2006).Sequence data did not indicate a close relationship of D. klovstadi with existing Isotominae genera.Stevens et al. (2006) therefore provisionally placed the species in Desoria.The affinity of D. klovstadi with the new genus is demonstrated here; consequently it is hereby transferred to Chionobora.

DISCUSSION
The new genus has the unique combination of characters in Isotominae of polychaetotic labium, lack of bothriotricha on body, no anterior chaetae on ventral tube and some T-chaetae on tibiotarsi lost.Two species, C. amila and C. klovstadi are known to possess these characters; both are restricted to high latitudes of the Southern Hemisphere.Sequence data for C. klovstadi did not resolve any affinity with Northern Hemisphere genera so it is possible that molecular analysis for C. amila might be similarly not illuminating.Ancient phylogenetic relationships may not always be revealed from sequence data.It may be significant that Tasmania was last adjacent to the part of North Victoria Land, where C. klovstadi occurs, 60 million years ago (Royer & Rollet, 1997).
The current climate of the Central Plateau is cold, windy and can be both wet and dry according to season.Annual rainfall here can vary from 3000 to 1000 mm from northwest to east.Average maximum temperatures are 18.7°C in summer and 5.5°C in winter while minimums are from -2°C in winter and 5.5°C in summer.(Storey & Comfort, 2007).Consequently both species of Chionobora live in extreme habitats, that is they are A-(adversity) selected and also possess a range of adaptive characters, typical for the habitat in which they are found.Correlates of adversity selection are given in Greenslade (1983) and include selection against dispersal if adapted to stable but predictably severe environments which may explain their restricted distributions.It is therefore most likely that vicariance and not dispersal is the cause of species divergence in this genus.
Glaciation on the plateau has occurred multiple times over the last two million years but it is believed the area west of Lake Augusta, where C. amila occurs, remained ice free and that alpine vegetation was present (Kieran, 1990;Colhoun et al., 1996;Kirkpatrick & Fowler, 1998).Shiel et al. ( 1998) note that at times these lakes "may have been refugia for aquatic species".Stevens et al. (2006) suggested that C. klovstadi is a relict persisting in Northern Victoria land (Eastern Antarctica) in refugia throughout glacial periods and Stevens et al. (2006) detected multiple glacial refugia for Chionobora in Northern Victoria Land.This could also be proposed for C. amila with both Chionobora species surviving with limited change over a long period in extremely cold habitats in stasis.This proposes an alternative strategy to (Fraser et al., 2014) who suggested, based on biodiversity modelling, that geothermal sites allowed species to persist throughout glacial cycles.
The geological formation on which C. amila occurs is part of a large area of Jurassic dolerite over nearly half of Tasmania.This rock type cracks as it cools and later becomes eroded resulting in a plethora of crevices providing protected refuges for invertebrates.
There are over 3000 lakes and tarns in the Central Highlands.The survey during which C. amila was found, sampled 120 lakes (Fulton, 1998) but C. amila was only found on nine of them.These lakes are all fairly small, within 25 km of each other and at an altitude of between 1000 and 1200 m in the southern part of the Plateau.Two are shown in Figs 20 and 21.There appears to be no apparent reason for C. amila being only found in a few of the lakes surveyed but there are a number of possible reasons such as incomplete sampling, paleohistorical factors, fire both accidental and managed, algal composition, as well as past grazing by stock and road construction leading to erosion into the lake and recreation (Storey & Comfort, 2007).It is known that the Plateau lakes are heavily used for trout fishing and that trout have been introduced to most lakes with some lake levels manipulated for hydroelectric schemes (Anon., 2002;Storey & Comfort, 2007).Also trout are known to impact deleteriously on aquatic invertebrates (Cadwallader, 1996;Davies et al., 2002;Elvey, 2002), however all but one lake (Meander) contained trout at the time of sampling (S.Chilcott, pers.com.).Most individuals of C. amila were found in Lake Meander.
Other examples of invertebrate aquatic taxa with endemic species and genera in the Central Highlands region are the fresh water aquatic invertebrates of Rotifera (Shiel et al., 1998); Mollusca (Ponder et al., 1993), Annelida (Pinder & Brinkhurst, 2000), phreatoicoid Isopoda (Knott, 1975), Ephemeroptera (Campbell, 1988) (see Greenslade & New, 1991).An additional threat to this fauna is that it is predicted that the region will experience reduced rainfall with climate change.The preservation and protection of such faunas, that provide information on the palaeohistory of habitats and faunas present today is clearly of a high priority.
Southern Hemisphere taxa.The presence of spiny or serrated chaetae on parts of furca and/or body is an important character in Isotominae genera of South Hemisphere.They are present in Tomocerura Wahlgren, 1901; Procerura Salmon, 1941; Paracerura Deharveng & Oliviera, 1994 and Acan thomurus Womersley, 1934 among others.The new genus lacks this character.Although the new genus differs in external appearance and ecology, it shares the rare character of two sublobal hairs with the monotypic euedaphic genus Azoritoma Greenslade & Potapov, 2008, described from within cushion plants on subantarctic Macquarie Island.

TaBle 1 .
Comparison of morphological characters of fifteen taxa of Isotominae.