Journal of the Royal Society of New Zealand

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Distribution, ecology, and conservation status of freshwater Idoteidae (Isopoda) in southern New Zealand

W. L. Chadderton , P. A. Ryan & M. J. Winterbourn

To cite this article: W. L. Chadderton , P. A. Ryan & M. J. Winterbourn (2003) Distribution, ecology, and conservation status of freshwater Idoteidae (Isopoda) in southern New Zealand, Journal of the Royal Society of New Zealand, 33:2, 529-548, DOI: 10.1080/03014223.2003.9517742 To link to this article: https://doi.org/10.1080/03014223.2003.9517742

Published online: 30 Mar 2010.

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Distribution, ecology, and conservation status of freshwater Idoteidae (Isopoda) in southern New Zealand

W. L. Chadderton1, P. A. Ryan2, and M. J. Winterbourn3

Abstract Three species of Idoteidae, Austridotea lacustris, A. annectens, and A. benhami, are known from the South Island of New Zealand and some of its outlying islands. All three have largely coastal distributions, with A. lacustris and A. annectens inhabiting both fresh and brackish water. They feed mainly on plant detritus and, to a lesser extent, algae and invertebrates. A. lacustris is known from Stewart Island, Campbell Island, Pitt Island, and the south of the South Island. On Stewart Island, many stream populations of A. lacustris occur immediately above the upper limit of tidal influence, but on Campbell Island, the species penetrates much further inland. A. annectens has been found on Stewart Island, Pitt Island, and the southern South Island as far north as Banks Peninsula. It lives in freshwater streams and brackish lagoons. A. annectens has a 1-year life cycle with young released in spring. A. benhami is known only from a few freshwater streams near Dunedin (South Island) and appears to be the most endangered of the three species. Habitat protection by vegetated riparian strips is proposed to reduce potentially harmful effects on its populations.

Keywords Austridotea; conservation biology; distribution; Idoteidae; Isopoda; life history; New Zealand; streams

INTRODUCTION The New Zealand freshwater fauna is notable for its paucity of Isopoda, and the literature indicates that most streams, rivers, and lakes contain no isopods at all. Freshwater species described from New Zealand belong to suborder Phreatoicidea, family Phreatoicidae; suborder Anthuridea, family Leptanthuridae; and suborder Valvifera, family Idoteidae, the first two containing both surface-dwelling (epigean) and subterranean (hypogean) species (Chapman & Lewis 1976; Wilson & Fenwick 1999). The family Asellidae, a prominent component of the freshwater faunas of Europe and North America, has not been recorded from New Zealand, although Chapman & Lewis (1976) and Horning et al. (1977) have reported an unidentified asellotan from an Auckland dune lake and Snares Island, respectively. Little has been published about the biology of any freshwater species of isopod in New Zealand. Although Nicholls (1937) proposed the generic name Austridotea, with subgenera Austridotea and Notidotea for the three New Zealand species and one from southern South America, he failed to designate a type species and, as pointed out by Poore (2001), the genus-

1Department of Conservation, P.O. Box 112, Hamilton, New Zealand. Email: [email protected] 22802 East 132nd Circle, Thornton CO 80241, USA. 3School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. R02010 Received 22 April 2002; accepted 7 October 2002; published 18 June 2003 530 Journal of the Royal Society of New Zealand, Volume 33, 2003 level names were therefore nomina nuda. Poore (2001) formalised the generic name Austridotea by providing a short diagnosis and designating Idotea lacustris Thomson as the type species. All three New Zealand species, Austridotea lacustris (Thomson), A. benhami Nicholls, and A. annectens Nicholls are considered to be congeneric members of the family Idoteidae (Poore 2001), not Chaetiliidae as proposed by Poore & Lew Ton (1993). The three New Zealand freshwater species of Idoteidae are the subject of this paper. Austridotea lacustris was first collected from Tomahawk Lagoon, near Dunedin (Chilton 1890), and subsequently has been recorded from other localities in Otago and Southland (Chapman & Lewis 1976), Stewart and associated islands (as A. benhami; Chadderton 1988, 1990), and subantarctic Campbell Island (Chilton 1909; Joy & Death 2000). A. lacustris appears to be absent from the Snares Islands, which lie between Stewart and Campbell islands (Fig. 1), and where the only isopods found in seven creeks surveyed by Horning et al. (1977) were a species ofParavireia (Sphaeromatidae) and an unidentified asellotan. Similarly, there are no records of freshwater idoteids from the Auckland and Antipodes islands. Several authors have suggested that A. lacustris may be primarily a brackish water species, or a freshwater species of recent marine origin (see Nicholls 1937; Marples 1962; Chapman & Lewis 1976). No aspects of its biology and life history have been published, although Marsh (1983), drawing on his experiences fishing in Southland, noted that isopods (apparently A. lacustris, which is shown in a colour photograph) are found in still water with silty or muddy beds and are often eaten by trout. Austridotea annectens has been reported from coastal creeks and streams on Stewart Island (Nicholls 1937; Chadderton 1990) and in the South Island as far north as Canterbury (Chapman & Lewis 1976). Like A. lacustris it is known from both fresh and saline water, including Waituna Lagoon (Riddell et al. 1988) and Lake Ellesmere (Waite 1981; Dawn 1995), both of which are brackish. Ryan (1986) found A. annectens to be common in the guts of short-finned eels (Anguilla australis) collected near the mouth of Waikewai Creek, Taumutu, at the southern end of Lake Ellesmere. Marshall & Winterbourn (1979) and Widdowson (2001) reported A. annectens as being abundant on stony substrata in the Leeston Drain and Birdlings Brook, two freshwater tributaries of Lake Ellesmere. Apart from these distribution records, little seems to be known about the biology of this species. The third species, A. benhami, is the least well known. Chilton (1891) considered it to be a variety of Idotea lacustris, but it was described as a new species of Austridotea by Nicholls (1937). All validated records of A. benhami come from the vicinity of Dunedin and the Otago Peninsula, and all are from freshwater sites. Because their distributions were apparently limited and little was known about their ecology, Collier (1993) listed A. annectens and A. benhami as of potential conservation interest. This action was supported subsequently by Tisdall (1994) who recognised A. annectens as one of 20 New Zealand freshwater invertebrates to be given high priority for further investigation. A. benhami was not given high priority because its distribution was assumed, erroneously, to include Stewart Island (records of A. lacustris had been attributed to A. benhami).

Fig. 1 Known distribution of Austridotea lacustris (closed circles), A. annectens (closed triangles), and A. benhami (open squares) on South, Chatham, Stewart, and Campbell islands. Inserts show location of the islands and details of distributions in the Lake Ellesmere, Otago Peninsula, and Horseshoe Bay (Stewart Island) areas. Closed squares show additional localities for A. benhami given by Chilton (1891). Campbell Island records from Joy & Death (2000). Chadderton et al.—Freshwater Idoteidae in southern New Zealand 531

Chatham Islands

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5km 532 Journal of the Royal Society of New Zealand, Volume 33, 2003

The purpose of the present paper is to draw together information on the distributions of the three species and aspects of their life histories and ecology. Collecting on South Island, Stewart Island, and Pitt Island (Chatham Islands), and the work of Joy & Death (2000) on subantarctic Campbell Island, has provided a much better understanding of their geographic distributions, habitats, and associations with other invertebrates. Additionally, gut content analyses have provided information on the diets of the three species, and the life cycle of A. annectens has been elucidated. Together, these data allow a better assessment of their conservation status and potential vulnerability to extinction.

METHODS Collections and surveys Searches were made for isopods in lowland streams, rivers, estuaries, and lagoons in southern South Island, Stewart Island, Codfish Island, Auckland Island, Campbell Island, Pitt Island, and Chatham Island between 1987 and 2001. On the South Island, collections were made in Southland including southern Fiordland and the Catlins, eastern Otago, and coastal Canterbury as far north as Lake Ellesmere. Extensive surveys by Chilton (1909) and Carpenter (1982, pers. comm.) of coastal rivers and streams on Banks Peninsula and in north Canterbury as far north as the Waipara River revealed no idoteids and these areas were not resurveyed. Collections were made in several ways, principally hand collecting and "kick-net" sampling. The latter involved disturbing streambed materials by hand and foot and collecting all invertebrates and plant debris in a hand net. Searches were made in flowing and still water, and on a wide range of substrata (sand, mud, gravel, cobbles, wood), amongst algae, and plant debris. Samples were preserved in the field with formalin or ethanol. Benthos samples were sorted in the laboratory and isopods and other invertebrates were identified. Most aquatic insects and were identified to genus or species except Diptera and Coleoptera (identified to family), and Oligochaeta (to class). The composition of invertebrate assemblages associated with the three isopod species was examined with detrended correspondence analysis (DECORANA). The analysis was carried out with PC-ORD (McCune & Mefford 1999) using presence-absence data for all taxa (except Idoteidae) collected at 35 sites. Water samples were collected in clean polyethylene bottles from many sites for the measurement of conductivity in the laboratory. Because some field sites were in remote areas, several days elapsed before measurements were made in some instances. Altitudes of collection sites and their distances from the sea were estimated from topographical maps (NZMS 260, 1:50 000). "The sea" was defined as the uppermost point on a river or stream known to be influenced by saline water. In addition to our own collections, distribution records and environmental data were obtained from several published papers (e.g., Taylor 1974; Joy & Death 2000) and other sources, e.g., theses and unpublished reports, as indicated in the text. The identities of isopod species reported in these studies were confirmed from specimens or photographs. Locations of isopod collection sites are given in Appendix 1.

Scanning electron microscopy Specimens of the three idoteid species were examined by SEM to check details of anatomical features used as taxonomic characters. were dehydrated in an alcohol series, air dried, mounted on aluminium stubs with conductive carbon paint, and sputter-coated with gold-palladium. Individuals were mounted on their dorsal and ventral surfaces, some with appendages removed so that underlying structures could be seen. Observations were made on a Leica 440 SEM at up to 150x magnification at 10-18 kV. Chadderton et al.—Freshwater Idoteidae in southern New Zealand 533

Gut content analysis Gut contents of 12-40 medium-large specimens of all three species from five localities were examined. Individuals were dissected individually on glass microscope slides and their gut contents teased out into a pool of lactophenol-PVA. A cover slip was added and slides were dried in an oven for at least a week. Slides were scanned under a compound microscope at up to 400x magnification, and the presence of seven food types: leaf (including grass fragments), wood and bark, sooty mould fungi, diatoms, filamentous algae, fine particulate matter (detritus and silt), and fragments, was recorded. Animal prey items were identified where possible from head capsules, mouthparts, and other appendages. Relative abundance of different food types was not measured objectively, but dominant materials were noted.

Life history determination Changes in population size structure and reproductive state of A. annectens were determined from 10 collections made in Waikewai Creek, a tributary of Lake Ellesmere, between September 1975 and August 1976. Samples were obtained by kick-sampling (net mesh 0.5 mm) within primarily gravel substrata. Isopods were preserved in the field and total body lengths were measured later with a micrometer inserted in the eyepiece of a dissecting microscope at lOx magnification. Individuals were classified as juveniles (body length < 6 mm; sex indeterminable), males, or females. Males were identified by the presence of an appendix masculina, while females were distinguished by the presence of brood plates medially on the bases of pereiopods. Broods carried by females were classified according to the stage of development of embryos. The four-stage system described by Jones & Naylor (1971) for the asellotan isopod Jaera albifrons was used. Fully developed juveniles retained within the brood pouch were designated stage 5. Egg counts were made on 20 females in November and their association with female size (body length) was examined by Spearman rank correlation.

IDENTIFICATION OF SPECIES The most detailed descriptions of A. lacustris, A. annectens, and A. benhami are those of Nicholls (1937), whereas taxonomic characters used to distinguish the species are given by Chapman & Lewis (1976). They include the form of the maxillipedal palps and pleotelson, the degree of setosity of the antennae (in males), the size and positioning of the eyes, and the general shape of the abdomen. Nevertheless, it is still easy to confuse A. lacustris and A. benhami, whose general appearance is very similar (Fig. 2). For this reason we are including the following observations and comments based on our examination of a wide range of material. In A. lacustris the pleotelson is separated from the free abdominal segments by two pairs of clefts of unequal lengths, whereas only one pair of clefts is found in the other two species (Fig. 3B,E,H). This character provides the most practical and least ambiguous way of identifying A. lacustris. The number of maxillipedal palp segments (five in A. annectens and A. benhami, and four in A. lacustris) is also a good practical character, although a groove subdividing "apparent lobes" of the fourth segment in A. lacustris can cause confusion (Fig. 3A,D,G). The shape of the head and size and positioning of the eyes are less satisfactory taxonomic characters since the differences between species are small and minor changes in orientation of specimens can change one's perception of these features. The presence of a thick setal fringe on the male antennae of A. lacustris, and its absence in the other two species, is also a sound character but the setae are shorter than the width of the antennal segments and do not always stand out. 534 Journal of the Royal Society of New Zealand, Volume 33, 2003

Fig. 2 Scanning electron micrographs of A, Austridotea lacustris (Little River, Stewart Island); B, A. annectens (Waikewai Creek, Canterbury); C, A. benhami (Lindsay Creek, Dunedin). Scale bars = 1 mm.

Austridotea annectens and A. benhami are easily distinguished on the shape of the pleotelson, which comes to a point in the former (Fig. 2B) but is evenly rounded in the latter (Fig. 2C). The distal segments of the uropods also differ in shape, as shown in Fig. 3F,I. Coloration is often an unreliable feature to use for identification due to intraspecific variation, but it can be useful in distinguishing the species. Thus, the general colour of A. lacustris tends to be grey or greenish grey, whereas A. benhami is a brownish grey and A. annectens is yellowish brown.

RESULTS Distribution and habitat Austridotea lacustris Austridotea lacustris is the most widely distributed of the three species. It is known from the southern South Island, including Fiordland, Stewart Island and adjacent Codfish Island, subantarctic Campbell Island, and Pitt Island (Chatham Islands) (Fig. 1). The northernmost records for the species on South Island are George Sound in the west and Otago Peninsula in the east. A. lacustris occurs on a wide range of substrata, especially stable cobbles, boulders, and logs. It can be particularly abundant on the undersides of submerged branches and logs and may aggregate in cracks and grooves in the wood. Individuals may also be found on sand, mud, gravel, and leaves, and they will swim actively in the water column when disturbed. Where present, A. lacustris is frequently the dominant macroinvertebrate numerically and in terms of biomass, especially at or directly above the saltwater/freshwater interface (Chadderton 1990, pers. obs.; Joy & Death 2000). Chadderton et al.—Freshwater Idoteidae in southern New Zealand 535

Fig. 3 Scanning electron micrographs of distal maxillipedal palp segments (top row), dorsal surface of pleotelson (middle row), and ventral post-abdomen (bottom row) of Austridotea lacustris (A-D) A. annectens (D-F), and A. benhami (G-H) showing characters used in identification (see text). Scale bars: A, D, G = 100 (im; B, E, H = 200 (im; C, F, I = 300 (im.

Most populations have been found close to the sea in flowing waters ranging from very small, shallow streams to large, meandering floodplain rivers. They often occur where flow is influenced by the tide and where saline water may penetrate, at least on spring tides. On Stewart Island most populations were restricted to the first 10-20 m reach of freshwater above the upper limit of saline water penetration. Although found most frequently near the coast, A. lacustris does penetrate inland (Fig. 4). The furthest inland it has been found is a remarkable 30 km, in a tributary of the Oteramika River, Southland (Ryder 1997). On Campbell Island it has been recorded up to 5 km from the sea at altitudes up to 400 m a.s.l. (Joy & Death 2000), and on Stewart Island, Chadderton (1990) collected it 13 km inland on the Rakeahua River and at 135 m a.s.l. in a small coastal stream. However, Cowie et al. (1978) did not find A. lacustris at 10 river, stream, and pond sites more than 5 km from the sea in Stewart Island's Freshwater valley. On Pitt Island, all A. lacustris populations were found within 1 km of the sea and at less than 130 m a.s.l. The predominantly coastal distribution of the species is reflected in the moderately high conductivities of water samples taken at some collection sites (Fig. 4). These indicate the incursion of seawater in some instances and probably a more general influence of the sea on aerosols and rain in others (Chadderton 1990). Joy & Death (2000) did not measure conductivity or salinity of stream water on Campbell Island. However, Taylor (1974) found that stream, tarn, and bog waters throughout the island were dominated chemically by sodium and chloride (mean concentrations 31 and 56 mg l–1, respectively), which were introduced from the sea in stormy weather. Despite appearing to favour waters with low-moderate conductivity/ salinity, Chadderton (1990) found that A. lacustris could tolerate the full range of salinities from distilled water to seawater for at least a week. 536 Journal of the Royal Society of New Zealand, Volume 33, 2003

Fig. 4 Frequency of occur- rence of records of Austridotea lacustris (solid) and A. annectens (open) in relation to A, conductivity; B, altitude; and C, distance from the sea.

<100 100-200 200-300 300-400 Conductivity {uS cm'1)

1-10 11-100 Altitude (m a.s.l.)

1-10 11-100 100-1000 Distance from sea (m)

Table 1 Invertebrate taxa most commonly associated with Austridotea lacustris and A. annectens in collections from South and Stewart islands.

A. lacustris A. annectens n = 20 n=12 Chironomidae (17) Potamopyrgus (12) Deleatidium (15) Oligochaeta (12) Oligochaeta (14) Tricladida (12) Potamopyrgus (11) Paracalliope (10) Paracalliope (10) Chironomidae (9) Austrosimulium (10)

The essentially freshwater nature of A. lacustris is borne out by an examination of the invertebrate taxa with which it is associated. Thus, at 20 sites on Stewart and South islands where community data were collected, the most commonly co-occurring associates were freshwater insects, snails, crustaceans, and oligochaetes typical of freshwaters (Table 1), and included 6 mayfly (Ephemeroptera), 17 stonefly (Plecoptera), and 19 caddisfly (Trichoptera) genera. The depauperate stream invertebrate fauna of Cambell Island is dominated numerically by A. lacustris, especially at high altitudes, and common associates include gammarid and eusirid amphipods, Chironomidae, a simuliid, and two stoneflies (Rungaperla species). However, several typically estuarine crustaceans, the grapsid crabs Helice crassa and Hemigrapsus crenulatus, the amphipod Paracorophium excavatum, and two species of Chadderton et al.—Freshwater Idoteidae in southern New Zealand 537

Fig. 5 Ordination of sites with 400 -, species of Austridotea present • lacustris on DECORANA axes 1 and 2 o annectens based on presence-absence of 300 * benhami macroinvertebrate taxa (35 sites with data available). The wide spread of points for A. lacustris CN and A. annectens indicates the % 200 H diverse nature of the associated o "o faunal assemblages. 100 .°o°o

0 < 100 200 300 400 Axis 1

Tenagomysis (Mysidae), also occurred with A. lacustris at several sites. The high taxonomic diversity of the invertebrate assemblages of which A. lacustris is a part (76 genera and higher taxa; Fig. 5) reflects the broad salinity tolerance and wide range of substrata inhabited by the isopods.

Austridotea annectens The first specimens of A. annectens were collected from a small stream flowing into Horseshoe Bay on Stewart Island (Nicholls 1937). Although the exact locality of this stream was not given, it was found in two streams entering this bay during our surveys. Despite extensive searching, A. annectens was found at only one other site on Stewart Island, where it and A. lacustris both occurred in low numbers. On the South Island it has been recorded from streams and lagoons in eastern Southland and Otago, and from Lake Ellesmere and three of its inlet streams (Fig. 1). However, in 2000 we were unable to find it in small streams flowing into the Mataura estuary, Tautuku Bay, and one of the Horseshoe Bay streams where it was present 10 years ago. Similarly, several searches of the lower Leeston Drain in the last 5 years have failed to locate A. annectens, which was common there 30 years ago (Marshall 1974). It also occurs in fresh and brackish water on Pitt Island in the Chathams group (Fig. 1), but has not been found on Chatham Island. The greatest distance inland at which A. annectens has been recorded is 5 km and the maximum altitude about 100 m a.s.l. (Fig. 4). Austridotea annectens occurs on a variety of substrata including stones, sand, and macro- algae. In tributaries of Lake Ellesmere, almost all individuals have been found on the undersides of stones (Marshall & Winterbourn 1979; Widdowson 2001), whereas Waite (1981) and Dawn (1995) collected them from the water column of Lake Ellesmere. Unlike A. lacustris it is never a dominant member of the macroinvertebrate community. The invertebrates most commonly found with A. annectens in freshwaters were snails, oligochaetes, flatworms, an amphipod Paracalliope, and chironomid larvae (Table 1). Fewer mayfly, stonefly, and caddisfly genera were found than with A. lacustris (6, 3, and 14, respectively), reflecting the generally more modified and often degraded nature of the coastal streams in which A. annectens was found. In estuarine habitats, mysids, H. crassa, P. excavatum, and Scolecolepides spp. (Polychaeta) were prominent associates. Like A. lacustris, the invertebrate communities including A. annectens were highly diverse (67 genera and higher taxa) (Fig. 5). 538 Journal of the Royal Society of New Zealand, Volume 33, 2003

Austridotea benhami The distribution of A. benhami appears to be limited to streams on and north of the Otago Peninsula and the Waters of Leith catchment north and west of Dunedin (Fig. 3). Collection sites on Lindsay and Ross creeks, tributaries of the Leith, were 5-6 km inland (200 m a.s.l.), but in Greenacres Stream, which flows into Macandrew Bay, large numbers of A. benhami were found from near sea level to 95 m a.s.l., 1.3 km upstream. At all three sites sampled on Greenacres Stream it was easily the most abundant macroinvertebrate. Chilton (1891) also found A. benhami at two sites north of Otago Harbour in the vicinity of Port Chalmers and Waitati, but we found only A. lacustris in this general area. Streams inhabited by A. benhami had clear water, moderately stable, often embedded substrata, including large cobbles and boulders, and flowed through mixed forested and pasture catchments with some housing developments. Conductivity of streamwater at all sites was < 100 (AS cm-1. Records of isopods from Lindsay Creek, Ross Creek, and a third tributary of the Leith (Poulters Creek) given by Hall et al. (2001) most likely refer to A. benhami as well. Unfortunately, specimens were not kept so this could not be confirmed. Common macroinvertebrates associated with A. benhami in benthic samples from Lindsay Creek and Greenacres Stream, which had similar faunas (Fig. 5), were the mayfly Deleatidium, beetle larvae (Elmidae), the snail Potamopyrgus antipodarum, several stoneflies, and oligochaetes.

Food Gut contents of isopods from five localities indicated that all three species were omnivorous and ingested a mixture of plant detritus, algae, and invertebrate prey items (Table 2). Specimens of A. lacustris used for gut content analysis were taken from a mixture of primarily forest streams on Stewart Island, and from the lower Waikawa River in the Catlins where it flows through farmland. Ninety percent of the 40 individuals from Stewart Island had ingested wood or bark and 63% contained animal fragments. The high incidence of sooty mould fungi is consistent with the abundance of ingested wood and bark fragments on which they were probably growing (Collier 1990). Prey organisms identified were leptophlebiid mayflies, including Deleatidium (12 guts), midge larvae (Diptera: Chironomidae, 10 guts), and blackfly larvae (Diptera: Simuliidae, 7 guts). Unidentified animal fragments were found in another 10 isopods, and the presence of A. lacustris in the guts of seven individuals

Table 2 Frequency of occurrence of food materials in the guts of Austridotea lacustris, A. annectens, and A. benhami from five localities. Percentages of guts examined containing each item are given. NR, not recorded; FPM, fine particulate matter.

A. lacustris A. annectens A. benhami Stewart Island Waikawa Waikewai Greenacres Lindsay streams River Creek Stream Creek Number of guts 40 12 16 20 12 Leaf (incl. grass) 8 50 44 100 100 Wood and bark 90 18 50 75 83 Sooty mould fungi 33 0 19 0 12 Diatoms 12 50 100 15 44 Filamentous algae 15 25 31 0 18 FPM (incl. silt) NR 92 100 100 92 Animals 63 18 20 20 0 Chadderton et al.—Freshwater Idoteidae in southern New Zealand 539 provided evidence of probable cannabilism. A. lacustris from the Waikawa River had gut contents dominated by leaf fragments (including grass), rather than bark or wood, while two contained mainly filamentous algae. A complex of fine detritus, diatoms, and silt was most abundant in the others. Deleatidium was the only prey item found. Austridotea annectens from Waikewai Creek, a tributary of Lake Ellesmere, appeared to be primarily benthic foragers, since mixtures of fine detritus, diatoms, and silt were found in all 16 specimens dissected. Some individuals had also ingested filamentous algae and sooty mould fungi, whereas leaf and wood fragments were found in about half the individuals. The only animal fragments seen were bits of copepods (Crustacea) in two guts, and a winged dipteran in one. All 32 individuals of A. benhami examined contained leaf fragments, some derived from grasses rather than trees. However, over three-quarters of them also contained fragments of wood and variable amounts of fine amorphous detritus and silt. The guts of some individuals were filled with leaf fragments but hardly any fine detritus or silt, suggesting that they had been feeding in clean leaf-packs. In contrast, the gut contents of other A. benhami comprised mainly fine detritus and silt, consistent with foraging on the streambed. Few diatoms or filamentous algae had been ingested by A. benhami and animal fragments were uncommon. Prey fragments found in the guts of four specimens from Greenacres Stream were two appendages, a rotifer, and bits of a Deleatidium nymph. Overall, gut content analysis indicated that all three isopods ingest a wide range of plant and animal materials, although our results suggest that A. lacustris is more carnivorous than the other two species. Variations in gut contents of individuals from different localities are likely to reflect differences in the availability of potential foods at a large scale (e.g., wood is more available in forest streams), and restrictions imposed by the immediate foraging site (e.g., leaf pack, silty bed, or algal mat).

Life history Austridotea annectens Austridotea annectens was univoltine and had a well-defined life cycle in Waikewai Creek, Taumutu (Fig. 6; Table 3). Very small juveniles (body length 2-3 mm) were first taken in field collections in November and were the main component of the population in December. Mean population size increased in the following months and was essentially bimodal in late autumn and winter when males had grown larger than females. Ovigerous females with stage 1 eggs were found in July. The mean development stage of eggs increased in subsequent months and juveniles were first found in marsupia in October. Adult females were also present in November and December, although the proportion that were ovigerous and their absolute numbers (as indicated by sampling effort) declined and none was taken in January.

Table 3 Percentage of ovigerous females of Austridotea annectens in field collections, and the median egg-stage present each month. NA, not applicable.

Sep Oct Nov Dec Jan Feb Apr May Jul Aug

Sample size 39 113 40 18 99 59 115 66 67 69 % ovigerous 54 96 50 33 0 0 0 0 79 87 females Median egg stage 2 4 2 4 NA NA NA NA 1 1 540 Journal of the Royal Society of New Zealand, Volume 33, 2003

September February

1 3 5 7 9 11 13 15 1 3 5 7 9 11 13 15

October April

1 3 5 7 9 11 13 15 1 3 5 7 9 11 13 15

November

1 3 5 7 9 11 13 15 1 3 5 7 9 11 13 15

December July 80 -, 60 %40 20 -

1 3 5 7 9 11 13 15 1 3 5 7 9 11 13 15

January August 40 -, 40 '/• 20 - %20 0 0 -I , 1 1 1 , r 1 3 5 7 9 11 13 15 1 3 5 7 9 11 13 15 Length (mm) Length (mm)

Fig. 6 Size distribution (body lengths) of Austridotea annectens in Waikewai Creek in 10 months, September 1975-August 1976. Black, males; white, females; cross hatched, juveniles. Chadderton et al.—Freshwater Idoteidae in southern New Zealand 541

Fig. 7 Size distributions of A, Austridotea benhami from Greenacres Stream, Otago Peninsula in November; and A. lacustris from streams on Stewart Island in B, August; C, December; and D, February.

40 .g 30- S 20 (U *• 10 • 0 a

20

15-

10

0

25- D

20 •

i

1 — S. °- I 5- I 0- ~i-H n 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Length (mm)

The average number of eggs carried by females in November was 43.9 (range 22-53, n = 20). Numbers of eggs per female was not correlated with female body length, which ranged from only 8.4 to 10.7 mm (r = 0.28, n = 20,P> 0.05).

Austridotea lacustris and A. benhami Systematic life history sampling programmes have not been undertaken for these two species, but some inferences can be drawn from the size structure of individuals in our larger collections (Fig. 7). Thus, the November collection of A. benhami from Greenacres Stream was unimodal and dominated by fairly small individuals. This size distribution is not unlike that found for A. annectens in September (Fig. 6), and suggests a univoltine life cycle with new individuals appearing in spring. In contrast, populations of A. lacustris from two streams on Stewart Island were strongly bimodal in August and December, whereas the size range of individuals present in a third population in February was very broad. In all 3 months very small isopods (2-3 mm) were present indicating that the period over which young may be released is much longer than observed for A. annectens in Canterbury. 542 Journal of the Royal Society of New Zealand, Volume 33, 2003

DISCUSSION The three New Zealand idoteids are predominantly coastal species that inhabit a wide range of freshwater and brackish water habitats in the south of the country. Although running water habitats occupied by many A. lacustris and A. annectens populations, in particular, are very similar, we have few records of pairs of species co-occurring. Exceptions are a single site on Stewart Island and streams on Pitt Island in the Chathams. Both species were present also in the Waikawa River system and the Waipori/Waihola complex, although we did not find them together. Like many other estuarine and freshwater isopods (Green 1968; Marcus et al. 1978) they feed primarily on plant detritus, although algae and invertebrates were also found among gut contents. Animals were most common in the diet of A. lacustris. Graca et al. (1993) noted the importance of fungi in the diet of the European freshwater isopod Asellus aquaticus; the presence of large-diameter sooty mould fungi in the guts of A. lacustris and A. annectens suggests that they may be ingested selectively by the New Zealand species, also. Naylor (1972) noted that the British marine isopod Idotea emarginata employed a raptorial mode of feeding, in which plant and animal food was macerated by the biting mouthparts while being held by the legs. A. lacustris was observed feeding on captured insect larvae in a similar way (Chadderton 1990). Austridotea lacustris is the most widely distributed of the three species, and was abundant on Stewart and Campbell islands. The absence of trout from almost all running waters on Stewart Island (Chadderton & Allibone 2000) may be one reason for its abundance there, since it is known to be eaten by trout in the South Island (Marsh 1983) where its distribution is much more patchy. Trout are also absent from Campbell Island whose isolation and uninhabited state may be important factors protecting the species. Chapman & Lewis (1976) suggested that it is typically a brackish water species, but our records indicate that it occurs most commonly in freshwater. Nevertheless, the distribution of A. lacustris is predominantly coastal and its tolerance of a wide range of salinities (up to full seawater; Chadderton 1990) suggests that its invasion of freshwater is recent. The recent evolution of freshwater races of another idoteid isopod, Mesidotea entomon, has been documented in the Baltic region where freshwater populations of this typically brackish water species have evolved since the last ice age (Croghan & Lockwood 1968). Freshwater populations of M. entomon retain physiological features of the brackish water races, including the ability to tolerate seawater. Obvious questions to ask are whether inland and island populations of A. lacustris have diverged genetically, and, if so, whether this is associated with greater adaptation to freshwater environments. In particular, isopods on geographically isolated Campbell Island have invaded freshwaters extensively and might be expected to differ genetically from other populations. The two other species of Austridotea have more restricted geographical distributions than A. lacustris. A. annectens is most common in the South Island, although individuals have been collected from a few freshwater sites on Stewart and Pitt islands. A. annectens also appears to have a stronger affinity for both saline and lentic waters, and was found in several variably saline lagoons (Lakes Ellesmere and Waihola, Waituna Lagoon, Mataura estuary). Nevertheless, it can also be common well inland in freshwater streams where saline water never penetrates. The life cycle of A. annectens in Waikewai Creek was characterised by males being larger than females, an adult sex ratio of about 1:1, the appearance of ovigerous females in mid-late winter, and the release of young in late spring/early summer. A characteristic of many isopod species is that males are larger than females (Kensley & Schotte 1989), but sex ratios can vary markedly, and Jones & Naylor (1971) found that females ofJaera albifrons outnumbered Chadderton et al.—Freshwater Idoteidae in southern New Zealand 543 males by up to 14:1. Release of young in the warmer months is consistent with the general condition found in British marine littoral isopods (Naylor 1972), although the timing of reproductive activity in isopods can be influenced by water temperature (Andersson 1969; Leifsson 1998). In the well-studied European freshwater isopod Asellus aquaticus, life cycle length can be 1 or 2 years and the period over which eggs are carried can be more drawn out and less synchronous than found in A. annectens (Steel 1961; Andersson 1969; Chambers 1977). Our body-length histograms for populations of A. lacustris on Stewart Island show small individuals present in August, December, and February, suggesting a more extended period over which young may enter the population in that species. Austridotea benhami is perhaps the most interesting of the three species. It was something of an enigma to Chilton (1891), who examined material from several streams on the northern side of Otago Harbour but refrained from describing it as a new species. Instead, he suggested it be considered a variety of lacustris (then placed in the genus Idotea), a decision overturned by Nicholls (1937) who described it as a new species. Austridotea benhami has a very restricted geographical distribution. All localities from which it has been recorded are close to Dunedin, and most are some kilometres inland and well above sea level. Its seaward distribution in river systems such as the Waters of Leith may be limited by urban development in the lower parts of the catchment, but as yet evidence to support or refute such a suggestion is lacking. Because its geographical range is so limited, and the catchments in which it is found are subject to agricultural and urban development, A. benhami is likely to be the most vulnerable of the three species. However, only A. annectens is listed by Tisdall (1994) and McGuiness (2001) as a species considered to be threatened (Category 1 species). This probably reflects a degree of confusion over the identity of A. benhami, which was mistaken for A. lacustris by Chadderton (1988, 1990), an error perpetuated by Collier (1992). In his compilation of data on aquatic invertebrates of potential conservation interest, Collier (1992) listed A. annectens from only three of the ecological regions of McEwen (1987), but it is now known from five (Canterbury Plains, Rakiura, Catlins, Makarewa, Otago Coast), whereas A. benhami is known from only one (Otago Coast). Using Collier's (1992) criteria it therefore falls into the rarest category (i.e., restricted distribution, and uncommon). Similarly, when considered with regard to threat of extinction (Molloy et al. 2002), A. benhami can be classified as "at risk" and "range restricted", whereas A. lacustris and probably A. annectens would be designated "not threatened". According to Collier (1993) the greatest threat to freshwater invertebrates in New Zealand has been loss and reduced quality of habitat caused by human-induced modifications to catchments, riparian and stream channel environments, water quality, and river flow. Similarly, Chadderton & Allibone (2000) argued that land-use changes, as well as the introduction of exotic fish, have brought about changes in the distributions of native, New Zealand fish. Angermeier (1995) emphasised that species with limited ecological and geographic ranges are vulnerable to extinction in aquatic and terrestrial systems, and if genetic diversity is to be conserved within the species, protection of a diversity of habitats is necessary (Ponder et al. 1996). These contentions are pertinent to the continued survival of the New Zealand idoteids, especially A. benhami. We believe the most effective way to conserve these species is through habitat restoration and management with emphasis on protecting the ecological integrity of entire systems (Angermeier 1995; Maitland 1995). It is difficult to address conservation issues relating to stream invertebrates above the habitat level, partly because the most pertinent aspects of their ecology are rarely understood adequately (Collier 1993). However, the protection of aquatic habitat has the potential to conserve natural assemblages 544 Journal of the Royal Society of New Zealand, Volume 33, 2003 of species that may include taxa of particular conservation interest. Because A. benhami has no free-living, or marine, larval stage, the whole life cycle is likely to be confined within a limited length of stony stream. Protection of reaches by the provision of a vegetated riparian zone or, better still, forested riparian reserves, should provide a first degree of protection by limiting stream habitat degradation through siltation and by providing a source of decaying plant material for this primarily detritivorous species. In contrast to A. benhami, A. lacustris is not classified as endangered using contemporary criteria (Collier 1992; Tisdall 1994; Molloy et al. 2002), and because it is widely distributed and often abundant in remote localities (Auckland, Chatham, and Stewart islands) its conservation status is not an issue. However, the disjunct distribution of A. annectens, its apparent disappearance from at least four sites in the last 30 years, and its absence from many seemingly suitable habitats along the east coast of the South Island suggest that it was probably more widespread in the past. For example, it might be expected to inhabit many more tributaries flowing into Lake Ellesmere than seems to be the case. Whether it will be possible to protect declining species such as A. annectens in small streams within intensively farmed catchments is debatable, and the long-term survival of the species may be in larger lagoons and tidal estuaries whose size may help limit habitat and water quality degradation.

ACKNOWLEDGMENTS We thank all those who assisted in the field, especially Richard Allibone, Jon Harding, Peter Ravenscroft, and numerous staff of the Department of Conservation offices on Stewart Island and in Southland. We also thank Ann Chapman, Gerry Closs, Mike Joy, Sandy King, Stephen Moore, Greg Ryder, Paul Sagar, and Joy Woods for contributing specimens, data, and other information on the distributions of species. Jan McKenzie took the scanning electron micrographs and compiled them into plates, and Chris Edkins produced the maps. Graham Fenwick and Gary Poore provided valuable comments that improved the scientific content of this paper. Initial fieldwork on Stewart Island by LC was assisted by grants from the New Zealand Lotteries Board, the New Zealand Forest and Bird Protection Society, and the New Zealand Department of Conservation.

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Appendix 1 Collection sites for the three species of New Zealand Idoteidae. Abbreviations of collectors names: WLC, W. L. Chadderton; MJW, M. J. Winterbourn; MAC, M. A. Chapman; DoC, Department of Conservation field staff.

Grid reference Location (NZMS 260) Dates Collector

Austridotea lacustris Stewart Island Maori River estuary E48/344615 25 Feb 2000 WLC Maori River E48/343614 5 Feb 1988, 25 Feb 2000, WLC, DoC 28 Mar 2000, 6 Apr 2000 Lee Bay Road stream E48/384598 24 Feb 2000, 30 May 2000 WLC Rakeahua River D49/188462 1989 (multiple collections) WLC Rakeahua River trib. D49/193458 31 Dec 1988 WLC Big Glory stream E49/348479 29 Nov 1988 WLC Ogles Creek D48/270508 1 Feb 1988 WLC Little River E48/370607 4 Feb 1988, 17 Jan 1996 WLC, MJW Little River beach E48/369608 4 Feb 1988 WLC stream New Xmas Village D48/263730 18 Jan 1989 WLC hut stream Slip Creek D48/095775 15 Jan 1989 WLC Boulder Beach D48/089774 15 Jan 1989 Bungaree swing E48/315649 20 Jan 1989 WLC bridge stream Deep Bay Stream E48/394558 16 Jan 1996 MJW Yankee River D48/190784 20 Jan 1989 WLC Yankee-Lucky stream D48/199773 20 Jan 1989 WLC Ryans Creek track E48/357562 20 Jan 1989 WLC stream Smoky Beach stream D48/144769 20 Jan 1989 WLC Pegasus cat camp D49/083262 31 Jan 1989 WLC stream Southwest Arm stream D49/226474 28 Nov 1988 WLC North Arm stream D48/294592 20 Jan 1988 WLC Lucky Point stream E48/231768 31 Dec 2000 WLC Vaile Voe E48/371561 Jun 1994 WLC Codfish Island Rodriguez D48/021682 18 Dec 1987 WLC Sealers Bay E48/015691 18 Dec 1987 WLC Pitt Island Waihere Bay Sht 2/078197 Feb 1994 WLC Waikuri Stream 31 Mar 2000 S. King Flower Pot stream Jun 1998 S. King Otago-Southland Nita Creek, C41/719656 24 Oct 1998 WLC George Sound Gut hut creek, B43/363284 17 Feb 1999 WLC Secretary Island Tahakopa River G47/3 68028 11 May 1988, 25 Feb 1999, WLC 4 Jun 2000 Taieri River H45/890647 11 Aug 1998 S. Moore Waikawa River G47/130943 1998, 29 Feb 2000, WLC, G. Ryder, 8 Aug 2000 DoC Oteramika Creek F46/827194 1997 G. Ryder (continued over page) 548 Journal of the Royal Society of New Zealand, Volume 33, 2003

Appendix 1 (continued) Grid reference Location (NZMS 260) Dates Collector

Thompsons Creek I44/238849 29 Nov 2000 WLC Tomahawk Creek I44/211756 29 Nov 2000 WLC Boulder Beach stream G. Ryder Lake Waihola 8 Dec 1997 MAC Campbell Island 19 streams See Joy & Death (2000) M. Joy A. annectens Stewart Island Horseshoe Bay, E48/383594 17 Nov 1988, 25 Feb 2000, WLC Back Road stream 30 May 2000 South Horseshoe Bay E48/387587 17 Nov 1988, 17 Nov 1999, WLC stream 17 Jan 1998, 30 May 2000 Southwest Arm, stream D49/226474 28 Nov 1988 WLC Pitt Island Waihere Bay stream 18 Apr 2000 S. King Flower Pot stream Jun 1998 S. King Otago-Southland Tahakopa River trib. 7 Dec 1998 MAC Florence Hill stream G47/390989 12 May 1988 WLC Toetoes Harbour F47/882949 11 May 1988 WLC stream Waipori River H45/876644 9 Mar 2000 S. Moore Waikouaiti River I43/246084 1 Mar 2000 S. Moore Waikawa River G47/145967 29 Feb 2000 WLC Waituna Lagoon F47/772956 5 Aug 1985, 13 Feb 1999 Riddelletal. 1998 Lake Waihola 8 Dec 1997, 3 Dec 1998 MAC Canterbury Waikewai Creek M37/585054 18 Apr 2000, 30 May 2000, MJW 4 Jun 2000 Leeston Drain M36/569133 1972 (multiple collections) J. W. Marshall Birdlings Brook M36/535143 May 2000, 21 Jun 2000 MJW Lake Ellesmere, M36/593120 1978-79 (multiple R. G. Waite Timberyard Point collections) Lake Ellesmere, M36/825130 1994 (multiple M. Dawn Kaituna Lagoon collections) A. benhami Lindsay Creek I44/198837 1 Jan 1998, 29 Feb 2000, G. Ryder, WLC, no date MAC Ross Creek I44/151823 3 Oct 2000 G. Ryder Greenacres Stream I44/234793, 29 Nov 2000 WLC 236791, 245790