Lepidurus arcticus

DISPERSAL Large Branchiopoda possess a remarkable ability to colonise new areas due to their dormant eggs within the egg banks (Longhurst 1955). Numerous studies have shown that branchiopod eggs are passively dispersed by mammals, birds, fish, amphibians, crayfish, insects, and wind (see Rogers 2014). When eggs are moved by predators like ducks and waders, which select similar habitats, this constitutes a ‘ directed’ passive dispersal vector. Additionally, eggs, larvae, and adults drifting downstream can also be considered ‘ directed’ passive dispersal, while wind acts as a ‘ random’ passive dispersal vector. Many branchiopods have developed the tactic of attaching their propagules to firm substrates, effectively reducing the likelihood of dispersal (Fryer 1996). This is also true in L. arcticus (Lakka 2013). This indicates that remaining in a favourable habitat may be more advantageous than risking the hazards of dispersal to other locations. An example of this is the slow colonisation of L. arcticus to newly established habitats near Aldegondabreen in Spitsbergen (Walseng et al. 2018). In Svalbard, small ponds are closely linked to the nearby terrestrial landscape, which often is inundated by snowmelt and rainfall, with passive water dispersal being possible (Borgstrøm et al. 2018). In the Scandes, the drift of larvae from upstream lakes into a reservoir is well documented (Brabrand et al. 2015). Downstream dispersal of adults of L. arcticus from a fishless headwater pond has similarly been observed in the Tornionjoki catchment in northern Finland (Lakka, unpubl.). Consequently, the establishment of L. arcticus in reservoirs downstream from their natural lower limit may also result from passive drift from upstream lakes (Dahl 1932; Aass 1969). Ponds and wetlands in Svalbard and lakes in the Scandes serve as important habitats for a diverse range of waterfowl and other bird species. At least 23 bird species are identified as potential dispersal vectors since they are frequently observed on or near water bodies (Lakka 2013, 2020; Håland 2012). Many of these birds prey on the adults, larvae and eggs of L. arcticus, thus serving as dispersal vectors. Water birds are more likely to travel directly between suitable habitats. Branchiopod eggs resist avian digestive enzymes, and passage through the alimentary canal can even enhance hatching (see Rogers 2014). Sticky eggs can be embedded in mud and attached to the fur of mammals or the feathers of birds (Neretina et al. 2020; Rogers et al. 2021). In Svalbard, eggs have been found at the foot of a ringed plover, Charadrius hiaticula Linneus, 1758 (see Lakka 2013). The distribution of similar haplotypes of L. arcticus across wide geographical ranges suggests high dispersal via migrating birds (Hessen et al. 2004). Lepidurus arcticus may also be actively dispersed by human stockings (Berg 1954; Aass 1969; Hesthagen 1979; Fjellheim et al. 2007). PREY-PREDATION RELATIONSHIPS Freshwater organisms play important ecosystem roles in linking terrestrial and aquatic detritus and periphytic algal production to higher trophic organisms such as fish. As an adult, L. arcticus is an omnivore species preying on detritus, bacteria, and different kinds of plants and algae, and it is also a predator of other crustacean species (Jeppesen et al. 2001), including conspecifics (Miller 1980; Lakka 2013, 2015). Evidence from a study of ten lakes on Hardangervidda conducted between 2000 and 2002, where the food web of brown trout was analysed using stable isotopes, revealed elevated δ 15 N levels indicating L. arcticus’ diverse diet in comparison with other invertebrates (Rognerud et al. 2003). In Arctic freshwater communities, L. arcticus dominates in size and biomass (Klemetsen et al. 1985; Dimante-Deimantovica et al. 2015, 2018). As a predator of smaller crustaceans, it significantly impacts the planktonic food web, as demonstrated in a study of Greenland ponds and lakes (Cristoffersen 2001). However, research on 75 ponds and lakes in Svalbard showed no substantial difference in taxa numbers between locations with and without L. arcticus (Walseng et al. 2018). Many lakes in the Scandes are renowned for their excellent fishing conditions, hosting large, high-quality brown trout (Figure 7). The abundance of crustacean food sources like L. arcticus, Gammarus lacustris G. O. Sars, 1863, and Eurycercus lamellatus (O. F. Müller, 1776) contributes to the brown trout’s substantial growth, often reaching one to three kilos or more (Qvenild et al. 2021). Arctic charr also attain big-sized specimens of excellent quality in lakes with these crustaceans. Lepidurus arcticus is highly sought for by fish. When this species is abundant, it positively influences the growth and production of the fish stocks, which are of considerable economic importance to Northern countries. In Lake Øvre Heimdalsvatn, L. arcticus accounted for 15 % of the annual energy intake of the brown trout population (Lien 1978). Instances of heavy predation show more than 200 – 300 specimens in a single brown trout stomach (Aass 1969; Borgstrøm 1970). Lake Sylvetjønne, located at high altitude in the Jotunheimen mountains, demonstrated remarkable yields of brown trout, primarily based on a diet of L. arcticus and G. lacustris, with total yields reaching 13 kg per hectare in 1941 – 1942 and individual specimens weighing up to 10 – 12 kg (Hesthagen 2005). Brown trout selectively prey on L. arcticus, with its significance as a food source increasing with the size of the fish (Qvenild & Hesthagen 2020). Conversely, an overstocking of juvenile brown trout can negatively affect L. arcticus populations. For example, in Lake Vesle Øljusjø, excessive stocking led to the species’ nearextinction, although it reappeared in brown trout diets after a decade without stocking (Aass 1969). In a comprehensive dietary analysis of brown trout populations across five lakes on Hardangervidda, the exceptionally strong 1997 year-class nearly eliminated L. arcticus and G. lacustris (Qvenild & Hesthagen 2020). The crustacean populations recovered once the brown trout numbers returned to normal levels. The absence of L. arcticus in High Arctic lakes is frequently attributed to the presence of Arctic charr, which is the only fish species present (Jeppesen et al. 2001; Presthus Heggen et al. 2010; Borgstrøm et al. 2018). In the Scandes, the invasive Eurasian minnow has populated many lakes and reservoirs since the 1960 s, thriving in habitats up to 1400 m a. s. l (Museth et al. 2007). As opportunistic foragers, minnows introduced to high-altitude lakes have been linked to reduced availability of brown trout prey (Museth et al. 2007; Borgstrøm et al. 2010). In Lake Øvre Heimdalsvatn, minnows appeared in 1969, leading to significant changes in the brown trout diet during the summer months between 1970 – 1972 and 1993 – 2005 (Lien 1978; Borgstrøm et al. 2010). Here, the Eurasian minnow has severely reduced L. arcticus populations, causing its virtual disappearance from the brown trout diet. On Hardangervidda, the Eurasian minnow was first observed in Lake Ørteren in 1973 and has since spread to at least 42 lakes in the northeastern plateau (Qvenild & Hesthagen 2019). The expansion continued westward, eventually reaching the headwater lakes in the tributary Sevra in the Lågen catchment. Numerous lakes in the neighbouring catchments, where both L. arcticus and G. lucustris are staple food organisms for brown trout, are easily accessible from the flat terrain near Stigstuv. In August 1999, a rotenone treatment was carried out, followed by another in September 2000, effectively eradicating the minnows. This action also caused immediate mortality within the invertebrate community (Fjellheim 2004). Remarkably, the diversity was almost completely restored within just one year. Lake Skjerja, formerly a prominent brown trout lake, experienced drastic declines in brown trout yield due to intense exploitation and subsequent minnow invasion, leading to a reduction in annual catches to only 32 % of the initial values (Qvenild et al. 2024). Despite fishing efforts to control minnows, the competition for food has negatively impacted brown trout production. In Lake Skjerja, the two crustacean species, G. lacustris and L. arcticus, were the basic food items. Fishing with baited traps on the minnows and the presence of a large fraction of predatory brown trout feeding on minnows have reduced the predation pressure on these two crustacean species, and they are still a part of the brown trout diet. In contrast, L. arcticus and G. lacustris no longer seem to be important in the brown trout diet in the nearby Lake Skaupsjøen, where no systematic reduction of Eurasian minnow is performed (Borgstrøm 2009). The balance of fish predation on L. arcticus is mainly determined by the abundance of fish and the availability of suitable refuges for the prey. Unlike other notostracan species that grow rapidly in temporary water bodies before they dry out or encounter increased predation (Longhurst 1955), this strategy is unfeasible in the High Arctic’s permanent lakes, which are often inhabited by stunted populations of Arctic charr dwarfs (Jeppesen et al. 2001). In these cold environments with clear water, the growth and development of L. arcticus are slow, making them particularly vulnerable to fish predation. Fish density also acts as a limiting factor in the permanent lakes of Bear Island and the Scandes. Nevertheless, in lakes where optimal refuges are accessible and fish abundance is low, L. arcticus can thrive. Thus, L. arcticus’ adaptability to various habitats seems to be broader compared to other notostracan species. Studies conducted on Hardangervidda and Northern Finland indicated a higher occurrence rate of L. arcticus populations in larger lakes with better access to refuges (Qvenild & Hesthagen 2019; Lakka 2021). Larger lakes also are generally colder than smaller ones. Birds are significant predators of L. arcticus. In Spitsbergen and Bear Island, species such as the purple sandpiper Calidris maritima Brünnich, 1764, the dunlin Calidris alpine (Linneus, 1758), and the arctic tern Sterna paradisaea Pontoppidan, 1763, frequently prey on L. arcticus in the shallow tundra ponds (Summerhayes & Elton 1923; Lakka 2015). When faced with these avian predators, L. arcticus exhibit behavioural adaptations (Lakka 2013). Observations in a pond with a dense L. arcticus population in Kapp Linné in Svalbard revealed that L. arcticus sought the shallowest areas when a red phalarope, Phalaropus fulicarius (Linneus, 1758), visited. The shallow and muddy waters (5 – 15 cm deep) hindered the red phalarope from performing its typical hunting behaviour of swimming in circles. The variations in the colour morphs of L. arcticus are believed to be an adaptation to avoid bird predation in addition to harmful UV radiation (Lakka 2013). In Lake Langavatnet on Hardangervidda, sea ducks, such as the black scoter Melanitta n. nigra (Linneus, 1758), have declined comparatively with the increase in brown trout abundance, which likely is a result of increased competition for their common food item, L. arcticus (Håland 2012). Simultaneously, piscivorous waterbird species like the Goosander (Mergus merganser Linnaeus, 1758), Red-breasted Merganser (Mergus serrator Linnaeus, 1758), Great Cormorant (Phalacrocorax carbo Linnaeus, 1758) and Black-throated Diver (Gavia arctica Linnaeus, 1758) grazing on the enhanced numbers of small brown trout increased. ENVIRONMENTAL CONDITIONS Lepidurus arcticus has demonstrated a remarkable ability to withstand significant environmental changes in the harsh alpine and Arctic environments since its emergence. Despite this, the species exhibits considerable sensitivity at various stages of its life cycle to numerous environmental stressors. Critical temperatures and thermal thresholds Temperature is a key environmental factor controlling the distribution of ectotherm species, including branchiopods. Typically, Triops species are more warmth demanding than Lepidurus (Fryer 1988). The cold-adapted L. arcticus can withstand water temperatures ranging from 1.8 ° C (Lakka 2013) to 19 ° C (Arnold 1966). Although L. arcticus can endure temperatures above 20 ° C for brief periods (Borgstrøm 2019), it is crucial that temperatures do not remain above 16.5 ° C (Lakka 2020). The ideal temperature for adult L. arcticus is around 10 ° C (Lakka 2013). In the Arctic, this optimal thermal condition is met for only short-duration periods, but it is more commonly encountered in Scandes lakes. Every species has an optimal temperature range for hatching efficiency. In a laboratory experiment in Svalbard, the highest hatching rate for L. arcticus was noted at 10 ° C (Pasquali et al. 2019). There was no hatching observed at 15 ° C and 25 ° C, likely because these temperatures surpassed the eggs’ physiological tolerance limits. Comparatively, the optimal hatching temperature of the more southerly distributed L. couessi was found to be 20 ° C (Hann & Loonsberry 1991). On the other hand, the more adaptable species, T. cancriformis, exhibited hatching within a broader range of 16 – 32 ° C, with an optimal range of 22 – 32 ° C (Kuller & Gasith 1996). Elevated temperatures above 15 ° C during the juvenile phase negatively impact survival (Pasquali et al. 2019). Abrupt changes in water temperature during the initial hatching phase, following ice break-up, can frequently occur. For instance, Lake Øvre Heimdalsvatn experienced a rise in water temperature from 4 to 14 ° C within just ten days in 2018 (Sildre, https: // sildre. nve. no). These temperature shocks are especially likely in small, shallow lakes and ponds, particularly in the Scandes region. During the moult stage, L. arcticus is also highly vulnerable (Thiéry 1997). Warm, oxygen-poor water can trigger an ‘ oxygen search behaviour’, where an adult swims to the surface, collects a small air bubble between their legs, and then swims back to the bottom (Lakka 2013). This behaviour exposes L. arcticus to predators such as birds and fish. The lower thermal threshold The time required for an organism to complete its life cycle can be measured in thermal units, known as degree-days. Laboratory experiments conducted with L. arcticus eggs collected in the autumn from lakes in southern Norway (Borgstrøm & Larsson 1974) and northern Finland (Lakka, unpubl.) suggest that a cumulative thermal energy of between 400 and 500 degree-days is necessary for the embryonic development to complete once the eggs are submerged in water. Fryer (1988) also noted that L. arcticus eggs could hatch without undergoing a drying or freezing period. In dry or frozen conditions, embryonic development is halted. For example, the shallow Lake Solvatnet in Spitsbergen freezes solid during winter (Pasquali et al. 2019). Eggs collected after the ice melted and incubated at temperatures of 5.0 ° C and 10.0 ° C began hatching after 15 and 7 days, respectively. Thus, eggs incubated at 5.0 ° C in Lake Solvatnet hatched more quickly than those kept constantly wet (Borgstrøm & Larsson 1974). This indicates that part of the embryonic development must have occurred during the previous summer or earlier. Only 64 % of the eggs in the Lake Solvatnet experiment hatched, highlighting the importance of the egg bank as a buffer against variable environmental conditions (Hann & Lonsberry 1991; Brendonck 1996). Lepidurus arcticus can tolerate very low temperatures close to freezing. However, a minimum thermal input is essential for the organism to develop into adults. In a study of six lakes on Hardangervidda, no adult L. arcticus was found in brown trout stomachs until approximately 400 degree-days were achieved (Qvenild et al. 2018). This equates to a mean summer temperature of at least 5 ° C from July 1 st to September 15 th. In the High Arctic, only ponds tend to be warm enough to support L. arcticus (Nowiński & Wiśniewska-Wojtasik 2006; Lakka 2013) in at least some seasons, enabling the establishment of an egg bank. After hatching, the postembryonic phase must be completed, and new eggs laid before freezing. In particularly cold environments, females seem unable to produce eggs (Olofsson 1918; Lakka 2015), thus restricting their altitudinal and latitudinal range. The heat sum required may vary in different climatic regions since adaptation to colder habitats is likely, as is revealed in Arctic charr (Koops & Tallman 2004; Jeuthe et al. 2016). However, information is lacking for L. arcticus. Late ice break-up and low water temperatures during a short growing season can significantly impede the growth and development of L. arcticus. This has been documented over several years on Hardangervidda (Halvorsen 1973; Pedersen & Scobie 1990; Simonsen & Valderhaug 1994; Borgstrøm 2016; Qvenild et al. 2018) and in ponds in Svalbard (Lakka 2013). Lepidurus arcticus was found to be smaller in colder ponds near glaciers compared to warmer ponds closer to the sea (Lakka 2013). In many ponds studied by Olofsson (1918), L. arcticus appeared to be absent, likely due to excessively low temperatures. The upper thermal threshold The lower altitude boundary for lakes containing L. arcticus in the Scandes signifies that beyond a certain thermal threshold, the conditions become unfavourable. Most Lepidurus lakes remain at mean air temperatures below the 10 ° C isotherm (Qvenild et al. 2021). This explains why the distribution appears to be dichotomous in the Scandes, splitting into northern and southern zones. The intermediate region is generally too warm to support L. arcticus populations. In the northernmost area, reaching up to 70 ° N, the 10 ° C isotherm falls below 250 m a. s. l, with few lakes situated above 300 metres in this region. Consequently, the likelihood of discovering new L. arcticus populations is minimal. Few Lepidurus lakes have a mean lake air temperature higher than 11 ° C in summer (Qvenild et al. 2021). Lakes with lake air temperatures at this level tend to have mean summer water temperatures close to 14 ° C. Hence, the upper thermal threshold for Lepidurus lakes was assumed to be close to this level. Excessive accumulation of thermal energy during the initial embryonic phase can lead to life cycle mismatches, resulting in eggs hatching too early under completely dark conditions of winter. In the Scandes, L. arcticus is rarely found in ponds, although it does occur (Berg 1954; Koli 1957; Halvorsen 1973; Hesthagen 1979; Blomkvist 1995; Walseng et al. 1996). Most small lakes and ponds have environmental conditions that are often too unstable for sustained survival. Fluctuating temperatures and acidic waters have adversely impacted these habitats (Blomkvist 1995; Fjellheim et al. 2002; Lakka 2013, 2020). Lepidurus arcticus populations are more commonly found in larger lakes where there is better access to refuges and colder water (Qvenild & Hesthagen 2019; Lakka 2021). Moreover, deeper lakes allow L. arcticus to evade warm surface layers by migrating to cooler depths. Further north, including Bear Island and Spitsbergen, the species predominantly inhabits coastal zones (Klemetsen et al. 1985; Lakka 2013). In these northern islands, the mean air temperature during summer typically does not exceed 5 ° C, ensuring that elevated temperatures do not limit its distribution. Water quality Acid waters low in calcium In addition to temperature, water quality plays a crucial role in determining distribution. These conditions are likely to interact with one another, as evidenced in T. cancriformis (Schönbrunner & Eder 2006). In a study on Hardangervidda, the absence of L. arcticus in the western region was attributed to low water temperature, a short growing season, and water that was low in pH and calcium (Qvenild & Hesthagen 2019). The melting snow in spring usually leads to a drop in pH levels during ice break-up in Arctic and alpine lakes (Fjellheim et al. 2002; Lakka 2020), which can be detrimental or even lethal for L. arcticus larvae, as shown in a laboratory study by Borgstrøm & Hendrey (1976). Hence, the juvenile phase is crucial in evaluating extinction risks. For crustaceans, rapid calcification of their exoskeleton immediately after moulting is essential, but this process can be hampered in regions with low calcium levels and poor water quality (Rukke 2002). This problem is further exacerbated by low pH levels, which normally interfere with low calcium concentrations. Multiple studies have shown that L. arcticus does not inhabit water bodies with pH levels below 6.1 (Borgstrøm & Larsson 1974; Borgstrøm & Hendrey 1976; Fjellheim et al. 2007; Lakka 2013). However, a recent analysis of 95 lakes on Hardangervidda, which host L. arcticus, revealed that pH levels ranged from 5.49 to 7.26, and calcium levels varied between 0.28 and 5.54 mg / L - 1 (Qvenild & Hesthagen 2019). Ten of these lakes had pH levels under 6.0 and calcium levels below 0.84 mg / L - 1, conditions which are thought to be critical for L. arcticus. The two lakes with the lowest calcium levels, Lake Svartavasstjørni and Svartavatnet, have been monitored each year. In 1993 – 1994, the pH was approximately 5.5, and the calcium level was approximately 0.4 mg / L - 1 (Tysse et al. 2022). An annual liming programme began in 1994, and the lost L. arcticus population was restocked over three consecutive years (1997 – 1999) (Fjellheim et al. 2007). The species has been observed nearly every year since then, despite the liming programme ending in 2013. Calcium levels have stabilised between 0.5 and 1.0 mg / L - 1, and pH values have been close to 6.0. Therefore, L. arcticus can survive in waters with calcium levels at approximately 0.5 mg / L - 1 and low pH values near 6.0. Proximity to refugia with better water quality is crucial for mitigating harmful pH fluctuations (see Fjellheim et al. 2007), as close to non-lethal conditions may inadvertently impact functional traits like growth and reproduction. Previous studies have indicated that L. arcticus likely became extinct in multiple regions, such as Finland (see Lakka 2020) and Northern Sweden (Lingdell & Engblom 2002). Recent research on the ponds examined by Koli (1957) suggests that the population has become extinct due to acidification (Lakka 2020). In the southernmost regions of the Scandes, specifically Setesdalsheiene, numerous populations of L. arcticus may have similarly faced decline and eventual extinction following the disappearance of brown trout populations observed in the mid- 1880 s (Enge et al. 2017). Humic waters The concentration of total organic carbon (TOC mg L - 1) in humic waters is an indicator of the runoff of external organic matter from surrounding areas. In shallow inland water bodies, carbon can accumulate at the bed of lakes or ponds, influencing the carbon flux. For lakes with clear water in the Scandes, TOC levels are typically low. On Hardangervidda, the TOC in nearly all Lepidurus lakes was less than 2.0 mg C L - 1 (Qvenild & Hesthagen 2019). Research conducted by Lakka (2013) in Svalbard in 19 ponds revealed TOC levels ranging between 0.9 and 52.0 mg L - 1. Despite these conditions, thriving populations of L. arcticus were observed, demonstrating the species’ tolerance to elevated TOC concentrations, at least temporarily. Salinity The hatching of many notostracan species is influenced by various environmental factors, including salinity (Brendonck 1996; Schönbrunner & Eder 2006). The salinity levels in water bodies where L. arcticus resides are typically very low. Salinity can exceed 30 ‰ in coastal regions of the Barents Sea. Research from Spitsbergen has shown that L. arcticus flourishes in ponds with salinities reaching up to 1.5 ‰ (Lakka 2013). Slightly saline water has been found to promote the growth of L. arcticus, an adaptation likely linked to survival in temporary ponds where salinity increases as they dry up. In these environments, L. arcticus must efficiently produce eggs before the pond desiccates. This assertion is backed by two observations from Lakka (2013). Firstly, faster growth to a larger size enhances reproductive success in L. arcticus, as larger individuals can produce more and bigger eggs. A low salt concentration (1 ‰) was found to significantly accelerate the second moulting process. Secondly, smaller individuals began producing more eggs in slightly saline temporary ponds compared to permanent ones. Arctic lowlands near the sea and river systems are generally former seabeds where sea spray is prevalent. This is also true for Svalbard, where populations of L. arcticus are located near the sea (Lakka 2013). The salinity tolerance of L. arcticus may have evolved in coastal seabed habitats where the primary water source was melting snow and glacier ice, with permafrost keeping pond bottom soil activity minimal. Lepidurus arcticus in regulated lakes In the Scandes, L. arcticus is frequently observed in regulated lakes. Out of 483 locations where L. arcticus is present, 88 are hydropower reservoirs, of which 25 are regulated more than 20 m, sometimes even 125 m (Qvenild et al. 2021). These reservoirs present highly variable environmental conditions that are often detrimental to many freshwater organisms. However, L. arcticus and another Branchiopoda, the Cladocera Eurycercus lamellatus, typically show an increase in abundance (Dahl 1933; Huitfeldt-Kaas 1935; Aass 1969). This is likely due to their drought- and freeze-resistant eggs, which may stay dormant in the dry drawdown zone. The relative significance of these two species as food for brown trout was illustrated in the reservoir Lake Aursjøen in the Reinheimen area (Hesthagen 2018; Lakka et al. 2020). During the period 1985 – 1989, L. arcticus dominated in the brown trout diet. Following a change in the regulation regime, the abundance of L. arcticus collapsed. The reason remains largely unknown, but both climate- and hydropower-driven changes may be important. The alternative food source, E. lamellatus, did not appear to adequately compensate for the caloric deficit resulting from the decreased consumption of the larger L. arcticus. This likely contributed to the observed decline in the condition, growth, and delayed maturation of the brown trout. The temperature of the water significantly affects the altitudinal distribution of L. arcticus, also in reservoirs (Qvenild et al. 2021). This was initially demonstrated through the impoundment history of Lake Pålsbufjorden in the Lågen catchment (Dahl 1932). This catchment drains numerous Lepidurus lakes located in the eastern fells of Hardangervidda. Lepidurus arcticus was not previously recorded in this downstream lake, but just a few years after the impoundment, it became established in the new reservoir. Dahl (1932) explained this phenomenon by the cooling of the drawdown zone, leading to an ‘ arctification’ of the lake. Specifically, the ‘ arctification’ might be attributed to the halted embryo development in the dry drawdown zone, which accumulates fewer degree-days than in the wetter zone below. This allowed L. arcticus to flourish in Lake Pålsbufjorden, a habitat that was initially too warm for this species. Comparable altitudinal shifts of L. arcticus have been observed in other lakes following the establishment of hydropower reservoirs (Aass 1969; Brabrand & Saltveit 1980). The reproductive success of L. arcticus in regulated lakes is heavily influenced by the timing of reservoir refilling. In Lake Mårvatn on eastern Hardangervidda, increased lowering of the water level during the winter of 1969 / 70 without subsequent refilling resulted in poor condition and low brown trout catches (Borgstrøm 1973). In preceding years, L. arcticus was the main food source, but it almost disappeared from the diet in 1970 and 1971, likely because the eggs remained dormant in the dry drawdown zone. Moreover, siltation resulting from erosion adversely affected the production of algae, which serves as the primary food for L. arcticus larvae. During 2009 – 2010, Lake Mårvatn was again permanently lowered, resulting in a similar outcome (Rognerud & Fjeld 2014). Proper management practices are thus crucial to maintaining healthy populations of L. arcticus and supporting the ecosystems that depend on them. In some reservoirs, the introduction of new fish species has caused L. arcticus to become a less significant food source. For instance, in Lake Stolsvatn in the Hallingdal mountains, L. arcticus was a primary food item for brown trout before the Eurasian minnow was introduced (Borgstrøm et al. 1985). The eggs, which are reddish or orange, are typically deposited in the littoral zone, and the sticky eggs are firmly glued to the bottom substrate. Upon hatching, the larvae are highly visible and exposed to predation from the schooling minnows. After several years, adult L. arcticus no longer constituted a significant part of the brown trout diet. Minnows did not replace L. arcticus in the diet, indicating that new fish species can cause increased food competition and diminish brown trout yields. Similar adverse effects have been observed in other hydropower reservoirs such as Lake Halnefjorden on Hardangervidda (Lehmann et al. 2008) and Lake Kaldfjorden in the eastern fells of Jotunheimen (Hesthagen 2022). In Lake Kaldfjorden, Eurasian minnow and whitefish were introduced in the 1970 s, resulting in L. arcticus being excluded from the brown trout diet. The disappearance of L. arcticus in the fish diet may serve as an indicator of an impending decline in a fish population. In some reservoirs, the Eurasian minnow seems to be less harmfull. Lake Vinsteren in Jotunheimen was impounded in the 1940 s, and L. arcticus was the predominant food item for brown trout (Aass 1969). After the Eurasian minnow was introduced in the 1980 s, the abundance of L. arcticus has differed. However, there has been no significant decrease in brown trout yield, likely because the minnows are more exposed to brown trout predation, given the absence of refuges in the drawdown zone (Hesthagen & Gran 2019). CLIMATE CHANGE Few regions on Earth have experienced more dramatic climate changes than the High Arctic, a trend that is projected to continue unabated. Over the last thirty years, temperatures in Svalbard have increased by at least 2 ° C above the 1961 – 1990 normal (Hanssen-Bauer et al. 2019). Of particular interest in this context is the rise in summer temperatures. In the Svalbard archipelago, data from five meteorological stations reveal that the summer air temperature has increased by 1.41 ° C in the first two decades of this century compared to the normal period (July 1 st to September 15 th). Similar data from six meteorological stations in the Scandes show a concurrent increase of 1.26 ° C, suggesting significant changes in summer temperatures in both Arctic and Alpine regions. The Svalbard archipelago The diversity of freshwater organisms in the still waters of Svalbard is notably low, even when compared to other High Arctic regions (Coulson et al. 2014). Climate change is causing glaciers to retreat, leading to the formation of new, untouched freshwater ecosystems. This rapid warming and lengthening of growing seasons may influence biodiversity and dispersal patterns, potentially attracting more warmthdemanding species. Currently, there are 31 documented crustacean species on the Svalbard archipelago, with at least seven being recent arrivals (Dimante-Deimantovica et al. 2018). This represents an increase of over 20 % in the total number of known species on the island. As water bodies warm, L. arcticus may extend its range into the colder regions of Svalbard, including new inland ice-free areas, and become more prevalent on the colder islands, such as Nordaustlandet and other eastern islands. Dry sediments in temporary ponds are susceptible to wind dispersal. Increased bird activity due to warming affects ponds and lakes (Walseng et al. 2018), and birds preying on L. arcticus may aid in egg dispersal. The eggs can adhere to fur and are resistant to avian digestive enzymes (see Rogers 2014). Similar haplotypes over large areas suggest significant bird-mediated dispersal (Hessen et al. 2004). The distribution of freshwater fauna along a gradient from the glacier edge at Aldegondabreen in Grønfjorden to the coastline probably mirrors the colonisation history from the last 80 years of melting in Svalbard (Walseng et al. 2018). This study found L. arcticus in six of the 22 surveyed sites, all located in ‘ older’ water ecosystems near the shoreline (Bjørn Walseng, pers. comm.). So far, these findings suggest a relatively low dispersal rate of L. arcticus over a short period. Currently, L. arcticus exists below its optimal temperature in Spitsbergen and could thus benefit from warmer summer temperatures and prolonged growing seasons. Consequently, in the clear-water ponds in Spitsbergen, L. arcticus may experience accelerated growth and development. Notably, the size of L. arcticus has significantly increased since Sars’ studies in 1896 (Lakka 2013). The clear-water lakes may warm considerably and resemble those in Bear Island, potentially leading to coexistence with Arctic charr. Lakes such as Straumsjøen and Nordre Borgdam may reach temperatures conducive to developing such populations. The Scandes Sediment remains of L. arcticus indicate that it once had a much broader distribution across Europe (see Økland & Økland 2003), suggesting a later recolonisation of colder lakes formed its current presence in the Scandes. In the Scandes, L. arcticus mainly inhabits the alpine zone, with 87 % of its lakes located above the treeline (Qvenild et al. 2021). The early Holocene warm period (7800 – 7600 BP) serves as an early indicator of potential increased precipitation linked to ongoing greenhouse warming (Dahl & Nesje 1996). During this time, a notable rise in the treeline occurred, which could have resulted in a considerable range retraction if L. arcticus followed this treeline extension. Thus, prolonged warm periods might pose significant challenges for L. arcticus in the Scandes, which is the southernmost edge of its range in Europe. A retraction from the lower part of its distribution is documented for another cold-water adapted branchiopod, Branchinecta palludosa (O. F. Müller, 1788) (Lindholm et al. 2015). In the southern Scandes, L. arcticus extends its distribution slightly above 1500 m a. s. l. In a warmer climate, L. arcticus might potentially thrive some hundred metres higher than this. Baseline studies and ongoing monitoring programmes are urgently needed (Qvenild 2022). The only long-term programme in the Scandes, involving repeated monitoring of environmental conditions and biota, is at Lake Øvre Heimdalsvatn (Brittain et al. 2019). Since 1993, L. arcticus has virtually been absent from the brown trout diet in this lake. Predation from brown trout and Eurasian minnow poses a significant risk to L. arcticus, compounded by notable warming since 1985 (Sildre, https: // sildre. nve. no). The average summer temperature has nearly reached 14 ° C, a critical threshold (Qvenild et al. 2021). Consequently, further warming could be detrimental to L. arcticus in this lake. Updated surveys in Lepidurus lakes with observations older than ten years would be invaluable and might reveal extinct populations, particularly in the lowermost lakes.

LIFE HISTORY TRAITS Reproduction and recruitment For many species, there is a shift from facultative parthenogenesis in temperate regions to obligate parthenogenesis in the Arctic (Hessen et al. 2004). Asexuality enhances dispersal capabilities, as single eggs or individuals can establish new populations. Lepidurus arcticus exhibits both parthenogenesis and sexual reproduction (Wojtasik & Brylka-Wolk 2010; Lakka 2015). Males are rarely found in studied populations. In Spitsbergen, males made up only 2.6 % of the total sample (N = 789) from 19 ponds (Lakka 2015). In the six ponds where males were present, the male-to-female ratio averaged 1: 16. Thus, sexual reproduction in L. arcticus may be more common than previously thought. Lakka (2015) documented males as far north as Reinsdyrflya (Kilneset at 79.70 ° N). Selective pressures affect the sexes differently. Male fitness improves through successful mate searching, while female reproductive potential increases with larger body sizes (Lakka 2015). Smaller males searching for females face greater risks due to the increased chance of encounters with cannibalistic females. Cannibalism, a form of extreme sexual conflict, may explain the low number of males in populations. Injuries caused by birds and conspecifics, including cracks and holes in carapaces and broken cercopods, were observed in L. arcticus populations in Svalbard (Lakka 2013). Maturity is determined by the carapace length when foot capsules (ovisacs) appear on the 11 th pair of legs (Miller 1980). In Spitsbergen, L. arcticus reached maturity at 4.0 mm (range 4.0 – 9.1 mm) in carapace length (Lakka 2013). Temperature, growing season length, and salinity influence the size of maturing females. In other studies, females reached sexual maturity at 5.0 to 8.9 mm in carapace length (Sømme 1934; Arnold 1966; Borgstrøm 1970). In Spitsbergen, egg size was studied in 14 populations (Lakka 2013). Sizes ranged from 0.2 to 1.1 mm, with the most common sizes being between 0.6 mm and 0.8 mm. Early in the season, small L. arcticus produced small to medium-sized eggs. When carapace length reached approximately 8 mm, they began producing eggs larger than 0.8 mm. Eggs sampled in some small ponds at Barrow in Alaska had a size of 0.71 mm (weight 33 ± 0.5 µg C) (Miller 1980). The number of developing eggs carried internally is linearly related to female length, with full-grown females (total length 25 – 30 mm) carrying up to 60 – 70 eggs in their lifetime (Miller 1980). Egg shape can be irregular before rounding inside the egg capsules (Lakka 2013). Externally, one to three eggs are carried at a time (Miller 1980), but in Svalbard, Lakka (2013) found females carrying up to twelve eggs. These eggs harden over a few days before release. In Spitsbergen, females mainly produced one egg when smaller than 8 mm in carapace length. On average, a single female produced 2.6 eggs (range 0 – 12 eggs), more than previously observed (Sømme 1934; Arnold 1966; Miller 1980; Vekhoff 1997). Embryonic development: egg laying to hatch Branchiopods have evolved various adaptations to endure the harsh and unpredictable conditions of both Arctic and arid temporary water bodies, which may freeze or dry out unexpectedly (Longhurst 1955). Their encysted embryos enable their progeny to survive periods of desiccation and freezing. These resting eggs, known for their remarkable longevity during dormancy, can withstand multiyear droughts, freezing temperatures, mechanical damage, oxygen deprivation, ultraviolet radiation, and digestion by predators (Rogacki & Brysiewicz 2021). When in a dry or frozen state, these eggs can remain viable for decades or even centuries without the need for rehydration (Brendonck et al. 2008; Hann & Lonsberry 1991). Embryonic development within an egg capsule is affected by temperature. During this phase, embryos may enter dormancy if the cysts undergo desiccation or freezing. It is essential for any organism to receive a minimum amount of thermal input for embryonic development (Gillooly & Dodson 2000). Only a portion of the eggs hatch upon rehydration (Hann & Lonsberry 1991; Fryer 1996; Rogers 2014; Pasquali et al. 2019). This may be explained with egg banks containing eggs of varying ages, with embryos being at different developmental stages. Furthermore, the incomplete hatching of egg batches, even from individual females, might be an adaptation to the highly variable environments found in ephemeral water bodies (Hann & Loonsberry 1991). Branchiopod eggs contain rhodopsin. When activated by water, this pigment reacts to light, stimulating the dormant embryo (Rogers 2014). In notostracan species, hatching is influenced by light, temperature, pH and salinity, as well as by some biotic parameters: nutrients, predators and population density (see Brendonck 1996; Schönbrunner & Eder 2006). Branchiopods are known to produce both dormant and subitaneous embryos (Fryer 1988; Hann & Lonsberry 1991). This adaptation enables eggs to either endure dry or frozen conditions without progressing in development or to continue developing in water based on temperature (Borgstrøm & Larsson 1974). Hatching in L. arcticus normally coincides with the melting of snow and ice break-up, leading to a rapid increase in water temperature followed by an illumination shock to the benthic eggs. From southern Norway, notable examples include larvae found in late June in Lake Stolsvatn (Borgstrøm 1970) and early August in Lake Litlosvatn (Simonsen & Valderhaug 1994), due to the rise in temperature and the illumination shock at ice break-up. Larvae have also been observed as late as August in newly ice-free locations on northern Hardangervidda (Halvorsen 1973).

Multiple sampling techniques, such as bottom samplers (Lakka 2020), sieves (Halvorsen 1973), artificial substrates (Fjellheim et al. 2007), benthic littoral kick samples (Fjellheim et al. 2007), traps and density samplers such as ‘ peltipelle-sampler’ (Lakka 2013), have been used. Visual observing is also frequently used (Lakka 2020). However, in lakes and ponds with fish, fish stomach analyses proved to be the most effective method, particularly when L. arcticus was present in low numbers or exhibited a patchy distribution (Fjellheim et al. 2007; Qvenild & Hesthagen 2019). The timing of sampling is crucial, as the abundance of L. arcticus varies throughout the season (Qvenild & Hesthagen 2019). Most fish investigations occur from August to October when L. arcticus is more abundant. In cold summers, L. arcticus may appear later and might not be visible until September, if at all. Detecting L. arcticus can be challenging, necessitating repeated investigations. Fish predation greatly diminishes L. arcticus populations, making detection difficult when fish are abundant. Moreover, in Arctic conditions, repeated sampling is essential, as a large portion of the population may be dormant in the egg bank.

Postembryonic phase: hatch to adult Lepidurus arcticus hatches as a non-feeding nauplius, undergoing a virtual metamorphosis at the first moult (Fryer 1988). It hatches in the lake sediment or among vegetation and possibly does not leave the bottom since it rapidly changes to the second instar, the first larval stage found in plankton samples (Borgstrøm & Larsson 1974). The second instar swims but does not feed because its mandibles are incapable of handling food (Fryer 1988). The third instar is also pelagic, but after these three stages, the larvae become benthic, living on or digging in the sediments. When feeding begins at stage 3, the mechanism involved is already adult-like (Fryer 1988). Both Lepidurus and Triops have a developed carapace from the earliest free-swimming stage (Møller et al. 2003). In a laboratory study by Miller (1980), growth was nearly continuous, with moults occurring every 48 to 60 hours at a constant temperature. Over the 60 days of the experiment, the animals underwent 24 to 30 moults. Adults typically eat their old cuticle after moulting (Lakka 2013). The more southerly species of Notostraca exhibit a rapid growth rate (Longhurst 1955; Rogacki & Brysiewicz 2021). Most branchiopods need a dry period for cyst hatching and are progressively eliminated at higher latitudes where low temperatures slow embryonic development (Brtek & Thiery 1995). Species, such as L. arcticus, inhabiting northern ponds, have a growth rate closely linked to water temperature and utilise much of the summer months to reach sexual maturity. In Svalbard, L. arcticus typically matures in August (Lakka 2015). In the Scandes, adults are generally observed in late July or early August (Borgstrøm 2019; Qvenild et al. 2018). In this region, peak abundance is typically noted from late August to September, after which the population ceases and dies. On Hardangervidda, L. arcticus has been found in brown trout stomachs as late as December (Sømme, 1934). Sars’ studies (1896) documented the largest males and females from Svalbard to be 13 and 24 mm, respectively. In 2010, the mean total length of L. arcticus males was 11 mm in Spitsbergen (Lakka 2013). Males were generally smaller than females, but they could reach up to 27.5 mm in total length. The largest female recorded in this study measured 39.4 mm, the largest L. arcticus ever noted. In a study in Lake Solvatnet in Spitsbergen, Mancinelli & Pasquali (2016) sampled adult L. arcticus early in August 2014. Adults with carapace lengths> 4 mm were randomly selected on-site and held in the laboratory for a week. The individual dry mass averaged 4.75 ± 0.22 mg (range 1.71 – 7.39 mg), corresponding to a total length of approximately 15 mm (range 9.5 – 18.5 mm), as estimated from TL = 0.015 * DW 2.12 (Miller, 1980).

Lepidurus arcticus as an indicator species While L. arcticus is relatively common in Norway, and in national Red List assessments it has been assessed to Near Threatened in Sweden and Endangered in Finland. Lepidurus arcticus is vulnerable to acid precipitation, fish predation and the ongoing global warming, which all may lead to habitat reduction and range retraction, making L. arcticus a valuable indicator species. The significance of this charismatic species has been acknowledged and included in the conservation guidelines for freshwater ecosystems (Lakka 2020). The Nordic countries have committed to the Kunming-Montreal Global Biodiversity Framework (CBD 2022), which sets a goal of conserving 30 % of inland waters by 2030. As a result, Arctic nations have a unique obligation to protect genuine Arctic species and their habitats.

DISTRIBUTION Lepidurus arcticus has a circumpolar distribution in the northern hemisphere, spanning both North America and Siberia (Rogers 2001; Hessen et al. 2004). The species was first described following a Siberian expedition in 1768, which included Peter Simon Pallas. Lepidurus arcticus is found as far north as 74 ° N in the eastern Novaya Semlya archipelago (Veckhof 1997) and extends to the Kuril Archipelago (Sayenko & Minakawa 1999). Rasshua Island (47 ° N) in the middle Kuril Islands is likely the southernmost limit of L. arcticus in Asia. These islands experience frequent summer fog and are snowcovered for much of the year. Lepidurus arcticus is also common in Iceland (Scher et al. 2000; Eiríksson et al. 2021) and Greenland (Sømme 1934; Røen 1962; Jeppesen et al. 2001). Dated sediment remains of L. arcticus show that the species once had a broader range, including Denmark (Økland & Økland 2003), Ireland, Scotland, and the Isle of Man (Mitchell 1957). Distribution in Svalbard The initial record of L. arcticus in the Svalbard archipelago dates back to 1878 in Advent Bay (Richard 1878, cited in Sømme 1934). By the 1930 s, it had been documented in at least 16 locations (Sømme 1934). Due to logistical challenges in Svalbard, most records from Spitsbergen come from areas near Longyearbyen and Ny-Ålesund (Figure 2), although some findings are from more remote regions. A few observations are reported from the eastern islands, including a pond in Snaddvika, Murchison Bay on Nordaustlandet (80.0 ° N, 18.8 ° E), which might be the northernmost record in Svalbard, albeit lacking an exact date and position (Sømme 1934). There is also an unspecified record from Barentsøya (Richard 1898, cited in Sømme 1934). Bennike & Hedenås (1995) found sediment remains of L. arcticus at four locations in Visdalen on Edgeøya in 1991. Jørgensen & Eie (1993) studied 17 locations on the Mosselhalvøya peninsula in 1977, recording L. arcticus in nine fishless ponds but not in eight lakes hosting Arctic charr (Salvelinus alpinus Linnaeus, 1758). The ponds near Polheim (79.89 ° N, 16.02 ° E) are the northernmost properly documented localities for L. arcticus in Spitsbergen, though Sømme (1934) noted a nearby finding in Sorgfjorden (at Treurenberg) at almost the same latitude, which is difficult to pinpoint. In 2010, Lakka (2013) found L. arcticus in one of the four ponds near Polheim. Further northern observations have been made from Reinsdyrflya (79.70 ° N) (Sømme 1934; Lakka 2013). Recent studies have thoroughly investigated the Lepidurus localities in the vicinity of Ny-Ålesund. In 2010, Lakka (2013) examined nine locations, finding L. arcticus in seven ponds. In 2015, Dimante-Deimantovica et al. (2015) studied eleven locations, with only one record of L. arcticus (information provided by Bjørn Walseng, pers. comm.). Calizza et al. (2022) investigated 18 locations, recording L. arcticus in nine, including three new habitats not previously studied (Dimante-Deimantovica et al. 2015; Lakka 2013). At least four L. arcticus localities (Storvatnet, Solvatnet, Tvillingvatnet, and Trehyrdingene) and two ponds without L. arcticus (Gluudneset and Goose Pond) were studied in all three projects (Calizza et al. 2022; Dimante-Deimantovica et al. 2015; Lakka 2013). Lepidurus arcticus was also detected in Lake Storvatnet in 1980 (Økland & Økland 2003). Lake Solvatnet has been studied on multiple occasions, with Lepidurus eggs collected for laboratory experiments in 2014 and 2015 (Mancinelli & Pasquali 2016; Pasquali et al. 2019). Despite the lack of findings in many locations, L. arcticus appears to be very common in the area, with an early observation near the Conway glacier on Kongsfjorden’s eastern shore (Sømme 1934). Lepidurus arcticus has long been known from the Isfjorden area, with thorough studies by various authors. In Adventdalen, both Sars (1885 – 91) and Richard (1898) (cited in Sømme 1934) reported L. arcticus from small ponds sampled in 1878 and 1897, respectively. In 1910, Olofsson (1918) found it in a lake (Teich XXI), likely the same location investigated in 1928 by Thor (1930), which he referred to as Isdammane. Lakes and ponds in this area served as drinking water and ice supplies for Longyearbyen, later dammed into the present Isdammen in 1960. In recent years, L. arcticus has been sampled in many localities nearby Isdammen (Lakka 2013; Dimante-Deimantovica et al. 2015). A find was noted six kilometres further into the valley at ‘ Engelskhytta’ (Thor 1930). Ponds in Longyeardalen also host L. arcticus (Lakka 2013), though it has not been observed annually in all city ponds since 2010 (Coulson, pers. comm.). On Diabasodden, northeast of Longyearbyen, a pond was positively investigated by both Olofsson (1918) and Dimante-Deimantovica et al. (2015). Expeditions have visited both sides of the Billefjorden area. On Kapp Napier, Olofsson (1918) failed to find the species in six ponds in 1910, but positive findings were made in 1970 and 2014 (Artsdatabanken; Dimante-Deimantovica et al. 2015). In Mimerdalen, three well-defined lakes repeatedly examined as water supplies for Pyramiden’s early settlement also yielded positive findings (Lakka 2013; Dimante-Deimantovica et al. 2015). In Lake Goluboye, two kilometres further into the valley, L. arcticus was found (Dimante-Deimantovica et al. 2015). Two ponds in this area also hosted L. arcticus. Surveys in the outer Isfjorden area include both Ymerbukta in the north and southern banks (Colesbukta, Grønnfjorden, and Kapp Linné). No lakes with Arctic charr have yielded positive findings despite extensive stomach analyses, except for anadromous Arctic charr caught in the outlet rivers of Straumsjøen (Aas 2007; Borgstrøm et al. 2018) and Lake Linné (Ebne 2009). Large, shallow lakes of Flydammane on Erdmans Tundra in Ymerbukta all hosted L. arcticus (Olofsson 1918; Dimante-Deimantovica et al. 2015), as did most ponds in Randvika (Dimante-Deimantovica et al. 2015), Aldegondabreen (Dimante-Deimantovica et al. 2015), Coles Bay (Olofsson 1918), and all at Kapp Linné (Lakka 2013). Thus, L. arcticus seems prevalent in small, fishless water bodies in the Isfjorden area, though year-to-year variation is significant. Further south, only three areas have been investigated. In Sveagruva, van Miejenfjorden, Olofsson (1918) found no records of L. arcticus in five freshwater locations. On the western coast, at Kapp Bruunodden, an old record exists (Sømme 1934). Some activity has occurred at the Polish station in Hornsund, with Janiec (1996) noting L. arcticus in three nearshore ponds in 1989. Genetic studies were conducted in two lakes at Fuglebergsletta by Wojtasik & Brylka-Wolk (2010), with a replicate in Fugledammen by Luoto et al. (2016). Wojtasik & Brylka-Wolk (2010) reported L. arcticus in 13 of 50 water bodies examined in Hornsund, though no further site details are provided. Thus, L. arcticus populations do exist in this area. Lepidurus arcticus is commonly recorded in numerous fishless water bodies in Spitsbergen, ranging from a 6 m 2 pond in Nybyen (Lakka 2013) to the 15 ha Flydammane on Erdmans Tundra (Dimante-Deimantovica et al. 2015). In total, L. arcticus has been documented in at least 81 fishless puddles and ponds in Spitsbergen, plus one locality on Nordaustlandet, one on Barents Island, and four sediment remains sites on Edge Island. Lepidurus arcticus is also noted in two outlet rivers that host anadromous Arctic charr, totalling at least 89 documented localities in Svalbard. Distribution in Bear Island Lepidurus arcticus is frequently found in Bear Island (Klemetsen et al. 1985). In a survey conducted in 1998 / 1999, Hessen et al. (2004) collected samples of L. arcticus from 29 ponds scattered across the island. Subsequently, in a more extensive survey of 109 ponds on the northeastern plateau, L. arcticus was detected in 71 (65 %) of them (Klausen 2012) (coordinates provided by Sigurd Einum, pers. comm.). Lepidurus arcticus is abundant in three rivers – Engelskelva, Jordbruelva, and Lakselva – all of which support populations of resident Arctic charr (Klemetsen et al. 1985). In Bear Island, L. arcticus is also present in lakes where it coexists with Arctic charr. Of the twelve lakes studied, all hosting Arctic charr, L. arcticus was found in eight (Klemetsen et al. 1985; Berg et al. 2010; Evenseth et al. 2005). Overall, L. arcticus is present in 114 locations: 103 fishless ponds, eight lakes, and three rivers with resident Arctic charr populations (Figure 4). Distribution in the Scandes The Scandes represent the southernmost limit of L. arcticus occurrences in Europe (Økland & Økland 2003; Qvenild et al. 2021). The species was first recorded in Norway by G. O. Sars during his mountain trip to Filefjell in 1863 (Sars 1864). In Sweden, an early record exists from the Sulitjelma area (Lilljeborg 1877, cited in Sømme 1934). By the early 20 th century, L. arcticus had been reported from at least 37 localities: 29 in Norway and 8 in Sweden (Sømme 1934). The first Finnish observation was from a pond near the Halti fells in 1955 (Koli 1957). A summary of records for the Norwegian mainland was provided by Økland & Økland (2003). A recent study from the Scandes reported L. arcticus in 391 natural and 88 regulated lakes, spanning latitudes from 59 to 71 ° N and elevations from 74 to 1524 m a. s. l (Qvenild et al. 2021). Norwegian localities extend further south and north than those in Sweden and Finland. On the Norwegian mainland, 315 natural lakes and 79 regulated lakes were recorded. Most Swedish records of L. arcticus were found in northern river systems, with 78 locations documented, including eight regulated lakes. In 2023, an additional record was made in a small pond (68.7024 ° N, 20.8518 ° E) with Arctic charr in Northern Sweden (Lakka, unpubl.). Finnish Lepidurus lakes in the Scandes are situated in the northwestern part of Finnish Lapland above 666 m a. s. l (Lakka 2020). Here, L. arcticus was found in seven localities, one of which is a regulated lake outside the Scandes (Lake Inarijärvi). Recently three more observations of L. arcticus are made: Lake Pitsusjärvi (69.2292 ° N, 21.2767 ° E), Lake Jápmajávri (69.1186 ° N, 21.4725 ° E), and Pond ‘ Luonnontutkija’ (69.1598 ° N, 21.1606 ° E) (Lakka, unpubl.). In total, 686 localities hosting L. arcticus are known in the Scandes. The distribution in the Scandes appears to be dichotomous, with a large northern area north of 64 ° N and a smaller southern ‘ island’ south of 63 ° N. Southernmost records are at altitudes ranging from 489 to 1524 m a. s. l, while north of 70 ° N, only 11 natural lakes are located below 327 m a. s. l. In this region, mountains are low, with few reaching more than 400 m a. s. l. The altitudes of natural lakes with L. arcticus decline towards the north, and regulated lakes are often at lower altitudes than natural ones. Of natural Lepidurus lakes, 87 % are situated above the treeline (Qvenild et al. 2021). In contrast to Svalbard, L. arcticus normally coexists with fish in the Scandes. Fish status was obtained in 379 of 391 Lepidurus lakes (Qvenild et al. 2021), documenting ten fish species. Most lakes host brown trout (89 %) and Arctic charr (> 23 %), particularly in the northern part. The invasive Eurasian minnow Phoxinus phoxinus (L., 1758) is now established in at least 9 % of Lepidurus lakes. Other species include grayling Thymallus thymallus (L., 1758), burbot Lota lota (L., 1758), perch Perca fluviatilis L., 1758, whitefish Coregonus lavaretus (L., 1758), ninespine stickleback Pungitius pungitius (L., 1758), northern pike Esox lucius L., 1758, and non-native American brook trout Salvelinus fontinalis (Mitchill, 1814). In lake Inarijärvi, outside the Scandes, three more species are documented: salmon Salmo salar L., 1758, three-spine stickleback Gasterosteus aculeatus L., 1758, and non-native lake trout Salvelinus namaycush (Walbaum, 1792). In a study of 124 natural lakes on Hardangervidda in southern Norway, L. arcticus was most frequently found at altitudes of 1100 – 1199 m a. s. l, with an occurrence rate of 80 % (Qvenild & Hesthagen 2019). A thermal deficit may limit the species at high latitudes and altitudes. On Hardangervidda, the highest lake hosting L. arcticus is Lake Kolsnutgryslane at 1386 m a. s. l. They may exist at higher altitudes, as recorded in Lake Rygghøtjønne at 1524 m a. s. l further north (Økland & Økland 2003). Cold water also limits reproduction in Svalbard populations (Olofsson 2018; Lakka 2015).