Gammarid amphipods (Crustacea) in Norway, with a key to the species

12 Fauna norvegica 2019 Vol. 39: 12–25. Gammarid amphipods (Crustacea) in Norway, with a key to the species Wim Vader1 and Anne Helene S. Tandberg2 Vader W and Tandberg AHS. 2019. Gammarid amphipods (Crustacea) in Norway, with a key to the species. Fauna norvegica 39: 12 – 25. Thirteen species in the amphipod family Gammaridae have been reported from Norway. This paper gives a survey of the distribution and habitat of all 13 species of the family Gammaridae occurring or expected to occur in Norwegian waters: both marine, brackish and fresh, including Svalbard, in addition to four species found in close neighbouring waters. It also provides a short history of the study of Gammaridae in Norway, as well as an illustrated identification key to all species in the area. doi: 10.5324/fn.v39i0.2873 . Received: 2018-11-28. Accepted: 2019-02-10. Published online: 2019-02-28. ISSN: 1891-5396 (electronic). Keywords: Gammarus, Marinogammarus, Amphipoda, identification key, habitat 1. Tromsø Museum – Universitetsmuseet, Universitetet i Tromsø, NO-9037 Tromsø, Norway. 2. Universitetsmuseet i Bergen, avd for Naturhistorie, PO Box 7800, NO-5020 Bergen, Norway. Corresponding author: Anne Helene S. Tandberg E-mail: [email protected] INTRODUCTION The many similar-looking amphipod species in the family Gammaridae, collectively called ‘marflo’ in Norwegian, are both widespread and numerous many places, in fresh water, brackish water, and on the sea shore. They are an important part of the various ecosystems, they are good indicators of water quality, and they are also often used in toxicology studies. To identify them to species, however, is not always easy. The usual identification method for amphipods we use in Norway, i.e. the combination of the always excellent illustrations in the old monograph by G.O.Sars (1890-95) with the keys and distribution data in the various papers by Stephensen (1928, 1929, 1935-42), fails dismally in this case: Sars (op. cit.) described only five species of Gammarus, three in sea water (G. locusta, G. campylops, G. marinus), one in brackish biotopes (G. duebeni) and one in fresh water (G. pulex). Now, however, we know 13 species in Norway (without, by the way, either G. campylops or G. pulex). The aim of this paper is 1. To give a short historical overview of the study of Gammarus species s.l. in Norway, 2. To provide an illustrated identification key to the Norwegian Gammaridae species, including a few species that have not yet been found here, but may occur, 3. To give references to good descriptions and illustrations of all species, and 4. To give a short review of the distribution and habitat of every species in Norway. There are some further freshwater amphipods in Norway, that traditionally have been reckoned in the Gammaridae s.l. These are the so-called ‘glacial relicts’ Gammaracanthus lacustris (Sars, 1867) and Pallasea quadrispinosa (Sars, 1867), that occur in some deep lakes in SE Norway; Pallasea has recently also been discovered on Jæren (Spikkeland et al. 2012). A marine Gammaracanthus species also occurs in Arctic waters. These are now considered to belong to the families Gammaracanthidae and Pallaseidae, and are not dealt with here. A further, non-indigenous freshwater amphipod has very recently been found on Jæren, i.e. Crangonyx pseudogracilis Bousfield, 1958 (Spikkeland et al. 2016); this species belongs in the family Crangonyctidae, and is also excluded. In recent years, many places in Europe, in both fresh and brackish waters, have had an influx of Ponto-Caspian species, especially in rivers and in the inner parts of the Baltic Sea. One of these is the ill-famous ‘killer amphipod’ Dikerogammarus villosus (Sowinsky, 1894). None of these species have hitherto been recorded in Norwegian waters and we have not included them in this paper, with the exception of the American immigrant Gammarus tigrinus Sexton, 1939. 13 Fauna norvegica 39: 12–25. 2019 Vader and Tandberg: Gammarid amphipods in Norway Sexton & Spooner, 1940 for the species of the stony littoral, and Lagunogammarus Sket, 1971 for the greater G. zaddachi group. Rivulogammarus is not an available name, having the same type species as Gammarus (Stock 1969a), while Lagunogammarus, although taken in use in her monograph by Tzvetkova (1975) is also no longer in use (Hou & Sket 2016). As shown below, the situation for Marinogammarus is more complicated. In this period several different classification and systematics models were taken in use in taxonomy: phenetics, cladistics, molecular classifications, and in our opinion a final classification of European gammarids has not yet been obtained. Stock (1968, 1969b, Pinkster & Stock 1970) worked on Marinogammarus (which he replaced by the older name Chaetogammarus, coined for a Black Sea species); he removed M. obtusatus to the Baikal genus Eulimnogammarus (a move no longer followed by anyone), and M. finmarchicus back to Gammarus s. str., close to G. duebenii. The latest revision is that by Hou and Sket (2016), and even though also that one has already drawn criticism and has not yet been followed by WoRMS (Horton et al. 2018), it is used here. They restored the genus Marinogammarus (still called Echinogammarus in WoRMS), but in a most interesting development these authors also declared M. stoerensis to be the probable sister group of all remaining gammarids and erected the genus Relictogammarus for this species (Hou & Sket 2016); also this species is accepted as Echinogammarus in WoRMS. The present paper deals with the following 17 species, of which 4(*) have not yet been recorded from Norwegian waters, but may conceivably occur there: * Gammarus crinicornis Stock, 1966 G. duebenii Liljeborg, 1852 G. inaequicauda Stock, 1966 G. lacustris Sars, 1863 G. locusta (Linnaeus, 1758) G. oceanicus Segerstråle, 1947 * G. pulex (Linnaeus, 1758) G. salinus Spooner, 1947 G. setosus Dementieva, 1931 * G. tigrinus Sexton, 1939 * G. wilkitzkii Birula, 1897 G. zaddachi Sexton, 1912 Marinogammarus finmarchicus (Dahl, 1938) M. marinus (Leach, 1815) M. obtusatus (Dahl, 1938) M. pirloti Sexton & Spooner, 1940 Relictogammarus stoerensis (Reid, 1938) Identification key In the construction of this key we have, as always in such cases, based ourselves for a large part on earlier work by Sexton & Spooner (1940), Reid (1944), Kinne (1954), Hynes et al. (1960), Stock (1966, 1967), Bousfield (1969) and Lincoln (1979), but in a few cases we have chosen new criteria. The key will work for History The development of gammarid classification in northern Europe in the last 100-150 years seems to have happened in definite stages. In the first stage a number of species was described: Gammarus kröyeri Rathke, 1843, G. poecilurus Rathke, 1843, G. duebeni Liljeborg, 1852, G. mutatus Liljeborg, 1855, G. batei Boeck, 1861 and G. lacustris Sars, 1863. In most cases there was no type material conserved and the descriptions are so vague and incomplete that it is now often practically impossible to identify these species. In the next stage practically all these species were again viewed as junior synonyms of the three classic Gammarus species of the area: Gammarus locusta, G. marinus and G. pulex. Characteristic for this stage is e.g. that Liljeborg already in 1855 reduced his own G. duebeni to a synonym of G. locusta, while Sars in 1892 no longer saw his own G. lacustris as different from G. pulex; both, as we now know, incorrect decisions. The third stage started with Sexton who, starting around 1910, over many years carried out a large number of rearing and crossing experiments with various Gammarus species in the Plymouth laboratory in England (see e.g. Sexton 1923, Sexton & Clark 1936). She found that the species were not at all so variable as everybody thought at the time: even when reared under quite different regimes, specimens at the same stage of development remained morphologically extremely similar. Sexton also described a few new species from brackish water, i.e. Gammarus zaddachi in 1912. She was clearly far ahead of her time, however, and it took more than 25 years for the leading European specialists (Chevreux, Schellenberg, Stephensen) to accept G. zaddachi as a valid species. In the 1930’s there was a change again, and new species were described regularly, usually without comparing them with the names from the first stage. In 1930 Gurjanova recognized Birula’s Gammarus wilkitzkii as ‘a good species’. In 1931 followed Gammarus setosus Dementieva and Rivulogammarus scandinavicus S. Karaman (= G. lacustris), in 1938 G. finmarchicus Dahl (in the same year also described from USA as G. greenfieldi Shoemaker), G. obtusatus Dahl and G. stoerensis Reid, in 1939 G. tigrinus Sexton, and in 1940 Marinogammarus pirloti Sexton & Spooner. In 1942 Sexton divided her Gammarus zaddachi into a ‘freshwater’ and a ‘saline’ form, in 1947 called G. z. zaddachi and G. z. salinus by Spooner. That same year Segerstråle (1947) described a third, more northern form as G. z. oceanicus, and all three forms were upgraded to species level by Kinne (1954). This almost completed the picture for NW Europe, except for the splitting of the Gammarus locusta group by Stock (1966, 1967), who i. a. showed that Sars’s G. campylops, until then looked at as a synonym of G. locusta, in reality represented an independent species, which he named G. inaequicauda (Stock & Kant, 1966). As we see, in this period also a few new genus names appear, besides the original Gammarus: Rivulogammarus S. Karaman, 1931 for freshwater species, Marinogammarus Vader and Tandberg: Gammarid amphipods in Norway 14 Fauna norvegica 39: 12–25. 2019 adult and subadult specimens, but is not reliable for juveniles and small immatures. For illustrations to the key, see figs 1 and 2. All illustrations are based on drawings originally presented in Sars (1890-95), Sexton & Spooner (1940), Sexton (1942), Stephensen (1949), Barnard (1959), Økland (1969), Pinkster (1970), Bousfield (1973) and Lincoln (1979). Key to the Gammarus/Marinogammarus/Relictogammarus in Scandinavian waters (Adult specimens) 1. a) U3 outer ramus with only 1 article. (Inner ramus 30-40% of outer ramus length. P7 basis posterodistally with only setae.) ............. Marinogammarus finmarchicus (Dahl, 1938) b) U3 outer ramus with 2 articles .................................... 2 2. a) U3 inner ramus more than half length of outer ramus first article (Figure 1) ........................................... 3 b) U3 inner ramus less than one third of outer ramus first article (Figure 1) .................................................... 16 3. a) Eyes oval, not higher than 2x width (Figure 1) ........... 4 b) Eyes “long and narrow”, clearly higher than 2x width (Figure 1) ............................................................... 7 4. a) Ventral margin of mandible palp 3rd article: setae not of equal length - producing an uneven fringe. (Peduncle A1 art 2 and 3 subequal in length) ....................... Gammarus wilkitzkii Birula, 1897 b) Ventral margin of mandible palp 3rd article: setae of equal length - producing a comb-shaped structure .... 5 5. a) Several plumose setae on P5-7, urosome and telson. Large arctic marine species ..........................Gammarus setosus Dementieva, 1931 b) No plumose setae on P5-7, urosome and telson. Boreal fresh water species ............................................... 6 6: a) Ep2 and 3 subequal in shape, hind corner acute. A2 flagellum in ♂ without brushes of sensory setae (Figure 1) ................Gammarus lacustris G. O. Sars, 1863 b) Ep2 and 3 not alike in shape: lower hind corner acute in Ep 3, but not in Ep2. A2 flagellum in ♂ with brushes of sensory setae (Figure 1) .......................................... Gammarus pulex (L, 1758) 7: a) Ventral margin of mandibular palp art. 3: all setation of similar type, either forming an uneven fringe, or an even comb-like structure ............................ 8 b) Ventral margin of mandibular palp art. 3: setae on inner part of margin uneven, on outer part of margin even. (All legs, antenna 2 and uropod 3 are heavily setose with long curled setae) .............................. Gammarus tigrinus Sexton, 1939 8: a) Ventral margin of mandibular palp art. 3: setae not of equal length, forming an uneven fringe ............. 9 b) Ventral margin of mandibular palp art. 3: setae of equal length, forming a comb-shaped structure ...... 11 9: a) A1 peduncle in ♂: 3rd article as long as 2nd article ( >40 mm). Large arctic under-ice species (Figure 1) .................... Gammarus wilkitzkii Birula, 1897 b) A1 peduncle in ♂: 3rd article not longer than 50-60% of 2nd article (< 25 mm) (Figure 1) .................10 10: a) P7 article 5: marginal setae obviously longer than spines (Figure 1) ....... Gammarus zaddachi Sexton, 1912 b) P7 article 5: marginal setae not longer than spines; most obviously shorter (Figure 1) .............................. Gammarus salinus Spooner, 1947 11: a) P7 article 2: posterodistal margin with setae only (Figure 1) ................ Gammarus duebenii Liljeborg, 1852 b) P7 article 2: posterodistal margin with both setae and spines (Figure 1) .................................................... 12 12: a) A1 peduncle: 3 setae groups on article 1, 4-5 setae groups on article 2. Mandibular palp ventral margin art. 3: all setae in the “comb” of equal length ..13 b) A1 peduncle: 1-2 setae groups on article 1, 1-2 setae groups on article 2. Mandibular palp ventral margin art. 3: setae in “comb” become gradually longer towards tip of article .......................................... 14 13: a) Several plumose setae on P5-7, urosome and telson. P7 basis posterior margin with several long setae. Setae on telson are 3x as long as the spines. (Figure 2) .............. Gammarus setosus Dementieva, 1931 b) No plumose setae on P5-7, urosome or telson. P7 basis posterior margin with few and short setae. Setae on telson 2x as long as the spines (Figure 2) ...............Gammarus oceanicus Segerstråle, 1947 14: a) Urosome segments with almost no visible “humps”. P7 basis 1.2 - 1.3 times longer than wide ........................... Gammarus crinicornis Stock, 1966 b) Urosome segments with clear “humps”. P7 basis more than 1.5x longer than wide .................................. 15 15: a) U3 inner ramus almost as long as outer ramus art. 1 (90-100%). Ep3 with several setae along hind margin. A2 in ♂ with calceoli (Figure 2) ...................................... Gammarus locusta (L, 1758) b) U3 inner ramus obviously shorter than outer ramus art. 1 (75-90%). Ep3 with 0-1 setae along hind margin. A2 in ♂ lacks calceoli (Figure 2) ....................... Gammarus inaequicauda Stock, 1966 15 Fauna norvegica 39: 12–25. 2019 Vader and Tandberg: Gammarid amphipods in Norway In this survey we have divided the gammarids of Norway into 8 groups. We have deviated from the traditional grouping on two points: we have joined Gammarus oceanicus (although originally described as a subspecies of G. zaddachi) with G. setosus, and also G. duebenii with Marinogammarus finmarchicus. Gammarus oceanicus and G. setosus are both uniformly coloured species with a northerly and amphi-atlantic distribution, while G. zaddachi and G. salinus are striped animals with a more southerly and purely eastern Atlantic distribution. Similarly G. duebenii and ?M. finmarchicus show many morphological and ecological similarities. The final taxonomic position of finmarchicus is still unclear, and many authors place it in Gammarus, close to G. duebenii. A. Gammarus locusta group In the older literature, also in Norway, practically all fully marine Gammarus were called G. locusta, although Sars (189095) realized there was a different species, which he called G. campylops. In 1967 Stock revised the Gammarus locusta group, and i.e. described four species from northern Europe, two of which have been found also in Norwegian waters; G. locusta s. str. and G. inaequicauda (=Sars’s campylops, but not campylops Leach), while a third, G. crinicornis, may still be found in the SW. All the species in this group are mainly sublittoral boreal marine or brackish water forms. 1. Gammarus locusta (Linnaeus, 1758) Descriptions and illustrations: Sexton 1942, pl. 3 figs 19-24; Spooner 1947, p. 7, figs 1, 2 A-B, 3; Kinne 1954, p. 409, figs 1-2pp, 4-6; Stock 1967, p. 11, fig. 1. Ecology and habitat: A marine, mainly sublittoral species of the algal belt, tolerates some lowering of the salinity, but not much, unless it is stable, as in the Baltic. Not often found in the intertidal during ebb. A useful review is Costa & Costa (2000) A very special case was the situation in Rauneforden near Bergen, where large amounts of kelp (mainly Laminaria) washed down to the bottom at 240 m. In this biotope Gammarus locusta was surprisingly common, and the animals grew very large (Vader, unpubl. obs.). Distribution in Norway: Not really well known because of earlier confusion, mainly with G. oceanicus, and the fact that this species mainly lives on the outer coast and largely sublittorally. Occurs quite commonly along the entire South and West coast, much more sparingly east of the North Cape. It does not penetrate into the fjords much, and in the North it is restricted to the outer coast. Surprisingly, recent studies have found the species present in Kongsfjorden, Svalbard (Berge & Shunatova, pers. comm.) 16: a) P7 article 2 posterodistal margin with setae only (adults < 10 mm). (U3 with almost only spines) (Figure 2) ....... Relictogammarus stoerensis (Reid, 1938) b) P7 article 2 posterodistal margin with both setae and spines (Figure 2) .................................................... 17 17: a) A1 peduncle: article 2 as long as article 1. Ep 3 hind corner rounded. (In ♂, the “hand” (propodus & dactylus) is much larger in P1 than in P2. The antennae have long and stiff setae, in every group one of the setae is a lot longer than the rest) (Figure 2) .................... Marinogammarus obtusatus (Dahl, 1938) b) A1 peduncle: article 2 much shorter than article 1. Ep 3 hind corner is rectangular or acute (Figure 2) . 18 18: a) Ep 3: hind corner very acute. A1 peduncle: article 1 shorter than articles 2+3. Spines on urosome form a continuous arc. U3: outer ramus in ♂ without plumose setae, in ♀ with plumose setae on inner margin (Figure 2) .................. Marinogammarus marinus (Leach, 1815) b) Ep 3: hind corner rectangular. A1 peduncle: article 1 almost as long as articles 2+3. Spines on urosomal segment 1 do not form a continuous arc. U3: outer ramus in both ♂ and ♀ with plumose setae on inner margin (Figure 2) .... Marinogammarus pirloti Sexton & Spooner, 1940 A survey of gammarid species in Norway The geographic distribution of gammarid species in Norway is not well known, as is the case for many other intertidal amphipods; most of the classical papers on Norwegian amphipods dealt mainly or exclusively with dredged material. In addition, older records can in general not be trusted, as the classification of gammarids has undergone vast changes in the last decades. The data used in this survey stem from the compilation by Vader et al. (1979, brought up-to-date by Brattegard & Vader (2018, unpublished)), from our own field observations in western and northern Norway, as well as from papers by Sars (1890-95), Stephensen (1935-42), Dahl (1946), Segerstråle (1947,1954), Spooner (1947), Steen (1951), Oldevig (1959), Brattegard (1966), Dennert (1973), Ingolfsson (1977), Palerud et al (2004), and a series of papers on freshwater gammarids by Økland (1965, 1969, 1970, 2011a,b). For the ecological data we have relied mostly on data from Western Europe, the series of papers from Newfoundland by Steele & V. Steele (1969, 1970, 1972, 1975; V. Steele & Steele 1970, 1972), and the Oslofjord studies of Skadsheim (1982, 1983, 1984). There has been much research also in the Baltic, but the situation there, with a lowered, but more or less constant salinity, is quite different from the situation in the intertidal, estuaries and pollens in Norway with their very changeable salinity. Vader and Tandberg: Gammarid amphipods in Norway 16 Fauna norvegica 39: 12–25. 2019 Figure 1. Illustrations to questions 2, 3, 6, 9, 10 and 11 from the key. 2a: Marinogammarus marinus illustration from Sexton & Spooner (1940). 2b: Gammarus wilkitzkii illustration from Barnard (1959). 3a: Gammarus wilkitzkii illustration from Barnard (1959). 3b: Gammarus tigrinis illustration from Lincoln (1979). 6a: Gammarus lacustris illustration from Økland (1969). 6b: Gammarus pulex illustration from Pinkster (1970). 9a: Gammarus wilkitzkii illustration from Barnard (1959). 9b: Gammarus salinus illustration from Lincoln (1979). 10a: Gammarus zaddachi illustration from Sexton (1942). 10b: Gammarus salinus illustration from Sexton (1942). 11a: Gammarus duebenii illustration from Lincoln (1979). 11b: Gammarus oceanicus illustration from Lincoln (1979). 17 Fauna norvegica 39: 12–25. 2019 Vader and Tandberg: Gammarid amphipods in Norway Figure 2. Illustrations to questions 13, 15, 16, 17 and 18 from the key. 13a: Gammarus setosus illustration from Bousfield (1973). 13b: Gammarus oceanicus illustration from Lincoln (1979). 15a: Gammarus locusta illustration from Lincoln (1979). 15b: Gammarus inaequicauda illustration from Sars (1890-95). 16a: Relictogammarus stoerensis illustration from Sexton & Spooner (1940). 16b: Marinogammarus marinus illustration from Sexton & Spooner (1940). 17a: Marinogammarus obtusatus illustration from Sexton & Spooner (1940). 18a: Marinogammarus marinus illustration from Sexton & Spooner (1940). 17b and 18b: Marinogammarus pirloti illustration from Sexton & Spooner (1940). Vader and Tandberg: Gammarid amphipods in Norway 18 Fauna norvegica 39: 12–25. 2019 100e, 111c, 119. Biology: Along the south and west coast and in the large fjords of western Norway this is a species of the low intertidal and shallow subtidal, where it is primarily found in the algal belt of Fucaceae and is generally very common. In Northern Norway, and still more characteristically in Svalbard, this species is also very often found under stones in the lower intertidal, even on mudflats with occasional algae-covered large stones (Weslawski 1994). Distribution in Norway: This is probably the most numerous Gammarus species on the Norwegian coast, where it also penetrates far into the fjords (Brattegard 1966, Vader 1977a). G. oceanicus is also common in Svalbard waters, but it is absent from the southern North Sea. It has recently been found further north than earlier in Svalbard waters, probably as a result of global warming (Węsławski et al. 2018) 5. Gammarus setosus Dementieva, 1931 Synonym: Lagunogammarus setosus auct. Description and illustrations: Stephensen 1935-42, p. 321, fig. 41; Segerstråle 1947, p. 241, fig. 7; Christiansen 1965, fig. 3; Bousfield 1973, p. 50, pl. 1-2. Biology: A mostly arctic marine, largely intertidal species. In the Arctic (Weslawski 1994), and also in Iceland (Ingolfsson 1977), this species is often found intertidally under stones, generally above the zone populated by G. oceanicus. In Newfoundland G. setosus is only found in the outlets of cool fresh-water streams in summer (V. Steele & Steele 1970), and this is precisely the biotope in which we have found this species in Finnmark, where it is found in the larger rivers, usually below the waterline. Distribution in Norway: This is basically an Arctic species, and it has been reported from only a few places in Northern Norway (Malangen, Hammerfest, Tana) (Vader 1977b); a record from the Oslofjord (Stephensen 1935-42, p. 323) we consider doubtful. The species has an amphi-atlantic, high northern distribution, and is common on Svalbard. C. The Gammarus zaddachi group The three species G. zaddachi, G. salinus and G. wilkitzkii appear to be closely related morphologically, zoogeographically and ecologically; however, the ‘under-ice high-arctic’ G. wilkitzkii seems quite different from the other two, that are confined to a quite restricted area in northwestern Europe: south to western France (Bretagne). All three are basically brackish water species. G. zaddachi and G. salinus are easily recognized compared to the species in groups A and B because of their colour pattern; they have a clear pattern of transverse dark stripes on a light brown background. 2. Gammarus crinicornis Stock, 1966 Synonyms: Gammarus plumicornis s. Pirlot (1939) and den Hartog (1964); non G. plumicornis Costa, 1853 Description and illustrations: Pirlot 1939, p. 54, figs 4-7 (as G. plumicornis); Stock 1967, p. 32, figs 14-16; Lincoln 1979, p. 263, fig 121. Ecology and habitat: A marine, mainly sublittoral species, often from sandy bottoms with many loose algae. In the Netherlands often in the surf zone off sandy beaches (Vader, unpubl. obs.). Not found in brackish water. Distribution: Not well known, not yet found in Norway. Occurs from W. Africa along the coasts to S. England and the Netherlands, also in the Mediterranean. 3. Gammarus inaequicauda Stock, 1966 Synonyms: Gamnmarus campylops s. Sars (1890-95); non G. campylops Leach, 1815. This species is very close to G. insensibilis Stock, 1966 Descriptions and illustrations: Sars 1890-95, p. 500, pl. 176-2 (as G. campylops); Stock & Kant 1966, pp 9-11, figs 1-2.; Stock 1967, p. 22, figs 7-8. Ecology and habitat: In Norway, this is a species of the shallow subtidal in protected, often densely vegetated areas of somewhat varying salinity. The intertidal in these areas is generally occupied by other Gammarus species, often G. salinus. Distribution in Norway: This species was originally described from Moss in the Oslo fjord. Its distribution is not yet fully known, but it seems to be quite widespread in the right biotopes along the south coast (Vader et al. 1984). Outside Norway, the only records still seem to be from Poland (Jazdzewski 1970) B. Gammarus oceanicus group. This is a group of quite large, unicoloured, basically marine Gammarus species, with a northerly and amphi-atlantic distribution. They are basically sublittoral, but are found more often intertidally than the species in group A, especially in the far north. Along the Norwegian coast they live primarily among algae, but in Arctic areas they are often found in Marinogammarus-like biotopes, intertidally below stones. 4. Gammarus oceanicus Segerstråle, 1947 Synonyms: Gammarus zaddachi oceanicus auct.; Lagunogammarus oceanicus auct. Descriptions and illustrations: Segerstråle 1947, p. 226, fig. 3 a-g; Spooner 1951, p. 130; Dunbar 1954, p. 765, fig. 31; Kinne 1954, p. 414, figs1-2, 4pp; Lincoln 1979, pp 243, 253, figs 19 Fauna norvegica 39: 12–25. 2019 Vader and Tandberg: Gammarid amphipods in Norway den Hartog 1964), although also there the species may occur intertidally under special circumstances, usually involving lowered salinity (Vader 1965). Distribution in Norway: Not well known. The species is common in the inner part of the Oslofjord (Skadsheim 1983, 1984), and it has also been found along the south coast (Dennert 1973, Vader et al. 1984). In the Hardangerfjord Brattegard (1966) found G. salinus only in the side fjords, while the northernmost records hitherto are from the Bergen area (Vader 1972). Vader (1977a) did not find G. salinus at all in the Sognefjord. G. salinus is a southern species, occurring along European coasts south to N. Spain (van Maren 1975); it is absent from Iceland (Ingolfsson 1977). 8. Gammarus wilkitzkii Birula, 1897 Synonym: Lagunogammarus wilkitzkii auct. Descriptions and illustrations: Sexton 1942, p. 601; Gurjanova 1951, p. 764, fig. 551; Barnard 1959, p. 120, pls 10-13. Ecology and habitat: This large and slow-growing species occurs primarily in the pack ice (Weslawski 1994), where it is omnivorous (Arndt 2002). In areas with seasonal ice cover the animals may live benthically in the ice free periods (Arndt et al. 2005). Distribution: This is a high-arctic species, occurring along the coasts of Siberia and northern Canada, as well as in the pack ice of the Polar Sea. It has never yet been found in Norwegian waters outside Svalbard. Svalbard distribution is mapped by Węsławski et al. (2018). D. The Gammarus tigrinus group 9. Gammarus tigrinus Sexton, 1939 Description and illustrations: Sexton 1939, pp 545-548, pls IV-VI; Bousfield 1973, pp 51-52p. 50, pl. IV-14; Lincoln 1979, p. 254, fig. 117. This species belongs to a group of primarily American brackishand fresh-water Gammarus species, which in many ways parallel the G. zaddachi group in Europe, i.a. by their conspicuous ‘tiger stripes’ (cf e.g. Bousfield 1969). G. tigrinus has no doubt been imported to Europe by man, but when and where is unknown. It was first discovered in Britain in the 1930’s and described as a new species by Sexton (1939). It was later recognized as identical to certain populations of the American freshwater species G. fasciatus Say, 1818 but this species was split by Bousfield (1958), with the brackish water populations retaining the name G. tigrinus. Later it was imported by fisheries people to both the Netherlands and Germany, in the hope that this would increase the food basis for freshwater fish; instead the species quickly developed into a problem (see i.e. Ruoff 1968). Its dispersal in the Netherlands 6. Gammarus zaddachi Sexton, 1912 (s. Spooner, 1947) Synonyms: Gammarus zaddachi zaddachi auct.; G. setosus balticus Dementieva, 1931; G. sarsi Reid, 1939; G. ochlos Reid, 1945; Lagunogammarus zaddachi auct. Descriptions and illustrations: Sexton 1942, p. 593, pl. 1, figs 1-7, pl. 2, figs 10-14 (as G. zaddachi, freshwater form); Segerstråle 1947, p. 231, figs 3h-j, 5; Spooner 1947, p. 20, figs 4A-B, 5A-C; Kinne 1954, p. 417, figs 14pp; Dennert et al. 1969, p. 23; Lincoln 1979, pp 239, 243, 249, figs 110a, 111a-b, 114. Ecology and habitat: Gammarus zaddachi is a brackish water amphipod, living subtidally or in the lower intertidal. It is quite common in the lower intertidal in the inner fjords (where G. duebenii occupies the upper intertidal), but on the outer coast it is generally confined to river mouths (Vader 1977a-b). Also there it lives mainly in the lower intertidal or shallow subtidal, while G. duebenii occupies the higher intertidal. Compared to areas further south (den Hartog 1964, Spooner 1947) G. zaddachi in Norway seems to occupy somewhat less saline waters. Migratory movements, quite conspicuous in French estuaries (Dennert et al. 1969, Stock 1966, Girisch et al. 1974), have not yet been studied in Norway. Distribution in Norway: Gammarus zaddachi occurs all along the Norwegian coast, in the right biotopes (Brattegard 1966, Dennert 1973, Vader 1977a-b). On the open coast it is restricted to the outlet of minor streamlets, where it may penetrate quite far inland (Vader 1977b). In the inner parts of the large fjords, often mesoor even oligohaline, G. zaddachi becomes the dominant amphipod in the lower intertidal, often together with G. oceanicus (Brattegard 1966, Vader 1977a). The species is absent from Svalbard. A very aberrant biotope was discovered by the Økland family (Økland et al. 2011): a population of G. zaddachi lives at 150 m depth in the freshwater lake Eikeren in inland southern Norway. 7. Gammarus salinus Spooner, 1947 Synonyms: Gammarus zaddachi salinus auct.; Lagunogammarus salinus auct. Descriptions and illustrations: Sexton 1942, pl. 1, figs 8, 9, pl. 2, figs 15-18 (as G. zaddachi, saline form); Spooner 1947, p. 20. figs 4C, 5D-E; Kinne 1954, p. 416, figs 1-4pp; Dennert et al. 1969, p. 23; Lincoln 1979, pp 243, 251, figs 111f, 115. Ecology and habitat: Elsewhere in Europe Gammarus salinus is known as the species concentrated in the areas with the largest oscillations in salinity (den Hartog 1964, Movaghar 1964, Meurs & Zauke 1988); this is in agreement with the situation in the inner Oslofjord (Skadsheim 1983). Here G. salinus lives in the lower intertidal, while the records from the south coast and the Bergen area mostly concern the shallow subtidal, as in the estuaries of Western Europe (Spooner 1947, Vader and Tandberg: Gammarid amphipods in Norway 20 Fauna norvegica 39: 12–25. 2019 pools, and even in oligohaline brackish water. Distribution: The Scandinavian distribution has been mapped by Segerstråle (1954). Gammarus pulex has in Sweden been found close to the border of Østfold in SE Norway, but as yet there are no Norwegian records. F. The Gammarus duebenii group Here we have combined G. duebenii and Marinogammarus finmarchicus, both somewhat odd man out in their genus, both morphologically and ecologically. Both are largely intertidal species, although duebenii has a much wider ecological tolerance, and both have an amphi-atlantic distribution. (Pinkster et al. (1970) split off the freshwater populations in Ireland and Brittany as G. d. celticus; this taxon is not considered here). In contradistinction to the species in the zaddachi and tigrinus groups, these species are not patterned, but unicoloured. 12. Gammarus duebenii Liljeborg, 1852 Synonyms: Rivulogammarus duebenii auct. (NB. The specific epithet is often written duebeni, and the author’s name Lilljeborg; both are incorrect.). Description and illustrations: Sars 1890-95, p. 507, pl. 177-1; Kinne 1954, p. 419, figs 1-4pp; Lincoln 1979, pp 243, 245, figs 111e, 112. Ecology and habitat: Gammarus duebenii is a very euryoecious species, and generally one can say that it occurs everywhere where other Gammarus and Marinogammarus species do not thrive (see e.g. Forsman 1951, Kinne 1953, den Hartog 1964). In Norway the main biotope is supralittoral rockpools, the mouth of small and medium large streamlets (here they can ascend quite a bit upstream and live at least temporarily in pure freshwater), and intertidally, under stones and among algae in mesoand oligohaline brackish waters, as in the inner part of fjords (Brattegard 1966, Vader 1977a). Gammarus duebenii has also several times been found in ‘freshwater’ lakes and pools in Norway (see Økland 1970, Kjærstad et al. 2016). As ‘marflo’ samples are often transported by fishermen between lakes and even from the shore to lakes, some of these occurrences may be not quite natural. Distribution. Common along the whole coast of Norway, but absent from Svalbard waters. A few freshwater occurrences. 13. Marinogammarus finmarchicus (Dahl, 1938) Synonyms: Gammarus finmarchicus auct.; Chaetogammarus finmarchicus auct; Gammarus greenfieldi Shoemaker, 1938; Echinogammarus finmarchicus auct. Description and illustrations: Dahl 1938, p. 125, figs 1-10 (Gammarus f.); Stephensen 1935-42, p. 344, figs 45-46 p.p.; Sexton & Spooner 1940, p. 656, figs 6,7; Bousfield 1973, p. has been followed in detail over the years, and it is clear that G. tigrinus is completely dominant to the indigenous Gammarus species in many habitats; it is i.e. less sensitive to pollution (see e.g. Nijssen & Stock 1966, Gras 1971). Ecology and habitat: In the USA Gammarus tigrinus is a species of oligohaline and mesohaline habitats in estuaries. In Europe it is found in similar biotopes, e.g. in the Baltic Sea, but also in stagnant oligohaline brackish and even in fresh water, where it often has ousted the local indigenous Gammarus species. Distribution: In Europe hitherto found in Ireland, Britain, Holland, Germany and the Baltic Sea. Gammarus tigrinus has not yet been recorded from Norway. E. The Gammarus lacustris group. This group contains the pure freshwater species. Morphologically the group is characterized by short oval eyes, and a short inner ramus of uropod 3. The two Nordic species are probably not especially closely related. G. lacustris has a holarctic distribution, while G. pulex is exclusively palaearctic. NB. The genus name Rivulogammarus S. Karaman, 1931, often used for this group, is unavailable (Stock, 1969a). 10. Gammarus lacustris Sars, 1863 Synonyms: Rivulogammarus lacustris auct.; Gammarus pulex sensu Sars (1890-95); non Cancer pulex Linnaeus, 1758; Rivulogammarus scandinavicus S. Karaman, 1931. Description and illustrations: Sars 1890-95, p. 503, pl. 177-2 (as G. pulex); Økland 1969, p. 132, figs 16-23. Ecology and habitat: Dealt with in detail by Økland (1969). G. lacustris lives in lakes, ponds and slow moving rivers at places where the water is not too acidic and too deficient in calcium. Distribution: See Segerstråle (1954) and Økland (1969, fig.2). This species occurs in most of Norway, but has not been reported from Østfold and Agder, while there are no reports from Lofoten-Vesterålen either. Around Bergen G. lacustris has been recently discovered by Bjerknes (pers. comm.). Outside Norway, G. lacustris has a very wide holarctic distribution at northerly latitudes. 11. Gammarus pulex (Linnaeus, 1758) Synonyms: Rivulogammarus pulex auct.; Gammarus pulex pulex auct. Description and illustrations: Schellenberg 1942, pp 2426, figs 6b, 7, 8 (as G. p. pulex); Pinkster 1970, pp 181, 183, 185, figs1 p. 179, figs 1-4. Ecology and habitat: Character species of rivers with not too strong currents, but it can also live in stagnant ditches and