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Bulletin Zoologisch Museum 0]NIVERSITEIT VAN AMSTERDAM Vol. 6 No. 3 1977 Furtherobservations on the range extension of the alienamphipod Gammarus tigrinus Sexton, 1939, in the Netherlands during the years 1974 to 1976 Jan Dieleman & Sjouk Pinkster extension Abstract showed a rapid range of this species (Nijssen & Stock, 1966, Pinkster & Stock, In the of 1967, autumn 1975 and in the spring of a of the of new survey range Gammarus tigrinus Sex- Dennert et al., 1968, Gras, 1971,and Lourens, ton, 1939, was carried out, especially at the bor- 1972). In a trade with 1973, new survey was spe- ders of the 1973 Considerable range. range exten- sions were found in the north-eastern of cial the of the local parts the emphasis on position gam- province of Groningen, the Kampereiland (province marid fauna in the area now inhabited by G . ti- of in the Deltaic and Overijssel), Region in some less isolated in more or areas the province of grinus (cf. Smit, During this survey only Zuid-Holland. In most of the invaded newly areas minor range extensions were found. Notable new G. tigrinus has replaced the indigenous Gammarus species. Sampling along the Dutch-German border localities were the Kralingse Plassen at Rotter- revealed that has no invasion taken place from any dam, where an isolated was and of the German population found, populations. A fungus was found which to be appeared pathogenic for G. tigrinus. the Haringvliet near Middelharnis, the southern- most record of G. tigrinus in the Netherlands and INTRODUCTION the first record for the Deltaic region. The most this striking result of survey was the disappear- After the introduction of the alien Gam—- of the amphipod ance indigenous species G. p. pulex (Lin- marus in the Netherlands duebeni tigrinus Sexton, 1939, naeus, 1758), G. Liljeborg, 1852, and G. series (probably in 1964) a of sampling zaddachi from extensive surveys Sexton, 1912, areas, now 22 inhabited by G. tigrinus alone. (Benthuizer Vaart, Stotrpwijkse Vaart, Langeraarse autuim in In 1975 and 1976 a new sampling sur- Pias, Kronme Mijdrecht). out the boundaries of the In the of vey was carried along province Groningen (northeastern distribution area of G. tigrinus as determined in Netherlands) the species has successfully invaded 1973. Mareover, intensive sampling was done along nearly all inland waters north of the line Gronin- the eastern border of the Netherlands, to check a gen-Winschoten-Nieuweschans, except for sane pools possible invasion of G. tigrinus from West Germany, and ditches behind the dikes of the Waddenzee. where this species was purposely introduced in the South of this line no gammarid fauna was found at rivers Weser and Werra (Schmitz, i960, Tesch & all, in spite of intensive sampling. Most probably Pries, 1963> and Ruoff, 1968), while recently new this is caused by the heavy organic pollution and found in localities were the Schlei (Bulnheim, the complete absence of oxygen during the sumner 1976). The total area, investigated during the months. 2. Netherlands there present survey, is indicated in fig. In the central part of the in is only a small number of new localities: the RANGE EXTENSION OF G. TIGRINUS IN THE AREAS Noordoost Polder, in the deltaic area of the river INVESTIGATED IN 1975 AND 1976 IJssel and in some places just inside the dikes along the former Zuyderzee (the parts now called the considerable During present survey range ex- Eemmeer and Veluwemeer). tensions have been found especially in the north- Intensive sanpling along the Dutch-German bor- eastern and southwestern parts of the country. The der revealed, that no invasion had taken place new localities, together with the distribution from any of the populations recently introduced area found by Smit are illustrated in fig. in West Germany. 1. Table I summarizes the exact localities, dates and chlorinities of the samples containing G. ti- THE INDIGENOUS SPECIES IN THE INVESTIGATED AREA grinus. Notable is the range extension in the south- Gammarus pulex pulex (Linnaeus, 1758) west on the islands of Voorne, Putten and Beijer- In the central part of the province of Zuid-Hol- almost fauna is land, an area without any ganmarid land there still an extensive area in which G. at all before the introduction of G. (Den is the dominant the tigrinus p. pulex species, with excep- Hartog, 1964, Smit, 1974). tion of some lakes in which G. tigrinus is the In the Haringvliet, where it was found only only gairmarid. This area is characterized by the once in G. has become in- of 1973, tigrinus a common presence many oligohaline, relatively unpollut- habitant and even inside the dikes on the island ed ditches with a well-developed and diverse flora of Goeree-Overflakkee the found in and fauna. In this of species was type habitat G. p. pulex can In and one single locality. stony exposed places build up dense populations. The area is now almost along the border of the Haringvliet G. tigrinus conpletely surrounded by waters containing G. ti- was often found together with G. duebeni. More to grinus and it will be quite interesting to inves- the east, in the Hollands Diep and the Biesbosch tigate further developments there. former tidal area) is (a freshwater there still Den Hartog (1964) expected a dispersion of G. no fauna at all. from the localities the Del- ganmarid p. pulex existing in The in isolated population the Kralingse Plas- taic region and the small Brabantian rivers into in (cf. sen Rotterdam Smit, has succesfully the Haringvliet, the Hollands Diep and the Bies- invaded the Bergscne Plassen and the Schiebroek- bosch after the closing of the sea arms and the sche Polder, further to the north at the city subsequent refreshing of these waters. However, boundary. the reverse seems to happen: some localities in In the central part of the of Zuid- Vilich G. found province formerly p. pulex was are now oc- Holland some formerly inhabited localities, found cupied by G. tigrinus whereas the Brabantian riv- abandoned the during 1973 survey, were again occu- ers are so severely polluted that they act as an by G. Noord A). In the effective barrier rather than pied tigrinus (Vliet, Geer, a way for disper- several same area new localities were discovered sion. So it seems most likely that G. tigrinus 23 the almost from the will soon occupy the Hollands Diep and Bies- has now completely disappeared in northern Deltaic the bosch since it is already abundant the Haring- region: it was found only on island vliet. of Goeree-Overflakkee at chlorinitiesrang- In the provinces of Priesland and Groningen G. ing from 3,000 to 10,000 mg/1. In the province of to G. it has been G. in p. pulex is gradually loosing ground tigri- Groningen replaced by tigrinus localities. nus (see also Smit, and now has almost com- many pletely disappeared. In the eastern part of the Netherlands, where used be it has DISCUSSION G. p. pulex to a common species, almost completely disappeared. The reason for this found cannot be found in the competition with G. tigri- In his 1973 survey Smit (197*0 only some nus but more likely in the progressive pollution minor range extensions of G. tigrinus, giving the of the water systems in this region. In the north- impression that the situation was stabilizing af- is in the ern part of the Veluwe area, where pollution ter the rapid expansion of G. tigrinus pro- less G. is still the severe until now, p. pulex com- ceeding years. During present survey, however, mon. considerable range extensions were found again in the south-western as well as in the north-eastern Gammarus duebeni duebeni Liljeborg, 1852 part of the distribution area. In the middle of the took Smit already noticed the spatial separation country the only considerable range extension of G. d. duebeni and G. tigrinus in the Buiten-IJ place in the Deltaic area of the river IJssel. Like in near Amsterdam. A comparable situation can be the preceding years, mainly oligo- found in Waterland (the area north of Amsterdam haline waters were occupied (table I), and there with many ditches, canals and small lakes with are still no records of G. tigrinus from running chlorinities of 500 to 1000 mg/1) where G. d. due- freshwater or from its original environment, the beni lives on exposed, stony places, and G. tigri- tidal belt (Bousfield, 1958) (Holland, 1976, men- nus lives in all other microhabitats. tioned one locality from a salt marsh along the In the Haringvliet, where G. tigrinus has be- very polluted Mersey estuary in England). In some is come a very common species, G. d. duebeni still areas G. tigrinus does not penetrate into clean, found but and and only on exposed stony places oligohaline ditches, where G. p. pulex is abun- even in these habitats it is rather scarce. The dant. The relatively low Cl-concentrations found salinity might have reached too low values to per- in these areas (130 to 400 mg/1) cannot be the mit successful reproduction here except during decisive factor in this respect, since G. tigri- winter the months (see Pinkster, 1975, for the com- nus was often found at much lower chlorinities. bined influence of temperature and salinity on re- Future observations as well as laboratory studies production) . will be necessary to find an explanation for this Compared with the situation found by Den Hartog phenomenon. (1964) G. d. duebeni now inhabits a more restrict- Pinkster (1975) already discussed, that in ed the northern Deltaic area in region. Only on oligohaline waters G. tigrinus has a much greater the island of Goeree-Overflakkee, where ditches reproductive capacity than G.

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