On members of the Gammarus pulex-group (Crustacea- Amphipoda) from western Europe by Sjouk Pinkster Institute of Taxonomic Zoology, University of Amsterdam, The Netherlands Résumé kraba, 1958), or an independant species (Koch, variabilité des Gammares du Wautier & Roux, La morphologique groupe 1835; Schellenberg 1934; 1959; pulex et la confusion concernant la position systématique Straskraba, 1962). The subsequent description of des différentes formes de ce groupe, déjà démontrées par the same species, G. fossarum, under other names l’auteur en 1970, l’ont conduit à donner des descriptions (e.g. G. delebecquei Chevreux & de Guerne, 1892; de différentes formes du leur varia- groupe et à discuter G. rambouseki S. G. bilité. Des expériences d’hybridation ont été faites entre pulex Karaman, 1931; pulex des populations morphologiquement différentes, étroite- danubialis S. Karaman, 1931) made the whole sit- ment liées ou bien intermédiaires entre G. pulex et of à, uation still more confusing. The introduction G. wautieri provenant de différentes localités d’Europe the generic name Rivulogammarus by Karaman, occidentale. Les résultats de ces expériences clarifient la 1931 and the description of supposed intermediate position systématique de ces populations. Les données forms between Gammarus and G. disponibles concernant l’écologie et la distribution des pulex fossarum sont fournies. différentes espèces (e.g. Dusaugey, 1955) contributed to the confu- sion. Abstract Roux, 1967 aided to clear the situation to some The morphological diversity within the Gammarus pulex- extent in proving that the so-called intermediate the confusion the group and concerning systematic po- forms between G. pulex and G. fossarum of Du- sition of the various forms belonging to this al- group, between saugey, 1955, were certainly no hybrids ready noticed by the author in 1970, formed the reason these two species but members of a good species, to give detailed descriptions of the different forms in this and discuss the within these that could be both on mor- group to variability clearly distinguished forms. have been between Hybridization experiments run phological and on physiological and ecological various morphologically different populations, closely grounds. However, at the other hand, Roux did related to, or intermediate between G. pulex and G. of this not give a complete description new spe- wautieri from different localities in Western Europe. cies, which he called G. wautieri. The results of these experiments, have an implication to the systematic position of these different populations. This situation instigated Stock, 1969 and Pink- When information is the and available, given on ecology ster, 1970 to attack the problem at the bottom, distribution of the various species. the first by clearing the generic position of G. pulex, the second by giving a redescription of the INTRODUCTION species and by stabilizing the nomenclature Gammarus the species, belonging to pulex- group through indication of a neotype. have been a source of confusion for quite a long At the same time systematic sampling was time, both amongst taxonomists as physiologists. started in certain areas where members of this the The causes of this confusion can be found in species-group occur. During the sampling, it be- of different very poor original descriptions the came clear that, apart from the classical species, species within the group, as well as in differences G. pulex (Linne, 1758), G. fossarum Koch, 1835, of opinion concerning the systematic position of G. wautieri A. L. Roux, 1967, and G. lacustris certain forms. G. O. Sars, 1863, several populations could be found, that showed distinct morphological de- viations So, Gammarus fossarum Koch, 1835, was alter- from these typical forms. At the there of nately considered a subspecies of Gammarus pulex one hand, was a group popu- (Linne, 1758), (e.g. Schellenberg, 1937, 1942; lations, all very close to, but clearly distinguishable Carausu, Dobreanu & Manolache, 1955; Stras- from G. fossarum, at the other hand there was a BIJDRAGEN TOT DE DIERKUNDE, 42 (2) - 1972 165 Without these the work could cluster of populations all very close to, or inter- (Z.W.O.). grants, present have been done. mediate between, G. pulex and G. wautieri. It is never Special thanks are due to Mr. H. H. Mittelberg, of the worth noting that most of these aberrant forms I.T.Z., Amsterdam, for technical assistance during the found in rather are some few, restricted, areas, experiments. viz. the French Alps (in the departements Isere, Prof. Dr. J. H. Stock has been of great help, because of his assistance both in the field and in the laboratory, Var and Alpes-Maritimes) and in the region and his critical remarks during the preparation of this around Montpellier (dept. Herault). The present for which manuscript, I am very grateful. series of author started a large hybridization ex- to clear the of these periments, systematic position THE CHARACTERS OF THE GAMMARUS different forms. In these experiments seven pop- PULEX-GROUP ulations were used, showing morphological affin- Within the genus Gammarus, the Gammarus pulex- ity to G. pulex and G. wautieri. One of the pop- group is characterized by the following characters: ulations, a typical G. pulex, came from the Neth- (1) small, kidney-shaped or rounded (al- other eyes, erlands; the six, including a population of less twice the ways than as long as wide), upper typical G. wautieri, came from southern France. margin of which is widely separated from the In addition, other members of the research team mid-dorsal line; (2) hands of gnathopods almost of the Institute of Taxonomic Zoology (ITZ), Am- equal in size, though different in shape: the first sterdam, started other series of hybridization ex- one is always pyriform, the second one has an periments (e.g. with populations morphologically almost transverse palm; (3) both gnathopods are close to G. fossarum, see Goedmakers, 1972). provided with a medial palmar spine in males; Where the results of these necessary experiments (4) P5 through P7 armed with spines only; (5) will be referred to. inner ramus of third uropod always at least half This article will be an attempt to bring more as long as the outer ramus; (6) first segment of clarity in the taxonomic status of the various exopod of third uropod armed with setae and "forms" within the Gammarus pulex-group and to spines along the outer margin. understand the importance of the certain slight Although some of these characters occur in morphological differences, which make this group other groups within the genus Gammarus as well, so difficult. Descriptions and illustrations are given this combination of features is quite characteris- of the western European species within the group. tic. So, the Gammarus balcanicus-group differs Variability will be discussed while figures are from the pulex-group in character (6) having the various given of those characters in which the pop- outer-margin of Ur 3 armed with spines only. ulations differ from each other. Where available, Gammarus roeseli Gervais could also be consid- the and information will be given on ecology ered as a member of the pulex-group, considering distribution of the various species. all the characters mentioned above, but never- ACKNOWLEDGEMENTS theless it is left because of the charact- out, very The author is indebted to Dr. Roy Olerod of the Natur- eristic teethlike projections on the metasome seg- historiska Riksmuseum of Dr. J. Forest Stockholm; to ments. of the Museum National d'Histoire Naturelle, Paris; to Dr. K. Jazdzewski of the University of Lodz, Poland; VARIABILITY to Dr. A. Skalski of the University of Czestochowa, Po- land; to Dr. W. Besch of the Landesstelle fur Gewasser- Within the Gammarus pulex-group, like in all other Dr. R. of the kunde, Karlsruhe, G.F.R.; to Margalef certain characters groups of gammarids, are very Instituto de Investigaciones Pesqueras, Barcelona, Spain; stable, while others can show an enormous and to for material. To many others, sending me prof. Dr. A. L. Roux for his help in collecting the right variability. after material for the hybridization experiments and for his As far as we could ascertain, studying ma- advice and interest in the to the enthu- personal work; terial of the various species, the following char- siastic team of B.Sc. students, working for their degree stable: acters within this group are more or less at the I.T.Z. for collecting materials and assistance with 1) The of the (always less than twice the fieldwork, especially to Miss A. M. C. Goedmakers, shape eye as Miss M. J. van Maren, Mr. R. Robelus and Mr. F. v. d. as long wide). Beld for their in and the help catching transporting 2) The structure of the mandible palp (which al- animals for the hybridization experiments. of the ways has a regular row spines on ven- wish for the Furthermore, I to acknowledge grants tral margin of the third segment). fieldwork from the University of Amsterdam, and of the The setation of the of A2 Netherlands' Organisation for Pure Scientific Research 3) flagellum (although 166 S. PINKSTER - GAMMARUS PULEX-GROUP in there be general stable, can exceptions, e.g. an effect that is intensified by the relative short in G. fossarum). antennae. The colour of live specimens is rather The hand of the from 4) shape of the gnathopods variable, greyish to red-brown or greenish, (the first one is always pyriform, the second depending on the composition of the bottom and one has an almost transverse in the in which found. The is always palm vegetation they are eye this group. Both gnathopods are provided rather small (fig. IB). with a medial palmar spine in males). The first antenna is relatively short (fig. 1C), 5) The ratio between the endopod and the first about 1/3 of the total body length of the animal.