ALBERT

Description: Migration of Gammarus pulex pulex (Linnaeus, 1758), G. fossarum Koch in Panzer, 1836, and Echinogammarus berilloni (Catta, 1878) has been studied in a small French chalk stream, the Slack. Three different approaches to investigate both up- and downstream migration were used: (1) migration survey, with a sampling program of migration at intervals of two weeks or a month at twelve localities in the river Slack; (2) continuous measurement of migration at three habitats with very stable, normal and very unstable environmental conditions, respectively, lying within 100 m of one another and populated by the same species, G. fossarum; (3) finally, marking experiments in order to identify and trace animals with a given behaviour. Both drift and upstream migration show a considerable microgeographic variation, which is larger for Gammarus than for E. berilloni. During the relatively warm year of 1975, the migration activity of E. berilloni was stronger than in 1974. Upstream migration was concentrated in early summer, while drift fluctuated during the year. Most animals migrated during the night, although the diel variation in drift was quite different from that in upstream migration. Water temperature and its diel fluctuations have a large effect on non-accidental migration. Changes in chemical composition of the water seem to be important as well. Light conditions have only a slight influence on migration patterns. Physical disturbance of the riverbed (for instance by wading cows or the scouring effect of spates) influences migration rather negatively. The mean size of migrating animals was larger than the average size of the standing crop. Upstream migrants were larger during hours of high upstream migration activity, while the animals that drifted in peak hours were usually smaller than those drifting in hours of low activity. Both up- and downstream migration proved to be a constant behaviour; most drifters of a particular night drifted again the following night and most upstream migrants moved again upstream after they had been marked. In particular our results on microgeographic and seasonal variation show clearly that a quantitative approach to migration would have been premature. Secondly, they make a direct correlation between production and drift unrewarding. The continuous measurement of migration showed that for this type of investigation field work is preferable to laboratory experimentation, since it gives more reliable results than those achieved under laboratory conditions.

Description: 2) These freshwater species can be classified in three artificial groups: (a) the G. pulex-group (species without dorsal carina and with dense setation on pereiopods 3 and 4 and uropod 3), (b) the G. balcanicus-group (species without dorsal carina and poorly setose pereiopods 3 and 4 and uropod 3) and (c) the G. roeseli-group (species with dorsal carina). These groups are merely artificial ones since transitive (intermediate) species do exist. Moreover, the origin of the species is not known, so that polyphyly is not excluded. 3) In the present work the Gammarus pulex-group is revised, based on rich material from Europe, North Africa, Asia minor and adjacent areas. 4) The taxonomic differences between the various species are usually small but distinct and stable. This is especially evident in mixed populations of two or more species. In those populations no intermediates between the taxa have been observed. 5) In some cases, morphological differences between two species are hardly discernible although reproductive isolation is present (e.g. G. fossarum and G. wautieri). 6) Hybridization experiments can solve taxonomic problems and test the taxonomic value of morphological differences between populations. Such experiments between many Asiatic and European populations might clarify their taxonomic status. 7) The taxonomic characters may largely be variable within one population as well as in different populations of the same species. 8) Characters that are very stable in one species can be largely variable in other species (e.g. presence of calceoli, length of rami of uropod 3). Nevertheless we can distinguish certain stable characters (e.g. the structure of the mandibular palp), but also instable ones (e.g. the number of dorsal and lateral spines on the urosomites) in all species. 9) Gammarus pulex has given rise to several isolated populations, adapted to subterranean life (being blind or having enlarged eyes). These populations are considered distinct subspecies. 10) Within some taxa (e.g. G. fossarum and G. p. pulex) morphologically aberrant populations can be found. Since these populations do successfully cross-breed and are sympatric they must be considered mere variations. 11) We had serious problems to determine the identity of several Gammarus species, especially from Asia Minor, because of the impossibility to obtain literature and type material of some Russian authors. (So we cannot exclude the possibility that our species described from Asia might be identical with a species formerly described by a Russian author). 12) For all species, except the most common ones, complete lists of all localities studied are given. Moreover, in 3 maps the distribution of the various species and subspecies is illustrated. 13) It was not possible to illustrate all morphological details of every taxon mentioned in the present work. Only G. pulex, the type species of the genus Gammarus and the nominal form of the entire group, is figured completely. For the other taxa, only those parts are illustrated that are fundamentally different from those of G. p. pulex.

Description: Migration of Gammarus pulex pulex (Linnaeus, 1758), G. fossarum Koch in Panzer, 1836, and Echinogammarus berilloni (Catta, 1878) has been studied in a small French chalk stream, the Slack. Three different approaches to investigate both up- and downstream migration were used: (1) migration survey, with a sampling program of migration at intervals of two weeks or a month at twelve localities in the river Slack; (2) continuous measurement of migration at three habitats with very stable, normal and very unstable environmental conditions, respectively, lying within 100 m of one another and populated by the same species, G. fossarum; (3) finally, marking experiments in order to identify and trace animals with a given behaviour.\nBoth drift and upstream migration show a considerable microgeographic variation, which is larger for Gammarus than for E. berilloni. During the relatively warm year of 1975, the migration activity of E. berilloni was stronger than in 1974. Upstream migration was concentrated in early summer, while drift fluctuated during the year. Most animals migrated during the night, although the diel variation in drift was quite different from that in upstream migration. Water temperature and its diel fluctuations have a large effect on non-accidental migration. Changes in chemical composition of the water seem to be important as well. Light conditions have only a slight influence on migration patterns. Physical disturbance of the riverbed (for instance by wading cows or the scouring effect of spates) influences migration rather negatively.\nThe mean size of migrating animals was larger than the average size of the standing crop. Upstream migrants were larger during hours of high upstream migration activity, while the animals that drifted in peak hours were usually smaller than those drifting in hours of low activity. Both up- and downstream migration proved to be a constant behaviour; most drifters of a particular night drifted again the following night and most upstream migrants moved again upstream after they had been marked.\nIn particular our results on microgeographic and seasonal variation show clearly that a quantitative approach to migration would have been premature. Secondly, they make a direct correlation between production and drift unrewarding. The continuous measurement of migration showed that for this type of investigation field work is preferable to laboratory experimentation, since it gives more reliable results than those achieved under laboratory conditions.

Description: Until 1974 Gammarus ibericus was known only from the type-locality in Spain. In that year, Goedmakers recorded the species from many localities in the Massif Central, France. Indeed, these populations show a great morphological resemblance to the Spanish ones. These French populations were used in cross-breeding experiments with other populations from southern France, without result. Attempts to hybridize these populations with the G. ibericus from the type-locality also failed to produce any offspring. Electrophoretic studies showed that the French and Spanish populations are very different from a biochemical point of view; these differences are much greater than between any of the known freshwater species (or populations) from western Europe. It is therefore decided that the populations from the Massif Central belong to a separate species: G. orinos n. sp. Careful examination of hundreds of specimens from both Spain and France showed that minor but constant morphological differences exist between the two species.

Description: La variabilit\xc3\xa9 morphologique des Gammares du groupe pulex et la confusion concernant la position syst\xc3\xa9matique des diff\xc3\xa9rentes formes de ce groupe, d\xc3\xa9j\xc3\xa0 d\xc3\xa9montr\xc3\xa9es par l\xe2\x80\x99auteur en 1970, l\xe2\x80\x99ont conduit \xc3\xa0 donner des descriptions de diff\xc3\xa9rentes formes du groupe et \xc3\xa0 discuter leur variabilit\xc3\xa9. Des exp\xc3\xa9riences d\xe2\x80\x99hybridation ont \xc3\xa9t\xc3\xa9 faites entre des populations morphologiquement diff\xc3\xa9rentes, \xc3\xa9troitement li\xc3\xa9es \xc3\xa0, ou bien interm\xc3\xa9diaires entre G. pulex et G. wautieri provenant de diff\xc3\xa9rentes localit\xc3\xa9s d\xe2\x80\x99Europe occidentale. Les r\xc3\xa9sultats de ces exp\xc3\xa9riences clarifient la position syst\xc3\xa9matique de ces populations. Les donn\xc3\xa9es disponibles concernant l\xe2\x80\x99\xc3\xa9cologie et la distribution des diff\xc3\xa9rentes esp\xc3\xa8ces sont fournies.

Description: 2) These freshwater species can be classified in three artificial groups: (a) the G. pulex-group (species without dorsal carina and with dense setation on pereiopods 3 and 4 and uropod 3), (b) the G. balcanicus-group (species without dorsal carina and poorly setose pereiopods 3 and 4 and uropod 3) and (c) the G. roeseli-group (species with dorsal carina). These groups are merely artificial ones since transitive (intermediate) species do exist. Moreover, the origin of the species is not known, so that polyphyly is not excluded. 3) In the present work the Gammarus pulex-group is revised, based on rich material from Europe, North Africa, Asia minor and adjacent areas. 4) The taxonomic differences between the various species are usually small but distinct and stable. This is especially evident in mixed populations of two or more species. In those populations no intermediates between the taxa have been observed. 5) In some cases, morphological differences between two species are hardly discernible although reproductive isolation is present (e.g. G. fossarum and G. wautieri). 6) Hybridization experiments can solve taxonomic problems and test the taxonomic value of morphological differences between populations. Such experiments between many Asiatic and European populations might clarify their taxonomic status. 7) The taxonomic characters may largely be variable within one population as well as in different populations of the same species. 8) Characters that are very stable in one species can be largely variable in other species (e.g. presence of calceoli, length of rami of uropod 3). Nevertheless we can distinguish certain stable characters (e.g. the structure of the mandibular palp), but also instable ones (e.g. the number of dorsal and lateral spines on the urosomites) in all species. 9) Gammarus pulex has given rise to several isolated populations, adapted to subterranean life (being blind or having enlarged eyes). These populations are considered distinct subspecies. 10) Within some taxa (e.g. G. fossarum and G. p. pulex) morphologically aberrant populations can be found. Since these populations do successfully cross-breed and are sympatric they must be considered mere variations. 11) We had serious problems to determine the identity of several Gammarus species, especially from Asia Minor, because of the impossibility to obtain literature and type material of some Russian authors. (So we cannot exclude the possibility that our species described from Asia might be identical with a species formerly described by a Russian author). 12) For all species, except the most common ones, complete lists of all localities studied are given. Moreover, in 3 maps the distribution of the various species and subspecies is illustrated. 13) It was not possible to illustrate all morphological details of every taxon mentioned in the present work. Only G. pulex, the type species of the genus Gammarus and the nominal form of the entire group, is figured completely. For the other taxa, only those parts are illustrated that are fundamentally different from those of G. p. pulex.