Gammarus Salinus Research Articles

Sea freshening may drive the ecological impacts of emerging and existing invasive non‐native species

AbstractAimThe spread of invasive non‐native species (INNS) will pose major threats to global biodiversity over the coming decades. However, predicting how key effects of climate change will influence the abilities of INNS to establish and exert ecological impact is a major challenge. One overlooked aspect of global change is the expected freshening of certain marine systems, which may interact with INNS and lead to drastic effects on community structure and stability.LocationBaltic Sea, Europe.MethodsHere, using three predatory amphipod crustaceans, we experimentally assessed how salinity reduction may affect the impacts of the emerging INNS, Pontogammarus maeoticus, relative to an existing INNS, Gammarus tigrinus and a trophically analogous native, Gammarus salinus. We quantified per capita impacts of the three species via the comparative functional response method (prey consumption over a range of prey densities) under a predicted seawater freshening scenario. We then combined amphipod functional responses with their life history traits to compare population‐level relative impact potential (RIP) on prey of the three amphipod species across salinities.ResultsFreshening substantially altered the predicted relative ecological impacts of both the INNS compared with the native. First, the functional responses of invasive P. maeoticus and G. tigrinus increased under freshening, while that of the native G. salinus decreased. Second, RIP became consistently higher for both the INNS compared to the native with increased freshening.Main conclusionsOur methods thus reveal potential for climate change via seawater freshening to drive large shifts in dominance and ecological impacts of INNS compared with natives. With the number of INNS introductions unlikely to saturate in the near future, we highlight the need to assess the impacts of potential future INNS, alongside established non‐natives and native species, in combination with abiotic changes associated with climate change.

Comparative analysis of the antennae of three amphipod species with different lifestyles

Crustaceans detect chemical stimuli in the environment with aesthetasc sensilla, which are located on their 1st antennae. With the transition to other environments, chemoreception faces physical challenges. To provide a deeper understanding of the relation between the morphology of olfactory organs and different lifestyles, we studied the peripheral olfactory system of three amphipod species, the marine Gammarus salinus, the blind subterranean freshwater species Niphargus puteanus, and the terrestrial Cryptorchestia garbinii. We compared the 1st and 2nd antennae of these species with respect to length and presence of aesthetascs and other sensilla. The females of N. puteanus reveal the longest 1st antennae in relation to body size. G. salinus shows the largest aesthetascs and the same relative length of the 1st antennae as male N. puteanus. C. garbinii has very short 1st antennae and reduced (putative) aesthetascs. Our findings show that the compensation of vision loss by olfaction cannot be generally assumed in animals from dark environments. Furthermore, the behaviour of C. garbinii indicates a chemosensory ability, despite the reduction of the 1st antennae. A comparison with other terrestrial crustaceans suggests that the loss of the olfactory sense on the 1st antennae in C. garbinii might be compensated with chemoreception by the 2nd antennae.

Diversity and distribution of genetic variation in gammarids: Comparing patterns between invasive and non-invasive species.

Biological invasions are worldwide phenomena that have reached alarming levels among aquatic species. There are key challenges to understand the factors behind invasion propensity of non‐native populations in invasion biology. Interestingly, interpretations cannot be expanded to higher taxonomic levels due to the fact that in the same genus, there are species that are notorious invaders and those that never spread outside their native range. Such variation in invasion propensity offers the possibility to explore, at fine‐scale taxonomic level, the existence of specific characteristics that might predict the variability in invasion success. In this work, we explored this possibility from a molecular perspective. The objective was to provide a better understanding of the genetic diversity distribution in the native range of species that exhibit contrasting invasive propensities. For this purpose, we used a total of 784 sequences of the cytochrome c oxidase subunit I of mitochondrial DNA (mtDNA‐COI) collected from seven Gammaroidea, a superfamily of Amphipoda that includes species that are both successful invaders (Gammarus tigrinus, Pontogammarus maeoticus, and Obesogammarus crassus) and strictly restricted to their native regions (Gammarus locusta, Gammarus salinus, Gammarus zaddachi, and Gammarus oceanicus). Despite that genetic diversity did not differ between invasive and non‐invasive species, we observed that populations of non‐invasive species showed a higher degree of genetic differentiation. Furthermore, we found that both geographic and evolutionary distances might explain genetic differentiation in both non‐native and native ranges. This suggests that the lack of population genetic structure may facilitate the distribution of mutations that despite arising in the native range may be beneficial in invasive ranges. The fact that evolutionary distances explained genetic differentiation more often than geographic distances points toward that deep lineage divergence holds an important role in the distribution of neutral genetic diversity.

The Application of DNA Barcodes for the Identification of Marine Crustaceans from the North Sea and Adjacent Regions.

During the last years DNA barcoding has become a popular method of choice for molecular specimen identification. Here we present a comprehensive DNA barcode library of various crustacean taxa found in the North Sea, one of the most extensively studied marine regions of the world. Our data set includes 1,332 barcodes covering 205 species, including taxa of the Amphipoda, Copepoda, Decapoda, Isopoda, Thecostraca, and others. This dataset represents the most extensive DNA barcode library of the Crustacea in terms of species number to date. By using the Barcode of Life Data Systems (BOLD), unique BINs were identified for 198 (96.6%) of the analyzed species. Six species were characterized by two BINs (2.9%), and three BINs were found for the amphipod species Gammarus salinus Spooner, 1947 (0.4%). Intraspecific distances with values higher than 2.2% were revealed for 13 species (6.3%). Exceptionally high distances of up to 14.87% between two distinct but monophyletic clusters were found for the parasitic copepod Caligus elongatus Nordmann, 1832, supporting the results of previous studies that indicated the existence of an overlooked sea louse species. In contrast to these high distances, haplotype-sharing was observed for two decapod spider crab species, Macropodia parva Van Noort & Adema, 1985 and Macropodia rostrata (Linnaeus, 1761), underlining the need for a taxonomic revision of both species. Summarizing the results, our study confirms the application of DNA barcodes as highly effective identification system for the analyzed marine crustaceans of the North Sea and represents an important milestone for modern biodiversity assessment studies using barcode sequences.

Separate and combined effects of habitat-specific fish predation on the survival of invasive and native gammarids

The North-American amphipod Gammarus tigrinus was observed for the first time in the northern Baltic Sea in 2003. The invasive amphipod has been particularly successful in some habitats (e.g. on pebbles) where it has become one of the most abundant gammarid species. We studied experimentally if the dominant fish Gasterosteus aculeatus preyed differentially on the exotic G. tigrinus and the native Gammarus salinus, if predation differed among habitats, and if one gammarid species facilitated predation on the other. The experiment demonstrated that (1) fish preyed more on the exotic G. tigrinus than the native G. salinus. (2) Predation did not differ among habitats. (3) Gammarus tigrinus facilitated the predation on G. salinus and this facilitation varied among habitats with significant effects on pebbles. Thus, the combined effect of habitat-specific fish predation and competition between gammarid amphipods is a possible explanation of the current range of G. tigrinus in the northern Baltic Sea. G. tigrinus seems to establish in habitats where it can significantly increase fish predation on the native gammarids.

Seasonal variability in the grazing potential of the invasive amphipod Gammarus tigrinus and the native amphipod Gammarus salinus (Amphipoda: Crustacea) in the northern Baltic Sea

Mesograzers are known to reduce the biomass of their host plant and modify the structure of the whole macrophyte community in many ecosystems. Thus, the introduction of an efficient mesograzer may destabilize macrophyte community and also affect the native grazers. We estimated how large proportion of macrophyte production are consumed by the alien gammarid G. tigrinus and the native gammarid G. salinus in the species poor ecosystem of the northern Baltic Sea. We analysed whether G. tigrinus consumes different diet as the native G. salinus and whether the effect of G. tigrinus on the survival of the native G. salinus is macrophyte species specific. Grazing experiments showed that there was a clear difference in the grazing rates of gammarids among the studied macrophyte species in summer and autumn but not in spring. The grazing rates were significantly higher in the prevailing macrophyte Pilayella littoralis as compared to other macrophytes. The grazing was inversely related to the diurnal net photosynthetic values of macrophytes. The gammarid amphipods potentially removed only a minor part of plant primary production except for summer and autumn when grazing of a few perennial species exceeded macrophyte production. Macrophyte species and presence of G. salinus had no effect on the survival of G. tigrinus. The presence of G. tigrinus, however, reduced the survival of the native gammarids within P. littoralis in summer. To conclude it is likely that both native and alien gammarid amphipods do not exert significant pressure on the macroalgal communities in the northern Baltic Sea. Competitive interactions between G. tigrinus and G. salinus within the prevailing macrophyte P. littoralis is the likely explanation of the decline of the native gammarid amphipods after the establishment of G. tigrinus in the northern Baltic Sea.

Genetic studies of Gammarus

The marine amphipods Gammarus salinus and G. oceanicus are both polymorphic at the enzyme loci glucosephosphate isomerase (Gpi), glutamic-pyruvic transaminase (Gpi), and mannosephosphate transaminase (Mpi). In addition, G. solinus is polymorphic at the glutamic-oxaloacetic transaminase locus Gotl. The variation at these loci is described for populations ranging from the Limfjord, Denmark, to the Baltic Sea at Askô, Sweden. In both species the Baltic populations and the Kattegat populations have diverged from each other. The populations are connected through gene frequency clines that can be smooth (at the Gpi locus in both species) or more irregular (at the Mpi locus in both species).

The subtidal macrobenthos in the mesohaline part of the Schelde Estuary (Belgium): influenced by man?

The macrobenthos of the subtidal, mesohaline zone of the Schelde Estuary (Belgium) was sampled in October 1996 and 1997 at 54 and 73 sampling locations respectively. Sediments ranged from silty to very coarse, with the dominant sediment type being silt (33% of all locations). Of the 35 macrobenthic species observed, only seven species occurred in more than 20% of the samples. The polychaete Heteromastus filiformis and Oligochaeta were most common. Multivariate techniques revealed three distinct communities, linked mainly with sedimentological factors: (1) a species-poor (nine species) community with a dominance of the amphipod Bathyporeia pilosa, a low mean abundance and biomass (86 ind m−2, 0.0189 g ash-free dry weight (AFDW) m−2), and a mean median grain size of 215±19 μm (fine sand); (2) a species-rich (22) community, with the small polychaete Polydora ligerica as indicator species, a relatively high mean abundance and biomass (2298 ind m−2, 1.395 g AFDW m−2, oysters excluded), a mean median grain size of 133±41 μm, and also the occurrence of sediments with hard substrates being characteristic for this community; (3) a community with an intermediate species richness (12), abundance and biomass (248 ind m−2, 0.249 g AFDW m−2), with H. filiformis and Oligochaeta as indicator species, and a median grain size of 76±9 μm. In the study area several typical brackish water species were observed (e.g. Polydora ligerica, Corophium lacustre, Gammarus salinus).Mean total abundance and biomass were very low, and the benthic communities appeared to be under stress, with a dominance of mainly small, subsurface deposit and surface deposit feeding opportunistic species. This is probably a combined effect of both natural physical and human-induced disturbance. Only sediments with hard substrates (e.g. rocks) seems to favour species richness, providing a shelter against physical disturbance.

Bioaccumulation of trace metals in marine and estuarine amphipods: evaluation and verification of toxicokinetic models

The suitability of the marine and estuarine gammaridean amphipods Gammarus locustra (Linneaus 1758), Gammarus zaddachi Sexton (1912), and Gammarus salinus Spooner (1947) from the Island of Heligoland and the Weser and Ems estuaries, northwestern Germany, as biomonitors was tested in a toxicokinetic study with the elements Cu, Zn, Pb, Cd, and Hg. The organisms responded with metal uptake upon exposure, and it was possible to estimate parameters of two-compartment or logistic regression models that were statistically different from zero. In most cases, the toxicokinetic models obtained were successfully verified in a second toxicokinetic uptake study for Cu, Zn, Pb, and Cd. Data for Cd were additionally verified with previously reported kinetic parameters. Results for Hg indicate that the organisms tested may not be suitable as biomonitors for this element. The calibration of gammarids as biomonitors was extended in a third toxicokinetic uptake study for Cu and Pb showing a linear increase from the control level to external waterborne metal exposures of 240 µg Cu·L-1 and 60 µg Pb·L-1, while Zn tended to level off above 120 µg·L-1 and Cd above 10 µg·L-1. These figures largely determine the range for which the models may serve as a predictive tool.

A Cohort Life Table for Gammarus salinus (Amphipoda)

A cohort of Gammarus salinus juveniles was reared from 2-3 d post hatching at 20°C and 14%o salinity without limitations in food and space. Survivorship, female size and age at breeding, number of eggs per clutch and clutch number were recorded. Depending on initial assumptions, differing life tables are treated. Associated rearing problems are discussed. The recommended approach is to start a cohort with juveniles at the age of release from their mothers, and count progeny at the same age. Counts were made weekly to provide survivorship data. The mortality rate was 54% the first week juveniles fend for themselves. Then the mortality rate decreased to ca. 5% per week, but increased again when breeding commenced. The last individual died in its 28th week (191 d old). Counting time in weeks, a life table including 25% egg mortality gave a net reproductive rate Ro = 5.7377, intrinsic rate of increase r=0.0933 and generation time T = 18.7155. These values were also calculated for 50% egg mortality. The reproductive value peaked after estimated generation time. Correlations between demographic and growth variables were partly masked by a possible cohort splitting in female size and reproductive effort (clutch size). Analyses showed negative correlations between maturation time and fecundity and between maturation time and growth rate. Directional selection on single traits may possibly be restricted by variable correlations between traits.

Regional variation in amphipod life history: effects of temperature and salinity on breeding

As a test for possible local evolution, Gammarus salinus Spooner (Amphipoda, Crustacea) populations from the Oslofjord, Norway, and from Limfjorden, Denmark, were raised from hatching at the combinations of 8, 12 and 20 °C and 6, 14 and 30%. salinity. At 8 °C all animals died before breeding. Only the Oslofjord amphipods produced a few broods at 12 °C. The Oslofjord specimens reproduced at all salinities at 20 °C, but first and most frequently at 14%. salinity, while those from Limfjorden almost exclusively reproduced at 14%. salinity. Egg volume increased when temperature and/or salinity decreased, egg dry weight increased when salinity increased. Adjusted mean egg number per clutch did not differ significantly between the Oslofjord cultures, but the two lowest means were in cultures with much lower survival. Adjusted means of the Oslofjord and the Limfjorden specimens differed significantly at 14%. Limfjorden females produced smaller broods of larger eggs at a larger body size and at a similar age compared with similarly raised Oslofjord females. Slopes of linear regressions of log egg number per clutch on log body size were never significantly different. Excluding possible hidden maternal effects, it seems that the Oslofjord and Limfjorden G. salinus have evolved differences in breeding traits. Reproducing Gammarus oceaniens Segerstr°le and G. salinus from the Oslofjord were collected and acclimated to continue reproduction at the combinations of 10 and 20 °C and 10 and 30%. salinity. Data corroborated the impression that temperature has a major influence on the determination of egg size, while clutch size is mainly regulated by energy available for breeding. The hypothesis that the balance between many small or a few large eggs per clutch at any time in a population may be genetically controlled was rejected. Physiological, ecological and evolutionary aspects of amphipod life history are discussed.

Life cycles of Gammarus oceanicus and G. salinus (Amphipoda) in the Oslofjord, Norway

In the Oslofjord the amphipods Gammarus oceanicus Segerstråle, 1947 and G. salinus Spooner, 1947 were estimated to live for a maximum of 15 months. All specimens which survived during winter died in spring, mainly in May. A G. oceanicus population living on a freshwater influenced shore contained smaller specimens, had a lower proportion of the female population in breeding condition and probably produced fewer broods than a population living on a fully marine shore. The differences are discussed in relation to environmental factors and the distribution of G. salinus. Gammarus oceanicus possibly was breeding from December to May, some females even in June, resulting in an estimated maximum of three broods from December to May. Gammarus salinus showed two breeding periods, the first from December to May, a few of these females even bred in June, while new females bred from June to October, giving estimated maxima of respectively three (December to May) and five broods. Gammarus salinus females entered a reproductive resting stage in September‐October. The sex ratio was mostly female dominated. A shift to male dominance was noted in one population and was related to possible infections. Information from the literature on longevity and breeding periods was compiled and compared to the Oslofjord data.

Seasonal Changes in Density, Composition and Reproductive Biology of Crustacean Populations in the Severn Estuary

Changes in the abundance and composition of crustacean populations in the plankton of the Severn Estuary (England) were followed for 25 consecutive months (1973-1975). The collections were made primarily near shore from the cooling intake of the Oldbury nuclear power station. Neomysis integer overwintered at comparatively low densities with all specimens measuring more than 5 mm in length. Inshore swarms of this species were detected early in the summer and most females carried either eggs or embryos. Praunus flexuosus invariably occurred less frequently than N. integer. During the summer, inshore aggregations of adults were probably formed and, although large numbers of berried females were observed at this time, the release of young possibly never occurred in the Oldbury region. Eurytemora affinis was observed only during periods of low salinity in winter with maximum reproductive activity taking place in December. The highest density of Eurydice pulchra occurred early in the summer due to the recruitment of young while, at other times, this species remained comparatively rare. The same pattern was observed in Gammarus salinus but since only a few females carried eggs at any one time reproduction must take place outside of the study area. Crangon crangon maintained greatest numbers during the summer and fall, being almost absent during part of the winter. The growth rate, size at maturity and period of reproductive activity of this species differed from earlier accounts on Severn populations.

Benthic invertebrates as indicators of organic pollution in the Western Baltic Sea

AbstractThe quantitative distribution of some important benthic invertebrates in a shallow inshore area of Kiel Bay (Western Baltic Sea) is described. This region is partly polluted by domestic sewage from the municipality of Kiel. Three groups of species are distinguished with reference to their densities and other population parameters in the different subareas: Progressive species of the 1st and 2nd order indicate a high and moderate to slight degree of pollution respectively by high population numbers. Regressive species are adverse indicators, absent or occurring in exceptionally low numbers in affected areas. In many cases the examination must be concentrated on sand bottom, because a series of species normally dwelling on aufwuchs build up dense populations in sand, if it is organically enriched.The polychaetes Capitella capitata, Nereis diversicolor and Polydora ligni are regarded as progressive species of the 1st order. The amphipods Corophium insidiosum and Gammarus salinus, the mussel Mytilus edulis, the polychaete Pygospio elegans, the snail Hydrobia ulvae, and some other invertebrates belong to the group of indicators for slight organic pollution. The main regressive species are the amphipod Bathyporeia sarsi and the mite Copidognathus fabriciusi.

The basis of food selection in some estuarine fishes. Eels, Anguilla anguilla (L.), whiting, Merlangius merlangus (L.), sprat, Sprattus sprattus (L.) and stickleback, Gasterosteus aculeatus L.

The factors influencing the selection of food by eels, Anguilla anguilla, whiting, Merlanglus merlangius, sprat, Sprattus sprattus, and stickleback, Gasterosteus aculeatus have been investigated by analysing collections made in the Severn Estuary for 1 year. Non–migratory (yellow) eels measuring from 19–5–56–5 cm in length fed mainly on either the decapod Crangon vulgaris or on the mysid Neomysis integer during the warmer months but ceased to feed in the winter. Whiting measuring between 2–5 and 15 cm fed almost exclusively on C. vulgaris, N. integer and the sand goby, Pomatoschistus minutus. Sprats fed chiefly on either the arnphipod Gammarus salinus or N. integer. Sticklebacks seldom contained any food in their stomachs despite the presence of large quantities of suitable prey and the high level of hunting efficiency. Gammarus salinus, although normally abundant in the environment, was usually ingested in disproportionately small quantities by all fishes except sprats reflecting its concealment among floating weeds and a selection against small (less than 1 cm) organisms. Because of the limitations of size and the unusually efficient escape reaction of larger animals, only young representatives of Crangon vulgaris were captured. The isopod Eurydice pulchra was rarely eaten by any of the fishes even though it was common in the environment. Burrowing polychaetes, mainly Nereis diversicolor, were never utilized possibly due to the difficulty involved in removing these organisms from the substrate. Neomysis integer was the most frequently consumed organism. Although the average length of prey ingested by the whitings increased with the size of fish, this was not the case for the other species. The proportion of non–feeding eels and the dry weight of stomach contents of this species depended on the temperature of the water but were not related to the availability of prey and tidal conditions whereas the feeding of whiting could not be related to any environmental parameter.

The basis of food selection in flounders, Platichthys flesus (L.), in the Severn Estuary

The factors influencing the selection of food by flounders, Platichthys flesus (L.), have been investigated by analysing collections made in the Severn Estuary for 1 year and the results of experiments. Flounders measuring between 6–0 and 35 cm fed heavily on the polychaete Nereis diversicolor in February and on the amphipod Gammarus salinus between February and April. Thereafter these species were replaced by the mysid Neomysis integer with the decapod Crangon vulgaris. Flounders shorter than 6.0 cm, fed mainly on Neomysis integer regardless of month. Numerous factors were involved in the choice of the food including (1) the maximum and minimum length of prey, (2) its spatial distribution in the water column, (3) its degree of concealment, (4) its motility and ability to escape predation, (5) conditioning of the flounders for certain foods, (6) the fish's swimming speed and (7) the turbidity and temperature of the water.Although the average length of ingested prey remained unchanged as the fish grew in size, an increase in the maximum and minimum lengths was observed. The percentage of stomachs that contained no identifiable remains in flounders greater than 6–0 cm averaged 80–95 % during part of the winter and 60% in the summer and between January and March. These changes partly reflected the influence of low water temperature on the metabolic rate and availability of prey. The dry weight of the stomach contents in flounders longer than 6–0 cm was lowest in winter but high values were recorded in the spring. This latter feature was probably because the rate of feeding was greater than that of digestion. During the summer, under more normal feeding conditions, the weight of the stomach contents remained relatively low compared to other fish populations. Flounders shorter than 6–0 cm always contained more food in their stomachs on a unit weight basis than larger individuals, reflecting metabolic rate and hunting efficiency.

The proximate and fatty acid composition of some estuarine crustaceans

The lipid content of Crangon crangon, Gammarus salinus, Neomysis integer, Praunus flexuosus and Orchestia gammarella collected from the reservoir of the Oldbury power station on the Severn Estuary ranged from 25 to 33% of the dry weight over the course of one year while the corresponding values for ash averaged 15–24%. Water accounted for 75–80% of the wet weight except in the case of O. gammarella (70%). The unusually high lipid values were probably connected with the highly favourable environmental conditions in the reservoir while both the size and reproductive condition of the animals were also important in determining the size of the lipid store. Phospholipids accounted for as much as 33% of the total lipid in C. crangon and for as little as 24% in N. integer and P. flexuosus. The predominant fatty acids in both the neutral and phospholipids of all species were 16:0, 18:1, 20:5 and 22:6.

Upstream and Downstream Migrations in Relation to the Reproductive Cycle and to Environmental Factors in the Amphipod, Gammarus Zaddachi

1. The real identity has been studied of juvenile gammarids, that were found in the fresh-water tidal region of the River Slack (France), 3 to 4 km inland of the mouth of its estuary. At the outset, these juveniles were (on basis of their morphology) considered to belong to Gammarus salinus. 2. Rearing demonstrated that the juveniles in question developed into adult Gammarus zaddachi. This indicates that the morphological criteria used in discriminating between G. salinus and G. zaddachi fail in juvenile material. Several morphological criteria are discussed and tested. It appears that a 100% certain key character does not exist, meaning that it is not possible to identify with absolute certainty all individuals in mixed populations of the two species. 3. At the same time, the resemblance of juvenile G. zaddachi to adult G. salinus provides us with a morphological feature allowing rapid distinction between adult and juvenile G. zaddachi in studies with large numbers of individuals. 4. In studies of the reproduction and migration of G. zaddachi, several types of nets were developed, both for anadromous and catadromous migrants. These nets agreed with one another in having the same catch capacity. 5. The activity of G. zaddachi shows a diurnal periodicity, the activity peak falling at night. This behavior results in a down-river migration (“drift”), which reaches its maximum 2 to 3 hours after sunset, slowing down later in the night, until the drift virtually stops at sunrise. No influence of the light intensity during the night (moon phase, clouds) was found. 6. During the periods with H.W.S., the direction of the stream reverses in the entire study area. In the parts of the river adjacent to the estuary, this stream reversal is accompanied by a rapid increase in salinity. In the more up-river reaches, no salinity changes occur, so that we find there a fresh-water tidal region. 7. All year round a certain part of the population of G. zaddachi living at a given place (the so-called standing crop) migrates down-river. During the autumn equinoxal spring-tides (and — though much less — during the spring equinox) mass migration takes place in up-river direction. This up-river migration takes place only at nights in which the stream reverses under the influence of the H.W.S. Practically exclusively juvenile animals participate in the up-river migration. 8. Per night, the maximum number of animals caught per net (= 30 X 50 cm) was 1,500 downstreamers and 6,000 upstreamers of G. zaddachi, which may roughly correspond with 30,000 downstreamers and 120,000 upstreamers per given section of the river. No other gammarid species in the river Slack shows migratory behavior. 9. Tagging experiments showed, that animals migrating up-river cover distances of 40 to 60 m per night. Downriver migrants cover at least 50 m and at most 80 m per night. 10. Several indications make it plausible that, though G. zaddachi uses the water currents for its migration, the up-river and down-river movements are not merely accidental transportation. Animals in the (physiological) phase of moving down-river continue to do so, irrespective of the direction of the stream. The same holds true for animals in the phase of moving in up-river direction. 11. The reproductive cycle of G. zaddachi is completed in one year. The young appear in spring, in autumn these juveniles grow adult, the first ovigerous females appear in December, the maximum production of eggs falls in the early spring. All adults die after the reproduction period. 12. The reproductive cycle is coupled with the migration cycle. The females participating in down-river migration can, dependent on temperature and salinity, successfully produce an offspring. At temperatures above 7°5C this happens only in the mixohaline parts of the river Slack. When the temperature drops below 7°5 C, egg production is also possible in the limnic reaches of the river. The juveniles pass the summer in the estuarine region. In this season an important portion of the up-river distribution area of G. zaddachi is depopulated. Repopulation of the limnic region by juveniles takes place in the fall, aided by the reversed water currents at H.W.S., thus completing the cycle.