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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.

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