Sex determination in Gamrnarus duebeni duebeni was analyzed by breeding experiments conducted with individuals obtained from various populations. Sex ratio of progeny depends on genetic factors and photoperiod. Generally, a preponderance of males was noted when offspring were raised under long-day photoperiods, whereas females prevailed under short-day photoperiods (Bulnheim, 1967(Bulnheim, , 1969. Based on these previous studies the critical daylength, the transition point at which the photoperiodic response switches, was estimated in specimens from a population of the Elbe estuary as ranging between 13 and 14 hours of light per day at 15 ~ C. The susceptibility to photoperiod is assumed to depend on the balance between a system of male and female determining genetic factors that act on a polygenie basis. From various coastal brackish-water populations thelygenie (all-female) strains could be selected. This maternally inherited sex-ratio condition is caused by the transovarially transferred microsporidians Octosporea efferninans and Thelohania herediteria which, independently of each other, exert a feminizing influence on the host's offspring. Either perfectly or imperfectly thelygenic females may occur. The latter produce eggs that are not all infected; hence their progeny are bisexual. As a consequence of the sex-determining influence of the microsporidians males are generally not parasitized. However, some males associated with O. effeminans were found in a strain derived from a population at Bornholm (Baltic Sea).Also, males infested with one or both of the two parasite species occur in some populations of G. d. eelticus, indicating that in these sex determination is not governed by the microsporidians concerned. The feminizing influence of the parasites may be affected by environmental factors. An increase of the ambient salinity level to 25-30 %o results in a disappearance of O. efferninans in the eggs released by infected G.d. duebeni femaies. Thus, the normally acting switch mechanism of sex determination is re-established. In addition, long exposures to low temperatures (~ 4 ~ C) may have an adverse effect on the vegetative stages of both microsporidians. Owing to this, infected females may produce eggs which are not all parasitized. Consequently, mixed progeny may arise. Studies on the incidence rates of the two microsporidians comprising 18 populations indicate that the parasites are widespread in their geographical range. Most populations studied in the Baltic Sea area and brackish-water habitats of the German North Sea coast are associated with either one or both of the two microsporidian parasites. Observations on the sex ratio of a population from the Elbe estuary, performed over several years, revealed considerable seasonal fluctuations. The percentages of infected females, however, remained at a fairly constant level during the study period. The results obtained are discussed with reference to the mechanisms of sex determination and differentiation as revealed in other amphipod speci...

In a macrogeographic survey, the population genetic structure of mussels from various regions of the Baltic Sea, a large semi-enclosed brackish-water basin, was examined with reference to

Comparative investigations on the physiological capacities of Gammarus locusta (L.), G. oceanicus Segerstråle, G. salinus Spooner, G. zaddachi Sexton and G. duebeni duebeni Liljeborg obtained from German coastal and estuarine areas were carried out under uniform experimental conditions. In order to assess the adaptation of these species to the abiotic conditions of their particular habitats, the following criteria were examined: (a) oxygen consumption as related to temperature, (b) time course of acclimation to a new steady state of metabolic rate following sudden temperature changes, (c) resistance to oxygen deficiency, and (d) resistance to aerial exposure. Considerable interspecific differences were found among the five amphipods studied. G. locusta reveals the highest O uptake rates and longest thermal acclimation periods as well as lowest resistance capacities to oxyten depletion and air exposure. The other species occupy an intermediate position as indicated in the above-cited order, but contrast with G. duebeni which exhibits comparatively high resistance capacities. The significance of the findings obtained is discussed in relation to the environmental requirements of the five crustaceans considered.

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