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Notes: Abstract Enchytraeus albidus is able to absorb dissolved14C-labeled neutral amino acids (glycine, L-alanine, L-valine,α-aminoisobutyric acid) and an amino-acid mixture from ambient water across the body surface against considerable concentration gradients. Saturation kinetics and susceptibility of glycine uptake to competitive inhibition by alanine suggest mediated transport. Absorption of neutral amino acids is an active process. Exchange diffusion of preloadedα-aminoisobutyric acid against external glycine orα-aminoisobutyric acid could not be detected. Results on inhibition of glycine uptake by a variety of low-molecular-weight substances indicate that glycine absorption is highly specific for neutral amino acids and somewhat less for basic amino acids; it is unspecific for non-α-amino acids, acidic amino acids, carbohydrates, and organic acids. Rates of transintegumentary net influx of glycine are nearly identical to14C-glycine influx, suggesting that only small amounts of amino acids are released, as compared with the capacity for uptake. Thus,14C-amino-acid influx data are used for characterization of the uptake system. Glycine uptake is positively correlated to external salinity. In fresh water, absorption is nearly zero; between 10 and 20 ‰ S, uptake increases markedly reaching maximum values at 30 ‰ S; these remain almost constant at 40 ‰ S. Transport constants and maximum uptake rates increase with rising salinities. Since absorption of glycine and L-valine is susceptible to sodium depletion, similar mechanisms presumably underly salinity-dependent uptake of amino acids and sodium-dependent solute transport. Oxygen consumption is not significantly modified by different external salinities. Estimates of nutritional profit gained from absorption of amino acids vary between 4 and 15 % of metabolic rate for glycine absorption and between 10 and 39 % for uptake of an amino-acid mixture, according to external concentrations (10 and 50 µM) and salinities (20 and 30 ‰ S).

Notes: Abstract 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 toMytilus edulis andM. galloprovincialis samples from the North Sea, Irish coast and southern Portugal. Electrophoretically detectable variation was analysed at 6 polymorphic enzyme loci (Ap, Est-D, Lap-2, Odh, Pgi andPgm). Evidence was provided of a remarkably large amount of biochemical genetic differentiation among ecologically and morphologically divergent mussel populations in the Baltic. Patterns of allele frequencies in low-salinity populations from the area of the Baltic Proper were demonstrated to be widely homogeneous but contrast strongly with those of the western Baltic, the latter resembling populations from marine habitats of the North Sea. Associated with a pronounced salinity gradient, the spatial heterogeneity in gene-pool structure is indicated by steep clines of allele frequency changes in the area of the eastern Danish isles. The adaptive significance of the observed allozymic variation is suggested. From genetic distance estimates, the subdivision of population structure is discussed in relation to the significant amount of differentiation detected withinMytilus populations to date and to the evolutionary time required for the divergence of Baltic mussel populations. The allozymic data provide evidence for the genetic distinctiveness of mussels from the low-salinity areas of the Baltic. Their position at the specific or subspecific level of classification requires further consideration.

Notes: Abstract Sex determination inGammarus 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, 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 polygenic basis. From various coastal brackish-water populations thelygenic (all-female) strains could be selected. This maternally inherited sex-ratio condition is caused by the transovarially transferred microsporidiansOctosporea effeminans andThelobania 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 withO. 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 ofG. d. celticus, 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 ‰ results in a disappearance ofO. effeminans in the eggs released by infectedG. d. duebeni females. 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 species, in particular to the interaction of genetic and non-genetic factors and the occurrence of monogeny phenomena.