Zur Ökophysiologie, Sexualität und Populationsgenetik litoraler Gammaridea — ein Überblick

Abstract

Comparative investigations on the physiological capacities in the euryhaline amphipodsGammarus locusta, G. oceanicus, G. salinus, G. zaddachi andG. duebeni were reviewed. In order to assess the adaptations of these species to the abiotic conditions of their environment, the following criteria were examined: oxygen consumption in relation to ambient salinity and temperature levels, respiratory responses following osmotic stress, resistance capacities to oxygen deficiency, resistance to aerial exposure and the simultaneous presence of hydrogen sulphide. Covering the range from marine to typically brackish-water inhabitants, the 5 species show adaptive responses in the above-mentioned order. Respiration is less intensely modified by external factors, and oxygen consumption decreases. Accompanied by faster rates of acclimation to new steady states of performance, resistance capacities increase. The significance of the findings obtained is discussed in relation to the environmental requirements of the amphipods considered. Based on breeding experiments, the sex-determining systems reported thus far inGammarus species are outlined. As demonstrated inG. duebeni, a more or less pronounced influence of external factors such as photoperiod may become effective. A preponderance of males was noted when offspring were raised under long-day photoperiods, whereas females prevailed under short-day conditions. In terms of the critical daylength, the light per day was estimated as being between 13 and 14 h (Elbe estuary population). Feminizing microporidians (Octosporea effeminans, Thelohania herediteria), which are transovarially transmitted, can interfere with the system of sex determination and sex differentiation of the host. As reflected in variousG. duebeni populations, they cause a maternally transferred sex-ratio condition by the production of all-female broods, thereby mimicking extrachromosomal inheritance. An increase of the salinity level to 25–30‰ results in a disappearance ofO. effeminans. In both microsporidians, long exposure to low temperatures (≤4°C) produces eggs which are not all parasitized. Furthermore, intersexuality can be induced by changing environmental factors. Microsporidian species have no influence on sex differentiation inG. duebeni celticus, G. salinus, G. locusta andG. pulex. Patterns of relative electrophoretic mobilities of proteins and the distribution of allele frequencies at polymorphic gene loci can be utilized for species diagnosis and for the evaluation of the relationships between different taxa, particularly at and below the species level. As exemplified by studies on several gammarids from marine, brackish and freshwater environments, inter- and infraspecific gene-enzyme variation is described. Electrophoretic investigations on natural populations of the euryhaline amphipodsG. zaddachi, G. salinus, G. tigrinus and others from different geographic areas provided evidence of considerable biochemical genetic variation. InTalitrus saltator- andTalorchestia deshayesii-populations the extent of variability based on micro-and macrogeographic aspects is illustrated. The large-scale genetic divergence is demonstrated by comparison of samples obtained from the Baltic, North, Atlantic and northern Mediterranean Seas.