Growth, Molting Frequency, Heart Beat, Number of Eggs, and Incubation Time in Gammarus Zaddachi Exposed To Different Environments

Abstract

Gammarus zaddachi from the Kleiner Kiel, Kiel, North Germany, were exposed to different temperature and salinity conditions in the laboratory and the rates of some life processes measured. All tests were performed under standard conditions (food, light, amount of water per individual, quality of water). The rates of growth, molting, heart beat, egg production and embryonic development proved to be similar in the same environment to those of the previously tested G. salinus (Kinne, 1960) from the same habitat. G. zaddachi and G. salinus, however, differ considerably with respect to these rate functions from a third Gammarus species of the Kleiner Kiel, namely G. duebeni. The latter exhibits much slower rates under the same conditions (Kinne, 1952b; 1953a, b). The intervals between subsequent molts are longer in G. zaddachi males than in females. No significant differences were observed in specimens exposed to the salinity levels 5‰, 10‰ and 30‰, respectively. Females kept without males tend to prolong progressively the molting intervals, beginning with the third or fourth interval after isolation. No relationship between body length and rate of heart beat could be established in 17 to 21 mm long males or in 14 to 16 mm long females. Such a relationship does, however, exist over a wider range of body length: the rate of heart contractions is definitely higher in immature specimens of a length of 5 to 6 mm. No significant differences of rates of heart beat in mature specimens were observed in 5‰ or 10‰. The number of marsupium eggs per female of a given length depends on the temperature, being higher at 14° to 16°C than at 18° to 20°C, and on the salinity. There exists a linear relationship between the number of eggs and the volume (cubed length) of the mother animal, except in senile females. In the absence of a male, there is a tendency to reduce progressively the number of eggs laid. A normal number of eggs is usually deposited only subsequent to a transfer of sperm. Along with the functional similarity between G. zaddachi and G. salinus there exists a striking structural similarity (Spooner, 1947; SegerstrAle, 1947; Kinne, 1954, etc.). The information available suggests that G. zaddachi and G. salinus represent two relatively young species which have only recently become genetically independent from each other (see also Spooner, 1947, 1951; and Kinne, 1954). The differences between these two species become apparent in terms of their distribution, ecology and interspecific sterility (Spooner, 1947; SegerstrAle, 1947; Steen, 1951; Kinne, 1954; Forsman, 1956) rather than in terms of the rate functions or morphological expressions studied. The physiological data available at this time are not sufficient for a detailed analysis. More elaborate experiments on the physiological potentialities of G. zaddachi and G. salinus may lead to the discovery of delicate differences and to a better understanding of their ecology and evolution.