Cycles annuels de Gammarus duebeni duebeni Liljeborg, 1852, et de Gammarus pulex pulex (Linne, 1758) le long de la cote française du Boulonnais

1. The life-cycles of Gammarus duebeni duebeni Liljeborg and G. pulex pulex (Linnaeus) have been studied in three stations (2 in fresh waters on the cliff coast, 1 in rockpools) on the French Channel coast, through monthly sampling during a good year. In every sample, the number of precopulae have been counted, and physico-chemical environmental factors, such as Ca++-contents, chlorinity, pH, and temperature, have been measured. The samples have been tested on homogeneity, after which the cephalic length was measured. Likewise, the sex and the stage of development (females either or not in reproductive stage, males, juveniles) have been determined. 2. In G. d. duebeni as well as in G. p. pulex there appears to exist a linear correlation between the total length of the body and the cephalic length. This correlation provides a means to study the age composition of the various populations through measuring the cephalic length. 3. For each of the three populations studied, life-cycle histograms were designed showing that: – Rockpool populations of G. d. duebeni (from Audresselles) reproduce in winter; – Populations of the same species living in trickles from a chalk-cliff (at Cran d’Escalles) reproduce in summer; – Populations of G. p. pulex living in a fresh stream on the cliff (at Pointe de la Rochette) reproduce in summer. 4. The temperatures in the rockpools of Audresselles and in cliff waters at Pointe de la Rochette show a yearly fluctuation correlated with the air temperatures. On the other hand, the trickles from the cliff at Cran d’Escalles show almost constant water temperatures all year round. The pH-values showed great variation and the measurements were considered too unreliable, so these remained out of consideration. The chlorinity of the cliff waters studied, showed some fluctuation, probably due to silt spray from the sea and to evaporation. In the rockpools, much greater fluctuations occured. The Ca++-contents in the cliff waters varied a little in an irregular way; in rockpools the fluctuations were stronger and runned parallel to those in the chlorinity. 5. The reproductive period of the three populations under study has been determined by: (a) the percentage of reproducing females, and (b) the numbers of precopulae in the monthly samples. The latter method proved to be handsome to obtain a rapid picture of the intensity and of the period of the reproduction. – G. p. pulex in a stream on the cliff at Pointe de la Rochette showed an intensive reproduction period in the spring and summer, and a resting period in the autumn and early winter. These different periods are possible induced by the combined influence of temperature and photoperiodicity. – G. d. duebeni on the cliff at Cran d’Escalles shows peaks in reproduction in the months of April and May; the reproduction almost stops in autumn. In this type of biotope, with its constant temperatures, the increase in day-length may induce the start of the reproduction periode. – G. d. duebeni in rockpools at Audresselles reproduces in autumn and winter; in summer the reproduction almost stops. It seems logic that this subartic/boreal species, living in a biotope where the temperature may rise to very high values in the summer months, concentrates its reproduction period in the colder part of the year.