A Summary of the Work on the AmphipodGammarus CheureuxiSexton Carried out at the Plymouth Laboratory (1912–1936)

Abstract

Thework began in June, 1912, simply as a study of the life-history of some of our common amphipods. The genus Gammarus was chosen, because of the number and the wide distribution of its species in the neighbourhood. Seven of these species, all black-eyed, from marine, estuarine, brackish and fresh waters, were kept in the laboratory for investigation until the first red-eyed specimens were discovered in one of the brackish species, later described asGammarus chevreuxi(see p. 360). Since the manner in which these red-eyed individuals occurred raised points of considerable interest, it was decided to confine investigation toG. chevreuxi, and a series of experiments was started. The different variations involving both the structure and the pigmentation of the body and of the eyes appeared from time to time, some behaving as simple mendelian characters and attributable to the presence of a single recessive gene, others with a more problematical hereditary basis. These variations have been recorded as they occurred (see bibliography). For ten years, no second mutation appeared, although frequent dredgings were made and the animals thus obtained were kept for several generations in laboratory conditions. Because of this apparent stability of character the wild Gammarus was regarded as a homogeneous population until, in 1922, it became increasingly evident that the results showing in the laboratory cultures could only be explained on the supposition that many recessive factors must be present in the natural conditions.The evidence steadily accumulated, but direct proof was very difficult to obtain partly because (as we found in our experience with the cultures) the recessive types are often less viable than the normal, and therefore probably less able to withstand the competition in the wild, and partly perhaps because we had no criteria by which to assess the influence of the laboratory conditions (changes of temperature, food, salinity, pressure, etc.) on the constitution of the Gammarus, and so could not decide whether a variation was caused by the inherent action of the recessive genes, or by the untoward environment producing a change in the action of the normal genes.