Lepley and Rice (1952) were astonished to observe that when put a maze, paramecia tend to alternate between left and right turns. This is usually considered as a behavioral marker of short-term memory. I quote the introduction: “The writers submit the following somewhat unconventional introductory statement. The planning of the experiments to be reported was done half in jest, and neither author, with any appreciable degree of confidence, expected that they would yield positive results. If scientists are permitted to be astonished by data, then the writers claim that privilege.”.

However, ten years later, Lachman and Havlena (1962) tried to reproduce the phenomenon and failed. But the two experiments differed both in the species used (P. caudatum and P. micronucleatum) and the dimensions of the mazes. In this paper, Harvey and Bovell reproduce the experiments with the two species, and mazes of different dimensions. They find that alternation behavior can be observed in both species, but only if the track length between the two turns is short enough (3 vs. 6 mm).

No particular interpretation is given, but they note correctly that, since paramecia swim in spirals, it seems rather unlikely that what the observer calls left and right makes much sense for paramecia, and therefore the alternation behavior may not actually be a signature of short-term memory.

My interpretation is the following. According to Jennings, Paramecium does not turn randomly. It turns towards the aboral side (away from the mouth). It only appears at random because Paramecium turns around its longitudinal axis and therefore the mouth constantly changes orientation. Without knowing where the mouth is, the observer would consider that the reorientation angle is random. Thus, whether Paramecium alternates between left and right in a maze should depend on the number of rotations around its body that has occurred between the two turns. When the track between the two turns is long, the number of rotations may be quite variable and therefore no reliable relation can be observed. But when it is short enough, one may observe either persistence or alternation in the turns, depending on the exact track length.