Rudiments of an ability for time measurement in the cavernicolous fish Anoptichthys jordani hubbs and innes (Pisces Characidae).

Rudiments of an ability for endogenous time-measuring are indicated a) by bimodal activity in the dark phase of LD-cycles of 16:16 or 24:24 h and b) by damped activity oscillations frequently following a transition from LD to constant conditions. These oscillations always have the same period length as the applied LD.