An annual cycle of flash induced luminescence in the euphausiid Thysanoëssa raschii

The euphausiid Thysanoëssa raschii (M. Sars) rarely luminesces spontaneously in the laboratory. The flash-stimulated luminescent response (FSLR) was therefore used to investigate seasonal changes in luminescent ability. Some observations were also made on the responsiveness of Meganyctiphanes norvegica. In the case of T. raschii both sexes showed maximum responsiveness from March through June and in some years to September. The strength of the tendency to respond was strongest in May and June. The FSLR delay was least during April, May and June. These results are interpreted in the light of the hypothesis that the form of the FSLR is the result of opposing inhibitory and excitatory components. It is tentatively concluded that luminescence is involved in behaviour related to spermatophore transference, which takes place from April to June.