Is the glucose concentration in the haemolymph a suitable indicator of circadian rhythmicity in Nephrops norvegicus (Decapoda, Nephropidae)?

The haemolymph glucose concentration shows marked fluctuations over a 24-hour cycle in several decapods. In the present study, the influence of the diel light cycle as a modulator of glucose concentration was evaluated in the haemolymph of Nephrops norvegicus in the northwestern Mediterranean. Haemolymph samples were collected from animals caught by bottom trawls at two depths, 100 m and 400 m, during two different seasons, October and June. A time series analysis showed the absence of a rhythmic pattern in the glucose concentration at either depth and in either season. Knowing that fishing stress increases haemolymph glucose concentration, laboratory experiments were also planned under controlled conditions, under different photoperiod regimes (day-night: 12-12, 18-6, 6-18). Again, no rhythmic fluctuations in glucose concentration were detected. Field glucose concentrations were higher than those determined in laboratory experiments (t-test, p less than or equal to 0.05), indicating a reference value of the stress for animals caught by trawling nets. Therefore, the absence of rhythmicity in both field survey and laboratory experiments suggests that this physiological variable is not a suitable indicator of the circadian rhythmicity in animals dwelling at a depth greater than 100 m. We propose that rhythms in locomotor and emergence behaviour and their influence on the underlying physiology may explain the glucose arrhythmicity in this species.