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

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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 ≤ 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. La concentración de glucosa en hemolinfa muestra marcadas fluctuaciones diarias en varios decápodos. En este estudio se evalúa la influencia del ciclo luminoso diario sobre la concentración de glucosa en la hemolinfa de Nephrops norvegicus del Mediterráneo oeste. La hemolinfa fue extraída de ejemplares capturados por artes de arrastre en profundidades de 100 y 400 m, durante Octubre y Junio. El análisis de la serie temporal de datos no mostró ningún patrón rítmico en la concentración de glucosa, ni por profundidad ni por época del año. Asumiendo que el efecto de estrés de pesca incrementa la concentración de glucosa, se realizaron también extracciones en condiciones controladas de laboratorio bajo distinto fotoperiodo (día-noche: 12-12, 18-6, 6-18). Tampoco se detectaron variaciones rítmicas. Los resultados muestran que las concentraciones de glucosa de los animales procedentes del campo fue superior a la de los animales de laboratorio (t-test, p ≤ 0.05) indicando un valor de referencia de estrés para animales procedentes de la pesca. La ausencia general de variaciones en la concentración de glucosa en la sangre, tanto de los animales procedentes del campo como los estudiados en laboratorio sugieren que esta variable fisiológica no es un buen detector de la ritmicidad diaria en Nephrops. Se discute el papel que el comportamiento locomotor y de emergencia de las madrigueras en Nephrops pueda tener en la arritmicidad diaria de glucosa en esta especie.

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