Circadian Oxygen Consumption Patterns in Continental Slope Nephrops Norvegicus (decapoda: Nephropidae) in the Western Mediterranean

The oxygen consumption of Nephrops norvegicus animals collected from the upper continental slope (400 m depth) in the northwestern Mediterranean was monitored under constant conditions of darkness and temperature. Two experiments were performed starting at the beginning of the expected day and at the beginning of the expected night phases, respectively. Mean oxygen consumption values recorded during the expected night were significantly higher than those recorded during the expected day. The slopes of the time series of oxygen consumption data of the two experiments were recalculated in consecutive 1-h intervals, being then averaged for corresponding 1-h time intervals. The plotting over a 24-h cycle of these mean hourly-values revealed a global nocturnal increase in the oxygen consumption in the laboratory. This result is discussed and compared with previously reported catch patterns accounting for emergence from burrows in the field, and locomotor and cardiac activity rhythms recorded in constant conditions in the laboratory in animals from the same depth. Nephrops norvegicus (Linne, 1758) is a burrowing decapod species distributed in the northeastern Atlantic Ocean and Mediterranean Sea (Farmer, 1975; Chapman, 1980). It mainly inhabits the continental shelf in the northeastern Atlantic and the upper continental slope in the Mediterranean (Abellò et al., 2002a, b). An endogenous circadian nocturnal rise in locomotor activity has been assessed in animals of this species kept in constant laboratory conditions in darkness (Atkinson and Naylor, 1976; Arèchiga et al., 1980; Aguzzi, 2002). Such a nocturnal rhythmicity is independent from the depth of sampling of tested animals (Chapman and Howard, 1979). At the same time, a rhythmic emergence behaviour, modulated upon light intensity cycles, was found in the field, as characterised by marked rhythmic variations in catches. Catch patterns differed with depth, suggesting that animals changed the characteristics of their emergence from burrows with depth. The comparison between laboratory and field results indicates that endogenous locomotor activity rhythms and emergence patterns present the same period and phase on the shallow continental shelf. There, animals emerge at night (Farmer, 1975; Moller and Naylor, 1980) in accordance with a corresponding increment in locomotor activity rates. As said, emergence rhythms increasingly differentiate with increasing depth, but not the endogenous locomotor rhythms. Thus, upper continental slope animals show an endogenous nocturnal increase in locomotor activity, but they emerge from their burrows during the day (Aguzzi et al., in press). Oxygen concentration near the bottom is an important environmental factor affecting Nephrops norvegicus emergence behaviour (Hagerman and Baden, 1988). Because of their burrowing habit, these animals can withstand very low oxygen concentrations when compared with other epibenthic crustacean species (Company and Sardà, 1998). According to Hagerman and Baden (1988) and Baden et al. (1990), the N. norvegicus populations of the North and Baltic Seas experienced cyclical events of hypoxia in the 1990s. A fall in oxygen tension at the sediment surface caused harder hypoxic conditions inside burrows (Gerhardt and Baden, 1998). This apparently incremented the duration of emergence, which in turn determined a rise in catches. These data suggested an alteration in the emergence behaviour of the animals under such environmental stressing condition (Hagerman and Baden, 1988). Fishery management of Nephrops norvegicus should carefully take into account studies on physiological fluctuations underlying rhythmic behaviour, in particular emergence from burrows, which deeply affect catchability of the 749 JOURNAL OF CRUSTACEAN BIOLOGY, 23(4): 749–757, 2003