Marine Biology (2000) 136: 387±396
Ó Springer-Verlag 2000
A. Legeay á J.-C. Massabuau
Effect of salinity on hypoxia tolerance of resting green crabs, Carcinus maenas, after feeding
Received: 30 October 1998 / Accepted: 15 October 1999
Abstract Mechanisms that can in¯uence the tolerance of hypoxia in brackish waters were studied in resting and fed crabs, Carcinus maenas, at 15 °C. Mortality, blood oxygenation, acid-base status and lactate concentration were analysed in fed crabs held in full-strength normoxic seawater (32.5& S) and then transferred for 24 h to a partial pressure of oxygen (Po2 ) of 3 kPa (1.4 mg l)1) and various salinities (17, 12.5, 10, 8& S). At salinity levels >10&, fed crabs tolerated Po2 values as low as 3 kPa in the ambient water and 0.5 kPa in their arterial blood for 24 h without switching to anaerobic metabolism. Only below 10& S did their blood-lactate content rise, leading to their death despite the fact that their blood O2-content was twice the control value measured in full-strength normoxic seawater and their blood Po2 did not decrease below values recorded at higher salinity levels. Addition of CO2 to 8& S water (CO2 partial pressure increasing from 0.1 to 0.3 kPa) decreased blood-lactate production and mortality, suggesting that at 10& S impairment of the O2 supply is limited by an excessive blood O2-anity. The results are discussed in terms of the distribution (O10& S) of C. maenas along salinity gradients in estuaries and bays.
Introduction Estuaries and bays are highly productive areas used by many ®sh, mollusc and crustacean species as feeding grounds and as reproduction and nursery locations Communicated by S. A. Poulet, Rosco A. Legeay á J.-C. Massabuau (&) Laboratoire d'Ecophysiologie et d'Ecotoxicologie des SysteÁmes Aquatiques, UMR 5805, Universite Bordeaux I and CNRS, Place du Dr. B. Peyneau, F-33120 Arcachon, France Fax: 0033 (0)5 5654 9383 e-mail:
[email protected]
(Setzler-Hamilton 1987). In these biotopes, human activities are expanding, with concurrent increasing stress. Stress, particularly that associated with eutrophication, aects the oxygen budget of the water and can cause hypoxic events with adverse eects on the fauna (Dethlefsen and Von Westernhagen 1983; Ocer et al. 1984; Rosenberg and Loo 1988; Pihl et al. 1991). Recently, however, substantial data have demonstrated the ability of various ®shes, molluscs and crustaceans to survive with a poorly-oxygenated internal milieu (McMahon 1985; Forgue et al. 1992a; Ellis and Morris 1995). An adaptation strategy whereby the partial pressure (Po2 ) in the arterial blood is maintained within a low and narrow range largely independent of ambient water Po2 (3 to 40 kPa), has also been described (Massabuau and Burtin 1984; Forgue et al. 1989, 1992b; Takeda 1990; Massabuau et al. 1991). Recent studies have described very slight blood-Po2 adjustments in crustaceans during increased oxidative metabolism. For example, it has been shown in the ``O2-sensitive'' lobster Homarus gammarus that under normoxia its arterial Po2 increases from only 1±2 to 2±4 kPa during the ®rst 5 h after feeding, and that this slight increase is sucient to fully support the muscular activity of the foregut (Clemens et al. 1998). For resting Callinectes sapidus at 23 to 25 °C (arterial Po2 ' 2 kPa), Gannon and Wheatly (1995) reported that a 15 min swimming eort required an increase in arterial Po2 to only '3.5 kPa. These observations suggest that at least some aspects of crustacean metabolism can be sustained with blood that is relatively poorly oxygenated, and raise questions as to which factors determine the survival and/or mortality of crustaceans during transient hypoxic events in the ®eld. In brackish waters, where respiratory conditions are especially stressful, there are various possible explanations for a lack of tolerance to hypoxia; a drop in salinity results not only in increased oxygen consumption (Schwabe 1933; Taylor 1977; Huni and Aravindan 1985), but also (through increased blood pH) in increased blood O2-anity (Truchot 1973). We therefore
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set out to examine interactions between osmoregulatory challenge, hypoxia and the post-prandial doubling of O2 consumption (induced by the speci®c dynamic action of food, SDA; Jobling 1981; Houlihan et al. 1990) in the crab Carcinus maenas. This crab is a typical inhabitant of these biotopes, with its arterial Po2 at the anaerobic threshold of
