The role of aerobic metabolism and intragel oxygen in hypoxia tolerance of three ctenophores: Pleurobrachia bachei, Bolinopsis infundibulum and Mnemiopsis leidyi

The quantitative importance of ctenophores, medusae, salps and other gelatinous organisms in pelagic ecosystems has been recognized widely for over two decades (Alldredge, 1984; Longhurst, 1985). The potential for gelatinous zooplankton to affect food web structure is illustrated dramatically in the Black Sea where a ctenophore, Mnemiopsis leidyi Agassiz, 1865, was introduced from North America and devastated fisheries (Kideys, 2002). In Chesapeake Bay, studies by Breitburg and co-workers (Breitburg, 1994; Breitburg et al., 1997) have shown that gelatinous organisms, including M. leidyi, can take refuge in hypoxic bottom waters where non-gelatinous zooplankton are unable to survive. This means that gelatinous predators are able to utilize the benefits of diel and tidal vertical migrations, but other planktonic organisms (e.g. crustaceans, fish larvae, bivalve larvae) are restricted to upper oxygenated layers. Hence, gelatinous organisms can have a perturbing effect on pelagic food webs in oxygen stratified estuaries as prey zooplankters are concentrated into oxygenated areas towards the surface making them more vulnerable to predation by ctenophores and medusae. Most research on physiological adaptations of pelagic animals to low oxygen environments has been undertaken on crustaceans and fish that inhabit oceanic oxygen minimum layers (Childress & Seibel, 1998). Oxygen minimum layers (OML) are wellknown phenomena that persist year round in layers 100s of metres thick over large expanses of ocean (Longhurst, 1967), and OML animals rely primarily on physiological adaptations in their aerobic metabolic physiology to survive there. These adaptations include increased gill surface areas, utilization of high affinity respiratory proteins, decreased oxygen diffusion distances and increased ventilatory rates (Childress & Seibel, 1998). In contrast to organisms in OML regions, benthic organisms that live in environments which undergo episodic hypoxia, for example intertidal molluscs, are usually considered to survive through anaerobic biochemical adaptations (Hochachka & Somero, 2002). Estuarine hypoxic zones are also different from the year-round stable low oxygen pelagic environments of OML, because many estuaries experience hypoxia only on a seasonal basis. Therefore, gelatinous zooplankters in estuaries experience a unique set of oxygen conditions, since they have the ability to migrate rapidly in and out of a low The role of aerobic metabolism and intragel oxygen in hypoxia tolerance of three ctenophores: Pleurobrachia bachei, Bolinopsis infundibulum and Mnemiopsis leidyi