Hypoxia tolerance in the copepod Calanoides carinatus and the effect of an intermediate oxygen minimum layer on copepod vertical distribution in the northern Benguela Current upwelling system and the Angola-Benguela Front

There is a growing concern that hypoxic and anoxic areas in the sea spread in extent and intensity, posing a severe risk to marine ecosystems and fisheries. Hypoxia may affect fish stocks directly or via detrimental effects on important prey species, such as zooplankton. A unique feature of the northern Benguela Current upwelling region and Angola-Benguela frontal system is a pronounced intermediate oxygen minimum layer (IOML) at 60-500 m depth with oxygen concentrations ≤ 1.4 mg O₂l^− 1 (minimum < 0.7 mg O₂l^− 1). Field studies during February-March 2002 demonstrated that the abundance of calanoid copepods and the biomass of mesozooplankton in general were severely reduced within the IOML. The dominant copepod Calanoides carinatus showed a bimodal vertical distribution with parts of the population either comprising all developmental stages concentrated in the surface layer (0-60 m), or copepodids C5 diapausing below 400 m depth apparently avoiding the IOML. Accordingly, abundances of other calanoid copepods were higher at the surface and below 300 m than in the centre of the IOML. The scarcity of planktonic life within the IOML raises the question whether this layer represents an effective barrier for zooplankton vertical migrations. Especially in C. carinatus, ontogenetic vertical migration plays a key role in the retention of the population within the productive upwelling region and for the rapid re-colonisation of plumes of newly upwelled water. To address this issue, the hypoxia tolerance of C. carinatus was determined in a series of laboratory-based, closed-bottle experiments in January 2005. Copepods were kept in gas-tight bottles and the decreasing oxygen concentrations were monitored to establish their minimum oxygen demands. Although copepodids survived apparently unharmed at surprisingly low oxygen concentrations of ca. 1.5 mg O₂l^− 1, they could not tolerate oxygen levels < 1.1 mg O₂l^− 1, implying that the core of the IOML, where O₂ concentrations are below this threshold, is uninhabitable for C. carinatus. In contrast, the IOML may represent a refuge from competition and predation for other copepod species specifically adapted to hypoxic environments.     

A revision of Atlantic species of Photostomias (Teleostei: Stomiidae: Malacosteinae), with a description of a new species

While one or possibly two species of the genus Photostomias have been recognized, an unpublished revision of the Malacosteinae suggested that there may be as many as six species worldwide. Our review of museum material revealed three taxa in the Atlantic alone: Photostomias atrox Alcock, 1890; Photostomias guernei Collett, 1889; and a new species described herein. Because of a paucity of Indo-Pacific material and a need to better document Atlantic biodiversity, we treat only the Atlantic species at this time. A key to the identification of Atlantic Photostomias is given.     

Comparative study of the spectral sensitivities of mesopelagic crustaceans

The spectral sensitivities of 12 species of mesopelagic crustaceans were studied by means of electrophysiological recordings. Nine of the species are vertical migrators, while 3 are not, and 9 species possess bioluminescent organs, while 3 are not bioluminescent. All species had a single peak of spectral sensitivity with maxima between 470 nm and 500 nm. There was no apparent correlation between sensitivity maxima and daytime depth distribution, migratory behavior, or the presence or absence of bioluminescent organs. With the exception of the hyperiid amphipod Phronima sedentaria, the spectral sensitivities of these mesopelagic crustaceans demonstrate a better match for maximum sensitivity to bioluminescence than to downwelling light. Accepted: 29 June 1999