Data were compiled from the literature on the critical oxygen level (Pcrit), a hypoxia-tolerance trait, comprising 96 studies covering 151 fish species from 58 families. Salinity, temperature, body mass, and routine metabolic rate were highly correlated with Pcrit. The effects of temperature, CO2, acidification, metals and feeding are also assessed.

Labs should cut plastic waste too Many governments now impose charges for single-use plastic bags and bottles. As responsible researchers, we should cut back on disposable plastics (see also G. Bistulfi Nature 502, 170; 2013). We estimate that the 280 bench scientists in our bioscience department generated roughly 267 tonnes of plastic in 2014 (data from University of Exeter Sustainability and Waste and Resource Management offices). That is equivalent to about 5.7 million empty 2-litre plastic bottles. Some 20,500 institutions worldwide are involved in biological, medical or agricultural research (where plastic disposal is likely to be heaviest), so that could equate to around 5.5 million tonnes of lab plastic waste in 2014roughly the combined tonnage of 67 cruise liners, and equal to 83% of the plastic recycled worldwide in 2012. We justify our use of disposables on the grounds of costs and time saved. Grant agencies therefore need to introduce incentives to reduce plastic waste, for example by funding lab washing-up and recycling facilities, and possibly to make greener lab practices a requirement in the grantapplication process.

SUMMARY Gymnotiform weakly electric fishes generate electric organ discharges (EODs) and sense perturbations of the resulting electric field for purposes of orientation, prey detection and communication. Some species produce oscillatory (‘wave-type’) EODs at very high frequencies (up to 2 kHz) that have been proposed to be energetically expensive. If high-frequency EODs are expensive, then fish may modulate their EOD frequency and/or amplitude in response to low-oxygen (hypoxic) stress and/or compensate for costs of signalling through other adaptations that maximize oxygen uptake efficiency. To test for evidence of an energetic cost of signalling, we recorded EOD in conjunction with metabolic rates, critical oxygen tension and aquatic surface respiration (ASR90) thresholds in Apteronotus leptorhynchus, a species found in high-oxygen habitats, and Eigenmannia virescens, a species more typically found in low-oxygen waters. Eigenmannia virescens had a lower mean ASR90 threshold and critical oxygen tension compared with A. leptorhynchus, consistent with field distributions. Within each species, there was no evidence for a relationship between metabolic rate and either EOD frequency or amplitude under normoxia, suggesting that there is no significant direct metabolic cost associated with producing a higher frequency EOD. However, when exposed to progressive hypoxia, fish generally responded by reducing EOD amplitude, which may reduce energetic costs. The threshold at which fish reduced EOD amplitude tended to be lower in E. virescens, a pattern consistent with higher tolerance to hypoxic stress. The results of this study suggest that wave-type fish reduce their EOD amplitude to reduce direct energetic costs without reducing metabolic rate under hypoxia.

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We used a common-garden rearing experiment to explore environmentally induced tolerance to hypoxia in the African mouth-brooding cichlid Pseudocrenilabrus multicolor. F(1) fish originating from three field populations were grown under low or high dissolved oxygen (DO), and their resting routine metabolic rate (RMR), critical oxygen tension (P(crit)), and marginal metabolic scope (MMS) were quantified. In a second rearing experiment, we compared the RMR of brooding and nonbrooding females of low-DO origin grown under low and high DO. Fish reared under low DO had a lower P(crit) than fish reared under high DO. There was also an interaction between treatment and gender; females had a higher P(crit) than males when reared under normoxia. Variation in RMR was driven primarily by population effects, and there was an interaction between treatment and population. Regardless of population or treatment, males had a higher MMS than females. Fish reared under low DO had a higher MMS than fish reared under high DO, except for the high-DO population in which there was no treatment effect. Brooding females had a higher RMR than postbrooding females regardless of the growth treatment, indicating an energetic cost to brooding. The results suggest a strong element of developmental plasticity in P(crit) across populations and both plastic and genetic components of variation in the RMR and MMS. This study also highlights the cost of parental care in mouth-brooding fishes, which may increase the fitness of the offspring at the energetic expense of the parent, a cost that may be elevated under hypoxia.

This study quantified the seasonal pattern of reproduction in a swamp-dwelling population of the African cichlid Pseudocrenilabrus multicolor victoriae. In the hypoxic waters of the Lwamunda Swamp, Uganda, P. multicolor was reproductively active throughout the year, even during the peak of the dry seasons. However, the degree of activity was seasonal, with rainfall providing a predictor of the percentage of ripe, mature females. There was no correlation between aquatic oxygen availability dissolved oxygen (DO) and either adjusted mean gonad mass or percentage of mature females, suggesting that DO is not limiting reproductive activity in this system. Reproductively mature females were larger during drier periods and may maximise their lifetime reproductive success by producing young throughout the year; but with a lower brooding efficiency. A comparison with Welcomme's (1969) study of a river-swamp system feeding Lake Victoria suggests that reproductive patterning is variable among populations of P. multicolor and may reflect adaptive response to chronically hypoxic conditions in the Lwamunda Swamp.

This study quantified variation in key life-history traits of the widespread African mouth-brooding cichlid Pseudocrenilabrus multicolor victoriae. Egg size, number, batch reproductive effort, size at maturity and brooding efficiency were compared among field populations across a wide range of dissolved oxygen (DO) concentrations from extreme hypoxia to normoxia. In the laboratory, a similar suite of characters was quantified in F1 of low- and high-DO origin reared under low or high DO. In general, females from low-DO habitats and females reared under low DO were characterized by a smaller size at maturity and no difference in batch reproductive effort when compared with females from high-DO habitats or females reared under normoxia. A trade-off between egg size and number was evident in the field and in the laboratory-rearing experiment, but the direction of the trade-off differed. Egg size was negatively correlated with egg number across field populations; females collected from low-DO sites generally had more, smaller eggs relative to females from high-DO sites. In the laboratory-rearing experiment, F1 females of high-DO origin produced larger, fewer eggs than F1 females of low-DO origin, lending support to the field results and suggesting a heritable component to these traits. There was also an element of developmental plasticity, F1 females raised under low DO produced larger, fewer eggs compared with F1 females raised under high DO (regardless of population) suggesting that DO may interact with other variables to determine egg size in the field.

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