Evaluating additive versus interactive effects of copper and cadmium on Daphnia pulex life history

A key challenge of standard ecotoxicological risk assessment is to predict the sub-lethal risk of multiple contaminants on aquatic organisms. Our study assessed the sub-lethal mixture toxicity of copper (Cu) and cadmium (Cd) on Daphnia pulex and included manipulations of food level and assessment of three genotypes. We investigated the interaction between essential (Cu) and non-essential (Cd) metals on ingestion rate, reproduction, maturation time, size at maturity and somatic growth rate of three D. pulex genotypes, over 21 days and under standard and high food conditions. We explored the potential interaction of the metals on ingestion and life history by implementing a response surface experimental design combining control and two levels of Cu and Cd and their combinations. Overall, both metals reduced ingestion rates, reduced reproduction, delayed maturation, reduced body size at maturity and lowered somatic growth rate. Our results further indicated pervasive interactions between the metals; numerous instances where the effects of each metal were non-linear; the effect of a metal varied by D. pulex food levels (ingestion rate and size at maturity), and the effect of a metal varied by genotypes (reproduction). Apart from the maturation time and somatic growth rate, our results suggest that life history traits are affected in non-additive ways by three factors that are often discussed and rarely estimated together: mixtures of metals, genotypes and resource levels. Our data that are derived from exposing daphnids to two metals highlight how metals interact with each other and the context of food resource and genetic variation. While interactions make it harder to generate predictions, and ultimately water quality regulations about the effects of metals, those detected in this study appear to be tractable.

Ocean acidification affects somatic and otolith growth relationship in fish: evidence from an in situ study

Ocean acidification (OA) may have varied effects on fish eco-physiological responses. Most OA studies have been carried out in laboratory conditions without considering the in situ p CO 2 /pH variability documented for many marine coastal ecosystems. Using a standard otolith ageing technique, we assessed how in situ ocean acidification (ambient, versus end-of-century CO 2 levels) can affect somatic and otolith growth, and their relationship in a coastal fish. Somatic and otolith growth rates of juveniles of the ocellated wrasse Symphodus ocellatus living off a Mediterranean CO 2 seep increased at the high- p CO 2 site. Also, we detected that slower-growing individuals living at ambient p CO 2 levels tend to have larger otoliths at the same somatic length (i.e. higher relative size of otoliths to fish body length) than faster-growing conspecifics living under high p CO 2 conditions, with this being attributable to the so-called ‘growth effect’. Our findings suggest the possibility of contrasting OA effects on fish fitness, with higher somatic growth rate and possibly higher survival associated with smaller relative size of otoliths that could impair fish auditory and vestibular sensitivity.

A microcalorimetric approach for investigating stoichiometric constraints on the standard metabolic rate of a small invertebrate

Abstract1: Understanding the determinant of metabolism is a core ecological topic since it permits to link individuals energetic requirements to the ecology of communities and ecosystems. Yet, besides temperature, the effects of environmental factors on metabolism remain poorly understood. For example, dietary stoichiometric constraints have been hypothesized to increase maintenance metabolism of small invertebrates, yet experimental support remains scarce.2: Here, we used microcalorimetric heat flow measurements to determine the standard metabolic rate (SMR) of Daphnia magna throughout its ontogeny when fed stoichiometrically balanced (C/P ratio:166) or imbalanced (C/P ratio:1439) diets.3: When fed a stoichiometrically imbalanced diet, daphnids were able to maintain the stoichiometric homeostasis within narrow boundaries. However, they consistently increased their SMR while decreasing their somatic growth rate. Our measurements unequivocally demonstrate that homeostatic regulation implies higher metabolic costs and thereby reduces the portion of energy that can be allocated to growth.4: Our study demonstrates that microcalorimetry is a powerful and precise tool for measuring the metabolic rate of small-sized organisms and opens promising perspectives for understanding how environmental factors, such as nutritional constraints, affect organismal metabolism.

A pilot tagging experiment on European hake (Merluccius merluccius): methodology and preliminary results

European hake (Merluccius merluccius) were caught alive in the northern Bay of Biscay in June/July 2002, using a codend specially designed to avoid crushing fish and also to retain water while hauling the trawl. In all, 1307 fish were tagged with anchor T-bar tags, injected with tetracycline, then released. The length range of the tagged and marked fish varied from 13 to 58 cm, and the modal size was 28 cm, at which length they were assumed to be 2–3 years old. A mean survival rate of 68.2% was estimated. Mortality was mainly caused by stress of capture and physical damage, and depended on the size of the catch and the depth of trawling. By the end of March 2003, 32 fish and three tags had been returned to the laboratory (a 2.7% return rate), the recapture data indicating that the probability of survival after release does not depend on catch depth or depth at location of release. For combined sexes, the somatic growth rate was estimated at 0.033 ± 0.019 cm day−1 (n=15). Males and females did not differ significantly in somatic growth rate, which were, respectively, 0.028±0.018 (n=6) and 0.033 ± 0.012 cm day−1 (n=6). This pilot experiment represents the first recorded mass tagging of European hake, which is known to be a very fragile species. The preliminary results indicate that it would be possible to carry out a large-scale tagging experiment of the European stocks in order to improve assessment and subsequent management decisions.