The Japanese eel (Anguilla japonica), is listed as 'Endangered' by the IUCN. Understanding eel riverine habitat is useful in considering conservation strategies. This study sought to determine the relationship between environmental DNA (eDNA) concentrations derived from Japanese eels, water quality, and river structure in three small rivers in Nagasaki, Japan. eDNA was detected at 14 of 15 sites (93%). The concentration of eDNA in brackish water was significantly higher than that in freshwater and was correlated with water depth. Eel occurrence throughout the river suggests a need to conserve a diversity of habitats.

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Numerous recent studies have documented ingestion of microplastics (MPs) by many aquatic animals, yet an explanation for misfeeding by fish remains unexplained. Here we tested the hypothesis that biofilm (biofouling) on MP surfaces due to exposure in the aquatic environment facilitates misfeeding in fish. Spherical polystyrene (PS) was cultured for 0 to 22 weeks in a freshwater environment to grow biofilm on the MP. Goldfish were employed in a simple feeding experiment with and without provision of genuine food at ecologically relevant MP concentrations. The absorbance (ABS), which is a proxy for biofilm formation increased exponentially within three weeks of initiation and reached a plateau after approximately five weeks. Although fish did not swallow the MPs, 'capture' occurred when food pellets were in the vicinity and significantly increased in probability with exposure time. Duration of capture also increased significantly with increasing exposure. These results suggest that the drift of MPs in aquatic environments may facilitate fish misidentification of MPs as edible prey.

Metabolic rate, the energy required per unit of time for an organism to sustain life, is influenced by both intrinsic and extrinsic factors. Despite the similarities among living organisms across the various domains of life, it has been observed that those adapted to deep-sea environments exhibit notable distinctions from those in shallower waters, even when accounting for size and temperature. However, as deep-sea organisms are infrequently kept in captivity for prolonged periods, investigations into their potential metabolic responses to food and temperature have yet to be conducted. In this study, we demonstrate the impact of food (specific dynamic action: SDA) and temperature (Q10) on the metabolic rate of the deep-sea isopod Bathynomus doederleini. Positive correlations were found between SDA parameters (peak, time to peak, duration, and factorial scope) and meal size in deep-sea organisms. The postprandial metabolic rate, at a meal size of 45.4%, increased by approximately 6.5-fold, and the duration was 20 days. Within the temperature range of their natural habitat, the overall Q10 was 2.36, indicating that a 10 °C increase would lead to a 2.4-fold increase in resting metabolic rate. The mean metabolic rate of this species, corrected for the equivalent temperature, was significantly 63% lower than the metabolic scaling rule for aquatic invertebrates. This low metabolic rate suggests that deep-sea isopods can survive for a year on a mere few grams of whale blubber at a water temperature of 10.5 °C. This information is crucial for understanding the metabolic strategies and consequences of adaptation to a deep-sea environment.

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