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Foraging depths of pelagic sharks: Insights from mercury stable isotopes

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Blue shark (Prionace glauca)

Abstract

The decline of shark populations in the world ocean is affecting ecosystem structure and function in an unpredictable way and new ecological information is today needed to better understand the role of sharks in their habitats. In particular, the characterization of foraging patterns is crucial to understand and foresee the evolution of dynamics between sharks and their prey. Many shark species use the mesopelagic area as a major foraging ground but the degree to which different pelagic sharks rely on this habitat remains overlooked. In order to depict the vertical dimension of their trophic ecology, we used mercury stable isotopes in the muscle of three pelagic shark species (the blue shark Prionace glauca, the shortfin mako shark Isurus oxyrinchus and the smooth hammerhead shark Sphyrna zygaena) from the northeastern Pacific region. The Δ199Hg values, ranging from 1.40 to 2.13‰ in sharks, suggested a diet mostly based on mesopelagic prey in oceanic habitats. We additionally used carbon and nitrogen stable isotopes (δ13C, δ15N) alone or in combination with Δ199Hg values, to assess resource partitioning between the three shark species. Adding Δ199Hg resulted in a decrease in trophic overlap estimates compared to those based on δ13C/δ15N alone, demonstrating that multi-isotope modeling is needed for accurate trophic description of the three species. Mainly, it reveals that they forage at different average depths and that resource partitioning is mostly expressed through the vertical dimension within pelagic shark assemblages. Concomitantly, muscle total mercury concentration (THg) differed between species and increased with feeding depth. Overall, this study highlights the key role of the mesopelagic zone for shark species foraging among important depth gradients and reports new ecological information on trophic competition using mercury isotopes. It also suggests that foraging depth may play a pivotal role in the differences between muscle THg from co-occurring high trophic level shark species.

 

Graphical abtsract

 

Highlights

– δ13C, δ15N, Δ199Hg and δ202Hg were determined in three pelagic shark species.
– Hg isotopes suggest that these shark species forage on mesopelagic prey.
– δ13C and δ15N overestimate overlaps between trophic niches.
– Differences in foraging depth better explain resource partitioning.
– Foraging depth influences mercury contamination level.

 

Reference

Lucien Besnard, Gaël Le Croizier, Felipe Galván-Magaña,David Point,Edouard Kraffe, James Ketchum, Raul Octavio Martinez Rincon, Gauthier Schaal. Foraging depth depicts resource partitioning and contamination level in a pelagic shark assemblage: Insights from mercury stable isotopes. Environmental Pollution, Volume 283, 2021, 117066. ISSN 0269-7491. https://doi.org/10.1016/j.envpol.2021.117066.

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Sound detection by the American lobster (Homarus americanus)

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Abstract
Although many crustaceans produce sounds, their hearing abilities and mechanisms are poorly understood, leaving uncertainties regarding whether or how these animals use sound for acoustic communication. Marine invertebrates lack gas-filled organs required for sound pressure detection, but some of them are known to be sensitive to particle motion. Here, we examined whether the American lobster (Homarus americanus) could detect sound and subsequently sought to discern the auditory mechanisms. Acoustic stimuli responses were measured using auditory evoked potential (AEP) methods. Neurophysiological responses were obtained from the brain using tone pips between 80 and 250 Hz, with best sensitivity at 80–120 Hz. There were no significant differences between the auditory thresholds of males and females. Repeated controls (recordings from deceased lobsters, moving electrodes away from the brain and reducing seawater temperature) indicated the evoked potentials’ neuronal origin. In addition, AEP responses were similar before and after antennules (including statocysts) were ablated, demonstrating that the statocysts, a long-proposed auditory structure in crustaceans, are not the sensory organs responsible for lobster sound detection. However, AEPs could be eliminated (or highly reduced) after immobilizing hairfans, which cover much of lobster bodies. These results suggest that these external cuticular hairs are likely to be responsible for sound detection, and imply that hearing is mechanistically possible in a wider array of invertebrates than previously considered. Because the lobsters’ hearing range encompasses the fundamental frequency of their buzzing sounds, it is likely that they use sound for intraspecific communication, broadening our understanding of the sensory ecology of this commercially vital species. The lobsters’ low-frequency acoustic sensitivity also underscores clear concerns about the potential impacts of anthropogenic noise.

Fig. 5. AEP responses from H. americanus to acoustic stimuli similar to the buzzing sounds they are known to produce. AEP responses from three lobsters (blue, purple and black curves) are shown.

Reference
Youenn Jézéquel, Ian T. Jones, Julien Bonnel, Laurent Chauvaud, Jelle Atema, T. Aran Mooney.Sound detection by the American lobster (Homarus americanus). Journal of Experimental Biology 2021 224: jeb240747. doi: 10.1242/jeb.240747 Published 25 March 2021
https://jeb.biologists.org/content/224/6/jeb240747

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Relationship between membrane n-3 HUFA content and mitochondrial efficiency

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The relationship between membrane fatty acid content and mitochondrial efficiency differs within- and between- omega-3 dietary treatments

 

Abstract

An important, but underappreciated, consequence of climate change is the reduction in crucial nutrient production at the base of the marine food chain: the long-chain omega-3 highly unsaturated fatty acids (n-3 HUFA). This can have dramatic consequences on consumers, such as fish as they have limited capacity to synthesise n-3 HUFA de novo. The n-3 HUFA, such as docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3), are critical for the structure and function of all biological membranes. There is increasing evidence that fish will be badly affected by reductions in n-3 HUFA dietary availability, however the underlying mechanisms remain obscure. Hypotheses for how mitochondrial function should change with dietary n-3 HUFA availability have generally ignored ATP production, despite its importance to a cell’s total energetics capacity, and in turn, whole-animal performance. Here we (i) quantified individual variation in mitochondrial efficiency (ATP/O ratio) of muscle and (ii) examined its relationship with content in EPA and DHA in muscle membrane of a primary consumer fish, the golden grey mullet Chelon auratus, receiving either a high or low n-3 HUFA diet. Mitochondria of fish fed on the low n-3 HUFA diet had higher ATP/O ratio than those of fish maintained on the high n-3 HUFA diet. Yet, mitochondrial efficiency varied up about 2-fold among individuals on the same dietary treatment, resulting in some fish consuming half the oxygen and energy substrate to produce the similar amount of ATP than conspecific on similar diet. This variation in mitochondrial efficiency among individuals from the same diet treatment was related to individual differences in fatty acid composition of the membranes: a high ATP/O ratio was associated with a high content in EPA and DHA in biological membranes. Our results highlight the existence of interindividual differences in mitochondrial efficiency and its potential importance in explaining intraspecific variation in response to food chain changes.

 

Figure 2: A golden grey mullet’s mitochondrial efficiency of ATP production under Low and High n-3 HUFA diet is related to the n-3 content (docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3)) of its membrane phospholipids, with individuals that have mitochondria with higher ability to make ATP had higher membrane n-3 content.

 

Reference

Salin K, Mathieu-Resuge M, Graziano N, Dubillot E, Le Grand F, Soudant P, Vagner M. 2020. The relationship between membrane fatty acid content and mitochondrial efficiency differs within- and between- omega-3 dietary treatments. Marine Environmental Research:105205.

https://doi.org/10.1016/j.marenvres.2020.105205

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New guidelines for the application of Stokes’ models to the sinking velocity of marine aggregates

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Numerical simulations of ocean biogeochemical cycles need to adequately represent particle sinking velocities (SV). For decades, Stokes’ Law estimating particle SV from density and size has been widely used. But while Stokes’ Law holds for small, smooth, and rigid spheres settling at low Reynolds number, it fails when applied to marine aggregates complex in shape, structure, and composition. Minerals and zooplankton can alter phytoplankton aggregates in ways that change their SV, potentially improving the applicability of Stokes’ models. Using rolling cylinders, we experimentally produced diatom aggregates in the presence and absence of minerals and/or microzooplankton. Minerals and to a lesser extent microzooplankton decreased aggregate size and roughness and increased their sphericity and compactness. Stokes’ Law parameterized with a fractal porosity modeled adequately size‐SV relationships for mineral‐loaded aggregates. Phytoplankton‐only aggregates and those exposed to microzooplankton followed the general Navier‐Stokes drag equation suggesting an indiscernible effect of microzooplankton and a drag coefficient too complex to be calculated with a Stokes’ assumption. We compared our results with a larger data set of ballasted and nonballasted marine aggregates. This confirmed that the size‐SV relationships for ballasted aggregates can be simulated by Stokes’ models with an adequate fractal porosity parameterization. Given the importance of mineral ballasting in the ocean, our findings could ease biogeochemical model parameterization for a significant pool of particles in the ocean and especially in the mesopelagic zone where the particulate organic matter : mineral ratio decreases. Our results also reinforce the importance of accounting for porosity as a decisive predictor of marine aggregate SV.

Sinking velocities vs. ESD (equivalent spherical diameter) for aggregates formed in each tank of the four treatments and comparison with theoretical expectations from different parameterizations of Stokes’ Law and the general Navier‐Stokes’ drag equation. P: phytoplankton; PZ: phytoplankton + microzooplankton (rotifers); PM: phytoplankton + mineral (calcite); PMZ: phytoplankton + mineral + microzooplankton. (a) Model 1, Stokes’ Law with constant porosities of 0% (dashed lines) and 99% (solid lines). (b) Model 2, general Navier‐Stokes’ drag law with constant drags of 1 (dashed lines) and 5 (solid lines), and a constant porosity of 99%. (c) Model 3, Stokes’ Law with a porosity scaled on a fractal geometry with coefficient a = 0.03 and D3 = 1.4 (dashed lines) and D3 = 1.8 (solid lines). (d) Model 4, general Navier‐Stokes’ drag law with a porosity scaled on a fractal geometry with coefficient a = 0.03 and D3 = 1.4 (solid lines) and 1.8 (dashed lines). See the text for details on drag calculation

Reference

Laurenceau-Cornec, E.C., Le Moigne, F.A.C., Gallinari, M., Moriceau, B., Toullec, J., Iversen, M.H., Engel, A., and De La Rocha, C.L. 2020. New guidelines for the application of Stokes’ models to the sinking velocity of marine aggregates. Limnol. Oceanogr. doi:10.1002/lno.11388.
Link to the journal:
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Methanogenic and fertilizing potential of aquaculture waste :

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towards freshwater farms energy self‐sufficiency in the framework of blue growth

The fisheries sector, particularly aquaculture, is a fundamental source of nutrition for humans, particularly in developing countries. The modern development of fish farming requires energy for production systems. This study investigates the potential of using organic wastes derived from fish fattening to produce on-farm energy through the process of methanization. Oreochromis  niloticus faeces methanogen potential was determined with (IFF) and without (UIFF) methanizer microbial inoculum. At the end of the manure methanation trials, the resulting digestates were tested as organic fertilizers for agriculture. The tests showed that inoculated fish faeces had faster biogas kinetics production compared with uninoculated fish faeces. In both cases, the produced biogas contained more than 60% methane (CH4) from the second week of incubation, indicating that it was of good quality. Furthermore, the total CH4 volume was twice as larger in IFF compared with UIFF. Biofertilizer tests showed no significant differences for most of the growth parameters in onion and tomato when compared to the unfertilized control, except in one case for tomato plants, which significantly increased its aboveground biomass. The results show that fish faeces are good methanogenic substrates conducive to energy recovery that could facilitate farm autonomy; however, valorization of the digestates as biofertilizer still requires extensive agronomic optimization. Based on our results, we estimate that equivalents of energy need of almost ten millions of people could be covered using the aquaculture potential in freshwater fish faeces biogas worldwide or that at least aquaculture farm energy self-sufficiency could be fostered.

 

Biogas production of fish faeces

Biogas production of fish faeces was monitored using the standard BMP test. After 9 weeks, fish faeces produced more biogas with the inoculum (IFF = 1100 mL) than fish faeces without the inoculum (UIFF = 900 mL) (see figure). Biogas production from substrate only (UIFF) followed an almost constant rate (resulting in linear accumulation) during the nine weeks of incubation.

References

Ndiaye, N.A., Maiguizo-Diagne, H., Diadhiou, H.D., Ndiaye, W.N., Diedhiou, F., Cournac, L., Gaye, M.L., Fall, S., and Brehmer, P. (n.d.). Methanogenic and fertilizing potential of aquaculture waste: towards freshwater farms energy self-sufficiency in the framework of blue growth. Reviews in Aquaculture (Early view). doi:10.1111/raq.12390.

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