Early ontogeny of the lesser sandeel (Ammodytes marinus).

BACKGROUND: Lesser sandeel (Ammodytes marinus) is widely distributed in North Sea ecosystems. Sandeel acts as a critical trophic link between zooplankton and top predators (fish, mammals, sea birds). Because they live buried in the sand, sandeel may be directly affected by the rapid expansion of anthropogenic activities linked to their habitat on the sea bottom (e.g., hydrocarbon extraction, offshore renewable energy, and subsea mining). It is, therefore, important to understand the impact of cumulative environmental and anthropogenic stressors on this species. A detailed description of the ontogenetic timeline and developmental staging for this species is lacking limiting the possibilities for comparative developmental studies assessing, e.g., the impact of various environmental stressors. RESULTS: A detailed description of the morphological development of lesser sandeel and their developmental trajectory, obtained through visual observations and microscopic techniques, is presented. Methods for gamete stripping and intensive culture of the early life stages are also provided. CONCLUSION: This work provides a basis for future research to understand the effect of cumulative environmental and anthropogenic stressors on development in the early life stages of lesser sandeel.

Movement patterns of temperate wrasses (Labridae) within a small marine protected area.

The movement patterns of three commercially important wrasse (Labridae) species inside a small marine protected area (~ 0.15 km2 ) on the west coast of Norway were analysed over a period of 21 months. The mean distance between capture and recapture locations varied between 10 and 187 m, and was species and season specific. The extent of movement was not related to body size or sex. These results imply that a network of small strategically located marine protected areas can be used as management tools to protect wrasses from size- and sex-selective fishing mortality.

Feeding habitat and silvering stage affect lipid content and fatty acid composition of European eel Anguilla anguilla tissues.

Lipids, particularly fatty acids (FAs), are major sources of energy and nutrients in aquatic ecosystems and play key roles during vertebrate development. The European eel Anguilla anguilla goes through major biochemical and physiological changes throughout its lifecycle as it inhabits sea- (SW), and/or brackish- (BW) and/or freshwater (FW) habitats. With the ultimate goal being to understand the reasons for eels adopting a certain life history strategy (FW or SW residency vs. 'habitat shifting'), we explored differences in lipid content and FA composition of muscle, liver and eyes from eels collected across Norwegian SW, BW and FW habitats, and at different lifecycle stages (yellow to silver). FW and SW eels had a higher lipid content overall compared to BW eels, reflecting differences in food availability and life history strategies. SW eels had higher proportions of certain monounsaturated FAs (MUFAs; 18:1n-9, 20:1n-9), and of the essential polyunsaturated FAs 20:5n-3 (eicosapentaenoic acid, EPA) and 22:6n-3 (docosahexaenoic acid) than FW eels, reflecting a marine-based diet. In contrast, the muscle of FW eels had higher proportions of 18:3n-3, 18:2n-6 and 20:4n-6 (arachidonic acid), as is typical of FW organisms. MUFA proportions increased in later stage eels, consistent with the hypothesis that the eels accumulate energy stores prior to migration. In addition, the decrease of EPA with advancing stage may be associated with the critical role that this FA plays in eel sexual development. Lipid and FA information provided further understanding of the habitat use and overall ecology of this critically endangered species.

Effects of Exposure to Low Concentrations of Oil on the Expression of Cytochrome P4501a and Routine Swimming Speed of Atlantic Haddock (Melanogrammus aeglefinus) Larvae In Situ.

Exposure to environmentally relevant concentrations of oil could impact survival of fish larvae in situ through subtle effects on larval behavior. During the larval period, Atlantic haddock (Melanogrammus aeglefinus) are transported toward nursery grounds by ocean currents and active swimming, which can modify their drift route. Haddock larvae are sensitive to dispersed oil; however, whether exposure to oil during development impacts the ability of haddock larvae to swim in situ is unknown. Here, we exposed Atlantic haddock embryos to 10 and 80 µg oil/L (0.1 and 0.8 µg ∑PAH/L) of crude oil for 8 days and used a novel approach to measure its effect on the larval swimming behavior in situ. We assessed the swimming behavior of 138 haddock larvae in situ, in the North Sea, using a transparent drifting chamber. Expression of cytochrome P4501a (cyp1a) was also measured. Exposure to 10 and 80 µg oil/L significantly reduced the average in situ routine swimming speed by 30-40% compared to the controls. Expression of cyp1a was significantly higher in both exposed groups. This study reports key information for improving oil spill risk assessment models and presents a novel approach to study sublethal effects of pollutants on fish larvae in situ.

Exposure to teflubenzuron negatively impacts exploratory behavior, learning and activity of juvenile European lobster (Homarus gammarus).

Infestations with salmon lice, a parasitic copepod, is a major problem in the salmon farming industry. Teflubenzuron is an in-feed pharmaceutical applied to control lice outbreaks; the standard medication is 10 mg per kg fish per day for seven days. Surveys reveal that teflubenzuron accumulates and persists in the sediment around fish farms and causes deformities and mortality in juvenile European lobster (Homarus gammarus), a species commonly found in the vicinity of salmon farms in Norway. To date, there is no information on sub-lethal effects of teflubenzuron on, for example, behavior. We conducted an experiment to assess possible difference in the shelter seeking behavior of teflubenzuron-exposed (N = 19) vs. not exposed (N = 19) H. gammarus juveniles. The teflubenzuron-exposed juveniles had been given very low concentrations, 1.7 µg per pellet twice per week for 113 days prior to this experiment. The concentration of teflubenzuron was estimated to be less than 1 ng/g lobster when they were tested in the behavior experiment. Animals were placed in a lane with a shelter at one end. Once a lobster had found and entered the shelter, they were repeatedly displaced back to the opposite end of the lane, for a total of 3 repeated runs per animal. Three of the exposed juveniles failed to settle in the shelter, and the remaining teflubenzuron-exposed animals took significantly more time to explore the environment and to find and recognize shelter. Furthermore, exposed lobsters also exhibited slower walking speed compared to the controls. These results demonstrate that teflubenzuron significantly reduces exploratory behavior, learning and activity of juvenile H. gammarus. Thus, exposure to teflubenzuron could increase predation mortality of juvenile lobsters in the wild.

Magnetic fields generated by submarine power cables have a negligible effect on the swimming behavior of Atlantic lumpfish (Cyclopterus lumpus) juveniles.

Submarine power cables carry electricity over long distances. Their geographic distribution, number, and areal coverage are increasing rapidly with the development of, for example, offshore wind facilities. The flow of current passing through these cables creates a magnetic field (MF) that can potentially affect marine organisms, particularly those that are magnetosensitive. The lumpfish (Cyclopterus lumpus) is a migratory species that is widely distributed in the North Atlantic Ocean and Barents Sea. It migrates between coastal spawning grounds and pelagic offshore feeding areas. We tested whether lumpfish respond to MFs of the same intensity as those emitted by high voltage direct current (HVDC) submarine power cables. Laboratory experiments were conducted by placing juvenile lumpfish in an artificial MF gradient generated by a Helmholtz coil system. The intensity of the artificial MF used (230 µT) corresponded to the field at 1 m from a high-power submarine cable. The fish were filmed for 30 min with the coil either on or off. Swimming speeds, and presence in the different parts of a raceway, were extracted from the videos and analyzed. Juvenile lumpfish activity, defined as the time that the fish spent swimming relative to stationary pauses (attached to the substrate), and the distance travelled, were unaffected by exposure to the artificial MF. The swimming speed of juvenile lumpfish was reduced (by 16%) when the coil was on indicating that the fish could either sense the MF or the induced electric field created by the movement of the fish through the magnetic field. However, it seems unlikely that a 16% decrease in swimming speed occurring within 1 m of HVDC cables would significantly affect Atlantic lumpfish migration or homing.

Atlantic cod (Gadus morhua) larvae are attracted by low-frequency noise simulating that of operating offshore wind farms.

The number and size of offshore wind (OW) turbines is increasing rapidly. OW turbines produce continuous, low-frequency noise that could impact marine fish dispersing/migrating through the facilities. Any such impact would be relevant for larval stages, which have limited possibility to swim away from OW facilities. If directional movement of fish larvae at sea is impacted by low-frequency continuous sound is unknown. We observe the behavior of Atlantic cod larvae (N = 89) in response to low-frequency sound while they are drifting in a Norwegian fjord inside transparent drifting chambers. We transmit 100 Hz continuous sound in the fjord, in the intensity range of OW turbines' operational noise, and measure the sound pressure and 3-D particle motion. Half of the larvae (N = 45) are exposed to low-frequency (100 Hz) continuous sound, while the other half (N = 44) are observed under the same conditions but without the sound. Exposure does not affect the routine and maximum swimming speeds or the turning behavior of the larvae. Control larvae orient to the northwest. In contrast, exposed larvae orient towards the source of low-frequency sound and particle motion. This provides a basis to assess how OW might impact dispersal in this species.

Co-exposure to UV radiation and crude oil increases acute embryotoxicity and sublethal malformations in the early life stages of Atlantic haddock (Melanogrammus aeglefinus).

Crude oil causes severe abnormalities in developing fish. Photomodification of constituents in crude oil increases its toxicity several fold. We report on the effect of crude oil, in combination with ultraviolet (UV) radiation, on Atlantic haddock (Melanogrammus aeglefinus) embryos. Accumulation of crude oil on the eggshell makes haddock embryos particularly susceptible to exposure. At high latitudes, they can be exposed to UV radiation many hours a day. Haddock embryos were exposed to crude oil (5-300 µg oil/L nominal loading concentrations) for three days in the presence and absence of UV radiation (290-400 nm). UV radiation partly degraded the eggs' outer membrane resulting in less accumulation of oil droplets in the treatment with highest oil concentration (300 µg oil/L). The co-exposure treatments resulted in acute toxicity, manifested by massive tissue necrosis and subsequent mortality, reducing LC50 at hatching stage by 60 % to 0.24 µg totPAH/L compared to 0.62 µg totPAH/L in crude oil only. In the treatment with nominal low oil concentrations (5-30 µg oil/L), only co-exposure to UV led to sublethal morphological heart defects. Including phototoxicity as a parameter in risk assessments of accidental oil spills is recommended.

Applying genetic technologies to combat infectious diseases in aquaculture.

Disease and parasitism cause major welfare, environmental and economic concerns for global aquaculture. In this review, we examine the status and potential of technologies that exploit genetic variation in host resistance to tackle this problem. We argue that there is an urgent need to improve understanding of the genetic mechanisms involved, leading to the development of tools that can be applied to boost host resistance and reduce the disease burden. We draw on two pressing global disease problems as case studies-sea lice infestations in salmonids and white spot syndrome in shrimp. We review how the latest genetic technologies can be capitalised upon to determine the mechanisms underlying inter- and intra-species variation in pathogen/parasite resistance, and how the derived knowledge could be applied to boost disease resistance using selective breeding, gene editing and/or with targeted feed treatments and vaccines. Gene editing brings novel opportunities, but also implementation and dissemination challenges, and necessitates new protocols to integrate the technology into aquaculture breeding programmes. There is also an ongoing need to minimise risks of disease agents evolving to overcome genetic improvements to host resistance, and insights from epidemiological and evolutionary models of pathogen infestation in wild and cultured host populations are explored. Ethical issues around the different approaches for achieving genetic resistance are discussed. Application of genetic technologies and approaches has potential to improve fundamental knowledge of mechanisms affecting genetic resistance and provide effective pathways for implementation that could lead to more resistant aquaculture stocks, transforming global aquaculture.

Magnetic fields produced by subsea high-voltage direct current cables reduce swimming activity of haddock larvae (Melanogrammus aeglefinus).

High-voltage direct current (HVDC) subsea cables are used to transport power between locations and from/to nearshore and offshore facilities. HVDC cables produce magnetic fields (B-fields) that could impact marine fish. Atlantic haddock (Melanogrammus aeglefinus) is a demersal fish that is at risk of exposure to anthropogenic B-fields. Their larvae drift over the continental shelf, and use the Earth's magnetic field for orientation during dispersal. Therefore, anthropogenic magnetic fields from HVDC cables could alter their behavior. We tested the behavior of 92 haddock larvae using a setup designed to simulate the scenario of larvae drifting past a B-field in the intensity range of that produced by a DC subsea cable. We exposed the larvae to a B-field intensity ranging from 50 to 150 µT in a raceway tank. Exposure to the B-field did not affect the spatial distribution of haddock larvae in the raceway. Larvae were categorized by differences in their exploratory behavior in the raceway. The majority (78%) of larvae were nonexploratory, and exposure to the artificial B-field reduced their median swimming speed by 60% and decreased their median acceleration by 38%. There was no effect on swimming of the smaller proportion (22%) of exploratory larvae. These observations support the conclusion that the swimming performance of nonexploratory haddock larvae would be reduced following exposure to B-field from HVDC cables. The selective impact on nonexploratory individuals, and the lack of impact on exploratory individuals, could have population-scale implications for haddock in the wild.

Increasing temperature and prey availability affect the growth and swimming kinematics of Atlantic herring (Clupea harengus) larvae.

Climate change will increase the magnitude and duration of warming events and the variability in the phenology and abundance of available prey to the early life stages of fish. These factors influence physiological, behavioral and ecological processes, impacting growth, development and survival. Using a fully factorial design with two prey-availability treatments (1200 prey items L-1 (high prey abundance) or 40 prey items L-1 (low prey abundance)) under three temperature regimes (8, 10 and 12°C), the swimming kinematics of 6-week old spring-spawning Atlantic herring larvae were examined using silhouette video photography. Higher temperatures combined with food limitation significantly decreased the growth and swimming kinematics of larval herring, with the most negative effect observed in larvae reared at 12°C and exposed to low food abundances. Specifically, larvae displayed reduced locomotory behaviors and reduced vertical movements. By contrast, larvae reared at high prey abundance and at 12°C displayed more active swimming and exploratory behavior, as evidenced by an increase in both locomotory behavior and vertical and horizontal turn angles, suggesting increased motivation to search for food. This research highlights the importance of determining to what degree fish larvae are sensitive to changes in temperature and how these changes might be further influenced by food availability.

Magnetic fields generated by the DC cables of offshore wind farms have no effect on spatial distribution or swimming behavior of lesser sandeel larvae (Ammodytes marinus).

In the North Sea, the number and size of offshore wind (OW) turbines, together with the associated network of High Voltage Direct Current (HVDC) subsea cables, will increase rapidly over the coming years. HVDC cables produce magnetic fields (MFs) that might have an impact on marine animals that encounter them. One of the fish species that is at risk of exposure to MF associated with OW is the lesser sandeel (Ammodytes marinus), a keystone species of the North Sea basin. Lesser sandeel could be exposed to MF as larvae, when they drift in proximity of OW turbines. Whether MFs impact the behavior of lesser sandeel larvae, with possible downstream effects on their dispersal and survival, is unknown. We tested the behavior of 56 lesser sandeel larvae, using a setup designed to simulate the scenario of larvae drifting past a DC cable. We exposed the larvae to a MF intensity gradient (150-50 µT) that is within the range of MFs produced by HVDC subsea cables. Exposure to the MF gradient did not affect the spatial distribution of lesser sandeel larvae in a raceway tank 50 cm long, 7 cm wide and 3.5 cm deep. Nor did the MF alter their swimming speed, acceleration or distance moved. These results show that static MF from DC cables would not impact behavior of lesser sandeel larvae during the larval period of their life although it does not exclude the possibility that later life stages could be affected.

Photo-enhanced toxicity of crude oil on early developmental stages of Atlantic cod (Gadus morhua).

Photo-enhanced toxicity of crude oil is produced by exposure to ultraviolet (UV) radiation. Atlantic cod (Gadus morhua) embryos were exposed to crude oil with and without UV radiation (290-400 nm) from 3 days post fertilization (dpf) until 6 dpf. Embryos from the co-exposure experiment were continually exposed to UV radiation until hatching at 11 dpf. Differences in body burden levels and cyp1a expression in cod embryos were observed between the exposure regimes. High doses of crude oil produced increased mortality in cod co-exposed embryos, as well as craniofacial malformations and heart deformities in larvae from both experiments. A higher number of differentially expressed genes (DEGs) and pathways were revealed in the co-exposure experiment, indicating a photo-enhanced effect of crude oil toxicity. Our results provide mechanistic insights into crude oil and photo-enhanced crude oil toxicity, suggesting that UV radiation increases the toxicity of crude oil in early life stages of Atlantic cod.

Welfare of aquatic organisms: is there some faith-based HARKing going on here?

Much of the literature on aquatic animal welfare is flawed by 4 non-mutually exclusive (and often inter-related) biases: under-reporting/ignoring of negative results, faith-based research and/or interpretations, Hypothesizing After the Results are Known (HARKing), and inflating the science boundary. These biases have an insidious impact on the credibility of the 'science' surrounding aquatic animal welfare. While concerns about the welfare of aquatic organisms are valid, research on this topic should be grounded in the scientific method, embrace negative results, avoid faith-based interpretations of experimental results and/or HARKing, and strictly respect the science boundary.

Gene expression and epigenetic responses of the marine Cladoceran, Evadne nordmanni, and the copepod, Acartia clausi, to elevated CO2.

Characterizing the capacity of marine organisms to adapt to climate change related drivers (e.g., pCO2 and temperature), and the possible rate of this adaptation, is required to assess their resilience (or lack thereof) to these drivers. Several studies have hypothesized that epigenetic markers such as DNA methylation, histone modifications and noncoding RNAs, act as drivers of adaptation in marine organisms, especially corals. However, this hypothesis has not been tested in zooplankton, a keystone organism in marine food webs. The objective of this study is to test the hypothesis that acute ocean acidification (OA) exposure alters DNA methylation in two zooplanktonic species-copepods (Acartia clausii) and cladocerans (Evadne nordmanii). We exposed these two species to near-future OA conditions (400 and 900 ppm pCO2) for 24 h and assessed transcriptional and DNA methylation patterns using RNA sequencing and Reduced Representation Bisulfite Sequencing (RRBS). OA exposure caused differential expression of genes associated with energy metabolism, cytoskeletal and extracellular matrix functions, hypoxia and one-carbon metabolism. Similarly, OA exposure also caused altered DNA methylation patterns in both species but the effect of these changes on gene expression and physiological effects remains to be determined. The results from this study form the basis for studies investigating the potential role of epigenetic mechanisms in OA induced phenotypic plasticity and/or adaptive responses in zooplanktonic organisms.

Glass eels (Anguilla anguilla) imprint the magnetic direction of tidal currents from their juvenile estuaries.

The European eel (Anguilla anguilla) hatches in the Sargasso Sea and migrates to European and North African freshwater. As glass eels, they reach estuaries where they become pigmented. Glass eels use a tidal phase-dependent magnetic compass for orientation, but whether their magnetic direction is innate or imprinted during migration is unknown. We tested the hypothesis that glass eels imprint their tidal-dependent magnetic compass direction at the estuaries where they recruit. We collected 222 glass eels from estuaries flowing in different cardinal directions in Austevoll, Norway. We observed the orientation of the glass eels in a magnetic laboratory where the magnetic North was rotated. Glass eels oriented towards the magnetic direction of the prevailing tidal current occurring at their recruitment estuary. Glass eels use their magnetic compass to memorize the magnetic direction of tidal flows. This mechanism could help them to maintain their position in an estuary and to migrate upstream.

The relationship between the moon cycle and the orientation of glass eels (Anguilla anguilla) at sea.

Links between the lunar cycle and the life cycle (migration patterns, locomotor activity, pulses in recruitment) of the European eel (Anguilla anguilla) are well documented. In this study, we hypothesized that the orientation of glass eels at sea is related to the lunar cycle. The European eel hatches in the Sargasso Sea and migrates across the Atlantic Ocean towards Europe. Upon reaching the continental shelf, the larvae metamorphose into glass eels and migrate up the estuaries, where some individuals colonize freshwater habitats. How glass eels navigate pelagic waters is still an open question. We tested the orientation of 203 glass eels in a transparent circular arena that was drifting in situ during the daytime, in the coastal Norwegian North Sea, during different lunar phases. The glass eels swimming at sea oriented towards the azimuth of the moon at new moon, when the moon rose above the horizon and was invisible but not during the other moon phases. These results suggest that glass eels could use the moon position for orientation at sea and that the detection mechanism involved is not visual. We hypothesize a possible detection mechanism based on global-scale lunar disturbances in electrical fields and discuss the implications of lunar-related orientation for the recruitment of glass eels to estuaries. This behaviour could help glass eels to reach the European coasts during their marine migration.

Silencing of ionotropic receptor 25a decreases chemosensory activity in the salmon louse Lepeophtheirus salmonis during the infective stage.

Chemoreception is critical for marine ectoparasites - such as salmon lice (Lepeophtheirus salmonis) - to identify and locate salmonid hosts. The molecular receptors that parasites employ to detect host-specific chemical stimuli from hosts (kairomones) have not been well characterised. In the present study, transcription of the sea louse Ionotropic receptor 25a (IR25a) was blocked to evaluate whether it functions as a chemical-perception related gene for a specific chemical cue from the Atlantic salmon host. Double-strand RNA interference (dsRNA) oligonucleotides were applied to salmon lice by in vitro transcription and then exposing salmon lice nauplii to dsRNA by soaking overnight. Silencing of the IR25a gene was confirmed by qPCR in experimental groups of knock-down copepodids (dsIR25a). Behavioural responses associated with host recognition were evaluated in dsIR25a sea lice after exposure to a peptide produced by the salmon host (Cath-2). The dsIR25a group decreased expression levels of IR25a by >7-fold with respect to the control group. This group was also 26% slower than the control group (control swimming speed was 69 mm/s, while the treated group was 51 mm/s). Since the swimming activity of salmon lice copepodids is associated with the activation of the chemosensory system, these results indicate that the L. salmonis chemosensory perception system was not fully activated due to gene silencing. The results of this study demonstrate the role of ionotropic receptor 25a during host recognition by sea lice.

Atlantic Haddock (Melanogrammus aeglefinus) Larvae Have a Magnetic Compass that Guides Their Orientation.

Atlantic haddock (Melanogrammus aeglefinus) is a commercially important species of gadoid fish. In the North Sea, their main spawning areas are located close to the northern continental slope. Eggs and larvae drift with the current across the North Sea. However, fish larvae of many taxa can orient at sea using multiple external cues, including the Earth's magnetic field. In this work, we investigated whether haddock larvae passively drift or orient using the Earth's magnetic field. We observed the behavior of 59 and 102 haddock larvae swimming in a behavioral chamber deployed in the Norwegian North Sea and in a magnetic laboratory, respectively. In both in situ and laboratory settings, where the magnetic field direction was modified, haddock larvae significantly oriented toward the northwest. We conclude that haddock larvae orientation at sea is guided by a magnetic compass mechanism. These results have implications for retention and dispersal of pelagic haddock larvae.