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

Effects of anthropogenic electromagnetic fields used for subsurface oil and gas exploration (controlled-source electromagnetics, CSEM) on the early development of Atlantic haddock (Melanogrammus aeglefinus)

Controlled source electromagnetics (CSEM) uses electromagnetic fields (EMF) to detect oil reservoirs. Atlantic haddock, Melanogrammus aeglefinus, is a commercially important demersal fish species that can potentially be impacted by such surveys due to potential overlap with egg distribution. In this study, haddock eggs were exposed to EMF, replicating CSEM survey conditions in a laboratory. Three different EMF intensities were used to replicate different distances between the EMF source and the organism. Exposures lasted for 15 min. A worst-case scenario, i.e. 1 h exposure at the highest EMF level was also carried out. None of the treatments caused malformations, mortality or affected hatching of eggs. However, EMF exposure induced tachycardia in newly hatched larvae and reduced the size of their yolk sac reserve. The effect was significant at the lowest EMF intensity (corresponding to 1000 m between the EMF source and the exposed subject) and increased with exposure time and intensity.

Time-compensated sun compass in juvenile sprat (Sprattus sprattus) reveals the onset of migratory readiness

Sprat (Sprattus sprattus) is one of the most commercially exploited fish species in the Baltic Sea and expresses a pronounced seasonal migration pattern. Spawning takes place, among other places, in the Kiel Bight and Kiel Fjord in early summer. Juvenile sprat leave the nursery areas in late summer/early autumn to move to their feeding and overwintering grounds. What kind of orientation mechanisms sprat use for migration is not known yet. This study shows that juvenile sprat can use a time-compensated sun compass, heading towards the northeast, in the direction of their proposed overwintering grounds in Bornholm Basin. The sprats tested at the end of August oriented themselves in the predicted direction, whereas the sprats tested at the beginning of August only showed a random orientation. For the first time, this demonstrates the onset of migratory readiness in juvenile sprat, indicating the preparation for starting their migration.

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.

Changes in Dendritic Spine Morphology and Density of Granule Cells in the Olfactory Bulb of Anguilla anguilla (L., 1758): A Possible Way to Understand Orientation and Migratory Behavior

Simple Summary

The olfactory bulb can process odour cues through granular cells (GCs) and dendritic spines, changing their synaptic plasticity properties and their morphology. The GCs’ dendritic spines density and morphology were analysed in Anguilla anguilla, considering the olfaction as a driver involved in fish orientation and migration. For the head and neck morphology, spines were classified as mushroom, long thin, stubby, and filopodia. Spines’ density decreased from juvenile migrants to no-migrant stages and increased in the adult migrants. Spines’ density was comparable between glass and silver eels as an adaptation to migration, while at non-migrating phases, spines’ density decreased. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb. Considering the role of olfaction in the orientation and migration of A. anguilla, the modification of environmental stimuli (ocean alterations and climate change) could represent contributing factors that threaten this critically endangered species.

Abstract

Olfaction could represent a pivotal process involved in fish orientation and migration. The olfactory bulb can manage olfactive signals at the granular cell (GC) and dendritic spine levels for their synaptic plasticity properties and changing their morphology and structural stability after environmental odour cues. The GCs’ dendritic spine density and morphology were analysed across the life stages of the catadromous Anguilla anguilla. According to the head and neck morphology, spines were classified as mushroom (M), long thin (LT), stubby (S), and filopodia (F). Total spines’ density decreased from juvenile migrants to no-migrant stages, to increase again in the adult migrant stage. Mean spines’ density was comparable between glass and silver eels as an adaptation to migration. At non-migrating phases, spines’ density decreased for M and LT, while M, LT, and S density increased in silver eels. A great dendritic spine development was found in the two migratory phases, regressing in trophic phases, but that could be recreated in adults, tracing the migratory memory of the routes travelled in juvenile phases. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb as an index of synaptic plasticity involved in the modulation of olfactory stimuli. If olfaction is involved in the orientation and migration of A. anguilla and if eels possess a memory, these processes could be influenced by the modification of environmental stimuli (ocean alterations and rapid climate change) contributing to threatening this critically endangered species.

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.

Magnetoreception and magnetic navigation in fishes: a half century of discovery

As the largest and most diverse vertebrate group on the planet, fishes have evolved an impressive array of sensory abilities to overcome the challenges associated with navigating the aquatic realm. Among these, the ability to detect Earth's magnetic field, or magnetoreception, is phylogenetically widespread and used by fish to guide movements over a wide range of spatial scales ranging from local movements to transoceanic migrations. A proliferation of recent studies, particularly in salmonids, has revealed that fish can exploit Earth's magnetic field not only as a source of directional information for maintaining consistent headings, but also as a kind of map for determining location at sea and for returning to natal areas. Despite significant advances, much about magnetoreception in fishes remains enigmatic. How fish detect magnetic fields remains unknown and our understanding of the evolutionary origins of vertebrate magnetoreception would benefit greatly from studies that include a wider array of fish taxa. The rich diversity of life-history characteristics that fishes exhibit, the wide variety of environments they inhabit, and their suitability for manipulative studies, make fishes promising subjects for magnetoreception studies.

An integrative investigation of sensory organ development and orientation behavior throughout the larval phase of a coral reef fish

The dispersal of marine larvae determines the level of connectivity among populations, influences population dynamics, and affects evolutionary processes. Patterns of dispersal are influenced by both ocean currents and larval behavior, yet the role of behavior remains poorly understood. Here we report the first integrated study of the ontogeny of multiple sensory systems and orientation behavior throughout the larval phase of a coral reef fish-the neon goby, Elacatinus lori. We document the developmental morphology of all major sensory organs (lateral line, visual, auditory, olfactory, gustatory) together with the development of larval swimming and orientation behaviors observed in a circular arena set adrift at sea. We show that all sensory organs are present at hatch and increase in size (or number) and complexity throughout the larval phase. Further, we demonstrate that most larvae can orient as early as 2 days post-hatch, and they swim faster and straighter as they develop. We conclude that sensory organs and swimming abilities are sufficiently developed to allow E. lori larvae to orient soon after hatch, suggesting that early orientation behavior may be common among coral reef fishes. Finally, we provide a framework for testing alternative hypotheses for the orientation strategies used by fish larvae, laying a foundation for a deeper understanding of the role of behavior in shaping dispersal patterns in the sea.

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.

Electric and magnetic senses in marine animals, and potential behavioral effects of electromagnetic surveys

Electromagnetic surveys generate electromagnetic fields to map petroleum deposits under the seabed with unknown consequences for marine animals. The electric and magnetic fields induced by electromagnetic surveys can be detected by many marine animals, and the generated fields may potentially affect the behavior of perceptive animals. Animals using magnetic cues for migration or local orientation, especially during a restricted time-window, risk being affected by electromagnetic surveys. In electrosensitive animals, anthropogenic electric fields could disrupt a range of behaviors. The lack of studies on effects of the electromagnetic fields induced by electromagnetic surveys on the behavior of magneto- and electrosensitive animals is a reason for concern. Here, we review the use of electric and magnetic fields among marine animals, present data on survey generated and natural electromagnetic fields, and discuss potential effects of electromagnetic surveys on the behavior of marine animals.