The Simrad EK60 echosounder dataset from the Malaspina circumnavigation.

We provide the raw acoustic data collected from the R/V Hesperides during the global Malaspina 2010 Spanish Circumnavigation Expedition (14th December 2010, Cádiz-14th July 2011, Cartagena) using a Simrad EK60 scientific echosounder operating at 38 and 120 kHz. The cruise was divided into seven legs: leg 1 (14th December 2010, Cádiz-13th January 2011, Rio de Janeiro), leg 2 (17th January 2011, Rio de Janeiro-6th February 2011, Cape Town), leg 3 (11th February 2011, Cape Town-13th March 2011, Perth), leg 4 (17th March 2011, Perth-30th March 2011, Sydney), leg 5 (16th April 2011, Auckland-8th May 2011, Honolulu), leg 6 (13th May 2011, Honolulu-10th June 2011, Cartagena de Indias) and leg 7 (19th June 2011, Cartagena de Indias-14th July 2011, Cartagena). The echosounder was calibrated at the start of the expedition and calibration parameters were updated in the data acquisition software (ER60) i.e., the logged raw data are calibrated. We also provide a data summary of the acoustic data in the form of post-processed products.

Coordinated gas release among the physostomous fish sprat (Sprattus sprattus).

Previous experimental studies suggest that the production of sound associated with expelling gas from an open swimbladder may play a role in communication. This would suggest non-random gas release. We used deployed echosounders to study patterns of gas release among a fjord population of sprat (Sprattus sprattus). The echosounder records concurrently revealed individual fish and their release of gas. The gas release primarily occurred at night, partly following recurrent temporal patterns, but also varying between nights. In testing for non-randomness, we formulated a data-driven simulation approach. Non-random gas release scaled with the length of the analyzed time intervals from 1 min to 6 h, and above 30 min the release events in more than 50% of the intervals were significantly connected.

Sleep walking copepods? Calanus diapausing in hypoxic waters adjust their vertical position during winter.

While hypoxia is generally associated with negative connotations, some animals may also take advantage of reduced oxygen concentrations. However, the dynamics of such processes for zooplankton are poorly understood. We made continuous acoustic studies of Calanus helgolandicus overwintering in hypoxic waters (Oslofjorden, Norway). Their apparent minimum oxygen tolerance was 0.2-0.3 mL O2 L-1 at 8°C. The copepods adjusted their vertical distribution in concert with the upward progression of hypoxia as oxygen contents declined in the course of winter. The hypoxic overwintering habitat largely excluded potential predators and mortality appeared low in early winter. As the copepod distribution shallowed in phase with declining oxygen contents at depth, mortality increased. In contrast to recent predictions, C. helgolandicus had sufficient energy reserves to sustain long-term overwintering. Termination of the overwintering phase in spring was gradual but appeared to accelerate during the development of the spring bloom. Enhanced oceanic deoxygenation with climate change may affect seasonally migrating copepods in unpredictable ways.

Pushing the limits of photoreception in twilight conditions: The rod-like cone retina of the deep-sea pearlsides.

Most vertebrates have a duplex retina comprising two photoreceptor types, rods for dim-light (scotopic) vision and cones for bright-light (photopic) and color vision. However, deep-sea fishes are only active in dim-light conditions; hence, most species have lost their cones in favor of a simplex retina composed exclusively of rods. Although the pearlsides, Maurolicus spp., have such a pure rod retina, their behavior is at odds with this simplex visual system. Contrary to other deep-sea fishes, pearlsides are mostly active during dusk and dawn close to the surface, where light levels are intermediate (twilight or mesopic) and require the use of both rod and cone photoreceptors. This study elucidates this paradox by demonstrating that the pearlside retina does not have rod photoreceptors only; instead, it is composed almost exclusively of transmuted cone photoreceptors. These transmuted cells combine the morphological characteristics of a rod photoreceptor with a cone opsin and a cone phototransduction cascade to form a unique photoreceptor type, a rod-like cone, specifically tuned to the light conditions of the pearlsides' habitat (blue-shifted light at mesopic intensities). Combining properties of both rods and cones into a single cell type, instead of using two photoreceptor types that do not function at their full potential under mesopic conditions, is likely to be the most efficient and economical solution to optimize visual performance. These results challenge the standing paradigm of the function and evolution of the vertebrate duplex retina and emphasize the need for a more comprehensive evaluation of visual systems in general.

Light penetration structures the deep acoustic scattering layers in the global ocean.

The deep scattering layer (DSL) is a ubiquitous acoustic signature found across all oceans and arguably the dominant feature structuring the pelagic open ocean ecosystem. It is formed by mesopelagic fishes and pelagic invertebrates. The DSL animals are an important food source for marine megafauna and contribute to the biological carbon pump through the active flux of organic carbon transported in their daily vertical migrations. They occupy depths from 200 to 1000 m at daytime and migrate to a varying degree into surface waters at nighttime. Their daytime depth, which determines the migration amplitude, varies across the global ocean in concert with water mass properties, in particular the oxygen regime, but the causal underpinning of these correlations has been unclear. We present evidence that the broad variability in the oceanic DSL daytime depth observed during the Malaspina 2010 Circumnavigation Expedition is governed by variation in light penetration. We find that the DSL depth distribution conforms to a common optical depth layer across the global ocean and that a correlation between dissolved oxygen and light penetration provides a parsimonious explanation for the association of shallow DSL distributions with hypoxic waters. In enhancing understanding of this phenomenon, our results should improve the ability to predict and model the dynamics of one of the largest animal biomass components on earth, with key roles in the oceanic biological carbon pump and food web.

Jellyfish distribute vertically according to irradiance.

We tested the hypothesis that the coronate jellyfish Periphylla periphylla distributes vertically according to a preferential range of absolute light intensities. The study was carried out in Lurefjorden, Norway, a fjord characterized by mass occurrences of this jellyfish. We collected data on the vertical distribution of P. periphylla medusa during day, dusk and night periods from video observations by a remotely operated vehicle in relation to estimated ambient light levels. Our results suggest that large P. periphylla (average size in catches ~9 cm diameter) avoided total irradiance levels above 5×10-3 µmol quanta m-2 s-1. Nearly two-thirds of the population stayed above irradiance of 10-7 µmol quanta m-2 s-1 during daytime, while some individuals occupied much darker water. Thus, part of the population appeared to distribute vertically and undertake diel vertical migration (DVM) according to a preferential range of light intensities.

Hypoxia Tolerance and Metabolic Suppression in Oxygen Minimum Zone Euphausiids: Implications for Ocean Deoxygenation and Biogeochemical Cycles.

The effects of regional variations in oxygen and temperature levels with depth were assessed for the metabolism and hypoxia tolerance of dominant euphausiid species. The physiological strategies employed by these species facilitate prediction of changing vertical distributions with expanding oxygen minimum zones and inform estimates of the contribution of vertically migrating species to biogeochemical cycles. The migrating species from the Eastern Tropical Pacific (ETP), Euphausia eximia and Nematoscelis gracilis, tolerate a Partial Pressure (PO2) of 0.8 kPa at 10 °C (∼15 µM O2) for at least 12 h without mortality, while the California Current species, Nematoscelis difficilis, is incapable of surviving even 2.4 kPa PO2 (∼32 µM O2) for more than 3 h at that temperature. Euphausia diomedeae from the Red Sea migrates into an intermediate oxygen minimum zone, but one in which the temperature at depth remains near 22 °C. Euphausia diomedeae survived 1.6 kPa PO2 (∼22 µM O2) at 22 °C for the duration of six hour respiration experiments. Critical oxygen partial pressures were estimated for each species, and, for E. eximia, measured via oxygen consumption (2.1 kPa, 10 °C, n = 2) and lactate accumulation (1.1 kPa, 10 °C). A primary mechanism facilitating low oxygen tolerance is an ability to dramatically reduce energy expenditure during daytime forays into low oxygen waters. The ETP and Red Sea species reduced aerobic metabolism by more than 50% during exposure to hypoxia. Anaerobic glycolytic energy production, as indicated by whole-animal lactate accumulation, contributed only modestly to the energy deficit. Thus, the total metabolic rate was suppressed by ∼49-64%. Metabolic suppression during diel migrations to depth reduces the metabolic contribution of these species to vertical carbon and nitrogen flux (i.e., the biological pump) by an equivalent amount. Growing evidence suggests that metabolic suppression is a widespread strategy among migrating zooplankton in oxygen minimum zones and may have important implications for the economy and ecology of the oceans. The interacting effects of oxygen and temperature on the metabolism of oceanic species facilitate predictions of changing vertical distribution with climate change.

Social behaviour in mesopelagic jellyfish.

Gelatinous organisms apparently play a central role in deep pelagic ecosystems, but lack of observational methodologies has restricted information on their behaviour. We made acoustic records of diel migrating jellyfish Periphylla periphylla forming small, ephemeral groups at the upper fringe of an acoustic scattering layer consisting of krill. Groups of P. periphylla were also documented photographically using a remotely operated vehicle (ROV). Although the adaptive value of group formation remains speculative, we clearly demonstrate the ability of these jellyfishes to locate and team up with each other.

Heterogeneous distribution of plankton within the mixed layer and its implications for bloom formation in tropical seas.

Intensive sampling at the coastal waters of the central Red Sea during a period of thermal stratification, prior to the main seasonal bloom during winter, showed that vertical patches of prokaryotes and microplankton developed and persisted for several days within the apparently density uniform upper layer. These vertical structures were most likely the result of in situ growth and mortality (e.g., grazing) rather than physical or behavioural aggregation. Simulating a mixing event by adding nutrient-rich deep water abruptly triggered dense phytoplankton blooms in the nutrient-poor environment of the upper layer. These findings suggest that vertical structures within the mixed layer provide critical seeding stocks that can rapidly exploit nutrient influx during mixing, leading to winter bloom formation.

The submarine volcano eruption off El Hierro Island: effects on the scattering migrant biota and the evolution of the pelagic communities.

The submarine volcano eruption off El Hierro Island (Canary Islands) on 10 October 2011 promoted dramatic perturbation of the water column leading to changes in the distribution of pelagic fauna. To study the response of the scattering biota, we combined acoustic data with hydrographic profiles and concurrent sea surface turbidity indexes from satellite imagery. We also monitored changes in the plankton and nekton communities through the eruptive and post-eruptive phases. Decrease of oxygen, acidification, rising temperature and deposition of chemicals in shallow waters resulted in a reduction of epipelagic stocks and a disruption of diel vertical migration (nocturnal ascent) of mesopelagic organisms. Furthermore, decreased light levels at depth caused by extinction in the volcanic plume resulted in a significant shallowing of the deep acoustic scattering layer. Once the eruption ceased, the distribution and abundances of the pelagic biota returned to baseline levels. There was no evidence of a volcano-induced bloom in the plankton community.

Vertical distribution and diel vertical migration of krill beneath snow-covered ice and in ice-free waters.

A bottom mounted upward looking Simrad EK60 120-kHz echo sounder was used to study scattering layers (SLs) and individuals of the krill Meganyctiphanes norvegica. The mooring was situated at 150-m depth in the Oslofjord, connected with an onshore cable for power and transmission of digitized data. Records spanned 5 months from late autumn to spring. A current meter and CTD was associated with the acoustic mooring and a shore-based webcam monitored ice conditions in the fjord. The continuous measurements were supplemented with intermittent krill sampling campaigns and their physical and biological environment. The krill carried out diel vertical migration (DVM) throughout the winter, regardless of the distribution of potential prey. The fjord froze over in mid-winter and the daytime distribution of a mid-water SL of krill immediately became shallower associated with snow fall after freezing, likely related to reduction of light intensities. Still, a fraction of the population always descended all the way to the bottom, so that the krill population by day seemed to inhabit waters with light levels spanning up to six orders of magnitude. Deep-living krill ascended in synchrony with the rest of the population in the afternoon, but individuals consistently reappeared in near-bottom waters already <1 h after the ascent. Thereafter, the krill appeared to undertake asynchronous migrations, with some krill always being present in near-bottom waters even though the entire population appeared to undertake DVM.

Surfacing behavior and gas release of the physostome sprat (Sprattus sprattus) in ice-free and ice-covered waters.

Upward-facing echosounders that provided continuous, long-term measurements were applied to address the surfacing behavior and gas release of the physostome sprat (Sprattus sprattus) throughout an entire winter in a 150-m-deep Norwegian fjord. During ice-free conditions, the sprat surfaced and released gas bubbles at night with an estimated surfacing rate of 3.5 times per fish day-1. The vertical swimming speeds during surfacing were considerably higher (~10 times) than during diel vertical migrations, especially when returning from the surface, and particularly when the fjord was not ice covered. The sprat released gas a few hours after surfacing, suggesting that the sprat gulped atmospheric air during its excursions to the surface. While the surface activity increased after the fjord became ice covered, the records of gas release decreased sharply. The under-ice fish then displayed a behavior interpreted as "searching for the surface" by repeatedly ascending toward the ice, apparently with limited success of filling the swim bladder. This interpretation was supported by lower acoustic target strength in ice-covered waters. The frequent surfacing behavior demonstrated in this study indicates that gulping of atmospheric air is an important element in the life of sprat. While at least part of the population endured overwintering in the ice-covered habitat, ice covering may constrain those physostome fishes that lack a gas-generating gland in ways that remain to be established.

Mesoscale eddies are oases for higher trophic marine life.

Mesoscale eddies stimulate biological production in the ocean, but knowledge of energy transfers to higher trophic levels within eddies remains fragmented and not quantified. Increasing the knowledge base is constrained by the inability of traditional sampling methods to adequately sample biological processes at the spatio-temporal scales at which they occur. By combining satellite and acoustic observations over spatial scales of 10 s of km horizontally and 100 s of m vertically, supported by hydrographical and biological sampling we show that anticyclonic eddies shape distribution and density of marine life from the surface to bathyal depths. Fish feed along density structures of eddies, demonstrating that eddies catalyze energy transfer across trophic levels. Eddies create attractive pelagic habitats, analogous to oases in the desert, for higher trophic level aquatic organisms through enhanced 3-D motion that accumulates and redistributes biomass, contributing to overall bioproduction in the ocean. Integrating multidisciplinary observation methodologies promoted a new understanding of biophysical interaction in mesoscale eddies. Our findings emphasize the impact of eddies on the patchiness of biomass in the sea and demonstrate that they provide rich feeding habitat for higher trophic marine life.

Inverse vertical migration and feeding in glacier lanternfish (Benthosema glaciale).

A bottom-mounted upward-facing 38-kHz echo sounder was deployed at ~400 m and cabled to shore in Masfjorden (~60°52'N, ~5°24'E), Norway. The scattering layers seen during autumn (September-October) 2008 were identified by trawling. Glacier lanternfish (Benthosema glaciale) were mainly distributed below ~200 m and displayed three different diel behavioral strategies: normal diel vertical migration (NDVM), inverse DVM (IDVM) and no DVM (NoDVM). The IDVM group was the focus of this study. It consisted of 2-year and older individuals migrating to ~200-270 m during the daytime, while descending back to deeper than ~270 m during the night. Stomach content analysis revealed increased feeding during the daytime on overwintering Calanus sp. We conclude that visually searching glacier lanternfish performing IDVM benefit from the faint daytime light in mid-waters when preying on overwintering Calanus sp.

Seasonal variations in vertical migration of glacier lanternfish, Benthosema glaciale.

The seasonal variations in glacier lanternfish (Benthosema glaciale) vertical distribution and diel vertical migration (DVM) were studied by use of a bottom-mounted upward-facing 38 kHz echo sounder deployed at 392 m depth and cabled to shore in Masfjorden (~60°52'N, ~5°24'E), Norway. Acoustic data from July 2007-October 2008 were analyzed, and scattering layers below ~220 m during daytime were attributed to glacier lanternfish based on net sampling in this, and previous studies, as well as from analysis of the acoustic data. At these depths, three different diel behavioral strategies were apparent: normal diel vertical migration (NDVM), inverse DVM (IDVM), and no DVM (NoDVM). NoDVM was present all year, while IDVM was present in autumn and winter, and NDVM was present during spring and summer. The seasonal differences in DVM behavior seem to correlate with previously established seasonal distribution of prey. We hypothesize that in regions with seasonally migrating zooplankton, such as where calanoid copepods overwinter at depth, similar plasticity in DVM behavior might occur in other populations of lanternfishes.

Distribution and diel vertical movements of mesopelagic scattering layers in the Red Sea.

The mesopelagic zone of the Red Sea represents an extreme environment due to low food concentrations, high temperatures and low oxygen waters. Nevertheless, a 38 kHz echosounder identified at least four distinct scattering layers during the daytime, of which the 2 deepest layers resided entirely within the mesopelagic zone. Two of the acoustic layers were found above a mesopelagic oxygen minimum zone (OMZ), one layer overlapped with the OMZ, and one layer was found below the OMZ. Almost all organisms in the deep layers migrated to the near-surface waters during the night. Backscatter from a 300 kHz lowered Acoustic Doppler Current Profiler indicated a layer of zooplankton within the OMZ. They carried out DVM, yet a portion remained at mesopelagic depths during the night. Our acoustic measurements showed that the bulk of the acoustic backscatter was restricted to waters shallower than 800 m, suggesting that most of the biomass in the Red Sea resides above this depth.

Diel vertical migration behaviour of the Northern krill (Meganyctiphanes norvegica Sars).

The prototype of Meganyctiphanes norvegica diel vertical migration (DVM) behaviour comprises ascent around dusk, feeding near the surface at night, and descent at dawn, explained as a trade-off between feeding and predator avoidance in an environment where both food and risk of predation is highest near surface. Light is the proximate cue, and daytime distribution is deeper in clear waters and sunny weather and nocturnal distributions deeper in moonlight. However, both internal state and external factors further affect and modify the diel migration pattern. While Meganyctiphanes migrates in synchrony to the surface at sunset, part of the population may descend soon after the ascent with individuals re-entering upper layers throughout the night. This has been explained with hungry individuals being prone to take larger risks and hence stay shallower, while satiated individuals seek shelter at depth. Females migrate closer to the surface than males of equivalent size, possibly due to their greater demand for energy to fuel egg production. Freshly moulted M. norvegica remain at depth throughout the diel cycle. This has been related to the fact that that krill do not feed during moulting, to reduced swimming capacity, and as a mechanism to avoid cannibalism whilst in a vulnerable condition. In some locations large parts of the population remain at depth at night. Such behaviour may incur access to demersal food sources, provide avoidance of predators, or can be a means to avoid horizontal transport to adjacent, unfavourable areas. Environmental gradients can arrest migrations of M. norvegica, yet the effect of physics is not always distinguished from associated biological properties, like subsurface maxima of phytoplankton located at pycnocline boundaries. Deeper nocturnal distribution when predators were abundant has been reported, and krill may adjust their distribution upwards when exposed to deep-living predators. Instantaneous escape to approaching predators is a common component of the anti-predator repertoire of Meganyctiphanes. Occasionally reported schooling behaviour that overrides normal DVM behaviour may serve anti-predation purposes, as well as being related to reproduction. M. norvegica can remain within confined areas, often defined by the bottom topography, even when exposed to strong currents. Behaviourally mediated retention may be accomplished by vertical migration in depth-stratified flows, but evidence for active use of DVM for the purpose of retention is so far circumstantial among M. norvegica. In several instances, large aggregations of krill that repeatedly occur in the same location appear to be accidental consequences of krill vertical migration behaviour interacting with the mean circulation and bottom topography, rather than representing active retention behaviour.

Trophic structure and community stability in an overfished ecosystem.

Since the collapse of the pelagic fisheries off southwest Africa in the late 1960s, jellyfish biomass has increased and the structure of the Benguelan fish community has shifted, making the bearded goby (Sufflogobius bibarbatus) the new predominant prey species. Despite increased predation pressure and a harsh environment, the gobies are thriving. Here we show that physiological adaptations and antipredator and foraging behaviors underpin the success of these fish. In particular, body-tissue isotope signatures reveal that gobies consume jellyfish and sulphidic diatomaceous mud, transferring "dead-end" resources back into the food chain.

Vertical migration, feeding and colouration in the mesopelagic shrimp Sergestes arcticus.

Intraspecific variation in vertical distribution, timing of vertical migration, and colouration of the mesopelagic shrimp Sergestes arcticus were studied in the >400 m deep part of Masfjorden, Norway. Very few individuals were caught in the upper strata during daytime, and larger individuals occurred deeper during the day than smaller ones. Vertical migration was prominent and no overall trend of increasing length with depth was found at night. Small individuals arrived in the upper layers earlier than larger ones. Animal colouration assessed by digital photography revealed significant variance in individual redness. Depth of capture was the most important factor explaining colouration, with increasing degree of redness with depth. Assessing the gut fullness of the transparent shrimps provided a rapid way of estimating feeding activity and showed that feeding took place mainly at night.