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Winter conditions in the NW Mediterranean cause instability of the water column and non-geostrophic dynamics, such as vertical mixing and convection are significant. These events involve nutrient supply to the photic zone that can sustain high productivity. In this study, we aim to investigate the role of winter hydrodynamics on the spawning strategies of Sardina pilchardus and Micromesistius poutassou. Data were obtained on two oceanographic cruises (February 2017 and 2018) off the Catalan coast. The occurrence of S. pilchardus eggs very close to the coast indicated a clear preference of the species for spawning in coastal areas. Preflexion and postflexion larvae exhibited a slightly wider distribution showing a clear association with the cold, less saline and more productive coastal waters. Preflexion larvae of M. poutassou were found on the upper slope and over the shelf, being offshore limited by the shelf/slope front present all along the slope. The front would act as a barrier preventing their dispersion towards the open sea. M. poutassou larvae in advanced developmental stages were located close to the coast in the productive shelf waters, with instabilities of the front contributing to larval transport from offshore waters to the coast. The vertical distribution of both species showed high variability, not only related with the daily cycle or developmental stage, but also with the vertical structure of the water column. Overall, the results provide some clues on how the spawning strategies of both species may evolve under future scenarios of higher winter-stratification, because of the global warming

Myctophids, commonly vertical migrators, and partial and non-migrant stomiiforms constitute most of the mesopelagic biomass, and transport organic matter throughout the food web from the surface to the ocean’s depths. Both the diet and trophic structure of twenty-nine species of mesopelagic fish collected around the Iberian Peninsula were studied through the analysis of stomach contents, quantifying food items with a high taxonomic resolution. The investigation covered oligotrophic to productive habitats, with sampling stations distributed in five discrete zones of the western Mediterranean Sea and the northeastern Atlantic Ocean. The geographic environmental conditions, migratory behavior, and species-specific body sizes allowed for the identification of some major feeding patterns for these fish communities. The trophic niche of migrant myctophids showed a high overlap, with copepods as the primary prey category. The diet composition of generalist myctophids (e.g., Ceratoscopelus maderensis and Hygophum benoiti) reflected the distinct zooplanktonic communities between zones. Large stomiiforms (Chauliodus spp., Sigmops elongatus) preferred feeding on micronekton, while the smallest stomiiforms (e.g., Argyropelecus spp., Cyclothone spp., Vinciguerria spp.) preyed on copepods and ostracods. Given the relevance of the mesopelagic fish communities for commercially exploited species and, therefore, for maintaining the sustainability of the fisheries’ activity in the zones analyzed, the information provided in the present study is essential for a better understanding of the biology and ecology of these species

Mesopelagic fishes play a central role in marine food webs linking primary consumers to top predators. In this study, measures of direct calorimetry were used to analyse the energy density (ED) of 34 mesopelagic species collected at 12 stations in the equatorial and tropical Atlantic. Mean ED ranged from 2.7 kJ g−1 wet weight (WW) for the lanterfish Lampanyctus nobilis to 8.7 kJ g−1 WW for the lanterfish Benthosema glaciale. This study includes species of the orders Myctophiformes and Stomiiformes and represents migrants, non-migrants and partial-migrants species. The majority of the species were grouped into the medium-energy quality category (ED from 4 to 6 kJ g−1); Myctophiformes showed higher energetic values than the Stomiiformes. For the different species, the ED values were discussed in relation to spawning period and energy allocation strategies for reproduction and growth and feeding and migratory behaviour, as well as the ecoregion of study. These values will be useful for future assessment of energetic transfer between trophic levels and energetic modelling of Atlantic ecosystems

Non-migratory bristlemouth fishes (Cyclothone spp.) are the most abundant vertebrates on Earth and play an important role in the biological carbon pump by remineralizing organic carbon in deep ecosystems. Acoustic data and net sampling are often used in combination to estimate fish and zooplankton biomass, but this procedure may be subject to several sources of error when applied to mesopelagic species. For instance, the allocation of echoes to species has often been biased by not considering Cyclothone spp. due to the use of nets targeting larger fish. Furthermore, the acoustic properties of the target organisms must be well understood to convert acoustic density into numerical density. The characteristics of a fish’s swimbladder are the most relevant features necessary to assess its acoustic properties. This study provides information on the swimbladder properties of six Cyclothone species inhabiting the meso- and bathypelagic layers in the North Atlantic Ocean and Mediterranean Sea, including swimbladder location within the body, fat tissue content, morphology, morphometry (only available for C. braueri and C. pseudopallida), and fish body-mass density (only available for C. braueri, C. pseudopallida, C. pallida, and C. pygmaea). The studied species showed a functional physoclistous swimbladder, with well-developed gas glands and rete mirabile and numerous capillaries in the case of the shallower species C. braueri and C. pseudopallida (mainly distributed from 400 to 600 m depth), and a fat-invested swimbladder in species with deeper vertical distribution (C. livida, C. microdon, C. pallida, and C. pygmaea). The fat content in the swimbladder (C. pallida and C. microdon) increased with depth and latitude, reducing the space in the swimbladder that could contain gas. Changes in swimbladder size and volume during growth were analyzed for shallower species, where swimbladder volume and equivalent radius followed negative allometric growth in relation to body length.

Different types and systems to discriminate plankton samples at different strata in the water column have been developed in recent decades. For sampling at sufficient depth, opening and closing zooplankton multinets are ideal because there is no contamination of one sample with organisms of the previous one. However, for bigger nets, such as those used to collect micronektonic organism, it is difficult to use multiple net units, and multiple cod ends are preferred because of their simplicity, but with the problem of sample contamination from having a common net passageway. We present here a cod-end Multisampler design, VERDA, that uses a carrousel-like system. Similar to some sediment traps, the system works like a revolver with six or eight compartments whose turning mechanism is triggered when the net arrives to a programmed depth level. This prototype was built with inexpensive and recycled components and electronics similar to Arduino® and Teensy PCB to carry out electronic control. The net we used for testing the equipment was a mid-size midwater trawl of ca. 30 m2 and total length of 58 m that works with a single towing cable and no doors. The overall system is useful for all type of ships, due to the relatively easy deployment operations and because the Multisampler does not need electrical cable or acoustics. In our case, we used a Marport® (Reykjavik Iceland) and Scanmar (Åsgårdstrand, Norway) sensors for real-time depth monitoring and opening distance

Dietary and predator–prey studies are more frequently relying on DNA metabarcoding methods, typically achieving results that have a better taxonomic resolution (e.g., species-level) than previous methods. With the continuous advancement in sequencing technology, what was previously accessible only as a large, fixed structure in a laboratory, which had a limited number of users, has now advanced to a small and readily usable device. In this study, we used the gut (content and lining) from juvenile lanternfish (Hygophum) specimens to compare the short-read sequencing capability of the portable Nanopore MinION with the Illumina MiSeq. Primers common in dietary DNA metabarcoding work (COI “Leray primers” and 18S rRNA V4 “Zhan primers”) were used, with an additional comparison of cost-effective COI “Lobo primers” (targeting the same COI fragment) for the proficiency in species detection of a broad range of taxa. Our results indicate high congruency between sequencing machines for, not only taxonomic assignments, but also relative read abundance of the main dietary items. We also identified that Nanopore sequencing is more cost-effective. The Lobo primers are comparable to that of Leray, but substantially reduce the primer set price without compromising detection of taxa. Using both COI and 18S broadened the taxonomic scope, providing greater prey detection. Overall, this preliminary study was successful in creating a foundation for future dietary work involving larvae and transformation stage fishes whereby the content of the gut need not be separated from the gut lining to detect prey. The Hygophum diet detected here aligns with previous research that suggests the main dietary items to be calanoid copepods, but using molecular methods, soft prey was more readily identified compared to studies using visual methods of identification of dietary items. Overall, this study found that Nanopore sequencing is suitable for short-read DNA metabarcoding and can provide rapid acces

This thesis presents novel results on carbon remineralization by the most abundant nonmigrator meso- and bathypelagic fishes on the earth (Cyclothone spp.), on the seasonality of migrant micronekton active flux, on the swimbladder properties of six Cyclothone species inhabiting the meso- and bathypelagic layers in the North Atlantic Ocean and Mediterranean Sea, and on the use of acoustic scattering as a proxy for mesopelagic abundance/biomass. Micronekton is mainly composed of cephalopods, crustaceans and fishes. These organisms represent an important resource for higher trophic levels and are a key component of the biological carbon pump, contributing through diel vertical migrations (DVM) in the case of migrants, and through remineralization in the case of non-migrants. The vertical migrators feed at shallow ocean layers and release carbon at depth through respiration, defecation, excretion, moulting, and mortality. Recent studies suggest that migrant micronekton transport similar amounts of carbon to migrant zooplankton. However, there is scarce information about biomass and carbon flux by non-migratory species in the mesopelagic and bathypelagic zones, and the seasonality of migrant micronekton active flux. In order to understand the contribution of carbon flux by non-migratory bristlemouth fishes (Cyclothone spp.) and partial migrator (Argyropelecus hemigymnus) we provide the first account of remineralisation by this community in the meso and bathypelagic zones from the oceanic upwelling off Northwest Africa to the south of Iceland (North Atlantic Ocean). We estimated that total carbon remineralisation in the meso and bathypelagic zone by these organisms was about 1%. We also studied the seasonal variability (June and October) of migrant biomass and respiratory carbon flux of numerically dominant pelagic crustaceans and mesopelagic fishes in the Gulf of California, a semi-enclosed sea (North East Pacific). Although this study included a fairly small fraction of

This thesis presents novel results on carbon remineralization by the most abundant nonmigrator meso- and bathypelagic fishes on the earth (Cyclothone spp.), on the seasonality of migrant micronekton active flux, on the swimbladder properties of six Cyclothone species inhabiting the meso- and bathypelagic layers in the North Atlantic Ocean and Mediterranean Sea, and on the use of acoustic scattering as a proxy for mesopelagic abundance/biomass. Micronekton is mainly composed of cephalopods, crustaceans and fishes. These organisms represent an important resource for higher trophic levels and are a key component of the biological carbon pump, contributing through diel vertical migrations (DVM) in the case of migrants, and through remineralization in the case of non-migrants. The vertical migrators feed at shallow ocean layers and release carbon at depth through respiration, defecation, excretion, moulting, and mortality. Recent studies suggest that migrant micronekton transport similar amounts of carbon to migrant zooplankton. However, there is scarce information about biomass and carbon flux by non-migratory species in the mesopelagic and bathypelagic zones, and the seasonality of migrant micronekton active flux. In order to understand the contribution of carbon flux by non-migratory bristlemouth fishes (Cyclothone spp.) and partial migrator (Argyropelecus hemigymnus) we provide the first account of remineralisation by this community in the meso and bathypelagic zones from the oceanic upwelling off Northwest Africa to the south of Iceland (North Atlantic Ocean). We estimated that total carbon remineralisation in the meso and bathypelagic zone by these organisms was about 1%. We also studied the seasonal variability (June and October) of migrant biomass and respiratory carbon flux of numerically dominant pelagic crustaceans and mesopelagic fishes in the Gulf of California, a semi-enclosed sea (North East Pacific). Although this study included a fairly small fraction of

This file provides a list of mesopelagic species of cephalopods, decapods and teleostean fish, likely to occur in the northeastern Atlantic Ocean (central and north) and Mediterranean Sea, according to literature

The Marine Biological Reference Collections (CBMR) are specialized mainly in Chordata (fish), Arthropoda and Mollusca, mostly from the Mediterranean Sea, but also with important contributions from the Atlantic Ocean and the rest of the world’s oceans

Most mesopelagic fishes perform large diel vertical migrations from the deep-sea zone to the surface. Although there is a trade-off between a higher food availability at the upper layers and an energy cost and predation risk, incursion towards the surface also implies a transport by currents, where the fish are exposed to a stranding risk on the coast. Here, we reported the first documented stranding of mesopelagic fishes along the southeast shore of Gran Canaria Island. Our study hypothesized that (1) the influence of the Canary Current, (2) the dominant incidence of the Trade Winds during summer, and (3) the presence of an upwelling filament coupled with an anticyclonic eddy south of Gran Canaria Island were the causative mechanisms of the strandings. Diaphus dumerilii (Myctophidae family) was the main species found as observed from an external morphological analysis using traditional taxonomy. The otolith contour analysis suggested the presence of other Diaphus spp. and Lobianchia dofleini. Nevertheless, the otolith morphological features described in the literature suggested that all the specimens were actually D. dumerelii. Errors in the identification were mainly due to the high intraspecific variability found in the otolith morphology. Even so, two patterns of oval and elliptic shapes were described with significant differences in its morphometry

Mesopelagic organisms play a crucial role in marine food webs, channelling energy across the predator-prey network and connecting depth strata through their diel vertical migrations. The information available to assess mesopelagic feeding interactions and energy transfer has increased substantially in recent years, owing to the growing interest and research activity in the mesopelagic realm. However, such data have not been systematically collated and are difficult to access, hampering estimation of the contribution of mesopelagic organisms to marine ecosystems. Here we present MesopTroph, a georeferenced database of diet, trophic markers, and energy content of mesopelagic and other marine taxa compiled from 203 published and non-published sources. MesopTroph currently includes data on stomach contents, carbon and nitrogen stable isotopes, major and trace elements, energy density, fatty acids, trophic positions, and diet proportion estimates for 498 species/genera. MesopTroph will be expanded with new data emerging from ongoing studies. MesopTroph provides a unique tool to investigate trophic interactions and energy flow mediated by mesopelagic organisms, and to evaluate the ecosystem services of this community

Sota les onades de l’oceà es belluguen una immensitat d’organismes. Molts els coneixem però, t’has imaginat mai que alguns arribin a ser tan petits que no poden ser vistos a ull nu? Submergeix-te en el mar d’espècies que es deixen portar pel vaivé de les onades. Descobriràs el plàncton, un món fascinant!

This study examines vertical distribution of the early stages of mesopelagic fishes in the south-eastern Indian Ocean and oceanographic factors that may affect it. This investigation along the 110 °E transect formed part of the second International Indian Ocean Expedition (IIOE-2). Depth-stratified samples to 500 m depth of early stages of mesopelagic fishes were obtained by means a multinet EZ-1 m2. This net captured larvae and transforming stages, but juveniles and adults were also caught, particularly at night. Diel vertical patterns of the different species and stages were analysed relative to the oceanographic conditions encountered at the 20 stations along the transect. This paper provides the ranges of vertical occurrence and weighted mean depth for larvae (135 taxa), and post-metamorphic stages (75 taxa) of fishes caught during the survey. Despite the varying vertical profiles of oceanographic parameters encountered, there were consistent patterns within species and families along the transect, with broader vertical distribution of larvae at the southern stations, where the upper mixed layer was deeper. During both day and night, larval stages of all the species were concentrated in the epipelagic zone, with some differences in terms of weighted mean depth (WMD) between taxa, but with no significant differences in vertical location between day and night. Transforming and juvenile-adult stages, however, presented a wider vertical distribution range than larval stages, and showed important differences among families and species. For example, Myctophidae and Phosichthyidae species presented the highest concentrations in the upper 50–100 m at night, and a much deeper distribution during the day, thereby indicating night vertical migration behaviour of transforming and juvenile-adult stages. By contrast, during these stages the Gonostomatidae Cyclothone spp. and the Sternoptychidae Argyropelecus spp. occurred deeper than the epipelagic layer during both day and n

As part of the second International Indian Ocean Expedition, an oceanic transect from 39.5°S to 11.5°S along 110°E in the south-eastern Indian Ocean enabled concurrent oceanographic measurements and sampling of early stages of mesopelagic fishes within the upper 500 m of the water column. Notable oceanographic features along the generally oligotrophic transect included an increase in surface water temperature from 12 to 28 °C from south to north and low salinities at both ends of the transect. High salinity Subtropical Surface Water was evident over the central part of the transect and low dissolved oxygen occurred at depth at the northern end where tropical waters were influenced by the Indonesian Throughflow. Throughout the study region larval fish stages were generally concentrated in the epipelagic zone. Species richness, abundance and diversity of fishes (n = 166 species) increased towards the tropical waters. The families Myctophidae (lanternfishes; 60 species), Phosichthyidae (lightfishes; 2 species) and Gonostomatidae (bristlemouths; 5 species) were numerically dominant. Clustering of integrated abundance data (0–500 m depth) revealed four major fish assemblages along the transect. When linked to oceanographic variables, multivariate multiple regression indicated that surface temperature, surface salinity and dissolved oxygen at 400 m depth provided the best fitting model explaining fish distribution patterns. Where possible, comparisons have been made with early data collected along this 110° E line in the 1960s during the first International Indian Ocean Expedition

The organic carbon resulting from photosynthesis in the upper ocean is transferred downward through the passive sinking of organic particles, physical mixing of particulate and dissolved organic carbon as well as active flux transported by zooplanktonic and micronektonic migrants. Several meso- and bathypelagic organisms feed in shallower layers during the nighttime and respire, defecate, excrete and die at depth. Recent studies suggest that migrant micronekton transport similar amounts of carbon to migrant zooplankton. However, there is scarce information about biomass and carbon flux by non-migratory species in the mesopelagic and bathypelagic zones. The non-migratory bristlemouth fishes (Cyclothone spp.) and partial migrator (A. hemigymnus) remineralise organic carbon at depth, and knowledge about this process by this fauna is lacking despite them having been referred to as the most abundant vertebrates on Earth. Here we show the vertical distribution of biomass and respiration of non-migratory mesopelagic fishes, during day and night, using the enzymatic activity of the electron transfer system (ETS) as a proxy for respiration rates. The study is focused on five Cyclothone species (C. braueri, C. pseudopallida, C. pallida, C. livida and C. microdon) and Argyropelecus hemigymnus. The samples were taken on a transect from the oceanic upwelling off Northwest Africa (20° N, 20° W) to the south of Iceland (60° N, 20° W). Cyclothone spp. showed, by far, the largest biomass (126.90 ± 86.20 mg C·m−2) compared to A. hemigymnus (0.54 ± 0.44 mg C·m−2). The highest concentrations of Cyclothone spp. in the water column were observed between 400 and 600 m and from 1000 to 1500 m depths, both during day and night. For the different species analysed, ETS activity did not show significant differences between diurnal and nocturnal periods. The total average specific respiration of Cyclothone spp. (0.02 ± 0.01 d-1) was lower than that observed for A. hemigymnus (0.05 ± 0.02 d-1).

Un relat que ens apropa a la ciència per generar consciència. Escoltem, compartim, descobrim i creem!

Mesopelagic fish populations are characterised by high species richness and abundance, and have been identified as important contributors to the active carbon fluxes in the open ocean. We report variability in communities of mesopelagic fish between five zones around the Iberian Peninsula, i.e. Balears and Alboran in the Mediterranean, and Cadiz, Lisboa and Galicia in the Atlantic. Day and night samples were collected from 7 layers of the water column with a midwater trawl fitted with a multisampler. Temperature and salinity regimes were very different on the Mediterranean and Atlantic sides of the peninsula, with much higher values through the entire water column in the Mediterranean, characterized by a strong pycnocline. The highest productivity was observed off Lisboa, where Chlorophyll a concentrations were two orders of magnitude higher than in any other zone. Samples from the western Mediterranean held 22 fish species, while 67 were found in the Atlantic. The lowest diversity and the highest dominance were observed in Balears, and the highest diversity in Cadiz zone. In all zones, but particularly in those in the Mediterranean, mesopelagic populations were dominated by a high number of small fish with low individual biomasses. The species Benthosema glaciale, Cyclothone pygmaea and Ceratoscopelus maderensis were common in the Mediterranean populations, whereas in the Atlantic, Cyclothone microdon/livida, Valenciennellus tripunctulatus, Ceratoscopelus warmingii and Benthosema suborbitale were the most common species. Temperature and salinity (both at surface and in the mesopelagic zone) were the main environmental factors explaining variability in assemblage composition. A persistent (day-night) deep scattering layer was observed using the vessel-based echosounder in all zones, and was comprised primarily of the gonostomatid Cyclothone spp. Night-time echosounder observations of scattering layers near the surface were observed in Balears, Alboran, Cadiz and Lis

The biological carbon pump is the process that transports carbon vertically out of the mixed layer in the ocean. Besides the sinking flux of organic particles, active flux due to the daily vertical migration of zooplankton and micronekton promotes a significant carbon transport not fully accounted for or understood in the world’s oceans. The diversity and abundance of epipelagic and mesopelagic species in the Gulf of California has been extensively studied, but the role of micronekton in carbon export has not yet been investigated. We studied the carbon flux promoted by juvenile and adult mesopelagic fishes and crustaceans (Decapoda and Euphausiidae) during the transition from the cold to warm period (June) and the onset of the warm season (October) in 2018. We provide the first estimation of migrant biomass and respiratory flux of the most abundant migratory species of mesopelagic fishes, decapods and euphausiids in the Gulf of California. The micronekton species collected accounted for a large biomass of mesopelagic fishes and pelagic crustaceans. The average migrant biomass estimates were 151.5 ± 101.2 mg C·m−2 during June and 90.9 ± 75.3 mg C·m−2 during October. The enzymatic activity of the electron transfer system (ETS) was measured as an estimate of their respiratory rates. Average specific ETS activity was significantly different between fishes and decapods, and between fishes and euphausiids (p < 0.05). The respiratory flux of fishes was predominant in the Gulf of California, followed by pelagic decapods and euphausiids. Seasonal changes in respiratory flux were observed for fishes (June: 6.1 ± 1.5 mg C·m−2·d−1; October: 3.2 ± 1.8 mg C·m−2·d−1) and decapods (June: 0.4 mg C·m−2·d−1; October: 0.7 ± 0.05 mg C·m−2·d−1). Respiratory flux estimation by crustaceans (decapods and euphausiids) and fishes together was 6.86 mg C·m−2·d−1 during June, and 4.21 mg C·m−2·d−1 during October 2018, suggesting a functional role of this large micronektonic fauna in the biologi

[EN] The early life history of fish is the period that elapses from spawning to recruitment into the adult population (Figure 1). These early stages –eggs and larvae– are a key component in the fish life cycle because their survival success determines both short- and long-term variations in population abundance. Most marine fishes produce thousands to millions of pelagic eggs, which in a few days hatch into larvae that spend weeks to months in the plankton. Fish larvae are characterized by their small size (a few mm to ca. 2 cm), transparency and limited swimming skills, and they display amazing shapes that differ greatly from those of adults (Figure 2). During the planktonic period, fish larvae interact with other planktonic organisms as both predators and prey (Figure 1): feeding upon small prey such as juvenile stages of copepods and, in turn, being preyed on by larger nektonic and planktonic organisms such as jellyfish. […], [ES] Las primeras etapas del ciclo vital de los peces comprenden el periodo desde la puesta hasta el reclutamiento a la población adulta (figura 1). La supervivencia de estos estadios tempranos del desarrollo, los huevos y las larvas, determina las variaciones, a corto y largo plazo, en la abundancia de sus poblaciones. La mayoría de peces marinos producen entre miles y millones de huevos pelágicos que en pocos días eclosionan dando lugar a larvas que pasan desde semanas a unos pocos meses en la comunidad planctónica. Las larvas de peces se caracterizan por su pequeño tamaño (entre pocos mm y unos 2 cm), su transparencia, una limitada capacidad natatoria, y una morfología, en ocasiones sorprendente, que difiere notablemente de la de los adultos (figura 2). Durante el periodo planctónico, las larvas interactúan con otros organismos del plancton, siendo tanto depredadores como presas (figura 1). Las larvas de peces se alimentan de organismos de pequeño tamaño, como estadios juveniles de copépodos (pequeños crustáceos), y, a su vez, son presa de otras especies más grandes que viven también en el medio pelágico, como las medusas. […], [CAT] Les primeres etapes del cicle vital dels peixos comprenen el període des de la posta fins al reclutament a la població adulta (figura 1). La supervivència d’aquests estadis inicials del desenvolupament, els ous i les larves, determina les variacions, a curt i llarg termini, en l’abundància de les seves poblacions. La majoria de peixos marins produeixen entre milers i milions d’ous pelàgics que en pocs dies eclosionen i donen lloc a larves que passen des d’unes setmanes fins a uns pocs mesos en la comunitat planctònica. Les larves de peixos es caracteritzen per tenir una mida petita (de pocs mm a 2 cm), la seva transparència, una limitada capacitat natatòria, i una morfologia, a vegades sorprenent, que difereix notablement de la dels adults (figura 2). Durant el període planctònic, les larves interactuen amb altres organismes del plàncton, sent tant depredadors com preses (figura 1). Les larves de peixos s’alimenten d’organismes de mida petita, com per exemple estadis juvenils de copèpodes (petits crustacis), i, al seu torn, són presa d’altres espècies més grans que viuen també en el medi pelàgic, com les meduses. […]

Microbial trophic steps were largely ignored in the estimations of trophic position of consumers. Recent developments in compound-specific stable isotope studies showed that the microbial exchanges may no longer be invisible. Using 24 species of mesopelagic and bathypelagic fishes collected in the North Atlantic, this study applied the analysis of nitrogen isotopes in amino acids to the estimation of their trophic position and of the contribution of microbial and metazoan trophic steps across depth layers. Isotope-based estimates agreed well with diet-based literature values, but the consideration of microbial steps reduced the mismatch between 0.5 and 0.8 trophic positions observed when only the metazoan food web is considered. Microbial trophic steps contributed between 6 to 21% to the overall trophic position of individual species. Body size was positively correlated with trophic position but not with the relative contribution of microbial trophic steps, except in the mesopelagic layer where the microbial contribution decreased with size. These results suggest that current isotope-based estimates of trophic position for marine consumers underestimate true trophic positions because they are based on metazoan-only trophic steps