Distribution, vertical migration, and trophic ecology of lanternfishes (Myctophidae) in the Southwestern Tropical Atlantic
Introduction
Lanternfishes (Myctophidae) are amongst the most abundant, widespread, and diverse fish groups in the world ocean (Gjøsaeter and Kawaguchi, 1980, Catul et al., 2011). This family includes 254 species that are generally small (mean maximum length of 90 mm) and are dominant in pelagic environments (Poulsen et al., 2013, Priede, 2017, Fricke et al., 2020, Cherel et al., 2020). Most myctophid species migrate vertically to the surface to feed at night and actively transport the carbon they ingest to deep waters during the day (Catul et al., 2011). Moreover, they play a significant role in energetic pathways by consuming zooplankton and providing forage for numerous epipelagic (e.g. tuna, mammals, squids, and diving seabirds) and deep-sea (e.g. viperfish, lancetfish, and moonfish) predators (Sutton and Hopkins, 1996, Cherel et al., 2010, Choy et al., 2013, Rosas-Luis et al., 2014, Eduardo et al., 2020b). These traits are crucial for connecting epipelagic and deep-sea ecosystems and oceanic carbon storage (Catul et al., 2011, Sutton, 2013, Cavan et al., 2019).
Many species of this notable fish group remain poorly known despite facing increasing risks of several kinds (St. John et al., 2016, Martin et al., 2020). While there is a lack of scientific information on the diversity and ecology of lanternfishes in most oceanic basins, the side effects of global warming (Levin et al., 2019), plastic pollution (Davison and Asch, 2011), and exploitation of deep-sea resources are accelerating (Hidalgo and Browman, 2019, Drazen et al., 2020). Further investigations on the ecology and conservation of lanternfishes are recommended. Research has already addressed important aspects of their taxonomy (e.g., Wisner, 1976, Nafpaktitis et al., 1977, Hulley, 1992, Martin et al., 2018), distribution (e.g. Costa et al., 2014, Olivar et al., 2017, Cherel et al., 2020, Sutton et al., 2020), morphometry (Tuset et al., 2018, Eduardo et al., 2020c, López‐Pérez et al., 2020), vertical migration (Watanabe et al., 1999, Olivar et al., 2012, Olivar et al., 2017, Annasawmy et al., 2018, Wang et al., 2019) and trophic ecology (e.g. Hudson et al., 2014, Bernal et al., 2015, Olivar et al., 2018, Contreras et al., 2018, Annasawmy et al., 2020, Czudaj et al., 2020, Bode et al., 2021). These studies demonstrated that lanternfishes present a broad range of vertical and feeding behaviours (Hopkins and Gartner, 1992, Watanabe et al., 1999, Catul et al., 2011). However, less attention has been given on how lanternfish species are scattered over different patterns of resource use (niche partitioning), as a means to avoid resource competition.
Understanding niche partitioning among lanternfishes is central to resolving the paradox between ecological theories demonstrating competitive exclusion and the fact that many of these species are morphologically and ecologically similar but do not drive one another to extinction (Schoener, 1974, Hopkins and Gartner, 1992, Finke and Snyder, 2008). Additionally, understanding niche partitioning helps clarify the coexistence of sympatric species and how resource use shapes their contribution to ecological processes (i.e., fluxes of carbon and nutrients; Brandl et al., 2020). Examining the use of resources in the unified framework of niche segregation, however, requires simultaneous information on biophysical and ecological aspects that are usually lacking. As an example, niche segregation typically occurs along three axes: diet (feeding ecology), space (habitat), and time (feeding chronology; Schoener, 1974). For lanternfishes, this information is sparse and restricted to just a few locations (Hopkins and Gartner, 1992, Hopkins et al., 1996, Hopkins and Sutton, 1998, Catul et al., 2011). Moreover, most previous studies addressing the trophodynamics of these species did not include predators and were based solely on stomach contents. Further approaches (e.g., stable isotopes, fatty acids, and genetics) should be included to provide a comprehensive picture of resource uses. Additional data on the trophic ecology of lanternfishes would help to clarify their trophic links and thus niche differentiation.
Another key point in the study of lanternfishes is how physical drivers shape their diversity and community structure. Variations in temperature, oxygen, and upper circulation processes play important roles in the ecology and movement of deep-pelagic species (Bertrand et al., 2010, Proud et al., 2017, Boswell et al., 2020). However, only a few studies, focusing on restricted locations, have been performed addressing the influence of these oceanographic variables in lanternfishes (Olivar et al., 2017, Milligan and Sutton, 2020). Oceanic islands and seamounts of the Southwestern Tropical Atlantic (SWTA), for instance, are interesting locations to study the influence of physical drivers on biological communities, as they sustain distinctive biodiversity and are considered as Ecologically or Biologically Significant Marine Areas (EBSAs; CBD, 2014). Additionally, this region includes different biogeographic provinces with contrasting thermohaline features, current systems, and water-mass properties, leading to diversification of biodiversity and ecosystems (Bourlès et al., 1999, Assunção et al., 2020, Dossa et al., 2021, Costa da Silva et al., 2021, Tosetto et al., 2021).
We conducted an integrative study of the ecology of lanternfishes, taking advantage of a comprehensive dataset collected in the SWTA. First, we assessed their vertical migration and trophic ecology by coupling information on their abundance, vertical distribution, habitat (oxygen, temperature and fluorescence), and trophodynamics. We did this by exploring the main trophic links of lanternfishes through the analysis of stable isotopes (carbon and nitrogen) of size fractions of zooplankton, gelatinous organisms, crustaceans, fish larvae, and epipelagic and deep-sea fish predators. Second, we assessed the importance of the influence of oceanographic features on lanternfishes by comparing their species composition, spatial distribution, and assemblage structuring under two different physicochemical scenarios. Finally, we discuss the functional roles of lanternfishes.
Section snippets
Study area
The study area is located off northeastern Brazil and includes the Fernando de Noronha Archipelago (03°50′S, 32°25′W), Rocas Atoll (03°52′S, 33°49′W), and seamounts of the Fernando de Noronha Ridge (Fig. 1). This region is divided into two areas with significant differences in currents and thermohaline structures (Assunção et al., 2020, Dossa et al., 2021, Costa da Silva et al., 2021). The first region, here named as Area 1, is mainly located along the Brazilian continental slope and
Oceanographic data
The thermal difference between the surface (29 °C) and 1000 m depth (4 °C) was 25 °C in both areas defined for the study. Area 1 was characterised by a weak thermal stratification, a deep thermocline (lower limit at 166 m), and a fluorescence maximum peaking at 100 m (Fig. 2). In this area, the water column was evenly oxygenated (Fig. 2) and subsurface salinity was high (>36.5). In contrast, Area 2 was characterised by a strong thermal stratification with a well-marked and shallow thermocline
Discussion
We investigated the vertical migration and trophic ecology of lanternfishes in the Southwestern Tropical Atlantic, as well as the influence of physicochemical factors on their horizontal structuring, by combining information on their species composition, distribution, stable isotopic composition, and habitat. We found multiple patterns of vertical and trophic behaviour, revealing multidimensional niches, underestimated trophic links, and several underlying mechanisms to avoid competitive
Conclusion
Lanternfishes are a highly diverse and abundant fish family of the Southwestern Tropical Atlantic, comprising at least 33 species and 40% of all mesopelagic specimens collected in our samples. This species composition is comparable to those found in the tropical and subtropical Atlantic, but with clear differences in dominance patterns. Species evaluated here showed weak horizontal structuring, suggesting that well-defined assemblages of lanternfishes are not maintained. Although the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank the French oceanographic fleet for funding the ABRACOS 2 survey at sea (http://dx.doi.org/10.17600/17004100) and the officers and crew of the RV Antea for their contribution to the success of the operations. We also thank the BIOIMPACT (UFRPE) and LIZ (UFRJ) students for their laboratory assistance and Pierre Lopez for editing Fig. 10. We are indebted to the two anonymous reviewers for providing very detailed comments and suggestions. Leandro Nolé Eduardo is supported by CAPES (grant
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