To unveil the ecological niche of pelagic top predator megafauna such as beaked whales, sperm whales and dolphins (but also seabirds, sharks, tuna and billfish), it is vital to know more about the prey they feed on, such as mid-water fish and cephalopods. These often lilliputian oceanic micronekton (with jellyfish and shrimp) migrate to the surface layers of the ocean at night and dive back to depths of 600 m or more during the day. This daily vertical migration, which occurs on a planetary scale, is recorded by acoustic probes as moving deep scattering layers. Because of this behaviour, mesopelagic organisms act as a carbon pump, transporting organic matter, nutrients and energy from the surface, where they feed, to the inner ocean. The impact of this biological process in mitigating the effects of climate change is known to be significant.
This cruise provided additional information on the mesopelagic community leaving in the Azores region.
Sampling the deep
Efficient sampling of micronekton organisms requires specific gears. During the RV Meteor M202 campaign, two types of standardised trawls were used. The RMT 8+1 (Rectangular Midwater Trawl) is a two-net setup designed to sample the midwater micronekton community simultaneously with its potential planktonic prey. The main net has an opening of 8 square metres, a mesh size of 4.5 mm; the associated plankton net has an opening of 1 square metre and a mesh size of 320 µm. Opportunistically, we also sampled the micronekton captured by the multinet, a 1 square metre, multi-opening, 9-net trawl with a mesh size of 300 µm designed to sample zooplankton at selected discrete depth layers.
Both trawls filtered the water column between 900 m and the surface, with an emphasis on the deep scattering layer. The nets were equipped with depth sensors.
From the 23rd to the 29th of July, four RMT8+1 and four multinet hauls, were made to the west of Terceira Island in the Azores.
Sorting and processing the catch
Once the nets are on board it’s time to get the animals out! The nets were carefully rinsed with seawater to ensure that no critters were missed. The catch was then sorted into fish, cephalopods, crustaceans, jellyfish and other organisms, such as polychaetes. Fish and cephalopods were identified to species wherever possible using the dichotomous keys (e.g. those issued by FNAM, 1986-1989, or in Sutton et al., 2020 for fish). The keys use morphological characters to distinguish species, and as most specimens were small, binocular optical microscopes were used. Once identified, samples were measured, counted, registered, labelled and frozen at -20ºC in small bags or microtubes for future analysis.
The catch of the day
A total of 2506 fish individuals were caught. As expected, the RMT8+1 caught 93% of the specimens and more than two thirds (69.5%) of these were caught with the 8 m2 net. Most of the fish from the Multinet (85%) were caught by the 4 nets that filtered the deepest layers sampled, i.e., between 375 and 675 m of depth.
About 80 different fish taxa were identified. Five hundred and thirty-three individuals (21.3% of the total) were assigned to 56 species. The most abundant fish with ~70% of the captured individuals belonged to the bristlemouth genus Cyclothone, believed to actually be the most abundant vertebrates on Earth.
The bristlemouth family (Gonostomatidae) accounted for 78% of the fish caught, mainly due to the aforementioned dominance of the 1937 Cyclothone spp. individuals. Lanternfishes (Myctophidae) and hatchetfishes (Sternoptychidae) followed, but their relative abundances were much lower (7.3% and 6% respectively). The Sloan’s viperfish Chauliodus sloani (66 individuals) and the half-naked hatchetfish Argyropelecus hemigymnus (54 individuals) were the second and third most abundant species.
The most diverse families in our catch were represented by lanternfishes (Myctophidae, 20 sp.), dragonfishes (Stomiidae, 8 sp.), bristlemouths and relatives (Gonostomatidae, 7 sp.), hatchetfishes (Sternoptychidae, 5 sp.), ridgeheads (Melamphaidae, 3 sp.) and lightfishes (Phosichthyidae, 2 sp.). These 45 species of the most typical mesopelagic fish families represent about 40% of the species of the same families known to occur in the Azores (46% of the myctophids and 25% of the stomiids). Two species (i.e., the lanternfish Taaningichthys bathyphilus and the hatchetfish Sternoptyx pseudodiaphana) have not been previously reported for this region. Three specimens of the apparently rare species Parabrotula plagiophtalma (Parabrotulidae) were also caught. Representatives of barreleyes (Opisthoproctidae), snipe eels (Nemichthyidae) and deep-sea smelts (Bathylagidae), among others, belonged also among the rare catches of this cruise.
Most fish were small, with an average size of 37.1 mm standard length corresponding to the limits of the deployed nets. The longest individuals were the anguilliform snipe eel Nemichthys scolopaceus (365 mm) and a Serrivomer sp. (327 mm). The elongated bristlemouth (Sigmops elongatus) was the most common species among these “giants”, with eight specimens between 146 and 267 cm, but a few other stomiids were also large, such as the scaleless black dragon fish (Melanostomias bartonbeani) and the snaggletooth (Astronesthes gemmifer). Most of the dominant small bristlemouth individuals (Cyclothone sp.) were not measured due to their sheer numbers. These trawls don’t sample the larger component of the size spectrum of fishes living in the midwater habitats.
Let’s talk tentacles
Besides catching vast numbers of various deep-sea fish, much less frequent were the cephalopods, which includes squids, octopods, cuttlefish and nautiluses. During M202 we caught a total of 34 cephalopods – compared to the quantity of fish they could almost seem less important. But to the contrary, cephalopods are one of the two main reasons we are on board of the RV METEOR. Azorean toothed whales – the other main reason – feed on squids and octopods in high numbers, especially beaked whales such as the goose-beaked whale Ziphius carvirostris mainly forage on squids during their deep-sea dives. Since we were researching beaked whales’ feeding grounds, the rather low catch numbers came as a surprise, however, squids are fast and intelligent animals which could easily avoid a slowly towed net. The majority of caught specimens were juveniles and often only a few centimetres long which made identification additionally challenging.
The most abundantly caught cephalopod group were the glass squids (Cranchiidae), which transparent and odd-looking members are a known diet component of beaked whales. We collected specimens belonging to the genera Liocranchia, Helicocranchia, Taonius and Galiteuthis. Additionally, we caught several specimens belonging to the Enoploteuthidae; a family of squids that is famous for the beautiful arrangements of numerous light organs across their bodies. A “fan-favourite” amongst the crew was Heteroteuthis dispar, a species of bobtail squid with huge eyes and a roundish body shape. Unlike other bobtail squids, this species lives in the free water column once reaching adulthood.
The 15 cephalopod taxa we identified also included specimens of three club-hooked squids, two chiroteuthid squids, two juvenile octopods and to our excitement one specimen of Bathyteuthis, a small and very rare deep-sea squid.
All cephalopod samples will be barcoded to obtain precise taxonomic identification and later, our collaborators will process them for calorific content measurements to understand how nutritious these animals are as prey for the Azorean cetaceans.
Further studies – midwater fish vision
The M202 cruise was also an opportunity to study the evolution of sensory systems in deep-sea fishes. Coming from an extreme environment, these fish often have fascinating adaptations that are unique and unparalleled in any other vertebrate species. Even in the dark depths, fish still need to find prey or a mate. Fish use vision to identify conspecifics, and in many cases, this is based on their unique bioluminescent patterns. Either they have a species-specific constellation of glowing organs (photophores) on their bodies, as in lanternfishes (Myctophiformes), or they differ in the shape and colour of their chin barbels, as in dragonfishes (Stomiiformes). After species identification, measurements and DNA tissue sampling, we focused on the fish eyes, dissecting their retinas and fixing them either in solution for subsequent transcriptomic (RNA content) analysis or in liquid nitrogen for even more detailed single-cell RNA sequencing. These genetic tools will allow us to determine the composition of the visual pigments and, to some extent, how these fish see.
Future studies
The fish caught during this cruise will be used for DNA barcoding , stable isotope analysis, microplastic content analysis and microbiome analysis.
We thank the cruise leader, Véronique Merten, for the opportunity to participate on the M202 campaign. We also acknowledge the crew members and our colleagues in charge of the gear operation, as well as those who helped with the catch sorting.
By Filipe M. Porteiro, Thomas Belaoud, Sophie V. Schindler and Zuzana Musilova