2018 Cruise - JC165

Searching for larvae in the deep sea!

So here I am again, about one year later, on the exact same site in the North Atlantic Ocean, the Porcupine Abyssal Plain Sustained Observatory (PAP-SO). This is among the oldest time-series stations in the world, which has been collecting data all the way from the surface of the ocean, across the water column to the seafloor, nearly 5 km deep. During just over three weeks, scientists, engineers and crew are working together to recover old instruments and deploy new ones in the ocean. The data collected with these instruments is transferred to the computers and laboratories onboard the ship, where they can look at the information and compare it with previous years.The kind of data collected are varied and include many chemical and physical parameters of the water, like oxygen, nutrients and temperature. During the cruise several other gear types (e.g. plankton nets, megacore, trawl and towed cameras) are used to collect biological information about small animals swimming around in the water (zooplankton) or buried in the sediment (benthic macrofauna and meiofauna), and larger organisms (megafauna) that live on the seafloor.

From such a comprehensive sampling program, scientists have been able to study patterns in diversity and abundance of the species and relate these patterns with changing ocean characteristics over the years. These long-term studies are very important to understand how natural changes, for example climate and food supply, and human activities (e.g. pollution, and overfishing) can impact the marine environment and its biodiversity. One critical aspect to know how marine animals survive and adapt to changing oceans is called population connectivity. In the same way that people travel around the world and move to new places, marine populations also exchange individuals among different geographic sites. While some of these movements are made by actively swimming between locations, others result from passive transport by oceans’ currents. But, how do species living on the seafloor disperse, and maintain populations across wide geographical areas? Most benthic animals have very limited movement, slowly crawling around (e.g. sea cucumbers and sea stars), or even staying at the same place for most of their life (e.g. mussels, anemones and sponges). However, many benthic species have a larval stage that can live in the water column and be transported hundreds or thousands of kilometres away from their parents.

Studying larvae from deep-sea species is extremely difficult because larvae are minute (a couple tenths of a millimetre) and very hard to collect in deep waters. Ocean observatories provide a great opportunity to test new sampling methods and standardize approaches, which may then be used simultaneously across the world to give us a global view of ocean’s health in space and time. With the goal to know more about larval diversity and distribution in the deep sea, I’ve started to deploy a series of larval traps in several deep-ocean moorings in the North East Atlantic and Mediterranean Sea. I also attached the traps to experimental substrates that larvae could use to settle and grow (see DY077 RRS Discovery cruise 2017), thus giving us a better understanding of population connectivity. Last Sunday, I collected the second set of samples from the sediment trap mooring at PAP-SO and preserved the samples onboard to observe them under the microscope when I return to my land-based laboratory at the University of Aveiro in Portugal.

recovery of larval traps
Figure 1.  Recovery of larval traps and colonization frames from PAP3 mooring

Last year, I also deployed traps attached to the Bathysnap mooring – a metal frame that hosts the time-lapse camera, but yesterday we couldn’t retrieve it. A malfunction of some kind has so far prevented the mooring from coming to the surface, so it is very likely still sitting on the seafloor. Next year, we may be able to bring an additional sonar to aid in locating and recovering the camera and also the larval traps and substrates. This misfortune may in turn reveal exciting new data since a longer time on the bottom will potentially allow more animals to fall in the larval traps and colonize the substrates. A new Bathysnap is now being prepared for deployment and I can’t wait to see what will come back next year.

larval trap recovery
Figure 2. Larval trap sample recovered from PAP3 mooring. 

 

 

Written by Luciana Génio.

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2018 Cruise - JC165

Baited beasties, or the scavengers of the deep.

Our first amphipod trap was deployed on Sunday morning and left to ‘soak’ on the PAP seafloor for about 40 hours before recovery. Amphipods are small crustaceans, shrimp-like in form but without a carapace, bearing different kinds of appendages on their thorax and abdomen, with impressive claw-like structures that can grip almost anything. The amphipods we collect at PAP are bentho-pelagic; they live on or close to the seabed and they are particularly ferocious! In fact, like piranha in the Amazon River, they can devour any ‘attractive’ prey, whether alive or dead. To attract these little deep-sea beasts, we use four dead mackerel as bait placed in funnels inside large cylindrical tubes mounted on a sampler that we simply call the “Amphipod Trap”. Deep seas are usually food-limited environments; benthic fauna relies mostly on the particulate organic matter that originates in surface waters and degrades through the water column before reaching the seafloor. We use the mackerel to simulate a natural food-fall that will appeal to scavengers.

amphipod_trap
Figure 1. Scientists waiting for the Amphipod Trap to come up to surface (left), and crew members trying to catch the trap with a hooked rope (right).

The trap is deployed from the afterdeck and sinks down to the Porcupine Abyssal Plain. A pair of bottom tubes containing the mackerel sits about 50 cm above the seafloor, and a top pair sits about 1 m above. When they smell the dead fish, the amphipods, many of the genus Eurythenes, swim into the funnels where they end up sampled. Depending on how many amphipods are trapped in the funnels and how long we leave the trap in the water, we sometimes only recover the bones of the fish (see DY077 Discovery 2017-cruise), and observe the largest specimens already eating the smallest ones. As if a whole mackerel was not enough!

Amphipod_trap_recovery
Figure 2. Amphipod trap recovery on the after deck. A mackerel was placed as bait inside each of the four funnels to attract the deep-sea amphipods.

To recover the trap scientists release a trigger that ‘calls’ the sampler to come up to the surface. Crew members then catch the trap using a hooked rope before it can be brought back on the afterdeck. It is then our turn, were we process the samples; we collect all individuals caught in the trap and preserve them in ethanol. This will allow morphological and genetic analysis once back at NOC in Southampton.

amphipods.jpg
Figure 3. Scientists part of the ‘Benthic Team’ picking out all amphipods trapped in the funnels before preservation with ethanol (top).
Example of deep-sea amphipods (bottom), many of the genus Eurythenes, collected on the PAP seafloor. Note the broad body-size range and morphologies of these little, necrophagous, creatures.

During this first deployment, the fish were not completely eaten, yet we collected a few hundreds of individuals. We aim at deploying at least two sets of samples during the cruise. NOC scientists have been collecting these deep-sea amphipods at PAP for over 30 years in order to assess any change in species abundance, diversity, and composition over time. These biological data are then related to local environmental factors such as food supply to the seabed and temperature that may explain the observed patterns of the baited beasties.

 

Written by Noëlie Benoist.