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.

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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!

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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.

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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.

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