What Lies Beneath

One of the most fascinating and romantic things about the marine world, is its endless mystery. The oceans are vast and deep, and largely unexplored, and there are many questions we still have about how animals survive in the great depths. However, as today's technology continues to advance, we are now able to explore further and deeper than we ever have before. So, have scientists seen sharks at the bottom of the sae? And what what can this teach us about life in the oceans?

Advancing submersible technology means we can now capture images of these roughskin dogfish feeding on a swordfish carcass on the ocean floor, some 453 metres down (Image Source: Deep Discoverer, Auster et al, 2020)

Deep Down

With the advancements of modern technology, we are now able to observe the community of organisms in offshore, deep-sea habitats like we never have before. Manned submersibles and deep-sea rovers (unmanned robots equipped with recording equipment) have allowed us to see so much more of this mysterious, alien world; we can explore new habitats, discover new species, and understand how energy and nutrients move through the ocean ecosystem (Higgs et al, 2014; Auster et al, 2020).

The remotely operated vehicle, Deep Discoverer (Image: Deep Discoverer, Auster et al, 2020)

Coastal habitats, with many "primary producers" (organisms like plants and algae, which incorporate energy into the environment by converting energy from the sun into matter) are highly productive, but comparatively, open-oceans and deep-sea habitats are often relatively low in nutrients. Yet, this does not mean that these environments are devoid of life. Migratory animals, like whales and sharks are able to move nutrients from one region to another. This is known as "nutrient translocation" (Higgs et al, 2014; Auster et al, 2020).

Nutrients are most often translocated by migratory animals eating in one place and defecating elsewhere. Less commonly a large amount of nutrients can be dumped into one spot very suddenly when an animal dies and their carcass sinks to the sea floor. These "food fall" events can support a plethora of organisms (Higgs et al, 2014; Auster et al, 2020).

Food-fall events are a vital source of nutrients for animals living in the deep oceans (Image: Deep Discoverer, Auster et al, 2020)

Dinner Bell

A remotely operated vehicle called Deep Discoverer was able to record fantastic footage of a food-fall event surrounding the carcass of a 2.3 m Atlantic Swordfish (Xiphius gladius) at 453 m depth, off the coast of South Carolina. The Deep Discoverer recorded images of several different species scavenging on the carcass, including two species of sharks; Genie’s Dogfish (Squalus clarkae) and Roughskin Dogfish (Cirrhigaleus asper), a cutthroat eel (Synaphobranchus sp.) and many different crabs (Callinectidae) (Auster et al, 2020).

Sharks are attracted to a carcass thanks to its smell, but also because of the noise that other excited scavengers make as they feast (Image: Deep Discoverer, Auster et al, 2020)

As many as 11 sharks were seen circling the carcass at once, where only solitary sharks had been seen very sporadically on previous dives. So how did all these sharks find this food source in the abyssal depths? They certainly could not see it in the dark (Auster et al, 2020).

The researchers agreed that odour was certainly a factor - with the sharks smelling the carcass over long distances - but they also suspect that sound played a significant role. When feeding, sharks beat their tails rapidly and move around the carcass a lot. Therefore, the arrival of the first shark at the carcass created an acoustic signal, which could be heard by other sharks that were nearby and they all came to join the party (Auster et al, 2020).

But it was not only sharks attracted by all this activity! The rover also recorded footage of the sharks themselves becoming the prey! A wreckfish (Polyprion americanus) was observed eating one of the smaller sharks which had arrived at the carcass to feed (Auster et al, 2020).

All You Can Eat Buffet

Observing how animals feed and behave around a food-fall event can teach us a lot about how oceanic ecosystems function and how nutrients are cycled through the oceans. The Deep Discoverer's footage showed that it is not only the carcass that creates a feeding opportunity around a food-fall event. "Lower trophic-level predators", like crabs and eels, and "mesopredators" (or middle predators) like the sharks, all came together to scavenge off the carcass, but "higher trophic level predators" were attracted to feed on the sharks themselves (Higgs et al, 2014; Auster et al, 2020).

Small sharks can become prey to larger predators attracted to all the commotion around a food-fall event (Image: Deep Discoverer, Auster et al, 2020)

This shows us that a complex and dynamic, temporary food chain springs up around a food-fall event and it does not only benefit scavengers, but also higher-level predators (Auster et al, 2020).

Hopefully as our technologies continue to advance, other rovers might be able to capture more wonderful footage like this, to help us to better understand the mysterious goings-on of the deepest parts of the ocean. It's like opening the door to an amazing and mysterious an alien world.

Carcasses sinking to the ocean floor provide food for a wide range of deep-sea animals, including sharks (Image: Deep Discoverer, Auster et al, 2020)

References

Auster PJ, Cantwell K, Grubbs D & Hoy S (2020). Observations of deep-sea sharks and associated species at a large food fall on the continental margin off South Carolina, USA (NW Atlantic). Journal of the Ocean Science Foundation, 35. Access online.

Higgs ND, Gates AR & Jones DO (2014). Fish food in the deep sea: revisiting the role of large food-falls. PloS one, 9:5, e96016. Access online.

By Sophie A. Maycock for SharkSpeak