What Lies Beneath

Scientists are always interested to understand how nutrients are cycled and moved throughout the marine ecosystem. Coastal habitats, with many "primary producers" (organisms which incorporate energy into the environment, by converting energy from the sun into matter, eg. plants), are highly productive, but comparatively, open-ocean or 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 (known as "nutrient translocation"), either through defecating or their death. This can support a plethora of organisms. It is just the location of the habitat which limits us from observing them...well... in the past it was challenging, but today it is very possible!

Roughskin dogfish feeding on a swordfish carcass on the ocean floor (Image Source: www.foxnews.com)

With the advancements of modern technology, we are now able to observe the community of organisms in offshore, deep-sea habitats. 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.

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

For example, a research group recently used a remotely operated vehicle called Deep Discoverer to observe a "food-fall" event in the deep off the coast of South Carolina, USA.

Food-fall events are sporadic occasions when a significant load of nutrients are deposited in the deep-ocean via the death of a large animal, like a whale. On this occasion, it was the carcass of a 2.3 m Atlantic Swordfish (Xiphius gladius) at 453 m depth .

Depth of the ocean floor at the Deep Rover study site (Auster et al, 2020)

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). As many as 11 sharks were seen circling the carass at once, including 9 individual Genie's dogfish and 2 Roughskin dogfish. Previous dives of the Deep Discover, had only found solitary sharks and they were only encountered at a low rate in this area (Auster et al, 2020).

Shark scavenging on swordfish carcass (Auster et al, 2020)

Therefore, the food fall event offered a significant opportunity for sharks to feed... such an attractive event that many sharks showed up from a large area all at once!

So how did all these sharks find this food source in the abyssal depths? Well, it might not be what you expected... the researchers agreed that odour was certainly a factor, but they also hypothesised that sound also played a significant role! When feeding, sharks and fish beat their tails rapidly and move around the carcass a lot. Therefore, the scientists suggested that the arrival of a large scavenger at the carcass created an acoustic signal, which could be detected by other animals nearby. Basically, when one shark found the carcass, all the others in the area heard and smelt the commotion and came to join the party (Auster et al, 2020).

Dogfish feeding on the ocean floor 453 m down (Auster et al, 2020)

What was also especially interesting for the scientists was the assemblage of different species round the carcass; alongside the 2 different species of sharks (a "higher trophic-level predator"), there were also many crabs (Callinectidae) and a cutthroat eel (Synaphobranchus sp.). These animals are "lower trophic-level predators", meaning they are also prey for the sharks.

But, 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. This shows that food-fall events do not only directly offer feeding opportunities, but the aggregation of many different species around a carcass also offers an opportunity for other species to turn up and predate upon the scavengers. A whole new community sprang up around the carcass (Auster et al, 2020).

Sharks exploring a food fall event (Auster et al, 2020)

Understanding food-fall events is critical for scientists to explain the translocation of nutrients in the deep ocean. Today, as technology continues to advance and our deep-sea rovers become increasingly effective, hopefully we will get more and more of these fantastic images of sharks in their natural habitats, far from where human eyes could ever normally see them.

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.

By Sophie A. Maycock for SharkSpeak.