The unexpected discovery of four unique deep sea fish-falls off the coast of Angola has revealed a feeding ecosystem considerably different than those typically associated with large animal carcasses.
Without sunlight, most creatures in the deep ocean are dependant on detritus from the surface as their primary source of food, such as dead plankton and fecal pellets produced by zooplankton. But every once in a while, the remains of large sea creatures sink to the bottom, offering a veritable bonanza for those animals who are ready to take full advantage.
A Shark and Three Rays
But natural fish-falls are rarely observed by marine biologists. So it was to the great fortune of Nicholas Higgs and his team the University of Plymouth's Marine Institute when a remotely operated vehicle (ROV) run by the oil and gas industry stumbled upon four deep-sea fish falls. Using its onboard camera, the ROV captured video footage of a whale shark (Rhincodon typus) and three mobulid rays (genus Mobula).
Observations showed that these carcasses can support scavenger communities on the deep sea floor for weeks to months at a time. But unlike larger marine mammal carcasses, these sites were unique in that they didn't appear to host characteristic "whale-fall" creatures. Normally, whale-falls are home to sharks, crabs, and shrimp-like amphipods (aka zombie worms).
"No evidence of whale-fall type communities was observed on or around the carcasses," write the researchers in PLoS, "with the exception of putative sulphide-oxidising bacterial mats that outlined one of the mobulid carcasses."
Of course, absence of evidence isn't proof, so further research is warranted.
Higgs's team mainly found scavenging fish, sometimes as many as 50 per carcass. Of these, there were three to four different types, most of them eel pouts. These eels eat smaller scavengers, like amphipods, who are attracted by the dead whale.
The team suspects that the carcasses of large animals provide as much as 4% of the total food that arrives on the seafloor in that area.
Our results suggest that in such areas the large food-falls can account for a significant proportion of carbon export to the deep-sea, approximately 10 times larger than previous estimates for a single taxon. This increased export is expected to result in a relatively high proportion of local surface primary production reaching the deep-seafloor, supporting a more abundant community of deep-sea scavengers.
What ever floats must eventually come down, particularly in this animal-rich area.
Read the entire study at PLoS One: "Fish Food in the Deep Sea: Revisiting the Role of Large Food-Falls."