Octopus then uses tentacles of jellyfish to defend itself
The open ocean is the largest habitat on Earth. Within this expansive and uninhibited realm, the marine creatures can grow to enormous sizes. Despite their size, many questions about their lives and behaviour remain unresolved.
The sheer expanse and depth of our oceans makes it difficult for us to study the animals that live there. Seemingly basic questions like what they eat is a challenging mystery that can take decades to solve. However, thanks to advances in underwater technology, it is now easier to observe deep-sea animals in their natural habitat.
A new study, published in Scientific Reports, by Henk-Jan Hoving of GEOMAR Helmholtz Centre for Ocean Research Kiel and Steve Haddock of Monterey Bay Aquarium Research Institute (MBARI) reveals a surprising truth about the deep-sea octopus Haliphron atlanticus: Its choice of prey is the jellyfish.
Much of this octopus' life is a mystery. In fact, it has been spotted by MBARI researchers only three times in 27 years. It is rarely caught alive; much of our knowledge about them comes from specimens caught in trawl nets.
This octopus lives in the deep pelagic ocean, and belong to a group of octopods called Argonautoidea. They are generally small in size, but the female Haliphrons are giants, measuring as long as four metres, and weighing up to 75 kilogrammes. In contrast, the males are only about 30 centimetres.
Using remotely operated vehicles (ROVs), MBARI scientists now have the ability to locate such elusive creatures like the Haliphron in the deep sea. With these devices, they can use high-resolution cameras to zoom in or to follow the animal to observe its behaviour.
On a recent cruise, Haddock's team chanced upon a Haliphron and followed it. Then, when it spread out its tentacles, they were suprised to see a large jellyfish firmly cradled in its grasp.
Later, Hoving and Haddock searched through the MBARI archives, and managed to find two records of the Haliphron. They saw that the animals were holding something in their tentacles. Unfortunately, it was difficult to see what the octopuses were holding, due to the cape-like webbing between their tentacles. At the very least, it was obvious that at least one of them was also carrying a gelaninous organism.
In addition, Hoving analysed the stomach contents of five specimens, and discovered that they all contained gelatinous zooplankton. For three of them, their stomach contents were easily identifiable as jellyfish.
This set of evidence – the sightings, stomach contents and literature observation – strongly proves that the haliphron atlanticus feeds on gelatinous zooplankton.
In the open ocean, gelatinous zooplankton are abundant. Thus, it makes sense for the haliphron, a large, slow-moving animal, to prey on them. Similar, other large pelagic animals like the leatherback sea turtles, tuna and large pelagic fish also eat jellyfish.
Although the “flesh” of these jelly-like animals are generally perceived to be nutrient deficient, the animals that eat them tend to be very big in size. In addressing this paradox, Haddock suggests that the net weight of the jellyfish may translate into substantial biomass, making up for the lack of nutrients.
In addition, Hoving and Haddock suggest that it is possible that after consuming the nutritious parts of the jellyfish, the female haliphron atlanticus uses their stinging tentacles as a tool.
Due to her large size, they can completely grasp a jellyfish within their webbed tentacles. Upon catching it, they bite through the bell of the jelly to get to the food within its digestive cavity. Although the jellyfish is dead at this point, its bell and fringe are still intact. Based on how the octopus is holding the jellyfish, Hoving and Haddock postulate that it is using the jellyfish's tentacles for defence or to capture more prey.
Link to the study: http://www.nature.com/articles/srep44952