The giant deep-sea octopus Haliphron atlanticus forages on gelatinous fauna

Feeding strategies and predator-prey interactions of many deep-sea pelagic organisms are still unknown. This is also true for pelagic cephalopods, some of which are very abundant in oceanic ecosystems and which are known for their elaborate behaviors and central role in many foodwebs. We report on the first observations of the giant deep-sea octopus Haliphron atlanticus with prey. Using remotely operated vehicles, we saw these giant octopods holding medusae in their arms. One of the medusae could be identified as Phacellophora camtschatica (the egg-yolk jelly). Stomach content analysis confirmed predation on cnidarians and gelatinous organisms. The relationship between medusae and H. atlanticus is discussed, also in comparison with other species of the Argonautoidea, all of which have close relationships with gelatinous zooplankton.

Scientific REPORTS | 7:44952 | DOI: 10.1038/srep44952 Another individual also had a yellow mass in its arms, but since the arms were never opened, the nature of the mass remained concealed. The large size of the specimen suggested it was a brooding female 16 , although our current observation may alter this interpretation. Both specimens are shown in Young 16 . Our third observation, from July 2013, involved a large female H. atlanticus (mantle width ~29 cm; Fig. 1). This specimen held the medusa Phacellophora camtschatica (egg-yolk jelly) which had lost all of its stomach and oral arms, and most of its tentacles. The observation of this H. atlanticus from the ROV (Fig. 1b) showed that the outer surface of the medusa bell was held by the arms and suckers. The animal was able to swim while holding the medusa, and when the arms were closed, the medusa was not visible. The Haliphron appeared to have bitten through the bell from the outside, as the beak was sometimes visible in the center of the subumbrella of the medusa (Fig. 1b).
Haliphron atlanticus specimens (n = 5) accessioned in the Hamburg Zoological Museum (Table S1) were examined and all specimens had body parts of gelatinous fauna in their crop and stomachs (Supplementary Figure S1). Two of the specimens had tentacles and lightly pigmented frilled material (one with fine tentacles), consistent with scyphozoan oral arms (Fig S1a), as we would expect from our in situ observation. Another specimen had brick red pigmentation and tentacles of a coronate scyphozoan (Fig S1b), similar to the previous report of a coronate medusa in gut contents 13 . One stomach had the remains of the stem and seven intact tentilla of a physonect siphonophore (Fig S1c), likely in the family Agalmatidae. The fifth specimen included the remains of a salp chain along with the only crustacean found in the stomach contents ( Fig S1d). This crustacean was identified as an amphipod of the genus Vibilia, which is known to specialize in parasitizing salps 19,20 . This record corresponds with the amphipod gut-contents previously reported 13 , which may be indicative of ingestion as "by-catch" when the gelatinous host of the amphipods is consumed.

Discussion
The only other information that existed previous to our observation was the finding of gelatinous fragments (identified to belong to a mesopelagic coronate medusa) and crustaceans in the crop of trawl captured H. atlanticus 13,18 . Our observations of two Haliphron holding on to a jellyfish combined with the finding of jellyfish fragments in the stomachs, in the literature and as observed here, strongly suggest that H. atlanticus feeds on gelatinous zooplankton. Other genera in the Argonautoidea are known to maintain symbiotic associations with gelatinous organisms, such as living in the barrels of salps or holding the tentacles of siphonophores. Such an association could be occurring with Haliphron as well, given that the clearest images show it holding the bell of the medusa with the fringe of tentacles intact, and the octopus' beak protruding through a slit in the center of the bell. Fully establishing that there is a feeding relationship beyond the behavioral association requires more than just witnessing the association, and we believe that the combined evidence supports that the octopus feeds upon the medusa, at least initially in the relationship: Our five specimens and the published record of gelatinous remains in the stomachs of net-caught Haliphron specimens 13 complements our in-situ observation that the oral arms and stomach of the medusa were missing from within the bell. This is similar to how fish 21 and sea turtles 22 feed on medusae, by initially targeting their more energy-dense oral arms and gonads.
In situ observations of Haliphron obtained previously repeatedly show that a mass is being held (and concealed) in between the arms 15 (https://youtu.be/sw8zl5vrAu8? t= 1m50s; Accessed 01/30/2017). Since the species is known to hold on to the eggs after spawning 17 , these masses may be eggs. Taking into account our current observations though, these concealed masses may also be the carcasses of gelatinous zooplankton.
There are other reports of cephalopods with gelatinous fauna in their stomachs 23 . Cnidaria, mostly Velella, accounted for more than 20% of the prey weight in market squid Doryteuthis opalescens 24,25 . The large onychoteuthid Onykia robusta had one Velella specimen in its jaws 26 . Vampire squid were reported with gelatinous material in their stomachs 7 . Sthenoteuthis pteropus consumed high numbers of Pyrosoma atlantica in the eastern tropical Atlantic 27 . Since these cephalopods (except vampire squid) are known to primarily consume crustacean and fish prey, feeding on gelatinous zooplankton may be incidental and part of their opportunistic feeding behavior. The observation of gelatinous prey ingestion by Haliphron adds to the variety of feeding strategies that can be found within the Cephalopoda.
The finding of H. atlanticus consuming medusae fits in the habits of argonautoid cephalopods, which apparently all have a relationship with gelatinous zooplankton at some point in their life. As juveniles, Ocythoe tuberculata are sometimes found inside the tests of large salps (e.g. Thetys vagina) 28 (J. Milisen, pers. obs.). Juveniles of Tremoctopus gracilis carry tentacles of cnidarians, often Portuguese man of war, which are held by the suckers on their dorsal arms and may function as defense or for capturing prey 29 . Argonauta argo Linnaeus, 1758 is regularly seen to attach to jellyfish 16,30 , and a detailed examination was performed of an Argonauta that was observed clasping and attacking a Phyllorhiza scyphomedusa 31 . Heeger et al. 31 wrote: "The formation of channels within the mesogloea of the jellyfish is of special importance. We conclude that the argonaut produced them by consuming parts of the exumbrella and mesogloea to connect to the gastral cavity of the medusa. This would enable the cephalopod to consume zooplankton from the gastrovascular system of the jellyfish via these channels while adhering to the exumbrella. Thereby the argonaut would use the efficient secondary mouth papillae of the oral arms of the jellyfish to collect prey items for itself. " The manner in which Haliphron holds the medusa in our observations is very similar to the description by Heeger et al. 31 . In Haliphron the medusa was held by the external bell with the oral part open within the folds of the arms, and the octopus's beak sometimes protruding into the subumbrellar space. Therefore, in addition to feeding directly on jellyfishes, Haliphron may target the stomach contents of the medusa, or even use the medusa as a tool to obtain more nutritious prey that are captured by the fringe of tentacles clasped within the octopus arms. The daytime distribution of Phacellophora in Monterey Bay is shallower than the few observations of Haliphron atlanticus (53.4 meters; n = 137). Whether Haliphron swims up at night to capture these medusae or eats them when they are dead and sinking down, remains speculation.
In situ deep-sea observations have provided new insights in the importance of gelatinous fauna in the ocean and its foodwebs 5,32 . An increasing number of large oceanic organisms (e.g. tuna) are found to feed on gelatinous zooplankton 33 . The importance of gelatinous zooplankton in diet studies may traditionally have been underestimated since digestion rates of gelatinous zooplankton are higher than squid and fish prey, and no hard parts are left to track ingestion 34 . The importance of gelatinous fauna in diets was long questioned because the caloric values of gelatinous tissue were thought to be too low. Haliphron has very low mass-specific metabolic rates, comparable to some medusae 35 , and to maintain its routine metabolism, this cephalopod does not require a lot of energy. Therefore the ingested gelatinous tissue may be sufficient to support its energetic requirements. Haliphron may also eat only the nutritious parts of the jellyfish, e.g. stomach contents or gonads. Selective feeding on the nutritious gonads of various large deepwater cnidarians has been observed in ocean sunfish 36 . If Haliphron indeed has a large jellyfish component in its ingested prey, it coincides with the observations that some of the largest oceanic predators (ocean sunfish, leatherback turtles), can attain very large sizes on a gelatinous diet.
In the North Atlantic H. atlanticus is an important prey of sperm whales, swordfish and blue sharks 33,34 . The observation of Haliphron feeding upon Phacellophora and other gelatinous organisms indicate that H. atlanticus may be a trophic link between gelatinous zooplankton and top predators. This further confirms the pivotal role that cephalopods and gelatinous zooplankton constitute in oceanic foodwebs, channeling energy from the bottom to the top of the oceanic food chain.