Most life in the deep sea environment relies on various pieces of food falling from the sunlit surface waters. This falling food can take many forms, much of it falls as marine snow, tiny dead and rotting animals, fish scales, and poop. At the end of their lives, larger creatures also fall to the seafloor, such as sharks, squid and whales. In a barren food desert, whale carcasses can become islands of feasting. Or copy this url:.
Standard definition video of chance encounters with large elasmobranch carcasses observed during Subsea 7 Hercules remotely operated vehicle ROV operations from the vessel M. Bourbon Oceanteam were analysed.
Four clips collected between 22 nd June and 14 th May , ranging from 29 seconds to 4 minutes 58 seconds in length were available for analysis. Full details of the video are shown in Table 1. The video data were collected as part of seafloor surveys of seafloor structures undertaken by BP Angola and partners in license block 18 offshore of Angola Figure 1.
The opportunistic nature of the data collection resulted in several limitations. No faunal samples were collected, so all taxonomic identifications are tentative Table 2 , based on consultation with a selection of taxonomic experts. There is no available indication of the sampling effort required to obtain these four observations and there was no inherent sampling design in the observations.
The industrial class ROVs that were used to undertake the surveys were not fitted with parallel lasers for determining scale, making it difficult to attain accurate size measurements of objects in the videos.
As a result the data reported here is primarily qualitative, using best estimates of size where necessary, based on previously published data. Quantitative values for comparison with previous studies were calculated by combining the occurrence data from the video with best estimates from the literature. Values for carcass size were used to estimate the time that each carcass had been on the seafloor, based on previously recorded rates of scavenging in the deep sea.
The mass and time values for each carcass were then integrated over the entire area bounding their occurrence convex hull to obtain an estimate of the mass flux to that area of seabed. Mass flux was converted to carbon flux according to empirically determined conversion factors reported in the literature. These calculations are sensitive to changes in the area that the carcass mass is integrated over and we chose the convex hull since it requires the fewest assumptions.
We consider this a conservative estimate since much of this area was not surveyed and may have contained further carcasses. During routine seabed surveys over the course of two years, the carcass of a whale shark and three mobulid rays were found by chance on the seafloor at bathyal depths on the Angola continental margin Figure 1. It is extremely rare to encounter natural food-falls; in five decades of deep-sea photography and exploration only nine vertebrate carcasses have ever been documented [10] — [12] , [15] , [27] , [37] — [40].
To find four in such close proximity is unprecedented, suggesting that large food-falls are common in the region. The cause of death of the animals identified here is unknown and most carcasses appear to have arrived at the seabed intact see below.
There is no targeted fishery for whale sharks and mobulid rays off Angola, but ship strikes and accidental entanglement are common sources of anthropogenic mortality [41] , [42]. Natural mortality is usually the result of opportunistic attacks by sharks and killer whales [43] — [45]. Whale sharks Rhincodon typus have only recently been documented in oceanic waters off of Angola, and appear to be more common in water depths over 1, m in this region [46].
This affinity for deep-water suggests that whale shark carcasses may be a common form of food-fall for deep-sea scavengers in this area. The remnants of a whale shark were found at a depth of m, resting dorsal-side up on the seafloor Figure 2A. Only the anterior part of the body remained, consisting of a fleshy head, pectoral fins, pectoral girdle and a portion of the spine trailing posteriorly.
Images have been enhanced. Originals and details of enhancements are available in Figure S1. Without an accurate scale we can only speculate on the actual size of the carcass. Of the 10 records of whale sharks off Angola reported by Weir [46] eight were estimated to be in the 5—7 m length-range. Additional records of large whale-sharks off Angola come from strandings records of individuals measuring If the mean length of the sighted sharks reported by Weir [46] is taken as representative we can estimate that carcass filmed here would be approximately 7.
With reference to the video footage, detailed anatomical measurements of an 8. The pectoral fin was 1. The carcass was attended by 18 zoarcids eelpouts , cf. Pachycara crassiceps , which have also been observed at baited camera traps in this area [49]. No active feeding on the carcass was observed, and most of the fish remained stationary on or near the carcass.
Although some zoarcids are thought to directly consume bait [50] , benthic fauna are their main prey [52] , [53] particularly small crustaceans such as amphipods [51] , [54] , [55]. Pachycara sp. Such grooves can bee seen in the head portion of the carcass, indicating that these fish have actively fed on the carcass Video S1.
Witte [50] noted that the dorsal part of the elasmobranch carcass was the primary site of consumption by all scavengers, which might explain why the posterior part of the carcass is missing. No other fauna were observed on or around the carcass, but the camera did not allow detection of low densities of macrofauna that may have been present.
This equates to a mass of 10— kg in weight, whereas individuals of Manta birostris are typically 4. The remains of an individual Mobula were found 1. The skeleton was mostly intact and articulated, with its anterior-posterior axis aligned in a southeast-northwest direction. There was very little flesh remaining, and most of the right wing and head region were covered in sediment.
The entire left wing was unsedimented, while the right was exposed revealing a fully articulated skeletal structure, held in place by connective tissue. The carcass was attended by at least 20 zoarcid fish, one of which appeared to be feeding to the right of the skull Video S2.
A singular hagfish, Myxine ios [59] was observed feeding on the left wing of the ray time in Video S2 , before swimming off to the west of the carcass. A grenadier, possibly Coryphaenoides marshalli or Coryphaenoides guentheri from depth distribution , was initially seen approaching the carcass from the west-southwest and abruptly changing direction towards the southeast when over the carcass.
It then paused and drifted for a few seconds before heading away to the east, without directly contacting the carcass. Throughout the observation a small benthic ray likely Bathyraja sp. At one point it approached the carcass and skirted its posterior flank, but did not seem to come into contact with it, and then moved away again. A synaphobranchid eel, likely Synaphobranchus kaupii , was observed slowly swimming directly over the carcass, but again did not interact with it before approaching the ROV and swimming away.
As this fish swam away another exactly like it was seen approaching the carcass, again from the northwest, and did not interact with the carcass. The only invertebrate observed was a starfish Asteroidea , lying a short distance to the west of the carcass.
The skeleton was articulated and bare of flesh, but both wings were covered in sediment, with only a small portion of the left wing tip protruding above the sediment. The remainder of the skeleton rested above the sediment and was attended by at least 54 zoarcid fish. Several fish were observed actively feeding on remnants of flesh inside the skeleton, but most were inactive, exhibiting roosting behaviour as they wait to prey upon small invertebrate scavengers Video S3.
An apparent zone of enrichment can be seen extending around the skeleton, demarcated by white mat, presumably made up of sulphide oxidising bacteria, common at whale-falls [17]. This mat represents the area of seafloor where organic matter from the carcass has become incorporated into the sediment and is being broken down anaerobically.
The video footage is not of sufficient quality to permit detection of individual macrofauna but no large aggregations of chemoautotrophic clams or other fauna typical of sulphide-rich sediments could be observed in or around the zone of enrichment. The relative size of the skeleton in relation to the zoarcids, coupled with much higher numbers of fish attending the skeleton suggests that it may be that of the larger manta ray, Manta birostris.
Other skeletal features also suggest this may be the case, but diagnostic features at the anterior end of the carcass are not visible in the video footage, so we are unable to confirm this M. Paig-Tran, personal communication. There is a single record of M. A third mobulid carcass Mobula sp. The skeleton was mainly articulated, with the exception of the left wing, the remains of which appeared to be deposited at the rear of the skeleton. Thin threads of flesh were hanging on the skull and a chunk of flesh at the posterior end of the spine.
The head of the carcass was pointing towards the north-northwest, indicating that it was not simply a resighting of carcass 1.
Furthermore, this carcass still had flesh visible suggesting it was more recently deposited than carcass 1. The carcass was attended by 13 zoarcid fish cf. Lycodes terranovae three of which were juveniles Video S4. No feeding was observed and fish remained stationary until disturbed by the ROV. The bottom of the ocean is a mysterious and harsh enviroment.
Animals down there have to deal with darkness, extreme cold, and incredible pressure. Yet despite these factors, an estimated 10 million or more species thrive in the deepest depths of the ocean. This forbidding realm supports the largest ecosystem on Earth, including the plunge of the Marianas Trench, in the Pacific more than 37, feet below sea level.
Because food is harder to come by in the deep sea, many of the creatures there have evolved novel ways to find and attract food.
One adaptation is bioluminescence, the ability of living things to produce their own light. Glowing animals can attract unsuspecting prey.
Consider the creature that Choy calls a galaxy siphonophore. It waits in the water for whatever swims into its orange curtain of tentacles.
And the black swallower fish: It uses its big jaw to swallow prey bigger than itself whole, like a snake. These different species show there are diverse ways to fill your belly in an unforgiving environment. One of the most common interactions that Choy and her colleagues observed were cephalopods like Gonatid squid, preying on fish. They are abundant in midwaters and play the role of both predator and prey in the food web.
Endowed with an insane metabolism, the voracious cephalopods are constantly eating.
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