Carrion that sinks to the seafloor represents a major energy transfer in the marine environment¹. Carrion is detected, located and consumed by marine scavengers², enabling the transfer of nutrients and energy back into pelagic and benthic marine food webs³. The aggregation of jellyfish carcasses on the seafloor (jelly-falls) are also a source of organic matter input to the benthos^4,5,6. Recently, increased frequencies of jellyfish blooms have been observed in a number of regions around the world, including in Norwegian fjords, wherePeriphylla periphylla is now highly abundant^7,8. A growing body of evidence suggests that the role of gelatinous zooplankton in the biological carbon pump in jellyfish-dominated fjords may be significant, and jelly-falls are actively scavenged here. For example, rapid scavenging on jelly-falls was first shown in Sognefjorden in 2012⁹, with scavenging on jelly-falls being dominated by Atlantic hagfish (Myxine glutinous), galatheid crabs (Munida sp.) and decapod shrimp (Pandalus borealis).

The Norway lobster (Nephrops norvegicus) is an economically important commercial species in the Atlantic¹⁰, and is common in Norwegian coastal regions¹¹. In Norway, the revenue from theN. norvegicus fishery exceeded 3 million US dollars in 2015¹². Although the species has been shown to predate and scavenge on a wide range of marine species, primarily decapod crustaceans and fish^13,14, it has never been observed scavenging on jelly-falls. Here, we describe the first photo-documentation ofN. norvegicus rapidly scavenging on jellyfish carcasses, and provide empirical evidence of the presence of an energetic pathway between jellyfish populations and this commercially-important species. The role of jelly-falls in the energy budget ofN. norvegicus is also estimated using values for energy input from jelly-falls andN. norvegicus energy demand.

Two baited underwater camera (BUC) deployments were made at 250 and 287 m across the outer fjord sill (061° 04.476′ N, 004° 59. 236′ E and 061° 04.087′ N, 005° 00. 378′ E respectively) of Sognefjorden, western Norway in June 2016. Both deployments lasted approximately 10 hours and were conducted during the day (09:10 to 19:07 UTC) and the night (22:25 to 08:02 UTC) respectively. Photographic still images of the jellyfish carcass and attending fauna were taken every 2 minutes by a deep-sea digital single lens reflex camera (Ocean Imaging Systems DSC 24000) system positioned 1.5 meters directly above a square bait plate (0.5 m²). For each BUC deployment, the bait plate was baited with two defrostedP. periphylla carcasses (~266 g ± 26, mean ± range). The number of scavengers from each species at the bait, the maximum number of scavengers observed at the bait at a single time (Max_N, a proxy for scavenger abundance) and the time to first scavenger arrival (t_arrival) were recorded from photographic images from each deployment. The flux of jellyfish material from the water column as jelly-falls (kJ m⁻² d⁻¹) to the seafloor was estimated using mass input data and bomb-calorimetry analysis described in previous studies^5,9. Jellyfish carrion flux rates were compared toN. norvegicus daily energy intake rates (kJ d⁻¹) using daily food intake data from a previous study that was adjusted for temperature using Q₁₀¹⁴. Also, estimates of the energy content ofP. periphylla tissue that were attached to each BUC (kJ g dry weight d⁻¹) were calculated based on the mass of jellyfish and bomb calorimetry analysis from⁹.

Data on the density ofN. norvegicus on the seafloor is required to determine the contribution of jelly-falls to their energy demand. Previous BUC studies have calculated the density of scavengers using the t_arrival method¹⁵. This model works well with abyssal t_arrival data sets, where t_arrival is generally longer (e.g. >100 minutes) than for datasets collected from shallower depth zones, as highlighted in a previous study¹⁶. Therefore,N. norvegicus seafloor densities in Sognefjorden were based on minimum and maximumN. norvegicus densities from other boreal coastal/fjord environments. The data used came from earlier studies in the Firth of Clyde (0.10 to 0.55 individuals m⁻², ¹⁷), Scottish sea lochs (Loch Torridon, (0.13 individuals m⁻², ¹⁸) and Loch Aline (0.18 individuals m⁻², ¹⁹)), the Irish Sea (0.13 to 0.31 individuals m⁻²)²⁰), the Kattegat and Skagerrak (0.2 to 0.4 individuals m⁻², ¹¹) and the North Sea (0.09 to 0.73 individuals m⁻²)²¹.

Data Availability Statement

No restrictions exist on the availability of material and data.

A number of different scavenger species consumed the bait in both BUC deployments. Hagfish (Myxine glutinosa) always arrived at the bait first (t_arrival = 2 minutes and 14 minutes). Other scavengers, such asM. glutinosa, P. borealis andMunida sp., also consumed the bait, but often declined in abundance whenN. norvegicus was present (Fig. 1a and b). A maximum of 1 N. norvegicus arrived and fed at the bait in deployment 1, while a maximum of 2 N. norvegicus were observed in deployment 2 (Fig. 2), with the firstN. norvegicus arriving 24 (first deployment) and 18 minutes (second deployment) after the lander reached the seafloor. In both deploymentsN. norvegicus consumed a large proportion (>50%) of the bait.N. norvegicus first removed what remained of the nutritional gonad tissue, and then continued to feed on the remaining mesoglea tissue.N. norvegicus feed during day-light and night hours. It was not possible to detect any influence of time of day on the abundance ofN. norvegicus (Fig. 1) owing to the low abundance of animals observed, yet this species has been observed to display diurnal patterns of emergence at the depths where we photographed it^22,23.

Maximum number ofNephrops norvegicus and other scavengers (inc.Myxine glutinosa,Munida sp. andPandalus borealis) observed in the BUC deployment at (a) 250 m (arrival time of BUC on seafloor 07:10 UTC; 19 June 2016) and (b) 287 m (arrival time of BUC on seafloor 20:25 UTC; 21 June 2016). Time represents the minutes that elapsed after arrival of the BUC on the seafloor.

Full size image

(A) TwoNephrops norvegicus feeding and approaching thePeriphylla periphylla bait; and (B) an individualN. norvegicus removing and feeding on a large portion of gelatinous mesoglea at the bait plate. White lines are spaced 10 cm apart.

Full size image

The energy intake rate of a 26 g N. norvegicus in the Firth of Clyde, Scotland, was found to be approximately 1.97 kJ d⁻¹ at a mean temperature of 11 °C¹⁴. At anin-situ temperature of 7.7 °C in Sognefjorden, Q₁₀-adjusted energy intake rates would be 1.6 (Q₁₀ = 2) to 1.4 (Q₁₀ = 3) kJ d⁻¹. Therefore, assuming that these energy intake rates are similar to that of theN. norvegicus individuals photographed in our study (N. norvegicus mass of ~29.8 g estimated from length-mass relationships), 50% of the jellyfish bait consumed in our experiments byN. norvegicus (mean energy content of 16.7 kJ g dry weight⁻¹) would provide enough energy for a singleN. norvegicus to survive for 90 (Q₁₀ = 2) to 103 days (Q₁₀ = 3).

Despite the relatively low energy content of jellyfish material²⁴, it is known to be an important food source to a variety of marine predators. For example, the leatherback turtle (Dermochelys coriacea) relies upon a diet of low energy-content gelatinous zooplankton²⁵, and salps are an important contribution to the diets of bentho-pelagic fish²⁶. Commercially exploited invertebrates have been recorded in traps baited with the giant jellyfishNemopilema nomurai²⁷. However, to the best of our knowledge, this is the first study that has directly photographedN. norvegicus feeding on a gelatinous organism, and attempted to quantify the importance of jelly-falls as an energy resource to this particular commercially-exploited invertebrate species.

Jellyfish are known to dominate several fjords along the Norwegian coast, including in Lurefjorden and Sognefjorden^7,8. In Sognefjorden, the abundance ofP. periphylla is high (100–300 individuals m⁻²), and biomasses here are several orders of magnitude higher than those in the open ocean⁷. This is also true for Lurefjorden, where large pelagic populations ofP. periphylla contribute (as jelly-falls) to an efficient jelly-pump that can be as important in transporting C and N as the classic phytodetritus pump⁵. Therefore, jellyfish carcass flux data from Lurefjorden was used to estimate how important jellyfish carcasses could be toN. norvegicus communities in Sognefjorden. The flux of jelly-fall material transported to the seafloor in Lurefjorden between November 2010 and November 2011 ranged from 12.5 mg to 72.8 mg C m⁻² d⁻¹ or 0.5 to 3.0 kJ m⁻² d⁻¹. The density ofN. norvegicus in similar environments in other regions of Northern Europe ranges from a minimum of 0.10 to 0.73 individuals m⁻²¹⁷. Therefore, assuming similar seafloor densities forN. norvegicus in Sognefjorden, and similar gelatinous carrion flux rates in both Lurefjorden and Sognefjorden, and a conservative consumption of half of the jellyfish, daily jelly-fall fluxes could provide 0.21 to 10.7 times the daily energy requirement forN. norvegicus in Sognefjorden. This high energetic contribution from jelly-falls toN. norvegicus clearly highlights a potentially important role of gelatinous material in the support of a commercially important species along the Norwegian margin. Even at highN. norvegicus densities (0.73 m⁻²) and low gelatinous flux rates (0.5 kJ m⁻² d⁻¹), jelly-falls potentially still provide almost a quarter of the daily energetic demands ofN. norvegicus populations. Although information onN. norvegicus stock sizes have not been collected within fjords, population-size information combined with the type of data presented here may enable the total number ofN. norvegicus that can be supported by jellyfish carrion to be calculated. This represents valuable information for fisheries management. Such estimates could be further improved with data onN. norvegicus foraging patterns, scavenging rates, and the contribution of other food sources to their diets.

This work demonstrates that jelly-falls can provide an important source of nutrition to a commercially-important species in Norway, and suggests that energy transfer pathways from jellyfish to benthic species may become more important in regions where jellyfish blooms presently occur, or are becoming more common (e.g. in numerous Norwegian fjords). There is evidence that the role of fish in some pelagic ecosystems may decline, and by inference, the transport of fish carrion to the seafloor, with increasing jellyfish biomass^8,28,29,30. Carrion supply influences deep-sea scavenger community dynamics and changes in the amount of fish carrion reaching the seafloor have been linked to the abundance of deep-sea grenadiers³¹. The findings presented here provide empirical evidence that a loss of energetic resources from fish (and other pelagic animal) carrion to deep-water scavengers could potentially be partially offset by sinking gelatinous material.

We would like to thank Captain Leon Pedersen of the RV “Solvik” for skillful assistance at sea, and Nick Higgs and an anonymous reviewer for suggesting improvements to the manuscript. This research was funded by the Norwegian Research Council grant:Combined effects of multiple organic stressors from jellyfish blooms and aquaculture operations on seafloor ecosystems (244572).

Authors and Affiliations

1. Akvaplan-niva, Fram Centre, 9296, Tromsø, Norway

   Kathy M. Dunlop

2. The Lyell Centre for Earth and Marine Science and Technology, Heriot Watt University, Edinburgh, EH14 4AP, UK

   Kathy M. Dunlop & Andrew K. Sweetman

3. International Research Institute of Stavanger, Prof. Olav Hanssensvei 15, 4021, Stavanger, Norway

   Kathy M. Dunlop

4. National Oceanography Centre, University of Southampton, Waterfront Campus, European Way, Southampton, SO14 3ZH, UK

   Daniel O. B. Jones

Contributions

Drs. K. Dunlop, D.O.B. Jones and Dr. A.K. Sweetman were all involved in the field data collection. Dr. K. Dunlop analysed the images and data, and all authors contributed to the writing of the manuscript. All authors reviewed the manuscript prior to submission.

Corresponding author

Correspondence toKathy M. Dunlop.

Competing Interests

The authors declare that they have no competing interests.

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