Direct evidence of an efficient energy transfer pathway from jellyfish carcasses to a commercially important deep-water species

Here we provide empirical evidence of the presence of an energetic pathway between jellyfish and a commercially important invertebrate species. Evidence of scavenging on jellyfish carcasses by the Norway lobster (Nephrops norvegicus) was captured during two deployments of an underwater camera system to 250–287 m depth in Sognefjorden, western Norway. The camera system was baited with two Periphylla periphylla (Scyphozoa) carcasses to simulate the transport of jellyfish detritus to the seafloor, hereby known as jelly-falls. N. norveigus rapidly located and consumed a large proportion (>50%) of the bait. We estimate that the energy input from jelly-falls may represent a significant contribution to N. norvegicus energy demand (0.21 to 10.7 times the energy required for the population of N. norvegicus in Sognefjorden). This potentially high energetic contribution from jelly-falls highlights a possible role of gelatinous material in the support of commercial fisheries. Such an energetic pathway between jelly-falls and N. norvegicus could become more important with increases in jellyfish blooms in some regions.

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 m 2 ). For each BUC deployment, the bait plate was baited with two defrosted P. 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 to N. norvegicus daily energy intake rates (kJ d −1 ) using daily food intake data from a previous study that was adjusted for temperature using Q 10 14 . Also, estimates of the energy content of P. 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 from 9 .
Data on the density of N. 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 method 15 . 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 study 16

Results and Discussion
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 as M. glutinosa, P. borealis and Munida sp., also consumed the bait, but often declined in abundance when N. 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 first N. norvegicus arriving 24 (first deployment) and 18 minutes (second deployment) after the lander reached the seafloor. In both deployments N. 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 of N. 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 .
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 °C 14 . At an in-situ temperature of 7.7 °C in Sognefjorden, Q 10 -adjusted energy intake rates would be 1.6 (Q 10 = 2) to 1.4 (Q 10 = 3) kJ d −1 . Therefore, assuming that these energy intake rates are similar to that of the N. 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 by N. norvegicus (mean energy content of 16.7 kJ g dry weight −1 ) would provide enough energy for a single N. norvegicus to survive for 90 (Q 10 = 2) to 103 days (Q 10 = 3).
Despite the relatively low energy content of jellyfish material 24 , 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 zooplankton 25 , and salps are an important contribution to the diets of bentho-pelagic fish 26 . Commercially exploited invertebrates have been recorded in traps baited with the giant jellyfish Nemopilema nomurai 27 . However, to the best of our knowledge, this is the first study that has directly photographed N. 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 of P. periphylla is high (100-300 individuals m −2 ), and biomasses here are several orders of magnitude higher than those in the open ocean 7 . This is also true for Lurefjorden, where large pelagic populations of P. periphylla contribute (as jelly-falls) to an efficient jelly-pump that can be as important in transporting C and N as the classic phytodetritus pump 5 . Therefore, jellyfish carcass flux data from Lurefjorden was used to estimate how important jellyfish carcasses could be to N. 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 of N. 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 for N. 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 for N. norvegicus in Sognefjorden. This high energetic contribution from jelly-falls to N. norvegicus clearly highlights a potentially important role of gelatinous material in the support of a commercially important species along the Norwegian margin. Even at high N. 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 of N. norvegicus populations. Although information on N. 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 of N. 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 on N. 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 8,28-30 . 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 grenadiers 31 . 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.