Not all jellyfish are equal: isotopic evidence for inter- and intraspecific variation in jellyfish trophic ecology

Abstract

Jellyfish are highly topical within studies of pelagic food-webs and there is a growing realisation that their role is more complex than once thought. Efforts being made to include jellyfish within fisheries and ecosystem models are an important step forward, but our present understanding of their underlying trophic ecology can lead to their oversimplification in these models. Gelatinous zooplankton represent a polyphyletic assemblage spanning >2,000 species that inhabit coastal seas to the deep-ocean and employ a wide variety of foraging strategies. Despite this diversity, many contemporary modelling approaches include jellyfish as a single functional group feeding at one or two trophic levels at most. Recent reviews have drawn attention to this issue and highlighted the need for improved communication between biologists and theoreticians if this problem is to be overcome. We used stable isotopes to investigate the trophic ecology of three co-occurring scyphozoan jellyfish species (Aurelia aurita, Cyanea lamarckii and C. capillata) within a temperate, coastal food-web in the NE Atlantic. Using information on individual size, time of year and δ13C and δ15N stable isotope values, we examined: (1) whether all jellyfish could be considered as a single functional group, or showed distinct inter-specific differences in trophic ecology; (2) Were size-based shifts in trophic position, found previously in A. aurita, a common trait across species?; (3) When considered collectively, did the trophic position of three sympatric species remain constant over time? Differences in δ15N (trophic position) were evident between all three species, with size-based and temporal shifts in δ15N apparent in A. aurita and C. capillata. The isotopic niche width for all species combined increased throughout the season, reflecting temporal shifts in trophic position and seasonal succession in these gelatinous species. Taken together, these findings support previous assertions that jellyfish require more robust inclusion in marine fisheries or ecosystem models.

Stable isotope‐based location in a shelf sea setting: accuracy and precision are comparable to light‐based location methods

“…Between-individual variances in jellyfish isotope compositions were estimated from jellyfish sampled both in 2011 (MacKenzie et al 2014) and in 2015 as 1Á69& and 1Á04& for d 13 C cor and d 15 N values, respectively. These between-individual variance estimates are similar to those provided for gelatinous zooplankton by Fleming et al (2015) and Nagata et al (2015), particularly when accounting for the marked effect of size on isotopic variance in the Fleming et al (2015) data. Total uncertainty in the assignment isoscape was given by: r 2 isoðx;yÞ ¼ r 2 k:isoðx;yÞ þ r 2 m:isoðx;yÞ þ r 2 bi:isoðx;yÞ eqn 2 where r 2 isoðx;yÞ is the pooled variance associated with the isoscape prediction, r 2 k:isoðx;yÞ is the variance associated with the spatial interpolation model, r 2 m:isoðx;yÞ is the variance associated with measurement error and r 2 bi:isoðx;yÞ is the variance associated with in situ between-individual variation.…”

“…Lion's mane jellyfish are opportunistic pelagic predators consuming a range of macrozooplankton and larval/juvenile fish. The jellyfish sampled in 2015 encompassed a relatively narrow size range from 80 to 240 mm bell diameter equivalent to a wet mass of c. 100-500 g, and no systematic size-related difference in trophic level between sampled individuals is expected (Fleming et al, 2015). Mass balance (Ecopath) modelling of the North Sea community (Mackinson & Daskalov 2007) estimates scallop and gelatinous zooplankton trophic levels as 2Á8 and 3Á6, respectively.…”

Coastal leatherback turtles reveal conservation hotspot

“…The small range of δ 15 N values for leatherback turtles observed in this study (3.4‰ when excluding a single individual of 15.1‰) suggests that this species was foraging on the same trophic level, whether in coastal or oceanic habitats. Indeed, leatherback turtles appear to be obligate gelatinous zooplanktivores and, even though jellyfish can vary greatly in δ 15 N values49, leatherback turtles likely target specific, and often larger, jellyfish species50. Conversely, loggerhead turtles exhibit a breadth of dietary preferences, including a variety of invertebrate and fish species18.…”

Population genetic analyses reveal distinct geographical blooms of the jellyfishRhizostoma octopus(Scyphozoa)

“…This inter‐annual persistence of an asexually reproducing sessile life stage can lead to the regular re‐occurrence of blooms in specific locations (Houghton et al ., , b; Lilley, Houghton & Hays, ), population structuring (Pitt & Kingsford, ), and eventual phylogenetic differentiation. As efforts to incorporate jellyfish more effectively into ecosystem and fisheries models gather momentum (Pauly et al ., ; Brotz et al ., ; Fleming et al ., ), such information is important when considering the temporal and spatial integrity of apparently isolated bloom events (Lee et al ., ).…”