Conceived and designed the experiments: LC IA AA. Performed the experiments: IA LC. Analyzed the data: LC IA. Contributed reagents/materials/analysis tools: AA AB LC IA. Wrote the paper: LC IA.
The authors have declared that no competing interests exist.
Stable isotopes of carbon and nitrogen were used to test the hypothesis that stomach content analysis has systematically overlooked the consumption of gelatinous zooplankton by pelagic mesopredators and apex predators. The results strongly supported a major role of gelatinous plankton in the diet of bluefin tuna (
An interest in gelatinous plankton has developed over the past decades after a long period of neglect by marine biologists
Avian and Rottini-Sandrini (1988)
Central to the top-down relaxation hypothesis is the hypothetical existence of a large community of pelagic predators that may opportunistically consume gelatinous plankton, thereby stabilizing their populations
Massive proliferations of gelatinous plankton in the Mediterranean have raised considerable public interest
Stomach content analysis has revealed the consumption of gelatinous plankton by several Mediterranean species of pelagic mesopredators and apex predators
All of the species sampled were caught for purposes other than research, except jellyfishes, salps, hyperiidean amphipods and euphausiids. No specific approval is required in Spain to undertake research on samples supplied by official channels and coming from by-catch of commercial fishing vessels. Loggerhead turtles, fin whales and bottlenose dolphins are protected by Spanish laws and hence samples were collected by the Marine Animals Recovery Center (CRAM), the organism officially designated by the Catalonian regional government to collect stranded marine animals, undertake necropsies and distribute samples among research groups.
Samples were collected from 2006 to 2007 in the northwestern Mediterranean, between the Iberian Peninsula and the Balearic islands. The area has supported very dense populations of gelatinous plankton since 2003, with pink jellyfish (
Potential prey were also sampled from the catch of commercial vessels operating in the same area (anchovy (
White dorsolateral muscle was sampled from all fish, as well as mantle from the cephalopods and carapace scutes from loggerhead sea turtles. Gelatinous plankton and crustaceans were fully homogenized. All of the species had a sample size of 5, except for blue butterfish, and copepod, hyperiidean and krill samples were collective. Samples were stored at −20°C prior to analysis.
Once thawed, tissues were dried at 60°C and ground to a fine powder, and their lipids were then extracted with a chloroform/methanol (2∶1) solution. Crustacean samples were split in two subsamples. One of them was treated with O.5 N ClH to remove the inorganic carbonates of the skeleton and avoid any bias in the δ13C. However, acidification may modify the relative concentration of N isotopes, so the other subsample was used to determine the δ15N value. All of the samples were weighed into tin cups, combusted at 1,000°C, and analyzed in a Flash 1112 IRMS Delta C Series EA Thermo Finnigan continuous flow isotope ratio mass spectrometer. A Carlo Erba Flash 112 elemental analyzer coupled to the isotope ratio mass spectrometer was used to measure the % C and % N of the dry weight. Stable isotope abundances were expressed in δ notation according to the following expression:
The proximate chemical composition of pink jellyfish, salps, mackerels and longfin squids was assessed to determine energy density. Once thawed, samples were weighed and dried at 100°C until a constant weight was reached. The moisture content was calculated by gravimetric difference between wet and dry mass
ANOVA and a Tukey post-hoc test, conducted with the PASW 17 software package, were used to test differences in the concentrations of stable isotopes of potential prey. As SIAR requires that the variability associated with sources is normally distributed
The Bayesian mixing model SIAR (Stable Isotope Analysis in R)
The model parameters were the following: the isotope ratios and the elemental concentrations of the potential food sources, the isotope ratio of tissue and the trophic shift, or isotopic enrichment, for carbon and nitrogen from prey to predator. Prey-to-predator isotopic enrichment for fishes, mammals and loggerhead sea turtles were taken from Reich et al. (2008)
Although SIAR incorporates uncertainty about diet-tissue isotopic discrimination factors in the form of standard deviation, we conducted a sensitivity analysis running SIAR for bluefin tuna with diet-tissue isotopic discrimination factors ranging from 1.1 to 2.3‰ for δ13C and from 2.2 to 3.4‰ and δ15N.
Data are usually shown as mean ± standard deviation (SD), but the feasible contribution of potential prey species to the diet is reported as the mean and 95% credibility interval.
Potential prey considered: pelagic crustaceans (solid squares), gelatinous plankton (empty squares), squid (solid triangles) and small pelagic and mesopelagic fish (empty triangles). Error bars show standard deviation.
Species | Common name | n | δ13 C | δ15 N | ||
mean | ±SD | mean | ± SD | |||
|
||||||
|
Copepods | A | −22.3 | 1.0 | 2.8 | 0.5 |
|
Fried egg jellyfish | 5 | −17.4 | 0.2 | 1.6 | 0.3 |
|
European anchovy | 5 | −18.5 | 0.6 | 9.8 | 0.8 |
Hyperiidae | Hyperideans | A | −19.0 | 1.2 | 5.6 | 0.5 |
|
Jewel lanternfish | 5 | −18.6 | 0.2 | 10.2 | 0.4 |
|
European common squid | 5 | −17.7 | 0.5 | 9.5 | 0.9 |
|
Krill | A | −20.8 | 0.7 | 5.2 | 0.4 |
|
Pink jellyfish | 5 | −17.8 | 0.6 | 5.6 | 0.5 |
|
European pilchard | 5 | −18.0 | 0.2 | 8.7 | 0.2 |
|
Salp | 5 | −19.7 | 0.6 | 3.9 | 0.3 |
|
European flying squid | 5 | −17.8 | 0.1 | 11.0 | 0.1 |
|
||||||
|
Bullet tuna | 5 | −18.1 | 0.3 | 9.5 | 0.5 |
|
Fin whale | 5 | −18.4 | 0.1 | 8.7 | 0.1 |
Loggerhead sea turtle | 5 | −16.3 | 0.4 | 10.1 | 1.7 | |
Loggerhead sea turtle | 5 | −17.6 | 0.2 | 6.7 | 0.4 | |
|
Dolphinfish | 5 | −18.3 | 0.3 | 9.8 | 0.7 |
|
Little tunny | 5 | −17.2 | 0.1 | 10.4 | 0.4 |
|
Leerfish | 5 | −17.1 | 0.3 | 13.1 | 1.0 |
|
Sunfish | 5 | −17.6 | 0.5 | 7.7 | 0.4 |
|
Bluefish | 5 | −16.9 | 0.3 | 14.8 | 0.4 |
|
Blue shark | 5 | −17.2 | 0.7 | 13.3 | 0.4 |
|
Atlantic bonito | 5 | −16.8 | 0.3 | 12.8 | 1.2 |
|
Mackerel | 5 | −18.5 | 0.9 | 11.4 | 0.4 |
|
Amberjack | 5 | −17.7 | 0.2 | 11.3 | 0.6 |
|
Striped dolphin | 5 | −17.3 | 0.4 | 12.1 | 0.8 |
|
Blue butterfish | 4 | −17.3 | 0.3 | 10.8 | 0.2 |
|
Spearfish | 5 | −17.8 | 0.4 | 10.1 | 0.7 |
|
Albacore | 5 | −17.8 | 0.4 | 11.0 | 0.4 |
Bluefin tuna | 5 | −18.3 | 0.3 | 10.3 | 0.6 | |
Bluefin tuna | 5 | −17.7 | 0.4 | 10.6 | 0.3 | |
|
Pompano | 5 | −17.5 | 0.4 | 11.2 | 0.3 |
|
Horse mackerel | 5 | −17.6 | 0.2 | 10.5 | 0.5 |
Swordfish | 5 | −17.8 | 0.3 | 11.4 | 0.4 | |
Swordfish | 5 | −17.8 | 0.7 | 11.2 | 0.2 |
:
The ratios of stable isotopes in bluefish, blue shark, leerfish, bonito, striped dolphins and neritic loggerhead sea turtles (
Solid circles represent the average stable isotope ratios of each consumer after correcting for diet-tissue isotopic discrimination and error bars show standard deviation. Other symbols show the average stable isotope ratios of potential prey: pelagic crustaceans (solid squares), gelatinous plankton (empty squares), squid (solid triangles) and small pelagic and mesopelagic fish (empty triangles).
A solid circle represents the average stable isotope ratios of whales after correcting for diet-tissue isotopic discrimination and error bars show standard deviation. Other symbols show the average stable isotope ratios of potential prey: pelagic crustaceans (solid squares), gelatinous plankton (empty squares), squid (solid triangles) and small pelagic and mesopelagic fish (empty triangles).
Species | Common name | Diet | References | |
Auxis rochei | Bullet tuna | F,C,E,H,(U),(Cn) | Mostarda et al. 2007 |
|
|
Fin whale | E | Laran et al. 2010 |
|
|
Loggerhead turtle | F,C,(U) | Tomás et al. 2001 |
|
Revelles et al. 2007 |
|
|||
|
Dolphinfish | F,D,H,C,(Cn) | Massutí et al. 1998 |
|
|
Little tunny | F,C | Kyrtatos 1982 |
|
Falautano et al. 2007 |
|
|||
|
Leerfish | F | Bennett 1989 |
|
|
Bluefish | F,C | Buckel et al. 1999 |
|
|
Blue shark | C,Ct,F | Henderson et al. 2001 |
|
|
Atlantic bonito | F,(U) | Kyrtatos 1982 |
|
Campo et al. 2006 |
|
|||
|
Mackerel | F,E,H | Kyrtatos 1982 |
|
|
Amberjack | F,C,E | Matallanas et al. 1995 |
|
|
Striped dolphin | C, F | Blanco et al. 1995 |
|
Meotti and Podestà1997 |
|
|||
Özturk et al. 2007 |
|
|||
|
Spearfish | F,C,(U),(Cn) | Castriota et al. 2008 |
|
Romeo et al. 2009 |
|
|||
|
Albacore | F,H,E,C,U,(Cn) | Consoli et al. 2008 |
|
|
Bluefin tuna | F,C,D | Morovic 1961 |
|
Kyrtatos 1982 |
|
|||
Orsi Relini et al. 1995 |
|
|||
Sanz Brau 1990 |
|
|||
Sinopoli et al. 2004 |
|
|||
|
Horse mackerel | E, F | Ben Salem 1988 |
|
|
Swordfish | F, C,(U),(Cn) | Chalabi and Ifrene 1992 |
|
Orsi Relini et al. 1995 |
|
|||
Romeo et al. 2009 |
|
The diet column reports the preys contributing at least 5% in weight or volume to stomach contents (F: Teleostei; D: Decapoda, H: Hyperiidea, E: Euphausiids; C: Cephalopoda, Cn: Cnidaria, Ct: Cetaceans; U: Urochordata). Consumption of cnidarians and urochordata representing less than 5% is reported in brackets.
In contrast, the ratios of stable isotopes in bluefin tuna, little tunny, spearfish and swordfish (
Solid circles represent the average stable isotope ratios of each consumer after correcting for diet-tissue isotopic discrimination and error bars show standard deviation. Other symbols show the average stable isotope ratios of potential prey: pelagic crustaceans (solid squares), gelatinous plankton (empty squares), squid (solid triangles) and small pelagic and mesopelagic fish (empty triangles).
Nekton 1: sardine. Nekton 2: anchovy, lanternfish, horse mackerel and longfin squid. Nekton 3: mackerel and shortfin squid. Results are shown as 95, 75 and 25% credibility intervals for each prey.
Solid circles represent the average stable isotope ratios of each consumer after correcting for diet-tissue isotopic discrimination and error bars show standard deviation. Other symbols show the average stable isotope ratios of potential prey: pelagic crustaceans (solid squares), gelatinous plankton (empty squares), squid (solid triangles) and small pelagic and mesopelagic fish (empty triangles). Nekton: anchovy, lanternfish, horse mackerel and shortfin squid. Results are shown as 95, 75 and 25% credibility intervals for each prey.
The concentration of stable isotopes in the remaining species suggested diets with varying combinations of fishes, cephalopods and crustaceans (
Solid circles represent the average stable isotope ratios of each consumer after correcting for diet-tissue isotopic discrimination and error bars show standard deviation. Other symbols show the average stable isotope ratios of their potential prey: pelagic crustaceans (solid squares), gelatinous plankton (empty squares), squid (solid triangles) and small pelagic and mesopelagic fish (empty triangles).
Solid circles represent the average stable isotope ratios of each consumer after correcting for diet-tissue isotopic discrimination and error bars show standard deviation. Other symbols show the average stable isotope ratios of their potential prey: pelagic crustaceans (solid squares), gelatinous plankton (empty squares), squid (solid triangles) and small pelagic and mesopelagic fish (empty triangles). Nekton: anchovy, lanternfish, horse mackerel and shortfin squid. Results are shown as 95, 75 and 25% credibility intervals for each prey.
Nekton: anchovy, lanternfish, horse mackerel and longfin squid. Results are shown as 95, 75 and 25% credibility intervals for each prey.
The proximate chemical composition and energy density of the considered potential prey are shown in
Pink jellyfish | Salp | Mackerel | Longfin squid | |
|
5 | 5 | 5 | 5 |
|
42±9 | 19±14 | 248±31 | 152±23 |
|
96.3±0.1 | 95.8±0.5 | 72.4±0.5 | 81.3±0.4 |
|
3.4±0.1 | 3.6±0.5 | 2.8±0.3 | 2.2±0.2 |
|
0.2±0.1 | 0.2±0.1 | 12.3±0.4 | 13.2±0.3 |
|
0.9±0.1 | 1.0±0.2 | 13.2±0.2 | 3.3±0.2 |
|
0.41±0.1 | 0.43±0.1 | 8.4±0.5 | 5.2±0.8 |
The use of stable isotopes for dietary studies relies on three major assumptions. First, that isotope fractionation from prey to predator is known. Fractionation is species and stage specific and controlled experiments in captivity are the best method to calculate diet-tissue isotopic discrimination factors. This type of experimental data were available only for the loggerhead sea turtle
The second assumption is that the variability in the ratios of stable isotopes of the potential prey is not obscured by migration between contrasting isoscapes. The western Mediterranean and the adjoining Atlantic differ in their isotopic baselines
The third major assumption is that differences in the concentration of stable isotopes in the potential prey are large enough to allow proper discrimination among potential prey. Although statistically significant differences existed between all the species of macrozooplankton considered in the present study, there was considerable overlap in their ranges, as was also true for nekton. As a consequence, the performance of SIAR in resolving diet breakup within those two groups was often poor. However, for several species, the results were unambiguous when the ratios of stable isotope were combined with published information about stomach contents.
On this ground, seven of the species considered here are unlikely to consume relevant amounts of gelatinous plankton: bluefish, blue shark, bonito, fin whales, leerfish, loggerhead sea turtles (in the neritic stage) and striped dolphins. Although detailed studies on the stomach contents of Mediterranean fin whales are missing, these cetaceans are thought to rely primarily on crustaceans
Fish and squid also dominate the stomach contents of bluefin tuna, little tunny, swordfish and spearfish
Albacore, mackerel, bullet tuna, dolphinfish, amberjack and horse mackerel also consume fishes and squids, but crustaceans are relatively abundant in their stomach contents (
Finally, stable isotopes confirmed the reliance of oceanic loggerhead sea turtles and ocean sunfish on gelatinous plankton. The differences in the ratios of stable isotopes of oceanic and neritic loggerhead sea turtles reported here are consistent with the satellite telemetry data reported by Cardona et al. (2009)
The overall evidence presented here suggests the existence of a guild of gelatinous plankton consumers including two specialists (ocean sunfish and loggerhead sea turtles in the oceanic stage) and several opportunists (bluefin tuna, little tunny, spearfish and swordfish. However, some further calculations are needed to demonstrate that massive consumption of gelatinous zooplankton by these species is energetically possible, considering the low energy density of gelatinous plankton (
The daily ration of captive bluefin tuna fed with fishes and squids ranges from 4.3% to 1.5% body mass, depending on tuna size
These quantities may seem large, but the biomass of gelatinous zooplankton in the epipelagic region of the Mediterranean Sea ranges usually 1–10 kg 100 m−3, with the biomass of the pink jellyfish reaching sometimes values as high as 24 kg 100 m−3
The results here reported demonstrate the plausibility that top predators control the abundance of gelatinous zooplankton, but do not prove it. Further research is needed to confirm that bluefin tuna, little tunny, spearfish and swordfish consume large amounts of gelatinous plankton across the Mediterranean. Stable isotope ratios from different regions and years with contrasting abundance of gelatinous zooplankton will be extremely useful as confirmatory evidence. The use of other intrinsic tracers, like fatty acids, can also be useful to precise the proportion of gelatinous in the diet of these species and perhaps would help to better resolve the consumption of gelatinous zooplankton by species like mackerel, bullet tuna or dolphinfish. Behavioral observations of tuna as they swim across jellyfish swarms will also be extremely helpful to understand how gelatinous plankton is handled and consumed. And last, but not least, detailed data on the demography of gelatinous zooplankton are urgently needed to allow modeling how the depletion of top predators might have be caused, together with climate forcing, recent jellyfish outbreaks.
Authors are grateful to Joaquim Puigvert and Oscar Jerez, skippers of the fishing vessels