# Jellyfish as an alternative source of food for opportunistic fishes

Raquel Marques, Corinne Bouvier, Audrey M. Darnaude, Juan-Carlos

Molinero, Cyrille Przybyla, Solenn Soriano, Jean-Antoine Tomasini, Delphine

Bonnet

► To cite this version:

Raquel Marques, Corinne Bouvier, Audrey M. Darnaude, Juan-Carlos Molinero, Cyrille Przybyla, et al.. Jellyfish as an alternative source of food for opportunistic fishes. Journal of Experimental Marine Biology and Ecology, 2016, 485, pp.1 - 7. 10.1016/j.jembe.2016.08.008. hal-01924189

HAL Id: hal-01924189

https://hal.umontpellier.fr/hal-01924189v1

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# Jellyfish as an alternative source of food for opportunistic fishes

Marques Raquel<sup>1,2*</sup>, Bouvier Corinne<sup>1</sup>, Darnaude Audrey M.<sup>1</sup>, Molinero Juan-Carlos<sup>3</sup>, Przybyla Cyrille<sup>4</sup>, Soriano Solenn<sup>5</sup>, Tomasini Jean-Antoine<sup>1</sup>, Bonnet Delphine<sup>1</sup>

## Abstract :

Although scientific interest on jellyfish ecology has substantially increased in the last decades, little is known on the role of potential predators shaping their population dynamics. Jellyfish were long considered as 'dead ends' within food webs, and therefore overlooked as potential food source for higher trophic levels, e.g. fishes. Here this question is tackled by using comprehensive laboratory experiments assessing fish predation on jellyfish. The approach included all the life stages (polyps, ephyrae and medusa) of *Aurelia* sp. versus more traditional aquaculture feeds in an easily farmed opportunistic fish, the gilthead seabream *Sparus aurata* (L.). Results revealed that all life stages of *Aurelia* sp. were accepted as a source of food by *S. aurata*, whose grazing pressure varies depending on the jellyfish life stage. Higher ingestion rates were observed on young stages (i.e. small medusa) indicating their higher vulnerability to fish predation and the potential negative impact this may have on *Aurelia* sp. population dynamics. These results provide new insights on the so far underestimated role fish predation can have on jellyfish population dynamics. In particular, opportunistic fish species, such as *S. aurata* may contribute to control jellyfish blooms, through top-down regulations of jellyfish biomass.

<footnote n="1"><sup>1</sup> Laboratoire MARBEC, Université de Montpellier, CC093, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France</footnote>

<footnote n="2"><sup>2</sup> Faculdade de Ciências e Tecnologia, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal</footnote>

<footnote n="3"><sup>3</sup> GEOMAR - Helmholtz Centre for Ocean Research Kiel, Marine Ecology/food webs, Duesternbrooker Weg 20, D-24105 Kiel, Germany</footnote>

<footnote n="4"><sup>4</sup> Ifremer, Unité Biologie des Organismes Marins Exploités, UMR 5119 Ecosym, Chemin de Maguelone, 34250 Palavas Les Flots, France</footnote>

<footnote n="5"><sup>5</sup> Université de Montpellier, Station Méditerranéenne de l'Environnement Littoral, 2 Rue des Chantiers, 34200 Sète, France</footnote>

<footnote>* Corresponding author : Raquel Marques, marques.rfs@gmail.com</footnote>
**Highlights**

► During bloom events, jellyfish might be a source of food for opportunistic fishes. ► All life stages of *Aurelia* sp. are used as a source of food by *Sparus aurata*. ► Higher ingestion rates of fish predation were observed on *Aurelia* sp. young stages. ► Opportunistic fish species might contribute to control jellyfish blooms.

**Keywords** : *Aurelia* sp., Predation, Fish, *Sparus aurata*, Ingestion rate, Energy pathways
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# 1. Introduction

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Jellyfish are ubiquitous components of marine food webs and acknowledged indicators of ecosystem change (Richardson et al., 2009). In recent decades, however, recurrent massive proliferations in temperate coastal waters (Brotz et al., 2012) warned on dramatic consequences for the management of ecosystem assets, including fisheries and tourism industries. Underlying mechanisms of jellyfish proliferations have been ascribed to climate changes and anthropogenic disturbances in the marine environment, i.e. mainly habitat modification and overfishing (Purcell, 2012; Richardson et al., 2009). In addition, while high human population densities persist expanding in coastal areas, the resulting anthropogenic stress continue fostering favorable conditions for jellyfish, thereby challenging the sustainable management of coastal resources (Pauly et al., 2009; Purcell et al., 2007; Purcell, 2012; Richardson et al., 2009).

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Research on jellyfish ecology has long focused on bloom drivers (e.g. Purcell, 2005). However, little is known in regards to causes impairing blooms, i.e. jellyfish mortality (Purcell and Arai, 2001), which is fundamental to understand their population dynamics. In particular, jellyfish mortality during early life stages may have a major effect on population recruitment, and subsequently on adults density (Lucas, 2001). For instance, recent findings provided new insights on the role predation on polyps and ephyrae may have on jellyfish outbreaks through enhancing jellyfish mortality (e.g. Ishii et al., 2004; Takao et al., 2014).

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Over decades, jellyfish were described as 'dead ends' in marine food webs, because of their high water content and low nutritional value (e.g. 2.3-3.6 KJ.g.dry mass⁻¹ for *A. aurita*, Doyle et al., 2007), although gelatinous organisms are often reported in lists of fish stomachs contents. For instance, some species, such as moon fish (*Mola mola*) and butterfish (*Peprilus triacanthus*), are known to prey exclusively
on gelatinous zooplankton (Arai, 2005, 1988; Ates, 1988; Mianzan et al., 1996; Purcell and Arai, 2001). Also, in periods of massive proliferation, jellyfish are further suggested as a non-negligible source of energy for fishes (Arai, 2005; Arai et al., 2003; Cardona et al., 2012), however quantitative estimations on the impact of fish predation on jellyfish are rare (Cardona et al., 2012; Milisenda et al., 2014). Hence, to date the strength of the trophic linkage between fish and jellyfish is barely known.

This study sought to examine the trophic relationship between fish and jellyfish. To tackle this question laboratory experiments were performed to explore the intensity of predation of coastal harvested fish species on jellyfish. The jellyfish *Aurelia* sp. was used as prey model and *Sparus aurata* as predator. The main aim was to assess predation on different life stages of *Aurelia* sp., and their respective attractiveness as food sources, when compared to prey types used in aquaculture. Therefore, it was intended to answer three main questions: i) What is the ingestion rate of jellyfish prey?; ii) Is the ingestion of jellyfish prey items fostered by their high availability/concentration?; iii) Are jellyfish positively selected? The approach provides evidence that jellyfish are more important as food source for fish than previously thought.

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## 2. Materials and Methods

The potential fish predation on jellyfish was investigated in laboratory conditions, bearing in mind the natural conditions occurring in the Thau lagoon, NW Mediterranean, 43°23′59.10″ N; 3°36′37.15″ E. This is a semi-enclosed brackish lagoon, which harbors a resident population of *Aurelia* sp. (Bonnet et al., 2012; Marques et al., 2015b), providing an ideal framework to understand the links between jellyfish and fish.
The gilthead seabream (*Sparus aurata*) is a common species in the northwestern Mediterranean and the main target of commercial fishery in Thau lagoon, representing at times up to 56.6% of total annual ‘capéchade’ catches (the most common fishing gear in Thau lagoon) (Crespi, 2002). Irrespective of age, most of gilthead seabream spend winter months at sea, where spawning occurs, and return to coastal lagoons around April (Audouin, 1962; Mercier et al., 2012). Although some adults spend significant amount of time in coastal lagoons (Mercier et al., 2012), in Thau Lagoon the population of *S. aurata* is composed mainly by juveniles under maturity age (2-3 years, 27-33 cm length; Lasserre, 1974 in Crespi, 2002). Moreover, gilthead seabream has been raised for decades in aquaculture farms worldwide and its life history is therefore well known (e.g. Moretti et al., 1999). All these criteria made it a particularly good candidate to obtain homogeneous groups of individuals with similar size.

The experiments were performed at the IFREMER institute (Institut Français de Recherche pour l'Exploitation de la Mer) in the research station of Palavas-les-Flots (France), from April to June 2014. A total of 433 *S. aurata* were obtained from the 'Les Poissons du Soleil' aquaculture farm. Two size classes were used in the experiments: 370 small individuals (Small Fish - SF: W of ca. 70 g; TL= 14-19 cm), and 63 large individuals (Large Fish - LF: W of ca. 200 g; TL= 20-25 cm). Fishes were acclimated in three 1500 L tanks filled with filtered sea water at 20-22°C. Every two days, fishes were fed with commercial dry pellet food for sea bream (B-Nature, Le Gouessant) at 1% of the fish biomass to meet their food requirements.

The predation on various prey items was compared. All preys were kept at 20±0.5°C. Live *Artemia* with about 1 cm length were collected in soft flats in Le Grau-du-Roi and maintained in 60 L tank with air supply. Live *Aurelia* sp. ephyrae and small medusae (Ø 1 cm) were obtained from 'Jellyfish Concept' company (Cherbourg,
France) and maintained in 15 L containers with air supply, fed with newly hatched
*Artemia*. Medium and large *Aurelia* sp. medusae (Ø 4 and 7-8 cm, respectively) and
colonies of *Aurelia* sp. polyps were collected in Thau Lagoon. Medium and large
medusae were collected with hand nets, while polyps, fixed on oyster or mussel shells,
were collected by SCUBA divers. They were all maintained in 60 L tanks with air
supply.

The experiment set up was composed by 24 separate tanks of 60 L (40 L of sea water) with shared water and air supply, which ensure identical temperature and salinity in all tanks. Experiments were performed at 20±0.5°C, since it is within the optimum temperature range for *S. aurata* (Feidantsis et al., 2009) and it also corresponds to temperatures at which blooms of *Aurelia* sp. occur in Thau lagoon (Bonnet et al., 2012; Marques et al., 2015a). Photoperiod was determined according to field conditions at the time of experiment (13 h of light and 11 h of dark period). As the maximum fish biomass recommended in *S. aurata* aquaculture farms is of 7 kg.m⁻³ (C. Pryzbyla pers. comm.), all experiments were performed with three SF or two LF per tank. Before each experiment, fishes were acclimated for four days in experimental tanks and maintained in starvation to ensure that all individuals empty their stomachs.

This study consisted in three different experiments, using three replicates for
each treatment, the monospecific diets, the gradient of concentration and the selectivity
experiments (Table 1).

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2.1. Monospecific diets

The main goal of the monospecific diets experiment was to compare *S. aurata*
ingestion rates for the various life stages of *Aurelia* sp. (polyps, ephyrae and different
sizes of medusae) with those for other types of food, live or not. As *S. aurata* were
acquired from aquaculture farms, dry pellets were used as control, since fishes were
previously reared with this diet. As the carbon content or dry weight of the prey items was not determined, the comparison of the different prey items was performed according to the concentration of prey per tank (item.tank⁻¹). Additionally, small, medium and large medusae were also compared according to their weight, determined according to theoretical calculations (Uye and Shimauchi, 2005).

The concentration of prey items in each treatment was calculated in order to meet 1% of fish biomass of dry pellets in each tank. Therefore, in SF and LF treatments, 44 and 80 pellets.tank⁻¹ were used, respectively. The same concentrations were then used for *Artemia* and *ephyrae*, since they have equivalent dimensions (0.7-1 cm).

As small, medium and large medusae have considerably higher dimensions as pellets, *Artemia* and *ephyrae*, these prey items were provided at lower concentrations: 50 and 1 prey item.tank⁻¹ of small and medium medusae, respectively. Large medusa was provided at the minimum possible concentration (1 prey item.tank⁻¹).

In order to mimic field conditions, one colony of polyps settled on oyster shell was provided. Consequently, the initial concentration of this prey item was not artificially fixed.

## 2.2. Gradient of concentration

The goal of the gradient of concentration experiment was to assess whether *Aurelia* sp. ingestion could be proportional to its availability in the field. Based on the results of the first feeding experiment, small medusae (Ø = 1 cm) were used for this test, using six different treatments (with initial concentrations of 5, 10, 15, 30, 40 and 50 item.tank⁻¹) for both size classes of *S. aurata*.
## 2.3. Selectivity experiment

Selectivity experiments allowed the assessment of the influence of simultaneous availability of high nutritional quality prey (crustacean) on the potential ingestion of early life stages of *Aurelia* sp.. Therefore, two composite diets were supplied for both SF and LF. Diet 1 was composed by ephyrae and *Artemia* with equal initial concentrations, while diet 2 was composed by polyps and *Artemia* (Table 1). Total prey concentrations were determined as previously described in monospecific diets experiment section and equally distributed by the prey items (i.e. 0.5% for ephyrae and *Artemia*).

The experiments were run for 2 h for all treatments, which is in accordance with previous laboratory experiments on fish predation on jellyfish (Arai et al., 2003) and allow an accurate estimation of ingestion rates, as further confirmed by a preliminary test. Since the experiment time was short, control experiment to determine prey mortality by predation from polyps/ephyrae was not required.

Prey items in each case were counted before and after the experiments, either by direct visual observation (for larger medusae) or under a dissecting microscope after filtering the water of the entire tank on a 200 µm mesh sieve. Polyps attached to oyster or mussel shells were photographed with a Cannon PowerShot G16 camera at the beginning and end of the experiment. Photographs were pre-treated with Adobe Photoshop CS2 Version 9.0, in order to improve contrast and polyps were counted by eye observation. In the end of the experiments, fishes were measured and weighted.

## 2.4. Calculations

Ingestion rate per gram of fish (*I*; prey item.g⁻¹fish.h⁻¹) was calculated according to the equation:
$$I = ((Ci - Cf)/t.n)/m$$

Where Ci and Cf are the initial and final concentration (prey item.tank⁻¹) of prey in the water, respectively, t is the experiment duration (h), n is the number of fish in each experimental tank and m is the weight of each fish (g). The results are presented as the mean I of each treatment (i.e. three replicates).

Ingestion rates of small, medium and large medusae were also compared according to their wet weight. Medusa biomass (wet weight; WW) was calculated based on medusa bell diameter (BD; cm), according to the equation of Uye and Shimauchi (2005):

$$WW = 0.0748 BD^{2.86}$$

Corresponding ingestion rates in biomass (Im; g.g⁻¹fish.h⁻¹) were calculated, according to the equation:

$$Im = I * WW$$

The results are presented as the mean Im for each treatment (i.e. three replicates).

## 2.5. Statistical analysis

In monospecific diets experiments, differences in S. aurata ingestion rates among prey types were tested by Kruskal-Wallis test, followed by post hoc multiple comparison test by Dunn test, with Bonferroni correction (α < 0.0018). The test of hypotheses explaining the S. aurata ingestion rates during the gradient of concentration experiment was done using a General Linear Model (GLM), including the initial prey concentration as predictor. Differences between SF and LF linear regressions of ingestion rates were tested with ANOVA test. Mann-Whitney-Wilcoxon test was performed to identify significant differences of ingestion rate between treatments in
selectivity experiments. All statistical analysis was performed using the software R 226
3.1.1 (The R Project for Statistical Computing 2014) and taking α < 0.05 as the limit for 227
statistical significance. 228

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230 **3. Results**

Predation activity was systematically observed during the feeding experiments 231
and occurred irrespective of *S. aurata* size class, the type of prey provided, and the tank.
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Furthermore, in several treatments, all the provided prey was consumed within the two 233
hours of experiment, indicating the suitability of the experiment time. 234

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236 3.1. Monospecific diets experiments

Predation activity on dry pellets (i.e. control) was consistently observed in all 237
experiments and occurred for both size classes of S. aurata, showing the suitability of 238
this type of food as control. 239

Both size classes displayed predatory activity over both benthic and pelagic 240
stages of *Aurelia* sp. life cycle (Fig. 1). Large medusae were not totally consumed by 241
any size class of fish, but bites on the edge of umbrellas were consistently observed. In 242
these cases though, the biomass of *Aurelia* sp. consumed by the fish was not 243
quantifiable with our protocol. 244

Ingestion rates for monospecific diets varied significantly according to the type 245
of prey offered, both for the SF (Kruskal-Wallis test, *p*<0.01) and for the LF (Kruskal- 246
Wallis test, *p*<0.01). Significant differences (Kruskal-Wallis test, *p* < 0.05) were 247
observed between the two size classes of *S. aurata*. The highest mean ingestion rates for 248
SF was obtained for small medusa (0.15±0.03 prey item.g⁻¹fish.h⁻¹), polyps (0.14±0.07 249
prey item.g⁻¹fish.h⁻¹) and *Artemia* (0.11±0.00 prey item.g⁻¹fish.h⁻¹), while ephyrae 250
(0.05±0.02 prey item.g⁻¹fish.h⁻¹), medium (0.003±0.001 prey item.g⁻¹fish.h⁻¹) and large medusa (no consumption) presented lower values (Fig. 1). The multiple comparison, though revealed that only medium medusa and large medusa presented significant lower values as control (Dunn post hoc test with Bonferroni correction, *p* < 0.0018), while the remaining were not significantly different. For LF, *Artemia* (0.11±0.00 prey item.g⁻¹fish.h⁻¹) and small medusa (0.07±0.00 prey item.g⁻¹fish.h⁻¹) were the most consumed preys, followed by polyps (0.04±0.02 prey item.g⁻¹fish.h⁻¹), ephyrae (0.03±0.02 prey item.g⁻¹fish.h⁻¹), medium (0.001±0.001 prey item.g⁻¹fish.h⁻¹) and large medusa (no consumption). Significant lower values than dry pellets were only revealed by large medusae (Dunn post hoc test with Bonferroni correction, *p* < 0.0018) (Fig. 1). Fish predation on medium medusae of *Aurelia* sp. was limited, when considering the quantity of individuals consumed, irrespective of fish life stage (Fig. 1). However, when considering ingestion rates in terms of biomass, *Aurelia* sp. medusae of medium size (Ø = 4 cm) proved to be at least as important as small ones as a source of food for *S. aurata*. Indeed, because one medusae of 4 cm bell diameter provides approximately the same wet weight of food (3.94 g) than 50 individuals with bell diameters of 1 cm (3.74 g), ingestion rates in terms of biomass were in fine similar between the two size classes of *Aurelia* sp. medusae, irrespective of the fish size class (Dunn post hoc test, *p* = 0.46; *p* = 0.36, for SF and LF respectively). It is worth noticing that the ingestion of *Aurelia* sp. small and medium medusae was consistently higher (Kruskal-Wallis test, *p* < 0.05) in the SF fishes (0.0112 ± 0.002 and 0.0116 ± 0.003 g.g⁻¹fish.h⁻¹, respectively) than LF (0.005 ± 0.000 and 0.004 ± 0.003 g.g⁻¹fish.h⁻¹, respectively), suggesting different food preferences for this species according to the size class.

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## 3.2. Gradient of concentration experiments

The results of ingestion rate obtained from the gradient of concentrations experiment suggested that the predation of *S. aurata* on *Aurelia* sp. individuals in the wild could be proportional to their availability, at least for the small medusae (Fig. 2). The ingestion rates increased with increasing concentration supply, for both size classes, showing a significant correlation (*p* < 0.01) and significant differences between the two size classes of fish (*p*<0.01). They reach the maximum value of 0.15±0.03 prey item.g⁻¹fish.h⁻¹ for SF and 0.07±0.00 prey item.g⁻¹fish.h⁻¹ for LF, which represents the ingestion of all available prey in the tank.

## 3.3. Selectivity experiments

Selectivity experiments showed that the attractiveness of *Aurelia* sp. as a prey depended on both its life stage (benthic or pelagic) and that of the fish (SF or LF) (Fig. 3). Although the results indicate higher ingestion rates for *Artemia* in both diets for SF, significant differences were obtained only when provided together with polyps (Wilcoxon test, *p* < 0.05). For LF, *Artemia* was significantly selected when compared with ephyrae (Wilcoxon test, *p* < 0.05). In diet 2, however, the reverse situation was observed with the polyps presenting higher ingestion values (Wilcoxon test, *p* < 0.05), despite the high variability in polyp ingestion rates among replicate tanks.

# 4. Discussion

The trophic interaction between fish and *Aurelia* sp. was assessed and found novel insights on the so far underestimated role fish predation can have on jellyfish population size. It is here hypothesised that oportunistic fish species, such as *S. aurata* (Escalas et al., 2015; Pita et al., 2002), might take advantage of local peaks in *Aurelia*
sp. densities to partially sustain their growth. This study provide evidences that jellyfish are not 'dead ends' of the food web, as they are actively predated when an alternative prey is not available, thereby favouring alternative energy pathways in food webs.

Both size classes of *S. aurata* feed on all life stages of *Aurelia* sp. offered as prey. This predator is an opportunistic feeder which commonly adapts its diet to the food resources available in its environment (Pita et al., 2002; Wassef and Eisawy, 1985) and can therefore prey on *Aurelia* sp. during blooms events. In contrast with former descriptions of the feeding preferences of this species (Escalas et al., 2015; Pita et al., 2002), the results provide evidence of the active predation of *S. aurata* on jellyfish. In line with this, several species of Sparidae family have been reported as jellyfish consumers (Ates, 1988; Mianzan et al., 1996), stressing the aptness of *S. aurata* as jellyfish predator.

The ingestion rates of both size classes of *S. aurata* varied significantly according to the type of prey offered. Firstly, it is worth noticing the high consumption of polyps by the SF. In the wild, younger individuals of *S. aurata* prey mainly on epibenthic polychaetes, small fishes, crustaceans and gastropods (Escalas et al., 2015; Tancioni et al., 2003), but with a clear dominance on bivalves in some habitats (Pita et al., 2002). As the polyps of *Aurelia* sp. in the experiments were provided fixed on the shells of living bivalves (oyster and mussel), it is possible that the actual target of fish predation was the settling substrate, rather than on the polyps themselves. Anyhow, bivalves are commonly colonized by polyps in the wild and particularly in Thau lagoon (Marques et al., 2015b), where bivalves play a crucial role in the development of *Aurelia* sp. benthic population. For instance, on submersed metal structures, oysters represent 90.4% of the biofouling that is colonized by the polys of *Aurelia* sp. in Thau lagoon (Marques et al., 2015b). Therefore *S. aurata* probably has a non negligible
impact on the benthic population of *Aurelia* sp. in this ecosystem, either by direct predation on its polyps or by indirect mortality when feeding on the bivalves.

With regard to the vulnerability of *Aurelia* sp. pelagic life stages the results revealed that the intensity of *S. aurata* predation depends on prey size range. That is, small medusa appeared to be preferred by both size classes, but more evident for SF of *S. aurata*, which showed ingestion rates as high as those observed with the control pelleted food or with live adult *Artemia*. In contrast, large medusae with 7-8 cm bell diameter were bitten but never fully consumed, while predation on medium sized medusae (Ø 4cm) was intermediate. Large medusae are highly vulnerable to capture by fish because their size increases their visibility in the water column and they have low escape capacities (Houde, 2001), which favour their consumption by fish. The preference of *S. aurata* for the smallest pelagic stages of *Aurelia* sp. may be therefore linked to prey size, which is largely controlled by mouth size in this species (Goldan et al., 1997; Russo et al., 2007; Wassef and Eisawy, 1985). Still, fish bites were observed in the edge of large medusae umbrella, suggesting that the quality of prey is not the hamper factor of its ingestion. Although just partially consumed, large jellyfish may provide a potential source of food for *S. aurata* and in turn, its bites damage the umbrellas of this jellyfish, reducing their capability to swim and capture prey.

Results showed that the small medusa (Ø 1cm) is the most consumed *Aurelia* sp. stage by *S. aurata*, however when considering the contributions according to prey biomass, both small and medium medusae appear suitable as food source, highlighting the vulnerability of *Aurelia* sp. early pelagic stages to fish predation. Indeed, *Aurelia* sp. within this size range are found from March to May in Thau lagoon (Bonnet et al., 2012), which matches the annual period for *S. aurata* individuals settlement in the
lagoon, either as post larvae or as sub-adults (corresponding to SF and LF size range) after a few months of overwintering at sea (Crespi, 2002; Mercier et al., 2012).

The observed trophic interactions between *S. aurata* and *Aurelia* sp. has wide implications for the population dynamics of the two species as the results showed that increasing availability of *Aurelia* sp. medusae boosts ingestion rates of *S. aurata*. In the experiments the fishes were exposed to higher concentrations of medusae than the maximum abundance recorded in Thau lagoon (3.30 x 10⁻³ ind.L⁻¹, Marques et al., 2015a), but in most tanks all the prey available was totally consumed. This suggests that even higher ingestion rates can be achieved if higher abundance of jellyfish are encountered in the wild as previously reported for other ecosystems (e.g. 2.48 x 10⁻¹ ind.L⁻¹ in Kertinge Nor, Denmark; Olesen et al., 1994). Such observations are in line with former empirical evidence of *S. aurata* behavior, which increase food intake when proteins and lipids levels in the diet provided are low (Santinha et al., 1999). Because *Aurelia* sp. medusae are mainly composed of water (ca. 96%) and have very low nutritional value (Lucas, 1994), higher volumes of these organisms must be ingested to meet predators energetic requirements (Cardona et al., 2012). In addition, as revealed here by significant differences of ingestion rates between the two size classes of fishes, larger predators may have a bigger impact on jellyfish population, as their food requirements are superior and greater numbers of jellyfish must be consumed. Nevertheless, the digestion rates of jellyfish (Arai et al., 2003) suggest higher ingestion as predator guts are cleared faster when feeding on these prey. Therefore the hypothesis is raised that important amounts of *Aurelia* sp. medusae are consumed punctually by both size classes of *S. aurata* each spring, contributing to the population size of the species in the Thau lagoon.
The intensity of trophic link however, is probably dependent on the relative densities of the various preys available in the lagoon at that time of the year. Indeed, the results of the prey selectivity trial pointed out that jellyfish are not preferred by *S. aurata* individuals when a type of prey with higher nutritional quality is equally available in the tank. Although carbon concentration was not estimated and considering that prey abundance was equally determined according to their size, it is realistic to assume that crustaceans represent a much important input of carbon as prey than jellyfish. Similar results were reported for the threadsail filefish (*Stephanolepis cirrhifer*), which was shown to preferentially ingest gelatinous prey when the high quality prey was less visible in the tanks (Miyajima et al., 2011). Likewise, prey accessibility was also pointed as a key factor in prey selectivity by *S. aurata* in the field (Pita et al., 2002). During bloom events, though, jellyfish dominance occurs as an outcome of their predation on zooplankton (Bonnet et al., 2012; Hansson et al., 2005; McNamara et al., 2013; Pereira et al., 2014; Purcell and Sturdevant, 2001), and can reduce the mesozooplankton stock by ca. 81%, as reported in Thau lagoon (Bonnet et al., 2012). Such shifting from high energetically zooplankton community to low quality jellyfish dominating system, might promote an adjustment of prey selectivity of *S. aurata* favouring the most abundant prey item, i.e. jellyfish. Jellyfish energy content also increases during the period of gonad maturation (Milisenda et al., 2014), which, in Thau lagoon, occurs from April to June (Bonnet et al., 2012). Hence, given the high ingestion rates that were observed for this life stage in the present study, massive concentrations of jellyfish during blooms have been suggested to satisfy fish energy requirements within a very small area, thereby minimizing energy waste in foraging and prey capture (Cardona et al., 2012; Mianzan et al., 1996).
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The results suggest that jellyfish like *Aurelia* sp. can be a non-negligible source of food for opportunistic fish species, which has important implications on energy fluxes within food webs and on the population dynamics of both the jellyfish prey and their predators. Furthermore, these laboratory experiments revealed that *Sparus aurata* could prey on benthic (by direct or/and indirect predation) and pelagic stages of *Aurelia* sp., which might have an important impact on *Aurelia* sp. by increasing its predation rates, when the availability of prey with higher nutritional quality is reduced in the wild.

These results offer insights on two important issues on jellyfish ecology; that is, fish, especially the opportunistic species, at least partially contribute to control jellyfish blooms, through top-down regulations by preying on different stages of their life cycle. The exploitation and consequent reduction of fish stocks is not only depleting jellyfish food competitors but also releasing jellyfish from the predation pressure. Purcell and Arai (2001) and Arai (2005) stated that predation by a large number of fish species with broad diets is more ecologically important than the predation by the relatively few specialized fishes with primarily gelatinous diets. Hence, this provides support on the role overfishing has in promoting jellyfish outbreaks (Roux et al., 2013). Second, during bloom events jellyfish may provide an alternative source of food which might favour the more opportunistic fish species. The energy from the lower trophic levels might therefore be diverted to fish species with less commercial value. Such modification of the energy pathways within the food webs should be further explored, included in ecosystem based models and considered in management strategies.
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561
562 **6. Acknowledgments**

563 The study was funded by EC2CO through the DYNAMO program (2012-2014)

564 (PI: D. Bonnet). Authors are grateful to many colleagues from IFREMER Palavas for

565 their support during laboratory experiments. Authors are also grateful to the anonymous

566 reviewers for their valuable comments.

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7. **Figure captions**

**Fig. 1:** Mean ingestion rates (in number of prey item) observed in SF (top) and LF (bottom) of *S. aurata* for all prey types during the monospecific diets experiments. Error bars represent standard deviations in each case. Letters indicate significant differences between groups after Bonferroni correction (*p* <0.0018). Note that the initial number of prey item offered (n) varied according to prey type (for more details, refer to **Table 1**).

**Fig. 2:** Mean ingestion rates (in number of prey items) observed for SF and LF of *S. aurata* for increasing initial abundances (n) of small medusae (Ø = 1cm) of *Aurelia* sp. in the tanks. Error bars represent standard deviations in each case.

**Fig. 3:** Mean ingestion rates (in number of prey items) observed in SF (top) and LF (bottom) of *S. aurata* for the various types of live prey (*Artemia* and *ephyrae* or polyps of *Aurelia* sp.) included in the two mixed diets used for the prey selectivity experiments. Error bars represent standard deviations in each case. Symbols above horizontal bars indicate when differences among groups were significant (*) or not (ns) at the risk level α = 0.05.

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**Table 1:** Initial concentration of prey (tank⁻¹ and L⁻¹) provided in the tanks during the different experiments, for SF and LF. Concentration of polyps represents the mean concentration of three replicates except for some treatments where one replicate was eliminated, as a result of polyps counting errors (*).

<table><thead><tr><th rowspan="3">Experiment</th><th rowspan="3">Prey type</th><th colspan="2">SF<br>(n= 3 per tank)</th><th colspan="2">LF<br>(n= 2 per tank)</th></tr><tr><th>prey item.tank<sup>-1</sup></th><th>prey item.L<sup>-1</sup></th><th>prey item.tank<sup>-1</sup></th><th>prey item.L<sup>-1</sup></th></tr></thead><tbody><tr><td rowspan="6">Monospecific diets</td><td>Dry pellets</td><td>44</td><td>1.10</td><td>80</td><td>2.00</td></tr><tr><td>Artemia</td><td>44</td><td>1.10</td><td>80</td><td>2.00</td></tr><tr><td>Polyps*</td><td>390</td><td>9.75</td><td>448</td><td>11.20</td></tr><tr><td>Ephyrae</td><td>44</td><td>1.10</td><td>80</td><td>2.00</td></tr><tr><td>Small Medusae (Ø1cm)</td><td>50</td><td>1.25</td><td>50</td><td>1.25</td></tr><tr><td>Medium Medusae (Ø4cm)</td><td>1</td><td>0.03</td><td>1</td><td>0.03</td></tr><tr><td>Large Medusae (Ø7-8cm)</td><td>1</td><td>0.03</td><td>1</td><td>0.03</td></tr><tr><td rowspan="6">Gradient of concentration</td><td>Medusae (Ø1cm)</td><td>5</td><td>0.13</td><td>5</td><td>0.13</td></tr><tr><td>Medusae (Ø1cm)</td><td>10</td><td>0.25</td><td>10</td><td>0.25</td></tr><tr><td>Medusae (Ø1cm)</td><td>15</td><td>0.38</td><td>15</td><td>0.38</td></tr><tr><td>Medusae (Ø1cm)</td><td>30</td><td>0.75</td><td>30</td><td>0.75</td></tr><tr><td>Medusae (Ø1cm)</td><td>40</td><td>1.00</td><td>40</td><td>1.00</td></tr><tr><td>Medusae (Ø1cm)</td><td>50</td><td>1.25</td><td>50</td><td>1.25</td></tr><tr><td rowspan="2">Selectivity</td><td>Ephyrae + Artemia</td><td>22 + 22</td><td>0.55 + 0.55</td><td>40 + 40</td><td>1 + 1</td></tr><tr><td>Polyps* + Artemia</td><td>215 + 22</td><td>5.38 + 0.55</td><td>400 + 40</td><td>10 + 1</td></tr></tbody></table>

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<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

**Fig. 1:** Mean ingestion rates (in number of prey item) observed in SF (top) and LF (bottom) *S. aurata* for all prey types during the monospecific diets experiments. Error bars represent standard deviations in each case. Letters indicate significant differences between groups after Bonferroni correction (*p* <0.0018). Note that the initial number of prey item offered (*n*) varied according to prey type (for more details, refer to **Table 1**).
<figure><img src="image_4.png" /><figcaption>**Fig. 2:** Mean ingestion rates (in number of prey items) observed for SF and LF of *S. aurata* for increasing initial abundances (n) of small medusae (Ø = 1cm) of *Aurelia* sp. in the tanks. Error bars represent standard deviations in each case.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3:** Mean ingestion rates (in number of prey items) observed in SF (top) and LF (bottom) of *S. aurata* for the various types of live prey (*Artemia* and *ephyrae* or *polyps* of *Aurelia* sp.) included in the two mixed diets used for the prey selectivity experiments.

Error bars represent standard deviations in each case. Symbols above horizontal bars indicate when differences among groups were significant (*) or not (ns) at the risk level *α* = 0.05.</figcaption></figure>