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Article

Diet of the Common Eagle Ray, Myliobatis aquila (Linnaeus, 1758) in the Northern Adriatic Sea

by
Lovrenc Lipej
1,
Riccardo Battistella
2,
Borut Mavrič
1 and
Danijel Ivajnšič
3,4,*
1
Marine Biology Station, National Institute of Biology, SI-6330 Piran, Slovenia
2
Independent Researcher, Via S. Stefano 47, 35042 Este, Provincia di Padova, Italy
3
Faculty of Natural Sciences and Mathematics, University of Maribor, SI-2000 Maribor, Slovenia
4
Faculty of Arts, University of Maribor, SI-2000 Maribor, Slovenia
*
Author to whom correspondence should be addressed.
Fishes 2025, 10(7), 311; https://doi.org/10.3390/fishes10070311
Submission received: 22 April 2025 / Revised: 13 June 2025 / Accepted: 17 June 2025 / Published: 1 July 2025
(This article belongs to the Section Biology and Ecology)
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Abstract

We studied the feeding habits of the common eagle ray (Myliobatis aquila) in the shallow northern Adriatic Sea. Altogether we analysed the contents of 122 stomachs of specimens caught as by-catch in the Gulf of Trieste and along the west Istrian coast. Shelled molluscs (N% = 75.17), mainly bivalves and gastropods, were the most prominent prey categories, while crustaceans, sipunculids, echinoderms and polychaets (N% < 10) represented considerably smaller numbers. With increasing size (and age) the eagle rays tend to become more experienced in preying molluscs and specialized to this prey category. The obtained results are in general in agreement with the limited existing reports on the diet of the common eagle ray in the Mediterranean Sea and adjacent areas.
Keywords:
food habits; myliobatid; ontogenetic shift; Adriatic Sea
Key Contribution: This research investigates the feeding habits of the highly endangered elasmobranch species of the common eagle ray (Myliobatis aquila) in the northern Adriatic Sea, an area considered as a critical nursery and reproductive habitat for various elasmobranchs. The findings of this study underscore the significance of this region as a vital feeding ground for the common eagle ray.

1. Introduction

The northern Adriatic Sea is considered as a productive, heavily exploited subbasin [1] whose marine communities are mainly influenced by intense fisheries [2], resulting in a dramatic 80% decline in elasmobranch landings over the past seven decades [3]. According to a recent assessment, 70% of Adriatic inhabiting elasmobranch species are regionally threatened, with 42 out of 59 species at risk [4]. A major problem for many elasmobranchs is bycatch, defined as the incidental capture of non-target species, which affect their populations also in the Slovenian coastal waters; among them are many batoid species [5].
Batoids are important predators in the sea. However, compared to the widely concerned sharks, batoids have received far less attention [6]. The common eagle ray Myliobatis aquila (Linnaeus, 1758) is a rather large-sized predatory fish, which inhabits the Mediterranean and eastern Atlantic, from the English Channel to South African waters [7]. It is a demersal and semi-pelagic ray species occurring in shallow coastal waters [8]. The common eagle ray exhibit gregarious behaviour during their reproductive period [9,10,11], making them particularly vulnerable to capture, with large groups being frequently caught by trawl or gillnets in a single haul [12].
In the Adriatic Sea, M. aquila used to be considered as a common species [13]. Different from many other elasmobranch species in this region [8] M. aquila was one of few species who did not face a decline according to Ferretti et al. [14]. Currently, the common eagle ray is classified as a ‘Critically Endangered’ species by the IUCN Red List, primarily due to declining catch trends and a limited number of recorded specimens. This indicates a significant population reduction of over 80% across the last three generations [12]. Among the elasmobranch species in the northern Adriatic Sea, M. aquila is the species who is involved in bycatch events most frequently [15,16]. Owing to its negligible commercial value, it is usually discarded alive at the sea, and it seems reasonable to hypothesize that the survival rate is high [15].
Knowing what a predator preys on can provide information about its habitat preference, distribution and its position in the food web. However, to date the feeding ecology of rays deserved only scarce scientific attention, since only about 30% of the diets of extant rays are known [17]. Despite its widespread distribution, the research on the trophic ecology of common eagle rays in the Mediterranean Sea is still scarce; only few studies were performed recently [18,19,20,21,22,23]. The ecological role and potential impacts of rays in this area, and in the adjacent northern Adriatic Sea, remain unknown. There is scarce information about the diet of rays in the Gulf of Trieste [24,25]. Moreover, no prior published information on the diet composition of the myliobatids from Slovenian waters, the broader Gulf of Trieste or the northern Adriatic in general is currently available. According to its regular presence in the area, the common eagle ray is supposed to play a relevant role in the trophic web.
The present paper studies the diet composition and feeding habits of the common eagle ray in the northern Adriatic Sea. Since the northern Adriatic Sea differs from the middle and the southern Adriatic Sea in terms of depth, productivity, benthic fauna and other ecological factors, we indirectly investigated whether these spatial traits can potentially be traced within the eagle ray’s diet. Furthermore, obtaining information about the food composition and feeding behaviour of species is necessary to understand the role and position of species in the food web better. From that perspective, the aims of this study were to (a) analyse the diet composition of the common eagle ray (M. aquila) and (b) to evaluate its diet related to sex and maturity stage (size class).

2. Materials and Methods

Specimens of common eagle rays, occasionally caught in the fishing nets as bycatch in the Gulf of Trieste and along the western Istrian coast (Figure 1) in the period from 2005 to 2015, were provided by local fishermen. Since the localities were close to each other, and captures of the common eagle rays occurred in shallow waters (<30 m), we did not analyse the effect of sites and depth on the diet.
After being defrosted in the lab, specimens were measured (disk width—DW) and weighed. Size categories were chosen based on reports that M. aquila reaches sexual maturity at 360–410 mm DW for males, and 480–580 mm DW for females [26]. The ontogenetic shift in the diet was evaluated by analysing two size classes: juveniles (DW ≤ 410 for males and DW ≤ 480 mm for females) and adults (DW > 410 mm for males and DW > 480 mm for females). The sex of each individual was determined. The assignment of sex for males was based on the presence of claspers. Differences in the common eagle ray’s sample concerning specimen length and weight per sex and size class factors were preliminary tested with Kruskal–Wallis’s statistics in the R statistical environment.
Since the stomach content analysis is still the main universal technique for sampling the diets of fish; the stomachs in our M. aquila sample were removed from thawed individuals and immediately stored in 70% alcohol. To determine the diet composition, stomachs of all specimens were opened, and all prey items were carefully removed. Prey items were identified to the lowest possible taxonomic level, enumerated, and weighed. Prey items were counted based on their typical parts such as shell and foot for bivalves, opercula for gastropods and some sipunculids, claws and legs for different crustaceans, carapaces for decapods, partial remains of polychaete skins with chetae, remains of holothurian bodies and skeletal remains and otoliths for teleost species. Prey identification was performed using identification keys such as Riedel [27], Falciai and Minervini [28] and others.
For the identification and biomass assessment of gastropods, we used their operculum, a species-specific disk that closes the gastropod shell aperture and thus protects the animal. A comparative collection of opercula of the Marine Biology Station (National Institute for Biology) was additionally used to support correct prey identification.
The length and width of each operculum found in stomachs were measured using an OLYMPUS-SZX16 microscope with the OLYMPUS-DP70 camera (OLYMPUS Optical CO.Ltd, Tokyo, Japan). To obtain a correlation diagram between the length of the operculum and the weight of the studied specimen of a particular species, several living specimens of gastropods, obtained by fishermen as bycatch, were used. The specimens were extracted from their shells and weighed. In the next step, their opercula were measured. Finally, we prepared a length–weight correlation diagram and thus evaluated the ingested gastropods in the diet of the common eagle ray. In some cases, average mass values reported in the bibliography were used.
For the assessment of the common eagle ray’s diet, the contribution of each prey taxon to the diet composition was estimated with three indices. The relative prey number (N%), the relative frequency of occurrence (FO%) and the relative biomass (W%) [29]. To assess the importance of prey species Pinkas et al. [30] combined the three mentioned indexes into the Index of Relative Importance (IRI) which is calculated as:
IRI = (N% + W%) × FO%,
whereas IRI% = (IRIi/Σ IRIij) × 100 [31].
IRIij is the fraction of prey (j) in the diet of the species (i). To test potential differences in prey composition (based on presence/absence data) regarding factors sex (male, female) and size class (juvenile, adult) simultaneously, the two-way multivariate permutation analysis of variance (PERMANOVA, 999 permutations, distance = Euclidean) was performed in the R statistical environment [32] by applying the vegan package [33]. To consider the influence of specimen size on prey composition, the PERMANOVA analysis was then repeated (with the same setting) for the factor sex by separately considering the size class (juvenile, adult). In this case, the betadisper and permutes functions were applied to the test group and permutation dispersion. These results were presented in the non-metric multidimensional scaling (NMDS) space in the R statistical environment. In addition to that, prey frequency tables were prepared by aggregating data into higher taxonomic groups considering both factors (sex and size class). Thus, chi-square statistics and the corresponding mosaic plots were designed using the vcd package [34] and the assoc and mosaic algorithms in the R statistical environment.
The common eagle ray’s diet was also described using the Shannon–Wiener diversity index: H’ = −(Σ pi × ln pi) [35], where H’ is diversity index and pi the proportion of the total sample belonging to each group. Prey diversity of the common eagle ray was additionally compared by sex and size classes.
To evaluate dietary overlap by sex and by size class, we used the Morisita–Horn index [36]:
CH = 2 × (∑pij pik)/(∑p2ij + ∑p2ik)
where CH is the simplified Morisita Index overlap [37] between species j and species k and Pij is the proportion of prey i of the total prey used by predator j, while Pik is the proportion of prey i of the total prey used by predator k, and n is the total number of prey. This index ranges from 0, when the diets are completely different, to 1 when the diets are identical. Values exceeding 0.6 are considered to overlap significantly.
To assess the trophic level of the common eagle ray, a TROPH index was calculated using TrophLab [38,39,40], downloadable from www.fishbase.org.
TROPHi = 1 + Σ G j=1 DCij × TROPHj,
where TROPHj is the fractional trophic level of prey j and DCij represents the fraction of j in the diet of the consumer species i.

3. Results

3.1. Biometrical Features

Altogether 122 specimens of M. aquila were measured, weighed and analysed. Eighty-seven (87) of them were males and 34 were females, while in one juvenile specimen we were not able to identify the gender. Males ranged from 232 to 620 mm in DW and weighed from 144 g to 2960 g. Females measured from 238 to 775 mm in DW and weighed from 180 g to 6460 g. Males and females showed significantly different lengths (Kruskal–Wallis’s test p value = 0.024; α = 0.05) and weight (Kruskal–Wallis p value = 0.0167; α = 0.05). On average, males were bigger (399.8 mm versus 340.4 mm) and heavier than females (1245 g vs. 771 g). If we consider the size at maturity which is from 360–410 mm DW for males and 480–580 mm DW for females, then the studied samples consist of juvenile and adult males and juvenile and adult females (Figure 2). The length (TL)—weight (W) relationship was statistically significant for both sexes considering the exponential functional relation and the 0.05 alpha level (males; y = 31.196e(0.0084x), R2 = 0.96; females, y = 46.767e(0.007x), R2 = 0.99).
From a total of 122 stomachs of the common eagle ray examined, 113 contained food, while 9 (7.38%; 5 females and 4 males) were empty. Altogether 1152 prey items were obtained from the stomachs. The number of prey items per stomach varied from 1 to 80 specimens, on average 9.44 ± 4.26 prey items per stomach (Figure 3). Although the mean number of prey items per stomach was nearly 10, more than 29% of full stomachs with food contained only one or two prey items. Most commonly one prey specimen was found in the stomachs (16.4% of studied specimens).

3.2. Overall Diet

The diet primarily consists of macrobenthic epifauna and infauna that inhabit a sedimentary bottom. Prey items isolated from the stomach analysis belonged to seven major groups: Bivalvia, Gastropoda, Polychaeta, Sipunculida, Decapoda (Natantia and Reptantia), Echinodermata and Teleostei (Table 1). The diet of the common eagle ray in our sample was composed mainly of molluscs (N% = 75.2; W% = 66.24, FO% = 66.4, IRI% = 93.80), while other prey categories such as different crustaceans (anomurans, ostracods, decapods, amphipods) (N% = 9.38), sipunculids (N% = 5.56), echinoderms (N% = 4.69) and polychaetes (N% = 3.04) represented considerably smaller numbers. Teleost fish represented less than 1% of prey items.
Molluscs were the dominant prey group also when we consider other three indices (FO%, W% and IRI%). The percentage of higher taxonomic groups is similar from the aspect of the relative abundance and the frequency of occurrence (Figure 4). The majority of prey were bivalves (N% = 52, W% = 42.22. FO% = 66.39 and IRI% = 85.85), which we failed to identify to lower taxonomic levels due to the crushed remains of shell. The second most important food category were gastropods, especially Cerithium vulgatum (N% = 11.37, W% = 16.99, FO% = 16.39 and IRI% = 6.38).
Other prey groups found in the stomach contents showed even lower IRI% values and were thus less important. Among gastropods, the number of specimens belong mainly to Cerithium vulgatus and Turritella communis. Some fish remains were also found in the stomachs. They represented less than 1% in terms of numerical abundance (N%) and relative importance (IRI%) but comprised 8.46% in terms of weight (W%). In terms of the relative importance of the prey, the preferential feeding category were bivalves (IRI% = 85.85), followed by gastropods (IRI% = 14.48). Other food categories were less than 5% in terms of relative importance of prey.

3.3. Diet Related to Sex and Size

For both sexes, molluscs constituted the main food source, while other prey groups can be considered accidental food. However, the chi-square test and the corresponding mosaic plot indicated significant differences (p < α; α = 0.05) in prey frequency regarding factor sex for the following taxonomic groups: Bivalvia, Gastropoda, Sipunculida and Echinodermata. On the other hand, the chi-square test results indicated stronger differences in prey composition considering factor size class (adult, juvenile) for the above-mentioned taxonomic groups. In terms of relative abundance, the diets of males and females overlapped (Figure 5). By considering the size class factor (juvenile, adult), differences in prey composition regarding relative abundance were more pronounced. Juvenile males preyed more frequently on gastropods and sipunculids, while adult males predominantly hunted down bivalves, and to a smaller extent crustaceans and echinoderms (Figure 5). Owing to the small number of female adults, the shift in prey composition from the juvenile to the adult phase is (Figure 5) less obvious. Anyway, there was a decrease in relative abundance of gastropods, bivalves and sipunculids, whereas taxonomic crustaceans and fishes increased.
From the perspective of relative importance (IRI%), the percentage of bivalves was considerably greater in males (Figure 6). However, more prey categories (in term of IRI values) were preyed by females, thus indicating a more diverse diet. For males, the shift in prey importance from gastropods in the juvenile phase, to bivalves in the adult phase, was observed and was, comparing to relative abundance, even more pronounced. A similar dietary trend was detected for females as well (having in mind that we obtained only three adult females); in the juvenile phase gastropods were the dominant taxonomic group, which were, in the adult phase, substituted by prey groups crustaceans and fishes.
Moreover, the average number of prey items, the average prey weight, the average meal size and the Shannon–Wiener diversity index were lower in males compared to females (Table 2), additionally proving a less diverse diet. The Morisita/Horn index values indicated the diet overlap between males and females (0.86) as well. Accordingly, the trophic levels of males (3.20 ± 0.39) and females (3.25 ± 0.39) were similar. However, the average number of prey and the average meal size were higher in juveniles. The average meal size varied from 10.01 to 14.6 g (in average 11.56 g). Since the average weight of the eagle ray obtained from of all studied specimens was 302.5 g, we can speculate that this represents approximately 3.31% to 4.65% of the predator body weight. The overlap between diets of juveniles and adults (Morisita/Horn index = 0.88) was confirmed also by similar trophic levels (juveniles = 3.23 ± 0.43; adults = 3.17 ± 0.24).
In the next step, the interdependence of both considered factors (sex and size class [juvenile, adult]) on the eagle ray’s diet were proved on the prey item presence/absence level too. The two-way PERMANOVA analysis indicated significant differences in prey composition by simultaneously considering both factors, sex and size. However, the eagle ray’s diet significantly overlapped (p = 0.094; α = 0.05; insignificant beta disper and permutes tests) for both sex categories if we compared the juvenile size class (Figure 7A). This result is in complete conjunction with our finding drawn from the prey relative abundance and importance data (Figure 5 and Figure 6).
However, the diet for juvenile eagle rays was characterized with high occurrences of gastropods such as Cerithium vulgatum and Turritella communis, and the sipunculid Aspidosiphon muelleri, while adult specimens prefer to prey on bivalves, but focus also on pagurids and holothurians (Figure 6). The juveniles had a diverse generalist diet which is probably the consequence that they were too young to reach an adequate experience in preying on optimal prey items and thus they should be considered as opportunistic feeders.
Differences in prey composition by factor sex were detected by comparing the adult size class (Figure 7B), where adult males more frequently preyed on bivalves in comparison to adult females that had higher frequencies of pagurids, anomurans and gastropods (results of simper analysis). However, this result could lead to misjudgements owing to small number of female specimens in the sample (n = 3).

4. Discussion

This paper represents the first quantitative assessment of the feeding habits of the common eagle ray in the northern Adriatic Sea. It shows that the common eagle ray (M. aquila) feeds on a large spectrum of prey categories; however, bivalves were preyed as the preferred prey category, which suggests that this species is a specialized benthophagous predator. All specimens were captured on the sedimentary bottom of the shallow Gulf of Trieste and along the west Istrian coast.

4.1. Feeding Habits

The prey items found in the diet are benthic dwelling animals, showing the benthophagous feeding habits of the common eagle ray. It predates mainly bivalves, being present in more than 66% of the analysed stomachs, and having the highest relative abundance (app. N% = 52) and index of relative importance (app. IRI% = 85). Eagle rays and other members of the family Myliobatidae have diets almost entirely composed of shelled preys like molluscs and decapods [41]. As large benthic elasmobranchs, eagle rays can have a drastic impact on molluscs and other invertebrates and have thus an important role in the structuring the benthic communities. They are recognized as durophagous benthic feeders, characterized by the presence of rigid jaws containing hard, fused grinding plates composed of flat teeth. These teeth interlock to efficiently crush hard-shelled organisms [42,43]. Valls et al. [21] showed that the diet of M. aquila is highly specialized in non-cephalopod molluscs (mainly bivalves) and anomuran crustaceans [44], supported also by other studies [18,19], which confirmed the predominant durophagy in eagle rays. According to Jardas et al. [20], who studied the feeding habits of the eagle ray in the Adriatic Sea, it exclusively preys on benthic invertebrates of the soft sea bottom. The eagle rays are continuous feeders which seek their prey on the surface of the substrate or target prey buried in the subsurface layer [6,44].
The presence of sipunculids in the diet of Myliobatis aquila was confirmed also by other authors. Jardas et al. [20] published that almost 23% IRI of prey captured by the eagle ray in the Croatian part of the Adriatic Sea were sipunculids, especially Aspidosiphon muelleri (nearly 16% IRI), which was recorded also in our study although in much lower percentage (<0.1% IRI, however N% = 4.51). This species has been reported among other habitats also in soft sediments, often inhabiting gastropod shells or polychaete tubes [45]. It is probable that its presence in the eagle ray diet is related to the hunting of gastropods, their shelter suppliers. However, we cannot exclude completely the fact that the consumption of sipunculids by eagle rays could be related to their high nutritional value and high protein content (43% of weight in Sipunculus nudus) as pointed out by Zhang et al. [46]. The consumption of wormlike prey such as polychaetes and sipunculids is an important and good nutritional source for the development of angular rough sharks [47].
Some holothurians were found in the stomachs of the common eagle ray. In his review of predators on holothurians, Francour [48] did not mention any data related to the eagle ray (or other batoid species) to prey holothurians. However, the holothurians percentage in the diet of the common eagle ray is low (PN 4.60%, IRI 1.60%). The eagle ray may misidentify them with their preferred prey category (bivalves) and thus caught them accidentally.

4.2. Ontogenetic Shift in the Diet

In terms of relative abundance, the differences between sexes where less pronounced, meaning that males and females occupy similar areas and/or encounter similar prey [49]. However, we noticed larger differences in the diet between males and females in terms of the IRI%. Additional statistical tests indicated that these differences were related to factors sex (male, female) and size (juvenile, adult) simultaneously. Thus, the eagle ray’s diet in our sample overlapped on the relative abundance, relative importance and prey composition levels by considering the juvenile size class. Differences on all three levels were evident in the adult size class phase where results regarding female specimen should be interpreted with caution owing to a small number. Male specimens were larger than females and had a less diverse diet. However, in many batoid species studied to date, males and females fed on similar prey items as previously evidenced in many studies of rays [24,25,50,51,52,53,54]. The TROPH values of male and female specimens of M. aquila did not differ considerably; however, the obtained values from 3.20 ± 0.44 to 3.25 ± 0.39 in our study were lower in comparison to the value 3.60, cited in www.fishbase.org [55].
The ontogenetic shift in the diet of the common eagle ray in the northern Adriatic Sea was detected on the prey relative importance index and prey composition levels, especially for males. While juveniles were preying on common epifaunal gastropods, more experienced adults focused their feeding habits on fossorial (endopsammal) bivalves. The shift in diet in various ontogenetic stages is characteristic for many sharks and rays [56]. Smaller (juvenile) specimens tend to prey on smaller prey items such as gastropods Turritella communis, Cerithium vulgatum and sipunculid A. muelleri. With the increasing size and maturity, the proportion of bivalves increased while the importance of gastropods and sipunculids diminished. Juvenile eagle rays had a more diversified diet compared to the adult specimens which was discovered also in other species of the genus Myliobatis, such as M. freminvillii [57]. With increasing size (and age) the eagle rays tend to become more experienced specializing in preying molluscs. The mean size of the prey tends to increase with the increasing size of the predator, a relationship resulting from optimizing the energy input needed for growth and sustaining higher metabolism [58]. As a predator grows it also gains experiences on how to prey on profitable, but difficult-to-catch prey [59].

4.3. Comparison with Other Studies

Analysis of feeding habits of the common eagle ray has been conducted in various parts of the Mediterranean Sea and adjacent areas [19,20,21,23,60]. The comparison of dietary composition in few available studies of this species have revealed considerable geographic variation in the diet; however, others have demonstrated considerable similarities. In all studies molluscs represented the main food category with more than 60% of prey items [19,20,22,23,60]. However, it should be mentioned that some studies are based on a very small sample [22,60] and are thus less suitable for comparison. In the Adriatic Sea, Jardas et al. [20] calculated that molluscs represented more than 68% of preyed items (Table 3). Bivalves dominated the mollusc assemblage with IRI% = 33.7, followed by gastropods with IRI% = 20.6, cephalopods with IRI% = 1.6, and scaphopods with IRI% = 1.1. Our data fit well with the data obtained by Capapé [19] in a comprehensive diet study in Tunisian waters with a substantial sample of more than 500 analysed stomachs, where the bivalves and gastropods revealed to be the major food categories of the common eagle ray. In his analysis Capapé [19] found a greater number of fish in comparison to our study, but almost no sipunculids. The most common fishes preyed upon were among others Clupea pilchardus, Engraulis encrasicolus and Argentina sphyraena, the first two being the most common fishes also in our study.
We were mostly not able to discriminate which bivalve species are taken by the common eagle ray; however, we may speculate that they are mainly fossorial ones. In fact, the myliobatids in general are known to prey on bivalves by excavating them from the sediments with their pectoral fin tips and subrostral lobe [61]. The high proportion of bivalves in the diet of the common eagle ray [44], which proves the importance of this prey category, was previously already reported by other studies that have confirmed bivalves as the major components in the diet of the eagle ray [18,19,20,62].
The present study shows that the common eagle ray is a specialized feeder, preying upon specific food items such as bivalves and gastropods. The variety of other prey categories observed in the stomachs was poor since they represent only a minor portion. The obtained data suit well with the fact that the common eagle ray is a continuous feeder which tends to ingest small prey at regular intervals resulting in high numbers of prey items in the stomach and with a low occurrence of empty stomachs [56]. In fact, in our study the number of prey items per stomach varied from 1 to 80 specimens (on average 9.44 prey items per stomach; Figure 3), while only 7.4% of stomachs were empty.
The results of this study provide basic information about the feeding habits of the common eagle ray, a ‘Critically endangered species’ [12] which has undergone a substantial population reduction in recent times. The majority of captured specimens of the common eagle ray, particularly females, were below the size of first maturity. This finding further underscores the significance of the northern Adriatic as a vital reproductive habitat for the common eagle ray. Some authors pointed out the fact that the northern Adriatic is an important nursery and reproductive habitat for many elasmobranchs and among them also for the common eagle ray [63,64]. It is possible that eagle rays (and relatives) select such areas because of the ample food resources available there [65], which was previously already confirmed in the very same area for some predators such as the Mediterranean shag (Gulosus aristotelis desmarestii) [66,67].

5. Conclusions

The present study shows that the common eagle ray (Myliobatis aquila) exhibits a high degree of trophic specialization, predominantly preying on molluscs, particularly bivalves and gastropods. The occurrence of other prey taxa in stomach content analyses was limited, suggesting their marginal contribution to the overall diet. Dietary diversity was notably greater in juveniles, indicating a more opportunistic foraging strategy during early life stages. However, with increasing body size, a pronounced ontogenetic dietary shift occurs, resulting in enhanced specialization towards predation of bivalves. As a moderately large benthic elasmobranch, the common eagle ray may exert considerable top-down control on molluscan assemblages, thereby influencing the structure and dynamics of benthic communities. While the northern Adriatic has previously been identified as an important habitat for the common eagle ray and related species, findings from this study suggest that the area also provides substantial foraging opportunities, underscoring its broader ecological significance for the species.

Author Contributions

Conceptualization, L.L. and B.M.; methodology, L.L. and R.B.; software, L.L., R.B. and D.I.; validation, L.L. and D.I.; formal analysis, L.L. and D.I.; investigation, L.L. and R.B.; resources, L.L.; data curation, L.L. and D.I.; writing—original draft preparation, L.L. and D.I.; writing—review and editing, L.L., D.I., R.B. and B.M.; visualization, B.M.; supervision, B.M. All authors have read and agreed to the published version of the manuscript.

Funding

This study was co-financed by the Slovenian Research and Innovation Agency (ARIS, grant numbers P1-0237 [Coastal Sea Research] and P6-0372 [Slovene Identity and Cultural Awareness in Linguistic and Ethnic Contact Areas in Past and Present]) and by the project Development of Research Infrastructure for the International Competitiveness of the Slovenian RRI Space—RI-SI-LifeWatch, co-financed by the Republic of Slovenia, Ministry of Education, Science and Sport and the European Union from the European Regional Development Fund.

Institutional Review Board Statement

Ethical review and approval were waived for this study since the material was obtained as bycatch by professional fishermen (SIC company) and are subject to European regulations on Fish Discards.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

Many thanks are due to Radoš Jenko, who provided us with occasional specimens entrapped as a bycatch in the Slovenian part of the Adriatic Sea in the period from 2005 to 2007, and Ariana Stojnić, SIC Company, for providing many Eagle ray specimens, caught as by-catch by fishermen along the west Istrian coast in the period from (2010 to 2016). We must extend our thanks also to our colleague Valter Žiža, the former head of the Piran Aquarium, who provided us with specimens from Slovenian waters. Special thanks also to Jernej Uhan, Francesca Garaventa, Martina Orlando Bonaca, Tihomir Makovec, Leon Lojze Zamuda and Domen Trkov for their help in biometrical measurements of some specimens. We would like to express our gratitude also to other members of the staff of the Marine Biology Station of the National Institute of Biology who helped us in the fieldwork and in the lab.

Conflicts of Interest

The authors declare no conflicts of interest.

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Fishes 10 00311 g001
Figure 1. Map of the study area in the northern Adriatic Sea with the localities where specimens of common eagle ray specimens were caught (red dots). Catch density is additionally presented as a heat map.
Figure 1. Map of the study area in the northern Adriatic Sea with the localities where specimens of common eagle ray specimens were caught (red dots). Catch density is additionally presented as a heat map.
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Figure 2. Size distribution of the analysed specimens of the common eagle ray (Myliobatis aquila) in the study area. Legend: F—females, M—males.
Figure 2. Size distribution of the analysed specimens of the common eagle ray (Myliobatis aquila) in the study area. Legend: F—females, M—males.
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Figure 3. Number of prey items per stomach of the analysed common eagle ray specimens from the northern Adriatic Sea.
Figure 3. Number of prey items per stomach of the analysed common eagle ray specimens from the northern Adriatic Sea.
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Figure 4. Relative abundance (a) and frequency of occurrence (b) of higher taxa in the diet of Myliobatis aquila in the northern Adriatic Sea.
Figure 4. Relative abundance (a) and frequency of occurrence (b) of higher taxa in the diet of Myliobatis aquila in the northern Adriatic Sea.
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Figure 5. Relative abundance of higher taxa in the diet of male and female specimens of the common eagle ray Myliobatis aquila in the northern Adriatic Sea regarding size class (juvenile, adult); * small sample size.
Figure 5. Relative abundance of higher taxa in the diet of male and female specimens of the common eagle ray Myliobatis aquila in the northern Adriatic Sea regarding size class (juvenile, adult); * small sample size.
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Figure 6. Relative importance IRI% of higher taxa in the diet of male and female specimens of the common eagle ray Myliobatis aquila from the northern Adriatic Sea regarding size class (juvenile, adult); * small sample size.
Figure 6. Relative importance IRI% of higher taxa in the diet of male and female specimens of the common eagle ray Myliobatis aquila from the northern Adriatic Sea regarding size class (juvenile, adult); * small sample size.
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Figure 7. Similarities/differences in prey composition in the diet of male and female specimens of the common eagle ray Myliobatis aquila from the northern Adriatic Sea regarding juvenile (A) and adult (B) size classes in the NMDS space.
Figure 7. Similarities/differences in prey composition in the diet of male and female specimens of the common eagle ray Myliobatis aquila from the northern Adriatic Sea regarding juvenile (A) and adult (B) size classes in the NMDS space.
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Table 1. Diet composition of the common eagle ray (Myliobatis aquila) from the northern Adriatic Sea. Legend: N%—relative abundance, F%—frequency of occurrence for a given prey item, W%—weight percentage and IRI% is the Index of relative Importance for each prey item, indet.—not identified taxa.
Table 1. Diet composition of the common eagle ray (Myliobatis aquila) from the northern Adriatic Sea. Legend: N%—relative abundance, F%—frequency of occurrence for a given prey item, W%—weight percentage and IRI% is the Index of relative Importance for each prey item, indet.—not identified taxa.
TAXAN%W%%FOIRI%
Mollusca
Bivalvia
Corbula gibba0.170.141.640.01
Laevicardium oblongum0.170.701.640.02
Bivalvia indeterminata52.0042.2266.3985.85
Gastropoda
Gastropoda indet.1.481.685.740.25
Aporrhais pes pelecani0.780.465.740.10
Cerithium vulgatum11.3716.9916.396.38
Gibbula sp.0.690.613.280.06
Turritella communis8.513.456.561.08
Cephalopoda
Cephalopoda indet.0.520.764.100.07
Mollusca indet.0.090.070.820.00
Sipunculida
Sipunculida indet.0.870.102.460.03
Sipunculus nudus0.171.141.640.03
Aspidosiphon muelleri4.510.5111.480.79
Polychaeta
Eunicidae0.430.040.820.01
Polychaeta indet.2.601.0616.390.82
Crustacea
Ostracoda
Ostracoda indet.0.090.000.820.00
Decapoda
Anomura indet.1.563.817.380.54
Diogenes pugilator0.170.031.640.00
Paguristes eremita2.087.787.381.00
Paguridae1.483.124.100.26
Decapoda—Natantia1.300.019.020.16
Processa sp.1.651.034.920.18
Decapoda Brachyura0.260.712.460.03
Thia scutellata0.432.751.640.07
Crustacea indet.0.260.532.460.03
Amphipoda 0.090.000.820.00
Amphipoda indet.
Echinodermata
Holothurioidea indet.0.090.000.820.00
Labidoplax digitata4.601.4915.571.30
Invertebrata indet.
Invertebrata indet.0.870.355.740.10
Teleostei
Teleostei indet.0.698.466.560.82
Table 2. Diet parameters of Myliobatis aquila from the northern Adriatic Sea for males, females, juveniles and adults.
Table 2. Diet parameters of Myliobatis aquila from the northern Adriatic Sea for males, females, juveniles and adults.
OverallJuvAdultsMalesFemales
average number of prey9.4410.697.319.3110.03
average prey weight (g)1.221.181.371.141.46
average meal size (g)11.5612.5810.0110.5914.6
Shannon–Wiener index1.491.951.441.71.97
Average Predator weight (kg)1.320.951.971.321.38
prey W/predator weight (%)2.592.832.162.283.51
TROPH index3.21 ± 0.403.23 ± 0.433.17 ± 0.243.20 ± 0.393.25 ± 0.44
Morisita/Horn index 0.880.86
Table 3. Comparison of Myliobatis aquila feeding habits from data obtained in various studies in Mediterranean and adjacent areas. The data are expressed in terms of relative abundance (% prey items). Different mollusc taxa are shaded in grey.
Table 3. Comparison of Myliobatis aquila feeding habits from data obtained in various studies in Mediterranean and adjacent areas. The data are expressed in terms of relative abundance (% prey items). Different mollusc taxa are shaded in grey.
AreaSea of MarmaraAzoresCroatian AdriaticSaros BayTunisian CoastsNorthern Adriatic
Source[22][60][20][23][19]this work
Anthozoa0.51
Nemertina 1.16 0
Bivalvia14.43 33.00 44.4252.34
Gastropoda 85.7030.3365.8617.2022.83
Scaphopoda46.28 3.99 0.00
Cephalopoda 0.811.220.77
Mollusca indet.13.88
Stomatopoda 0.00 2.82
Decapoda3.9814.309.739.7611.178.95
Polychaeta14.95 4.696.106.423.04
Sipuncula 15.76 0.775.56
Echinodermata 0.00 1.284.69
Ascidiacea 0.00 1.28
Pisces5.66 0.609.7613.860.69
other 7.32 1.91
sum100.00100100100100100
number of specimens151016585523122
empty stomachs6124 9
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Lipej, L.; Battistella, R.; Mavrič, B.; Ivajnšič, D. Diet of the Common Eagle Ray, Myliobatis aquila (Linnaeus, 1758) in the Northern Adriatic Sea. Fishes 2025, 10, 311. https://doi.org/10.3390/fishes10070311

AMA Style

Lipej L, Battistella R, Mavrič B, Ivajnšič D. Diet of the Common Eagle Ray, Myliobatis aquila (Linnaeus, 1758) in the Northern Adriatic Sea. Fishes. 2025; 10(7):311. https://doi.org/10.3390/fishes10070311

Chicago/Turabian Style

Lipej, Lovrenc, Riccardo Battistella, Borut Mavrič, and Danijel Ivajnšič. 2025. "Diet of the Common Eagle Ray, Myliobatis aquila (Linnaeus, 1758) in the Northern Adriatic Sea" Fishes 10, no. 7: 311. https://doi.org/10.3390/fishes10070311

APA Style

Lipej, L., Battistella, R., Mavrič, B., & Ivajnšič, D. (2025). Diet of the Common Eagle Ray, Myliobatis aquila (Linnaeus, 1758) in the Northern Adriatic Sea. Fishes, 10(7), 311. https://doi.org/10.3390/fishes10070311

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