The diet analysis of G. melastomus confirmed that several mesopelagic resources, including cephalopods, bathypelagic fish, and decapods, provide the primary food source for this species, and contributed to filling the knowledge gap highlighted by D’Iglio et al. (2021) for the central Tyrrhenian Sea. Food habits of blackmouth catshark revealed changes during its ontogenetic development, with an increasing diversity along size. Due to this, the diet of small and young individuals mainly consisted of fish, whereas the diet of intermediate-sized individuals (15–35 cm) also included decapod crustaceans. The diet of large individuals resulted in a more balanced composition with similar proportions of crustaceans, cephalopods, and fish.
The diet of G. melastomus exhibited a high diversity in species composition, since it mainly included bathypelagic (Myctophidae) and demersal (Chlorophthalmidae) fish in the smaller individuals. Prey diversity increased in the individual of intermediate size, and additionally included demersal crustaceans (Sergestidae) and cephalopods (Sepiolidae), together with pelagic Euphausiidae. The most diversified diet was observed in adult individuals, and comprised demersal crustaceans (e.g. Penaeidae, Sergestidae, Polychelidae), as well as pelagic crustaceans (e.g. Euphausiidae, Pasiphaeidae, and Hyperiidae amphipods), which may have been captured during their daily migrations in mid-water (Onsrud and Kaartvedt 1998; Elder and Seibel 2014). Moreover, adult individuals prey on many bathypelagic (Stomiidae, Myctophidae) and demersal fish (e.g. Merlucciidae, Macrouridae, Soleidae, Bothidae). Finally, cephalopods included both bathypelagic (Histioteuthis bonnellii) and demersal species (Rossia macrosoma and Heterotheutis dispar), as well as they are an important food source for other bathyal selachians (Bello 1997; Rey et al. 2005; Fanelli et al. 2009; Valls et al. 2011; Darna et al. 2019; D’Iglio et al. 2021).
The presence of typically benthic preys, such as fish of the families Bothidae and Soleidae, as well as the crustaceans of the family Paguridae, indicate that G. melastomus is a benthic feeder and scavenger, as reported by other authors (Anastasopoulou et al. 2013; Taleb Bendiab et al. 2016; Barría et al. 2018). In addition, the presence of bathypelagic species highlighted that G. melastomus behaves as a supra-benthic predator capable of moving from the bottom to catch prey in the bathypelagic environment, confirming the behaviour previously reported for G. melastomus in other areas of the Tyrrhenian Sea (D'Iglio et al. 2021).
Small individuals were the only ones that displayed preys with a high specific abundance and occurrence, as the Costello model revealed, and such evidence is an indication of their specialization. In fact, preys with a low specific abundance and presence were only occasionally consumed by a few individuals. In contrast, individuals of intermediate and large size did not display a close trophic relationship with specific prey species, since no dominant species were found in their diet. This result suggests that, for most of its life cycle, blackmouth catshark has a generalist behaviour and, accordingly to the prey ecological habits, the results confirmed that it actively catches preys on the bottom, as well as in the supra-benthic layer, feeding on all available preys, as also reported by other authors (Anastasopoulou et al. 2013; Barria et al. 2018; D’Iglio et al. 2021).
Further differences in diet related to depth distribution and size are also revealed by the CA analysis and were consistent with the behaviour of both preys and predator. Based on the prey composition of the stomach contents, the differences among the size groups were confirmed and could be explained by the vertical movement of blackmouth catshark. Indeed, the dynamics of the species suggest that younger individuals move through the water column towards surface waters. Specifically, the smaller blackmouth catshark are more active near the continental plateau (250–450 m depth), while the adult individuals are found at depths between 450 and 900 m. Our results are also in agreement with other studies which highlighted variations in the blackmouth catshark habits and composition of their preys during their growth in relation to the availability of the resources (Carrassón et al. 1992; Olaso et al. 2005; Fanelli et al. 2009).
Our study allowed to show that blackmouth catshark ingest a wide variety of plastics in terms of shape, colour, and size. Particularly, there was a high proportion of microplastics (1–5 mm), and transparent filaments and films were the most abundant particles found in the samples. Regarding the causes of the observed pattern, we hypothesize that the major source of plastic contamination is related with plastic stored on the sediments, where films and filaments are known to be the major components, as a consequence of their greater sinking capacity if compared to spheres and fragments (Chubarenko et al. 2016). Conversely, low-density particles (polystyrene spheres found in the samples) tend to float (Hidalgo-Ruz et al. 2012), but they can even sink after biofouling. Therefore, this can change the weight of the specific particle, making it easier to detect them within deep-sea sediments (Eriksen et al. 2014; Alomar and Deudero 2017; Valente et al. 2019). Finally, several polymers of polyamide have been found in the stomach contents of G. melastomus. As this polymer is the main component of the nylon used in the ropes and fishing nets (Deopura et al. 2008), the wastes from fishing gears appeared a considerable cause of plastic pollution in the Tyrrhenian Sea, in line with other studies in deep-sea areas (Pruter 1987; Murray and Cowie 2011; Lusher et al. 2013; Neves et al. 2015; Güven et al. 2017; Welden and Cowie 2017).
In summary, significant sources of plastic pollution were identified through the GAM model, which was used to disentangle the role of different factors in the ingestion of plastic by G. melastomus. Based on this, two predictors were identified as major contributors affecting the number of plastic items ingested. Indeed, out of all the potential sources of variability considered (i.e. fish size, sex, depth of capture, distance from the coast, short-term trophic level, stomach fullness, and amount of MaP in terms of the number of items and total weight), only the distance from the coast and the total weight of MaP on the seafloor significantly affect the number of plastic items ingested by G. melastomus. The decreasing amount of plastics associated with the distance from the coast could be interpreted as the move away from the source of pollution. This variable was one of the major factors that negatively affect the amount of waste on the seabed along the coast (Coll et al. 2012; Steer et al. 2017; Sbrana et al. 2020; Franceschini et al. 2021). Accordingly, the coast of the Tyrrhenian Sea (GSA9) hosts a series of fishing grounds along a narrowed shelf that receives plastic waste through some important sources including the rivers Tiber and Arno (Inghilesi et al. 2008; Montreuil and Ludwig 2013; Montuori et al. 2016; Crosti et al. 2018).
Similarly, the amount of MaP present on the bottom is positively correlated with the ingestion by G. melastomus. This second factor deserves an additional reflection since, in this study, the amount of MaP was associated with the ingestion of smaller particles and not directly associated to the MaP. It is well known that plastic waste at sea is undergoing a process of fragmentation and miniaturization, so it is logical to expect that large quantities of MaP can generate corresponding quantities of MeP and MiP (Crawford and Quinn 2017; Chamas et al. 2020). Similar studies have already identified positive relationships between MiP ingestion and MaP hotspots at the sea bottom (Alomar et al. 2020; Franceschini et al. 2021). Particles, however, increase their ability to be transported as they become smaller (Zhang 2017), and thus oceanographic factors can greatly affect the spread and accumulation of different sizes of plastic particles. Leaving aside the oceanographic dynamics of plastic pollutants, the biomonitoring of their abundance and distribution as revealed by the analysis of stomach contents in non-commercial species, such as the blackmouth catshark, could represent an important approach to assessing the risk of contamination of fisheries landings of commercial species. A future study could examine whether the pattern returned by the blackmouth catshark could serve as a proxy for those of other species, including key resources for fisheries, given that the samples collected during this study were taken over important fishing grounds (Russo et al. 2018).
In conclusion, this study sheds light on two related topical issues: trophic ecology and plastic pollution. Several novel insights have been gained into the feeding ecology of G. melastomus in the central Tyrrhenian Sea, which is an area where this predator is very abundant and commonly caught as a by catch in bottom trawl fisheries, but adequate information is lacking. Additionally, this study allowed us to examine the ability of G. melastomus to provide information about the amount of macroplastics accumulated on the sea bottom. The dietary approach, which begins with a description of the organism's feeding habits, allows further exploration of plastic ingestion with great potential and versatility for most biological or ecological studies (Baker et al. 2014; Mahesh et al. 2019). The opportunistic feeding behaviour of G. melastomus affects the incidental ingestions of numerous plastic particles likely confused for other preys, or indirectly ingested by feeding, in line with the results of previous research (Anastasopoulou et al. 2013; Cartes et al. 2016; Alomar and Deudero 2017; Valente et al. 2019). The identification of plastic in the stomach contents of a certain species can indirectly reflect the presence of this pollutant in the marine environment (Cardozo et al. 2018), and our results indicate a correlation between the amount of macroplastics present on the seabed and the frequency of ingestion of plastic pieces by this species. Given this, and considering the generalist behaviour of blackmouth catshark, its abundance and distribution, and interaction with plastic hotspots, we consider, in line with Fossi et al. (2018), this species as a suitable candidate for developing a monitoring programme for the presence of plastics on the seabed, as requested in the Marine Strategy Framework Directive for European waters.