Understanding the characteristics of blooms of the zooplanktivorous invasive predator M. leidyi is increasingly important, due to its ongoing successful invasion of new regions and its potential impact on zooplankton densities and ecosystem production. The top-down effect of the predation pressure on zooplankton, which is especially significant during intense blooms of M. leidyi, can favor a substantial decrease in zooplankton and a correlated increase in phytoplankton (Shiganova 1998; Finenko et al. 2006; Tiselius and Møller 2017), accompanied by a decline in fish stocks, as already experienced in the Black and Caspian Seas (Shiganova and Bulgakova 2000).
Considering the importance of Venice Lagoon as a nursery area, the massive blooms experienced in the last years in this habitat raise concerns regarding its already ongoing and future effects on ecosystem production and ecosystem services. Hence, given the importance of this area within the "blue economy" with various business categories falling under this definition, such as environmental regulation, fish farming and fishing, providing additional insights into the potential impact of invasive species on the ecosystem, on which also human activities gravitate, is crucial to reduce the gap between economic demand and environmental protection.
In this study, Mnemiopsis was found to be present at all 16 investigated stations in the Venice Lagoon, which are representative of different environmental conditions and showed a seasonal persistence (at different life stages), hence tolerating the measured temperature range of 3.0 - 30.5°C. These findings confirm its high ecological plasticity, which makes it a successful invader and highlights the need to improve our knowledge about this species, including its feeding preference. Spatial differences in abundance found within the lagoon may be driven not only by prey availability, but also by hydrodynamic processes that accumulate M. leidyi in specific areas. Seasonal differences are evident with highest abundances in terms of biovolume [ml/m³] detected during summer (July-October) with temperature as the main abiotic driver, likewise stated by many authors, e.g., Kremer (1994), who mentioned temperature and prey abundance as key factors affecting its seasonal patterns. Other factors that make semi-enclosed lagoons especially vulnerable are potential low oxygen levels that can occure especially during summer (Bernardi-Aubry et al. 2020). However, M. leidyi, as other gelatinous species, can potentially benefit from it as they are generally more tolerant to hypoxia compared to their preys. Decker et al. (2004) showed a reduced jumping frequency of the copepod A. tonsa favoriting capture rates as it makes less-tolerant prey more vulnerable to predation in hypoxic waters.
Several authors have studied the feeding preferences of M. leidyi in the past. However, to our knowledge, this is the first study applying DNA metabarcoding based on NGS technologies to investigate its dietary composition. The primary benefit of this method compared to morphology-based identification in analyzing the feeding preference is the detection of also partially digested prey and cryptic species. However, the (relative) quantification of prey items that are more effortlessly digestible, e.g., soft organisms like fish larvae, or that have been ingested beforehand, may be underestimated.
The literature, with morphology-based identification, indicates that M. leidyi’s diet often reflects the composition of ambient preys (e.g., Javidpour et al. 2009; Madsen and Riisgård 2010; Granhag et al. 2011). Copepods are often dominating the diet of M. leidyi, but also meroplanktonic larvae of polychaetes, mollusks, decapods and barnacles are fed (e.g., Kremer 1979; Purcell et al. 2001; Colin et al. 2015). In our study, the diet of M. leidyi was very variable, but mainly included copepods, decapods, cladocerans, gastropods, bivalves and polychaetes, but also echinoderms, Nemertea and cnidarians, hence a composition that characterizes a typical lagoon community. During winter, the dietary composition shows a peak in polychaete larvae, in consistency with Larson (1987) and McNamara et al. (2010), which reported the ingestion of polychaetes larvae by Mnemiopsis. However, this noticeable difference of the winter samples may also be a result of higher uncertainty due to the smaller sample size (see biovolume during winter).
Similarly to Decker et al. (2004) and Roohi et al. (2010), also in our study A. tonsa was the most abundant copepod species, both in-situ and in the gut content. However, in general, copepods and cladocerans were less represented in the gut content than in-situ, while decapod and mollusk larvae were more abundant in the gut content, indicating a preferential feeding on the latter ones. In fact, due to the capture mechanisms of Mnemiopsis, less mobile organisms such as mollusks seemed to be a very vulnerable prey of M. leidyi, which is consistent with the literature (e.g., Madsen and Riisgård 2010; Marchessaux et al. 2021). Nevertheless, species-specific differences in mobility are of importance as well. Within copepods, for example, smaller species like Oithona nana and O. davisae or Euterpina acutifrons seemed to be captured preferentially. In comparison, the larger species Temora stylifera, being potentially faster, are less abundant in the gut content as they may escape from M. leidyi more easily. It has to be kept in mind, especially regarding the holoplanktonic copepods, that DNA metabarcoding does not allow to differentiate between life stages. Therefore, more than size differences between copepods species, the actual life stage of each species at that specific moment may have a more significant effect on the vulnerability of particular species to the feeding pressure of M. leidyi. The diet of Mnemiopsis is known to differ at different life stages. While larvae and post-larvae consume primarily microphyto- and microzooplankton prey like dinoflagellates or ciliates (Sullivan and Gifford 2004), adults feed on a variety of holo- and meroplankton organisms (Shiganova and Bulgakova 2000). In this study, a standard sampling net with a mesh size of 200 µm was used to collect the in-situ zooplankton community. The ingested preys may include zooplankton smaller than 200 µm, like nauplii or bivalve larvae, which might be underestimated in the sampled zooplankton community. However, as in this study, only adult Mnemiopsis individuals above 1.5 cm were included in the gut content analysis, the use of a standard mesozooplankton net with a mesh size of 200 µm should not have a strong bias of the comparison of the in-situ zooplankton community with the gut content. The selectivity of the 200 µm sampling net could be another explanation for the higher relative abundance of small sized organisms in the gut content compared to the in-situ zooplankton assemblages. Hence, the additional use of e.g., an 80 µm plankton net to better describe the smaller size fraction of the community could be beneficial (Pansera et al., 2014).
As previously mentioned, the VL represents an ecosystem of huge ecological but especially socio-economic importance. It is not only a vitalnursery area for fishes, but it is also an area for mussel, clam and crab aquaculture. On the one hand, in this study, no significant correlation between the in-situ abundance of fish larvae or eggs and its abundance the gut content was found, indicating no direct predation on fish larvae or eggs. This is probably explained by the dominance of benthic fish species in the VL, like Zosterisessor ophiocephalus, and the fact that the spawning time may not coincide with the major blooming period of Mnemiopsis (Franzoi et al. 2010). Moreover, the reproductive strategy of lagoon resident fish species is adapted to prevent seaward flushing of eggs and larvae by spawning demersal eggs attached to the aquatic vegetation or other substrates, while the planktonic larval stage is reduced or lacking (Dando 1984). Therefore, rather than direct predation on fish eggs and larvae, competition for zooplankton may have an impact on the fish stock in the VL, where socio-economic functioning is also affected by the clogging of fishermen’s nets by Mnemiopsis (Palmieri et al. 2014). On the other hand, the impact Mnemiopsis seems to have on the meroplanktonic compartment of the zooplankton community may increase the pressure on the local economy and industrial production. In fact, while in other geographic areas the major concern regarding the arrival and large blooming of Mnemiopsis refers mainly to the fish stocks and its associated economy, in the VL and the Northern Adriatic coasts, Mnemiopsis’ impact may be greater on the meroplanktonic compartment, hence on the mussel, clam, and crab fishery and aquaculture.