1. Cestum veneris Lesueur, 1813 (Ctenophora) : a Rare Guest in the Northern Adriatic SeaManja Rogelja, Martin Vodopivec, Alenka Malej, 2022, original scientific article Abstract: We report the occurrence of the ctenophore Cestum veneris Lesueur, 1813 in the Gulf of Trieste (northern Adriatic Sea). Three individuals were found in March 2022, representing the first documented record in the scientific literature in more than 130 years. They were observed and photographed in the sea, and two specimens were brought to the Piran Aquarium, where their behaviour was observed. Historical and recent literature on the occurrence of this species in the Adriatic Sea was reviewed. The occurrence in the Gulf of Trieste is likely related to the transport of C. veneris with the currents, and therefore we present simulation results for the period January-February. Despite the relatively rare detections near the coast, this species is probably widespread in the offshore waters of the central and southern Adriatic.
Keywords: winter gelatinous plankton, Ctenophora, distribution, Gulf of Trieste
Published in DiRROS: 31.07.2026; Views: 139; Downloads: 85
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3. Long-Term Study of Zooplankton Biomass in the South-Eastern Part of the Gulf of Trieste (Adriatic Sea)Jan Malej, Tjaša Kogovšek, Martin Vodopivec, Janja Francé, Patricija Mozetič, Matevž Malej, Alenka Malej, 2025, original scientific article Abstract: We present a dataset of zooplankton biomass spanning more than four decades (1974–2019). The dry mass of zooplankton ranged from below 10 to almost 100 mg m-3, was highest in 1989–2002 and declined thereafter. The interannual variability of zooplankton dry mass in the period 1993–2018 was compared with the regional atmospheric index WeMOi, chlorophyll a and frequency of jellyfish blooms. Increased dry mass values were associated with a positive phase of the regional atmospheric index WeMOi, higher chlorophyll a and a lower frequency of jellyfish blooms.
Keywords: net zooplankton, dry mass, time series, chlorophyll a, jellyfish blooms
Published in DiRROS: 24.06.2026; Views: 212; Downloads: 153
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4. Review of Eastern Adriatic hydromedusae: unravelling two centuries of recordsIvona Onofri, Davor Lučić, Alenka Malej, Barbara Gangai, 2026, review article Abstract: The Eastern Adriatic Sea is biogeographically complex, yet knowledge of its hydromedusae is fragmented across two centuries of uneven sampling and shifting taxonomy. This review integrates historical faunistic records (pre-1950), mid-century programmes (1950–2000), modern quantitative time series (post-2000), and citizen science observations to compile an updated checklist of 98 non-siphonophoran hydrozoan taxa. Records are synthesised across eight sub-regions, although the most continuous research has focused on the Northern Adriatic and the open South Adriatic. The clearest long-term signal is in the Northern Adriatic, where diversity collapsed by >60% from the 1960s to the 1980s, largely through the loss of meroplanktonic taxa with benthic polyp stages under eutrophication-driven hypoxia. Since 2000, oligotrophication coincides with a partial recovery, marked by the re-emergence of meroplankton and episodic intrusions of oceanic holoplankton (including Trachymedusae) linked to circulation regimes (BiOS). For the open South Adriatic, bathymetric distributions and diel vertical migration patterns are synthesised to characterise a persistent offshore core. Taxonomic updates and information on non-indigenous and bloom-forming taxa are provided. Methodological biases and gaps, especially polyp-stage ecology and spatial sampling voids, are highlighted, and routine DNA barcoding is recommended. The checklist provides a baseline for tracking change in a shifting ecosystem.
Keywords: gelatinous zooplankton, ecological collapse and recovery, citizen science, meroplankton, taxonomic checklist
Published in DiRROS: 02.03.2026; Views: 394; Downloads: 268
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5. Spatially explicit individual-based model reveals the mauve stinger jellyfish distribution in the Mediterranean SeaMartin Vodopivec, Alenka Malej, 2025, original scientific article Abstract: The mauve stinger Pelagia noctiluca is a holoplanktonic oceanic jellyfish known for its painful sting and large outbreaks at irregular intervals. As jellyfish field observations are almost exclusively limited to coastal regions, there are many unknowns about the spatial and temporal dynamics of most species. We approach the problem using a spatially explicit individual-based model for the mauve stinger in the Mediterranean Sea. The newly developed module for the OpenDrift Lagrangian particle tracking tool includes diel vertical migrations, fooddependent growth modeled on Michaelis–Menten kinetics, and custom mortality related to shallow waters and stranding. In the first part of the study, we analyze the resulting parameter space searching for solutions that ensure a stable Pelagia noctilucapopulation in the western Mediterranean Sea. In the second part, we conducted several 21-year simulations. Despite its relative simplicity, the model reveals for the first time the dynamic spatio-temporal distribution of Pelagia noctiluca. We demonstrate how spatially explicit individual-based model can be used to generate a realistic spatio-temporal distribution of marine zooplankton, even with a limited set of observations and loosely constrained parameters. The model code is easy to modify and freely available.
Keywords: individual-based model, agent-based model, zooplankton, Lagrangian particle tracking, ocean circulation, jellyfish
Published in DiRROS: 05.06.2025; Views: 1363; Downloads: 1044
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6. Macrojellyfish (Scyphozoa and Ctenophora) in two enclosed marine systems (the sea of Marmara and Adriatic sea) : a reviewMelek Isinibilir, Alenka Malej, Davor Lučić, Martin Vodopivec, 2024, independent scientific component part or a chapter in a monograph Abstract: Although geographically well apart, there are interesting similarities between the ecosystems of the Sea of Marmara and the Adriatic Sea. In this study, following an evaluation of the ecological problems as well as oceanography, productivity, plankton biomass of these seas, macro-jellyfish ecology of the two ecosystems were discussed comparatively. The Sea of Marmara and the Adriatic Sea are among the areas where increases in jellyfish and mucilage events have been observed in recent decades. Environmental problems such as global warming, oxygen depletion, eutrophication, overfishing, intense maritime traffic, and habitat destruction are causing opportunistic and alien species to increase in both abundance and number of species. As biodiversity declines due to habitat destruction, so does the ecosystem’s means and ability to combat the invasion of alien species in both ecosystems. Accordingly, disruption of lower food chains can put pressure on the entire ecosystem and affect socio-ecological prospects. The number of species of Scyphozoa and Ctenophora was found to be higher in the Adraitic Sea compared to the Sea of Marmara. Mass occurrences with a clear seasonal pattern and related to the geographical location were observed for Aurelia aurita, Rhizostoma pulmo, Chrysaora hysoscella and non-native Mnemiopsis leidyi in the Marmara Sea. However, although A. aurita and R. pulmo show widespread blooms according to the seasons and regions in the Adriatic Sea, C. hysoscella and M. leidyi produced blooms limited to a few areas. Negative and positive interactions of macrojelyfishes with human were also considered. Semi-enclosed seas such as the Sea of Marmara and the Adriatic Sea are representative of many temperate coastal waters where climate change and anthropogenic stressors are increasing, and the ecosystem has changed from a fish-dominated to a jellyfish-dominated ecosystem.
Keywords: jellyfish, eutrophication, diversity, introduce, fisheries
Published in DiRROS: 07.05.2025; Views: 1125; Downloads: 837
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7. Feeding habits of the invasive ctenophore Mnemiopsis leidyi in the Gulf of Trieste (Adriatic Sea)Borut Mavrič, Danijel Ivajnšič, Davor Lučić, Alenka Malej, Lovrenc Lipej, 2025, original scientific article Abstract: The diet of the invasive ctenophore Mnemiopsis leidyi in the Gulf of Trieste was examined during its peak occurrence in the summer and early-autumn months (July to October) from 2017 to 2019, through the analysis of stomach contents. Altogether 506 specimens were individually caught for the analysis. A total of 3215 prey items were isolated and identified. Copepods emerged as the primary prey (relative abundance 66.7%), followed by cladocerans (7.7%), and bivalve larvae (6%). Notably, specimens of M. leidyi constituted a significant portion of the diet (5.4.%), providing further evidence of cannibalism within this species. Copepods were also the most commonly occurring prey items in the diet of M. leidyi. Most of them were represented by calanoid and cyclopoid nauplii (48.2%), followed by a harpacticoid M. norvegica (28.3%), and calanoids (26.8%). Other frequently occurring taxa were bivalve larvae (19.3%), M. leidyi (18.7%), and cladoceran Penilia avirostris (16.1%). The rate of cannibalism peaked in July, coinciding with a period of limited food availability. Additionally, the study revealed that fish eggs and larvae were infrequently found in the stomachs of M. leidyi. However, the presence of massive aggregations of M. leidyi may impact microzooplankton populations in late summer or autumn, potentially leading to competition with small pelagic fish.
Keywords: bioinvasion, cannibalism, comb jelly, competition, diet, Mediterranean Sea
Published in DiRROS: 10.04.2025; Views: 1242; Downloads: 693
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8. Jellyfish modulate bacterial dynamic and community structureTinkara Tinta, Tjaša Kogovšek, Alenka Malej, Valentina Turk, 2012, original scientific article Abstract: Jellyfish blooms have increased in coastal areas around the world and the outbreaks have become longer and more frequent over the past few decades. The Mediterranean Sea is among the heavily affected regions and the common bloom - forming taxa are scyphozoans Aurelia aurita s.l., Pelagia noctiluca, and Rhizostoma pulmo. Jellyfish have few natural predators, therefore their carcasses at the termination of a bloom represent an organic-rich substrate that supports rapid bacterial growth, and may have a large impact on the surrounding environment. The focus of this study was to explore whether jellyfish substrate have an impact on bacterial community phylotype selection. We conducted in situ jellyfish - enrichment experiment with three different jellyfish species. Bacterial dynamic together with nutrients were monitored to assess decaying jellyfish-bacteria dynamics. Our results show that jellyfish biomass is characterized by protein rich organic matter, which is highly bioavailable to ‘jellyfish - associated’ and ‘free - living’ bacteria, and triggers rapid shifts in bacterial population dynamics and composition. Based on 16S rRNA clone libraries and denaturing gradient gel electrophoresis (DGGE) analysis, we observed a rapid shift in community composition from unculturable Alphaproteobacteria to culturable species of Gammaproteobacteria and Flavobacteria. The results of sequence analyses of bacterial isolates and of total bacterial community determined by culture independent genetic analysis showed the dominance of the Pseudoalteromonadaceae and the Vibrionaceae families. Elevated levels of dissolved proteins, dissolved organic and inorganic nutrient release, bacterial abundance and carbon production as well as ammonium concentrations characterized the degradation process. The biochemical composition of jellyfish species may influence changes in the amount of accumulated dissolved organic and inorganic nutrients. Our results can contribute insights into possible changes in bacterial population dynamics and nutrient pathways following jellyfish blooms which have important implications for ecology of coastal waters.
Keywords: sea, medusae, research programmes, DGGE, Pelagia noctiluca, gel electrophoresis, oligotrophic waters, Adriatic Sea, jellyfish, gelatinous zooplankton, bacteria, flavobacteria
Published in DiRROS: 26.03.2025; Views: 3549; Downloads: 995
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9. State of the art of the marine non-idigenous flora and fauna in SloveniaLovrenc Lipej, Borut Mavrič, Martina Orlando-Bonaca, Alenka Malej, 2012, original scientific article Abstract: Authors provide the state of knowledge on marine non-indigenous species in waters off Slovenia. According to published records and authors unpublished data at least 15 non-indigenous species were up to date recorded in the Slovenian part of the Adriatic sea. Eleven species were considered as established and four species as casual. The vectors of introduction are various, comprising Lessepsian migration, mariculture and shipping. The majority of non-indigenous taxa were recorded in the mediolittoral belt and coastal lagoons. The low number of non-indigenous species so far recorded in the Slovenian part of the Adriatic sea could be explained by various factors. Among them, the most reasonable are the lack of scientific attention, the small proportion of Slovenian Sea and low winter temperatures, which represents a physiological barrier for the survival of newcomers. The number of recorded species is far from being satisfactory. Therefore, we expect that the list of non-indigenous species will be enlarged in the nearby future.
Keywords: biologija, vegetacija, ribe, favna, morje, Jadransko morje, biodiverziteta, Sredozemsko morje, neavtohtone vrste, Gambusia hoolbroki, Sardinella aurita, severni Jadran, obalno morje, morske alge, alohtone vrste, meridionalizacija, bioinvazije, varstvo narave, Tržaški zaliv, biološki kazalci
Published in DiRROS: 26.03.2025; Views: 867; Downloads: 607
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10. Is it possible to determine the economic impact of jellyfish outbreaks on fisheries? A Case Study - SloveniaBojan Nastav, Matevž Malej, Alenka Malej, Alenka Malej, 2013, original scientific article Abstract: The interdependence between the economy and the environment is becoming a fact of increasing importance. Productive coastal areas have been recognised as one of the most valuable ecosystems from an ecological and socio-economic point of view. In this paper we focus on the massive presence of jellyfish in the northern Adriatic and their effect on the Slovenian economy. Our results indicate that high jellyfish abundance in 2004 resulted in a reduction of fish catch, value added, gross income, and employment in the fishing industry. Moreover, the government and the EU have acknowledged the impact of jellyfish on the fishing industry by allocating financial help to the fishermen involved. We attempted to assess other factors influencing the fishing industry but none were statistically significant. The input-output analysis has not revealed a significant impact on the entire Slovenian economy presumably due to the small contribution of the fishing industry to Slovenian GDP. Our work is a first attempt to relate ecological changes such as jellyfish outbreaks in the northern Adriatic to the wider economy and we suggest that such a methodology can be applied to other countries/regions and to other natural phenomena affecting the economy.
Keywords: morje, objavljeni prispevki na konferenci, morsko ribištvo, trajnostni razvoj, gospodarski aspekti, meduze, množično pojavljanje, distribucija, numerično modeliranje, klimatske spremembe, hipoteze, Pelagia noctiluca, Jadransko morje, gospodarske politike
Published in DiRROS: 26.03.2025; Views: 1295; Downloads: 948
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