Scyphomedusae and Ctenophora of the Eastern Adriatic: Historical Overview and New Data

Abstract

One of the obstacles to detecting regional trends in jellyfish populations is the lack of a defined baseline. In the Adriatic Sea, the jellyfish fauna (Scyphozoa and Ctenophora) is poorly studied compared to other taxa. Therefore, our goal was to collect and systematize all available data and provide a baseline for future studies. Here we present phenological data and relative abundances of jellyfish based on 2010–2019 scientific surveys and a “citizen science” sighting program along the eastern Adriatic. Inter-annual variability, seasonality and spatial distribution patterns of Scyphomedusae and Ctenophore species were described and compared with existing historical literature. Mass occurrences with a clear seasonal pattern and related to the geographical location were observed for meroplanktonic Scyphomedusae Aurelia solida, Rhizostoma pulmo, and to a lesser extent Chrysaora hysoscella, Cotylorhiza tuberculata and Discomedusa lobata. Holoplanktonic Pelagia noctiluca also formed large aggregations, which were seasonally less predictable and restricted to the central and southern Adriatic. Four species of Ctenophora produced blooms limited to a few areas: Bolinopsis vitrea, Leucothea multicornis, Cestum veneris and the non-native Mnemiopsis leidyi. However, differences between Adriatic subregions have become less pronounced since 2014. Our results suggest that gelatinous organisms are assuming an increasingly important role in the Adriatic ecosystem, which may alter the balance of the food web and lead to harmful and undesirable effects.

Long-term Monitoring of Carnivorous Gelatinous Macrozooplankton in the Area of Dubrovnik-Neretva County (Croatia)

“…Values represent the highest frequency of jellyfi sh occurrence in a given month, regardless of the number of reports received. Several authors have already used similar methods to present the results of macrozooplankton research [27,43,16,48,62,79,54].…”

“…Furthermore, annual semi-quantitative abundance was calculated from the monthly datasets using the following equation previously used by [62]: The blue button P. porpita is a fl oating colony of hydroids inhabiting mainly the superfi cial part of the water column as neuston. The species has been found in the Atlantic Ocean, Indo-Pacifi c Ocean and Mediterranean Sea [15,38], including the Adriatic Sea [35,69].…”

“…R. pulmo is one of the most abundant jellyfi sh in the Mediterranean Sea [13] often appearing in large numbers in recent decades, with consequences for fi sheries and tourism [55]. Blooms have been recorded most frequently in semi-enclosed and highly productive marine ecosystems, such as the Northern Adriatic Sea [43,62].…”

“…Otherwise, the number and occurrence of this previously rarely seen species is increasing throughout the Mediterranean Sea, especially along the eastern Adriatic coast [52]. The largest number of fi nds and the greatest abundance were found in Boka Kotorska Bay [84,62].…”

“…Most of the jellyfi sh data in the Adriatic Sea relate to the northern and western Adriatic coasts, and there is much less information for the southern parts. Much attention has been paid only to the mass occurrences of the scyphomedusa Pelagia noctiluca in the 1970s and 1980s [62], as well as to the phenomenon of consistently high numbers of Aurelia relicta in the marine lake on the island of Mljet [10,51,80,1,44]. Apart from the studies on P. noctiluca blooms and the intensive research on A. relicta in the lake of Mljet Island, the only records of jellyfi sh macrozooplankton refer to the presence/ absence of certain species in this region [29] and the occurrence of some species in the period 1995-2001 [9].…”