Biol. Mar. Mediterr. (2014), 21 (1): 2-9 A. MALEJ 1 , T. KOGOVšEK 1,2 , S.-I. UYE 2 1 Marine Biology Station Piran, National Institute of Biology, Fornac ˇe, 41 - 6330 Piran, Slovenia. alenka.malej@mbss.org 2 Graduate School of Biosphere Science, Hiroshima University, Japan. MEDUSAE - PEST OR RESOURCE FOR HUMANS? MEDUSE - PESTE O RISORSA PER L’UOMO? Abstract - Not so long ago medusae were considered a trophic dead-end and a pest to humans while recently it has become increasingly clear that they may play a significant role in the pelagic food web and may also be a valuable resource to humans. Our contribution will highlight the double role medusae play as they present both opportunities and challenges for humankind. We also review medusa role in pelagic food webs. Key-words: jellyfish blooms, socio-economic impacts, medusa fisheries, ecological role. Introduction - The benefits of medusae as a marine resource are traditionally recognised in Asian cultures. Eating medusae has been a tradition in China for more than a thousand years and they are considered a delicacy. In the rest of the world medusae are prevailingly viewed as stressors both for humans and marine ecosystems. Fascination with medusae and negative public perceptions are not new in western cultures and ancient mythology depicted them as frightening creatures. With the increase in sea-based human activities and the more frequent bloom phenomena in many coastal areas around the world this negative perception has been accentuated. Medusae blooms are not of recent origin and fossil evidence points to the existence of mass occurrences on a geological time scales (Cartwright et al., 2007). However, several studies indicated more severe and frequent outbreaks over the past few decades in different coastal areas (Brotz et al., 2012; Kogovšek et al., 2010; Roux et al., 2013; Uye, 2008). Conversely, Condon et al. (2013) who analysed long-term trends in gelatinous taxa within Cnidaria, Ctenophora and Thaliacea concluded that there was no robust evidence for a global increase in jellyfish. The strongest non- random trend found was an approx. 20-year periodicity and among 37 analysed data sets which included only time series exceeding a decade 27% experienced increases, 16% showed decreases while the remaining data trends were not clear. Although no significant increase was found over the whole observational period (1874-2011), results indicated a weak yet significant increase since 1970. The rise in jellyfish populations in coastal areas seems to be both a symptom of the cumulative deterioration of the ecosystems as well as the outcome of combined climate and anthropogenic stressors. Although there is no clear direct evidence that anthropogenic drivers are responsible for increases, a large amount of correlative evidence suggests such connections and, globally, six of the top ten highly disturbed marine systems (Halpern et al., 2008) coincided with locations that have had jellyfish blooms (Purcell, 2012). While global trends of medusae blooms and causes of population fluctuations require further rigorous research to clarify trends there is also an immediate need for assessment of the socio-economic impacts of medusae blooms (Gibbons & Richardson, 2013) and the development of management strategies to cope with problems (Lucas et al., 2014). The detrimental effects of medusae blooms to humans are widely acknowledged, conversely, the benefits and diverse role of medusae in the ecosystem have been largely overlooked. In our contribution we are focusing on scyphomedusae (Scyphozoa, Cnidaria) and highlight both positive and negative impacts that they may have for humans, we also briefly review their ecological role in marine systems.