ORIGINAL ARTICLE Hydrozoan species richness in the Mediterranean Sea: past and present Cinzia Gravili 1 , Cristina Gioia Di Camillo 2 , Stefano Piraino 1 & Ferdinando Boero 1,3 1 Dipartimento di Scienze e Tecnologie Biologiche e Ambientali, Universita ` del Salento, Lecce, Italy and CoNISMa -Consorzio Nazionale Interuniversitario per le Scienze del Mare 2 Dipartimento di Scienze della Vita e dell’Ambiente, Universita ` Politecnica delle Marche, Ancona, Italy 3 CNR-ISMAR, Genova, Italy Keywords Hydrozoa; Mediterranean Sea; nonindigenous species; zoogeography. Correspondence Dr Cinzia Gravili, Laboratory of Zoology and Marine Biology, Dipartimento di Scienze e Tecnologie Biologiche e Ambientali, Di.S.Te.B.A., Universita ` del Salento, Via Prov.le Lecce-Monteroni, 73100 Lecce, Italy. E-mail: cinzia.gravili@unisalento.it Accepted: 2 October 2012 doi: 10.1111/maec.12023 Abstract The Mediterranean hydrozoan fauna (Siphonophora excluded) comprises 400 species; most (68%) occur in the Atlantic Ocean, 20% are endemic to the Medi- terranean, 8% are of Indo-Pacific origin, and 4% are non-classifiable. There are 69 nonindigenous (NIS) species in the basin: 44% of these are casual (recorded just one or very few times), 28% established (widely recorded in the basin), 6% invasive (established NIS that are able rapidly or largely to disseminate away from the area of initial introduction, having a noticeable impact on the recipient com- munity), and 22% questionable (of doubtful taxonomic status). Entry through the Suez Canal and range expansion through the Gibraltar Strait, often enhanced by ship traffic, appear to be the main processes for recent species introductions, but uncertainties remain for many NIS. Species additions immediately result in larger local or regional species pools, but the newcomers might impact on popu- lations of native species, altering extinction probabilities. A more reliable evalua- tion of the species pool can be accomplished by adding new species when they enter the taxonomic record (i.e. the records of any taxon in all types of literature), and by removing species that have not been found for a ‘reasonable’ time (e.g. several decades). Of the 400 non-siphonophoran hydrozoan species known to occur in the Mediterranean Sea, positive records in the last 10 years are available for 156 species (39%), whereas records of the remaining 244 species are older than a decade: 67 species have not been recorded for 41 years, 13 for 31–40 years, 79 for 21–30 years, and 85 for 11–20 years. Introduction Species lists and distribution records are fundamental to biodiversity research (Costello et al. 2001; Mora et al. 2011). Biodiversity research has a long history in the Mediterranean Sea, which is one of the best known seas globally. Yet, reliable fauna inventories and easily accessi- ble regional species lists are only available for a few tax- onomic groups. Lists of nominal species, such as the ERMS (European Register of Marine Species; Costello et al. 2001), do not provide resolution of records at the regional scale (Boero 2002; Occhipinti-Ambrogi et al. 2011). Both historical and ecological factors contribute to high biodiversity in the Mediterranean (Bianchi & Morri 2000; Bianchi 2007). In particular, pronounced seasonality is thought to facilitate the spatial coexistence of species by creating niche differentiation in time, and by allowing the occurrence of species with temperate and tropical affinities in the same basin (Coma et al. 2000). The biodiversity of the Mediterranean Sea is changing dramatically, partly linked to global warming, which is thought to contribute to the establishment of tropical nonindigenous species in the basin (Coll et al. 2010; Marine Ecology 34 (Suppl. 1) (2013) 41–62 ª 2013 Blackwell Verlag GmbH 41 Marine Ecology. ISSN 0173-9565