Hydrozoan species richness in the Mediterranean Sea: past and present

Abstract

The Mediterranean hydrozoan fauna (Siphonophora excluded) comprises 400 species; most (68%) occur in the Atlantic Ocean, 20% are endemic to the Mediterranean, 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 community), 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 populations of native species, altering extinction probabilities. A more reliable evaluation 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.

Looking for long-term changes in hydroid assemblages (Cnidaria, Hydrozoa) in Alboran Sea (South-Western Mediterranean): a proposal of a monitoring point for the global warming

“…Also, the total number of species did not show significant changes between the compared periods. This is in agreement with other studies dealing with long-term changes in hydroid assemblages with global warming that suggest that the number of species tends to remain stable, but the structure and composition of the assemblages change Gravili et al 2013). Although Peña Cantero and García Carrascosa (2002) carried out a faunistic study with a higher sampling effort (time, depth range and number of samples) than our study in 2007, the samples in 2007 provided a good representation of the global diversity of hydroid assemblages.…”

First record of Aequorea macrodactyla (Cnidaria, Hydrozoa) from the Israeli coast of the eastern Mediterranean Sea, an alien species indicating invasive pathways

“…The report of A. macrodactyla in the Red Sea (Schmidt 1973) is old, more than 40 years ago, and there have been no further records of A. macrodactyla. Moreover, Gravili et al (2013) proposed to remove species from the list of taxonomic records that have not been found for a reasonable time (several decades), and we concur.…”

“…Only 8% are considered Indo-Pacific and these are mainly restricted to the eastern Mediterranean basin. Twenty years later Gravili et al (2013) presented more or less the same picture for non-siphonophoran hydrozoan fauna distribution in the Mediterranean Sea. From the 400 species they recorded, as much as 68% originate from the Atlantic Ocean, 20% are endemic to the Mediterranean, 8% are Indo-Pacific and the other 4% are defined unclassifiable.…”

“…A year later in 2012, Mamish et al (2012) observed Aequorea globosa near the coast of Syria and considered its appearance an indicator of climate change in the eastern Mediterranean marine environment. Gravili et al (2013) published a comprehensive review to update the diversity poll of species of what is known as a non-Siphonophora Hydrozoa (NSH) in the Mediterranean, which are traditionally studied as a separate group as their polyp and/or medusa stage is distinctive in global distributions (Kramp 1961;Kramp 1965). In this recent work A. macrodactyla was absent from the lists of Mediterranean species but mentioned as being present in neighboring areas: the Gulf of Eilat, the Gulf of Aden and the Red Sea .…”

Sertularia marginata (Cnidaria: Hydrozoa) in the Mediterranean: an alien species in expansion?

“…A natural dispersion into the Eastern Mediterranean through the Suez Canal does not seem probable (Picard, 1958a;Morri et al, 2009). But entry through the Strait of Gibraltar and dispersion to the Eastern Mediterranean using its own means also seems unlikely, given that the species had never been detected at any intermediate point before 2007 (Boero & Fresi, 1986;De Vito, 2006;Puce et al, 2009;Gravili et al, 2013).…”