The distribution of living species and communities is determined by a combination of large-scale biogeographic history, migration patterns and environmental conditions (Wiens and Donoghue 2004). The biodiversity of the Mediterranean Sea is therefore primarily shaped by its geographical position and the main connections with surrounding oceans, its geological history and the prevailing oceanographic conditions. The Mediterranean is the largest of the seas peripheral to the main oceans, being located between three continents and occupying an elongated basin of 4000 km length (Tyler 2003). It is naturally connected to the Atlantic Ocean through the Strait of Gibraltar (320 m depth) and to the Black Sea through the Dardanelles Strait (103 m depth). Since 1869, it has also been artificially connected to the Red Sea by the Suez Canal. Between six and five million years ago, the connection between the Atlantic and the Mediterranean Sea through the Strait of Gibraltar was closed, giving rise to the Messinian Salinity Crisis (Manzi et al. 2013, Vasiliev et al. 2017). This salinity crisis involved the complete disappearance of former deep benthic fauna, which means that the present deep Mediterranean organisms have originated from Atlantic migrations since 5.5 million years ago (Lugli et al. 2015).

The Mediterranean Sea is considered a biodiversity hotspot, with an estimate of about 17000 marine species (Coll et al. 2010). Most organisms presently living in the Mediterranean have an Atlantic origin, but tropical species have entered the basin for decades, either actively through the Suez Canal (Lessepsian migration) or the Strait of Gibraltar, or passively mainly through ship transport (Coll et al. 2010). During the last few decades, climate change has also affected the distribution and relative abundances of Mediterranean marine species (Bianchi and Morri 2004, Lejeusne et al. 2010, Lasram et al. 2010).

A gradient of species richness from the northwestern to the southeastern Mediterranean is driven by different oceanographic conditions between the two basins (Coll et al. 2010). The main differences are related to a decrease in productivity, as well as an increase in temperature and salinity, from the western to the eastern basin (Danovaro et al. 1999, Turley et al. 2000). As a result of such contrasting conditions, the western and eastern basins display biological similarities to the Atlantic ecosystem (higher number of cold-temperate species) and the Indo-Pacific ecosystem (higher number of subtropical species), respectively (Coll et al. 2010).

In this paper we investigate whether the different environmental conditions between the western and eastern basins are reflected in the cephalopod assemblages found throughout the Mediterranean. Cephalopods represent excellent case studies for analysing species-environment interactions owing to their high sensitivity to environmental conditions (Pierce et al. 2008, Rodhouse et al. 2014). They have short life spans (1.5–2 years at most) and a high population turn-over, so they show rapid responses to changes in external conditions. Although there are several studies on cephalopod assemblages relative to Mediterranean areas in both the western (e.g. Quetglas et al. 2000, González and Sánchez 2002, Fanelli et al. 2012) and eastern (e.g. Lefkaditou et al. 2003, Krstulović Šifner et al. 2005, 2011) basins, all of them are restricted to local scales and differences in sampling and data analysis prevent the comparison of results in most cases (Gaertner et al. 2013).

Like all other faunal components, the bulk of the current Mediterranean teuthofauna comes from the Atlantic Ocean (Bello 2003). Up to now, a total of 70 cephalopod species have been reported in the Mediterranean (Bello 2008, 2016) but only 53 of them are represented by well-established populations (Bello 2003). Ten cephalopods are endemic or quasi-endemic in the Mediterranean (Bello 2003, and pers. comm.), accounting for 14.3% of its teuthofauna.

In this work, data from standardized bottom trawl surveys carried out during the last 22 years by EU Mediterranean countries were used. The wide spatiotemporal scale of these scientific surveys and the data standardization used during the sampling and data processing allow a reliable comparative analysis of the cephalopod assemblages inhabiting the whole Mediterranean basin. In addition to investigating spatial differences in cephalopod assemblages, the available database will also be used to analyse temporal changes in species composition during the last two decades.

A total of 47 different cephalopod species have been taken from the Mediterranean Sea during the last 22 years, accounting for 67.1% of the cephalopods recorded in the basin up to now. This percentage highlights the high number of cephalopods that have not been captured by bottom trawl gears during the study period—one third of recorded species—despite the large spatiotemporal sampling (23749 sampling stations). Although the westernmost bioregion (Iberian-Lions; B1) held a higher number of cephalopod species (S=42 vs 36) than the easternmost bioregion (Aegean Sea; B5), there was no clear trend in species richness throughout the Mediterranean, as has already been reported in a recent study (Keller et al. 2016). Whereas Mangold and Boletzky (1988) suggested a general decrease in the species richness from west to east, Bello (2003) noted that this theory should be revised according to available updated information. Even for the three main biogeographical provinces traditionally used to divide the Mediterranean (the western and eastern basins and the Adriatic Sea; Mangold and Boletzky 1988, Lejeusne et al. 2010), the species richness obtained with our data was the same in both the western and eastern basins (S=43), but was lower in the Adriatic (S=35). As already reported for fish (Gaertner et al. 2007, Granger et al. 2015), the absence of a west-east decreasing trend suggests that primary production (i.e. food availability) is possibly not the major factor explaining large-scale patterns of species richness of demersal cephalopods. At a global scale it was found that although net primary productivity at the ocean surface seems to drive diversity patterns of pelagic cephalopods, coastal species diversity can be predicted by climate (SST) and non-climate (spatial area) variables (Rosa et al. 2008).

Given that cephalopods are highly sensitive to changing environmental conditions (Pierce et al. 2008, Rodhouse et al. 2014), it was expected to find spatial differences in the species composition over the Mediterranean. However, our results revealed no differences in species composition among bioregions, but only changes in the species’ relative abundances, which led to significant differences between all pairs of bioregions in intermediate waters (upper and middle slope assemblages). Assemblages from the upper and middle slope, for instance, are dominated by the squid lllex coindetii in the easternmost bioregions (B3, B4, B5), whereas sepiolids predominate in the remaining bioregions. Assemblages from the shelf were also significantly different between all pairs of bioregions except in two cases, the Iberian-Lions (B1) vs Tyrrhenian Sea (B2) and the Ionian Sea (B3) vs Strait of Sicily (B6). The homogeneity of shelf assemblages in the B1-B2 bioregions might be related to the counter-clockwise circulation of Atlantic waters in the western basin, which produces a basin-wide cyclonic gyre through the Tyrrhenian vein (Millot and Taupier-Letage 2005) that might hinder exchanges of low-moving life stages such as cephalopod paralarvae. The strategic situation of the Strait of Sicily might explain the differences found between the B1-B2 and the B3-B6 assemblages: the surface isotherm of 15°C for February (the coldest month in the year) follows quite closely the biogeographic boundaries of the Strait of Sicily, and biotic differences between the two basins are probably due to differences in temperature regime, i.e. a physiological barrier separating the western and eastern Mediterranean (Bianchi 2007). In addition, the Strait of Sicily is also characterized by high mesoscale activity (Nieblas et al. 2014) and high fishing rates (Colloca et al. 2017). The lower slope was the most homogeneous stratum since it had the highest number of non-significant pairwise comparisons. This faunal homogeneity is a general feature of deep-sea waters (McClain and Hardy 2010, Rex and Etter 2010) and is proably related to the homogeneous conditions and the circulation pattern at such great depths (Millot and Taupier-Letage 2005). Results also revealed that the Aegean bioregion (B5) was significantly different from all other bioregions, a finding which might be related to its high spatial bathymetric variability (Millot and Taupier-Letage 2005, Nieblas et al. 2014) and low fishing exploitation rate (Colloca et al. 2017). Compared with the rest of bioregions, for instance, the broader upper slope of the Aegean Sea was characterized by higher abundances of the cuttlefishes Sepia elegans and S. orbignyana, which is in accordance with the sustainable exploitation exerted in the area with a high prevalence of small-scale fisheries (Colloca et al. 2017).

In most bioregions (Iberian-Lions, Tyrrhenian, Ionian and Adriatic) the species richness showed a slight decrease with depth from the shelf to the middle slope before plummeting to the lowest values on the lower slope. The Aegean Sea and the Strait of Sicily, however, did not show that decreasing trend and their richness values at the deepest stratum were much higher than those of the remaining bioregions. Such high values might be due to the fact that the lower slope stratum obtained from our cluster analysis for these two bioregions encompassed a much wider depth range than the rest of the bioregions. Our results did not show the hump-shaped trend of species richness with depth reported in previous cephalopod studies carried out in the Mediterranean (González and Sánchez 2002, Krstulović Šifner et al. 2011, Keller et al. 2016).

Except in the deepest stratum of the Adriatic Sea (B4), there were no clear dominant species in any cephalopod assemblage, but a rather homogeneous mix of different species. This indicates a continuous substitution of species with depth rather than discrete assemblages separated by distinct boundaries. In all bioregions, a number of eurytopic (generalist) species appeared in most strata (e.g. A. veranyi, S. oweniana, I. coindetii) and some specialized species characterizing specific strata appeared in shallow (e.g. E. moschata, S. officinalis) and deep (e.g. B. sponsalis, Histioteuthis spp.) waters. On the lower slope of the Adriatic Sea, two deep-sea squid species (T. eblanae and H. reversa) accounted for up to 75% of the cephalopod assemblage. This finding agrees with the well-known comparatively poor deep-sea teuthofauna of the Adriatic, despite the fact that it reaches 1200 m depth in the southern basin and has a relatively wide connection with the Ionian Sea (Mangold and Boletzky 1988, Bello 2003, Keller et al. 2016).

As stated above, the bulk of the current Mediterranean teuthofauna comes from the Atlantic Ocean (Bello 2003). However, a comparison of the cephalopod assemblages from the westernmost bioregion (Iberian-Lions; B1) with those from the Gulf of Cadiz (Silva et al. 2011), an Atlantic area adjacent to the Strait of Gibraltar, revealed great differences in species contribution. Such differences in species relative abundance might be related to the contrasting oceanographic conditions in these two areas (Millot and Taupier-Letage 2005, Tanhua et al. 2013).

Despite claims of biodiversity loss as a result of high anthropogenic impacts in the Mediterranean (Danovaro 2003, Calvo et al. 2011, Vasilakopoulos et al. 2014), the species composition of the cephalopod assemblages has not changed during the last 22 years. As in the case of the spatial analysis, temporal differences were only found in the species’ relative abundance of three bioregions: Iberian-Lions, Ionian and Strait of Sicily. The Strait of Sicily was the only bioregion showing significant differences in all four bathymetric assemblages, whereas the Iberian-Lions and the Ionian bioregions displayed differences in three and two assemblages, respectively. As stated above, both natural factors (15°C isotherm boundary, complex topography, high mesoscale variability) and anthropogenic factors (high exploitation rates) might explain the existence of differences at all depths in the Strait of Sicily. The remaining bioregions (Tyrrhenian, Adriatic and Aegean Seas) showed no long-term temporal differences in species’ relative abundance, a finding which might be related, at least in the Tyrrhenian and Aegean Seas, to reduced interchanges of water masses owing to the main topographic characteristics (Millot and Taupier-Letage 2005).

Previous studies of cephalopod assemblages in the Mediterranean confirm our results. A long-term study carried out in the western basin (Fanelli et al. 2012) found no significant differences in the bathyal cephalopod assemblages or in the species abundances between the two time periods analysed (1985-92 vs. 2007-10). In a shorter-term study (2000-2007) from the Gulf of Cadiz, the species composition also remained unchanged and only the relative abundance of different groups varied (Silva et al. 2011). The lack of changes in species composition during the last two decades also applies to fish (Granger et al. 2015), which might indicate that the demersal ecosystem was already altered before the beginning of our time series or that noticeable changes will only be revealed at longer temporal scales (Granger et al. 2015, Keller et al. 2016).

Given that temperature is a key driver of the biogeographic distribution (Puerta et al. 2014) and habitat selection (Lauria et al. 2016) of cephalopods, it is expected that climate change will have significant effects on many temperate species (Hastie et al. 2009). Climate change projections for the Mediterranean indicate that it might be an especially vulnerable region (Giorgi and Lionello 2008, Albouy et al. 2013). In fact, a consistent warming trend has already been reported in the 1982-2016 period (Pastor et al. 2017). As a whole, 25% of the Mediterranean Sea continental shelf was predicted to experience a total modification of endemic species assemblages by the end of the 21st century (Lasram et al. 2010). Some cephalopod populations from the Mediterranean show an increasing trend in abundance that might be related to the global change (Doubleday et al. 2016, Keller et al. 2017). Currently, however, none of the Lessepsian species (Octopus aegina, O. cyanea, Sepioteuthis lessoniana and Tremoctopus gracilis) reported in the Mediterranean up to now (Bello 2016) were found in our samples. As most Lessepsian species inhabit depths shallower than those prospected during the MEDITS and/or non-trawlable grounds, they in principle are not prone to being captured by trawling gears. A few Lessepsian fish species have been recorded in the MEDITS surveys in the south Aegean and Cretan Seas (Peristeraki et al. 2017), but no Lessepsian cephalopod species have been recorded so far, although some of them have well-established populations in the eastern Mediterranean (Zenetos et al. 2011; Lefkaditou, unpublished data). It is therefore expected that these Lessepsian migrants will spread westwards, as some fish species have already done (Bianchi 2007, Calvo et al. 2011), probably leading to spatiotemporal differences in cephalopod assemblages through the Mediterranean, at least during the westward spreading phase. Our study has shown, however, that spatiotemporal differences during the last two decades only affected some specific bioregions and were restricted to variations in the relative species abundance from a common pool of species inhabiting the whole Mediterranean.