Benthopelagic shrimps have a widespread geographic and bathymetric distribution from high latitudes in both hemispheres to intertropical waters (Crosnier and Forest 1973, Casanova and Judkins 1976, Serejo et al. 2007). Many continental slope shrimps show cyclic movements associated with the photoperiod, as shown in some pasiphaeid and sergestid shrimps (Froglia and Giannini 1982, Cartes 1993a, Aguzzi et al. 2007). This vertical daily migration behaviour shown by some of these species provides them with an important role in the transfer of matter and energy from the upper primary productive layers of the ocean, where these species tend to feed during the night, down to the epibenthic community of the continental slope, where they dwell during the day (Cartes 1993b, Herring and Roe 1988, Naylor 2010). Benthopelagic shrimps are thus a fundamental food item for fish, other crustaceans and cephalopods with nektobenthic habits on the continental slope and deep sea (Garrison and Link 2000, Fanelli and Cartes 2008). Moreover, some of these species play an important ecological role and have potential for exploitation as commercial target species, as is the case of Pasiphaea japonica (Nanjo and Ohtomi 2009). Studies of the biology and ecology of pasiphaeid shrimps have been conducted in a few regions, such as the Japan Sea (Nanjo 2007, Nanjo and Ohtomi 2009), the Mediterranean Sea (Orsi-Relini and Relini 1990, Company et al. 2001), and the northeastern and southeastern Atlantic Ocean (Matthews and Pinnoi 1973, Gibbons et al. 1994, Kensley and Schotte 2006).

The family Pasiphaeidae has a worldwide distribution, and over 90 species are known to date (Hayashi 1999, Tavares and Cardoso 2006, De Grave and Fransen 2011). In the northeast Atlantic up to 18 Pasiphaeidae species are known to occur, eight of them belonging to the genus Pasiphaea (Casanova and Judkins 1977, d’Udekem d’Acoz 1999, Koukouras 2000), but only two species of Pasiphaeidae are present in the Mediterranean Sea, namely Pasiphaea sivado (Risso, 1816) and Pasiphaea multidentata (Esmark, 1866).

Pasiphaea sivado is a benthopelagic caridean shrimp commonly captured as a by-catch by demersal trawling on the upper slope across the eastern Atlantic and the Mediterranean Sea (González-Gurriarán and Olaso 1987, Abelló et al. 2002) down to a maximum depth of around 800 m (Abelló et al. 1988, 2002). Biological studies on this species have been mainly performed in the western Mediterranean, where it has been reported to reproduce continuously throughout the year, although peaking in autumn-winter, and a longevity of up to two years has been estimated (Company et al. 2001, 2003). The species has been shown to predate mainly on euphausiids, calanoid copepods, and epibenthic peracarid crustaceans (Lagardère 1972, Cartes 1993b).

Pasiphaea multidentata inhabits benthic boundary layers on the middle and lower slope down to 2000 m depth (Cartes 1993c, Abelló et al. 2002, Tecchio et al. 2011), with juveniles inhabiting shallower waters than adults (Company et al. 2001). It also performs vertical migrations into upper water layers during the night, mainly but not exclusively restricted to juveniles (Cartes 1993a, Aguzzi et al. 2007, Simão et al. 2014). In the northwestern Mediterranean the species shows a marked seasonality in reproduction, with ovigerous females being only present from September to February, and it reaches an estimated longevity of around 3.5 years (Company et al. 2001, Ramirez-Llodra et al. 2007). It is an active nocturnal feeder on benthopelagic crustaceans such as Gennadas elegans, P. sivado, P. multidentata, sergestid shrimps and small mesopelagic fish such as myctophids and Cyclothone spp. (Cartes 1993a).

The main objectives of this paper are to analyse the main characteristics of the bathymetric and geographic distribution, population size structure and some reproduction-related characteristics of both P. sivado and P. multidentata in the western Mediterranean, as well as to relate the patterns obtained with geomorphologic and hydrographic characteristics.

The information obtained during the studied series of trawl surveys was used to analyse the distribution patterns of density and biomass of the two species of the genus Pasiphaea present in the western Mediterranean throughout the southern and eastern coasts of the Iberian Peninsula down to depths of around 800 m. Furthermore, their population size structure was also described. While the overall depth range distribution of both P. sivado and P. multidentata found in the present study falls within the ranges described in the literature for these species (Cartes 1993c, Koukouras et al. 2000, Fanelli et al. 2007), several patterns were studied in detail given the broad geographic range of the surveys (Bertrand et al. 2002).

Most of the previously available information on distribution patterns, size structure and other population characteristics of the two species was restricted to the northernmost sampled area: the Catalan coasts (Abelló et al. 1988, 2002, Cartes 1993a,b,c, Company et al. 2001, 2003, Aguzzi et al. 2007, Ramirez-Llodra et al. 2007). Orsi-Relini and Pinca (1990) and Orsi-Relini and Relini (1990), respectively, provided information on reproduction and trophic interactions in the Ligurian Sea (NE of the western Mediterranean basin). No biological or population information are available from other areas within the distribution range of the two species. Our results have shown the occurrence of marked differences in both distribution and population characteristics between the southwestern areas of the Alboran Sea, located in the Algerian basin of the western Mediterranean, and the northeastern area, the Catalan Sea in the Catalano-Balearic basin.

Thus, the geographic distribution of densities of P. sivado has first shown a heterogeneous distribution pattern with two main nuclei: one in the Alboran Sea, especially in its western area, and one along the Catalan coasts in the northeast. Additionally, the bathymetric distribution in the WALB sector was markedly different from that in the other geographic sectors. In this area, the species occurred in both much shallower and deeper waters, reaching markedly higher densities at depths between 100 and 250 m, where it is largely absent in the remaining sectors. The bathymetric distribution of P. multidentata extended to shallower waters in the WALB sector. This pattern was especially evident in P. sivado, and is in agreement with the occurrence of temporal upwellings located along the northwesternmost region of the Alboran Sea, in the area around Malaga (Vargas-Yáñez and Sabatés 2007). These upwellings are responsible for the occurrence of high primary production cells which have been shown to provide plankton blooms followed by high epibenthic shrimp secondary production (Fanelli and Cartes 2004). The occurrence of these upwelling cells close to the coast is caused by the interaction of the permanent strong eastward inflow of Atlantic water into the Mediterranean through the nearby Strait of Gibraltar. Inside the Mediterranean Sea, the inflow is influenced by the steep continental slope of the southern Iberian continental margin, and intense local westerly winds (Millot, 2005, Vargas-Yáñez and Sabatés 2007), which contribute to offshore displacement of the surface water, allowing the inflow of deeper, cooler and nutrient-richer waters to shallower areas.

Within the sampled depth ranges, densities of P. sivado have been shown to be much higher than those of P. multidentata, in agreement with Company et al. (2001), who suggested that the greater fecundity output of P. sivado would support its higher population densities when compared with P. multidentata. Both juvenile and adult P. sivado are known to perform upward vertical migrations at night, while remaining on the bottom or close to it (epibenthic layer) during the day (Cartes 1993a, Aguzzi et al. 2007). In this way they can be considered benthopelagic species. Night-time migrations into the water column have also been reported for P. multidentata, but mainly restricted to juveniles, with the apparent exception of the adults (CL>30 mm), which were assumed to perform bathymetric displacements along the seabed (Cartes 1993a, Cartes et al. 1993, Aguzzi et al. 2007). However, recent research has shown that adult P. multidentata are also able to migrate vertically in the water column (Simão et al. 2014). The day-time sampling schedule of the present trawl surveys would accordingly be suitable for sampling both juveniles and adults of the two species.

The relationship between mean density and percentage occurrence by depth stratum was used to delimit depth strata showing high figures of both density and occurrence, which could be assumed to be the optimal depth ranges for the species. These were clearly located between 300 and 500 m in P. sivado, and deeper than 500 m (down to the deepest sampled depth, 800 m) in P. multidentata. It must be emphasized that the sampling schedule clearly encompassed the whole bathymetric distribution range of P. sivado in the study area (Abelló et al. 1988, 2002, Cartes et al. 1994), whereas in P. multidentata it did not reach the deepest distribution of the species, which has been reported to occur down to 2261 m depth (Cartes 1993c) in the Mediterranean.

Both species increased in size with depth, as shown in Company et al. (2001), with juveniles being found in much shallower waters than adults. In P. sivado, populations in the Alboran Sea reached larger sizes than those in the northwestern Mediterranean and in intermediate sectors. This implies that the population dynamics in these areas are different, and probably linked to the high productivity of the Alboran Sea (Fanelli and Cartes 2004). Recruitment in P. sivado was detected throughout the study area, in agreement with the main autumn-winter reproductive season of the species observed in the Catalan Sea (Company et al 2001), and was mainly located at depths shallower than 300-400 m. By contrast, recruitment in P. multidentata was not present in the Algerian basin sectors (Alboran Sea and Alacant regions), while it was marked in the Catalano-Balearic basin. The reproductive season in P. multidentata, from studies made in the Catalan Sea (Company et al. 2001, Ramirez-Llodra et al. 2007), is centred in late autumn-winter. The absence of recruitment in spring in the geographic sectors belonging to the Algerian basin may imply that seasonality of reproduction in this species significantly differs between the populations inhabiting the two basins.

The analysis of the available environmental information clearly detected differences between the two species with respect to temperature and salinity on the bottom, as well as between juveniles and adults of the two species. Temperature and salinity windows (25-75 percentiles) of adults of the two species clearly overlapped, but the salinity range of P. multidentata was narrower than that of P. sivado, and much narrower than that of juveniles of the species, while the temperature range was contiguous but non-overlapping at a limiting temperature of 13.25°C. By contrast, the temperature-salinity window of P. sivado juveniles was much larger, and more widely overlapping, than that of the adults, which were associated with higher salinities. This finding clearly showed that in both species, but particularly in P. sivado, juveniles had a wider thermohaline window, indicating they are able to cope with a wider variability in temperature and salinity conditions. A close dependence on a restricted salinity range has also been shown in deep-sea and continental slope crustaceans, such as Parapenaeus antennatus and Aristeus antennatus (Guijarro et al. 2008, 2009), while species displacements related to movements of water masses have also been found to occur at wide temporal and geographical scales (Cartes et al 2009, 2011), linked to relatively small density changes in seawater masses.

The differential distribution pattern between juveniles and adults of both P. sivado and P. multidentata (Company et al. 2001) could be related, as in other caridean shrimps inhabiting the continental slope (Company and Sardá 1997, Carbonell and Abelló 1998), to trophic resource partitioning between the two ontogenetic phases, which would allow a lesser degree of intraspecific competition. The dietary overlap between these species has been shown to be low due to the different size spectra of their respective prey items (Cartes 1993b). Adults of P. sivado and juvenile P. multidentata share a similar body size range, and are both important food items for P. multidentata adults (Cartes 1993b). Thus, size segregation would also be helpful to avoid intraspecific predation.

The present study has shown that in both P. sivado and P. multidentata, the populations inhabiting the Alboran Sea were clearly differentiated from those in the Catalano-Balearic basin concerning their bathymetric distribution, density, maturity size, and population size structure. The populations in the intermediate Alacant sector showed more affinities with the Alboran Sea populations than with the northern populations, especially in P. multidentata, which is consistent with their general affinity for the Algerian basin.