Abundance patterns of intertidal caprellids associated with the alga Corallina elongata were studied along the Iberian Peninsula. Nineteen stations were selected. The Mediterranean coast showed higher seawater temperature and conductivity and lower dissolved oxygen and turbidity than the Atlantic coast. Cover of C. elongata was negatively correlated with seawater temperature and decreased towards the Mediterranean sites. Number of caprellid species and abundances were higher in stations in the Strait of Gibraltar, probably related with the currents' dynamic around this area. In general, caprellids associated with C. elongata were more abundant along the Mediterranean than along the Atlantic, in spite of the lower algal cover in Mediterranean stations. Caprella hirsuta and C. liparotensis were restricted to the Mediterranean coast whereas C. penantis was only found in the Atlantic and the Strait of Gibraltar. Although C. grandimana has been reported along the Mediterranean, its presence on Corallina elongata was restricted to the Strait of Gibraltar. Oppositely, Caprella liparotensis, which usually inhabits Atlantic and Mediterranean intertidal ecosystems, was only present in the Mediterranean and did not coexist with C. penantis along the Atlantic. Probably, competence process among C. grandimana, C. hirsuta, C. liparotensis and C. penantis are regulating the distribution patterns on the intertidal alga Corallina elongata. On the other hand, canonical correspondence analysis showed a clear preference of Caprella penantis at sites with high oxygen concentrations, whereas C. hirsuta was mainly influenced by higher temperature and conductivity.

The distribution and abundance patterns of the caprellids (Crustacea: Amphipoda) associated with the alga Stypocaulon scoparium (L.) Kützing were studied on a spatial scale along the Iberian Peninsula. Fourteen stations and five environmental factors were considered (seawater temperature, conductivity, dissolved oxygen, turbidity and pH). The Atlantic coast was characterized by lower temperature and conductivity as well as higher values of oxygen and turbidity than the Mediterranean coast, probably associated with a higher swell. Cover of S. scoparium was higher in the Strait of Gibraltar than in the remaining stations, coinciding with maximum values of number of caprellid species and Shannon diversity, while abundances were higher in the Mediterranean. Caprella acanthifera and C. liparotensis were dominant at the stations of the Strait of Gibraltar, C. penantis and C. danilevskii were restricted to Atlantic stations, while C. grandimana and C. hirsuta were very abundant in all the Mediterranean stations; Deutella schieckei, C. acanthifera discrepans, Phtisica marina and Pseudoprotella phasma, were only found at some Mediterranean sites. Multivariate analysis showed that some species, such as C. penantis, significantly correlated with oxygen concentrations, while other species, such as P. phasma, D. schieckei and Phtisica marina were associated with stations with higher values of conductivity, temperature and turbidity. Number of caprellids associated with S. scoparium was significantly higher than in other dominant algae from the Mediterranean and the Atlantic, probably due to the optimal morphological structure of this seaweed for caprellids to cling.

The objectives of this study were to describe the peracaridean fauna associated to the algae Corallina elongata from the Strait of Gibraltar, and explore possible biodiversity and biogeographical patterns of variation along the north–south and Atlantic–Mediterranean axes across the Strait of Gibraltar. Twenty-five stations were selected along the north and south coasts of the Strait to cover the broadest possible range of human pressure and environmental conditions, including both natural rocky shores and artificial breakwaters. The alga Corallina elongata was selected as substrate, and the peracaridean crustaceans were identified to species level and classified in geographical distribution groups. Univariate and multivariate analyses were conducted to explore and confirm patterns of variation. Forty peracarid species were collected, most of them with an Atlantic–Mediterranean distribution (67%) with only the gammarid Parhyale eburnea, being an endemic Mediterranean species. The most common species collected during the present study were the gammarids Hyale stebbingi, Jassa marmorata, Stenothoe monoculoides and Ampithoe spp., the caprellids Caprella grandimana and C. penantis, the isopod Ischyromene lacazei and the tanaid Tanais dulongi. The number of species per station and the diversity index were significantly higher in the stations located along the north side of the Strait of Gibraltar. However, the two-way ANOVA discarded differences between north and south due to the type of substrate (natural versus artificial) and degree of human pressure. Based on peracaridean assemblages, the Strait of Gibraltar behaves as a whole and homogeneous region, with a very similar faunal composition in all stations and there is not a clear gradient of species substitution from the Mediterranean to the Atlantic stations. Provided that we selected the same substrate in all stations, and that statistical analyses revealed that differences between north and south stations were not due to environmental factors such as anthropogenic stress or type of substrate, we should look to historical biogeographical reasons to explain the higher diversity in the north side of the Strait of Gibraltar.