Molluscs from the Gazul mud volcano and its adjacent areas in the northern Gulf of Cádiz were studied using differ­ent sampling methods. This mud volcano has vulnerable deep-sea habitats and a potential high biodiversity. A total of 232 species were identified from the taxocoenosis and thanatocoenosis, of which 86 are new records for the Spanish margin of the Gulf of Cádiz, three of them are new records for Spanish waters and two species are new to science. The high species richness observed could be related to the combination of different sampling methods, the study of the thanatocoenosis, the high habitat heterogeneity and the geographical location of the Gazul mud volcano between different biogeographical regions. The best-represented species were Bathyarca philippiana, Asperarca nodulosa, Leptochiton sp., Astarte sulcata and Limopsis angusta. The thanatocoenosis harboured, with low frequency, species that are typical of northern latitudes, species indicating past seepage, species from the shelf and species restricted to particular hosts. The taxocoenosis found in different areas of Gazul (the mud volcano edifice, erosive de­pression and adjacent bottoms) generally displayed significant differences in multivariate analyses. Furthermore, the environmental parameters related to environmental complexity and food availability displayed the highest linkage with the molluscan fauna.

A checklist of marine Mollusca recorded in Spanish jurisdictional waters is presented, based on a thorough literature search and a limited input of recent field work. The list is detailed according to the five demarcations of the Marine Strategy Framework Directive (NOR, Spanish north coast; SUR, Spanish coast of the Gulf of Cádiz; ESAL, Strait of Gibraltar and Alboran Sea; LEBA, East coast of Spain and Balearic Islands; CAN, the Canary Islands). The list differentiates coastal species living from the supralittoral zone to the shelf break, deep-sea benthic or demersal species, and holoplanktonic species, and also distinguishes between non-indigenous species and species endemic to a particular demarcation. A total of 2466 marine Mollusca species have been reported from Spanish waters, of which 1126 are recorded only from waters surrounding the Iberian Peninsula, 498 are recorded from the Canaries only, and 842 are shared. That total number represents around 5.5% of the global marine molluscan richness. Shore and shelf species are the most numerous (1837), followed by deep-sea benthic species (490) and pelagic species (139), but the contribution of the deep-sea species is most noteworthy in NOR. All eight molluscan classes were represented, but solenogastres and scaphopods were notably more represented than usual in NOR. Species richness in SUR and ESAL was higher than would be expected from their areas, supporting the claim that Andalusian waters have an exceptionally diverse molluscan fauna. Thirteen species collected from INDEMARES cruises are here reported for the first time in Spanish waters. The species recorded in Spanish seas represent more than half of the 4340 species of Mollusca recorded within the scope of the European Register of Marine Species, making this the highest species count in European seas. The Canary Islands demarcation holds by far the largest number of endemic species (127), but special mention is needed regarding the ca. 20 species endemic to the Strait of Gibraltar, a highly anomalous chorotype in the marine realm. The number of non-indigenous species (38) is surprisingly low compared with that of species known in the Mediterranean Sea, amounting to hardly 1.5% of the total and supporting the view that a high number of native species tends to reduce invasion success. The list is seen as a much needed tool for the identification of priority areas for biodiversity conservation, but requires permanent attention and curating in order to remain up-to-date.

Many studies have focused on the importance of the periphyton as a food source for different species of molluscs associated with vegetation (e.g. seagrass beds, photophilic algae), such as the gastropods Gibbula spp. (Peduzzi, 1987; Mazzella & Russo, 1989), Jujubinus striatus (Linnaeus, 1767) (Peduzzi, 1987; Hily et al., 2004), Rissoa membranacea (J. Adams, 1800) (Fredriksen, Christie & Boström, 2004) or the cephalaspidean Haminoea orbygniana (de Férussac, 1822) (Malaquias et al., 2004). Some of these microalgal grazers (e.g. J. striatus, R. membranacea), which are dominant in beds of Zostera marina or Cymodocea nodosa, may also form stable populations in other similar habitats, such as algal beds of Caulerpa prolifera (Rueda & Salas, 2003), and consequently their dependence on seagrass beds could be questioned. Contrary to terrestrial ecosystems, in marine ecosystems few molluscan species seem to feed directly on plant tissues (e.g. seagrasses) (Fishlyn & Phillips, 1980; Carlton et al., 1991; Zimmerman et al., 2001), although these may represent an important food source for some species of vertebrates (e.g. fishes, dugongs, sea turtles) and sea urchins (Hemminga & Duarte, 2000). No detailed information is available on seagrasses as a direct food source for any molluscan species living on European coasts.

The macrofauna of molluscs associated with soft bottoms in the Bay of Cádiz (southern Spain) was studied monthly from February 1994 to January 1996. The number of species (63) is high for a soft bottom and is related to environmental characteristics (growth of macrophytes) and the biogeographical setting of the studied area. Corbula gibba (∼90%) was the dominant species in this taxocoenosis together with the gastropod Nassarius pygmaeus and the bivalves Pandora inaequivalvis, Parvicardium exiguum and Macoma melo. The most frequent species during the two years was also the bivalve Corbula gibba (100%) followed by the gastropods Nassarius pygmaeus, Tricolia tenuis, Rissoa membranacea and the bivalve Macoma melo. Total abundance of the taxocoenosis in both years reached higher values in spring. The dynamics of the ecological indices such as diversity or evenness, and the species richness showed a similar pattern in both years, with low values of diversity and evenness together with high species richness in spring and summer months and the reverse in autumn and winter. The qualitative correspondence analysis of monthly samples shows an ordination related to seasonality in both studied years, however the values of Jaccard's coefficient do not indicate significant boundaries among the monthly samples. The quantitative correspondence analysis shows an ordination and grouping of samples related to the biology of species, particularly with the recruitment of C. gibba, the dominant species. The existence of similar trends in the structure of the taxocoenoses over both years, and the seasonality highlighted by the qualitative correspondence analysis, seems to indicate a certain stability of the ecosystem.