Thalassas, 27 (2): 23-35 An International Journal of Marine Sciences PATTERNS OF SPATIAL DISTRIBUTION OF CEPHALASPIDEANS (MOLLUSCA, GASTROPODA) IN SUBTIDAL SOFT BOTTOMS JUAN MOREIRA(1), ANTÍA LOURIDO(2,*), EVA CACABELOS(2,**) & JESÚS S. TRONCOSO(2) Key words: Cephalaspidea; benthos; assemblages; sediment; Iberian Peninsula; Atlantic Ocean. ABSTRACT any given kind of sediment (coarser sandy sediments or fine sand-mud); other species such as Retusa Cephalaspidean gastropods are common truncatula showed, however, eclectic patterns of components of shallow soft-bottom benthic distribution, which might be related to their life cycle assemblages; they are, however, often overlooked in and other factors such as availability or preference numerous studies because of the small size of many of prey. species. The diversity, composition and distribution of cephalaspidean assemblages at three different bays INTRODUCTION located in NW Iberian Peninsula are described from quantitative data. In general, patterns of composition The Cephalaspidea constitutes a widespread and of cephalaspidean faunas varied across locations; diverse group of opisthobranchs (Wägele, 2004), which in two out of three locations there were no patterns are well represented in marine soft bottoms (Franz, of distribution that could be related to sedimentary 1970; Howard et al., 1994). In fact, many species composition. Some species seemed to be present in are provided with a cephalic shield, which helps the animal to burrow in the sediment (Fretter & Graham, 1954). The taxonomic status and family composition (1)Departamento de Biología (Zoología), Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, of the Cephalaspidea has been questioned in the last E-28049 Madrid, Spain. years (see, for example, Haszprunar, 1985; Mikkelsen, e-mail: [email protected] 1996; Wägele, 2004; Wägele & Klussmann-Kolb, (2)Departamento de Ecoloxía e Bioloxía Animal, Facultade de Ciencias, Campus de Lagoas-Marcosende s/n, Universidade 2005; Malaquias et al., 2009a,b). Here, we retain the de Vigo, E-36310 Vigo, Spain. term “Cephalaspidea” to refer to a number of families Actual address: traditionally considered within this taxa, such as (*)Instituto Español de Oceanografía, Centro Oceanográfico de Acteonidae, Retusidae, Ringiculidae, Haminoeidae, A Coruña, E-15001 A Coruña, Spain. (**)Laboratory of Coastal Biodiversity-CIIMAR, Rua dos Bragas Philinidae and Cylichnidae, which are mostly found 289, 4050-123 Porto, Portugal. on soft bottoms (García et al., 2008). 23 JUAN MOREIRA, ANTÍA LOURIDO, EVA CACABELOS & JESÚS S. TRONCOSO Figure 1: A. Location of the studied areas in Galicia (NW Iberian Peninsula). B-D. Location of sampling sites and distribution of sedimentary types at the Ría de Aldán (B), Ensenada de Baiona (C) and Ensenada de San Simón (D). 24 PATTERNS OF SPATIAL DISTRIBUTION OF CEPHALASPIDEANS (MOLLUSCA, GASTROPODA) IN SUBTIDAL SOFT BOTTOMS Figure 2: Total number of species (S) of cephalaspideans per sedimentary type at each location (A) and mean abundance (+SD) per site in each sedimentary type at each location (B). GR, gravel; VCS-CS, very coarse/coarse sand; MS, medium sand; FS-VFS, fine/very fine sand; MU, mud. *, one sampling site in total. Black bars, Aldán; grey bars, Baiona; white bars, San Simón. Cephalaspideans may be common in soft bottoms The molluscan fauna of the Galician rías (eastern and seagrass beds (Sprung, 1994; Gosliner, 1995; Atlantic, NW Iberian Peninsula) is particularly rich, Rueda et al., 2009) but are easily overlooked if not both on hard and soft substrata (e.g. Rolán, 1983; sampled with the appropriate methodology (Rueda et Rolán et al., 1989; Olabarria et al., 1998; Moreira al., 2009); this is mostly due to the small size of many et al., 2005; Troncoso et al., 2005; Lourido et al., species (Collignon, 1960). Some cephalaspideans 2006; Cacabelos et al., 2008). This is mostly due, are herbivores although carnivorous habits are more on the one hand, to the variety of intertidal and widespread within the group; carnivore species subtidal habitats and, alternatively, to the particular mostly feed on foraminiferans, polychaetes and oceanographic regime of the rias, characterized by bivalves (Rasmussen, 1973; Berry, 1994a; Wägele upwellings between March- April and September - & Klussmann-Kolb, 2005; Malaquias et al., 2009a). October (Álvarez -Salgado et al., 2000); the latter For instance, predation by philinids and retusids result in a high primary production and therefore in may greatly influence the population dynamics of an important food supply for benthic fauna (Blanton their prey, such as snails and clams (Morton & Chiu, et al., 1987; Figueiras et al., 2002). Despite their 1990; Barnes, 1999); some introduced species of ecological importance, few attention has been paid philinids might also represent a potential risk for to the distribution and diversity of cephalaspideans maintenance of populations of indigenous species in the Galician rías. Determining the patterns of due to competition for trophic resources (Gosliner, spatial distribution of benthic populations and their 1995). On the other hand, several cephalaspideans relationship with sediment variables, including show seasonal and interannual fluctuations in those of taxa influencing the dynamics of other their presence and abundance as do other benthic species, is important to understand the processes invertebrates (Seager, 1982; Berry, 1994b). In which determine the structure and evolution of addition, they may serve as potential bioindicators of assemblages and for an adequate management of the quality of the benthic environment. For example, the natural marine resources (Malaquias & Sprung, Retusa obtusa (Montagu, 1803) has been shown to 2005). Therefore, in this paper we describe the resist high concentrations of organic matter, lipids composition of the cephalaspidean faunas in a range and heavy metals in polluted soft-bottom in harbours of shallow-water sediments at several locations within (Guerra-García & García-Gómez, 2004). the Galician rías, and test whether there are any 25 JUAN MOREIRA, ANTÍA LOURIDO, EVA CACABELOS & JESÚS S. TRONCOSO Figure 3: Total abundance per sedimentary site (%; bars) and mean abundance per site (indiv. 0.28m2 + SD; line) of cephalaspidean species (>20 indiv.) at the Ría de Aldán (AL). GR, gravel; VCS-CS, very coarse/coarse sand; MS, medium sand; FS-VFS, fine/very fine sand; MU, mud. *, one sampling site in total. relationship among patterns of distribution and those oriented towards West and therefore exposed to the of granulometric composition. influence of winter storms. MATERIALS AND METHODS Sampling was done in December 1995 (Baiona), July 1997 (Aldán) and November 1999 (San Simón). Studied areas Geographic coordinates and abiotic features of sampling sites may be found in Moreira et al. (2005; The three studied locations correspond to inlets Baiona), Lourido et al. (2006; Aldán) and Cacabelos or small embayments at the mouth of the Ría de Vigo et al. (2008; San Simón). Sites were located at depths (Ensenada de Baiona; 42º07’N-42º09’N, 08º49’W- of between 2-12 m (Baiona), 3-42 m (Aldán) and 2-28 08º51’W) and the Ría de Pontevedra (Ría de Aldán; m (San Simón). Sediments ranged from gravel to 42º16’N-42º20’N, 08º49’W-08º52’W), and an inlet mud in Baiona and from very coarse sand to mud in in the innermost part of the Ría de Vigo (Ensenada Aldán and San Simón, with a predominance of sandy de San Simón; 42º17’N-42º21’N, 8º37’W-8º39’W) sediments at the first two locations and that of muddy (Figure 1). The mouth of the three locations is sediments at the latter. 26 PATTERNS OF SPATIAL DISTRIBUTION OF CEPHALASPIDEANS (MOLLUSCA, GASTROPODA) IN SUBTIDAL SOFT BOTTOMS Sampling (centroids) were classified by cluster analysis based on the group-average sorting algorithm. Clusters of The same sampling methodology was used at the sites determined as statistically significant by profile three locations. Five replicate samples were taken at test SIMPROF (P<0.05) were considered as having each site by using a quantitative Van Veen grab with a similar fauna (Clarke et al., 2008). Non-metric a sampling area of 0.056 m2; a total area of 0.28 m2 multidimensional scaling (nMDS) was used to produce was therefore sampled at each site. Samples were the ordination of centroids; values of selected abiotic sieved through a 0.5 mm mesh and fixed in 10% features were further superimposed to detect visually buffered formalin for later sorting and identification any related pattern in that ordination. Multivariate of the cephalaspideans. Empty shells were neither analyses were done through the PRIMER 6 software identified nor counted. An additional sediment package (Clarke & Gorley, 2006). sample was also taken at each site to determine the granulometric composition, grain-size median The possible relationship between the (Q50), sorting coefficient (So), calcium carbonate cephalaspidean fauna and abiotic variables content (%) and total organic matter content (%). The (granulometric fractions, grain-size median, sorting following sedimentary fractions were considered: coefficient, organic matter, calcium carbonate, gravel (>2 mm), very coarse sand (2-1 mm), coarse depth) at each location was explored using the BIO- sand (1-0.5 mm), medium sand (0.5-0.25 mm), fine ENV procedure (PRIMER). All variables expressed sand (0.25-0.125 mm), very fine sand (0.125-0.063 in percentages were previously transformed by log mm) and silt/clay (<0.063 mm). Calcium carbonate (x+1) and