Intertidal communities are found at the boundary between sea and land and are influenced by tidal flooding and consequent dynamic periods of emersion and immersion (Coutinho et al. 2016; Legrand et al. 2018). During low tides, seawater can get retained in eroded patchy depressions of rock, i.e. rockpools, representing mesocosms of marine life (Legrand et al. 2018). Compared to the surroundings, the environment inside these rock pools is substantially less harsh due to the lower amplitude of physicochemical fluctuations (temperature, salinity, pH, and dissolved O2), offering refuge, feeding areas, and nurseries for several marine species (Underwood and Jernakoff 1984; Dias et al. 2014; Vinagre et al. 2015; Mendonça et al. 2018). The study of rockpools is of extreme importance due to their proven vulnerability to climate change, namely global warming, sea level rise, and overall anthropogenic activities that can lead to an abrupt biodiversity loss in these coastal areas (Fairchild et al. 2018; González-Murcia et al. 2020).
Whereas intertidal rocky shore platforms have been well studied, intertidal rock pools therein have received relatively less attention, particularly on their community dynamics (Martins et al. 2007; Mendonça et al. 2018). However, there has been growing interest in determining the biotic (e.g. competition, predation, recruitment) and abiotic (e.g. tidal isolation, desiccation, salinity) factors structuring their communities (Metaxas and Scheibling 1993, Arauújo et al. 2006, Brendonck et al. 2015, Bezerra et al. 2017). Within these communities, the mollusc group is particularly understudied with only a few examples in literature (e.g. Underwood 1976, Breves and Moraes 2014, Souza and Matthews-Cascon 2019) and hence, the target group of our work.
Rockpool molluscs play essential ecological roles therein including acting both as prey and predator, indirectly influencing the topography via grazing and, shaping the habitat resulting in a complex benthos community, which in turn attracts other intertidal organisms to the area (Martins et al. 2007; Gartner et al. 2013; Souza and Matthews-Cascon 2019). Marine molluscs are also currently threatened by future ocean acidification conditions as it is predicted that they will produce thinner shells of lower structural integrity (Fitzer et al. 2018), thus highlighting the importance of collecting data about this taxon.
Almost no environmental factors are known to affect the habitat selection of molluscs in intertidal rock pools. Previous studies (Underwood and Skilleter 1996; Marsh et al. 1978; Carvalho et al. 2021) proved that pool size does not affect species richness or abundance within rock pools, namely in molluscan assemblages (Souza and Matthews-Cascon 2019), but this is still a matter of debate as other authors reported differing results (Martins et al. 2007). Other studies on molluscs in rock pools have evaluated grazing intensity (Noel et al. 2009), nutrient and sediment effects (Atalah and Crowe 2010), and trophic relationships (Masterson et al. 2008, Breves and Moraes 2014). Recently, Sousa and Matthews-Cascon, (2019) showed that variation of molluscan assemblages between pools within the same site is not due to different pool sizes but rather due to the association between these organisms and the substrate of the pool. Hence, little is still known in the literature regarding the factors that drive their diversity and distribution in these habitats.
Our study is one of the few existing to focus on rockpool molluscs community patterns, particularly at a large scale between shores of differing exposure to wave action. This last factor has been shown to be significantly important for other pool taxa such as crustaceans (e.g. Carvalho et al. 2021). Our study also provides the first focused assessment of rockpool molluscs in Portugal, with previous studies being limited to fish (e.g. Compaire et al. 2022) and amphipods (Carvalho et al. 2021), hence allowing to set a database baseline for comparison with future research. We tested the hypothesis that molluscan diversity and abundance vary between shores of differing wave action (sheltered and exposed). We also assessed the relationship between the overall rockpool community composition and the presence of molluscs and evaluated whether physical rockpool structure (size, depth, and width) influences the abundance and diversity patterns of molluscs.