Discussion
Our main results are straightforward: the community of proseriates in the swash levels was different from those in the shoaling and subtidal levels mostly due to the presence of certain functional traits related to adaptation to hydrodynamics. This was expected given that the zoological literature highlighted the presence of highly specialized interstitial species in swash levels of reflective beaches, bearing unique combination of traits, also in other groups of animals (Herranz et al., 2019; 2021, Jörger et al., 2009, Martínez et al., 2021; Worsaae et al., 2012). However, we here support those observations by showing that the proseriate communities in the swash level bear a narrower and non-random combination of traits, compared to those in the shoaling and the subtidal levels. We also quantified how those specialized species shape the taxonomic and functional diversity patterns across beach levels using an extensive dataset. Overall, species taxonomic richness was not affected by differences across beach levels but traits significantly differed across beach levels, leading to differences in the functional richness across levels, explained by differences in the frequency of traits related to hydrodynamics. These results, together, may provide insights into the underlying process related to environmental filtering, at least in beach proseriates.
The differences in the overall functional space across beach levels depended on the higher frequency of traits that were previously described as adaptations against hydrodynamic stress, rather than on the presence of fewer species in the swash level. This contradicts the view of sandy beaches, and particularly the swash level, as marine deserts in which few animal species can survive (McLachlan, 1983), and, instead, suggests that beach habitats might act as a potential cradle for evolutionary innovation. Based on our results, we hypothesized that selection of specialized adaptive features in the swash level, along with the limited dispersal capability of many interstitial species, might have even favoured processes of ecological speciation and the radiation of certain interstitial lineages. This hypothesis warrants further exploration using appropriated phylogenetic comparative methods (Nosil, 2012), given that it is in contrast with previous phylogenetic studies on proseriates. Indeed, proseriate species adapted to the conditions in the swash level belong to different families (Curini-Galletti, 2023; Scarpa et al., 2017)