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)