Patterns of functional diversity: species traits vary across
levels—yet, a large proportion of the variance remains unexplained
Taxonomic and functional community composition varied across beach
levels. The functional space of the swash level had different properties
than the functional space of the shoaling and subtidal levels, with
differences relying on the different frequency of traits that are linked
to beach morphodynamics. Communities in the swash level had a
significantly higher proportion of species with flat bodies, ventral
ciliary sole, cephalic sensory area, and brain capsule, as expected if
proseriate species living in the swash level had to withstand the effect
of waves (Curini-Galletti et al., 2023). Species with a more regular
arrangement of the adhesive glands along the body, as it is achieved by
flattening the body, combined with the mucous secretion of the ventral
ciliary sole, seems to maximize the adhesion capabilities. The presence
of flat bodies, with a ventral ciliary sole, cephalic sensory area, and
brain capsule characterizes the species of the genus Otoplana(Ax, 1956). Interestingly, this genus has been historically
highlighted as the most specialized animal occurring in the swash level
of sandy beaches in the Atlantic-Mediterranean are: the swash level was
called “Otoplanen -zone” by early meiobenthologists (Gerlach,
1953; Ax, 1956).
Adaptations and responses of meiofauna to different beach morphodynamics
are known for several groups. A higher taxonomic diversity of annelid
taxa was found in reflective than in dissipative beaches (Di Domenico et
al., 2009), suggesting that more species of worms could be adapted to
the strong action of waves. Studies focusing on a single species of
annelids have shown how Saccocirrus pussicus probably sticks to
the sand grains using the glands in the rear end of the body and move
along with them across beach morphodynamic zones (Di Domenico et al.,
2014). Other species of meiofauna cope with turbulence by digging deeper
into the sediments, such as certain kinorhynchs (Herranz et al., 2019;
Mello et al., 2019). Moving to deeper sediments is a general response of
meiofaunal communities to cope with turbulence in the sea (Moreno et
al., 2006).
Notwithstanding such a pattern of relationships between community
composition, trait space, and environmental variables, when we analysed
the relative contribution of environmental factors and geographical
distances to explain the variance of community composition, a large
proportion of variance remained unexplained. A reason for the
unexplained variance might be connected to overlooked relevant
environmental predictors: oxygen, depth of the redox zone, or the
presence of certain nutrients mediates by bacterial production could
have been relevant. Indeed, bacterial communities have been proposed to
determine the presence of certain meiofaunal species, by acting as
specific food sources or by providing additional nutrients to the fauna
via the excretion products (Gray, 1966; 1967). Another reason behind the
unexplained variance might be due to the role of species interactions at
the community level, either amongst proseriates or with other
interstitial species that share the habitat with them. Indeed, the role
of species interactions is known to be relevant in structuring community
composition (Vieira et al. 2022). Yet, we could not include such
additional abiotic and biotic predictors in our analyses because we did
not have such information. Early studies have suggested that species
interactions might play a negligible role explaining that the
distribution patterns of macrofaunal taxa across the beach gradients,
which instead rely on the integrated response of different species
towards multiple environmental drivers, such as granulometry and wave
exposure (Brazeiro, 2001; Papageorgiou et al., 2006). However, this
hypothesis assume that the environment negatively affects the richness
in the swash zone, excluding most species from surviving in there. Given
that, at least for proseriates, we found a rich community of
functionally specialized taxa living in that beach level, we might
assume that species interactions might play an important role that
remains to be quantified. Indeed, in other ecosystems it has been
debated whether it is possible to quantify the effect of ecological
filtering, given the confounding factor of competence (Cadotte &
Tucker, 2017)