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)