Patterns of taxonomic diversity: many new species but no differences in species richness across beach levels
We did not find significant differences in the number of species across beach levels. The number of species was significantly affected by environmental variables shaping beach as habitats: granulometry and eastward superficial velocity. Granulometry, in addition to determining available space among the sand grains for meiofaunal species, is a proxy for beach exposure, since finer sediments tend to be washed away by strong waves (Remmache et al., 2020). Indeed, granulometry strictly influences the morphodynamic stage of a beach, with its dimensionless fall velocity being correlated to mean grain size in our dataset (Pearson r = 0.73). Another significant predictor of species richness of western Mediterranean proseriates, eastward superficial velocity, provides a complementary support for the idea that different types of beaches may host different numbers of species: beaches with coarse or very coarse grain size and stronger winds hosted more species in our dataset.
Interestingly, when looking at other environmental drivers of the differences in species richness separately for each level, the number of species in the swash level was negatively affected by precipitation (Supplementary Table S5). This is an evidence that richness in marine meiofauna might be negatively affected by freshwater discharge in the upper layer of the beach through precipitation: it makes sense to expect a negative effect of freshwater on a group of almost
One potential caveat to our estimates of richness can be found in the fact that our species identities were entirely based on morphology. Morphological identifications tend to underestimate the number of meiofaunal species in many groups, given that may meiofaunal lineages present conserved, simplified morphologies with few morphological characters available for species identification (Tang et al., 2012; Jörger & Schrödl, 2013). This is somehow different in Proseriata, given that the species of the group have complex reproductive structures with sclerotized spicules that provide very useful morphological characters for species identification (Curini-Galletti et al. 2023). Indeed, we found 93 new species out of the 152 species recorded in our survey. As impressive as this number might look considering the taxonomic knowledge available for other ecosystems, it is not so different from numbers obtained in previous meiofaunal surveys (Martínez et al., 2019; Curini-Galletti et al., 2012; Jörger et al., 2021). The presence of so many new species might be influenced by some of the species traits potentially connected to low dispersal capability (Curini-Galletti et al., 2001) but most likely reflects the low sampling effort that has been historically performed for most interstitial groups and the little taxonomic knowledge we have on several meiofaunal groups (Fontaneto et al., 2012; Rubio-López et al., 2023).