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).