Introduction
Life is not a beach for those animals that survive in the rough
ecological conditions found in marine sandy beaches. Early beach
ecologists categorized beaches amongst the harshest aquatic ecosystems
on Earth (McLachlan et al., 1993), and even dubbed beaches asmarine deserts of sand and salt water (McLachlan, 1983). Beaches,
as ecotones between the ocean and the land, indeed represent steep
environmental gradients largely defined by exposure to tides and waves
(Brown & McLachlan, 1990; Brazeiro, 2001). Temporally dynamic changes
of waves and level of exposure on the one hand, and beach morphology on
the other hand, affect each other, driving the morphodynamic evolution
of a beach over cycles that might extend for long temporal scales,
ultimately influenced by changes in regional climate, sea level, tidal
regime, and long-term geological processes (McLachlan & Defeo, 2018).
Beach morphodynamic stages range from dissipative—waves break far from
the coast, favouring progressive profiles and flatter slopes—to
reflective—waves directly reach the beach front producing short and
steep slopes (Short and Wright, 1983; Short, 1996). Because energy is
released progressively on dissipative beaches, environmental conditions
are more benign in them, and become harsher as the reflective condition
increases (McLachlan et al., 1993, 1995). As a result, the number of
species, as well as density, abundance and biomass of fauna, increases
from reflective beaches to dissipative beaches (McLachlan & Defeo,
2018). In addition, within each beach, the number of species is known to
be lower in the wave-incidence zone (McLachlan et al., 1981, 1993, 1996,
McLachlan 1990, Jaramillo et al., 1995), with a pattern that seems to be
robust at global scale for large, macrofauna species (McLachlan &
Dorvlo, 2005; Defeo & McLachlan, 2005). Such a pattern is considered to
arise from the congruent but independent responses of different species
living on the beach to the harsh physical environmental conditions of
the wave-incidence zone of beaches (Noy-Meir, 1979; McLachlan, 1990).
Yet, sandy beaches might look deserted at the first glance because most
of their biodiversity is represented by small meiobenthic organisms,
which take advantage of the habitat available in the small spaces
amongst the sand grains (McLachlan and Defeo, 2017). Meiofauna, defined
as a heterogeneous group of small animals passing through a sieve of
0.5mm mesh (Giere, 2009), is a crucial component of beach ecosystems,
not only in terms of number of species and abundances, but also because
of its role in carbon cycling, sediment transportation, and interstitial
water circulation (Schratzberger and Ingels, 2018). Therefore, in
contrast to their minute size, the importance of meiofaunal organisms is
larger than their size would suggest as meiofauna is involved in many
processes directly linked to the ecosystem services that beaches provide
to human societies (Harris and Defeo, 2022), mostly by shaping
biogeochemical processes (Bonaglia et al. 2014, 2020; Bonaglia &
Nascimento, 2023). Understanding the responses of meiofauna to different
environmental stressors is then crucial not only from a theoretical
perspective, but also to design strategies to preserve those services
upon local and global anthropogenic perturbations (Defeo et al., 2021).
And here things get interesting, because in contrast to macrofauna,
meiofaunal organisms exhibit a wider range of responses to beach
environmental parameters (Moens et al., 2013; Venekey et al. 2014; Maria
et al. 2018), with some species even preferring areas of strong
hydrodynamic disturbance. Counterintuitively, meiofauna, in contrast to
macrofauna, is generally more diverse and abundant in reflective than in
dissipative beaches (Gheskiere et al. 2005, McLachlan et al., 2018). In
reflective beaches, there are even species that prefer the hydrodynamic
turmoil of the reflective wave-incidence zone than deeper and far less
agitated subtidal areas (Di Domenico et al., 2009; 2013).
Problematically, incongruent species-specific responses across many
meiofaunal groups may hamper any attempt to identify changes in
taxonomic diversity patterns across beaches or beach levels, thereby
masking the overall effect of stressors on beach meiofaunal communities,
even if, for example, hydrodynamical gradients might exert ecological
filtering on meiofauna similarly to what is known to occur on
macrofaunal species (e.g. Albuquerque et al., 2007; Moreno et al., 2006;
Sevastou et al., 2011). These difficulties in understanding meiofaunal
responses to stress are increased by the lack of trained taxonomist as
well as the large number of undescribed species and the conserved
morphology that many meiofaunal lineages exhibit (Jörger & Schrödl,
2013; Fontaneto et al., 2015). Altogether, the role of environmental
stressors on beach communities might be better characterized using
functional metrics, which attempt to identify and quantify the traits
that explain each species differential response across levels, in
addition to and/or regardless of the species identity (Martínez,
García-Gómez et al., 2021).
A quick browse over the pictures and drawings in any specialized
identification guide (Schmidt-Rhaesa, 2022) immediately highlights that
taxonomical diversity metrics might only capture a small part of the
diversity of meiofauna. Meiofaunal species, far from representing a
bunch of small worm-like creatures, exhibit a remarkable diversity of
shapes, sensory structures, reproductive organs, and swimming
capabilities, even within a single family or genus. Descriptions of
different species-specific adaptations to interstitial life style in
meiofauna populate the specialized zoological literature (Martínez et
al., 2013; 2015, Polte & Schmidt-Rhaesa 2011, Herranz et al., 2019,
2021, Jörger et al., 2009); sometimes experimentally proved in model
species (Armonies 1988, Boaden, 1963, 1968). What remains to be
quantified is how the frequency of different traits might affect the
ecological response of meiofaunal communities across beach hydrodynamic
gradients. This is challenging, not only because very different
structures might perform the same function across meiofaunal phyla, but
also because many meiofaunal groups need to be studied alive, making any
multi-taxon study across a large geographical area very complicated
(Leasi et al., 2018, Martínez, Eckert et al., 2020). These problems can
be alleviated by targeting a model group of organisms with enough
taxonomic and functional diversity, focusing on a habitat that maximize
hydrodynamic gradient but reduces other confounding environmental
factors.
Our goal here is to quantify the response of meiofauna to beach
hydrodynamic gradients, using communities of proseriate flatworms in 116
Western Mediterranean reflective beaches. Proseriate flatworms exhibit a
remarkable diversity in beaches in terms of number of species as well as
morphological and behavioural characters, some of which have been
proposed as functionally important to cope with turbulence and
hydrological stress (Reise, 1988). Even before the first studies on
beach morphodynamics (Wright and Short, 1982), the upper, swash level of
reflective beaches was known as “Otoplanen -zone”, due to the
abundance of proseriates, and particularly Otoplana species,
which dominates the swash level of Atlantic-Mediterranean beaches
(Gerlach, 1953; Ax, 1956). We here focused on reflective beaches, so we
can establish a comparable zonation across all sampled beaches, avoiding
the confounding factors introduced by different hydrodynamic stages due
to reefs, barriers, or progressive slopes. By selecting the Western
Mediterranean Sea, we not only delineate a common pool of species for
the study, but we also avoid the confounding effect introduced by ample
tidal fluctuations of beaches on open oceans. Our overall hypothesis is
that species responses to hydrodynamics depend on the presence of
certain traits, with the main rationale included in the four alternative
scenarios, depending on whether the environmental condition of the swash
zone affects species richness or not; whereas it selects for specific
combination of species traits (Figure 1). In order to select amongst
these four scenarios, we first investigate the drivers for species
richness, specifically testing whether the number of species differs
across beach levels. Then, we explore the drivers for species
composition, focusing on whether species composition across beach levels
depends on species traits. Third, we compare the properties of the
functional space of each beach level, under the assumption that the
swash zone exhibits a lower functional richness and higher species
functional contributions than the shoaling and subtidal levels. Finally,
we aim to explain these overall differences by the presence of a higher
frequency of traits related to hydrodynamics in the species in the swash
level.