INTRODUCTION

Fluctuating salinity is one of the major natural stress factors for aquatic biota because it determines their habitat suitability and distribution (Lambret et al., 2021). In land-sea transition zones, such as supralittoral rockpools, salinity fluctuation is intense because of a wide variation range from freshwater to concentrated seawater as a result of the combined effect of marine and terrestrial water inputs (Telesh et al., 2013; Vinagre et al., 2015). This splash zone is also influenced by wave action and wind, with surf and salt spray wetting rocks. These fluctuations correlate with other environmental changes (Ganning, 1971). Thus increasing salinity, temperature, dissolved oxygen and UV occur as aerial exposure time prolongs (McAllen et al., 1998; Mirón-Gatón et al., 2022; Telesh et al., 2013), and is more marked on the Mediterranean coast due to low tide amplitude (Izquierdo & Mikolajewicz, 2019). During drier seasons, Mediterranean rockpools can be salt-saturated for several days to months, and concentrations can exceed 200 g L⁻¹ (author's field data). These fluctuations may be exacerbated by the climate change effect (Balogh & Byrne1, 2021; Kaplanis et al., 2020; Smale et al., 2019), and by other non-climatic drivers, such as infrastructure development, agriculture, industrialisation and anthropogenic habitat degradation (2021; Cavraro et al., 2022; IPCC, 2019; Newton et al., 2012). For all these reasons, rockpools have been considered good model systems for investigating climate change impacts and their inhabitants as sentinels of global warming (Kaplanis et al., 2020). In these habitats, organisms are physiologically adapted to cope with drought and osmotic stress, mainly to the increase of salinity in drying pools. Therefore, studies of tolerance responses of the species living in these extreme environments have shown considerable interest in them for ecological and conservation purposes.

The macroinvertebrate communities that inhabit rockpools are characterised by poor species richness of exclusive halotolerant and halophilic fauna (Villastrigo et al., 2020), where Crustacea and Mollusca of marine origin, and Insecta (Diptera, Coleoptera and Hemiptera) of continental origin, are the dominant inhabitants (Margalef, 1949). Remarkably, some species show extreme euryhalinity and cope with rapid transitions between freshwater and saltwater (Harrison et al., 2012; McAllen et al., 1998). They must possess effective hypo-osmoregulation mechanisms to live in hyperosmotic media, such as reduced extracellular water loss, maintaining body water content by drinking, ion excretion (Pallarés et al., 2015; Velasco et al., 2018), and hyper-osmoregulatory capability in dilute media (Bradley, 2009).

Despite the importance of physiological salinity tolerance studies and their relations with vertical zonation patterns, most current knowledge has been acquired by focusing on intertidal organisms (e.g., Hoyaux et al., 1976; Iwabuchi & Gosselin, 2020; McMahon, 2003), and very little attention has been paid to supralittoral species. Of them, crustacea have been the most widely studied, such as some harpacticoid copepod species of Tigriopus Norman (e.g., Bonello et al., 2018; McAllen et al., 1998; McAllen & Taylor, 2001; Ranade, 1957), the crab Armases miersii (Rathbun) (e.g., Anger, 1996; Anger et al., 2000; Charmantier et al., 1998; Torres et al., 2007), and several species belonging to the genus Ligia Linnaeus (e.g., Todd, 1963; Wilson, 1970; Zhang et al., 2016). Other salinity tolerance studies have been conducted on mollusc littorinid species (e.g., Muraeva et al., 2017; Sundell, 1985) and, to a lesser extent, on insects, like those carried out on the larvae of some mosquito species (Margalef, 1949), the collembola Anurida maritima (Guerín) (Witteven & Joosse, 1987), and the beetle Ochthebius quadricollis (Hase, 1926; Jacquin, 1956).

Ochthebius (Coleoptera: Hydraenidae) is one of the most specious water beetle genera (more than 500 species) to live across the complete salinity gradient (Millán et al., 2014; Villastrigo et al., 2019, 2020), and only a few are known to spend their entire life cycle in supralittoral rockpools worldwide (Sabatelli et al., 2016; Villastrigo et al., 2022), and different congeneric species are able to co-exist in the same pools (Mirón-Gatón et al., 2022; Villastrigo et al., 2022). As comparative studies into the thermal tolerance (Mirón-Gatón et al., 2022) and life cycle (Velasco et al., 2022) of two co-existing species (O. quadricollis and O. lejolisii) in the western Mediterranean coast have recently highlighted thermal niche and life-cycle differences, we also expect them to display differences in salinity tolerance. These and other potential differences in niche specialisation can help to understand their spatio-temporal co-existence patterns and to assess the viability of the populations of both species when faced with present and future global changes (Bozinovic et al., 2011; Carbonell et al., 2012). We applied the Grinellian concept of niche, where the fundamental niche is defined as the range of abiotic conditions that influences a species' positive intrinsic growth rates (physiological tolerance) and, within it, the species can only effectively occupy a subset of the entire range of favourable conditions, which is referred to as the species' realised niche (Arribas et al., 2019; Soberón, 2007).

The aim of the present study was to compare the realised and fundamental saline niches of water beetles O. quadricollis and Ochthebius lejolisii to predict their viability against expected changes in salinity in their habitats. Our specific objectives were to: (i) define from field distribution data the saline realised niches of adults and larvae in each species; (ii) define the fundamental niches of the adults, larvae and eggs of both species with the data obtained from salinity tolerance laboratory experiments; (iii) determine the concordance between the realised and fundamental niches of both life-cycle stages and species. We predicted that O. quadricollis would show greater salinity tolerance than O. lejolisi in concordance with its greater thermotolerance (Mirón-Gatón et al., 2022), and the eggs and larvae of both species would be more halotolerant than adults due to their null or lower mobility, respectively. We also expected to find some differences between the realised and fundamental saline niches in the studied species and stages due to the diverse response to the combined effect of salinity and other environmental stressors in natural media.