Discordant pattern between realised and fundamental saline niches in two supralittoral Ochthebius species (Coleoptera: Hydraenidae)

1. Determining the saline niche of the congeneric and co-existing species that inhabit supralittoral rockpools subjected to extremely fluctuating saline conditions is a crucial concern to understand their spatial distributions and occurrence, and to predict their ability to face increasing environmental instability due to climate change. 2. Therefore, we compared the realised and fundamental saline niches of Ochthebius quadricollis and Ochthebius lejolisii (Coleoptera: Hydraenidae) on the Mediterranean coast of the Iberian Peninsula. 3. A realised niche was determined using field data on the abundance of the adults and larvae of both species. 4. A fundamental niche was addressed with laboratory experiments by measuring the survival of the adults and larvae and the egg-hatching success exposed to different salinity treatments (35, 60, 90, 110, 140 and 170 g L-1). 5. Our study found a discordant pattern between realised and fundamental niches. Both species are euryhalines and support extreme salinity fluctuations from 1.80 to almost 140 g L-1, and O. quadricollis larvae in the field withstand up to 180 g L-1. 6. However, physiological tolerance was greater in O. lejolisii than in O. quadricollis, and O. lejolissi adults survived in the laboratory at 170 g L-1 for more than 3 days with 30% egg-hatching success, whereas O. quadricollis showed a maximum survival time of 2 days and no egg-hatching success at that salinity. 7.The larvae and eggs of both species were more tolerant than adults. 8.The two studied species exhibited high physiological capacity to tolerate extremely fluctuating salinity, which will be exacerbated by climate change.