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As global ocean temperatures are rapidly increasing, marine environments are changing, forcing its inhabitants to adapt. The most common response of marine animals to their warming environment, is to relocate to cooler waters by either migrating along the Earth’s latitudinal lines or by relocating to deeper waters. Evidence for shifts in depth distribution due to warming waters is limited, as deepening can also be a result of ecological constraints and studies focussing on depth shifts are limited to species inhabiting high latitude regions. Here, authors present four patterns for changes in species depth distribution, in response to warming oceans. The pattern followed, depends on the thermal preference of each species. 1. Conservatism, or no change in depth preference occurs. 2. Shift, where both the minimum and maximum depths of a species deepen. 3. Compression, where only the minimum depths are deepened and maximum depth is maintained. 4. Expansion, where the minimum depth is maintained, and the maximum depth is deepened. The strong climatic gradient of the Mediterranean Sea (increase in water temperature from the West to East) provided the ideal region for examining shifts in depth distribution of marine species. Data on the depth range of 236 marine species were collected from literature, including fish, cephalopods and malacostracans. Species of these taxa had various thermal preferences ranging between 18–28 °C across the Mediterranean Sea. Further environmental variables of the Mediterranean Sea were extracted from the Bio-ORACLE database. The OBIS database was used in combination with other databases such as Fishbase, SeaLifeBase and GBIF to extract species traits that can be attributed to a shift in depth distribution, including taxonomic affinity; depth affinity; overall depth range; and thermal preference and range. Authors report convincing changes in minimum, maximum and range of depth among species, implying a prominent deepening response of species across the Mediterranean Sea. Deepening was found to be highly dependent on species traits, therefore, species traits were extracted from OBIS and other databases to contribute to the explanation of changes in depth. The results clearly showed that, with an increase in minimum sea surface temperature (SSTmin), cold-water species relocate to deeper waters by increasing their minimum depth while maintaining or also increasing their maximum depth, depending on the taxa. Authors reiterated the generality of species’ tendencies to shift their depth distribution, which is essential to consider when predicting marine organisms’ response to warming oceans.
Reference: Chaikin, S., Dubiner, S., & Belmaker, J. (2022). Cold-water species deepen to escape warm water temperatures. Global Ecology and Biogeography, 31(1), 75-88. https://doi.org/10.1111/geb.13414