Surviving the heat: Long-term growth patterns of Adriatic gobies reconsructed from otolith analysis

Marine fishes are increasingly affected by climate warming and anthropogenic stressors, particularly since the beginning of the Industrial Revolution. The widely accepted Temperature-Size Rule (TSR) predicts that aquatic ectotherms grow faster, mature earlier, but attain smaller adult sizes under warmer conditions. However, its universal applicability remains controversial, and growth responses vary among species, ontogenetic ages and ecosystems. Fossil fish remains offer a unique opportunity to understand long-term growth and body size variability, providing historical baseline data, predating significant human impacts. Otoliths,  incrementally grown CaCO₃ structures in the inner ear of teleost fishes, are well-preserved in the fossil record and exhibit species-specific morphologies. These biominerals preserve detailed records of growth, age, life history, and environmental conditions in the form of daily, seasonal, and annual growth bands that can be analyzed through sclerochronological analyses. Our study focuses on the black goby (Gobius niger Linnaeus, 1758), a non-commercial, resident demersal species that is highly abundant in temperate shelf regions of the Atlantic and Mediterranean Sea, today and in the geological past. We use fossil otoliths from Holocene sediment cores off the coast of Piran (Slovenia) in the northern Adriatic Sea and modern otoliths from living populations caught in the same area to test whether anthropogenic climate warming has altered the growth patterns of this species, as predicted by the TSR. To assess growth patterns over the past millennia, sclerochronological analyses, including light microscopy and backscatter electron imaging of incremental records, are combined with radiocarbon dating on the same specimens. Our first results reveal no significant differences in overall growth patterns between fossil and modern populations. However, modern otoliths exhibit greater structural complexity and stress-related features, such as vaterite formations, alongside more variable growth patterns. Our study highlights the importance of integrating fossil and modern data to examine long-term growth trends and to expand our knowledge beyond commercially important and charismatic species to inform conservation and management strategies today.