Life in Extreme Conditions: Diet and Condition of the Extremophile Fish (Teleostei: Cyprinodontiformes) in a Thermal Rheocrene Spring.

This study aims to understand the bioecological traits of an extremophile fish, , in order to explore how it survives and colonizes extreme habitat conditions. To achieve this, the bioecological characteristics-condition, diet, and feeding strategy-of inhabiting the Tuzla thermal spring, which is characterized by extreme habitat conditions in terms of physicochemical water parameters, were studied. Among the physicochemical parameters measured, salinity and temperature were remarkably high, ranging from 23.7°C (in winter) to 42.7°C (in summer) and from 47.7 ppt (in autumn) to 60.7 ppt (in winter). A total of 248 fish individuals were collected from the thermal spring, and the diet analyzed seasonally consisted of 17 different food items, and their importance values (MI% and IRI%) varied seasonally. According to the F%, IRI%, and MI% values, the population fed mainly on Diatom, Cyanobacteria, and Diptera, resulting in niche breadth with low values ranging from 0.25 to 0.54. The extreme conditions of the thermal spring indicated that the environment was poor in terms of macroinvertebrate diversity (six taxa), and fish showed food selectivity (E) for Chironomidae and Ceratopogonidae in winter, spring, and summer and for Ephydridae in autumn (E > 0; positive selectivity). Seasonally influenced fish condition was represented by high values in summer (K = 1.43 ± 0.14) and was positively correlated with both water temperature and the increasing importance of the genus in its diet. This study provides the first comprehensive insight into the seasonal diet and condition of , shedding light on its survival strategies in harsh ecological conditions.