SMI variations in olms: a comparison between cave and spring populations

Adaptations to cave life, such as blindness, depigmentation, and drastically reduced metabolism, are defining traits of stygobiont species that typically limit their survival in surface habitats. However, the olm (Proteus anguinus), a fully stygobiont amphibian, has been observed in spring environments. Our earlier findings revealed that its occurrence in springs is influenced by specific factors, including hydroperiod and the absence of potential predators. Building on these insights, we conducted a study to assess whether Proteus anguinus benefits from accessing the trophic resources available in spring habitats. Between June 2020 and the present, we sampled 64 springs and 12 caves in the Monfalcone province (Italy), documenting 73 individuals (36 from springs, 37 from caves). Each individual was photographed, measured, and weighed to calculate their Scaled Mass Index (SMI). Concurrently, we assessed macrobenthos communities by counting organisms within a 30x30 cm plot across 2x2 m² areas to estimate available trophic resources. Generalized Linear Mixed Models (GLMMs) were used to analyse the relationships among SMI, sampling location, and trophic resource availability. Our findings reveal that olms from springs exhibit significantly higher SMIs compared to their cave-dwelling counterparts. Moreover, the SMI of spring-dwelling olms was positively correlated with the abundance of trophic resources. These results suggest that P. anguinus benefits from inhabiting surface environments, with its presence in spring habitats likely driven by the greater availability of food resources.