Unexpected guests: epizoic diatoms living on the exoskeleton of four Ephemeroptera genera

The study of epizoic diatoms living on aquatic insects is an unexplored and challenging field of research, offering interesting perspectives. We analysed the epizoic diatom flora living on the exoskeleton of four sympatric genera of Ephemeroptera (Ecdyonurus Eaton, 1868; Ephemera Linnaeus, 1758; Habroleptoides Schoenemund, 1929; Baetis Leach, 1815) collected in a low order stream in North-West Italy. Epizoic diatom communities were very rich and diverse, although no obligate epibionts were observed. Fifty-one diatom species were found on the investigated Ephemeroptera. Among the most abundant: Fistulifera saprophila (Lange-Bertalot and Bonik) Lange-Bertalot, 1997 dominated the sample collected on Baetis; Navicula gregaria Donkin, 1861 was abundant on Ephemera; Cocconeis fetscheriana Stancheva, 2022 was very abundant on Ecdyonurus, while Achnanthidium minutissimum (K & uuml;tzing) Czarnecki, 1994 dominated on Habroleptoides. The taxonomic and functional differences detected in the four epizoic communities offered useful information on hosts' habits, as well as microhabitat and niche exploitation. Low profile and adnate diatoms, able to colonise fast flowing waters, were mostly present on Ecdyonurus according to the reophilous nature of this macroinvertebrate. Motile diatoms, generally closely related to fine sediments, characterised the flora living on Baetis and Ephemera. Diatom colonisation seem strictly correlated to habitat preferences of mayflies.