A SEASONAL DYNAMICS AND POPULATION STATUS OF THE STENO-ENDEMIC KARST FAIRY SHRIMP CHIROCEPHALUS CROATICUS (STEUER, 1899) (BRANCHIOPODA, ANOSTRACA) FROM SLOVENIA

We investigated the presence and ecology of Chirocephalus croaticus (Steuer, 1899) during three successive seasons (spring and autumn 2018, spring 2019) at Lake Petelinje (SW Slovenia). Although the presence (duration, abundance) of this fairy shrimp strongly differs among different seasons and both years, we showed that in a season with favourable conditions (spring 2018) its abundance increases rapidly at the end of the season as the shrimps persist in small pools until the water dries out. The species may appear again during the autumn, however, its abundance and persistence seem to be much lower and shorter than during the spring, respectively. Nevertheless, persistence and abundance may also differ strongly among the two successive spring seasons (less shrimps found during the shorter season of 2019 than in 2018). Larvae appear first, and are most abundant at the beginning of the season, and are followed by adult males (protandry) and females, which become most abundant at the end of the season. Also, larvae seem to be more abundant on exposed bottom (gravel path), while adults are much more abundant in the shelter of a submerged meadow. Ovigerous females had 9-37 cysts (fertilized eggs) in their brood pouch, and the position of differently developed eggs changed during the season; the presence of females without eggs during mid- and at the end of season indicates the production of thin-shelled summer, and thick-shelled winter cysts, respectively. In contrast with the water decrease that is in strongly negative association with the abundance of the fairy shrimps (lake size - abundance: r = -0.56, P<0.001), no other environmental parameters were highly associated ( r <= 0.38|) with species abundance. Although the local population (one of only two currently known) seems to be in good condition, appropriate conservation action should be taken to prevent the decrease of this population in the future.