The study of cyst hatching, performed on 5 cyst types under lab conditions, …

• Abstract
• Background
• Results
• Discussion and conclusion
• Methods
• Abbreviations
• References
• Figures

A total of 19 cyst morphotypes were found (only 13 are given in Figure 1) in the 6 hypersaline lakes studied. Seven of them have been identified at the genus level, also due to the successful hatching experiments carried out in laboratory conditions. On the average, the number of cyst morphotypes extracted from the sediment cores was more than double the number of the active organisms found in the plankton over the study time [see Additional file 1]. The richest cyst bank was that of Vecchia Salina (17 morphotypes), and the poorest was that of lake Koyashskoe (5 morphotypes).

The most common cyst types were Turbellaria cocoons, which were present in all the sediment samples. Regarding abundances, the cyst bank of lake Pantano Grande at Vendicari (Sicily, Italy) was the richest, and Koyashskoe was the poorest (average on three multilayered cores from each lake, 10812 and 504 cysts/100 cm³ respectively). Generally the cyst content in upper sediment layers was higher than that in the lower ones, but in the Vecchia Salina (Apulia, Italy) the difference among the three layers considered (0–3, 3–6, and 6–9 cm) was not evident (3924, 2634, and 3284 cysts/100 cm³ respectively) and in Khersonesskoe the trend was reversed (4747, 4937, and 6088 cysts/100 cm³) (Figure 2).

Some species as Moina salina (Cladocera), Artemia urmiana [14] and Artemia sp. (Anostraca) were exclusive of deeper layers in the lakes studied. The cyst assemblage composition demonstrated highly significant differences among lakes (ANOSIM, R value = 0,947, P***). This result is clearly shown by MDS plot (Figure 3): samples (represented by cyst types and abundances) from each hypersaline lake grouped separately one another.

The SIMPER procedure associated with MDS of all samples identified the main species responsible for the biotic characterisation of each lake [see Additional file 2]. Artemia sp. eggs characterised the "cyst bank" of Nartë saltworks (SIMPER, similarity percentage: 73%), A. urmiana eggs characterised the Koyashskoe lake (SIMPER, similarity percentage: 69%), Copepoda Calanoida eggs (unidentified) and Hexarthra fennica eggs characterised the Khersonesskoe lake (SIMPER, similarity percentage: 84%), while Turbellaria cocoons and Hexarthra fennica eggs were the main responsible for the average similarity of Vecchia Salina, Pantano Grande and Pantano Roveto in Italy (SIMPER, similarity percentage: 72%, 70% and 75%, respectively).

Laboratory experiments under controlled conditions let us to estimate the hatching success of some species (Figure 4). In Fabrea salina (Ciliophora) 100% of hatching was obtained in equinox conditions (12 h light, 12 h dark; at a salinity of 46‰ after 1 week). In Hexarthra fennica (Rotifera) the maximum hatching (93%) was obtained in summer light conditions (14 h light; 10 h dark; at a salinity of 46‰ after 2 weeks). In Brachionus sp. (Rotifera) a hatching peak (66,7%) occurred after 3 days at a salinity of 46‰, under summer light conditions. Artemia sp. (Crustacea Anostraca) showed the maximum hatching-rate under summer conditions (67% at a salinity of 36‰ after 3 days). Moina salina (Crustacea Cladocera) showed an hatching peak (70%) occurring at a salinity of 26‰ after 2 weeks, in summer light conditions.