The aim of this study was to investigate the physiological mechanisms which define the individual cyclomorphic stages and presumably enable H. crispae to withstand large fluctuations in abiotic factors as temperature and salinity. We focused on the two predominant stages, the active and pseudosimplex 1 (P1) stage. The active stage tolerates large shifts in external salinity. Total body volume of single specimens (350-500 µm) was estimated from microscopical images following salinity transfers from 20 ppt (control) to 2 ppt, 10 ppt and 40 ppt. Our results show that animals in this stage experience large changes in total body volume, yet exhibit a regulatory volume decrease/increase over a 48 h period. Hemolymph osmolality was measured by melting point depression in a nanoliter osmometer. In animals kept at 20 ppt the hemolymph osmotic pressure was 926 ± 29 mOsm/kg (n = 6). This value changed to 330 ± 50 mOsm/kg (n = 6), 584 ± 68 mOsm/kg (n = 6) and 1297 ± 32 mOsm/kg (n = 6) during exposure to 2 ppt, 10 ppt and 40 ppt respectively. At any given external salinity the active stage hyper-regulates, indicating the excretion of dilute urine. Animals in the P1 stage are freeze tolerant. Survival at sub-freezing temperatures depends on cooling rate. Differential scanning calorimetry revealed that ice formation proceeds rapidly at a crystallization temperature of - 19 °C at which more than 60% of the water content freezes. The very low crystallization temperature indicates ability for extensive supercooling and excludes the presence of ice nucleating agents.