Ecology and thermal tolerance of the marine tardigrade Halobiotus crispae (Eutardigrada: Isohypsibiidae)

Tardigrades form an important component of meiofaunal communities across the globe. However, our knowledge on tardigrade ecology is very limited. Here, we report the results of 21 field samplings of the marine tardigrade Halobiotus crispae collected over a period of 74 months at the locality of Vellerup Vig, Denmark, with the aim of providing novel insights into its ecology. Uniquely, H. crispae is characterized by the presence of seasonal cyclic changes in the phenotype of the animal, i.e. cyclomorphosis. Our sampling data include (i) total number of animals extracted, (ii) dominant cyclomorphic stage found, and (iii) important environmental parameters such as temperature, salinity and pH. Our accumulated data constitute a tentative model for the annual fluctuations in animal density, which reveals an annual peak in abundance during the months of February and March. In contrast, tardigrade density appears to decrease in response to increasing temperatures during late spring/early summer. The thermal tolerance of H. crispae was therefore investigated experimentally for the pseudosimplex 2 stage, active stage and pseudosimplex 1 stage, revealing LTmin values of 24.6 degrees C, 14.2 degrees C and 24.7 degrees C, LT50 values of 30.6 degrees C, 29.6 degrees C and 30.8 degrees C, and LTmax values of 35 degrees C, 35.8 degrees C and 35 degrees C, respectively. In general, our thermal tolerance data reveal that (i) the entry into the pseudosimplex 1 stage represents an adaptive strategy that enables the survival of the Vellerup Vig population during periods of heat stress, and (ii) temperature is an important factor in limiting the geographic distribution of H. crispae.