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

Abstract

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 LT_min values of 24.6°C, 14.2°C and 24.7°C, LT₅₀ values of 30.6°C, 29.6°C and 30.8°C, and LT_max values of 35°C, 35.8°C and 35°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.

Keywords:

• Animal density
• tardigrade
• geographic distribution
• thermal tolerance
• global warming

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

We would like to thank Martin McNaughton and Jonas Thormar for help during sample collection and for pictures of the locality, as well as Hans Ramløv (Roskilde University) for loan of the Isefjord Laboratory (Vellerup Vig). RMK was supported by the Danish Natural Science Research Council (grant no. SNF 21-04-0047; FNU 272-08-0576) and the US National Science Foundation (AToL, Tree of Life, grant no. EF #0531757), and NM was supported by the Carlsberg Foundation (grants no. 2006_01_0534 and 2011_01_0539) and a Freja-Programme grant (Faculty of Science, University of Copenhagen).

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark