Occurrence of tardigrades and physiochemical conditions in rock pools by the Baltic Sea

Background: Rock pools are eroded depressions in bedrock that provide temporary aquatic habitats. Their frequency of desiccation depends on geographic location and shape, and temperature, salinity, pH, and dissolved oxygen are important physiochemical variables that affect communities of organisms in rock pools. Tardigrades have evolved evolutionary adaptations that allow them to survive in many habitats with varying and extreme abiotic conditions, including desiccation. Their occurrence in rock pools, however, have rarely been investigated. The purpose of this study was to investigate the occurrence of tardigrades and the physiochemistry conditions in rock pools by the Baltic Sea in southeast Sweden.

Methods: Samples of benthic material were collected from 32 rock pools at three sites near the town Karlshamn in late May and early July 2021. Physiochemical measurements of pH, temperature, salinity, and dissolved oxygen of the water in the pools were made in the field using a portable mustimeter.

Results: Tardigrades occurred in 19% of the rock pools and represented five eutardigrade genera (Ramazzottius, Hypsibius, Macrobiotus, Isohypsibius, Milnesium). In most of the pools where tardigrades were found only few (100 specimens were found (Ramazzottius). Rotifers were observed in 56% of the samples and were often abundant, while nematodes were observed in only 9% and were less abundant. Physiochemical variables showed some variation both within and among the three sites but there were no general differences between rock pools with or without tardigrades. However, rock pools with tardigrades tended to be shallower than pools without tardigrades, indicating that more desiccating-prone rock pools may be more favourable habitats for tardigrades.

Conclusions: The study shows that tardigrades are part of the micro-invertebrate fauna in rock pools. The tardigrade fauna in this habitat deserves more studies in general, as well as closer investigation into the tendency of more frequent occurrence of this animal group in shallow rock pools. The occurrence of tardigrades in rock pools provide interesting opportunities for many different experimental and non-experimental studies on, e.g., environmental stress, population dynamics, and dispersal of tardigrades in a small-scale natural habitat.