0044-5231/01/240/03–04-409 $ 15.00/0 The Nature of Selection on Anhydrobiotic Capacity in Tardigrades * K. Ingemar JÖNSSON Department of Theoretical Ecology, Lund University, Lund, Sweden Abstract. Tardigrades are well known for their capacity to enter an anhydrobiotic state, and remain ametabolic for several years. Yet the evolutionary background, and the forces of natural selection that act on anhydrobiotic capac- ity have remained unexplored. In this paper, I review the basic problem of anhydrobiosis and the phenotypic traits that may be the main targets of natural selection for improved anhydrobiotic capacity. I also make an attempt to put anhydrobiosis into a life history perspective, and discuss this life form in the perspective of energetic and genetic constraints. Key words. Tardigrada, anhydrobiosis, cryptobiosis, natural selection, adaptation. 1. INTRODUCTION The ability of some micrometazoan taxa (Rotifera, Nematoda, Tardigrada) to undergo repeated periods of desiccation, in which the organism loses most or all of its water with a dramatically reduced metabolism as a consequence, has been known for 300 years (KEILIN 1959). While desiccation tolerance during embryonic stages is found in other animal taxa, rotifers, nema- todes and tardigrades show this ability at all life history stages. SPALLANZANI (1776) was apparently the first to study anhydrobiosis in tardigrades, and his experi- ments were followed up by several others during the 19 th century, in particular by DOYÈRE (1842) and F. A. POUCHET (see BROCA 1860). In the early part of the 20 th century, numerous experimental studies were done by RAHM (1923, 1926) and BAUMANN (1922), with much focus on the question whether organisms in anhydro- biosis should be considered as dead (i.e. ametabolic) or alive (i.e. with maintained metabolism). PIGON & WEGLARSKA (1953) attempted to resolve this long- standing controversy by directly measuring the respira- tion of anhydrobiotic tardigrades, and documented a very low rate of oxygen turnover. Although this sug- gested that anhydrobiotic tardigrades were not strictly ametabolic, later work on anhydrobiotic cysts of the crustacean Artemia (e.g., CLEGG 1973, 1986) have established the current paradigm that considers anhy- drobiosis as a truly ametabolic state. Anhydrobiosis represents a latent life form qualitative- ly distinct from metabolically active life forms by the absence of energy turnover and replication, which in turn prohibits any processes of reproduction and growth. The only life history characteristic that the active and latent states have in common is therefore survival. Thus, anhydrobiotic life forms offer organ- isms nothing more than survival, which nevertheless is of paramount importance, since it provides the oppor- tunity of continuous persistence in an otherwise hostile habitat. However, indirectly anhydrobiosis may affect also reproduction and growth, by withdrawing energy from potential use in the latter functions. Anhydrobiot- ic periods also have an obvious impact on the genera- tion time, which in turn influences the potential rate of evolution. Consequently, anhydrobiotic life stages may have strong effects on the evolution of life histories. Despite a considerable interest in anhydrobiosis, the evolutionary background to this biological phe- nomenon has received little attention, and there exist very few studies on the nature of selection on anhy- drobiosis. This is true both in general, and regarding tardigrades. The main purpose of this review is there- fore to encourage biologists to consider evolutionary Zool. Anz. 240 (2001): 409–417 © by Urban & Fischer Verlag http://www.urbanfischer.de/journals/zoolanz * Contribution to the 8th International Symposium on Tardigrada, Copenhagen, Denmark, 30 July–5 August 2000.