Correlations between cryptobiosis and other biological characteristics in some soil animals

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Abstract
Attention is focused on soil rotifers, gastrotrichs, nematodes and tardigrades which are active solely when enveloped by a water film and survive freezing and desiccation by virtue of their ability to enter cryptobiosis. All these groups share some morphological and biological characteristics, some of which have a manifest adaptive significance, while in others the adaptive value is less evident. Lastly, some of these features still want elucidation, e.g. the maintenance of continuous thelytokous parthenogenesis even in lineages very rich in species (in rotifers and gastrotrichs entire orders are so characterized) over extremely extended periods of time. From the analysis of characteristics common to the groups surveyed, some problems, for which a unitary solution is formulated, have emerged: a) the marked uniformity in morphology and size of the four groups; b) the greater morphological uniformity of soil and freshwater forms with respect to marine ones; c) the existence of a greater number of superspecific taxonomic units, that is, of evolutionary lines, in the sea than in freshwater or soil (rotifers alone are an exception here); d) the existence of a less common pattern of geographic distribution than usually believed, as concerns animals subject to passive transport; e) constancy in cell number; f) incapability of regeneration; g) maintenance of lineages of so great antiquity as to have given rise to entire orders, all of them absolutely parthenogenetic. The author advances the hypothesis that a physiological property, i.e. cryptobiosis, acquired as an adaptation to a new environment to colonize, namely the soil, would have permitted or favoured, either directly or indirectly, the establishment and maintenance of other characteristics. One of these, parthenogenesis, would in turn have strongly delayed evolution; these groups would have changed slightly and extremely slowly during their evolutionary history, also because, by virtue of cryptobiosis, they almost completely avoid selection by the environment. In fact, these animals are active only under favourable environmental conditions, which is as good as to live consistently in a stable environment. This truly exceptional situation, by screening these animals almost totally from selection by the environment, might have made the continuous rise in genetic variability unnecessary. This would explain the maintenance of absolutely parthenogenetic lineages over extremely long periods. Once this impact of cryptobiosis on evolution is admitted, many facts hitherto obscure can be satisfactorily accounted for and elucidated by linking them to one another in a fairly logical sequence of cause and effect.