Phylogenetic significance of egg shell variation in tardigrades

Abstract

Freshwater and semiterrestrial tardigrades produce either smooth shelled eggs laid in the exuvium or ornamented eggs laid freely, in contrast to the more primitive marine heterotardigrades which have smooth shelled free-laid eggs. Data are presented on the observation of free-laid ornamented eggs in the primitive non marine heterolardigrade genus Oreella and in a number of eutardigrade genera including Eohypsibius, Amphilus, Minibiotus and Macrobiotus. These findings have provided a sound basis for the formulation of a hypothesis in which the evolution of the tardigrade egg can be explained by two separate events. The first is the acquisition of ornamentation and the second is the utilization of the exuvium for oviposition with subsequent loss of egg ornamentation. This hypothesis is consistent with new taxonomic arrangements within eutardigrade families and genera (based on character) of the animals) which have led to a more uniform assortment in terms of egg types. This means that egg type (smooth or ornamented) becomes a valid taxonomic character at the generic level.