PHYLOGENETIC SIGNIFICANCE OF EGG-SHELL VARIATION IN TARDIGRADES

Freshwater and semiterrestrial tardigrades produce either smooth shell ed eggs laid in the exuvium or ornamented eggs laid freely, in contras t to the more primitive marine heterotardigrades which have smooth she lled free-laid eggs. Data are presented on the observation of free-lai d ornamented eggs in the primitive non marine heterotardigrade genus O reella and in a number of eutardigrade genera including Eohypsibius, A mphibolus, 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 fi rst is the acquisition of ornamentation and the second is the utilizat ion of the exuvium for oviposition with subsequent loss of egg ornamen tation. This hypothesis is consistent with new taxonomic arrangements within eutardigrade families and genera (based on character) of the an imals) which have led to a more uniform assortment in terms of egg typ es. This means that egg type (smooth or ornamented) becomes a valid ta xonomic character at the generic level. (C) 1996 The Linnean Society o f London