Hie reproductive cycle of Plumatella casmiana (Phylactolaemata: Plumateliidae)

Freshwater bryozoans are unique in their reproduction in that they produce a great variety of asexual propagules; among this group, Plumatella casmiana has developed the highest degree of complexity. It produces three types of statoblasts, two of them with heavily chitinised valves – the sessoblasts and floatoblasls – while the leptoblasts show a very weak degree of chitinisation. Sessoblasts are fixed to the substrate by an attachment apparatus, whereas tloato blasts and leptoblasts develop a swim-ring that enables them to drift in the water. While sessoblasts and floatoblasts undergo an obligate dormancy, the thin-walled leptoblasts as well as the sexually produced larvae develop further within a few hours. A study focussing on the role of the different kinds of propagules in F. casmiana was carried out in the Laxenburg pond in Eastern Austria (48°4’ N, 16°22’ E) where this species was found to be the dominant bryozoan over a period of several years. From May to September, colonies were collected weekly from artificial substrates that had been exposed in this waterbody for one year. The colonies were narcotised, preserved, stained and examined by light microscopy for their gonadal condition and their production of sessoblasts, floatoblasts and leptoblasts. Results on the investment of P. casmiana in its different modes of propagation show that although testes and ovaria are present from late May to late August, the development of fertilized eggs in the embryosacs was rarely detected. Asexual reproduction and in particular the propagation by leptoblasts seems to play the major role in the reproductive cycle throughout the year.