Convergent evolution of encrusting calcareous tubeworms

Abstract

In the present paper we trace the patterns of convergent evolution of encrusting calcareous polychaetes and tentaculitoids with respect to the morphology and function of their tubes. Both clades have morphologically similar tubes. Six of the eight basic morphotypes of encrusting tubeworms occur both in calcareous polychaetes and in tentaculitoids. All six encrusting tentaculitoid tubeworm morphotypes distinguished have analogues among serpulid and sabellid polychaetes. Both tentaculitoids and polychaetes with calcareous tubes have a similar plesiomorphic morphotype, which is characterized by substrate parallel, irregular growth away from the initial point of encrustation. Dominance of similarities in the order of evolution of morphotypes in the calcareous tubeworms indicates that despite genetic differences, ecological similarities (suspension feeding) and similarities in the tube material (calcareous), as well as body plan (worm-like) had a strong control over the evolution of calcareous tubeworms. The calcareous tubeworm life modes likely appeared in the order of increasing dependence on the type of substrate. In general, possible antipredatory strategies (spines, keels, cryptic and endobiotic mode of life) of calcareous tubeworms are evolutionarily old. The plesiomorphic general hard substrate encrusters are evolutionarily the most successful and have the longest stratigraphic range both in tube-dwelling polychaetes and tentaculitoids. Innovations, such as attachment of the tube by basal projections were developed in some microconchid tentaculitoids but not in calcareous polychaetes; budding tubes occur in both taxa, however, in microconchid tentaculitoids it was a brief evolutionary event, whereas in polychaetes such pseudocolonies have a long stratigraphic range. The ability to live in brackish and fresh waters by some tentaculitoids and tube-dwelling polychaetes seems not to be associated with any particular tube morphotype.