Microgeographical differentiation between morphotypes of Trididemnum orbiculatum (Tunicata: Ascidiacea) in southeastern Brazil

Ascidians, like most encrusting organisms, present polymorphisms in shape and color that are frequently attributed to the environmental conditions in which the adults live. In southeastern Brazil, the ascidian Trididemnum orbiculatum presents 2 morphotypes that occur along different parts of the rocky shoreline. Between the intertidal and subtidal zones, the morphotypes differ in numerous characters, such as the pigmentation of zooids, growth pattern of the colony and abundance of spicules. We assessed the differentiation between the morphotypes using morphological and allozyme data. Characters traditionally used in ascidian taxonomy did not separate the morphotypes into distinct species, but the intertidal morphotype presented consistently smaller zooids, larvae and colony thickness, possibly caused by the harsh conditions of this zone. Although most of this divergence could be caused by phenotypic plasticity, clear genetic structure was observed between morphotypes. We propose that the morphotype from the subtidal zone constitutes a different species from that in the intertidal zone. In addition, morphological and genetic data suggest that colonies from the intertidal zone constitute more than one genetic entity.