SANTELICES, B.*; ALVARADO, J.L.; P. Universidad Católica de Chile, Santiago; P. Universidad Católica de Chile, Santiago: Applying the concept of metamorphosis to the crustose-to-erect thallus transition of macroalgae

Metamorphosis is broadly defined as a more or less radical morphological change between two multicellular life stages within an organism life phase, often marking the transition from pre-reproductive to reproductive stages. It involves structural re-organization and major physiological changes, generally under the control of endogenous and exogenous factors, often resulting in changes in habitat use. This concept has been applied to the crustose-to-erect thallus (hereafter ‘CET’) transition of some red algae and this study evaluates the tenability of such hypothesis. In contrast to the metamorphosis of some invertebrates, the CET transition of red algae does not involve radical morphological changes nor complete morphological remodeling; it may be restricted to only one of the two life phases in species with alternation of generations and often involves only slight microhabitat changes. On the other hand, the CET transition may separate pre- and reproductive stages, the onset of the morphological change (e.g. axis differentiation) appears regulated by endogenous signals (growth factors) and the axial growth is affected by genetic and environmental factors. Furthermore, equivalent processes exist in green and brown algal species and paleontological data on taxa with reliable fossil records (e.g. coralline algae) suggest a later evolutionary origin for erect axes from species previously conformed only by crustose morphologies. Thus, the CET transition of red algae partially approaches the broader concept of metamorphosis, it is taxonomically widespread, within each group it seems to be evolutionary derived and it allows separation of functions within the plant, the crustose base holding the often limiting substratum and erect axes maximizing propagule production and dispersal.