Dynamics and evolution of branching colonial form in marine modular organisms

Branching colonial form is a common pattern for many sessile marine modular organisms. Bifurcation-based ordering systems are unrealistic to explain colonial development. Branching is sub apical in modular organisms such as gorgonian corals and form is the product of a dynamic process maintaining a constant ratio between mother and daughter branches. This dynamic process was modeled using a recurrence expression accounting for branching and ecological/physiological effects. Colony growth preserves shape but is a logistic growth-like event due to branch interference. The mother branch size frequency distribution follows a scaling power-law dependence suggesting self-organized criticality. The hallmark in the evolution of branching among modular organisms is the existence of convergent colonial forms. Molecular phylogenetic analyses from both nuclear and mitochondrial ribosomal genes (16S and 18S) revealed that branching in octocorals have evolved multiple times. Phylogenetic analyses among 28 Caribbean gorgonians, using three mitochondrial coding regions (ND2, ND6 and msh1), also uncovered multiple origins of pinnate and reticulate species. Using phylogenetic contrasts and graphic modeling, I found evidence of morphologic integration at two levels of colonial organization. Despite the implicit modularity from gorgonian corals constructed by replicated polyps, there may be an emergent level of integration for characters product of branching during colony development. This is the first suggestion of an emergent level of integration and modularity produced by the branching process itself and not entirely the module replication process. This study presents for the first time a theoretical basis to understand the branching as an alternative to bifurcation-based approaches as well as evolutionary approaches on the complex nature of branching and colony form.