Evolution and dynamics of branching colonial form in marine modular cnidarians: gorgonian octocorals

Abstract

Multi-branched arborescent networks are common patterns for many sessile marine modular organisms but no clear understanding of their development is yet available. This paper reviews new findings in the theoretical and comparative biology of branching modular organisms (e.g. Octocorallia: Cnidaria) and new hypotheses on the evolution of form are discussed. A particular characteristic of branching Caribbean gorgonian octocorals is a morphologic integration at two levels of colonial organization based on whether the traits are at the module or colony level. This revealed an emergent level of integration and modularity produced by the branching process itself and not entirely by the module replication. In essence, not just a few changes at the module level could generate changes in colony architecture, suggesting uncoupled developmental patterning for the polyp and branch level traits. Therefore, the evolution of colony form in octocorals seems to be related to the changes affecting the process of branching. Branching in these organisms is sub-apical, coming from mother branches, and the highly self-organized form is the product of a dynamic process maintaining a constant ratio between mother and daughter branches. Colony growth preserves shape but is a logistic growth-like event due to branch interference and/or allometry. The qualitative branching patterns in octocorals (e.g. sea feathers, fans, sausages, and candelabra) occurred multiple times when compared with recent molecular phylogenies, suggesting independence of common ancestry to achieve these forms. A number of species with different colony forms, particularly alternate species (e.g. sea candelabrum), shared the same value for an important branching parameter (the ratio of mother to total branches). According to the way gorgonians branch and achieve form, it is hypothesized that the diversity of alternate species sharing the same narrow variance in that critical parameter for growth might be the product of canalization (or a developmental constraint), where uniform change in growth rates and maximum colony size might explain colony differences among species. If the parameter preserving shape in the colonies is fixed but colonies differ in their growth rates and maximum sizes, heterochrony could be responsible for the evolution among some gorgonian corals with alternate branching.

Character lability in deep-sea bamboo corals (Octocorallia, Isididae, Keratoisidinae)

“…They appeared to have been gained and lost several times. A similar result was found by Sánchez (2004) for branching patterns of shallow Caribbean octocorals, where reticulate branching arose 3 different times on the phylogeny. France (2007) also found this same pattern between an MSH1 phylogeny and branching patterns (branched versus unbranched) in some keratoisinid genera.…”

“…salamanders, Chippindale et al 2004, Mueller et al 2004. Character lability has also been documented in marine organisms such as gastropods (Collin & Cipriani 2003, Collin et al 2007), sponges (Borchiellini et al 2004) and shallow-water octocorals (Sánchez 2004). Given the pattern of character lability shown by Sánchez (2004) in shallow-water octocorals, it is reasonable to expect a similar outcome in deep-sea octocorals such as bamboo corals.…”

The Potential for Evolutionary Responses to Cell‐Lineage Selection on Growth Form and Its Plasticity in a Red Seaweed

“…4 in Santelices 2004) that intraclonal variation translated to larger scales by recurrent cycles of cell-lineage selection and ramet formation may partly explain the persistence and remarkable variability of clonal seaweed populations. Selection in such systems may frequently shape morphological traits through their effects on size, given the demographic consequences of clonality (i.e., life histories that are based more on size than on age ;Caswell 1985;Tanner 2001) and that sessile taxa may chiefly interact with their environment via clonal growth and morphology (Sánchez 2004). Some argue, moreover, that morphological plasticity afforded by modular construction is more likely than mosaicism to help such taxa combat environmental change (Tuomi 2004).…”

New records of the genera <em>Leptogorgia, Pacifigorgia</em> and <em>Eugorgia</em> (Octocorallia: Gorgoniidae) from Ecuador, with a description of a new species

“…Despite the rebirth of invertebrate systematics due to the ever-increasing availability of DNA sequence characters (Mallet and Willmott 2003, Sánchez 2004, Due-ñas and Sánchez 2009), evaluation of the effectiveness of molecular barcodes in octocorals is largely hindered by lack of knowledge regarding species boundaries in these organisms. One reason for this is that the rate of octocoral mitochondrial gene evolution is very slow.…”