Adapting the growth‐form concept to geniculate coralline algae (Corallinales, Rhodophyta)

Geniculate coralline algae (Corallinales, Rhodophyta) are a diverse group of calcifying macroalgae, loosely united by flexible upright axes formed by alternating calcified (intergenicula) and uncalcified segments (genicula), a trait that has independently arisen several times. To standardize terminology, the concept of growth forms (i.e., the external gross morphology of a specimen) was developed previously for non-geniculate (without joints) corallines, but no such efforts have been undertaken for geniculate corallines. Here, we adapted the growth--form concept to geniculate corallines. We propose that, based on their branching pattern and two- or three-dimensional branch arrangement, geniculate corallines can be grouped into seven growth forms: unbranched (unbranched uprights), feather-like (complanate, pinnate branching), fan-like (complanate, dichotomous branching), irregular (complanate or multiplanar without a clear branching pattern), whorled (multiplanar, verticillate branching), arbuscular (multiplanar, dichotomous branching), and plumose (multiplanar, pinnate branching). We have provided a dichotomous key to assign specimens to different growth forms and discussed examples of each of these growth forms as well as the concept of intergrades between them. Additionally, we have provided a glossary of terms used in the growthform definitions and for intergenicular morphology. These tools will be a helpful resource for ecologists, taxonomists, beachcombers, and anyone interested in these comparatively little-studied but diverse macroalgae.