Geniculate (alga) in the context of "Corallinales"

Coralline algae are red algae in the order Corallinales, characterized by a thallus containing calcareous deposits within its cell walls, giving it hardness. The colors of these algae are typically some hue of pink, or another shade of red, but some species can be purple, yellow, blue, white, or gray-green. Typically, these algae grow in a crustose manner (encrusting rocks and other hardscape); in the intertidal zone of rocky shorelines, and within coral reefs, these algae appear as an abundance of colorful patches on rock surfaces. Unattached specimens (maerl, rhodoliths) may form relatively smooth compact balls, or forming warty to fruticose thalli.

The red algae belong to the division Rhodophyta, within which the coralline algae form the order Corallinales. There are over 1600 described species of nongeniculate coralline algae. The corallines are presently grouped into two families on the basis of their reproductive structures. Most are marine, though one species lives in freshwater; Pneophyllum cetinaensis.

Maerl (also rhodolith) is a collective name for non-geniculate coralline red algae with a certain growth habit. Maerl grows at a rate of c. 1 mm per year. It accumulates as unattached particles and forms extensive beds in suitable sublittoral sites. The term maerl originally refers to the branched growth form of Lemoine (1910) and rhodolith is a sedimentological or genetic term for both the nodular and branched growth forms (Basso et al., 2015). The terms rhodolith and maerl are used in very similar ways. A study in 2023 clarifies that maerl refers to only living, branched coralline thalli, while rhodolith includes unattached coralline red algae, both dead and alive.