by Michael Rasser

There are different types of large scaled growth form features among the encrusting coralline algae; you may also call them sediment types because that's what they are.

There are two basic types: unattached and attached encrusting coralline algae.

Unattached means that the algal thalli are not fixed to the substrate; i. e., they are living unattached on the hard or (mostly) soft substrate. 

The best known type of unattached encrusting coralline algae are the rhodoliths. If we have wide and thick accumulations of rhodoliths with only few detritus, we call them rhodolith pavements. Rhodoliths are unattached nodules predominantly (i. e., >50%) consisting of nongeniculate encrusting coralline red algae. They are sometimes defined by the size (more than 20 mm or 10 mm, respectively), and sometimes by the existence of a nucleus. But actually the size and the nucleus are not relevant.

Sediments, which are predominantly composed of unattached coralline algal branches, rhodoliths, and their detritus, are called Maerl. Today they are restricted to the Mediterranean and the Northern Atlantic, as well as scarce records from the Southern American Atlantic coast. In deeper occurrences, peyssonneliacean algae contribute to the maerl formation.

Attached growth means that the algal thalli are fixed to a substrate. For example, they can form simple encrustations on tropical reefs (some say that coralline algae are the main binding agents in coral reefs).

Attached crustose coralline algae can even form coralline algal reefs and comparable structures:

  • Algal ridges represent a late successional stage of coral reef development forming a constructional cap over coral as the reef approaches sea level.

  • Algal cup reefs, which are also called boilers or breakers, are cup-shaped algal bioherms, mostly arising from Pleistocene rocks.

  • Trottoirs are known from the Mediterranean and Northern Atlantic. They are intertidal bioherms, usually growing on steep rock shores, but can also form algal "micro-ridges".

  • Coralligéne de plateau is known from the Mediterranean and can develop from soft bottoms.

Read more about this in the Palecology & Facies section!