This is the summary of a discussion on rhodoliths posted to the Algae-L and CALCALGA Listserver:

Question of Michael Rasser:

Dear colleagues,
I would like to raise a discussion on growth form features of calcareousred algae (Rhodophyta).

"Rhodoliths" are very important carbonate producers in the Cenozoic. They are defined as unattached nodules composed of calcareous (i. e., with true calcified cell walls) red algae (mostly Corallinaceae, but also Peyssonneliaceae are involved). The published rhodolith definitions include two size limations: one limit is > 10 mm in diameter, the other > 20 mm. These size limitations are artificial, without doubt. They reflect, however, a certain size-dependent hydrodynamic "behaviour" of rhodoliths!

Recently, I submitted a paper on Eocene peyssonneliacean algae to a palaeontological journal. These algae can form subglobular nodules, usually with a diameter of 2 - 3 mm (!), composed of uni-layered crusts, without any nucleus (see reference and link below). One of the reviewers suggested to call these nodules "rhodoliths"!

My questions are:
1. Is it reasonable to call these nodules rhodoliths?
2. Do we need "artificial" size limitations for the definition of rhodoliths?

Images of the nodules in question are available at:
Reference: RASSER, M. W., 2000. Coralline red algal limestones of the Late Eocene Alpine Foreland Basin in Upper Austria: component analysis, facies and palecology. - Facies, 42: 59-92.

1. Answer from Daniela Basso, Milano (

From my point of view, there is no doubt: your Eocene peyssonneliacean nodules ARE rhodoliths. 1) Bosellini & Ginsburg 1971 gave a clear and complete definition of their new term "rhodolith" (originally spelled "rhodolite", and then corrected into rhodolith, see Ginsburg & Bosellini, 1973 Jour. Geology 81: 239).

From Bosellini & Ginsburg 1971, Jour. Geol., 79 (6): 669-682 (see page 670): " this paper, algal nodules are defined as follows: nodules formed through the action of algae before or during the deposition of the enclosing sediment ...and whose size...falls between 2 and 250 mm - the granule, pebble and cobble fields. .....For nodules and detached branched growths with a nodule form composed principally of coralline algae we propose the term "rhodolite" FROM THE CLASS NAME OF THE ALGAE".

Therefore, even if Bosellini & Ginsburg did not explicitely included peyssonneliaceans, the latter do form rhodoliths, from the class name, mechanism of formation and kind of nodule. The cited size originally specified in the definition includes your nodules. Moreover, in my opinion, we do not need any dimensional limit in the definition of rhodolith: since it is genetical, how should I manage with a 260 mm nodule or with a 1,8 mm nodule? Are they something really different? I think that terms are useful to shorten descriptions of more complex situations. What should I do in describing a sedimentary environment where all sizes of coralline mixed with peyssonneliaceans nodules are present? Should I call them in a different way because a little bit larger or smaller then the definition? It is a non sense!

Therefore I suggest to avoid arbitrary separation in function of size (especially those different from the authors' definition).

2. Answer from Kirsty Brown, Adelaide (

I just attended a talk by Dr Philippa Unwins on material <200nm in size, which, according to to top microbiologists is too contain the material needed for 'life'. However, she has almost undisputable evidence that these organisms are alive. This makes me think that putting any size limitation on a definition of an organism as obsolete.

The nodules you have do indeed sound like rhodoliths. Maybe conditions were not favourable for larger growth forms, or they are the 'seed' of what would have grown into what we more commonly know as rhodoliths. They definately appear to have an inner core - what is it composed of? I have been unable to access your paper as yet, so the answer may be in that.

Maybe a term such such as micro-rhodoliths could be employed.

3. Answer from Bob Hooper

I object to lower size limits in the definition of rhodoliths. My objections are based on their biological provenance. Our mature rhodoliths of Lithothamnion glaciale can average 100 to 300mm diameter but they arise frequently from tiny (<5mm) apical fragments broken from their parent by ice, storms or bioerosion.

4. Answer from Curt Pueschel

Most authors dealing with modern members of the Corallinales state that a rhodolith develops around a particle of foreign material that serves as a nucleus for growth of the alga. Sometimes layering is included in the description. Some authors discriminate marl (maerl) as broken off free-living branch fragments, presumably without a foreign-body nucleus. Both growth types are unattached and moved about by water motion and they often intergrade. Often the authors question the value of attempting to make a distinction between the two types. Size ranges generally are not mentioned.

You asked for opinion. My own view is that the term rhodolith might be used for any calcified red algal thallus that can grow in nature in a persistent unattached state and whose morphology is altered as a result of the lack of attachment. Members of the Peyssonneliaceae pose the additional problem of varying in the extent of their calcification, but I see nothing wrong with the concept of a peyssonneliacean rhodolith for an extensively calcified, free-living thallus.

5. Answer from Rafael Riosmena-Roriguez


Do we need "artificial" size limitations for the definition of rhodoliths?

6. Answer from Davide Bassi

I agree with the other CALCALGA members about the rhodolith definition. (...) I would not restrict the rhodolith definition to the size of the "nodule", therefore anything ("free-living") made up by corallines would be a rhodolith.

7. Second question of Michael

Except for a few colleagues, there seem to be a general consensus that rhodoliths should NOT be defined by the size; and they can NOT be defined by the existence of a nucleus (because non-nucleated rhodoliths can exist).

In summary, most colleagues say that rhodoliths are just free-living (i. e., unattached) calcareous red algae. In my oppinion, this definition is not sufficient! This definition includes a single, LIVING coralline algal branch with only one or two growth directions, which was, for example, produced by the disruption of a rhodolith.

1. Is this single branch a rhodolith?
2. If not, do we need a nodular structure to call it rhodolith?
3. What is a nodular structure in this respect - do we need growths into all directions? Do we need a (sub)spherical morphology?
4. In Florida and in the Red Sea area, there are fragile, branched, non-nucleated rhodoliths (for example: with several transitions from - let's say - simple branches to rhodoliths, which partly cannot be called 'nodules'. When does a such a branch become a rhodolith?

I do not want to be too 'sophisticated', but I think that these are some important nomenclatural questions.

8. Answer of Davide Bassi

with regard to the definition of rhodolith,

>This definition includes a single, LIVING coralline algal branch with
>only one or two growth directions, which was, for example, produced by
>the disruption of a rhodolith.

An algal branch cannot have "only one or two growth directions" as it can growth along the maximum axis (longitudinal direction) and perpendicular to it (transversal direction). The latter is obviously a multidirectional growth (the algal branch increases its trasversal diameter in different points of the longitudinal axis). As the growth cannot be considered constant (meaning equal in all the growth directions at the same time), the algal branch develops different forms (i.e., clavate, subcilindrical, 8-like form) which have different hydrodinamic properties.

>1. Is this single branch a rhodolith?

No, because it is just a fragment of a rhodolith (all the coralline branch fragments making the maerls are not rhodoliths). It is "potentially" a rhodolith if: (a) the coralline still grows, (b) it is unattached. The biotic and abiotic factors control indeed the susceptibility of such coralline branch to transport and, therefore, to further developments to become a rhodolith.

>2. If not, do we need a nodular structure to call it rhodolith?

Yes. "Nodular" structure does not point a "spherical" form but a "multidirectional" growth (with also different growth rates, see below). Such a multidirectional growth needs a "free-living mode of life".

>3. What is a nodular structure in this respect - do we need growths into >all directions? Do we need a (sub)spherical morphology?

Yes, we need to recognise multidirectional growths. "Multidirectional" growths do not mean that the growth was equal in all the directions at the same time (this would make a subspherical morphology) but only that the rhodolith, during "its life", was able to growth in different direction with different growth rates.

>4. In Florida and in the Red Sea area, there are fragile, branched,
>non-nucleated rhodoliths (for example:
>with several transitions from - let's say - simple branches to
>rhodoliths, which partly cannot be called 'nodules'. When does a such a
>branch become a rhodolith?

I would call it rhodolith when it has an unattached mode of life: this means that it is able to survive to the movement. Therefore, the coralline/s keeps/keep up its/their growth independent to the movement.

9. Answer of David Ballantine

Michael Rasser: I have done some research on shallow-water nucleated rhodoliths in Puerto Rico [Ballantine, D.L., A. Bowden-Kerby and N.E. Aponte. 2000. Cruoriella (Peyssonneliaceae, Rhodophyta) rhodoliths from shallow-water back reef environments in La Parguera, Puerto Rico (Caribbean Sea). Coral Reefs 19: 75-8]). I would certainly regard a coral fragment with a single thickness of the encrusting algae as being a true rhodolith. It is a developmental stage. I think the key to defining them is that they are composed of calcareous algae and they are not attached (even though they may become immobilized). Bosence (1983. Description and classification of rhodoliths (rhodoids, rhodolites). In: Peryt T (ed.) Coated Grains. Springer Verlag New York. pp. 217-224) provided discussion on the wide range of forms and composition that rhodoliths may take.

10. Answer of Bill Woelkerling

I have several personal thoungts on the subject. Firstly, I think it useful to have a term that encompasses all free living algae regardless of size or shape. Rhodolith seems like a good term for this purpose, but perhaps you have another, better term. Secondly, you may see merit from an ecological point of view in recognizing different categories of 'rhodoliths' and thus there may be merit in developing a system of 'rhodolith' growth-forms. I have not thought about this in detail, but two papers that might be useful in looking into this are Dan Bosence's 1983 paper on the subject in Coated Grains and the 1993 paper I co-authored on growth forms in nongeniculate corallines.

11. Answer of Daniela Basso

as I already told you, I suggest to refer to the original definition given by Bosellini & Ginsburg (except for size limitation). Small branches and their fragments are also included in the definition, and I personally agree completely with this. There are too many terms of transition between the fragment of branch and a (sub)-sphaerical rhodolith ! It would be necessary to create dozen of new terms, whose application I imagine difficult and highly subjective, especially to non-specialists. Let's remember the old way to describe maerl, with many defined forma ...that were criticized and mostly abandoned after Bosence's works. You can find the same species as a crust, a sphaerical rhodolith or a fragment of branch! Why shall we complicate this matter with an overflowing nomenclature?