Case Studies

Antarctic Umbilicaria

Rex Filson recorded two Antarctic species of Omphalodiscus (but none of Umbilicaria) in his 1966 monograph about the lichens of Mac. Robertson Land and Plate 33 in that book shows paintings of two specimens that had been identified as Omphalodiscus decussatus and Omphalodiscus antarcticus. You can see that plate reproduced here, with the two upper paintings showing Omphalodiscus decussatus and the other two Omphalodiscus antarcticus. This case study was prompted by my wish to find out the current names of the two specimens shown on that plate.

Over time the names of species may change. For example, as a poorly known group of organisms is investigated in more detail it may be found that two species, previously thought to be very closely related, are in fact not so closely related and warrant being placed in different genera. Perhaps what had been considered to be just one species is in fact found to be two or what were thought to be two distinct genera are found to be identical. Given that Filson's book is now several decades old it made sense to check the status of the species names that he'd used. Before continuing it's necessary to pose two distinct questions:

What are the current names of the species Omphalodiscus antarcticus and Omphalodiscus decussatus?

What are the current names of the specimens depicted on Plate 33 and labelled as Omphalodiscus antarcticus and Omphalodiscus decussatus?

When the description of a species is first published that description will include mention of the type collection, namely the actual specimen (or specimens) on which the description of the new species is based. As new tools or techniques are developed or as doubts arise over the nature of a particular species there may be cause for a re-examination of a type collection. If, after such a re-examination, the new evidence says that the type collection warrants a different name then we'd say that the species has had a name change since the name of a species is tied to the type collection for that species. Nowadays such name changes are recorded on various databases, which usually makes it fairly easy to answer the first of the above two questions and I'll get to that shortly.

The answer to the second question takes a bit more effort. Sometimes when people identify specimens they make mistakes and in such cases it would of course be wrong to change the caption on an illustration to whatever is the current name of the species given in the caption. So, did Filson make a simple mistake when he labelled that painting as species of Omphalodiscus and is that why the answer to the second question is not so simple? No. Rather, since the publication of Filson's book there has been an increase in knowledge about Antarctic lichens and this better understanding has shown that some careful thought is needed before coming to any conclusions about the specimens featured in Plate 33. Moreover, the quest to find the current names for the specimens shown in that plate has given a good example of why you should be wary of blindly replacing any occurrence of a species' older name with its current name. All in all, good grounds for writing this case study.

Current names of
Omphalodiscus antarcticus and Omphalodiscus decussatus

A description of the genus Umbilicaria was published in 1789 and over the years many additional species of Umbilicaria were described. Decades ago one lichenologist thought that the species Umbilicaria decussata differed enough from the bulk of the species of Umbilicaria to warrant the establishment of a new genus. A formal description of the genus Omphalodiscus was published in 1934 and this genus was based on Umbilicaria decussata, which necessitated a change in that species' name to Omphalodiscus decussatus. In effect the old genus Umbilicaria was split with some species staying in Umbilicaria but others taking new identities in Omphalodiscus. In 1939 a pair of lichenologists described the new species Umbilicaria antarctica and in 1950 another lichenologist judged this species to be better placed in Omphalodiscus, so that year saw a published name change to Omphalodiscus antarcticus. However, later investigation led lichenologists to conclude that the supposed differences between the genera Umbilicaria and Omphalodiscus were not really significant enough to warrant two ' Umbilicaria-like' genera and the species of Omphalodiscus have been subsumed in Umbilicaria. Two ways to describe this process are to say that Omphalodiscus has been sunk into or synonymized withUmbilicaria. When it comes to publishing descriptions of new genera or species - or to sinking species or genera - there are rules to follow and these are set out in the International Code of Botanical Nomenclature (or ICBN). Unless there are certain exceptional circumstances an older published name takes priority over a name published later - hence the use of Umbilicaria rather than Omphalodiscus.

When it comes to the current name for Omphalodiscus decussatus there's no problem - it's Umbilicaria decussata - the name before the split of the genus Umbilicaria. As noted above, before becoming Omphalodiscus antarcticus that species had been known as Umbilicaria antarctica and the latter is once again widely used as the current name, though you may also see the species sometimes referred to as Umbilicaria rufidula. Why is that? Essentially it comes down to a debate about an interpretation of one of the ICBN rules. That legalistic debate is irrelevant for this case study, the important point being that, for each school of thought, it is easy to find the current name for the species Omphalodiscus antarcticus.

Now, what about...

...the specimens shown on Plate 33?

Given all of the above, do Filson's paintings show Umbilicaria decussata and Umbilicaria antarctica (or rufidula)? Not necessarily. There is still the question about how Filson used the names when identifying Antarctic specimens and Filson's 1975 paper gives food for thought. In that paper Filson wrote that previously he'd...

... referred all the rhizinate Umbilicaria specimens from Mac. Robertson Land, Antarctica to Omphalodiscus antarcticus...Kashwidani (1970) and Lindsay and Brook (1971) have similarly referred rhizinate Umbilicaria specimens from other regions in Antarctica to O. antarcticus.

Lindsay (1972) suggested that two species were involved; Umbilicaria antarctica from the Antarctic Islands and Peninsula region, and U. aprina, a Northern Hemisphere species, on the Antarctic Continent. This present study supports this suggestion confirming the presence of U. aprina in mainland Antarctica.

Amongst the species of Umbilicaria the rhizinate ones have short, blackish string-like bundles of hyphae (called rhizines) on the undersides of the foliose thalli. The erhizinate species lack such rhizines. After this paragraph there are enlarged views of parts of Filson's paintings of Omphalodiscus antarcticus. On the left you can see the smooth brown upper surface of part of the thallus as well as a side view of one lobe (arrowed) showing a blackish, 'furry' underside. Below you can see a large area of the underside of a thallus, with numerous short, black streaks covering that underside. Those black streaks and that 'furry' layer are the rhizines which are dense in this species. Incidentally, on the right you can also see part of the thallus cut, to show the white fungal tissue as well as a narrow green layer of algal cells near the upper surface.

Umbilicaria

Initially Filson, and other lichenologists, had thought there was just one rhizinate Omphalodiscus (or Umbilicaria) in Antarctica. However, by 1975 Filson had realized that there were at least two rhizinate species in Antarctica and in 1987 he published a more detailed study of Antarctic Umbilicaria, based on the examination of over 350 specimens, in which he concluded that there were five Antarctic Umbilicaria species - four rhizinate and one erhizinate. The one erhizinate species was Umbilicaria decussata. In the same 1987 paper Filson noted that two of the rhizinate species had dense rhizines and two had sparse (sometimes only marginal) rhizines. The two with dense rhizines were Umbilicaria aprina and Umbilicaria rufidula. Filson also included distribution maps of the five Antarctic Umbilicaria species. Of the four rhizinate species only Umbilicaria aprina was found in Mac. Robertson Land and Umbilicaria rufidula was known only from the Antarctic Peninsula area, on the opposite side of the continent. The caption to the Plate 33 painting does say the featured specimen was collected at Mawson, which is in Mac.Roberston Land.

Putting all that together would lead to the following conclusions:

The current name for Omphalodiscus decussatus is Umbilicaria decussata and the painting labelled as Omphalodiscus decussatus in Plate 33 represents Umbilicaria decussata.

The current name for Omphalodiscus antarcticus is Umbilicaria antarctica (or Umbilicaria rufidula) but the painting labelled as Omphalodiscus antarcticus in Plate 33 represents Umbilicaria aprina.

The current names for the species present no problems but there are two important caveats to the conclusions about the paintings. The first is that the detective work has been based on the Filson papers given in the references below and on the detail shown in Plate 33. In the Mac.Roberston Land book no specimen number is cited for either painting. If it were possible to link the paintings to particular herbarium specimens there would be physical evidence against which to check the above conclusions as to the two species depicted in the paintings. The second caveat is that the conclusions about the paintings are based on the state of knowledge in 1987. There have been more studies of Antarctic Umbilicaria since then.

In his 1987 paper Filson mentioned thallospores, asexual propagules produced on the undersides of the thalli of the five Umbilicaria species he documented. He had not previously included any thallospore details in his species descriptions and in the 1987 paper they did not play an important role in identifying the five species. Geir Hestmark of Norway studied the asexual propagules of Umbilicaria in considerable detail and, in a paper published in 1990, showed that they had substantial taxonomic value at the species level. Hestmark referred to the asexual propagules as thalloconidia and there's a little more about Hestmark's work further on this web page.

A paper published in 2004 recorded eleven Antarctic Umbilicaria species (eight with rhizines, three without) and included an identification key to all eleven Antarctic species. Three of the species (not all erhizinate) lacked thalloconidia or produced them rarely whereas thalloconidia were clearly present in the other eight species. The paper made fundamental use of rhizines and thalloconidia in its identification key so that, in the absence of any information about thalloconidia in the paintings of Filson's Plate 33, it becomes more challenging to be sure of the names of the specimens depicted in that plate. The species in the 2004 paper that were additional to the five in Filson's 1987 paper were recorded from the side of the continent opposite to the Mawson area - but that's a reflection of more lichenological work on the opposite side of the continent rather than proof of the absence of all additional species from the Mawson area. In the 1992 paper by Sancho and others, the authors noted that:

It is remarkable that three species of Umbilicaria previously unknown from Antarctica occur in a small area of Livingston Island. This gives an indication of the floristic richness of South Bay, Livingston Island...It also demonstrates how poorly known much of the Antarctic flora is, even in apparently well-studied lichen groups.

Since additional species have been found in Antarctica after 1987 and given that the nature of the thalloconidia has become an important identification feature, the earlier evidence from just the Plate 33 paintings and the previously known species' distributions cannot be considered as strong as it was once thought to be. Perhaps the strongest statement that can be made about Filson's Plate 33 is that it has not been proven that the specimens featured there are not Umbilicaria decussata and Umbilicaria aprina. This inconclusive statement is not a cause for pessimism. It's simply a consequence of the great improvement in the knowledge of Antarctic (and indeed the world's) Umbilicaria species since 1987. It's fairly common to find that, as knowledge improves, illustrations or written descriptions that were once thought sufficient to identify species are found to be deficient. This Antarctic Umbilicaria story also shows why sometimes it can be misleading simply to replace an older species name with the easy-to-find current name of a species.

Thalloconidia

Hestmark noted that about a third of Umbilicaria species possessed thalloconidia and he studied the thalloconidia of 18 species in detail. In his 1990 paper he illustrated the way thalloconidia were distributed on the thalli of those species.

Here are the distribution patterns for the rhizinate species in the study:

thalloconidia

These diagrams, based on one of Hestmark's illustrations, depict stylized rhizines (which Hestmark termed rhizinomorphs) growing from the lower cortex (both shown in grey) with the thalloconidia shown in black. Here are the explanations:

1. Thalloconida cover the lower cortex but not the rhizines.

2. Thalloconidia cover the lower cortex and the rhizines.

3. Thalloconidia are produced mainly, or exclusively, on short, peg-like rhizines.

4. Thalloconidia are produced in bundles on branching rhizines.

5. Thalloconidia are produced solitarily on branching rhizines.

6. Rhizines disintegrate into bundles or chains of thalloconidia.

Here are the thalloconidia distribution patterns Hestmark found in the erhizinate species:

thalloconidia

The diagrams, once again based on one of Hestmark's illustrations, depict stylized thallus undersides. The red cross indicates the position of the central holdfast that attaches the thallus to the substrate, black indicates the areas with thalloconidia and grey those parts of the lower cortex free of thalloconidia. The distributional patterns were:

1. Lower cortex completely covered with thalloconidia.

2. Only a marginal zone lacking thalloconidia.

3. Only a central area lacking thalloconidia.

4. A central area and a marginal zone lacking thalloconidia.

5. Thalloconidia patchily distributed.

Hestmark found that the distributional patterns as well as the features of the thalloconidia themselves could help distinguish species. For example, he noted that Umbilicaria aprina and Umbilicaria africana had often been confused - but they had quite distinct thalloconidia.

References