How river hildenbrandia ceased to be a bioindicator, or even a hildenbrandia

There is no denying that there are species that are more popular than others. In conservation, even the notion of charismatic species has become accepted – it is easier to get people interested in protecting the giant panda, wolf or orca than it is to get people interested in protecting the egg-laying quadruped or the brittle cove. Wetland ornithologists film ruffs and campaign for aquatic warbler conservation. Foresters and hunters are recalling their contributions to bison conservation. Phycologists find it more difficult – the public mainly learns about algae in the context of toxic cyanobacteria blooms or golden algae. Within a certain niche, however, it can be said that the charismatic species for Polish phycologists, and by extension somewhat more widely also for botanists and hydrobiologists, is the river hildenbrandia.

Three unique features of hildenbrandia

River hildenbrandia doesn’t form blooms, it just stains submerged rocks a vibrant red color. This, by the way, made it of interest to people thousands of years ago. The builders of Stonehenge imported such stones for ornamental and perhaps ritual purposes. Of course, they didn’t realize that the bloodstains were the work of an algae related to sea lettuce. Scientists only discovered this in the first half of the 19th century. At first it wasn’t even certain whether it was an alga or a lichen – especially since they were then considered close groups of cryptic plants. Unlike most crayfish, which have the form typical of a seaweed, hildenbrandia has the form of a shell closely adhering to a stone with a barely noticeable thickness.

Carnivores are not well known in Poland. Almost all their species live on the coasts of the oceans. Few can endure the conditions of waters as sugary as the Baltic, and true exceptions live in inland waters. And among such is the river hildenbrandia, which is reflected in its species epithet. Perhaps because of this uniqueness, it became an object of special interest to Polish phycologists in the first half of the 20th century. In the streams of the Tatra Mountains it was first discovered by Boleslaw Namyslowski in 1918. Ten years later Karol Starmach described slightly more sites clustered in the Carpathians and Pomerania. It was then that he began to study its development.

All species of hildenbrandia are distinguished among crayfish by their lack of sexual reproduction. Admittedly, scientists to this day are cautious about prejudging this fact, assuming that observations can still surprise them. Freshwater species of the genus stand out even more because their spores have not been found, so they must reproduce by division. Starmach, observing the river hildenbrandia in the wild and growing it in the lab, discovered that this division takes place by means of special propagules. This is considered a discovery of world renown.

Hildenbrandia an indicator in Polish waters

The inland dwarf larvae that live in Poland (in addition to hildenbrandia frogroot or lemanea) are most often found in mountain streams and somewhat hydrologically similar Pomeranian streams. The crustaceous habit of river hildenbrandia is not accidental – it allows it to adhere to the bottom and makes it difficult to be torn off by a rushing current, so it is mainly found in well-oxygenated places, from which all pollutants are quickly washed away. As a result, it has been associated with clean waters and considered a bioindicator. In the popular opinion, proof of water purity is the presence of crayfish (in view of the invasion of American species, this is a completely outdated truth), and in the opinion of hydrobiologists and botanists, river hildenbrandia plays such a role.

Due to such a limited occurrence, river hildenbrandia was among the first algae to be included in the list of protected plants in 2004. Poland’s adoption of the Water Framework Directive meant that its bioindication role ceased to be of merely scientific value and became an element of administration. During analyses, however, it turned out that its presence correlates not so much with oligotrophic waters as with mesotrophic ones. What’s more, although some of the historical sites have disappeared, reports began circulating among naturalists about the discovery of new ones in unexpected places, including in still waters. Thanks to state environmental monitoring, it has even been found in the middle Vistula River.

A team of scientists from the Adam Mickiewicz University in Poznan decided to gather this information, and it turned out that in the first two decades of the 21st century, river hildenbrandia was discovered at about two hundred new sites. Admittedly, water quality, especially in terms of saprobia, has improved somewhat compared to the end of the previous century, but not that much. River hildenbrandia may not tolerate heavily polluted waters, but some pollution does not bother it. It’s hard to say whether the exponential increase in the number of sites is the result of its expansion or a more accurate recognition of the existing condition. Probably both, although given how distinctive it looks, it’s hard to believe that it had previously been overlooked inordinately.

New relative

Recently, however, there have been further scientific reports that may confuse the interpretation of the results. Well, the genus hildenbrandia, through its abandonment of sexual reproduction, is relatively poorly differentiated genetically, making few of its species distinguishable. Researchers, comparing species from different parts of the world under the microscope, have often concluded that they are the same species. This applies even more strongly to freshwater species. There have been disputes among phycologists as to whether American specimens described as Hildenbrandia rivularis (i.e., river hildenbrandia) are actually representatives of Hildenbrandia angolensis,or perhaps not necessarily. At the same time, the genus Hildenbrandia in the traditional view has no modern relatives. It is the only genus of the hildenbrandia family, and this is the only family of the order Hildenbrandia.

This picture changed at the turn of the century, when the close genetic affinity of the puzzling genus Apophlaea came to light. The genus had hitherto been attempted to be matched to various groups of kelp until it was finally revealed that the genus is essentially a Hildenbrandia distinguished by its symbiosis with the fungus Mycophycias apophlaeae. Further research showed that Apophlaea and Hildenbrandia are not sister genera, but the former forms one branch of the phylogenetic tree nested among the hildenbrandia. Thus, one must conclude that either such a genus should not be separated and its species considered hildenbrandia, or consider a new approach to hildenbrandia itself.

Last year, a paper was published analyzing the genetic aspects of various specimens included in the genera Apophlaea and Hildenbrandia. Its authors confirmed that the former forms a single branch and can be distinguished. It also turned out that the morphological and ecological similarity of all currently known freshwater species of Hildenbrandia is not a coincidence, such as convergence, but is related to affinity. They form a distinct enough branch to be distinguished as a new genus, which the authors did.

Since last year, the algal bases have been enriched with the genus Riverina, and one of its representatives is Riverina rivularis, which replaced Hildenbrandia rivularis. Other branches of the tree in the genus Hildenbrandia were left by the authors for future analysis. It can be expected that it will soon be divided into several or perhaps a dozen new genera.

Analyses will certainly also cover the genus Riverina more closely. For now, two species are fairly well defined in it, which were already described several years ago on the basis of molecular data – Riverina (so far Hildenbrandia) jigongshanensis and Riverina (so far Hildenbrandia) tahitiensis. Riverina rivularis has been distinguished, but there is a problem with its new definition. Specimens hitherto included in Hildenbrandia rivularis form several lineages, not necessarily the most closely related. It will probably have to be decided which of them will keep the name, and which will turn out to be new species. After 200 years since the first description, it is difficult to make a clear decision. According to the rules, these should be the specimens that served to first define the species, i.e. from a stream in Kongens Møller, Denmark. The question remains whether their preparations have been preserved.

What does the future hold for river hildenbrandia?

The conclusions of this analysis indicate that what even to specialists appears indisputably to be river hildenbrandia may not really be it. Even the best microscope may not be sufficient for diagnosis. So this puts all previous ecological analyses into question. Perhaps the specimens observed in the past by Starmach, and now by other Polish researchers, belong to different species of hildenbrandia, or rather riverina (or, due to the rules of modern Polish, perhaps rivalry?; given that perhaps the last biologist who tried to use Polish names for algae was Zbigniew Podbielkowski, who died a few years ago, it is unlikely that anyone will change the name).

Perhaps this is also true of modern specimens from different locations, such as the Vistula and the Nysa Klodzka. Maybe some species are very sensitive to pollution and are almost extinct, while others are tolerant and are spreading. Without molecular analysis, it’s hard to tell. In the case of plants that have abandoned true sexual reproduction, such as various dandelions or hawkweeds, today hundreds or thousands of micro-species are distinguished, which almost no one can distinguish. This may also be the case with river hildenbrandia.

In the article, I used, among other things. z:

[1] Namysłowski Bolesław, Mikroflora źródeł podreglowe = (La microflore des sources subalpines) Kosmos. R. 47: 1922 pp. 205-232

[2] Starmach Karol, On the reproduction of the crayfish Hildenbrandia rivularis (Liebm.) J. Ag., “Acta Societatis Botanicorum Poloniae,” 21 (3), 1952, pp. 447-474.

[3] Jakubas-Krzak Emilia, Gąbka Maciej, Panek Piotr, Kowalski Wojciech, Lisek Daniel, Smoczyk Michał, Rybak Andrzej, ” The red alga Hildenbrandia rivularis is a weak indicator of the good ecological status of riverine habitats,” Ecological Indicators, 147, 2023, p. 109918, DOI: 10.1016/j.ecolind.2023.109918.

[4] Vieira Christophe, Brooks Cody M. , Akita Shingo, Myung Sook Kim, Saunders Gary W., Of sea, rivers and symbiosis: Diversity, systematics, biogeography and evolution of the deeply diverging florideophycean order Hildenbrandiales (Rhodophyta), “Molecular Phylogenetics and Evolution”, 197, 2024, p. 108106, DOI: 10.1016/j.ympev.2024.108106, PMID: 38750675