A few weeks ago I described some of the algae that I found during a visit to the Avgás Gorge (pictured above) in Cyprus, including a chain-forming Ulnaria (see “Cypriot delights …”). I’ve now had a chance to prepare cleaned valves from this material so we can take a closer look.
The chain-forming habit had already led David Williams to suggest Ulnaria ungeriana (Grunow) Compère 2001 and more detailed observations have confirmed this. This is a species that was actually first described from Cyprus (actually Northern Cyprus) and it was also recorded quite extensively during a survey of the island’s diatoms a few years ago. Unfortunately, some of the key diagnostic characters – such as small marginal spines and striae composed of single rows of pores – cannot be seen with light microscopy but the former, at least, can be inferred from the chain-forming habit. Note, too, how the long chains that dominated the population in the live state, fell apart when the sample was cleaned with oxidising agents and I did not see more than three cells joined together in the new preparation.
Ulnaria ungeriana from Avgás Gorge, Cyprus, April 2018. Scale bar: 10 micrometres (= 1/100th of a millimetre).
The Ulnaria ungeriana cells are mostly about 100 – 150 mm long and 7-8 mm wide, with a striae density of 9-10 / 10 mm. They have parallel sides, narrowing to rostrate to slightly sub-capitate ends, and central areas that reach to the valve margin and which are slightly longer than they are broad. Unfortunately, most of these characteristics overlap with those of Ulnaria ulna in all but most recent identification guides. This species was first described by Nitzsch in 1817; it would have been one of the more conspicuous diatoms visible with the relatively basic equipment available at the time, with a magnification of about 150x. His drawings are of live cells, mostly in girdle view, which means that many of the details which modern diatomists use to discriminate species are not apparent. Moreover, the material on which these drawings are based is no longer available so we cannot go back to this in order to ascertain the characteristics of the original Ulnaria ulna and, to increase the confusion yet further, it is possible that Nitzsch has illustrated more than one species (see the reference by Lange-Bertalot and Ulrich below).
It would be, in short, very easy to look at a population of Ulnaria ungeriana in the cleaned state and match it to the descriptions of Ulnaria ulna which, under various names, have appeared in the identification literature over the past 100 years or so. You might just detect the small marginal spines if you have a good microscope and know what you are looking for. In the live state, however, the ribbon-like colonies are a very distinctive feature yet these do not survive preparation, putting anyone who only encounters this species on a permanent slide at a distinct disadvantage. It is a good example of how examination of live material can add valuable information to an understanding of a diatom species yet, inevitably, many diatomists make little time for examination of their samples before dropping them into their bubbling cauldrons of oxidising agents.
High magnification views of the ends and central portions of Ulnaria ungeriana valves. Scale bar: 10 micrometres (= 1/100th of a millimetre).
What do we know about the ecology of Ulnaria ungeriana? Our survey of Cypriot streams a few years ago yielded 11 records, forming up to four percent of all diatoms in the sample. This means it is both less widespread and less dominant in samples than some other Ulnaria species. It was often found along with other Ulnaria species, in particular U. mondii and, though generally not associated with reference sites (one out of the 11 records), it was mostly found in relatively clean conditions. It was also associated with sites with high conductivity, which corresponds with the limestone geology that we saw in the Avgás Gorge. On the whole, these environmental preferences are similar to those of other Ulnaria species from Cyprus that we’ve studied (see reference in earlier post).
The last question is perhaps the hardest to answer. What benefit does the chain-forming habit confer upon Ulnaria ungeriana? Ulnaria often forms tufts of upright cells sharing a common pad of mucilage at the base, and it is often (but not exclusively) found as an epiphyte on other plants. We can’t rule out the possibility that the Ulnaria ungeriana chains are not also attached at one end, but it is also possible that the chain-forming habit means that they are easily entangled with the Chara and filamentous green algae that I described in the earlier post. Both mucilage pads and entangled chains fulfil the same role of keeping the alga in the same spot in the stream, particularly where there are other plants and filamentous algae to offer extra protection from the current.
There is some speculation in the final couple of sentences but that’s never a bad thing for an ecologist. If nothing else, it provides me with a reason to return one day …
Ecological preferences of Ulnaria ungeriana at running water sites in Cyprus. a. pH; b. conductivity; c. total nitrogen (TN) and d. total phosphorus (TP). Arrows indicate the mean value for each variable, weighted by the relative abundance of Ulnaria ungeriana in the sample.
Krammer, K. & Lange-Bertalot, H. (1991). Süsswasserflora von Mitteleuropa 2 Bacillariophyceae, 3 teil: Centrales, Fragilariaceae, Eunotiaceae. Spektrum Akademischer Verlag, Heidelberg, Berlin.
Lange-Bertalot, H. & Ulrich, S. (2014). Contributions to the taxonomy of needle-shaped Fragilaria and Ulnaria species. Lauterbornia 78: 1-73.