Water mosses

Many species of water mosses can be found in the northern Bothnian Bay. Here most of the species are Fissidens fontanus, with some Oxyrrhyncium speciosum thrown in to the right upper corner. Polyps (hydra) cling on to the lower water moss “branches”. Photos: Metsähallitus

Mosses are funny macrophytes. Most often people think of Sphagnum bogs with cloudberries or swamps of some kind when talking about mosses, or beautiful green and soft Boreal forest floor cover with blueberries growing all over them. Or if you mention aquatic or water mosses, people will think of running waters, rivers and streams. 

Oxyrrhyncium speciosum was just found to be part of the Finnish water moss family in the Bothnian Bay. It was recognized a few years earlier on the Swedish side. Photo Metsähallitus.

There are, however, many species of water mosses in the northern Bothnian Bay. A few species like Fontinalis antipyretica can be found across most of the coastal areas of the Baltic Sea in Sweden and Finland. It looks quite different across the sea being large, dark green and lush in the SEAmBOTH area but smaller, thinner and lighter green, like it wouldn’t really be doing too well, in the Kvarken area. It is, after all, a freshwater species that just happens to tolerate brackish water to some extent, so it’s not a big surprise that it fairs better in the north were the water is almost fresh. 

Fissidens fontanus (left) and Fissidens osmundoides (right) seen through a microscope. Photo Metsähallitus.

Since bladderwrack Fucus species are missing from the northern Baltic Sea due to the low salinity conditions, water mosses have taken their place on rocky reefs. Water mosses are the larger macrophytes, which grown on hard substrates and they are the only larger macrophytes on reefs around these areas. 

In the early years of Metsähallitus underwater inventories in the north Bothnian Bay on the Finnish side we found approximately 4-5 water mosses that we were able to identify. The University of Oulu helped us a lot since our microscope was not, and still isn’t, adequate enough to reveal the tiniest identification clues from the water moss leaves. It seemed that we could never get the species right and at some point we acquired the university, whether they would like to organize us a species identification course on water mosses. That was a mistake since it turned out, that there might be as many as 20 different aquatic mosses to be found in the northern Bothnian Bay. It had been better living in a blissed ignorance than to wait for completely new species to appear on every dive. The same thing happened on the Swedish side – last year they sent 19 samples of water mosses for identification and they turned out to be samples of as many as 8 different species! 

Water mosses are usually attached to rocky or other hard bottoms. Here it looks like they grow from sand, but that’s because the rock is covered by sand. Photo: Metsähallitus.

Some of the species have eluded the science for the longest time. When I started as a marine biologist in 2006 in Metsähallitus, a water moss called Rhyncostegium riparioides was found everywhere. This common water moss looked exactly like another one, called Leptodictyum riparium, and I confused the two all the time in the beginning. I took our samples to the University of Oulu every time we encountered this species and every single time it came back as Rhyncostegium riparioides. Someone else had obviously mistaken it to Leptodictyum before since that was apparently the only species found in the Bothnian Bay National park during the 1990s. In a few years, Rhyncostegium turned into Platyhypnidium as the scientists learned more about it and the species skipped genuses. About 10 years went with this identification until someone started to suspect something and Turku University started to do DNA sampling on the moss. It turned out that the species was something else altogether, a new species for the Finnish side of the northern Bothnian Bay even though it was already previously identified from the Swedish side. Oxyrrhyncium speciosum found its way to the northern Bothnian Bay. You can read more about this in Finnish  http://metsahallitusmerella.blogspot.com/2019/04/eraan-vesisammalen-tarina.html or from a scientific paper: Huttunen, S. & Ignatov, M.S. 2010: Evolution and taxonomy of aquatic species in the genus Rhynchostegium (Brachytheciaceae, Bryophyta). Taxon 59: 791-808. A new publication will come out about the DNA identification of this species in the new future. 

Fissidens fontanus looks like a bird feather. It can be identified already underwater. No other species looks quite like this one. Photo: Metsähallitus

There are some success stories with the aquatic mosses from the SEAmBOTH area as well. A tiny Fissidens fontanus was previously regarded as being regionally threatened even though it could be found everywhere. If you dived to a dept of 1-6 meters to a bottom with at least a few rocks and the place wasn’t too sheltered, you were bound to find Fissidens fontanus (and Oxyrrhyncium speciosum as well) with a certainty of about 90 %. The reason why this small water moss was thought threatened was that it was almost only by diving that you could get hold of it. It grows deeper than one can wade and it won’t really come up in a rake sample, and it won’t show or be identifiable in a video, and our team was really the first one that started scientific diving around these waters in 2006. We reported every finding dutifully and were very happy to see the species dropped from the Red species list of Finland in March 2019. Now we have enough data to say that Fissidens fontanus is not regionally threatened, it was just thought so because it was small and elusive. 

If you want to know more about the water mosses and their habitats, reefs, you can read through the SEAmBOTH blog about reefs https://seamboth.com/2018/06/08/unique-habitats-reefs/  

Essi Keskinen 

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