Having the nerves (2 of them) to live in a cave

Leaf of Cyclodictyon laetevirens
Photo by Sean Edwards

The costa or nerve is one of the most fascinating structures of moss gametophytes. The structure may be single (e.g. Plagiomniun, Orthotrichum), highly expanded (e.g. Paraleucobryum), double (e.g. Cyclodictyon), very prominent with dorsal lamellae (e.g. Polytrichum) or completely absent. The taxonomic value of the costa is widely recognized among bryologists, for example the presence of a double costa is one of the defining features of the family Pilotrichaceae.

Among the members of Pilotrichaceae, Cyclodictyon is one of the most charismatic. I came across C. albicans during my undergraduate studies because it grows in close association with the hornwort Nothoceros vincentianus, near streams in Panamanian cloud forests. The shortly-acuminate bordered leaves and the double costa are a dead give-away for the species. In Europe there is another Cyclodictyon species, C. laetevirens, commonly named the bright-green Cavemoss. The species is particularly noteworthy in England due to its rarity. Forming glossy, dark green patches, C. laetevirens is restricted to a few maritime caves in the Cornish region, where the temperature is unlikely to ever fall below freezing.

Cyclodictyon laetevirens.
        Photo by Sean Edwards

Ron Porley’s book on rare English bryophytes contains a candid anecdote on the discovery of C. laetevirens in England. The species was first collected in 1840 by the botanist J. Ralfs. Unaware of Ralfs’ collection, W. Borrer also collected the plant in one of the Cornish caves. He realized this was the first record of the species for England and when he returned home he asked a local cleric to collect more of the species to distribute to colleagues.  The zealous cleric wanted to ensure that nobody but his friend collected the precious plant – and so he deliberately wiped out the entire population! Borrer became preoccupied about the fate of the plant, probably with some feelings of guilt, and asked other colleagues if they had come across the species. He discovered that Ralfs had collected it in a different sea cave, and the two men established a life-long friendship. The species appears to be restricted to very few sea caves in England, with a 1998 survey reporting it to cover a total area of 30 ×70 cm on Ralfs’ original cave, growing with Conocephalum conicum (Porley 2013). The species is also found in very few localities in Jura, Scotland.

A major concern for this endangered species is the vulnerability of its habitat, with sparse protection to Cornish caves and potential sea-level rises due to global warming. The cave-dweller is, however, kept ex-situ at the bryophyte threatened facility at RBGK.

Thanks to R. Porley for his comments and help in obtaining pictures of C. laetevirens from S. Edwards.