The floating liverworts of Essex
KEN ADAMS
School of Health & Biosciences, University of East London, EI 5 4LZ
Introduction
Four species of floating aquatic liverworts, all belonging to the Marchantiales, are found in the
British Isles, and all but Riccia canaliculata have been recorded from Essex. R. rhenana and R.
canaliculata are extremely rare in Britain, and sporadic in appearance. Whereas R. canaliculata
produces sporophytes in this country, R. rhenana sporophytes are unknown, and its sporadic,
ephemeral occurrences here suggests aquarium throw outs. This account summarises all the historical
and contemporary Essex records known to the author, and is detailed with repeat site occurrences
in order to build up a picture of persistence and mobility. Two of the species that occur in Essex are
unexpectedly indirectly endangered by the occurrence of the al ien aquatic Crassula helmsii. Attempts
to remove this plant, (mostly unsuccessfully), by the complete scouring of ponds, has resulted in
the loss of Ricciocarpos natans and Riccia fluitans from several sites in the county in recent years.
R. fluitans probably occurs in far more ponds in the county than recorded here, and the author
would welcome detailed information on additional occurrences.
Ricciocarpos natans (L.) Corder
An unmistakeable floating liverwort of neutral to highly base-rich stagnant ponds and abandoned
canals, frequently in association with Riccia fluitans. Very rare and probably decreasing in the
U.K., but virtually world-wide in distribution and in some countries very abundant. Not found in
Scotland, 56 x 10km sq records post 1950 in England/Wales/Ireland, but it seldom persists for
more than a few years at any one site, so these figures give an over estimate of its occurrence at any
one time. The thalli fragment into 1-2 furcate portions, that sink to the bottom in late autumn, and
rise up again in the spring, though like Riccia fluitans they may not do so every year. If the pond
dries up during the growing season, the stranded thalli continue to grow, but retract their floatation
scales and may narrow down their bifurcations to resemble Riccia fluitans. Sporophytes arc said to
be very rare in Europe and there do not seem to be any records for the U.K. It is therefore a mystery
as to how it is dispersed in this country. Fragments of thalli on muddy birds' feet seem the most
likely source of propagules, though odd spores leading to single sex clones might be brought over
in this way from parts of the world where it does produce sporophytes. In America, it is apparently
monoecious, and fruits abundantly. Smith suggests that our clones may be dioecious, which might
explain the lack of sporophytes. Paton, however, notes that sexual organs have not been found in
Britain either. Elsewhere, the antheridia and archegonia apparently occur along the mid-line of the
lobes, and the sporophyte is spherical and immersed in the upper surface, the mature spores being
blackish-brown. Being so uncommon a plant, it is possible that sexual organs have been overlooked.
It has been exterminated on Epping Forest first of all by the dredging of Earl's Path, and then
Goldings Hill Upper Pond, not apparently by removing all the thalli, but by changing the nutrient
status of the water. It is seems likely to meet the same fate in Dagnam Park, where ponds are
similarly being dredged to reinvigorate them. There are fifteen 1km sq records from Essex, though
it is sporadic and seldom likely to be present at more than a couple of sites in the county at any one
time.
Essex Naturalist (New Series) 21 (2004)
I 43