Northern Ireland's Priority Species

Physcomitrium sphaericum – dwarf bladder-moss

Physcomitrium sphaericum

Physcomitrium sphaericum (Hedw.) Brid.
Family: Funariaceae

P. sphaericum is an attractive small acrocarpous moss that grows on mud exposed by falling water levels of lakes and reservoirs.

In brief

  • In Northern Ireland it is known only from County Antrim, at Copeland Reservoir and North Woodburn Reservoir
  • The habitat is on partly bare mud exposed by falling water levels within the inundation zone of reservoirs (and elsewhere, lakes, ponds, or rivers)
  • The species begins to grow only after water levels have fallen, so it can only be found in years when water levels remain low; it is only identifiable with capsules, which may mature from late summer and autumn into the winter
  • It is rare and there are no other populations known in Ireland
  • The main threats to the species are likely to arise from changes in water-level management, especially if water levels are kept permanently high to benefit angling or water sport’s interests.

Species description
A small moss that grows as scattered plants or small patches, at most 4mm high. The leaves are ovate to ovate-lanceolate, entire, unbordered, with nerve ending below the apex. Leaf cells are quadrate to hexagonal above, rather large and lax as in other species of the Funariaceae. The capsule is spherical when immature, hemispherical after the lid falls off, held erect on a seta 1 to 2mm long. The spores are rather large, 24 to 32 microns in diameter. The immature capsule is partly covered by a symmetrical (mitriform) calyptra which develops several longitudinal splits.

Life cycle
The plants are short-lived annuals that grow from spores when water levels fall and mud is exposed to the air. The leafy stems grow to full size within a few weeks, after which the sporophytes mature within a few months. The plants are autoicous (female inflorescence at shoot tip, male inflorescences on branches). Capsules mature from late summer through autumn into the winter. Spores may survive underwater for many years, germinating only when falling water levels eventually expose them to the air.

Similar species
Although there are several small acrocarpous mosses that colonise bare mud, P. sphaericum is distinctive because of the capsule shape, almost spherical when mature, then hemispherical after the lid falls off. The untoothed leaf margins, lax leaf cells, short seta (1 to 2mm) and spore size (24 to 32 microns) are also important distinctions from related species.

How to see this species
In Northern Ireland it is known only at two reservoirs in County Antrim — Copeland Reservoir and North Woodburn Reservoir. The species can only be found in years when low water levels expose the sediments of the reservoir inundation zones, which should be searched from August to November. Relevant access permissions should always be sought prior to visiting any sites.

Current status
Known in Northern Ireland only by the populations discovered in 1999 at Copeland Reservoir and North Woodburn Reservoir in County Antrim. Surveys at several other reservoirs in the province have not revealed additional populations, although high water levels have prevented adequate surveys at some of them. There are no other populations known in Ireland. Elsewhere, the species is recorded from scattered localities in England, Wales, continental Europe and across Asia eastwards to Japan.

Why is this species a priority in Northern Ireland?

  • It is rare and there are no other records from Ireland; thus Northern Ireland is its Irish stronghold
  • It is an Irish Red Data Book Species classed as Vulnerable.

Threats/Causes of decline
The main potential threat would be from changes in water-level management, especially if water levels were to be kept permanently high to benefit angling or water-sports. However, persistently low water levels would lead to the habitat of the species becoming shaded by taller plants. Other risks could arise from eutrophication of the water or disturbance on the reservoir banks.

Conservation of this species

Current action

  • The two populations known in County Antrim were discovered during bryophyte surveys by EHS in 1999
  • The water supply authorities have been informed of the significance of the reservoir margins for plant conservation.

Proposed objectives/actions

  • Maintain the number of viable populations of the species
  • Maintain the range of the species.

What you can do
Study of mosses is a specialist pursuit since microscopic study is usually needed for reliable identification. Accurate appraisal of combinations of characters is necessary for identification of many species. The Field Studies Council provides courses introducing bryophytes and bryology. The British Bryological Society welcomes beginners to its field trips and indoor meetings.

Further information

British Bryological Society

Field Studies Council

Furness, S.B. and Hall, R.H. (1981). An explanation of the intermittent occurrence of Physcomitrium sphaericum (Hedw.) Brid. Journal of Bryology 11: 733-742.

Hill, M.O., Preston, C.D. and Smith, A.J.E. (1994). Atlas of the bryophytes of Britain and Ireland. 3. Mosses (Diplolepideae). Colchester: Harley Books.

Holyoak, D.T. (2003). The distribution of bryophytes in Ireland. Dinas Powys, Vale of Glamorgan: Broadleaf Books.

Smith, A.J.E. (2004). The moss flora of Britain and Ireland. 2nd ed. Cambridge: Cambridge University Press.

Text written by:
David Holyoak