Callitriche platycarpa Kütz., Various-leaved Water-starwort
Account Summary
Native, rare. European temperate.
1881; Stewart, S.A.; Carrick Td, on the Western Plateau uplands.
Throughout the year.
Fermanagh occurrence
A taxonomically difficult and, locally, apparently rare perennial species with just three voucher specimens and a total of 26 records in the Fermanagh Flora Database. It would therefore be foolish to pretend to know much about the real local occurrence of this species which can easily be confused with the definitely much more common C. stagnalis (Common Water-starwort). Having said that, C. platycarpa has been recorded in 18 Fermanagh tetrads, 16 of them with post-1975 records.
The presence of stem scales and clearly winged fruits that are as wide as long, separates C. platycarpa from all other Callitriche species except C. stagnalis, from which it can only reliably be separated by the pollen shape. The fruit wing is generally much wider in C. stagnalis, but there is some overlap with C. platycarpa. The bluntly triangular pollen grains of C. platycarpa can only be properly seen under 100-400 × magnification (Lansdown 2008).
Preferred habitats
The typical habitats are in shallow, still, slow-flowing or sheltered waters, or in muddy ground by lowland lakes, rivers and ditches. C. platycarpa appears sparsely scattered across Fermanagh in both acid and base-rich areas. It might be slightly more prevalent around Lough Erne, but the distribution data do not really support this contention, since its presence here could merely reflect the recording bias created by excessively detailed lake survey recording carried out in this area by the EHS Habitat Survey Team.
Preston & Croft (1997) indicate that, like the morphologically similar C. stagnalis, this species has both aquatic-perennial and terrestrial-annual life forms and, likewise, it too has a very wide ecological amplitude in terms of both water/soil chemistry and shade/open sunlight. Plants of C. platycarpa can be shy of flowering or apparently totally sterile, thus denying botanists the fruits necessary to confirm the species identification.
British and Irish occurrence
Preston & Croft (1997) believe C. platycarpa is grossly under-recorded in B & I, but with the identification problems being significant (see the accounts by R.V. Lansdown in Rich & Jermy (1988) and in the BSBI Handbook 11, Water Starworts, Callitriche of Europe (Lansdown (2008)), it is hardly likely that they can really tell this for certain. It is perhaps more of a hunch, but Lansdown (2008) is of the opinion that C. stagnalis is over-recorded at the expense of C. platycarpa.
Visual comparison of the hectad maps in Preston & Croft's (1997) book, Aquatic plants in Britain and Ireland, with those in the New Atlas (2002) displays a considerable recent increase of this species, especially in parts of C & S England. Although the distribution pattern still remains rather patchy, becoming more coastal in Scotland north of the Glasgow-Edinburgh conurbations, it is felt that the species increase might be real, rather than reflecting recorder effort. The apparent increased presence may result from widespread habitat eutrophication since the 1970s, which this species can tolerate better than many other aquatic species, since a similar circumstance has previously been observed in southern Sweden where C. platycarpa appears to be a recent arrival (R.V. Lansdown, in: Preston et al. 2002; Lansdown 2008).
European occurrence
On the continental mainland, C. platycarpa occurs mainly in W Europe, throughout France extending north into S Sweden and east to the Czech Republic. It has also been reported from a few sites in the Iberian Peninsula. It appears to be extending its range to the N & E (Lansdown 2008).