Classification
 Nomenclature
Scientific Name:
Leptodictyum riparium (Hedw.) Warnst., Krypt.-Fl. Brandenburg, Laubm. 878 (1906)
Synonymy:
  • Hypnum riparium Hedw., Sp. Musc. Frond. 241 (1801)
  • Amblystegium riparium (Hedw.) Schimp. in Bruch et al., Bryol. Eur. 6, 58 (1853)
Lectotype: Europe. (Designated by Hedenäs & Geissler, 1999.) Not seen.
  • = Amblystegium riparium var. debilinerve Dixon & Sainsbury, J. Bot. 71: 249 (1933)
Isotypes: N.Z., Waikato River, just below Ātiamuri, on shallow soil on rock at edge of river, 29 April 1928, K.W. Allison 63 (herb. G.O.K. Sainsbury 510), CHR 604760!, WELT M030301!, M005810!
Etymology:
The epithet means frequenting the banks of rivers or streams.
 Description

Elements in the following description are taken from Kanda (1976).

Plants yellow- to bright-green, sometimes black in submerged forms, weakly lustrous when dry. Stems prostrate or free-floating, irregularly and rather loosely branched in a single plane, in cross-section with thick-walled outer cells grading into thin-walled parenchymatous inner cells and with a small and indistinct central strand. Branches variable in length, usually with leaves more complanate than the stems. Stem and branch leaves similar, rather distant, wide-spreading, distinctly or moderately complanate, and ± twisted longitudinally when dry, extremely variable in shape and size, narrowly to ± broadly ovate-lanceolate, gradually tapered to a narrowly acuminate or a short ± acute apex, often variable in shape on a single plant, variable in length, (2.0–)2.5–3.5 mm, not channelled, very weakly decurrent and with some cortical cells often stripping off with leaves, entire; mid laminal cells linear-rhomboidal, mostly 65–95 × 9–11 µm and c. 6–9:1, not porose, the lumina much broader than the cell walls; basal cells shorter and wider; alar cells not or very weakly differentiated from adjacent basal cells, rectangular or subquadrate, plane. Costa single, rather thin, extending c. ½–⅔ the leaf. Pseudoparaphyllia and axillary hairs not seen. Paraphyllia absent.

Autoicous. Perichaetia scattered on stems, the inner leaves erect, sheathing the setae, oblong-lanceolate and costate. Perigonia gemmiform, very small (c. 0.5 mm), and scattered on stems. Setae commonly 10–15 mm (or longer in submerged forms), weakly flexuose, red, smooth; capsules short-cylindric from a weakly defined neck, strongly curved, c. 2.5 mm; mouth oblique and constricted when dry; exothecial cells mostly oblong-hexagonal, firm-walled; stomata not seen; operculum conic. Exostome pale yellow-brown, the teeth cross-striolate below on outer surface and distinctly bordered; endostomal segments from a high basal membrane, keeled and perforate, alternating with 1–2 nodose cilia. Spores 12–18 µm, very finely papillose.

 Illustrations

Smith 2004, fig. 255, 9–12; Crum & Anderson 1981, figs. 454–457, 458 J–O.

 Recognition

Leptodictyum riparium is often recognised as much by its habitat and overall habit as by its few distinguishing microscopic features. Confusion is most likely with straight-leaved and submerged forms of Drepanocladus aduncus and D. brachiatus. The absence of secund leaves at the branch/stem apices in Leptodictyum riparium combined with more distinctly complanate branch leaves, less narrowed leaf bases, the tendency for leaves to twist around their longitudinal axis when dry, and lack of inflated alar cells will distinguish most collections. In all instances the alar cells of L. riparium are plane, whereas those of Drepanocladus spp. tend to be excavate. In populations which are poorly developed or heavily encrusted with unicellular algae confident determination is sometimes not possible.

Confusion is also likely with Campyliadelphus polygamus and not all material can be distinguished with confidence. Leptodictyum is confined to lowland situations while C. polygamus occurs in lowland to alpine elevations. The shoots of L. riparium are usually distinctly complanate and sometimes weakly secund distally, while those of C. polygamus are neither complanate nor secund. In L. riparium the leaves are ± evenly tapered from base to apex and the upper portions are flat (either moist or dry); those of C. polygamus are unevenly tapered and channelled above (either moist or dry). Both species can have spirally twisted leaves, but this is more distinct and constant in L. riparium. The leaves in L. riparium can also be longer (2.0–3.5 mm) than those of C. polygamus (c. 1.8–2.3 mm).

 Distribution

NI: N Auckland, including offshore islands (LB), S Auckland, Gisborne (Poverty Bay, Lake Waikaremoana), Hawke’s Bay, Wellington; SI: Nelson (Lake Rotoroa, other unlocalised localities), Canterbury (mostly near Christchurch), Otago (eastern districts).

Nearly cosmopolitan. Recorded from mainland Australia (Scott & Stone 1976). Streimann & Klazenga (2002) recorded it from Tasmania but it was not cited from that island by Dalton et al. (1991). Recorded from Kerguelen by Smith (2004). Widespread in the temperate northern hemisphere and in tropical regions. The possibility that this species is adventive in N.Z. cannot be excluded.

 Habitat

Mostly attached to wood in wet situations, less often attached to rock or soil. Often on stumps or logs beneath willows at stream or lake margins or in swamps; frequently growing in depressions which are flooded only at intervals of high water. Also occurring in clear, spring-fed streams (as at Christchurch and probably at Onehunga, N Auckland L.D.). Rarely attached to rocks "in fast flowing stream(s)" as at Waipapa River (S Auckland L.D.). Occurring from near sea level to at least 610 m (Lake Waikaremoana, Gisborne L.D.), but with most localities <100 m elevation.

 Biostatus
Indigenous (Non-endemic)
 Notes

Sometimes growing with aquatic species of Fissidens (F. berteroi, F. rigidulus), L. riparium is also sometimes associated with both Nitella (as at Lake Wairarapa, Wellington L.D.) and Chara (as at Lake Rotoroa, Nelson L.D.). Mixtures with Drepanocladus sp. and other aquatic mosses occur and can be perplexing.

Collections by R. Mason from a "fast flowing stream" (Waipapa River, S Auckland L.D., CHR 106224) are coarser and more robust than the bulk of examined material, with median cells up to 120 µm and c. 10:1. At Lake Rotoroa (Nelson L.D.) L. riparium has been collected at depths of between 17–22 m as part of an assemblage of 7 moss species, two thalloid hepatics, and Chara australis (de Winton & Beever 2004).

A few North I. collections, including R. Mason 7542 (Waitoa River, S Auckland L.D., CHR 106208) and K.W. Allison 4954 (Ngā Awapurua, Hawke’s Bay L.D., CHR 578605), are black in colour. However, a collection from the Waipapa River, Gisborne L.D., R. Mason 7732 (CHR 106224) is intermediate between this black form and more representative material.

 Bibliography
Bruch, P.; Schimper, W.P.; Gümbel, W.T. 1853–1855: Bryologia Europaea seu genera muscorum Europaeorum monographice illustrata. Vol. 6. Schweizerbart, Stuttgart.
Crum, H.A.; Anderson, L.E. 1981: Mosses of Eastern North America. Columbia University Press, New York.
Dalton, P.J.; Seppelt, R.D.; Buchanan, A.M. 1991: An annotated checklist of Tasmanian mosses. In: Banks, M.R.; Curtis, W.M. (ed.) Aspects of Tasmanian Botany – a Tribute to Winifred Curtis. Royal Society of Tasmania, Hobart. 15–32.
de Winton, M.D.; Beever, J.E. 2004: Deep-water bryophyte records from New Zealand lakes. New Zealand Journal of Marine and Freshwater Research 38: 329–340.
Dixon, H.N.; Sainsbury, G.O.K. 1933: New and rare species of New Zealand mosses. Journal of Botany, British and Foreign 71: 213–220, 244–251.
Fife, A.J. 2014: Amblystegiaceae. In: Heenan, P.B.; Breitwieser, I.; Wilton, A.D. Flora of New Zealand — Mosses. Fascicle 1. Manaaki Whenua Press, Lincoln.
Hedenäs, L.; Geissler, P. 1999: Lectotypification of Hedwig names: holarctic pleurocarpous mosses. Candollea 54: 417–432.
Hedwig, J. 1801: Species Muscorum Frondosorum descriptae et tabulis aeneis lxxvii coloratis illustratae. Barth, Leipzig.
Kanda, H. 1976 ("1975"): A revision of the family Amblystegiaceae of Japan I. Journal of Science of the Hiroshima University: Series B, Division 2 (Botany) 15(2): 201–276.
Scott, G.A.M.; Stone, I.G. 1976: The Mosses of Southern Australia. Academic Press, London.
Smith, A.J.E. 2004: The Moss Flora of Britain and Ireland. Edition 2. Cambridge University Press, Cambridge.
Streimann, H.; Klazenga, N. 2002: Catalogue of Australian Mosses. Flora of Australia Supplementary Series. Vol. 17. ABRS, Canberra.
Warnstorf, C. 1904–1906: Laubmoose. Kryptogamenflora der Mark Brandenburg un angrenzender Gebiete herausgegeben von dem Botanischen Verein der Provinz Brandenburg. Gebrüder Borntraeger, Leipzig.