Callitriche insularis Lansdown 2022, sp. nov.

12. Callitriche insularis sp. nov.

Type: — FRENCH POLYNESIA. AUSTRAL ISLANDS. Rapa, Stream draining taro patch by village, Ahurei, 8 July 1934, H. St. John & J. Maireau 15430 (US 610318!).

Etymology: —Named because of its occurrence only on French Polynesian islands.

Diagnosis: —This species can be distinguished from all other Callitriche species by the presence of stem and leaf scales, ♀ and ♂ flowers together in some axils, filament length, yellow pollen, and the lack of bracts, combined with the size and colour of mature fruit.

Description: —Stem and leaf scales present. Leaf bases connate. Linear leaves unknown; expanded floating or submerged leaves unknown, leaves of terrestrial plants expanded 2.0– 3.2 mm long × 0.9–1.8 mm wide, venation simple, petiole 1.2–2.7 mm long; ♀ and ♂ flowers together in an axil opposed by a ♀ flower. Bracts apparently lacking. Styles erect, caducous ≤1.0 mm long. Filament erect, 0.1 mm long; anthers quadrilocular, 0.1–0.2 mm long × 0.1–0.2 mm wide; pollen yellow. Fruit substrumose, subsessile, slightly wider than high, uniform golden-brown when mature, 0.6–0.8 mm long × 0.8–1 mm wide, winged throughout.

Illustrations: — Fig. 10b.

Recognition: — C. insularis can be distinguished from all other Callitriche species in the region by the small, broadly winged fruit which are golden-brown when mature. It is most similar to C. muelleri but that species has leaves with a single tooth on each margin which is lacking in C. insularis.

Distribution: —Native. Known only from Rapa Island and its satellite Tauturau, French Polynesia.

Habitats and Ecology: —Specimens of Callitriche insularis have been collected from a stream draining a taro patch, from a moss-covered seepage from a basalt outcrop above the sea and from the margin of a stream flowing through marshes. 1–300 m elevation.

Conservation Status: — Callitriche insularis is known from only a very small number of specimens and is estimated to have an AOO significantly less than 20 km 2, such that it is very vulnerable to factors such as local landuse changes. It is therefore classed as Vulnerable D2 (IUCN 2012).

Specimens examined: — FRENCH POLYNESIA: Tauturau Island, 18 April 2002, K.R. Wood 9681 (NY, PTBG); St. John 15461 (K).