Posidonia robertsoniae J.Kuo & Cambridge

Rhizome laterally compressed, 1–2 mm thick; internodes variable in length, usually 1 or 2 long internodes, 2–3 cm long, followed by a series of short internodes, 0.5–1 mm long. Leaves 1 or 2 per shoot. Leaf sheath 9–14 cm long; margins overlapping for almost entire length; ligule 1 mm long; auricles 2 mm long; sheath fibre bundles (apart from those adjacent to epidermis) occurring in a distinct layer separated from adaxial epidermis by 2 cell layers. Leaf blades slightly biconvex in T.S., linear, 60–150 cm long, 2–4.5 mm wide; apex obtuse, rarely tapering; longitudinal veins 6–9; epidermal cells rectangular, with finely corrugated margins, l:w 1.5–4; epidermal cells cross-section height:width ratio 2. Inflorescence of 3 or 4 spikes, each with 2 or 3 flowers; peduncle 20–30 cm long. Anther 3–4.5 mm long, 1–1.2 mm diam.; anther connective 5–7 mm long. Ovary 2–2.5 mm long; stigma 1–1.5 mm high, loosely papillose with 2 crests. Fruit falcate, 2.5–3 cm long. Seeds pyriform, 12–15 mm long.

Found from 0.5–20 m deep in semi-sheltered as well as high wave energy areas, forming small to large clumps. growing adjacent to other Posidonia species within P. ostenfeldii species group.