Rhizome laterally compressed, 2–5 mm thick; internodes variable in length, usually 1 or 2 long internodes, 1.5 cm long, followed by 2–4 short internodes, 0.5–2.5 mm long. Leaves 2 or 3 per shoot. Leaf sheath 8–20 cm long; margins overlapping for 3/4 its length; ligule 1 mm long; auricles 2 mm long; fibre bundles abundant amongst parenchyma tissue. Leaf blade biconcave, linear, to 120 cm long, 2.5–7 mm wide, coriaceous; apex spathulate to rounded; longitudinal veins 7–9 (–11); epidermal cells oval to rectangular with smooth margins, l:w 1–3; epidermal cells cross section height:width ratio 3–5. Inflorescence of 3 or 4 spikes, each with 2–5 flowers; peduncle 25–40 cm long. Anther 2.5–4 mm long; anther connective 4–6 mm long. Ovary 1.5–3.5 mm long, 1.2–1.4 mm diam; stigma 1–2 mm high, irregularly papillose with 2 crests. Fruit broadly elliptic, laterally compressed, 3–4 cm long. Seeds pyriform, 10–12 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.
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