Neptune grass

This is not an alga but a true flowering plant that returned to the sea, endemic to the Mediterranean and found nowhere else on earth. It grows in tufts of five to eight flat ribbon leaves, about one centimetre wide and commonly 30 to 80 cm long, bright green when young and darkening with a fur of epiphytes as the season goes on. The tufts sit on woody rhizomes that creep sideways and pile slowly upwards, and over centuries the dead rhizomes, roots and trapped sediment build a raised terrace called matte, which is why a meadow often sits noticeably higher than the sand beside it.

A Posidonia meadow is the nursery and pantry of the Maltese inshore sea. Seahorses, pipefish, juvenile bream and wrasse, cuttlefish eggs, the noble pen shell and an enormous cast of small invertebrates depend on it, and it releases oxygen and locks away carbon at a rate no other Mediterranean habitat matches. In autumn it flowers and later drops olive-shaped floating fruits that Maltese beachgoers find washed up and mistake for real olives.

Around the Maltese islands it forms extensive meadows from around a metre down to about 40 m wherever the water is clear and the bottom is sand or matte, and it is the habitat you swim over on the way to most shore-dive reefs. In winter, storms strip old leaves and pile them on beaches in springy brown banks called banquettes - these look like rubbish but are the beach's own defence against erosion and a habitat in their own right, and clearing them accelerates sand loss.