Occurrence and fruit and seed biology of Halophila tricostata Greenway (Hydrocharitaceae)

Halophila tricostata Greenway appears to be endemic to eastern Queensland, Australia, and occurs between 14�11'S and 23�45's. It was found at depths from 1.4 to 30 m in well sheltered habitats, including in shallow coastal sites near mangrove-lined estuaries, on the lee side of continental and coral- reef islands, and on some commercial prawn-trawling grounds within the Great Barrier Reef lagoon. It grows on predominantly fine mud substratum in small monospecific meadows or mixed with other tropical seagrasses, mostly other Halophila species. Field observations indicate that H. tricostata is an annual angiosperm and produces an estimated 70 000 seeds m-2 year-1. Halophila tricostata is dioecious. The plant has a horizontal rhizome bearing an erect shoot with eight to twelve nodes and a root at each rhizome node. Except at the first two or three nodes, the mature plants produce a reproductive organ at each node of their rarely branched erect shoot. The reproductive organs and fruits develop and mature acropetally along the erect shoot. There are 24-60 seeds, with a mean of 41 seeds, per fruit. The seed has a coiled embryo protected by a cotyledon, and an enlarged hypocotyl. The hypocotyl acts as a nutrient store and contains starch, protein and lipid. The seed covering consists of pericarp remains and two thin cuticular layers of seed coat. The surface of the seed covering has numerous fine protrusions. The seed covering becomes loose and is discarded during germination, exposing the hypocotyl. The surface of the hypocotyl develops hair-like unicellular structures during seedling development. The majority of the seeds begin to germinate at 26-28�C after two weeks of culturing, but germination is not synchronized. The culturing of H. tricostata seedlings beyond the three-leaf stage was not successful.