The P. ostenfeldii complex typically form patchy meadows with mixed species in open ocean or rough water sublittoral habitats (Cambridge 1975). They are characterised by their long, thick, leathery leaves and long leaf sheaths that are deeply buried. These characters appear to be associated with strong wave movement and mobile sand substratum typical of the environments in which they are found (Kuo and Cambridge 1984).

Campey et al. (2000) implies that vegetative morphological characters, upon which five species of the P. ostenfeldii complex were erected, was not effective for the identification of species from multiple samples from any one location.

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There is no specific population information for Posidonia denhartogii. The overall population is thought to be stable.

Stable

Threats may include coastal development by industry, pipelines, communication cables, mining and dredging. Additional threats include pollution from eutrophication causing seagrass overgrowth and smothering of epiphytes, aquaculture, farming and direct physical damage by recreational and commercial boating activities (Green and Short 2003).

It is recommended that the whole P. ostenfeldii complex be reanalyzed and that further morphological and genetic work is carried out (Campey et al. 2000).