Top views of spicate flowers of A. ilicifolius and both subspecies of A. ebracteatus, subspp. ebracteatus and ebarbatus
Right view, mature seed pods of A.ebracteatus
2 species in Australia, one with 2 subspecies
Acanthus L. (1753) is the only genus with mangrove inhabitants in the family Acanthaceae Juss., a family of chiefly tropical herbs, shrubs, and small trees with conspicuous zygomorphic flowers, and capsular fruits with hardened shells. Acanthus is a large genus of some 300 species in tropical Asia and Africa with a centre of diversity in the Mediterranean. It is often distinguished from related genera by spiny leaves, spicate terminal inflorescences, two bracteoles and uniform anthers. Three species, A. ebracteatus, A. ilicifolius, and A. volubilis Wall. are recorded in mangrove habitat, but they lack consistent diagnostic features. Mangrove Acanthus species occur either as an under canopy of various mangrove associations, or in frontal thickets on stream edges of recently accreting estuarine banks.
Although mangrove Acanthus do occur in lower estuarine locations, they grow most commonly in middle to upper estuarine areas, in both dense frontal thickets and as undercanopy patches to the high water margin.
Derivation of Genus Name
‘Acantha’ means thorn or thistle (in Greek), and refers to the spiny leaves of some species.
Acanthus species growing in mangroves are distributed across the Indo-West Pacific from India and China, through Asia and Indonesia to the Philippines, western Pacific, New Caledonia and tropical Australia. Two species are recorded in mangrove habitats of northern Australia.
Key to Australia's Acanthus species
Two species of Acanthus are recognised in Australia’s mangroves across the northern coast from Western Australia, Northern Territory and Queensland. They are distinguished by flower colour, inflorescence shape, and the presence or absence of bracts and bracteoles at the base of flower buds and fruits. Both species, at times, have spiny leaves, but not always.