Avicennia
Avicennia, genus comprising at least eight species of trees or shrubs in the family Acanthaceae. Avicennia has a wide geographical distribution, with members found in intertidal estuaries along many of the world’s tropical and warm temperate coasts. Their fleshy, leathery leaves are opposite and entire and feature salt-exuding glands on the upper and lower surfaces. Similar to other mangrove species, members of Avicennia have pneumatophores (respiratory roots) that project above the mud to facilitate gas exchange for the submerged primary roots. The small bisexual flowers are sessile (lacking a petiole and attached directly to the stem) with a persistent calyx and produce achene fruits with a relatively large seed. Avicennia exhibits cryptovivipary, in which the embryo emerges from the seed coat but remains in the fruit before falling from the parent plant. The floating propagules are commonly dispersed aquatically, and the fruits of some species can be viable for up to a year in salt water without rooting.
Avicennia plants have a number of ethnobotanical applications. The wood is commonly used as fuel and for construction, and the leaves can be fed to cattle as fodder. The fruit is used as an insect repellent, and the salt exuded from the leaves is edible. Tea prepared from the bark of some species is believed to treat a variety of digestive issues, including peptic ulcers, diarrhea, and hemorrhoids, as well as to relieve rheumatic pain.
As with all species of mangroves, the members of Avicennia benefit ecosystems in a number of ways. For example, in addition to serving as a physical buffer between marine and terrestrial communities, Avicennia plants prevent erosion, stabilize shorelines, and provide habitat for a diverse array of terrestrial organisms. Furthermore, many species of coastal and offshore fish and shellfish rely on mangroves as their breeding, spawning, and hatching grounds. Because of their high salt tolerance, black mangroves are often among the first species to colonize mud and sandbanks flooded by seawater, but an increase in coastal development and altered land use has led to a decline in the populations of all Avicennia species, with several listed as vulnerable on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species.