black mangrove propagules!

(blog posts for this project begin with a retroactive entry)

20 January 2009

This bag is filled with black mangrove (Avicennia germinans) propagules that a fish breeder in Texas shipped to me in exchange for some livebearers. Black mangroves produce propagules via cryptovivipary, a process through which the seed sprouts and begins to grow while still attached to the mother plant. Many other mangrove tree species use this same strategy. The reasons for the predominance of cryptovivipary among mangroves are not fully understood, but may have to do with the harsh seashore conditions in which mangroves grow: perhaps the large propagules are better able than small seeds to withstand salty water, intense sun and buffeting waves. In some mangrove species the propagules are covered with buoyant husks and may float long distances before settling and sprouting.

black mangrove propagules

Black mangrove occurs in costal areas in the tropical Atlantic, including the east coasts of Central America and South America, the Caribbean and Africa’s west coast. In the US it is found in Florida, Texas and Louisiana. Black mangrove is physiologically adapted to purge salt through leaf glands, so it can grow in saltwater to brackish conditions where few other plants can survive. The muddy substrates in mangrove areas are also rich in nutrients and organic matter. In response to the highly anoxic (oxygen-starved) conditions that develop under these circumstances, black mangrove roots grow upright pneumatophores, pencil-like structures that emerge from the mud and bear numerous oxygen-conducting pores. Small black mangrove pneumatophores are visible beneath mature plants in the picture below.

Avicennia germinans mature plants, image courtesy of U.S. Geological Survey, South Florida Information Access (SOFIA)

Of the three mangrove tree species that occur in the United States, red mangrove (Rhizophora mangle) apparently tolerates the saltiest conditions and can grow well in full seawater. Black mangrove and white mangrove (Languncularia racemosa) generally grow at higher intertidal zone locations in somewhat fresher water.

Red mangrove is already a popular plant for aquariums with brackish water fish and as an element in reef aquarium refugiums. Black mangrove and white mangrove have seen little use in marine tanks and these species might poorly tolerate full-time exposure to saltwater. The author’s searches also produced no accounts of their use in freshwater aquariums. This experience with A. germinans is a test and might reveal an interesting new plant for riparium displays.

black mangrove in planter

Most of the propagules went into shallow trays filled with water and saturated gravel. I planted more than thirty in hanging planters. The hobbyist who sent me the box of propagules indicated that they sprout best when lying on the substrate surface. His experiences also suggested that unlike red mangrove, A. germinans tolerates pruning well. Since wild mangroves usually grow in full sun, it will be interesting to see how black mangrove performs under fluorescent T5 lighting.

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