Black mangrove propagules washed up in the wrack line

As summer comes to a close on Sanibel Island, our beaches become littered with what look like exploding green lima beans in the wrack line. These are in fact the seeds of the black mangrove (Avicennia germinans), a hardy, salt-tolerant tree that is native to our shores. Black mangrove trees flower in summer and produce seeds in the late summer and early fall.

Like most mammals, but relatively unique in the plant world, black mangroves are viviparous—they bring forth live young. The seeds ripen and grow while still attached to the parent tree, and then drop into the water fully germinated. They float on the tides for an indefinite period of time before contacting soil and taking root in a new home in the intertidal zone.

Floating propagule

The intertidal zone is a stressful place to make a living, and its inhabitants must have specialized adaptations in order to survive. Because black mangroves get their water from “drinking” saltwater through their roots, one of their main challenges is dealing with salt. Black mangroves get rid of excess salt by excreting it through glands on the bottom surface of leaves – if you’ve ever touched a black mangrove leaf, you may have felt this crusty layer of salt under your fingertips.

The soils where black mangroves live are often waterlogged and oxygen-poor. To supplement the oxygen supply to the tree, black mangroves grow pneumatophores.  These are breather tubes that protrude above the ground and act somewhat like snorkels, providing additional oxygen to the roots. Networks of pneumatophores also form important protected nursery areas and attachment surfaces for marine organisms.

Here at Sanibel Sea School we love a challenge. If you find something strange and intriguing on the beach, let us know! We would love to try to help you identify it. Check back for more information about the cool creatures that live on our shores!