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Surf and Turf

Black mangroves’ adaptations help them thrive in coastal ecosystems.


On the Lighthouse Lakes Paddling Trail near Aransas Pass, paddlers can glide through a maze of black mangroves as roseate spoonbills fly overhead and redfish swim below.

Black mangroves (Avicennia germinans) have developed a remarkable set of adaptations to survive in this environment that’s part marine and part terrestrial.

These tropical woody shrubs live in water that’s much saltier than most plants can tolerate by excreting salt through glands in their leaves. Salt crystals often form on the surface of the leaves. 

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Their snorkel-like roots, called pneumatophores, are unique as well.

“These roots grow up out of the ground and are used to exchange gases,” says TPWD coastal ecologist Jackie Robinson. “There’s no oxygen in the sediments where they live, so they use those pneumatophores to get oxygen from the atmosphere.”

Reproductive adaptations enable mangrove seedlings to germinate while still attached to the parent tree, increasing the chances of survival in this adverse environment.

“When they go to make seeds, they don’t really make a fruit,” Robinson says. “It’s like a seed that’s already begun to open up on the plant itself, so once that reproductive part falls off into the water, it’s ready to grow a root and make a new plant.”

Black mangroves form dense thickets with a web of tangled roots. The root systems trap sediments, helping reduce erosion and improve water quality. Mangrove forests can reduce the impact from storms and protect shorelines from damaging wind and waves. They provide important habitat for fish and nesting sites for birds.

Over the past 90 years, black mangroves have been expanding their northern range limit along the Texas coast. During much of this time, periodic hard freezes limited the growth of individual plants and kept range expansion somewhat in check.

The reduced frequency of such freeze events along the Texas coast during the past two decades has enabled individual plants to grow much larger and allowed greater expansion of black mangrove habitat into salt marshes and tidal flats.

Mangrove forests and salt marshes are both highly productive coastal ecosystems, and researchers are working to understand the ecological changes that happen when mangroves move in. 

 Laura Adams

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