Mangrove Adaptations A unique feature of mangroves is their ability to survive in an extremely hostile environment. Plants have to constantly fight the intrusion of salt into their body; tides alternately expose and submerge at the plants; soil is oxygen deficient and propagation in the constantly changing environment is quite a challenge. Mangroves have developed interesting adaptations to thrive in this dynamic environment.

Salinity Tolerance MechanismsSome mangroves filter out salt before it enters the plant body; some actively throw out salt through the leaves; some others accumulate salt in their leaf tissue, preventing them from interfering with the metabolic activities. Some mangrove species adopt a combination of these strategies. In species like Avicennia, which throw out salt with the help of salt glands in their leaves, one can observe salt crystals on the leaf surface. Species which accumulate salt in leaf tissue will have glabrous leaves.

Salt deposition seen on Avicennia leaf

Specialised rootsMany species of mangroves have breathing roots all around the tree, to absorb oxygen directly from the atmosphere. In the case of Avicennia spp, they are pencil straight and numerous. Sonneatia alba has stouter, peg-like pneumatophores, whereas those in Sonneratia caseolaris are quite large, reaching up to 2 m in height.

Pneumatophores of Avicennia spp

Pneumatophores of Sonneratia caseolaris

Rhizophora plants elevate themselves above the ground with an extensive network of stilt roots, whereas Bruguiera spp. has knee roots, so named due to their similarity to folded knees. Knee roots crop up from the ground at regular intervals and bend back again in a very unique pattern. This again is a mechanism to capture oxygen from the atmosphere.

Knee roots in Bruguiera spp,Stilt roots in Rhizophora mucronata

ViviparyMembers of the Rhizophoraceae family (Rhizophora, Bruguiera, Ceriops and Kandelia) exhibit a unique technique of propagation called vivipary. The seed, instead of remaining dormant for a while, germinates on the tree itself and comes out as a long and pointed ‘propagule’. It remains attached to the tree and carries out photosynthesis on its own. After detaching from the tree, it floats around for a while, before settling down at a suitable place.

A partial form of vivipary, known as cryptovivipary is exhibited by Avicennia, wherein the seed does germinate on the tree, but does not come out of the fruit in the form of a propagule.

Propagules of Rhizophora, Bruguiera, Ceriops and Kandelia