Adaptations to
The tidal Zone

Australia’s mangroves show remarkable adaptations dealing with frequent saltwater inundation and varying climatic zones from arid to very wet. Mangroves share specialised attributes for growing with excess salt and saturated air-less soils. They also have special growth strategies to facilitate establishment and regeneration. One special attribute promoting sustainability and dispersal of mangroves is their unusual production of live young - vivipary. Such attributes have kept mangroves from extinction for more than 50 million years, and enabled them to occupy tidal areas around the world.

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Origins of Mangroves

Fossilised pollen

Distinctive pollen grains of Rhizophora stylosa (lips), Nypa fruticans (spiky hamburger) and Sonneratia caseolaris (bipolar).

Origins map

Map of Eocene position of continental fragments and locations of mangrove fossils surrounding the ancient Tethys Sea.

Mangroves, as we know them, have been around for at least 50 million years, and possibly a lot longer. For instance, the origins of Avicennia have been linked to the opening of the Atlantic Ocean extending back almost 100 million years. In any case, species like Rhizophora and Avicennia were widely distributed by 50-55 million years ago. At that time the ancient Tethys Sea, a shallow equatorial sea full of coral reefs and islands, was fast closing as the massive continental fragments continued to shift and jostle across the globe. This process of continental drift appears to have played a pivotal role in determining both the diversity and type of mangroves found today, as well as their current global distributions. Mangroves have not evolved and dispersed uniformly, and there was no common centre of origin. However, mangrove plants share common adaptations, individual species have evolved differently and developed at different rates. Today’s distributional patterns are useful fingerprints from which we might trace the origin of each species. Current distributions of individual taxa show numerous instances of unusual occurrences and absences that demonstrate finite dispersal limitations, especially across open water. Furthermore, there are some genetic discontinuities that occur despite the lack of any current dispersal barriers. In each case, such unusual distribution patterns provide important clues and evidence of past geological and climatic conditions that tell us something about the origin and evolution of mangroves.

Right - fossilised leaves of Nypa from the Eocene period found near Strahan, in southern Tasmania. photo: Mike Pole

Daintree seedlings

Spreading Seed to Distant Shores

Mangrove species are dispersed by water-buoyant propagules (fruits, seeds and hypocotyls), allowing them to take advantage of estuarine, coastal and ocean currents both to replenish existing stands and to establish new ones. The ability to accomplish this differs for each species. Furthermore, dispersal ability varies significantly among species with comparable distributional ranges. A good example is shown with the widely distributed global genera Rhizophora and Avicennia. Propagules of Rhizophora have considerable longevity at sea surviving over 3-4 months, while those of Avicennia last only up to 3-4 weeks. Such differences profoundly affect dispersal range, and this is shown by the much greater eastward extent of IWP Rhizophora than Avicennia eastward across the Western Pacific. Each mangrove species differs in its establishment success and growth development rates, and each has unique tolerance limits and growth responses. In practice, species are influenced by the interplay of different factors along key environmental gradients that are conveniently considered at four geographic scales — global, regional, estuarine and intertidal. Global distributions are affected largely by temperature while regional distributions are affected also by rainfall. By comparison, local distributions upstream in estuaries and across the tidal profile are influenced by a much greater number of factors including: species presence, rainfall, salinity, catchment area and tidal range. Mangroves respond to all these factors and their presence at any location demonstrates the effectiveness of past and current dispersal and establishment strategies of respective species.

Right - Rhizophora seedlings colonise accreting banks often deposited from runoff into downstream estuarine reaches.

  • Floating seed Floating keeled capsule of Heritiera littoralis.
  • Exploded seed capsules Exploded capsule fragments and seeds of Excoecaria agallocha.

Community Volunteers

A key feature of MangroveWatch is its close partnership between community volunteers and scientists from the James Cook University’s Mangrove Hub. Together they are systematically recording basic data as video and still imagery for assessments of estuarine habitat health.

Armed with expert support, training and advice, MangroveWatch volunteers in key regions are actively contributing to the monitoring of local estuaries and shorelines. An important goal in this phase of the program is to develop a network of like minded groups with the aim of producing public documents that describe important issues affecting local estuaries and mangroves, and their overall health.

Getting Involved

If you would like to find out more about us or if you like to initiate your own MangroveWatch group within your area, please contact someone at the Mangrove Hub. We will be happy to help.

  • Mangrove Hub Facilitator
  • Dr Norm Duke
  • MangroveWatch Ltd
    ABN: 44 153 297 771
  • PO Box 1250,
  • Elanora Q 4221
  • Mangrove Hub Email

Mangrove Watch Brochure

You can download our fact and information sheet (see link below) to get more information about the MangroveWatch programs.

Mangrove Watch Brochure