Learn about maldivian giant corals and coral reefs

Giant corals: How they live, their habitat, their health and competing organisms

This section will help you gain some knowledge on the reefs and on corals highlighting those features that might be useful during your mission to locate Giant Corals.

Corals

Coral reefs cover less than 1% of the ocean surface, yet they support 25% of the known marine life. They are living structures made of hard corals, animals belonging to a group of invertebrates called Cnidaria.

Aside from a few genera, most of them form colonies made of thousands of polyps growing a hard calcium carbonate skeleton and creating coral reefs in the tropical areas of the whole world. A colony originates either via sexual reproduction or from the fragmentation of a larger coral. Colonies then grow in size as each polyp divides itself into daughter polyps, thus potentially growing indefinitely. One of the largest, if not probably the largest, giant corals colonies in the world has been found in American Samoa. “Big Momma” is a Porites sp. measuring over 6 meters in height, 41 meters in circumference, and likely over 500 years old (Brown, D.P. et al., 2009). Maldives probably and hopefully host many similar colonies.

Growth forms and growth

Giant corals, and corals in general, can grow in different forms related to their most common reproductive method and lifespan. Terms to describe them are very evocative, allowing for easy grouping:

• Branching corals: they form thin tree-like colonies growing upwards and generally showing a fast growth rate, e.g., an average 100-150mm/year in the Indo-Pacific (Dullo, 2005)
• Columnar corals: they form columnar cylinders which are not branch out
• Tabular corals: they grow horizontally on table-like surfaces
• Massive corals: massive refers to their dense shape and hemispherical to spherical shape. They are slow-growing species, sometimes growing between 4 and 100 mm/year (Dullo, 2005), and although they can reach massive sizes, this is not always the case.
• Encrusting corals: they grow following the substrate they adhere to, forming a thin crust of skeleton over it
• Foliose corals are thin corals flattened horizontally and form whirl-like structures made of several unifacial plates.
• Free-living corals are made of single polyps or colonial organisms not attached to the substrate.

Habitat

Corals can be found in tropical areas of the world where temperatures range between 20 to 32°C (depending on different regions). They also need light to grow since they host photosynthetic symbionts in their tissue (algae called zooxanthellae). Through their limestone skeletons, they can build giant bioconstructions even forming islands, atolls, and archipelagos (such as the Maldives). Corals create different habitats that host thousands of marine species. Generally, certain coral growth forms are generally more common than others in each area, depending also on hydrodynamics, light availability, and sedimentation. Within coral reefs, different areas with defined boundaries can be found:

• Lagoon: areas of calm waters, sometimes devoid of corals or presenting several patch reefs
• Reef flat: a large and shallow, mostly barren area strewn with detritus with an elevation varying very little
• Reef front: this zone extends from the reef crest to the shelf break, sloping to depths of even 50m. It is generally a narrow area exposed to waves
• Reef slope: it is the outermost zone of the reefs. It can slope to over 100m

Health and competing organisms

Coral reefs are under threat, and with them, we might lose some of the oldest and most resistant colonies, giant corals. IUCN Red List of Endangered Species has highlighted how reef-forming scleractinian corals are the category (amongst birds, mammals, amphibians and cycads) which is moving faster towards a trajectory of extinction (IUCN 2020) with over 36% of coral species (cnidarian and hydrozoan) being threatened (IUCN 2022). They are projected with high confidence to decline by 70-90% with a 1.5°C global warming, and there is a high likelihood of species extinction and biodiversity loss (IPCC 2023). In particular, coral reefs are extremely vulnerable to climate change (Hoegh-Guldberg et al. 2019) with risks deriving from higher water temperatures, increased severity of storms, ocean acidification, and possible phase shifts (from coral-dominated reefs to algal-dominated reefs) reducing the potential for reef recovery.

In the Maldives, where our project began, global threats coupled with local threats such as land reclamation (Fallati et al. 2017), coral diseases (Montano et al. 2015) and corallivores outbreaks (Bruckner & Coward, 2018; Saponari et al. 2018, Montalbetti et al. 2019). In recent years, Maldivian coral reefs have seen major declines in health and coral cover. Before the 1998 mass bleaching event, reefs averaged 40% coral cover and dropped to less than 2% in 1998 (Pisapia, C., et al., 2016) to then recover to almost 38% in 2009 (Pisapia, C. et al., 2016) and presented a coral community similar to pre-1998 by 2014 (Morri, C., et al., 2015). Following 2016, bleaching event reports indicate communities with 6-11% coral cover (Perry, C. T., & Morgan, K. M., 2017; Pisapia, C. et al., 2019) and changes in the dominant coral genera. In certain locations, massive corals have become the dominant growth forms after bleaching events due to their resistance. Nonetheless, they are subject to possible bleaching, diseases and predation by competing organisms.

Bleached corals appear white in colour, with tissue present on the skeleton and transparent polyps. Bleaching results from disrupting the symbiosis between coral polyps and the photosynthetic microscopic algae (zooxanthellae) found within their tissue. These are responsible not only for corals’ bright coloration but also for providing them with most of the energy necessary for their growth. In high temperature conditions, the symbiosis is broken and coral starve to death unless the original conditions are restored.

Predation generally leaves scars on the tissue of the colony and most of the times predators are found nearby. Predators and competing organisms are various: the Crown of Thorns Seastar (Acanthaster planci), the Pin-cushion Seastar (Culcita sp.), the sea snail Drupella sp., worm snails in the family Vermetidae (Dendropoma spp.), different species of sponges, and certain species of fish biting off pieces of the colonies e.g., parrotfish (Scaridae), damselfish (Pomacentridae), Triggerfish (Balistidae), Butterflyfish (Chaetodontidae). Outbreaks of some of these predators have caused in the Maldives large scale mortality events targeting preferably branching corals, but also moving towards massive corals following the lack of branching species post bleaching events (Saponari et al. 2018).

In terms of competing organisms, accounts of the sponge Terpios hoshinota have been reported from the Maldives (Montano S., et al., 2015) and the organism is known for overgrowing corals and undergoing outbreaks.

Predators have been reported to have several effects on corals: they can cause complete or partial mortality of corals (Rotjan, R. & Lewis, S., 2008), be the vector of a disease or leave lesions that may cause infections (Nicolet, K. J., 2018). The first report of coral diseases in the Maldives dates back to 2012 (Montano, S. et al., 2012) and since then five diseases have been found in the region (Montano, S. et al, 2016). Given the slow-growth of certain coral morphologies and the mass mortality that diseases are causing  (Walton, C. J. et al. 2018), key to conservation is the early detection of any disease.

The most common diseases found in the Maldives are:

• Skeletal Eroding Band (SEB), recognised by black specks clustered within the corallites of a bare skeleton
• Black Band Disease (BBD), identified by a dark band between live tissue and exposed white skeleton
• Brown Band Disease (BrB), showing a brown band found between a narrow white stripe following the live tissue, and the white skeleton.