A Christmas Coral

Arguably one of the most incredible areas of coral reef worldwide?

A blog all coral scientists must read

By Dr April J Burt, Conservation Scientist working with the Seychelles Islands Foundation

On an island, far far away there is a place that everyone needs to know about – especially coral scientists. Why? Because this place is both spellbinding and theory-generating and just plain exceptional. This place continues to create the excitement in me that only Christmas morning as a child or, as an adult, mind-blowing nature now has the power to create.

The magical and secret place I want to share is located at Aldabra Atoll, one of the largest atolls in the world. Aldabra is 300 km north of Madagascar (see map below). This giant atoll is part of Seychelles but, at over 1000 km from the main inhabited islands, is truly isolated. To put its size into context you could fit New York’s Manhattan Island inside the lagoon. The atoll is surrounded by a fringing reef and the atoll’s ring is broken up by channels. This means the tidal regime there empties and fills this lagoon twice daily, creating some of the fastest currents in the world. This regime and the geomorphology of the atoll have led to an incredible variety of habitats created by pockets of unique chemical and physical conditions

Location of Aldabra Atoll, Seychelles & aerial view across Picard island looking down the length of the lagoon.

Pockets of wonder

One of these pockets is situated inside the lagoon close to one of the smallest channels, known as Johnny Channel (or Passe Gionnet). The channel current enters the lagoon and funnels around a series of islets that have been sliced up by thousands of years of fast-flowing hydrodynamic action. Fringing the channel and wrapping around the islets are dense mangrove stands and these mangroves host one of the largest frigatebird breeding colonies at Aldabra (see pic and video below). Above everything towers a vortex of spiraling life that cannot but create a sense of wonder for all those below it. Guano rains into the crystal-clear waters, swirling in shimmering shards down into the seascape below the waves.

Map showing the islands of Polymnie and Malabar split by the small channel known as Johnny Channel (or Passe Gionnet) and the islet known as ‘Gros Ilot Gionnet’. High-density nesting of frigatebirds on the mangroves around Johnny Channel.

Pocket of wonder #1

On one side of an islet called ‘Gros Ilot Gionnet’ (see map) directly opposite the channel, you first encounter what looks like a carpet of pale orange, stretching around the islet and extending out across the lagoon floor. When you get your snorkel and mask on and look below the water, the ‘carpet’ becomes a sea of extended tentacles, as far as the eye can see and reaching all the way up to the mangrove roots. This scene is indescribable, so luckily I have pictures and video…

Coral carpet extending up to the roots of the mangrove trees and inhabited by juvenile fish and rays

The type of coral making up this living blanket is called Goniopora. The colonies have numerous daisy-like polyps that extend outward from the base, each tipped with 24 stinging tentacles which surround a mouth. These corals are often found in lagoons and areas with turbid water conditions, their large and extended tentacles make them efficient feeders. Their ability to gain more energy from self-feeding in this way is thought to potentially give them an advantage over corals that are more reliant on their symbiotic algae for energy, when mass coral bleaching occurs.

The coral carpet is a sea of Goniopora tentacles moving rhythmically in the current

Why is this scene so magical? It is not only rare nowadays to see 100% coral cover extending unbroken over such a large area, it is also rare to see this from Goniopora. I have only ever seen a similar scene at a pinnacle in a lagoon in the Chagos archipelago, and that was many much bigger boulder colonies that were clearly separated by substrate or other corals. In addition, it is truly magical the way the coral reaches the base of the mangrove roots, creating a glorious seascape ‘meadow’ of coral and trees, dotted with juvenile fish.

Coral meadow meets the forest

Pocket of wonder #2

On the other side of the islet is something quite different: At first it appears to be only rocks and rubble but once your eyes focus you see that, instead of rubble, the seabed is covered with small, stunted or ‘bonsai’ (as I like to call them) coral colonies of foliose Pavona. These colonies are sometimes only 5-10 cm high but are jampacked bumper-to-bumper. Sadly, I don’t have pictures of this area yet because it was only on a coral scouting trip in November and with the tide rushing out of the lagoon that I had the luck to encounter them! But here is an example of a foliose Pavona coral from somewhere else…

What does this mean?

At high tide, the water level around these islets is about 1-1.5 m and Aldabra’s tidal range is 3 m, so at extreme low tides much of this area is exposed or covered by just centimeters of water. What is clear is that while the outer reefs have undergone extreme heat stress and subsequent coral mortality at depths of over 25 m, this site has corals that are not only surviving, but thriving in high temperatures and high UV exposure levels. In these highly dynamic and stress-prone areas there is clear selection for these two species.

This one location reflects the ghosts of corals past, present and future…

The ghost of corals' past can be seen in the fossilised coral reef that the islands and islets are made from.

The ghost of corals' present is written in the broader landscape, with the outer reefs of Aldabra still recovering from the 2016 global bleaching event where mass mortality resulted in 52% hard coral cover reduction.

The ghost of corals' future lies within these pockets of wonder, with corals that can adapt to and become resilient against the impacts of climate change. With the dedication of the world’s coral scientists to investigate these natural wonders and translate science into action, there is a great deal to be hopeful for.

Have you seen such places? Where was your most mind-blowing coral experience?

The Aldabra lagoon, Johnny channel fregate colony and the Goniopora wonderland @SIF