Two different reef personalities

Today this post tells you about two very different reefs a few nautical miles apart: Undine and Mackay Reefs offshore of Cape Tribulation and how we tackled the vastly different conditions.

Undine Reef and its intriguing coral

During our time at Undine Reef where the conditions were overcast and the water clarity poor, Chris focused her photos on two types of corals: The Black Sun Coral and Gorgonians. These are not only striking but also quite interesting species. So here is a bit of a marine biology lesson.

Most corals get a large percentage of their food from the algae that inhabit their tissues. In this carefully constructed arrangement, the algae donate some of the sugars they produce through photosynthesis to the coral animal in exchange for housing. Environmental stressor such as increased temperatures can cause the coral to expel these algae in a process called coral bleaching, something we have seen a lot of as you know.

Black Sun Coral

But here is what we found: there is one type of coral that is immune to bleaching: the Black Sun Coral.

The Black Sun Coral (Tubastrea micrantha) unlike other corals is not photosynthetic, but heterotrophic. This means that instead of receiving most of the energy from the symbiotic algae within their tissues, it eats other plants or animals for energy and nutrients. It extends long tentacles at night to catch passing zooplankton. Black Coral will therefore not bleach when temperatures get too high! It may well be the coral of the future.

Black Sun Coral is often found a bit deeper down, in 5 to 10m depth, so you have to work a little harder for your photo! But when you get close, you realise that the Black Coral may look black at a distance, but it is actually a shade of dark green.

Black Sun Coral
Black Sun Coral about 6m deep


Another striking type of coral is the Gorgonians. They look like plants, but they are actually animals belonging to the same subclass as soft corals, sea anemones and sea pens. They consist of individual tiny polyps that form colonies. A gorgonian colony is supported by an internal central skeleton but the exact structure of a gorgonian colony varies a lot between the various species. Many species grow erect with a flat and branched growth pattern that makes them resemble fans. But there are many species that come in other shapes and you can for instance find encrusting gorgonians, bushy gorgonians and whip-shaped gorgonians, those long curly stick-like organisms which can be several meters long.

The gorgonian is a filter feeder. Each polyp is equipped with eight tentacles which are used to catch planktons and other types of organic matter brought to it by the currents. To make the process more efficient, the “fan” of the gorgonian will be oriented across the prevailing current. We love seeing the lacy fan shaped Gorgonians, with their brilliant colours. The ones at Undine Reef were mainly rusty red.

Mackay Reef & Cay – the way a reef should look

Just a short 5nm further north from Undine is Mackay Reef. Just like Undine, it has a lovely cay and varied reef. Being a popular place with commercial tour boats, you can end up with 20 people ashore at low tide or floating around the shallows during the day. So we dinghied to the cay first thing in the morning to take Bengie for a little trot and to get an aerial view with the drone before the crowds arrived.

Later in the morning, it was time for a snorkel. None of these wanders in the shallows with the noodle brigade for this one (the tourists from the commercial boats are given a noodle to help them float). We were taking Murray and Maree to the windward side of the reef wall, at the drop off! The tide was perfect, close to low, the light was bright, the visibility was good. As soon as we “turned the corner” we were confronted by the big beasties: Giant Trevally, Whitetip Shark, Spotted Sweetlips. But what really was evident was the health of the coral, the amount of cover, the colours, the variety. “Yes, just as we remembered it”. It was a relief and an absolute pleasure to roam along the wall and observe. Maree was comfortable and powering ahead of the team, Chris and Murray were head down, bum up taking photos. Wade was happy too, spearing us a coral trout and a snapper for dinner. This slideshow shares some of the treasures we observed.

  • Blackback Anemonefish
  • Anemonefish amongst clams
  • Blueline Surgeonfish
  • Sixband Angelfish
  • Lunartail Bigeye
  • Pink Anemonefish

Off to the Ribbon Reefs!

Encouraged by what we have seen at Mackay Reef, a location still very close to the mainland, we are excited by what is to come on the very outside edge of the Great Barrier Reef. We are now at Lena Reef, the furthest north we have ever been, at the start of the Ribbon Reefs. These are the long, skinny reefs that lie between Cooktown and Lizard Island on the edge of the continental shelf of the northern GBR. We have at least another week of beautiful weather, perfect for exploration.

From here on, we may not have any internet coverage. So you know the drill, check our satellite link to track our movements!

10 thoughts on “Two different reef personalities

  1. Wow, so many beautiful underwater photos! The MacKay reef side looks great, much better than others. The Black Sun Coral is amazing, and a wee bit creepy too! How can you guys get internet service offshore? Directional antennas?

  2. Thanks for the info about the different corals, fascinating. Great to finally see reef which is not destroyed, beautiful. Enjoy the Ribbon Reefs

    • Thanks Sue, glad the info was interesting. The reef is not in great condition but we are finding large bommies rather than the lee side of the Ribbon Reefs are better.

  3. Very interesting lesson about the Black Sun and the Gorgonians. Your photos are excellent! Keep up the good work! 🙂

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