Extensive Coral Bleaching at Semakau

Apart from the finding the dead sharks in the drift net, we came upon another rather alarming scene while conducting the survey at Semakau.

Many of the corals appeared bleached!


At the reef edge, the situation was especially bad. I would estimated at least 80 percent of the corals having some signs of bleaching! You can see that all the hard corals in the above photo have turned white.

Coral bleaching happens when there is significant loss of zooxanthellae (a type of algae living in the corals) in corals. These zooxanthellae are the ones that gave most corals the characteristic brownish tones. They can photosynthesize and some of the food produced will be leaked to the corals. in the meantime, the corals provide shelter and nutrients (their metabolic waste) for the algae. Coral bleaching can occur if there is an increase in sea water temperature, and if the corals cannot recruit new algae in time, they may die.


Moving away from the reef edge to the middle of the reef flat, the situation appeared to be better, but I would still estimated at least 30-50 percent of the corals either partially or completely bleached. Just compare the whitish coral on the right, with the bluish-brown one on the left.


And not just the hard corals, a few soft coral colonies appeared more whitish as well, instead of the healthy brownish colour.


And rather shockingly, even the zoanthids were bleached. The Palythoa mutuki above on the left were all bleached, compared to the healthier ones on the right.


There was even a bleached Magnificent Anemone (Heteractis magnifica).


A healthy Magnificent Anemone normally has brownish tentacles, like the one above. If you look carefully, you can also find a Bulb Tentacle Sea Anemone (Entamacea quadricolor) on the right.


Semakau is one of the few places in Singapore with huge numbers of Magnificent Anemones. Just look at the photo above - there were so many of them!


Another sea anemone that can be found on Semakau is the Hell's Fire Sea Anemone (Actinodendron sp.). This sea anemone supposedly gives painful stings, but I am certainly not going to test if this is true.


I spotted a few Tube Anemones (Order Ceriantharia), which we were still not sure of the exact species.


There were a number of huge Barrel Sponges (Xestospongia testudinaria) too!


There were a few Acanthozoon Flatworms (Acanthozoon sp.). This flatworm is very common in the region, but is still not described by scientist and thus does not have a species name.


This little flatworm is probably a Pseudoceros sp.


There were a few white Fan worms (Family Sabellidae) too. These worms live in tubes, and use their fan-like tentacles to filter plankton to eat.


At the rocky shore area, I found several Spotted-belly Crabs (Ozius guttatus). These crabs have huge claws for cracking shells.


Another crab we saw was the Soldier Crab (Dotilla myctiroides). This crab used to be very common, but now we seldom see them on mainland Singapore. They can still be found in huge numbers on some of our islands though.


The Furry Sea Hare (Stylocheilus sp.) appeared to be in season, and we saw hundreds, if not thousands of them! It's simply impossible to count how many there were. I am really glad that we decided to conduct survey on this part of the shore, which we seldom visit, as I had never seen this sea hare on the other shore of Semakau which we normally visit.


And many of them form long mating chains! Sea Hares are hermaphrodites, meaning each slug has both male and female reproductive structures. The former is located on the "neck", while the latter is located at the back within the mantle. Hence, when they form mating chains, the one in front plays the role of the female, while the one at the back plays the role of the male.


And just when I thought they were all the same, I found a number of them looking too hairy to be the Furry Sea Hares. They were the similar-looking Hairy Sea Hare (Bursatella leachii) - another first-time sighting on Semakau for me!


To differentiate between the two, look at the body closely. The Furry Sea Hare has lots of lines on its body, but not the Hairy Sea Hare. The former appeared less hairy too. And, in the photo above, the one on the left is a Hairy Sea Hare, while the one on the right is the Furry Sea Hare. Interestingly, both sea hares feed on cynobacteria (or blue-green algae).


We found this cute little Leaf Slug (Elysia ornata). I have not seen this on Semakau for at least 3 years! This slug feeds the sap of green algae, and is able to retain the chloroplast (organelles that perform photosynthesis) and use them to make food through photosynthesis later! This is truly a solar-powered slug!


Yet another solar-powered slug that we found was this Blue Dragon (Pteraeolidia ianthina). But instead of feeding on macro algae, it feeds on hydroids, which are related to corals and harbour zooxanthellae.


Another slug which we saw was this huge Forskal's Sidegill Slug (Pleurobranchus forskalii).


We also found a few Burrowing Giant Clams (Tridacna crocea). One of them does not appeared to have been tagged by Mei Lin, who's doing a research on them. So guess I'll forward her the GPS when I returned to office next week.


There were many octopuses too, and they can change their colour to blend into the surrounding.


There were a few Dragonfish Sea Cucumbers (Stichopus horrens). Did not see them during my last trip to this particular shore, so it was a rather pleasant surprise for me too.


Marcus found a Stonefish Sea Cucumber (Actinopyga lecanora). This sea cucumber apparently was collected in some places for the sea cucumber trade.


This pinkish brown sea cucumber, Holothuria fuscocinerea, is usually found hiding under rocks or corals.


There were several juvenile Copperband Butterflyfish (Chelmon rostratus).


I saw a Snake Eel (Pisodonophis crancrivorous) looked really graceful swimming around with its slender body.


Here's another look at the reef when the sun was up, with a few unbleached corals.

I really hoped that the ones that were bleached will be able to survive and return to their former glory soon.