First Morning Semakau Walk of the Year

It was about 4.45am in the morning when we reached Marina South Pier last Sunday. And why were we here at such an early hour? This was our very first morning Semakau walk of the year!

My group name was Great-billed Heron, and I had Mindy with me as my assistant doing on-the-job training.


It's been a while since I last seen acorn worms (Class Enteropneusta) at Semakau. The above was an acorn worm's cast. Not sure if they are seasonal or what, but somehow they didn't seem to be as common as they used to. This worm got its name from its acorn-shaped proboscis. To feed, this worm swallows sand or mud and digest the organic matter or microorganisms within. The processed sand or mud will then be excreted during low tide coiled up.


The polka-dot nudibranch (Jorunna funebris) is commonly spotted at Semakau. "Nudibranch" means "naked gills", refering to the exposed flower-like gills on the back of most species. We have seen them feeding on blue sponges before.


Here's my group crossing the seagrass meadow.


And immediately at the edge of the meadow, we saw a knobbly sea star (Protoreaster nodosus)! We have been seeing many knobbly sea stars these days, though most of the time they were usually a distance away from the usual route that we took during our guided walks. Thus, it was really nice to see one right in the middle of the route.


Near the huge population of sand-sifting sea stars (Archaster typicus), I found this moon snail (Polinices mammatus) leaving a trail as it slid along just below the sand surface, probably seeking little snails or clams to feed on.


Once again, we saw the unidentified sea star that we first spotted during the launch of Project Semakau!


Near a sandfish sea cucumber, we saw this little black blenny (Suborder Blennioidei) hiding in a crevice under a rock.


An octopus decided to put up a show for us, as it wandered around a little tidal pool, changing colours along the way. Octopuses have specialised skin cells which allow they to change their color, opacity, and reflectiveness to blend in with the surrounding and also to communicate with other octopuses.


Many of the tidal pools had at least a fanworm (probably Sabellastarte indica). The feathery appendages of the worm allow it to filter for tiny organic particles in the water, which it feeds on.


It was rather amusing that both Mindy and I took quite a while to find a hairy crab (Pilumnus vespertilio), since its supposed to be so common at the coral rubble area. Studies in the early 1980s revealed that this crab is sometimes poisonous, probably from a red algae it feeds on. There are also suggestions that this crab also feeds on zoanthids sometimes, which makes it poisonous sometimes too.


While we were walking towards the reef crest, we heard a loud explosion. Seemed like they were conducting a live-firing on Pulau Senang.


Nearer to the reef edge, our hunter-seekers found us a few flatworms (Acanthozoon sp.). This is a very common flatworm in the region, but somehow has not been described yet.


They also found us another nudibranch, a Gymnodoris rubropapulosa. This nudibranch feeds on other slugs, even those of its own kind!


The resident fluted giant clam (Tridacna squamosa) somehow looked bigger than before. This clam can grow to about 40cm wide, and frankly I think this one is almost there. While it feeds on plankton and tiny organic particles in the water, they also obtain nutrients from the algae living inside it, which can photosynthesize.


There was a huge colony of dead man's finger soft coral (Sinularia sp.). This coral has lots of little polyps living together in a shared leathery tissue.


Several hard corals were spotted too, including the mushroom coral (Fungia sp.) on the right, and the sunflower mushroom coral (Heliofungia actiniformis). While most other hard corals live in a colony stuck to the substrate, the above 2 are solitary and free-living. They are called mushroom corals because in their juvenile stage, they are attached to the substrate on a stalk, resembling mushrooms.


I lost count of the noble volutes (Cymbiola nobilis) I spotted laying eggs.


On the way back, I brought my participants to the edge of the seagrass meadow to see one of the bigger gigantic carpet anemones (Stichodactyla gigantea). This one did not have any anemonefish living with it though.


Here's another shot of our group.


Giving our walk a nice closure was this little mangrove horseshoe crab (Carcinoscorpius rotundicauda). Horseshoe crabs are very ancient animals, and have been around for more than 400 million years! That's why they are often called living fossils.