Semakau Walk on 17 Aug 2008

We were out on Semakau on a guided walk again. This time round, my group name was "Volute", and I had D with me doing on-the-job training.

It was a very pleasant morning when we set off from Marina South Pier - cooling, but not windy. On the boat, we were talking about it being full-moon, and so it should be rather bright relatively when we make our way to the intertidal area. So you can imagine how surprised we were when we saw this:


The first thing that came to my head was - did I remember wrongly and we're not supposed to be having full moon today? And then, it suddenly dawn on me that we're witnessing a lunar eclipse! Wow!

That was certainly a very nice start for the guided walk! Anyway, as per most of the walks I led, I couldn't really spare much time to take photos. So please pardon the quality :P


As usual, the resident gigantic anemone (Stichodactyla gigantea) was waiting for us at the same spot, and so was the anemone shrimp. Not sure what has happened to the bigger female shrimp though, as I had been seeing the male only for the past 2 trips. Anemone shrimps (Periclimenes brevicarpalis) are commonly seen on several species of sea anemones at our reefs. The shrimps usually scavenge for food among the tentacles of the anemone. When there is insufficient food, it may actually feed on the anemone’s tentacles! They protect themselves from the anemone’s stinging cells by coating their body with the mucus secreted by the anemone.


And in the very same tidal pool, I saw this kite butterflyfish (Parachaetodon ocellatus). I've seen such butterflyfish feeding on carpet anemones before. Was it happily feeding when the tide went down too quickly, thus trapping it in this little tidal pool? Hmm...


Another resident anemone we saw was this stinging anemone (Family Aliciidae), which supposedly gives painful stings. Been seeing it around the same spot for the past few months! Seemed to have grown bigger though :P


It was certainly a morning for Acanthozoon flatworms (Acanthozoon sp.), and I saw at least 10 of them. Being very flat and fragile, they tear very easily, so please avoid handling them. Fortunately, they can regenerate lost body parts, and I'll assume that to be a painful process.


We saw several sunflower mushroom corals (Heliofungia actiniformis) too. They have long tentacles, and are thus often mistaken to be anemones.


We saw several species of nudibranchs (Order Nudibranchia) too, and I'll just highlight some of them here, such as the funeral nudibranch (Jorunna funebris) above. This one looked really fat. Wonder if it's pregnant or something. Anyway, "nudibranch" means "naked gills", refering to the flower-like gills on the back of many species of nudibranchs.


Nudibranchs are also hermaphrodites, meaning they have both male and female reproductive systems. When they mate, they usually fertilise each other in a 69 position, and it appeared that the glossodoris nudibranch (Glossodoris atromarginata) above were doing just that!


This little fellow looked very much like a flatworm, but is in fact a discodoris nudibranch (Discodoris boholensis)! Look closely and you may be able to see the gills on its back.


We also had an uncommon sighting- a swallowtail headshield slug (Chelidonura pallida)! This was the second time I've seen this slug on Semakau! Like other headshield slugs (Order Cephalaspidea), the swallowtail headshield slug has well-developed headshield, which is a broadening at the head used to plow beneath the sand surface and help prevents the sand entering the mantle cavity. The term "swallowtail" refers to the characteristic long split in the tail, of which the left side is always longer than the right. These slugs feed on tiny acoel flatworms living on coral reef substrate.

Headshield slugs "smell" or sense their prey with a pair of round, bristle covered structures on either side of the head. These sensory structures are used also by the slugs to find mates too!


We saw several species of sea stars too. The above are sand-sifting sea stars (Archaster typicus) in a shallow tidal pool. Interestingly, the one with four arms kept sliding over the pair which were getting ready for fertilisation. Not sure what was it trying to do.


We were quite luck to saw a cushion star (Culcita novaeguineae) too. This one appears to be a different individual from the previous 2 that we've seen on our walks, as the markings and colours on its back are different. Cushion stars feed on corals.


And one of the highlights of the day will be seeing the knobbly sea star (Protoreaster nodosus). This is probably the biggest sea star in Singapore based on body width, and can grow to about 30cm wide. This sea star apparently feeds on micro-organisms on seagrass or sediment surface, but is known to feed on soft corals, sponges, clams etc in captivity as well. However, they are not known to survive long in captivity, which suggest that the latter may not be the preferred natural diet.


We also saw this knobbly sea star again which didn't look too healthy. We saw it during our last walk which it already look rather unwell, but before that we had not seen it for more than a year. Several of its knobs appeared to be missing.


And here's a group shot of the the Volutes...


We had quite a bit of luck with sea cucumbers too, and saw 4 out of the 5 sea cucumbers commonly seen on Semakau. The above is a dragonfish sea cucumber (Stichopus horrens). This interesting sea cucumber gets very stress when it's out of water for too long, and in fact, is known to somewhat "melt" and disintegrate if left in the hot sun for too long, and may eventually die unless it's returned to water before it's totally gone.


This sandfish sea cucumber (Holothuria scabra) is the one often served in Chinese restaurants. They must be probably treated to remove their toxin before they can be consumed though.


This pretty ocellated sea cucumber (Stichopus ocellatus) which has lots of "eyespots" on its body, which are basically dark papillae surrounded by a white ring.


This worm-like thing is actually a synaptid sea cucumber (Family Synaptidae). When fully stretched, it's probably about 2m long!


My group name was "Volute", and we're quite lucky to spot 3 noble volutes (Cymbiola nobilis) with their eggs. It's always nice to see animal eggs here, as it just shows how much alive our shores are! The juveniles will eventually hatch as little snails crawling out of the egg capsules.


We also saw a spiral melongena (Pugilina cochlidium) laying eggs! This snail supposedly feed on barnacles.


Our hunter-seeker also found us a mangrove horseshoe crab (Carcinoscorpius rotundicauda). This is a living fossil that existed even as far back as more than 400 million years ago! Known to be scavengers, Mangrove horseshoe crabs are also known to feed on worms and clams. The mouth is located on the underside, and interestingly, they have no teeth! Basically, they grind their food with their legs as they move around!


We also saw a little egret (Egretta garzetta) hunting for fish, rushing around the among the shallow tidal pools to flush out its prey.

On the whole, this was certainly a great day with lots of interesting sightings!