Wednesday, May 13, 2009

The Secret Side of Sentosa 130509

All, if not most of us, attribute things like beach volleyball and suntanning sessions to the sunny island of Sentosa. So what's this about Sentosa having a secret side? Well, not so well-known is the existence of marine life that has made the beach-goers haven their home.

Had the chance today to explore an islet off Siloso beach and more. And indeed, they were full of life!

This echinoderm - the rare Laganum Sand Dollar (Laganum depressum) - was the first of the 'firsts' for me today.

Though the water was murky, ZL spotted this baby horseshoe crab drifting in the water. During a previous shore exploration at Pasir Ris, we saw one just like it. Wonder what makes them green!

Sea cockroaches (Ligia sp.) are very common at rocky shores. They can be seen scurrying back and forth, here and there, over rocks. They remind me very much of Plankton in Spongebob Squarepants, but Plankton really is a plankter (singular form of plankton).

The rocks were encrusted with noticeable number of oysters (Family Ostreidae).
Oysters are filter feeders and will open their shells a little during high tide to sieve food particles from the water.

Attached to the side of a rock was this flowery soft coral (Family Nephtheidae). Both hard and soft corals belong to the same phylum of Cnidaria, but differ most significantly in that the former builds calcium carbonate skeletons while the latter do not.

The shores are colourful! Ascidians, such as the ones above, come in a variety of colours and can be colonial or solitary. The two pink blobs above are separate colonies of ascidians.

This Spotted Moon Crab (Ashtoret lunaris) was the second crab I saw on the islet. Its legs are paddle-shaped, which allows it to burrow very quickly into the sand.

Bet you're wondering what happened to the first crab... While exploring the rocky area, I heard a scrabbling sound, turned to look, and saw that a brownish crab had fallen from higher up and landed on one of the lower rocks. More scrabbling later, and it disappeared from view.

These Common Sea Stars (Archaster typicus) appeared on the sandy shore, seemingly out of the blue. Quite amazing, really. We had no idea that they were there, hiding in the sand, till they decided to make an appearance. Two of the sea stars have paired, getting ready to pseudocopulate (meaning that their reproductive organs do not actually touch). They can remain paired for up to a few months, before sperm and eggs are released into the water.

Third encounter with a crab. I have no idea what species it is. Found it lying wrong-side up in the sand and flipped it over. Its pincers have a reddish tip.

This prawn apparently jumped out of a hole near the Common Sea Stars.

Though the terms 'prawn' and 'shrimp' are largely used interchangeably, there is a difference! Prawns and shrimps may look alike, but upon closer inspection, the difference can be elicited by the arrangement of the second abdominal segment. For prawns, each of their abdominal segments (and thus the second abdominal segment) is overlapped by the preceding one. In shrimps, the second abdominal segment overlaps both the preceding and following ones. Read more about their differences here.

Soon, we left the islet and made our way to Tanjong Rimau to continue exploring :)
The beach which we cut across had a lot of 'wrinkles' which never ceases to grab my attention.

This fan shell (Family Pinnidae) was lying on the beach, in full view. As these molluscs typically burrow into the sand and anchor themselves to the substrate by strong byssus threads, it was really weird to see one lying out in the open. The valves were slighly open, and we could see the soft body of the animal. Can't tell if it was dead or alive. My guess is that someone or something dug it up.

In one of the modules that I took this semester, we learned a bit about the caves and cliffs (I was so surprised when I learned that Singapore actually has cliffs!) found at Tanjong Rimau. It's one thing to learn from textbooks, and another to actually see it for yourself! As you can tell, I was rather excited to be there :)

For the module (The Biophysical Environment of Singapore), we used a textbook entitled Singapore's Biophysical Environment by Lu Xi Xi, Wong Poh Poh and Chou Loke Ming. Here's an extract from it pertaining to Tanjong Rimau:

"Two major types of cliffs are left in Singapore. Sedimentary rock cliffs are located on the northwest coast and on some southern islands. If they are active, i.e. subject to coastal processes, they are bare of vegetation and are almost vertical. If they are inactive, they are usually protected by vegetation and appear more in the form of steep slopes. Some cliffs have developed a ramp or a platform in the intertidal zone, sometimes covered with sand. Caves may form where rocks are more susceptible to erosion, for example in the interbedded conglomerates and sandstones on Sentosa."

The picture above shows just a few examples of the caves and cliffs of Tanjong Rimau. A closer look at the caves showed that all, if not most, are more like recesses (and don't extend all the way in) or crawl-spaces. The cliffs, on the other hand, are cut by a lot of grooves and the layers of rock are evident. Pretty :)

This is what the exposed intertidal area looks like during low tide: sandy beach and rocky shore.

More crab sightings! Looking very much like a (movable) piece of substrate was this Common Hairy Crab (Pilumnus vespertillo). Their body has long, silky hairs which trap sediment, enabling these crustaceans to camouflage very well with their surroundings. They are also affectionately known as teddy bear crabs (no prizes for guessing why!).

I was stunned for a moment when I saw this crab. It's another 'first' for me! Have heard quite a lot about it, but have never actually seen one until today. The red colouration of the Mosaic Reef Crab (Lophozozymus pictor) already serves as a warning - stay away! It is the most poisonous crab in Singapore, and cooking it will not destroy the toxins.

We came across a fair number of corals, both hard and soft. This is a picture of a boulder coral - a favid hard coral.

Adding sporadic splashes of blue to the rocky shore were these blue sponges. Though sponges and ascidians look alike, they can be told apart by the absence or presence of oscules (the 'holes' of a sponge through which water is pumped). Sponges belong in Phylum Porifera, a term which translates into 'pore-bearer'.

Nice to see, but not nice to touch though. Sponges typically have sharp microscopic structures known as spicules in their body, which can get embedded into one's skin. Ouch.

This is a sponge, not a soft coral. Thanks SY for pointing it out!

Despite being rather common, I don't think I've seen the Polka-dot Nudibranch (Jorunna funebris) till today! 'Nudibranch' literally means 'naked gills', which is what nudibranchs have. On the posterior end, the feathery-like structures are its gills. Its black-tipped rhinophores are located on the anterior side.

Here's another favid hard coral.

We saw two of these tiny Blue-lined Flatworms (Pseudoceros sp.) today, with WW spotting the first one. Marine flatworms swim by undulating the sides of their body.

Also highly poisonous is the Red Egg Crab (Atergatis integerrimus). It's my first time seeing this crab in the flesh, so to speak.

Curiously enough, I saw two Mosaic Reef Crabs and two Red Egg Crabs. A Floral Egg Crab (Atergatis floridus) also made an appearance. These three crabs hail from the Family Xanthidae, a group of poisonous crabs.

Sponges side by side. They come in a myriad of colours!

I think this is an anemone coral (Goniopora sp.).

I'm not sure what this is. A soft coral?

These favid hard corals were huge.

The rocky shore was covered with a lot of this filamentous green algae.

Looking rather contorted was this leathery soft coral (Family Alcyoniidae).

This is the Phyllodesmium briareum nudibranch which only seems to make its appearance on Sentosa. It harbours zooxanthellae - microscopic single-celled algae - in its body which can photosynthesize. It's mutualism at work for the algae are protected and the mollusc receives nutrients as a product of photosynthesis.

Marine snails have what is known as an operculum - a trapdoor of sorts - which they can use to seal their shell. We saw a few operculums lying on the shore.

Leathery corals can resemble many things. This one here looks like an omelette, don't you think?

We decided to follow the coast of Tanjong Rimau and made our way around. Spotted this Reef Bristleworm which was directly in my path. Lucky I didn't step on it! It's important to always watch where you're going, in order to minimise damage to the shore.

My list of 'firsts' for today had another addition with the sighting of onchidiums (Onchidium sp.). Like the Common Hairy Crab, they resemble the substrate very closely and are difficult to spot unless they move!

Have not seen a fan worm in a long time. This small orange fan worm was found together with the zoanthids in a small pool.

The zoanthids in the pool. They resemble anemones and thus it's easy to get both mixed up.

We could hear and see the tide rushing in.

The rocky shore.

This is a frilly anemone, probably a Plain Frilly Anemone (Phymanthus sp.).

I thought I'd leave these Lightning Dove Shells (Pyrene fulgurans) to the last. Most are dark brown/black with the distinguishing white zigzag pattern on the shell.

Pretty shells naturally catch the eye of many, and inevitably, many are collected for personal or commercial reasons. When empty shells are removed from the shore, hermit crabs are deprived of a home. Shelled gastropods - live ones - are often killed for their shells and over-collection has played a significant role in the reduction of the number of organisms. What good are shells to us except to admire and display?

On a brighter note, today was a really interesting trip with good company. Thanks KS for organising the trip!

For more about today's trip, check out Kok Sheng's posts at and

For more about the islets of Siloso, check out Ivan's posts at and


  1. Great findings! The dove shells are lovely!

  2. :D now I know why Tg Rimau is ur first love. There are so many things to discover :)

  3. The soft coral is a sponge. Laminodysides herbacea if I am not wrong.

  4. Hi SY. Thanks for pointing that out! Couldn't find a match when I googled 'Laminodysides herbacea' though. The closest one is Lamellodysidea herbacea which is a blue encrusting sponge but can't find any pics of it.