Guided walk at Semakau on a (not so) Sunny Sunday afternoon

On my first guided walk in 2010, we were threatened by an overcast weather. Thankfully, we were only douse with a slight drizzle at the end of the walk.

Together with a fellow GV, we were attached to a group of Sec 3 students from Punggol Sec School. Hopefully, the 'Nudibranchs' who felt that they were forced to come for the trip initially would now feel glad that their teachers had forced them to come. =P

Our first few animals of the day were small crabs scampering on the sandy shore, including this cute Soldier Crab (Dotilla myctiroides). Instead of flattened bodies, their bodies are round and they can walk forward unlike other crabs. Soldier Crabs typically move in large troops, probably how its name is derived, but unfortunately this scene is not so common anymore. It feeds on detritus in the sand by scooping sand to its mouth using its pincers. The shifted sand is then discarded in the form of a small round ball.

With some careful observation, we were able to see orange dots darting into burrows. As usual, the male Porcelain Fiddler Crab (Uca annulipes) were eager to attract attention by waving their single huge pincer, as if playing the fiddle. During their mating season, their pincers are clad in even brighter colours to create an alluring appearance (a similar behaviour observed of man too). Apart from being a great asset in courtship, the pincer is actually quite a handicap as it requires more to maintain. Having said so, it is quite a feat to be able to survive with this 'liability', thus females are likely to be attracted to those with larger claws.

Much larger in size as compared the Soldier Crab & Fiddler Crab, the Swimming Crab (Thalamita pelsarti) lifted up its pincers aggressively as we crowded around the container. Swimming crabs are easily identified by their last pair of legs that are paddle-liked (vs the rest of its legs that are pointed) which enables it to swim very well. Their speed and agility, coupled with their long and strong pincer, helps it to catch fish and other fast moving prey.

 

Our first Sea Cucumber of the day is the Synaptid Sea Cucumber (Family Synaptidae), whom some thought was a worm! This is the longest Sea Cucumber that can grow up to 3m long! This cute small fellow is probably a juvenile, and was gingerly lashing it's feathery feeding tentacles  in the water to feed on small particles. Their body wall is very thin and is thus very delicate, so its important that we do not handle them. Instead of tube feet, they have hooked spicules to stick to things. I wonder if this is the inspiration in the invention of the velcro.

Everyone had a chance to experience the cooling water of one of the longest stretch of Tape Seagrass in Singapore. The Seagrass habitat is important to the entire ecosystem as it provides shelter for many young marine animals. This is in addition to its dutiful role of giving oxygen to our environment when it photosynthsizes. 

The Sandfish Sea Cucumber (Holothuria scabra) surely must make an appearance, especially when CNY is around the corner - this is the species that some human beings deem as a delicacy. However, they must be processed before it is edible as it is mildly poisonous. It is a good burrower in sand, probably thats how it got its name. Unlike us, sea cucumbers circulates water through its body to transfer nutrients and thus can get very stressed out when they are out of water. As such, we should always leave these animals in their natural habitat.

Everyone was familiar with the movie 'Finding Nemo' and was quick to identify the Anemone, which has sticky tentacles with stingers that inject a toxin affecting smaller animals. While these stings do not hurt us very much (except that of the Hell's Fire Anomone), they can leave welts on sensitive skin, which is why we always caution participants in touching any animals.

We didnt see a clownfish but instead an Anemone Shrimp (Periclimenes brevicarpalis) which coats itself with mucus produced by the anemone to prevent being stung. This 'free loader' not only feeds on left over food from the anemone, and also gets protection from predators, however does not return any favour to the anemone.

We were really lucky to see the Moon Snail (Polinices mammilla) as it is usually more active at night. Round and pearly white, just like the moon, it is a ferocious predator of other bivalves and snails. It suffocates its prey by wrapping its huge body around its subject. If this fails, it can secrete an acid to soften the victim's shell, creating a hole with some help from its radula to feed on the prey.

The hunter-seekers also found us a Sand collar which is the Moon Snail's egg mass. The Moon Snail lies at the centre of the collar, turns round and round like a ballet dancer while combining its eggs, mucus and sand in a thick sheet which hardens to form the collar. The Sand Collar disintegrates after the eggs hatch, so we must not touch an intact Sand Collar in case we destroy the thousands of living eggs it contains.

Flatworms like this Acanthozoon sp.are unsegment worms, and are named so because its really very flat! Because it is very flat, it can move into any crevices to hide but are also very fragile and tears easily, so we should never handle them. It doesnt have a respiratory or blood circulation system, instead, oxygen diffuses across its body. While it may look like an elegant swimmer, this carnivore is a predatory as it can inject digestive juices resulting in a liquidfied victim which it will drink up.

Glad to see the mascot of our group - Polka Dot Nudibranch (Jorunna funebris). Nudibranch translated in latin means 'Naked Gills' and refers to the exposed feather-like rhinophores on its back, said to have sensory functions in detecting prey/ mate. A type of sea slug, Nudibranchs are born with shells but loses them as they become adults. Thus, species like the Polka Dot Nudibranch acquire poisons from the blue sponge it feeds on as a defense mechanisim; others develop bad tasting glands to repel predators. Most of the nudibranches are clad in bright colours to warn predators of its poisonous or bad tasting nature.

Here is another species of Nudibranch, the Gymnodoris rubropapulosa,

and the Phyllid Nudibranch (Phyllidiella nigra) which is said to be very poisonous. While we show our emotions when stressed, this Nudibranch can release toxins into the water when they are stressed, which could possibly kill other marine organisms.

Taking the lead in Semakau's gallery of seastars is the Sandsifting Sea Star, which got its name from its behaviour of burrowing in the sand.  This pair that we saw was getting ready for copulation by practising external fertilisation. Although close to each other with alternating arms, their reproductive organs do not meet. They can stay in this position for a while to wait for the best opportunity to release sperms and eggs outside of their body when the tide comes in.

I was really happy to see the 2 juevenile Cushion Star (Culcita novaeguineae). I was sharing with the group that the yellow Cushion Star was slimmer the last few times I saw it, am happy that it is putting on weight and on its way to adulthood! Its round shape and thick calcified body walls makes it more difficult for fish and other predators to bite it.

Wowing the group with its size is our Project Semakau icon, the Knobbly Sea Star (Protoreaster nodosus). Named after the knobs on its body, this Sea Star can grow as wide as 30cm. While they may look quite dangerous with its knobs and bright colours, they are not poisonous. Similar to the Cushion star, its heavy body that is calcified makes it difficult to be preyed on. Interestly, their stiff-looking arms are quite flexible just like a gymnast! Due to the aquarium trade and loss of habitat, these pretty Sea Stars are unfortunately endangered on our shores.

Another good indication of the good health of our shores, this Noble Volute (Cymbiola nobilis) was laying eggs and we were careful not to disturb it in case we cause any destruction.

Not quite evident in the blur picture that I took, I shall play cheat by using a previous photo taken at Semakau. Although often covered with algae, the Noble Volute actually have a very beautiful shell which has also resulted in overcollection and thus the vulnerability of the species. Another fierce predator, the Noble Volute presses its foot on the prey. When the suffocating victim tries to open its shell to breathe, the Volute then capitalises on the chance to attack & feed. 

The Upside-down Jellyfish (Cassiopea sp) has a reason for its posture. Algae lives on the underside of the Jellyfish amongst its tentacles. In order for the algae to photosynthesize, the jellyfish remains in the upside-down position for the algae to get sunlight. While the jellyfish provides shelter and home, the algae shares the food it makes with its host. A win-win situation!

The resident Oscellated Sea Cucumber (Stichopus ocellatus) awed many of us with its sheer size. By far, this is the largest I've seen at Semakau. The eye rings on the body are said to have sensory function to help it move around.

Our 4th Sea Cucumber of the day was the Stonefish Sea Cucumber (Actinopyga lecanora). Unlike the Sandfish Sea Cucumber which has rough skin, the Stonefish Sea Cucumber is much better at beauty care with its smooth surface, looking just like a very well-polished stone.

The tide was low enough for us to meet the smiley resident Fluted Giant Clam (Tridacna squamosa). As the name suggests, this is one of the largest bivalves. Unlike other clams, the Giant Clam's fleshy body plays host to the algae. This is also why the mantle is usually exposed to sunlight to allow the algae maximum resources to make food. This is another animal kingdom example of mutualism as the clam provides a home for the algae, while the algae in return, shares the homemade food.

Presenting the Nudibranchs who were such wonderful participants. Certainly hope that they enjoyed the walk as much as I do. =)