… if you value your fingers! But then again, not all the crabs that you see below aren’t suitable to be sold on the shiny silver platters for your 8-meal course. Those that come from mangrove areas are usually the ones caught, though the blue swimmer crab has been eaten as such delicacy.
(Left): Thunder Crab (Myomenippe hardwickii) – so called because of the belief that only by the clap of a thunder, the crab will free you from its pinch – that is if your fingers get in between its pincers in the first place. It’s pretty common but prefer the shelter under rocks. At low tide, it will either crawl into a burrow or peer its way out from beneath a rock. With green eyes rimmed a crimson red, it hunts for clams and snails during the high tide before rendering them to tasty chow with its large, powerful pincers. What’s terrifying to the molluscs is nothing but delicious cuisine for us humans. We eat them – with the pincers particularly favoured!
(Right): Spoon-pincer crab (Leptodius sp.), with its spoon-tipped pincers (not obvious in the picture) are commonly seen on the shores and islands southern of Singapore, whether it’s amongst coral rubble or near the living reefs. Instead of sharp, pointy tips, the black pincers have white, spoon-shaped tips, which are presumably used to scrape off their meal (algae and detritus) encrusted on rocks. Imagine trying to eat pudding with chopsticks in place of the usual dessert spoon. =)
(Left): Here we have a Sponge crab (Family Dromiidae) that’s on a sponge – an upside-down position compared to its usual form. Using its pincers, the Sponge crab will snip out a bit of a living sponge (sometimes ascidians) that’s just right for its body. Then using its last pair of slender legs that’s bent over its back, it holds the sponge atop – with a firmer grip provided by the leg’s sharp, little claws. As the living sponge continues to grow, the crab will continue to trim it to a suitable size. So unless it moves (and slowly too!), it really takes a trained eye to spot this well-camouflaged decapod. Besides providing an invisibility cloak of sorts, the sponge is also a good deterrent for potential sponge crab diners. Taking a bite of the crab (or the sponge in this case) will usually make the predator swear off sponge crabs – unless desperation takes the upper hand. :)
(Right): Flower crab (Portunus pelagicus) – all-time favourite seafood (or just one of them anyway) is another common crab, but difficult to spot as you can see (from the picture) that it tends to bury itself beneath sand and mud.
Males (like this one) are more beautifully-patterned and usually have bright blue legs and claws. On the other hand, females tend to sport colours like dull green and brown. Also, the boys have longer pincers – twice or three times longer than the width of their body. While the males possess a more pointed triangular shaped abdomen, the females are broader as it’s used to carry eggs.
Being a predator, the pincers are armed with sharp spines to snag swimming creatures. Never mind that the prey are motile and quicksilver in water, this crab that’s fully aquatic in nature is just as limber. Reason being that its last pair of legs are paddle-shaped and can rotate like propellers. And that’s why the Flower crab is also known as the Swimming crab.
In a pose that seems to say “Hug me!”, this body of the bulky-built Thunder Crab’s (Myomenippe hardwickii) was found amongst a bed of lush seaweed, an appropriate final resting place – till the tide washes in, that is.
What looked like it might have been a young Spider conch (Lambis lambis) is actually a different mollusc altogether. It’s thought to be the juvenile of the Lambis lambis (Class Gastropoda)because it lacked the spines of the a typical adult conch.
(Left): Another gastropod that we bumped into besides the supposed Spider Conch is the Blue-spotted nudibranch (Dendrodoris denisoni). But unlike the usual gastropods, nudibranchs lack the external protective shell common to snails and slugs. But compared to most nudibranchs (from the Greek “naked gills”), the Blue-spotted lacks the special set of minute jagged teeth (radula) that acts like a saw for breaking up food in typical sponge-eating nudibranchs. Instead, it releases digestive liquid, dissolves the sponge to soup-like texture before sucking it all up.
(Right): Another gastropod found was this small, air-breathing onchs (Family Onchididae). Onchs, unlike nudibranchs, have simple lungs or modified gills, allowing them to survive on terra. Sea slugs like the nudibranchs are limited to aquatic environment as they respire via gills. At high tide, they crawl away from their favourite spot on the rocks at shore to burrow into sand and mud; breathing from the air bubble trapped under the folds of their mantle. As the tide ebbs, these shell-less creatures crawl back onto the rocks and start grazing off the algae and lichen. To reduce water loss when out in the sun, they rely on their tough skin to do the work.
And my first time seeing an octopus, alive and kicking, on the shores of Singapore! Other times were limited to either a glimpse of one of its limbs or a dead body lying limply on a sun-baked shore. Octopi are also molluscs (or soft-bodied creatures) (Family Cephalopoda). Though these intelligent creatures have no internal nor external skeleton, they have a beak (similar to a parrot’s beak) held at its mouth to kill and decimate prey. It’s also used crack shells and inject poisons and digestive fluids into shellfish and other prey. Each of its arms have double rows of suction cups, each of which consist of ‘taste sensors’ to identify food.
Though they have good eyesight, octopi are deaf (not that it will be useful underwater or above it). Here are some of the ways which octopi use for locomotion purposes. There’s the slow crawling method, followed by ‘walking’ (2 alternating arms behave in a rolling gait while the remaining arms are utilized for camouflage). Then, there’s the usual swimming motion and the faster jet-propulsion technique. And last but not least, octopi have been known to also ‘fly’. I kid you not. Though not a usual travelling method, octopi have been observed to blast themselves out of water completely to escape predators [3]! How cool is that. :)
To my delight, another gem which Pulau Sekudu yielded was the gorgeous Blue Dragon (Pteraeolidia ianthina). Another first! An aeolid nudibranch, the Blue dragon has evolved a method of capturing and ‘farming’ its own fresh batch of zooxanthellae (golden-brown algae), which proliferate and convert the sun’s energy into sugars before passing a portion to the nudibranch [1]. An aeolid nudibranch is a type of nudibranch in which the mantle is extended into long finger-like projections (ceratas), rather than a feather-like external gill on the dorsal (or top) surface.
Also, aeolid nudibranchs have developed an amazing strategy to protect themselves from predators. They extract nematocysts from the coelenterates (comb jellies etc) on which they feed and store them in the special sacs at the tips of their ceratas. The nematocysts are used by coelenterates as stinging weapons to catch food and for defence. Aeolids ‘steal’ these weapons and discharge them when needed in their own defence [4].
Some other creatures which can rival the beauty of the nudibranchs are marine flatworms (Family Pseudocerotidae) such as the one above. Okay, so this example might not be a jaw-dropping beauty, but it does come from a family which consists of bella donnas like these. Unsegmented, flatworms are extremely flat (usually less than 1mm thick). Of course being flat, respiration is a cinch as oxygen can diffuse easily from any part of its body. Nutrients can also diffuse rapidly from the central gut to the entire body.
Marine flatworms are hermaphrodites, that is they both possess male and female reproductive organs. In some cases when two flatworms meet, they exchange sperm by simply insert their needle-like penis anywhere in the body of the partner. But in other species, each flatworm refuses to be the ‘girl’ and tries to impregnate the other without getting impregnated as producing eggs require more energy. In short, they need to eat more! This results in a bout of 'penis-fencing' when such flatworms meet! I have yet to come across such an interesting display but one can only hope. =)
And speaking of eggs, the white casings in the left picture belong to the Spiral melongena (Pugilina cochlidium) while on the right are eggs bore by the squid.
All in all, it was once again a smashing trip down the Southern shores of Singapore. A big thanks to Ron and LK also for providing me an eye-opening opportunity like this. My gratitude to Ron, July and KS also for their blog entries as all three provided some information pertaining to the identification of the critters above (and the 3 entries before). :)
References:
1. http://www.seaslugforum.net/factsheet.cfm?base=pteriant
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