04 September 2011

Sentosa rocks with special crab

It's the first evening low tide of the year! We zipped past all the glitzy night spots on Sentosa on a late Saturday night, and headed for where things really rock! 
Sentosa's natural shores! With stretches of natural sandy areas, rocky shores, natural cliffs, coastal forest and reef flats.

While the rest explored the shore thoroughly, I plodded slowly to focus on the rocky shore. And came across some creatures that I've not observed closely before. I saw geckoes (two of them) and a centipede on parts of the rocks that would be covered at high tide. I also saw these creatures on Changi last month. Does this mean these two animals are naturally found on the intertidal rocky shore at low tide? Wow! I need to look out for these on our other rocky shores too!
I also saw these small speedy polychaete worms (about 10cm extended out of their hiding place). They do look like centipedes but are not. They retracted instantly into their hiding places when they sensed me, and certainly after a flash photo. There were also lots of tiny little critters in the seaweed mats, and also some long-legged insects that look similar to the ones I saw at Changi. I really should take more time to properly look closely at our rocky shores!
My mission for the trip was to take proper photos of snails commonly seen on our rocky shores. Such as these beautiful Nerite snails (Family Neritidae), which can look very similar. It's not very easy to tell them apart from the upper side especially since snails of the same species can have many different colours and patterns. So it's best to look at the shape of the shell.
They are more easily distinguished by their undersides. From left to right they are the Polished nerite (Nerita polita), Waved nerite (Nerita undata), and the last two are Chameleon nerites (Nerita chamaeleon). Here's a comparison of some common nerites.
These two kinds of snails are also often mistaken for one another. The difference is more obvious when we look at their underside. On the left column, the Toothed top shell snail (Monodontia labio) has a thin yellow 'door' to seal the shell opening. While the Ribbed turban snail (Turbo intercostalis) has a thick convex 'door'. Here's a comparison of top and turban snails.
I came across the smallest Giant top shell snail (Trochus niloticus) that I've ever seen! These snails can grow to an enormous size, with a base of 8-15cm in diameter!
Here's a closer look at the Giant top shell snails (left column) compared with the more common and never-grow-bigger Spotted top shell snail (Trochus maculatus).
Along the way, I noticed the rock pools were teeming with tiny gobies and snapping shrimps!
The very shy Coral ghost shrimp (Glypturus sp.) creates large long smooth sides burrows in solid rubble. I don't know how they manage it. We hardly ever see more than the tips of their orange pincers before they slide away into hiding.
The rocky shores are swarming with sea slaters (Ligia sp.) as usual. These are not cockroaches! They are not even insects. They are crustaceans like crabs and shrimps, but are very happy scrambling around out of water.
I only had a brief glimpse at a tiny portion of the reef. I saw various hard corals small and medium sized that seemed to be in good health.
I also saw one tiny Leathery soft coral, some patches of corallimorphs, in sheltered pools there were fanworms and zoanthids. On the reef flats, many patches of brightly coloured and healthy looking sponges.
There were also many other kinds of crabs on the rocky shores. I saw several Spoon pincer crabs (Leptodius sp.), four large Spotted belly forceps crabs (Ozius guttatus), many fierce Red-eyed reef crabs (Eriphia ferox) and lots of swimming crabs of all kinds (Family Portunidae). There were also many Purple climber crabs (Metopograpsus sp.).
Just as we were about to leave, Marcus found this special crab on the high shore!
We think this crab is Ocypode cordimanus, which I've been wanting to see for a very long time! Thanks Marcus!
Compared to the more commonly seen Long-eyed ghost crab (Ocypode cerathophthalmus) (below), our Sentosa crab doesn't have pointy things on top of its eyes and lacks the dark brown markings on the back.
Horn-eyed ghost crab (Ocypode ceratophthalmus)
The rest of the team saw a lot more stuff and I'm sure we'll hear soon about their finds. One of them, a baby stonefish. Thank goodness no one stepped on it or its larger friends.

Sadly, we came across seven of these freshly laid crab traps on the sandy shores.
Hope the crabs and marinelife on this marvellous stretch of Sentosa won't be too badly affected by these traps.

Posts by others on this trip
  • Debby shared on facebook videoclips of an octopus showing amazing colour changes, and a photo of a baby stonefish. With full story on the Pulau Hantu blog.
  • Jocelyne with more about octopuses, shrimps, crabs and more! 
  • Kok Sheng with baby stonefish, a look at the corals and more.

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