10 November 2010

Seaweed city at Sentosa

A feathery green carpet covers Sentosa's reef flat when we visited last night!
Teeming amongst the delicate fronds of the Hairy green seaweed (Bryopsis sp.) are all kinds of creatures big and small.

There are of course, lots of Bryopsis slugs (Placida dendritica) among the seaweeds! These tiny slugs suck the sap out of these seaweeds and usually are the same colour as their food.
There were also lots and lots of these Woolly leaf slugs (Elysia cf. verrucosa).
I saw this tiny little thing. I have no idea what it is. It looks like some kind of slug. Kok Sheng also saw a pretty tiny slug that I've not seen before.
Other creatures in the feathery forest were lots of Red-nose shrimps (Periclimenes sp.), several of another kind of shrimp I've not seen before, many tiny crabs and small fishes too. Under the feathery curtain on the sea floor were many larger swimming crabs (Family Portunidae).
There were also lots of these flattened critters that are probably some kind of isopod.
These tiny creatures are part of the food chain and feed larger animals on the shore. A tiny Longspined scorpionfish (Paracentropogon longispinis) was patiently stalking among the seaweeds. It isn't as well camouflaged on the seaweed as it is on the rocks!
A snapping shrimp (Family Alpheidae) scurries through the tangle of fronds and disappears before I could take a closer look at it. It doesn't look like the kind I often see.
Although the Hairy seaweeds look similar, a closer look reveals that some clumps have long unbranched filaments, while others are feathery. They come in all shades of green and some are beautifully bluish.
What a relief to see that the Tape seagrasses (Enhalus acoroides) on Sentosa are still long and lush. Unlike those at Cyrene Reef we saw recently.
But I also see clumps of Tape seagrass which look like they have been cut off near the base. Oh dear.
It was dark by the time I got to the rare Nyireh mangrove trees (Xylocarpus rhumphii) that grow here. I noticed the leaves of the Mother Tree were turning yellow! While I've read about how the leaves of Nyireh batu (Xylocarpus mollucensis) wither yellow, I've not come across reports that X. rumphii does the same.A closer look at the leaves changing colour. I do hope this is just a natural seasonal thing for the tree and not a sign of something going wrong. The other smaller X. rumphii nearby didn't have yellow leaves.
My purpose of this trip to Sentosa was to try to take better photos of some common snails, especially the Nerite snails (Family Neritidae). Nerite snails are very common on our rocky shores, even on man-made rock walls. They are cute, active snails with long tentacles and a pajama striped body. But it's hard to tell them apart just from the upper shell patterns.
These snails are more easily differentiated by looking at their undersides. After gathering a bunch of snails from the rocks, indeed, it was simpler to identify them by their undersides! The ones with more white undersides turned out to be Chameleon nerites (Nerita chamaeleon) while those with more yellowish undersides turned out to be Scaled nerites (Nerita histrio).Chameleon nerites (Nerita chamaeleon), as their name suggests, have shells in a wide variety of patterns and colours. The shell opening is whiter with tiny 'teeth'. The 'door' (called the operculum) that is used to seal the shell opening has a pimply surface.
Scaled nerites (Nerita histrio) also come in a variety of shell patterns. The shell opening is more yellowish and there is usually a yellow stain near the tiny 'teeth'. The operculum is pimply.
Waved nerites (Nerita undata) also come in various shell patterns and colours. Compared to the two previous nerites, the shell has a more pointy spire and the shell opening has 2-3 blunt 'teeth'. The operculum is pimply.
The beautiful Polished nerites (Nerita polita) are not as common as the other kinds. The shell is more oval than round and come in lovely patterns. The operculum is smooth and has no pimples.
I also took photos of other common snails like these Toothed top snails (Monodontia labio). They come in a variety of colours too, but all have a single 'tooth' at the shell opening and a thin operculum.
Planaxis snails (Planaxis sulcata) on Sentosa are large and fat!
I'm still not too sure what these little snails are. There are many on the rocky shore near the low water mark. I think they are some kind of creeper snails (Family Cerithiidae).
When the snail dies, the empty shell is a valuable home for hermit crabs. Sentosa has some endearing Land hermit crabs (Coenobita sp.). They are very busy at night!
There was one very large pretty purplish lilac hermit crab. When I approached, it promptly retracted into its shell, which was too small for it! When it emerged, it rolled over but soon righted itself and scrambled away.
Living in a coiled up shell stuck to a hard surface, these Worm snails (Family Vermetidae) are often mistaken for worms. But like other snails, they too have a tiny operculum to seal up the shell opening.
There were lots and lots of tiny dove snails on the Tape seagrasses. I think these are all Dotted dove snails (Euplica scripta).
Among some of the intriguing encounters I had was this little insect on the rock wall. At first I thought it was a large mosquito. But a closer look suggests that it's a water strider (Halobates sp.), one of the few insects that live on the sea. What was it doing on the rock wall? Laying eggs? Having a rest?
Alas, I didn't get a chance to check the shore for coral bleaching. The tide wasn't low enough and the Hairy seaweeds made it difficult to walk safely on the reef flat. But I did see two healthy looking Pore corals (Porites sp.).

As always, I haven't enough energy to do a shore properly during low tide. My back hurt after all that bending to take a closer look at the many fascinating creatures on this shore!

2 comments:

  1. The tiny green slug looks like a Sacoglossan from the Family Juliidae.

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