28 April 2009

Changi beach is full of crab!

Crabs on our shores are often overlooked as our attention is focused on more striking creatures.
Many of our crabs are well camouflaged, well hidden or can make a quick getaway. But here's some crabs that I saw today on the shore.

Just I stepped on the shore, I met this Horn-eyed ghost crab (Ocypode ceratophthalmus). It seemed to be eating a bristleworm. Eew.
It was before sunrise and the shores were busy with these crabs. During daylight you hardly ever see them.
This pretty Reticulated moon crab (Matuta planipes) is often seen on Changi. It has complicated designs on its shell, and colourful markings on its limbs. All the legs of a moon crab are modified into paddles. These are not much used for swimming, but function more like spades to bury itself in an instant into the sand.I also saw the less decorative Spotted moon crab (Ashtoret lunaris), which is quite commonly seen on many of our shores. It was holding on to a small crab. At first I thought the two crabs were getting ready to mate, something I've seen before.But a closer look (at home, when I was processing the photos) suggests that something more sinister might have been in store for the little crab, as it was NOT a Spotted moon crab!Another very common crab on many of our shores is the Flower crab (Portunus pelagicus). These swimming crabs belong to the Family Portunidae, and their last pair of legs are paddle-shaped and rotate like little boat propellers so they can zoom about in the water. They are also often buried in the sand. Tiny ones are also often seen. There were a lot of these crabs on Changi, though most were rather small.
Near an abandoned drum were two swimming crabs with purple legs (Charybdis sp.). These crabs are more commonly seen near reefs and rocky areas and less often encountered in seagrassy areas like Changi.
And here's another kind of swimming crab. I'm not really sure which kind it is. It might be a small Red swimming crab (Thalamita spinimana).The elbow crabs (Family Parthenopidae) are small and superbly camouflaged. Thus often totally missed. But there are LOTS of them at Changi. Which is why I am careful to avoid stepping on seagrasses where possible, and try to stick to bare sand.These crabs have enormously elongated pincers that are many times longer than the width of their bodies!

I also saw a tiny sponge crab (Family Dromiidae) using an ascidian as a camouflaging cap. There were also lots of signs of Sand bubbler crabs (Scopimera sp.) and possibly Soldier crabs (Dotilla sp.) on the shore. But I didn't have the energy to stalk a hole to catch a photo of one today. Spotted box crabs (Calappa philargius) and Pebble crabs (Family Leucosiidae) were seen in the past on this shore, but I didn't see any today. But Kok Sheng did see the Pebble crab!

A special 'crab' I saw today was the Mangrove horseshoe crab (Carcinoscopius rotundicauda).
This animal, however, isn't a crab! It isn't even a Crustacean! The horseshoe crab is a strange, ancient creature that has been around since before the dinosaurs. It is more closely related to spiders and scorpions of the Class Arachnida.

The difference is more obvious on the underside. It has weird legs and appendages, as well as flappy things that are its book gills. The white circular things are slipper snails (Crepidula walshii) that have settled on its shell. These strange snails also settle inside the shells occupied by hermit crabs. Slipper snails can change gender. They are usually found in pairs. The larger one is the female and the smaller one male. When the smaller one grows big enough, it will change into a female!

Here's more about fishes and other sightings, and hairy slugs seen on Changi today.


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