learn only 3 things about them ...
| They are everywhere on the sandy shore at low tide. Don't
step on them!
They are very shy. To see them, wait quietly without moving.
most crabs, they can run forwards, backwards as well as
seen? This amusing tiny ball-shaped crab with enormously
elongated pincers is sometimes seen on natural sand bars. While the
shy crab itself is seldom seen (it hides as soon as it senses footsteps
far away), the balls of sand that it leaves on the shore at low tide
indicate where it is active. To spot one, you will have to wait quietly
next to a burrow. Stay low and avoid casting a shadow over the burrow.
In a few minutes, it and its neighbours will appear. If you stay still,
they will go about their amusing business
Features: Body width to about 1.5cm. Body spherical, greenish eyes on short
thick reddish stalks. Pincers very long, slender and folded downward
with the claws pointed towards itself. The crab is well adapted for
life out of water: it can absorb air through special parts of its
legs which are thinner. It also absorbs water from the sand through
silky hairs on the abdomen. Unlike most crabs, the soldier crab can
run forwards as well as sideways. And it can move very fast indeed!
Dotilla wichmani looks like other sand bubbler crabs. It is smaller (body width up to 1cm) and prefers
sandier areas not wandering far from its hole.
Dotilla myctiroides is larger (body width up to 1.5cm) and is found in muddier areas often
moving around in large groups at low tide. This habit of 'trooping'
in numbers gave these crabs their common name.
Chek Jawa, Nov 04
burrow with small and big
sand balls around the opening.
Chek Jawa, Feb 06
built just before the incoming tide.
Tanah Merah, Nov 11
|Burrowing in a bubble of air: A
study found that Dotilla myctiroides in wet semi-fluid
sand builds an 'igloo' of sand pellets by rotating itself, in order
to seal a bubble of air around itself in the wet sand. Beneath the
sand, the crab continues to scoop sand from below it and plaster the
sand above the bubble of air. In this way, the crab in its bubble
of air is able to burrow downwards. In more stable sand, the crab
builds a vertical burrow.
What does it eat? The soldier
crab eats the thin coating of detritus on sand grains. Sand grains
are scraped up with the downward pointing pincers and brought to the
mouthparts that then sift out any tiny food particles. The shifted sand is then discarded in a little ball. The bigger untidy
balls are sand pellets dug out of the burrow.
Chek Jawa, Jun 07
Creating an 'igloo' in wet sand
with the incoming tide.
Status and threats: One of our
soldier crab species (Dotilla myctiroides) is listed among
the threatened animals of Singapore due to loss of our natural beaches.
While somewhat common on Chek Jawa, they are not commonly seen elsewhere
in Singapore. Like other creatures of the intertidal zone, they are affected by
human activities such as reclamation and pollution. Trampling by careless
visitors can also have an impact on local populations.
crabs on Singapore shores
|Other sightings on Singapore shores
Chek Jawa, Nov 17
Shared by Abel Yeo on facebook.
Berlayar Creek, Oct 15
Shared by Loh Kok Sheng on flickr.
St. John's Island, Jan 09
Shared by Loh Kok Sheng on his
Lazarus Island, Feb 09
Shared by Loh Kok Sheng on his
Pulau Semakau South, Feb 16
shared by Heng Pei Yan on facebook.
Pulau Pawai, Dec 09
Pulau Sudong, Dec 09
Pulau Senang, Aug 10
shared by Loh Kok Sheng on his
- Satoshi Takeda,
Masatoshi Matsumasa, Hoi-Sen Yong, Minoru Murai. 15 June 1996.
construction by the ocypodid crab, Dotilla myctiroides
(Milne-Edwards) (Crustacea; Brachyura): the role of an air chamber
when burrowing in a saturated sandy substratum Journal of
Experimental Marine Biology and Ecology Volume 198, Issue 2, Pages
- Ng, Peter
K. L. and Daniele Guinot and Peter J. F. Davie, 2008. Systema
Brachyurorum: Part 1. An annotated checklist of extant Brachyuran
crabs of the world. The Raffles Bulletin of Zoology. Supplement
No. 17, 31 Jan 2008. 286 pp.
- Lim, S.,
P. Ng, L. Tan, & W. Y. Chin, 1994. Rhythm of the Sea: The Life
and Times of Labrador Beach. Division of Biology, School of
Science, Nanyang Technological University & Department of Zoology,
the National University of Singapore. 160 pp.
- Ng, P. K.
L. & Y. C. Wee, 1994. The
Singapore Red Data Book: Threatened Plants and Animals of Singapore.
The Nature Society (Singapore), Singapore. 343 pp.
- Jones Diana
S. and Gary J. Morgan, 2002. A Field Guide to Crustaceans of
Australian Waters. Reed New Holland. 224 pp.