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Phylum Arthropoda > Subphylum Crustacea > Class Malacostraca > Order Decapoda > Brachyurans > Superfamily Ocypodoidea
Soldier crab
Dotilla sp.
Family Dotillidae
updated Oct 2016

if you 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.
Unlike most crabs, they can run forwards, backwards as well as sideways.

Where 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 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.

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.

Sometimes mistaken for the sand bubbler crab. The soldier crab is larger and has longer, more slender pincers.

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.

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.

Chek Jawa, Nov 04


Spitting out a sand ball.
Cyrene Reef, Aug 11


Typical burrow with small and big
sand balls around the opening.

Chek Jawa, Feb 06


'Igloo' built just before the incoming tide.
Tanah Merah, Nov 11

Chek Jawa, Jun 07

Creating an 'igloo' in wet sand
with the incoming tide.


Soldier crabs on Singapore shores

Photos of Soldier crabs for free download from wildsingapore flickr

Distribution in Singapore on this wildsingapore flickr map


Berlayar Creek, Oct 15
Shared by Loh Kok Sheng on flickr.

St. John's Island, Jan 09
Shared by Loh Kok Sheng on his blog.
 


Lazarus Island, Feb 09
Shared by Loh Kok Sheng on his blog.

Pulau Semakau South, Feb 16
Photo shared by Heng Pei Yan on facebook.
 


Pulau Pawai, Dec 09

Pulau Sudong, Dec 09

Pulau Senang, Aug 10
Photo shared by Loh Kok Sheng on his flickr.

Dotilla species recorded for Singapore
from Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore
in red are those listed among the threatened animals of Singapore from Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore.

  Family Dotillidae (previously Ocypodidae)
  Dotilla myctiroides (VU: Vulnerable)
Dotilla wichmanni

Links

References

www.flickr.com
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