10 January 2010

Horn-eyed ghost crabs (Ocypode ceratophtalmus) of Koh Yao Noi


When visiting Koh Yao Noi, an island near Phuket, Thailand, in 2007 I was so fortunate to encounter and observe Horn-eyed ghost crabs, Ocypode ceratophtalmus (Pallas, 1772)(NL: Gehoornde spookkrab), on the beach.


I saw something moving on the beach, like a leaf blown by the wind. Just a leaf. Some time later it was something ghostlike, a swift moving transparent thing: the first ghost crabs. Very agile and - especially the small specimens - very well camouflaged. I saw one, and then more and more of these crabs scavenging the beach.


More


and more.


This is where they are hiding and appearing: self made burrows.


A young specimen leaving its burrow.

Features

Adults are easily recognised because of the long horned eyes. It gives them a funny appearance. Younger specimens are less easy to recognize: they lack the horned eyes. Poore (2004; see literature): ‘The stridulating organ, a rough area on the inside of the large cheliped (claw) is a more reliable character to differentiate the species.’ With this stridulating organ they can make sound. I can’t remember hearing a particular sound or it should be like the noise cicadas and later on the day crickets are making. Its carapace width is 40 mm and rather rectangular.


Captured to have a closer look. The only way to capture the ghost crab is to let it ‘escape’ into water, where it’s less agile. Observe the typical rectangular carapace.


All alone in the big ocean. Horn-eyed ghost crabs are semi-aquatic. They need water for reproduction and respiration. Staying too long in water they will drown.


After his more or less involuntary swim he touched ground: a few seconds later he ran away with amazing speed.


Excellent 360º sight.


A young(er) ghost crab without the typical eye-extensions.

Biology
Generally scavengers, they also feed on bivalve molluscs and will attack and eat turtle hatchlings and cannibalise members of their own species (Richmond, 2002). I can not remember a lot of hostility or seeing crabs with amputated claws or legs as is quite usual with species like Grapsus. When one came too close, the other crab(s) ran away. Maybe it is just a matter of having enough ‘lebensraum’.


I saw a lot of crabs sifting sand for organic matter.

Ghost crabs make deep burrows above the high tide mark. It was a real treat to observe the ghost crabs making or improving their burrows.


Poore (2004): ‘They rarely leave the burrow during the day. At night however they run quickly over the beach where they scavenge and are known to forage almost a kilometre away from the sea.’ The Ko Yao Noi-ghost crabs were very active during the day. Maybe there aren’t many predators on the island, so they can be bolder. And there was enough organic matter to scavenge on the beach at ebb tide.


The first excavation. The sand is blocking a perfect (360º) view. Dangerous: predators like birds - coming from the right position and keeping low - could ambush.


So throw it a bit further away.


This crab, making or renovating a burrow, is clever. He doesn’t want the sand blocking his sight around the burrow. So he makes balls of sand and moves them out of sight.


After all this hard labour he was ‘dancing’ on the sand near the burrow. Not for joy: by trampling on the sand the surface will get even smoother. And maybe it gets more solid, so the entrance of the burrow will stay firm.


An entrance with typical smoothing patterns. The sand thrown out of the burrow is still moist.


Typical treading patterns near the entrance of a burrow.

These crabs are well adapted to living in burrows. Branch (1983): ‘Ghost crabs have a cavity on either side of te body between the bases of the third and fourth legs, which is fringed with hairs. These hairs serve to suck water from damp sand and pass it through the cavity to the gill chamber. They also serve to filter the water to exclude sand or mud.‘

Distribution
Its distribution is the Indo-Pacific, so for me as a Dutchman it is as exotic as it looks.

Searching the web
I found an interesting and extensive report of the off-road vehicle-impacts on beaches in Australia. With a frontphoto of a crushed horn-eyed. Click here.

Literature:

  • Branch, M. & G., 1983. The living shores of Southern Africa. ISBN 0869771159.
  • Edgar, G.J., 1997. Australian Marine Life, the plants and animals of temperate waters. ISBN 187633438X.
  • Poore, G.C.B., 2004. Marine Decapod Crustacea of Southern Australia. ISBN 0643069062.
  • Richmond, M.D., 2002. A field guide to the seashores of Eastern Africa and the Western Indian Ocean Islands. ISBN 9158687831.


Hiding in its burrow in the evening.

3 comments:

Nicole said...

Awesome post, Info and photos!
I was just looking up the 'Horn eyed crabs' and found you that way.
It's tough for me to understand that people these days still drive along the beaches. I thought it was the case only here in Kuwait, but obviously not :/

Andrew Mitchell said...

That is really good information. I also think that they tunnel into the faces of beaches below high tide level and their tunnels are flooded at high tide. I would love to find out how they would survive. Although I live on a beach and can see these animals almost everyday, I did not know about the dance. There is also something special about Asia. I saw locals flicking small dead cockle half-shells so that they rolled down the face of the beach like a wheel and large ghost crabs would come out in the middle of the day and chase the shells. The game was for the person to dive at the tunnel and cover it up before the ghost crab could get back. Usually, the ghost crab won. In Australia, the crabs hide all day and are never bold.

Mick Otten said...

Andrew: Thanks for your complementary information.