Identifying Madagascar Ghost Crabs

According to Sakai & Türkay, 2013, Madagascar has 5 sp. of Ghost Crab (although @martinmandak
a 6th in the distinctly pink-kneed O. ryderi on an island thats technically part of Madagascar). The are:
O. ceratophthalma
O. cordimana
O. pallidula
O. pauliani
O. madagascariensis

The first 3 have very large distributions across the Indo-pacific.

O. ceratophthalma is the only one with 'horns' (ocular styles) which makes them easy to spot (but careful, only adults have horns, juvenile ghost crabs are pretty much hopeless to get to species unless you can examine characters almost never visible in iNat photos (reproductive organs or the stridulating ridge on the palm of the hand).

The large claw of both O. cordimanus and O. pallidula are relatively smooth. I'm still not confident what O. pallidula looks like so I'm not totally sure how to separate these (lots of mis-ID'd O. pallidula photos out there I suspect). But if you look in Eastern Australia where only O. ceratophthalma and O. cordimanus are present you'll get a good feel for what O. cordimanus looks like.

The remaining two species are Madagascar endemics or close to that (O. madagascariensis is also found across the the Mozambique Channel along the coast of southern Africa). I wasn't sure how to identify them, but I was aware of some observations of 2 distinctive ghost crabs on Madagascar that I hadn't seen from anywhere else on iNat. So I decided to try to reconcile them with Sakai & Türkay, 2013 and the type specimens (here and here

Bumps on the back

O. madagascariensis has larger bumps (tubercles) on the back that are pretty much the same size across the back. In contrast, O. pauliani has larger bumps towards the side edges of the back and smaller bumps in the middle. This is pretty easy to spot in several of the more in-focus close up photos

Bumps on the larger claw

Both O. madagascariensis and O. pauliani have claws with much larger bumps (tubercles) on them than O. cordimanus or O. pallidula. But of the 2, O. pauliani has a much rougher looking claw with larger fewer bumps. The bumps on O. madagascariensis are smaller and aligned more in rows especially towards the tip of the claw. O. pauliani just gives the impression of a much more tricked out claw than O. madagascariensis (annoying the Sakai & Türkay, 2013 specimen is missing its claw)


Sakai & Türkay, 2013 mentions both of the characters above and they are visible in the specimens. But as usual they don't mention the distinctive color patterns which are so easy to spot in iNat photos but I guess don't preserve well in specimens. In this case adult O. pauliani seem to have striking colors with pale heads with bright orange eyestalks and mouthparts. O. madagascariensis seems much more uniformly sandy colored by comparison.

I feel pretty confident in my conclusions, but would definitely appreciate hearing from the expertise of others. For both of these crabs there don't appear to be any labeled photos of living animals on the web, which is kind of depressingly shocking, so I could be wrong in my logic linking these living animals to the specimens and museum descriptions. But assuming I'm right, its exciting to be able to finally add labels to these striking photos - especially O. pauliani which we have a few obs of and they are beauties for sure!

Publicado el enero 13, 2023 09:54 MAÑANA por loarie loarie


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