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Spider Crabs
All spider crabs belong to the family Majidae.
Where commensal crustaceans occur on specific hosts and nowhere else (to our present knowledge) identification is simply a matter of visual recognition of the host and, in most cases, the commensal can be identified.
However, with more and more divers becoming environmentally minded and the upsurge in imagery interest due to digital cameras we are now all involved in recording the many (often complicated) relationships between hosts and their commensals.
This welcomed investigative nature by divers has led to a great deal more knowledge being available and whereas once a species may have only been known from a single host in the past, today the records are being rewritten.
Some time ago I found what I thought to be a different species of Black Coral Spider Crab in Milne Bay. Over 25 years the crabs were always found on a specific species of Branching Black Coral at Wahoo Point, Milne Bay, Papua New Guinea. Upon investigation of the genus Xenocarcinus I noticed there were several species of these commensal spider crabs, Xenocarcinus depressus Miers, 1874 and Xenocarcinus tuberculatus White, 1847, Xenocarcinus conicus.
Xenocarcinus depressus lived on sea fans and X. tuberculatus lived on black coral sea whips. From my own observations I knew that the former was present in shallow water and the latter below 20 metres. Nothing in the scientific literature suggested that either species lived in branching Black Corals and as mine appeared visually different it was worth further investigation.
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During our collaboration I discovered that the original type species drawing and description of Xenocarcinus tuberculatus does not match the crab we all assume to be this species, so, here we go again. For the time being, and until this can be sorted, the crabs remain with the same names. Many thanks to Peter Davie and the Queensland Museum for the chance to get this one sorted.
The Depressed Spider Crab Xenocarcinus depressus 20 metres, Bali, Indonesia. This individual appears to have transferred living flesh from its host to its body and leg joints (appearing as the same colour).
(Photo Neville Coleman)
Another example where the Depressed Spider Crab Xenocarcinus depressus has induced the hosts tissue to grow on its shell surface, 18m, Bali, Indonesia.
(Photo Neville Coleman)
Male Depressed Spider Crab Xenocarcinus depressus at five metres from Gove, Northern Territory, Australia.
(Photo Neville Coleman)
A very well-developed typically coloured and patterned Depressed Spider Crab Xenocarcinus depressus from 12 metres LIzard Isl. GBR.
(Photo Neville Coleman)
Photographed at 20 metres in branching Black Coral Antipathes sp. this Conical Spider Crab Xenocarcinus conicus has the typical white central back blaze and brown body colour to blend in with its host.
(Photo Neville Coleman)
The object of my investigation. The Wahoo Point, Milne Bay PNG version of the Conical Spider Crab Xenocarcinus conicus found at 10 metres on a different species of branching black coral Antipathes sp. appeared at the time (to me) as different, but it keyed out to be just a variation.
(Photo Neville Coleman)
The Tuberculate Spider Crab Xenocarcinus tuberculatus on a black coral sea whip Stichopathes sp. from 40 metres, Bali, Indonesia. This species appears very characteristic with a shiny shell and a pattern that mimics its hosts polyps.
(Photo Neville Coleman)
A colour variation of the Tuberculate Spider Crab Xenocarcinus tuberculatus from 20 metres on a black coral sea whip from inside a wreck at Loloata Island, Papua New Guinea.
(Photo Neville Coleman)
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One of the most difficult spider crabs to get to look remotely like a crab is the master of camouflage Spidery Spider Crab Camposcia retusa. It covers itself entirely with sponges and only comes out at night. (Bali Indonesia)
(Photo Neville Coleman)
Only found in association with the Spiky Soft Corals Dendronephthya spp. Oates's Soft Coral crab Hoplophrys oatsii is one of the best mimics in the World of Water. It occurs across the Indo - Pacific, but is not always easy to find.
(Photo Neville Coleman)
This is a very common species but generally so covered in marine growths that it is not recognised. However, soon after they cast their shell the real identity of the Rough Spider Crab Schizophrys aspera can be seen.
(Photo Neville Coleman)