10 replies to this topic

[Andy H]

OK, I know that the pic below is a zebra crab on a species of toxic urchin.

What I'd like to know from a more knowledgable diver/biologist, is if the relationship with the urchin is symbiotic?

reason I ask is that the first time of seeing these guys was in the Lembeh strait recently where the guides said that the way of finding them was to look for an urchin that was half eaten - as the one is illustrated below. Sure enough, all of the adult crabs that I found were on similar urchins.

My question is, do these crabs literally eat their host? Otherwise what is another explanation?

I'm really intrigued by this so thanks in advance for anything,
Have a wet 2004,
Andy

[frogfish]

I'm not a marine biologist, but Debelius (Crustacea Guide of the World) says that Zebrida adamsii "seems not to harm its host".

However, that's not necessarily true of the Coleman shrimp, Periclimenes colemani, which is often found on the same toxic fire urchins together with Zebra crabs. This shrimp is only found on the Fire urchin Asthenosoma varium - Zebra crabs will inhabit several different urchin species - and it occupies a place on the urchin that it cleares by removing tube feet and spines.

I've never seen them clear a patch as large as the one in your photo, however.

Off to Secret Bay tomorrow! I'll let you know if I see any zebras eating A. variums.

Robert Delfs

[Art]

Hi

I am currently preparing a review of symbioses in decapod crustaceans

first of all the terms SYMBIOSIS and SYMBIOTIC are often used incorrectly for what should be called MUTUALISM and MUTUALISTIC

SYMBIOSIS means etymologically "live together" without any assumption on the nature of the association, and includes MUTUALISM (advantages to both partners or symbionts), COMMENSALISM (advantage to one symbiont, no harm to the other) and PARASITISM (advantage to one symbiont, damage to the other)

there are intermediate and transitory states between these 3, it's actually more like a spectrum from the parasitic end to the mutualistic, with frequent shifts from one to another state: normally the presence of a symbiont is advantageous to the host, but in some situations (due to environmental changes, overpopulation of symbionts etc.) the symbiont can act like a parasite (e.g., feeding on the tissue etc.), and the association quickly becomes disadvantageous to the host, in other situations (inferstation by other parasites or microbes) the symbiont acts like a mutualist as it helps to maintain the host in good health - again, in its own interest ! nothing is free in this world ...

regarding Zebrida and eumedonid crabs there are several ecological/biological studies, I give only those dealing with Zebrida:

Daniel, A. & S. Krishnan. 1979. A parthenopid crab, Zebrida adamsii White, 1847 inhabiting interspaces of spines of the sea urchin, Salmacis virgulata L. Agassiz, 1846. Bulletin of Zoological Survey of India, 1998, 1(2): 171-175.
Doki, R. 1972. Zebrida adamsii attached to a sea urchin, Asthenosoma ijimai. Nakiseibutu, 14: 25.
Mori, A., Y. Yanagisawa, Y. Fukuda & P.K.L. Ng. 1991. The complete larval development of Zebrida adamsii White, 1847 (Crustacea, Decapoda, Brachyura), reared in the laboratory. Journal of Crustacean Biology, 11(2): 292-304.
Suzuki, K. & M. Takeda. 1974. On a parthenopid crab, Zebrida adamsii on the sea urchins from Suruga Bay, with a special reference to their parasitic relations. Bulletin of National Science Museum, Tokyo, 17(4): 287-296, pl. 1.
Utinomi, H. 1969. On the occurrence of a rare crab Zebrida adamsi White (Brachyura: Parthenopidae) in the sea of southern Kii region. Nankiseibutsu, 11: 37-40. (in Japanese).

Suzuki & Takeda treat the crabs as parasites, and your observations seem to confirm it: crabs (usually a pair) obviously feed on epithelial tissues of urchins. Crabs possibly use urchin's pigments and toxines for protection. I think normally crabs' impact on the host is very slight, but sometimes, as here, it appaears to be rather significant, although I don't really know how fast the urchin is able to regenerate the lost tissues, maybe it's not so bad as it looks

there is a small possibility that the grazed area was not caused by crabs, but I don't think so

and yes, Periclimenes colemani is also a parasite as it clears a particular area of the urchin from pedicellaria and smaller spines, creating a sort of "living room"

on the other side, the crinoid crab Harrovia albolineata from the same family (Eumedonidae) has not been confirmed as a parasite and is probably a simple commensal, using the feather star as both permanent shelter and feeding place, but not consuming host's tissues - but then again one can argue that the crab steals food from the crinoid (in form of organic particles trapped by the crinoid arms) and is a kind of "kleptoparasite" ... whatever !

hope this helps

cheers
Art

[Art]

small correction to avoid confusion:

"normally the presence of a symbiont is advantageous to the host ..." I forgot to add that it is just an example ! obviously the presence of a parasitic symbiont is not advantageous at any time, although even here some people have argued that it may "keep fit" the immune system !

cheers
Art

[lindai]

I for one would like to thank Art for his great reply. I am always interested in learning more about the marine life, and it is difficult to obtain information like this. Thanks Art for taking the time to respond to this and other posts!
Regards, Linda I.

[Cybergoldfish]

I'll second that Linda!
I'm not sure that the damage to the urchin was down to the crab, it looks more like predation by starfish, snail or fish. Whilst in lembeh several of these guys were getting picked on by hogfish, and more obviously by mimick octopus.
Would the zebra not be ruducing its protection offered by the host?
I've noticed the presence of Zebras in Tube anemone and sea apples too.

[Cybergoldfish]

It seems an awesome task for these little pincers to do that kind of damage.
Pix are for scale...

[Andy H]

Thanks Art for an informative reply.

I was also sceptical due to the amount of damage on the urchins, though it did vary from individual to individual. And this was one of the worse cases I found.

I did see them on a few other types of urchin but at the time never looked for this damage. Though it definitely wasn't so apparent.

Incidentally, my favourites were the juveniles (only a few millimatres long) found on the 'radiata' urchins:-

[Art]

Hi guys

>>> I've noticed the presence of Zebras in Tube anemone and sea apples too.

hmmm ... are you sure these were zebra crabs ??? the swimming crab Lissocarcinus laevis is also brown-black with white markings, and is known to associate with tube anemone, its congeneric L. orbicularis is also B&W-coloured, and associates with various sea cucumbers, Bohadschia spp., Stichopus spp. etc. (to my knowledge not sea apples though)

by the way I am searching for good pictures of sea cucumber associates: the eel-like fish fish Carapus spp. and various crabs

re the impact of Zebrida on their hosts: personally I believe that the crabs are kind of neutral commensals with frequent shifts to parasitism (when they feed on host's tissues) and mutualism (when they defend or clean their host)

I am not sure that the grazed area on Alex' photo was caused by Zebrida, but it is possible, sea stars usually don't attack toxic sea urchins, attacks by fishes would look different .. maybe it's a kind of microbial infestation producing those dead patches on urchins ? but on the other hand, it's probably not by chance that the crabs are found mostly on grazed urchins, according to locals in Lembeh ..

cheers
Art

[Art]

ooops, it was ANDY's photo !!! sorry

[Cybergoldfish]

I think there may be another photo of the zebra/tube anem. lurking around; taken in Gila Banta... One will search!

I have some assfish shots lurking too!