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Author Topic: 1 in 5 crabs ... question need help!  (Read 7937 times)

Offline uksceptic

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1 in 5 crabs ... question need help!
« on: 25/08/2009 22:05:01 »
Hello all,

I am a regular listener to the Naked Scientist podcast but must confess I don't post on here often. I work in TV and am currently working on a nature documentary and my director has put this fact in the script that I need to fact check and I am almost certain it is wrong!

Crabs are a particularly fascinating seashore dweller. 1 in 5 of them live in shells they’ve taken over from mussels.

Any help from any experts would be much appreciated. I have emailed a Crustacean biology dept but it being the Summer I haven't had any responce as yet. This is my take on it;

I cannot find any specific information to confirm or refute this comment but it is almost certainly not true. Most crabs grow their own shells as soft tissue underneath their existing shells which they then moult once they have out grown them. They then hide while their new soft shell now exposed to the elements, hardens. It is mainly hermit crabs that take over shells from other creatures and even then it is most likely empty mollusc shells not mussels. Mollusc’s account for a huge amount of species so this fact maybe true of mollusc shells; specifically from the gastropoda class of molluscs.


 

Offline JimBob

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1 in 5 crabs ... question need help!
« Reply #1 on: 25/08/2009 22:27:44 »
Google hermit crabs and fiddler crabs - to my knowledge, hermit crabs are the only ones that take over mollusks shells. Fiddler crabs have HUGE population numbers - millions on one beach alone. I doubt this fact is correct. But then again, I am not a population biologist specializing in crustaceans. Many many other types pf crabs other than hermit crab species are around so I am with you on this?

I am moving this to the proper section as well. No real problem; just house-keeping.
 

Offline RD

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1 in 5 crabs ... question need help!
« Reply #2 on: 26/08/2009 07:42:14 »
There are tiny crabs which reside in the living mussel ...

Quote
The pea crab, Pinnotheres pisum, is a small crab in the family Pinnotheridae that lives symbiotically in oysters, clams, mussels and other species of bivalves.
http://en.wikipedia.org/wiki/Pea_crab

As they are minute there are probably a helluva lot of them.


Quote
Of 10 New Zealand green-lipped mussels taken in Four Fathom Bay, Pelorus Sound, New Zealand, eight hosted pea crabs of various sizes. Their presence does not harm the host, which may be harvested and consumed as usual.
http://en.wikipedia.org/wiki/New_Zealand_pea_crab
« Last Edit: 26/08/2009 08:45:53 by RD »
 

Offline Don_1

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1 in 5 crabs ... question need help!
« Reply #3 on: 26/08/2009 09:09:24 »
There are over 6500 species of crab, of which over 500 are Hermit crabs. The Pea crab, I think, may be counted as a single specie under the family name of Pinnotheridae, but there are a vast number of these tiny crabs which lead a symbiotic life with just about every bi-valve.

I would not be overly surprised to find your director is correct.

It would be interesting to know for sure, so please do post the answer, if you manage to get a definitive one.

Thanks uksceptic.
 

Offline LeeE

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1 in 5 crabs ... question need help!
« Reply #4 on: 26/08/2009 16:15:53 »
I don't think we should be talking about crabs that have a symbiotic relationship with bivalves as the crabs neither 'take over' the bivalve shell, supplanting its original occupant, and nor do they occupy empty bivalve shells, which is what specifically what the statement "1 in 5 of them live in shells they've taken over from mussels." implies.  [my empthasis]

The "1 in 5" figure needs to be clarified too; does this mean one in five species or one in five individuals?  It is almost certainly the latter that was the intended meaning, in which case you need to research the total populations for all the different species of crab and then see if the population of discarded mollusc shell dwellers is one fifth of the total crab population across all crab species.

I think this is one of those questions which, once it has been clarified, needs some solid numbers to be properly answered and where instinctive answers may be misleading.
 

Offline Don_1

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1 in 5 crabs ... question need help!
« Reply #5 on: 26/08/2009 16:55:00 »
The Pea crab lives within the bivalve shell. The crab benefits from the protection of the shell, while the bivalve bnefits from the crab keeping the inside of the shell clean.
 

Offline LeeE

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1 in 5 crabs ... question need help!
« Reply #6 on: 26/08/2009 17:18:37 »
The Pea crab lives within the bivalve shell. The crab benefits from the protection of the shell, while the bivalve bnefits from the crab keeping the inside of the shell clean.

Don, I don't want to sound too FOGgy but how is this relevant to taking over a shell?  The fact being disputed doesn't apply to symbiots but specifically refers to "shells they’ve taken over from mussels".

Afaik, although crabs may inhabit mussel shells in a symbiotic relationship with the mussel, none of them take over the shell.  In fact, I'm not aware of any crab species that takes over mussel shells - the only shells that are taken over are empty mollusc shells, which is what the original poster pointed out - bivalve shells are not suitable for crab occupation once they're empty because they're the wrong shape and would quickly separate into their two halves as the crab moved around.
 

Offline Don_1

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1 in 5 crabs ... question need help!
« Reply #7 on: 26/08/2009 17:33:44 »
I do beg your pardon LeeE, I just re-read the question "Crabs are a particularly fascinating seashore dweller. 1 in 5 of them live in shells they’ve taken over from mussels."

No. I don't think this is right, symbiotic relationships, yes.

Take over, no. The bivalve shell does not lend itself to a take over by a crab.
 

Offline uksceptic

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1 in 5 crabs ... question need help!
« Reply #8 on: 26/08/2009 20:38:17 »
Thanks very much for all your feedback.

The vagueness of the statement is not clear, which doesn't help.

First of all we are specifically talking about crabs that take over the discarded/empty shells of other creatures. So any crabs that form symbiotic relationships should be discounted. As should fiddler crabs which grow their shells. To my knowledge the only type of crab that uses shells from others is the hermit crab. If I am wrong in this please correct me!

I guess my question is are their enough hermit crab numbers to say that 1 in 5 crabs are hermit crabs?

I am fairly sure the end of that 'fact' is supposed to be molluscs not mussels unless anyone out there has seen a host of hermit crabs running around in empty mussel shells!?

 

Offline Don_1

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1 in 5 crabs ... question need help!
« Reply #9 on: 27/08/2009 08:12:12 »
Around 1 in 14 of the known crab species are hermits.

On reflection, I think that estimating the population of more than 6500 species of crab would prove impossible. So this figure of '1 in 5' must fall a million miles short of a 'stab in the dark'.
 

Offline LeeE

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1 in 5 crabs ... question need help!
« Reply #10 on: 27/08/2009 16:46:44 »
Perhaps one way of addressing the problem is to consider the size of the crabs.  For a crab to occupy a discarded or empty mollusc shell, the crab must be within a certain size range i.e. not too small that it can't move the shell and not too large that it can't get inside it.

Now in general, the smaller an animal is, the more of them there tend to be, so if there are species of crabs that are too small to use discarded shells there are likely to be many more of them than there are larger crabs that are able to use the shell.  The number of crabs that are too large to use a shell should be correspondingly less common than those that are small enough to use a shell.

If this is so, then a plot of crab numbers against crab size is likely to be logarithmic-like and should give you a very rough approximation of how many crabs out of the total would fit inside a discarded shell.  This doesn't tell you how many crabs actually use discarded shells, because just being within a suitable size range doesn't mean that a crab has to use a discarded shell, but it would set an upper limit that you can check against that one in five figure.
 

Offline AllenG

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1 in 5 crabs ... question need help!
« Reply #11 on: 29/08/2009 04:24:12 »
There are over 6500 species of crab, of which over 500 are Hermit crabs.

6500 ÷ 500 = 13

"Crabs are a particularly fascinating seashore dweller. 1 in 13 of which are spices that live in abandoned mollusk shells."


 

Offline Don_1

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1 in 5 crabs ... question need help!
« Reply #12 on: 30/08/2009 11:57:49 »
I think the actual number of known crab species is more like 6800 and I don't think this includes freshwater and terrestrial crabs, just the marine species, although I wouldn't swear to that.
 

Offline chris

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1 in 5 crabs ... question need help!
« Reply #13 on: 01/09/2009 18:52:46 »
Crabs are exquisitely sensitive to calcium concentrations, which they can "taste" with specialised chemoreceptors. They select their shells based on the amount of calcium leaching from them - in experiments in which hermit crabs were offered a choice between synthetic (replica) shells and real shells which differed only in calcium release they could taste the difference (they'd obviously been to Sainsbury's)!

I suspect that at least part of their shell-choice is down to this parameter.

Chris
 

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1 in 5 crabs ... question need help!
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