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Let's Talk About ~Crabs~

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NaH2O

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Don't be shy....let's talk about your crabs. The good, the bad, and the ugly of these critters. Why are they sometimes labelled as "evil"? Let's also hear about good experiences for balance. What crabs have better reputations in reef aquaria?

Let's start off with hermit crabs. Although, not "true crabs", these little guys are quite interesting. Hermit crabs are true decapods, but they don't really look like crabs. They have the "hard" front, but their back end is soft. This soft portion is protected by a snail shell (or the like), which they insert their body in. To differentiate between "true crabs" and hermits....hermits follow the rules of the front appendages being modified claws, and instead of 4 pairs of walking legs....hermits appear to only have 3 pairs of walking legs. The 4th pair is just in front of the abdomen, and is used to keep the shell on, which does not usually allow them to be visible.

For sure, hermit crabs are popular in the trade. Some species are "better" than others. But a crab is a crab, which means they are omnivorous scavengers, so one must be prepared to find an occasional specimen munching on a prized possession, stealing a snail shell from the snail, or pulling other hermit crabs out of thier shell....because the shell appeared better. Which brings me to my next point. When adding hermit crabs to an aquarium, provide empty shells in order for the hermits to shuffle between. They will be less likely to steal from each other. According to Anthony Calfo and Bob Fenner in Reef Invertebrates, most hermits ultimately cannot be recommended in any significant number for mixed invertebrate displays. This means, to me, that some cleaner packages offer too many hermits for a system. Has anyone put a very large number of hermits in one system?

Someone once asked Dr. Shimek what he thought of hermits in a reef tank, and here was his response (Hermit Crabs in a Reef Tank)

The masses of hermits that folks seem to be having in their tanks create a decidedly abnormal situation, as such herds are simply not found on most reefs. Their crawling over animals such as corals and sponges is abrasive and destructive. They also remove food from sessile animals such as corals. Additionally, hermit crabs are at least partially predatory and the bigger ones are particularly a problem in this regard. Some of them are significant predators on small snails, other small crustaceans, and each other. When folks see hermits gleaning materials from the rock surfaces, they are seeing the hermits pick off small animals as well as small plants. This results in an abnormal surface that has only the animals and plants that are hermit crab resistant living on it. Additionally, at least some of them appear to be predatory on some of the worms living in the sand, and I think this is decidedly a problem.

Upon the removal of hermits from my reef (and their replacement on the sand by Nassarius snails as scavengers) I have noticed a significant new growth of smaller polyped animals, as well as small feather duster and other tube worms. I have not noticed an increase in algae. Basically, I feel like the hermits are an unnecessary complication in high numbers. Some species I think don't belong in an aquarium under any circumstances, but others like the small blue legs are probably benign if there are only one or two in a system.
Let's hear your experience when talking about hermits. I've found Scarlet hermits to be less aggressive. I currently have 3 scarlet hermits in my tank, and they seem to be picking at the detritus on the live rock (however, after reading Dr. Ron's quote above, I'm not so sure). I do keep empty shells for them to change into.
 

Angelscrx

Import Fish
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Ettrick, VA
Man that is kind of depressing since I love my little hermits both red and blue legs! I know that the better informed we are the better we can make our systems so this is the kind of good news that can be bad! (I hope that made sense?) I did have an emerald crab that didn't bother a thing until I bought some really expensive blue and purple zoos and he decided to give them a taste! he was exiled to the sump/fuge. My hermits don't seem to bother anything but each other but I have read that blue legs tend to find Nassarius (or as my wife calls them Nascar) snails a nice snack and then steal their shell. That is the only down side to them that I have seen. The red legs I don't keep in large numbers in my 55g I have four. In my 150g I think I will switch over to Nassarius snails and drop the blue legs. I will have a shallow sand bed and don't really want it littered with shells. My 2 cents :)
 

NaH2O

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Jan 25, 2004
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One point Dr. Ron made in the above quote, that I don't necessarily agree with is this: "Their crawling over animals such as corals and sponges is abrasive and destructive." I can kind-of see what he is getting at, but I don't know how destructive a walk across a coral can be.
 

gobie

dave the gobie
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Dec 2, 2004
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those points are why when I had a crash in my reef last fall and lost most if not all of my lower inverts. I went with a more diverse selection than the mass algae/ detritus packages that are avalable. Often times they offer blue legged hermits w/ astrea snails. and i guess this works to some extent when the snails die the hermits move into their shells. this type of mass mix works for some.but not for me. Blue legged hermits are too predatory for my tast, Specially upon snails and crabs. Even the few left in my tank are constantly fighting eachother. I have never witnessed my scarlets or zebras do this but wouldnt doubt that they do. most of my algae removal is relyed upon my sallylightfoot crab. I threw a piece of live rock that was in my fuge. it had caulerpa on it both grape and Prolifera in 24 hours the sally lightfoot picked it clean. it seamed to leave the sponges and tunicates and worms alone.
 

The R/C Man

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Feb 20, 2004
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Spokane WA
What bothers me the most is the itching........... Oh, Ummmmmmm, Never mind! :D Just kidding.......

My emerald crab does a pretty good job a munching on the algae in the tank. He does tend to get a hold of my fire fish's fin every once and a while though. For the most part he is good.

I enjoy my larger hermit crabs for their colors but I am getting a little fed up with having to stand my frags back up all the time... :rolleyes:

Generally speaking all the crabs that have been in my tank have been pretty well behaved. They are definetly a good part of the clean up crew....
 

NaH2O

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Jan 25, 2004
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LOL, Greg!

Let's talk a little about Emerald/Mithrax Crabs (Mithraculus sculptus) and Sally Lightfoot Crabs (Percnon gibbesi). Size and behavior come in a wide range with crabs, and these are no exception. Remember, crabs are omnivorous scavengers.

Emerald Crabs are often purchased for use on nuisance bubble algae (i.e. valonia). As The R/C Man posted with his experience, emerald crabs can also be a threat to small fish. Some have wonderful experiences of emeralds doing a great job on nuisance algae, however they are omnivores, and that must be considered. Perhaps start out with a smaller sized crab, then once it misbehaves, banish to the sump. I have one in my tank....very small, around the size of a nickel. Does a good job picking at algae, but I have been watching it pick at foraminiferans, too.

Sally Lightfoots are another popular crab, and as gobie has experienced, can be a good algae eater. Small individuals will feed on nuisance algae and scavenge meaty foods. Larger individuals have been known to go after small fish, like emeralds.

Is there anyway to tell if a crab is going to be "reef safe"??? What can we look at when trying to determine whether or not a crab will be an algae eater?
 

cronicreef

Active member
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Jul 18, 2004
Messages
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I had a larger emerald crab awhile ago but after seeing him move a larger pc of LR back and forth trying to get under it, I got him out of the tank. He also liked to move my frag around on the rock work. Ate up a few of my hermits too.

Also I notice that emerald grow really fast. The one I had doubled in size after one molt. I couldn't believe it.

So emerald crabs aren't by anymeans my favorite.
 

gobie

dave the gobie
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Dec 2, 2004
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Auburn
picking crabs is like picking fish there is a rule of thumb but you are never truly going to know till you get it in your tank. the rule of thumb is hairy crabs are bad and smoothe ones are good. i dont know how accurate that statement is. thre is a good artical about crabs in the Marine Fish and Reef 2005 annual. if you dont read this mag you should at least give it a shot it has such contributers as CHARLES DELBEEK, MARTIN A. MOE JR., DR RON SHIMEK, JULIAN SPRUNG.
 

dgasmd

Fragologist Magnus
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Nov 30, 2003
Messages
473
Location
Florida
Crabs? Well, there was that girl from Venezuela...... :shock: :D Sorry, wrong forum.............

I for one am not a fan of crabs at all. NEver had them actually and was happy without them. Then, I decided to follow someone's advice against my better judgement and got 300 small one and 300 snails. Now, 3 months later, 90% of the crabs are MIA and 60% of the snails are dead. In retrospect, they never seemed to do much more than the snails did. They do something snails don't do though and that is eat the other snails :( :( Snails in good numbers always seem to do a much better job at cleaning in my opinion, but that is another thread altogether.

In the future, no more crabs ever. And for those of you wondering why, I have 3 words for you:Penicillin, penicillin, penicillin!!! :lol: :lol:
 

wrightme43

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bowling green ky
I bought the package from reeftopia with 100 sm blue legs, 100 astrea, 30 cerith, and 100 nasisrius. Or something like that. LOL The blue legs have wiped out the cerith snails, a good portion of the nasisrius, and a good portion of the astrea. On the flip side my moray eats the blue legs when he can sneek up, knock them over, and rip them out of the shell before they can get away. I know its wrong though I kind of enjoy watching it. LOL I also have one emerald crab left that I can not catch. He is the best at getting away I have ever seen. His brothers and sisters are in the quar. tank, for pulling polyops off the acros, or at least that is what it looked like to me. I like crabs, but I dont think any of them are totally reef safe. To me its a trade off, I like to watch them, I like the way they clean up after the fish, I like the way the pick at any algae that is growing. I dont like the way they kill and eat snails, I dont like them crawling on my corals. I am just going to leave it alone for now, my tank is doing well, so its best not to tamper with it. LOL Steve
 

Maxx

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Actually, I really kind of dig the acro crabs. THOSE are reef safe.
Nick
 

NaH2O

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Jan 25, 2004
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gobie brought up the point of Hairy vs. Smooth crabs. It may be true in some cases, but a better indication of whether a crab will behave appropriately, is to look at the shape of the crab's claws. Form follows function (to quote Anthony Calfo), so looking at the claws will tell you if they are bad. But again, we are talking about omnivorous scavengers, so any hitchhiker, IMO, get it out.

Acro crabs: Here is a thread, where a once thought acro crab turned bad. Is this bleaching?.

Here is my thought on acro crabs: Where would an acro predator most likely be found? Probably on an acro. I think keeping an eye on these guys is a wise thing to do, or if you have a colony going south, consider the crab. I imagine in the majority of cases, the hitchhiking crab in an acro colony is commensal, and won't cause any damage. Observation is so important to our tanks.
 

lunchie

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Jan 27, 2005
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95
Location
Georgia
Wow :eek: You guys have me scared to death about having crabs and I JUST put two emeralds in my tank this weekend! Of course there is nothing in our tank yet because it is a brand new tank -- see my photo gallery -- We were at the stage of algae and had to do something! So I pose the question:

If you don't use crabs for algae control then what do you use? Please tell me something besides snails :p
 

NaH2O

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Just keep your eye on them, lunchie. I already knew about crabs capabilities and added an emerald all the same. I've discriminated against him, and he is constantly under my watchful eye.

Snails are a help - a good variety. Also, algae is part of a new tank, which is sometimes tough on patience. Nutrient control is the best bet. Use of RO/DI water, and a good skimmer is helpful. Here is a thread about snails Let's Talk About ~Snails~.
 

Ed Hahn

Life is A Highway...
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Jan 27, 2004
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Kennewick, Wa
There was this one time in Band Camp, ...The girll said they would help eat detritus, and kill my outbreak of abalonia. :oops: oh, emerald crabs..sorry..
 

lunchie

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Georgia
Please...Something besides Snails

Nothing against the little guys...but there must be something besides snails to help control algae... :D
 

Maxx

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Jul 31, 2003
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Long spinned Sea urchins.........unless you have an acrylic tank.
Nick
 

wrightme43

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bowling green ky
Nick, is that because they can eat into the tank walls and scratch it up? Or some other reason? Just wanted to know for future reference. Thanks Man
 

Maxx

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Its primarily because of the way they eat....their mouth parts are different than the radula of snails, and will cause damage to acrylic tanks...I've seen it in person. I could kinda see them stiffening up when being harrassed by something in the tank and potentially scratching the tank with their spines, but I really kinda doubt it.
Mainly the design of their mouth/teeth.

Nick
 

gobie

dave the gobie
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Dec 2, 2004
Messages
366
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Auburn
Lunchie If You Are Looking Into A Fish That Eats Algae A Fox Face Rabbit Does A Nice Job. And Another Thing I Forgot To Post About My
Sallylightfoot Is She Is Very Territorial, Kills Emeralds And Other Sally's But Dosen't Eat Snails, Hermits, Or My 6" Squamossa Clam.
 
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