Mystery Crab - can you identify it???

[marine_hawaii]

So two questions. This kind of crab is plentiful in the fore reefs in Hawaii. Anyone know what kind of crab it is?

Related question: for a reef tank with live-sand bottom, how much "turning" of the sand is a good thing?

This little crab digs into the sand, and sets to filtering it. He'll then come up and move to a new location. I'd guess that on a standard 55gal, he'd dig up the top 2" of sand across the entire bottom of the tank in a week or so.

This crab does not damage inverts or play bulldozer. It doesn't bury under rocks, etc. Just in the open sand area.

It buries itself slowly, there is barely any cloud of "dust" stirred up by it's action. And when it runs it's filter, the water coming out is clear.

This crab is currently in a tank with about 1~2" of sand on the bottom. A resident mantis shrimp digs down to the glass once a month or so. All inverts are doing well. Good coraline algae growth (no kalkwasser supplemented).

[Aloha Corals]

Knobby Box Crab (calappa gallus)

Native to Hawaii & Indo

AC-Kevin

[marine_hawaii]

Thanks! Good ID.

Any thoughts on plenum digging?

[marine_hawaii]

Some info I found:

Calappa gallus


Exclusively a marine species
The species is generally observed in a sandy/silty area.
The species is generally seen moving in its environment.
The species is generally found on or close to the seafloor.
The species can be found between 1 and 15 Meters.
Uncommon Species

General behaviour of the species
Species generally solitary
Hides in the sand.
The animal is rather indifferent to the diver, though it remains on its guard
Maximum size : 8 cm
The animal is a hunter low in the food chain.
Diet : Omnivorous
The animal can show variable colouring
The species can be eaten but is of no particular culinary interest

[Personal data:

These crabs are quite common in fore-reef areas in Hawaii. They are shy but fairly easily spotted during low tide. It is rare to find a crab smaller than 2" or larger than 4". All seem to be about the same size (3").

]


Another pic:

[Poseidon]

As a rule I keep crabs OUT of my tank, especially ones with large POINTY claws.

[Aloha Corals]

As a rule I keep crabs OUT of my tank, especially ones with large POINTY claws.

Agreed. I got one little bugger in one of my frag tanks that only comes out at night. I try, but i just can't catch the darn thing. That's what i get for not sifting the live sand before putting it in the tank.

[Doodaa]

I've only had one hitchhiker crab and I caught him using the jar with bait trick. Got a wide mouth jar which in my case was a pint sized wide mouth mason jar, put some bait in it, set it in the tank at a slight angle propped up against the rock I saw him the most often hiding, and the next morning he was in there. The jar has to be at a slight enough angle not to scare him from going in, but severe enough that once he's there, he can't crawl back out...also has to be deep enough that he can't hang on to the lip and get the food without falling in. I used a meaty blend of food...Ocean Blend's Formula One frozen cube.

[marine_hawaii]

Ok, I'm in total agreement that MOST crabs should be caught and crushed (er, flushed). But SOME crabs are good. How about blue-leg and red-leg hermit crabs?

So in that SOME crabs are not as terrible as their bulldozer/home-wrecker cousins, the question is:

Is the Knobby Box Crab the best "sand sifter" you can get for your sandy bottom reef tank?

First some quotes from some good articles:

From "Live Sand: Beautiful & Functional"
By Bob Fenner

Should you periodically stir your Live Sand, even vacuum it? In my opinion, yes. A bunch of infauna will die consequently, but the effects of sifting are warranted: removal, reshuffling of mulm, release of trapped gasses, reassortment of life forms... all make stirring, vacuuming "worth it".

And you don’t have to do all the work yourself. Biological helpers in the sand-sifting category abound. Various gobies do greater or lesser substrate disturbing, and Goatfishes (Family Mullidae) and Jawfishes (Family Opistognathidae) make this a full time career.

http://www.waltsmith.com/media/pdfs/sand.pdf
or
LiveSand


The Plenum - Could It Work For You
By Peter Cunningham

The upper levels in the substrate will become home to various organisms which can either be introduced via the aquarist or are organisms which have relocated from their home in the live rock. These are very important, the same as they are in a deep sand bed. Their job is to keep the upper levels of substrate ‘turned over’ and clean from both detritus and waste.

The Plenum - Could It Work For You | Aquarists Online | Aquarium Fish Resources And Information

...

So these two articles seem to be in agreement that "turning over" or "sifting" of the sand in the bottom of a reef tank is a necessary thing.

I've seen a lot of different critters who "sift" sand. But all are just amateurs compared to the Knobby. From my observations:

This crab spends the majority of its time buried with only it's eyes sticking out above the sand. When it is feeding, a constant flow of water (and sometimes fine sand particles) can be seen shooting up an inch or so from the top of the crab (just below the surface of the sand). This plume tends to be very clean (I guess it would depend on how much muck or very fine sand is in your tank thou).

When the Knobby crab moves, it runs along the sand poking it's legs about 3/8" into the sand and turning that layer (looking for food, I would assume). As a shy crab, if it sees you while it is roaming around, it will immediately dig into the sand using a pumping/jackhammer method (sounds bad! but it only stirs up a bit of sand) and gets less shy over time.

This crab is NOT interested in the live rock in the tank. I've had two Knobbys in two different tanks. One with large live rock, another with smaller pieces. The large rock tank has more sand bottom, and the small rock tank is stuffed full of live rock. In neither case did the Knobby crab ever go near the live rock, burrow beneath it, climb on it, etc. Both only roam around in the sand area.

As for the claws on this crab, they are more like "sheilds" than "pincers". They are practically ornamental as far as thier use is concerned. Each claw covers the front of the crab like a sheild. The pincer area is very small and quite weak. I've never put two Knobby crabs in together, so I couldn't say if they would fight or not. I've never seen two together in the wild, so my quess would be they don't get along very well (might make for an interesting "battle tank", but the local blue crabs here in Hawaii are way more aggressive, so I'd opt for them if setting up such a tank!).

So without drawing this out too long and lenthy, my observations of this crab lead me to believe that is is the best "sand sifter" out there. And despite the (well deserved) terrible reputation of many other crabs, these crabs are sand-centric -- they just aren't interested in anything except filtering sand.

So for a reef tank in need of some sand cleaning, detritus removal, general sifting, how can this crab be beat?

Bob Fenner talks about gobies as being good sand sifters. I've had a few different gobies, and they were horrible, tunneling, bulldozing sand MOVERS. Kobby crabs don't bulldoze, don't dig tunnels or pits, they sift the top 3/8" surface constantly and also submerge themselves in the sand for deeper filtering activities.

My guess would be that Bob has never seen a Knobby crab in action (although his reef articles are otherwise top knotch).

One final thought. When the Knobby crab travels along the surface, just checking out the top layer for food, it creates a very interesting surface pattern. On the surface of the sand, the larger particles of sand (pea sized pieces of coral rubble, if there) are brought to the top and arranged in what looks like a very nicely raked zen garden.

Any crab that can do that and doesn't bother with the liverock or other inverts is a winner in my book.

So any thoughts about how this crab would be a bad recruit for a reef tank?

[Ninong]

Some info I found:Calappa gallus
Maximum size : 8 cm
The animal is a hunter low in the food chain.
Diet : Omnivorous
All seem to be about the same size (3")

Attached is a photo of Calappa gallus (yellow box crab). This one is a male, carapace is 1.75"x2.5". While this crab may be acceptable in a fish-only marine aquarium, it's not a good choice for a reef aquarium. This should be obvious from one look at it's claws, which are designed for opening gastropod shells. The Calappidae feed on small crustaceans (e.g., copepods) and other zoobenthos as juveniles but move on to gastropods as they get larger. One of these crabs would decimate your sand bed infauna as well as your snail population. IMO it's not a good choice for a typical reef aquarium.

[Ninong]

There is no evidence to support the notion that a plenum is any better than a deep sand bed. Some people choose to install a plenum in their reef aquariums but more people choose a deep sand bed without a plenum.

Other people choose to operate their reef aquariums with no sand bed at all.

[marine_hawaii]

Thanks for the posts Ninong, but maybe that's a different kind of knobby crab that the specimin that I have in my tank. The knobby crab I'm referring to has claws which could only be described as "ornamental". There is no way that the ineffectual claws could open up a marine snail shell.

As 2 crabs do not equal an acceptable sampling group, the next time I'm out on the reef, I'll pickup the knobby crabs I come across and let em pinch me. Typically I see about a dozen or so crabs (which I normally just ignore). So while less than a perfect sampling, I'd bet my fingers that the pinch wont amount to much.

Having worked with lobsters for several seasons back east, I know that "catcher" and "crusher" claws are normally specialized. This species of knobby crab has claws which are pretty much identical. In fact, the claws, when closed, are only bearly touching at the tip, with none of the "sissor" action of a catcher claw or the "grip" action of a crusher claw.

In any case, from my close observations of this crab, it has become readily apparent that the claws are more of a shield which the crab holds in front of it's face at all times. Unlike many other crabs on the reef here, it does not "wave" it's claws in defensive posture. It's claws are always held tight to it's "face". The claws are also paper thin and conform to the front of the shell (I'll try to get some good photos of what I'm talking about the next time I'm on the reef).

The photo above looks like the claws are massive, and maybe that the case for that species. But I'll post another shot of the claws to show how tiny they are.

...

So the question I'd like to get back to is how much "sifting" of live sand is good for a reef tank? Can it be "over sifted"? As these crabs don't dig deeply, but do continually sift thru the top 1" of sand (whether plenum or DSB), would this continual rotation have any negative influence on the de-nitrification we are trying to achieve in in the hypoxic zone?

[marine_hawaii]

Here's a semi-blurry shot. Notice how the pinching part of the claw is very small and does not completely close.

[Ninong]

I'm only going by the identification provided by Aloha Corals as Calappa gallus that you said was a "good ID."

I really can't see your pictures all that well. I'm basing what I said on the assumption that it is supposed to be Calappa gallus, or at least a member of the Family Calappidae (box crabs).

Maybe you and Aloha Corals can start over.

[marine_hawaii]

Good point. I guess it's not Calappa gallus.

[marine_hawaii]

I'll post what I find for anyone following along (anyone who can help id the crab in the top photo on this page, many thanks!)


The below crab has similar traits, but seems to be quite a big larger and the coloration is completely different.

Shamefaced Crab
Calappa granulata

One of the most amusing of the local crustacea, the shamefaced crab does indeed seem to be hiding its visage as if guilty of some social gaffe. In fact, those heavy claws which protect the more delicate parts of the crab are specially designed for peeling sea snails apart.

[marine_hawaii]

Another similar crab.



Shameface Crab - Calappa calappa
Photo Details Photographer: Ethan Daniels (see this users gallery)

The face of a Shameface Crab (Calappa calappa) on the ocean floor in Palau, Micronesia

[marine_hawaii]

This one is also similar:




Calappa hepatica (Linné, 1758). - Tuamotu, Moruroa atoll, coll. Poupin (leg. of this sp. to M. Tavares collections) sand 2 m, carapace width 55 mm. Copyright J. Poupin.

2

Calappa hepatica (Linnaeus, 1758). Male carapace 28.9x47.5 mm, Clipperton, coll. Le Chuiton, June 1996. Coloration altered by the preservative. Photo L. Albenga.

3

Calappa hepatica (Linnaeus, 1758). Tahiti, Lagoon at night, 0.5 m. Det. Bacchet/Poupin. Copyright P. Bacchet.

4

Calappa hepatica (Linnaeus, 1758). - Moorea fieldwork December 2006, Haapiti mangrove, December 2006. Copyright J. Poupin.

5

Calappa hepatica (Linnaeus, 1758). - Wallis Island, Halalo mangrove, st. 24, male 29x44 mm. Copyright J. Poupin.

[marine_hawaii]

That last photo by J. Poupin shows the kind of claws I was talking about. More like a shield than something that could open up a marine snail.