JMT's Crabs Per Tank Guidelines

This is where you discuss the conditions of your crabitat -- temperature, humidity, substrate, decorating, etc.
User avatar

Topic author
Jedi Tech Support
Jedi Tech Support
Posts: 1803
Joined: Sun Jan 11, 2004 3:05 pm
Location: Nerima district of Tokyo, Japan

JMT's Crabs Per Tank Guidelines

Post by JediMasterThrash » Tue Nov 27, 2012 10:19 pm

First lets explore what type of space your tank needs.

(Initially this assumes a normal tank without a substantial amount of 2nd level area. Later on I will address that, but you can generalize up to a 50% increase in the number of crabs with a substantial 2nd level)

Basically, each crab needs room/individual space for the following: digging/burrowing, hiding/resting, and climbing.

As a whole for the tank, you also need 2 water bowls (saltwater and freshwater) and a food bowl.

So, take a look at your tank after you've got the three bowls in it. The rest of the floorspace is currently substrate. You'll need room for at most 1/2 of the crabs to be buried at any given time, and for their burying spot to not be overcrowded itself. I first suggest not placing any of the bowls in the corners, because crabs like to bury in the corners (actually they like to bury up against something, which would be the corners, sides of tank, and sides of deeper bowls).

That's a starting number. Now you need to add playthings/hidey places/climbing toys/decor. After you have all your decor in, you need to make sure that there is room for every crab to hide and feel secure. This could be inside of a plastic cave or log, behind some cork or cholla, or amongst the leaves of a fake vine.

Between these two things, you should be able to get a good feel for how many crabs you can fit comfortably in your tank (and what sizes).

You can use these things to gauge how overcrowded you are on your own, and how many crabs you think you can get before upgrading your tank. If you want to get more crabs, look at your tank, and see if you can find a spot available for him to hide, and see if you can pick an unused spot of substrate that he could dig in. If you can't find these places, then your tank isn't big enough for more crabs.

Observed Crabs Per Gallon Guidelines.

I use the size of the shell, rather than the size of the crab, to simplify sizing.
JMTs Shell Size Categories: wrote: Teeny = marble
Small = quarter-50c piece
Medium = 50c piece-golf ball
Large = golf ball-tennis ball
Jumbo = baseball
These are my sizes. There is no widely held consensus on what constitutes different sizes.
JMT's Crabs Per Gallon: wrote: 5g: teenies only.
10g: 2 large OR 4 medium OR 6 small OR 8 teenie
20g: 3 large OR 6 medium OR 9 small OR 12 teenie
30g: 4 large OR 8 medium or 12 small OR 16 teenie
40g: 5 large OR 10 medium OR 15 small OR 20 teenie
55g: 2 jumbo OR 6 large OR 12 medium OR 18 small OR 24 teenie
75g: 3 jumbo OR 8 large OR 16 medium OR 24 small OR 32 teenie
92g: 4 jumbo OR 10 large OR 20 medium OR 30 small OR 40 teenie
You can of extrapolate combinations of sizes from this. For instance, if you have only 1/2 the listed number of larges, then you would also have room for 1/2 the listed number of mediums.
Some Extrapolations: wrote: 10g: 2 large OR 4 medium OR 6 small OR 8 teenie
----> 1 large AND 2 medium
----> 2 medium AND 1 small AND 2 teenie
30g: 4 large OR 8 medium or 12 small OR 16 teenie
----> 2 large AND 4 medium
----> 1 large AND 2 medium AND 4 teenie
55g: 2 jumbo OR 6 large OR 12 medium OR 18 small OR 24 teenie
----> 1 jumbo AND 6 medium
----> 1 jumbo AND 1 large AND 1 medium AND 2 small AND 3 teenie
Substrate Depth Considerations

The substrate should be twice as deep as the biggest crab. So you need room in the tank to have 6in of substrate, and still have enough height above that for crabs to climb decorations, so obviously a 10g isn't tall enough to accommodate jumbos.

Tank Height Considerations

The tank needs to be at least 24in deep before I'd put jumbos in it, since they need 6in+ deep substrate, and big stuff to climb on, and not have the stuff they climb on reach the top, since they are strong enough to push open the lid and escape (ask Christa!)

Second Levels

If you have a good second level, you can increase these numbers by 50%, since there is more second-level space to crawl and hide in, and with some of the crabs on the second level, there's more open substrate space to dig in.

Water Bowl Considerations

Any crab medium and above needs deeper/weighted water bowls, because they'll topple them when they dig if they are too light.

Also keep in mind that if you want to avoid regular baths (and many experienced crabbers believe regular baths and flushing their shell-water are too stressful), you'll need to have water bowls big enough for your crabs to wade in, and submerge their shell. And the water bowl needs to have some method for smaller crabs to crawl back out once they get in. Sponges work great, but plastic mesh or coral are other good options.

I personally find that in order to have water bowls big enough to allow self-bathing, you'll need to have at least a 30g tank with a second level, or a 55g tank. So I can't personally recommend anything smaller than that. These poor crabs used to have free reign of the beach and mangroves, it's not fair to keep them in a tiny cage all their lives. Smaller tanks do make good ISOs however.

Also, if you give your crabs exercise external to their tank, or give them regular baths, never dig up buried crabs to do this. Only take crabs visible on the surface. I also recommend that you don't deep-clean more often than once every 3 months, to give your crabs a chance to bury and destress/molt without being disturbed too often by being dug up, cleaned (crabs maintain the salinity and acidicy of their shell water, cleaning it out before a crab molts is not a good thing in this crabber's opinion).

A Surface Area Analysis of Crabs Per Tank

It's been my experience, having varying numbers of crabs over the years, that roughtly no matter how many crabs you have, about 1/3 will be buried/digging at any given time. Either for fun, molting, pre-molt, or destressing. Roughtly 1/3 will be visible, and roughtly 1/3 will be on the surface, but hidden.

So, there are three numbers to look at when determining your maximum # of crabs, and use the smallest of the three.

The first is the surface area of the substrate to determine what 1/3 of your crabs could be underground. This could vary from 9 square inches for teenies to 100 square inches for jumbos. My 92g has 1017 square inches surface area. Divide by an average of about 36in^2, that gives 28 crabs.

The second is the # of surface hiding spots for the 1/3 of your crabs that are up but hidden. This can include corners, spots under moss or plants, and places inside shelters. I estimate at least 30 easy hiding spots, especially since up to 5 can easily hide inside one log shelter (in fact, when I had them in a tub while I was deep-cleaning my tank, I had about 30 crabs jammed solid like a tetris into a single plastic cave. It was amazingly hilarious, and completely physically impossible as far as I could tell. What happens if the guy in the back decides he wants out? I guess start pinching and hope for the best).

The third is the # of open spots for crabs to climb/bask. Adding a second level is good for increasing this. I estimate about 30 good places to climb.

So the minimum number is 28, multiplied by 3, says I could have 84 crabs. I currently have 52, and can definitely say the tank doesn't look crowded.

Counting the 3 levels (substrate, surface, and 2nd level), that's 1017*3in^2 / 42 crabs = 58in^2 per crab. That means that each crab has roughly 8in x 8in space of its own.

The surface area analysis for a 10g tank would be 10in x 20in times 2 (buried plus surface) = 400in^2, divided by 36in^2 per crab equals 11 crabs, which again is about twice my average crabs per gallon guideline.

Caveat On the Surface Area Analysis

"What?" You say? Somebody set us up the bomb? 84 crabs? You must be nuts! Bear in mind that this was an average of 36in^2 per crab, predominately small crabs. Obviously you can't fit 84 medium, large, or jumbos in there. Consider 30 teenies, 30 small, 20 medium, and remember that this tank is 3-feet radius corner. One time I binged on 18 teeny tinys I saw at a pet store, and they literally made no dent in the tank population. They just disappeared straight into the woodwork (or chollawork).

Comparing the Surface Area Analysis to Crabs Per Gallon Numbers

I'll use my tank as an example:
92g: 4 jumbo OR 10 large OR 20 medium OR 30 small OR 40 teenie
Given my distribution, this equates to 2 large, 5 medium, 15 small, 6 teenie, add 50% for my second level, this totals 42 crabs. Compare the crabs per gallon result of 42 crabs (which equates to 1 crab on average per 8in x 8in square of surface) to the surface area analysis of 84 grabs (which is 1 crab on average per 6in x 6in square of surface). (all times three levels in my case).

The surface area analysis is generally about twice the crabs per gallon analysis. There is no one right answer. But being more conservative and easier to understand, I'd stick with the crabs per gallon numbers for now. But more advanced crabbers with large tanks can start looking more at the surface area analysis.

Crabs Growing Up

I would not listen to the "But when they all grow up to be jumbo's you won't have room" argument. Except for one person who only had two crabs, nobody yet has had all of their crabs grow up to be jumbos. I figure when that day comes, you'll be happy to get them a larger tank.

Teenies and smalls grow to be mediums in a few years
(See my "Putting an Age To Crabs" article) ... ge#p711437

However, it's going to be 20 years before your mediums reach jumbo-hood. It's not going to happen overnight. So don't worry about that now.


I'd say my crabs per gallon guideline may conservative for larger tanks. But I think it's a good starting point. I think if you have a larger tank it becomes easier to go beyond the guidelines, because each crab in theory can have the entire tank to roam, and it's only the 1/3rd that are buried that will collide, and the bigger the tank the deeper the substrate and they are less likely to run into each other.

But in the end, it really is on a per-tank basis, and you have the best ability to estimate how many crabs can fit into your tank. Look at how big your crabs are, how much substrate they have, and how many hidey spots and climbing areas they have. And see if you can find a spot for every crab.

And remember that sometimes in nature, it's 50 crabs per gallon! ... G-7963.jpg

Move Zig for great justice!

Stuck-up, half-witted, scruffy-looking crab-herder since '92.