I thought I would provide some input on this thread. I have two decades of experience with crabs, and have experimented with a dozen different water pools.
For 18 years, and 250 crabs, I have never once had a crab get into something that they could not get themselves out of.
It really is absolutely amazing. The number of times that I have been scared for my crab, thinking they were stuck and would die. The number of times I thought I'd have to saw something apart to save them. The number of times I thought I'd have to rescue them from a deep pool. But after time after time watching them always somehow be able to get themselves out of whatever they got themselves into, I'm no longer worried.
I've have 5 crabs lodged in the center of a 18in cholla log with a narrow hollow center, only accessible at the ends, in such a manner that I could not physically get them out. Two days later they had all freed themselves (magic!).
http://home.comcast.net/~JediMasterThra ... nkleft.jpg
I've had crabs lodged up inside the branches of my large plastic bonsai tree, and somehow they get back out.
I've had crabs get their shells completely cemented with wet/dried sand 6 inches underground during a molt, and the crabs were able to abandon their shells, crawl up 6 inches of substrate naked, climb up to the top baskets and find a new shell, and it's happened a dozen times.
The issue with crabs and heat rocks is a different puppy altogether. Crabs are cold blooded and cannot sense heat like we do, so they don't know if they are too hot or too cold. Just if they are comfortable or not. That's why you can put sick crabs in the freezer and they will freeze without any struggle (not recommended!), and why they will burn themselves on heat rocks. Cold blooded crustaceans will usually just slow down body functions in cold temperatures, and speed up activity in hot temperatures (possibly why you see high activity when your tank is at a high temperature - it doesn't necessarily mean they enjoy it).
I used these step-down shallower pools for a long time (you'll have to look a bit to see the pools in the pictures).
http://home.comcast.net/~JediMasterThra ... /pool1.jpg
http://home.comcast.net/~JediMasterThra ... ltpool.jpg
The pools were shallow step-down on two sides, with the largest depth (1.5in) on the back side. Crabs would often fall off the rim into the deep side, and I was amazed at how they would try, unsuccessfully to get out via the high edge all the time, rather than crawl up the stairs for an easy out. It never made sense to me. But always eventually they'd figure out how to get out. I put the sponges in just to make sure the little babies could get out. I'd often see crabs perched on the top of the sponges, but they were just eating them. My crabs were always eating sponge. It was a running joke in my house.
I would manually clean out the pools every week and replace water often. I wanted to get aeration to improve water quality, so I started getting waterfalls.
For a while I used this waterfall:
http://home.comcast.net/~JediMasterThra ... lclose.jpg
The basin at the bottom is roughly 3 inches deep with water, and very narrow. I was always worried a large crab would get down there and not be able to get out (or a small one for that matter). As you can see in the picture, I added sponges so that any small crab would have a way to crawl out. I watched many times crabs fall off the waterfall and end up all the way at the bottom, but they always managed to get out with ease.
I later acquired this tabletop fountain (it's the one that looks like a staircase):
http://home.comcast.net/~JediMasterThra ... eplat3.jpg
The basin is also 3 inches deep. In this one, I used some sponges, and also filled a bunch of shells and coral in the bottom. I'll never put shells or coral I care about in a water bowl again though, as they got permanently stained from the cocofiber and poo. I would occasionally find crabs completely submerged in the pools, and it is often on purpose (though sometimes on accident too lol).
But aeration alone is insufficient, so I got filters. i found a tupperware at a garage sale to house the waterfall, so that there was a large enough basin for the filters. Cycling water is not effective below half-gallon, so you really need large pools in order to effectively filter it (5-gallons is idea for cycling water. with half-gallons I still have to do a major water change every month to prevent nitrate buildup):
http://home.comcast.net/~JediMasterThra ... eWater.jpg
In the case of the above pictures, the only way out of the 3in deep pools was the sponges. I'd still get nervous when I saw a crab in the bottom of the pool, but they always managed to get out. They didn't always use the sponge. They would often crawl out by using the 120v cord for the filters. In fact, this was their preferred method. In fact, I think they just plain enjoyed climbing on the 120v cord. It probably gave them a good buzz. Especially the crab that snipped it (and survived!).
I put my fingers in the water to pull the filter out, and got shocked! I touched again and there was definitely a shock in the water (like touching an electric fence back at the farm, when you're trying to convince your cousin that it's safe to touch the electric fence). Well that's how I discovered the cord had been snipped. Apparently it doesn't "phase" the crabs.
Later on I created safety-nets out of plastic mesh. This way I no longer had to keep sponges in the pools for the crabs to use to crawl out.
http://home.comcast.net/~JediMasterThra ... /Pools.jpg
Now they can easily crawl up and down the sides easily. In fact, they enjoy being able to hang onto the sides, because this allows them the opportunity to dip their shells in the water exactly as far as they want. Also, the mesh "ladder" extends on the outside as well. This allows me to place the pools up on stilts, so that they do not use up any molting/burying substrate. The crabs have 4 inches of substrate under the pools to bury in.
This picture shows the "stilts", which are the cheapest set of mugs I could find at the goodwill
http://home.comcast.net/~JediMasterThra ... P_2876.jpg
http://home.comcast.net/~JediMasterThra ... P_2884.jpg
Land crabs will drown if submerged for a length of time. But anecdotal unintentional experiments (usually forgetting about crabs in baths) has shown that they can survive 90 minutes to 3 hours submerged without ill effects (but DON'T try this at home kids!). The modified gills still do absorb some oxygen through water, so they won't drown within minutes like most mammals.
Shallow pools are definitely fine, so long as the crab can shovel water into their shell from the water, that's all they need. But many members have had crabs that enjoy submerging (I personally have had only a few). In my experience, and that of many others, there has never been any noted detrimental affects from willful or accidental submerging (forced bathing excluded). My crabs mostly prefer to hang out on the edges and drink from there. But providing them the large pool does no harm to them, and it is extremely helpful in keeping the humidity up, and also keeping the surrounding substrate moist. And as noted before, larger pools are required for effective filtering, and as long as you provide means of escape (ladders, sponges, etc), a submerged crab will find their way out.
Most crabs that crawl to the bottom of a water bowl they could get out of, but died, were likely weak and dying already. I've witnessed weak or PPS crabs crawl into the water bowls and die several times. This is one of the reasons in my "how to make an ISO" article I recommend only using shallow water bowls for ISO. This way sick or weak or PPS crabs can't submerge. So yes, it is best to prevent weaker crabs from submerging. But healthy crabs should have no problems with it.
http://www.hermitcrabassociation.com/ph ... hp?t=76148
There are always exceptions to every rule. There will be healthy crabs that drown in deep water they could have crawled out of. There will be crabs that survive in an open air kritter killer full of woodchips for 4 years. Someone will have a murderous crab that eats molters, others will never have aggression with hundreds of crabs.
I guess my real conclusion is to remember what I wrote in this article:
"To Understand Why There Is No Right Or Wrong To Crab Care"
http://www.hermitcrabassociation.com/ph ... hp?t=57785&
Basically every 'tat is different. Every home is different. Every climate is different. Every crab is different. Your mileage may vary.
It was very informative and I enjoyed reading it.
