HCA: Hermit Crab Association

I did some research a while back on just that, and based my current crabitat on it.ClimatologyThe gulf cost regions (PP, and presumably similar E climatalogy) is:Summer daily variance:Humidity: 60-90% (predominantly 65-85%)Temperature: 70-100oFWinter daily variance:Humidity: 60-90% (predominantly 65-85%)Temperature: 60-80oFAffects Of FluctuationFirst thing to note: High daily fluctuation, which indicates that constant temperature and humidity is unnatural. In fact, steady states allow for hamrful bacteria to develop unabated. Fluctuations keep things better in check.Second thing to note: Humidity is not directly perceived by hermit crabs. Rather, relative humidity is a measure of how close to saturation the water vapor content of the air is. The lower the number, the more water can evaporate and the quicker it will evaporate. As hermit crabs need to keep their gills moist, low relative humidity cuases them to dry out faster, which requires that they either stay clamped in their shells to use their shell water to keep their gills moist (less activity), or they bury into moist substrate to create microclimates of higher humidity to keep their gills from drying out (also perceived as less active crabs).So, higher humidity means the crabs can be more active and out without drying out their gills. In practice, it seems that upper 70's is a sweet spot for most crabs.However, climate is multivariabled. Crabs are also observed to be more active and healthy in higher temperatures, even in the presense of lower humidity. Through thermoregulation, they will use their shell water to keep cool as necessary. But their shells serve as insulators as well. So if it is too hot, they will appear more active, or to hang out of their shells more, to cool off. The evaporating water from the higher temps and lower humidity serves to cool them off well.Some species of crabs (straws and Es) appear to enjoy warmer temps as their sweet spot (upper 80's, lower 90's), while other species appear to prefer upper 70's, lower 80's.Not the same, but related to temp, is light. And hermit crabs are observed to increase activity with UV light, and this is probably a natural result of most animals requiring some UV light for vitamin D synthesis.ConclusionSo, my conclusion is that a crabarium should provide daily fluctuation in temperature and humidity with averages near 80oF/75%. I double the amount of heat lights during the daytime to acheive this. A crabarium should provide a source of UV light, which if nothing else, kills bacteria and mold. And a crabarium does not necessarily need to have high humidity in all areas of the tank, as long as there is a water supply and damp substrate to provide microclimates. I tend to actually have a lower average relative humidity because of this. And the crabarium should be allowed to have a lower average temperature during winter months, by up to 10oF (but keep the average at least 75oF, but if it drops to lower 70's at night in the winter, it's OK). Providing insulation on the bottom and sides of the substrate (but not UTH) is probably something we should do.Variances Within the CrabariumThe daily fluctuation should cover the whole tank - we're not talking about gradients, which would be variations among different areas of the tank at the same time.So if you have a RH gauge in a fixed location, that gauge's reading should go up and down during the course of the day. I have a flukers digital, and it stores a min/max, and I occasionally reset it, and then check it in a couple of days to see what my min/max ranges are (both RH and temp).And RH is the correct gauge to measure how fast moisture will evaporate. Absolute humidity is probably fairly constant during the day in my tank. It's the change in temperature that drives changes in RH, since warmer air can hold more water vapor. Near the ocean, the sun will evaporate more ocean water, which will drive up AH, but we don't get that effect in our tanks as much, unless you have daytime-only bubblers or misters.Gradient or micro climate is still important for other reasons, because the crabs need places to go to obtain the conditions they want at that moment. Nature naturally fluctuates; it's crab's natural behavior to move around or bury to compensate.Also, a note, those daily variances are maximums. It won't go up and down 30oF every single day. Crabs don't live in Minnehsootah, where it's well known to go from 70oF and shorts to 10oF and blizzard in under 24 hours. ^_^Inverse VariationsActually I've thought I might be going backwards in my tank, with the higher RH at night (due to lower temperatures). This is because the absolute humidity is constant in my tank, so RH is just inversely proportional to temp. However, I think that at least near the beach in the wild, the sun radiating on the ocean during the day might increase absolute humidity enough to sometimes raise RH during the day. But it could depend on many factors. So far my crabs haven't been walking upside down, so it seems to be working out all right. Smaller TanksThe main issue with smaller tanks is: 1. Temp. With a larger tank, I can take over 100w of overhead lighting, so it's easy to set extra 25 or 15watters on timers. However, for a smaller tank, 15 or 25w might be all it can take, so it's hard to find an inbetween for day/night fluctuations. A combination of a low-side-mounded UTH and light may work. 2. Humidity. With a larger tank, there's room for larger bowls, which add humidity when the lights heat them up. With a smaller tank, you may be pressed for space for larger bowls. However, a bubbler or humidifier set on timers may work. Whatever you choose to try, report back on what you did and your results!GradientsThe humidity fluctuations listed about show that they could daily swing from 60% to 90%. Humidity will naturally gradient in the tank as well, being higher near damp substrate and water with bubblers/falls, and lower near lights and openings. I have not yet noticed any particular species variation in humidity requirements, but if you check in the caresheets forum, lots of members have their own opinions on the best ranges for each species. Air QualityAs for stagnant air, you are absolutely correct, though that's a topic I've dealt with in different posts. Air needs to be circulated. For a small tank, propping the lid enough works, as there isn't much air to diffuse. For larger tanks, more needs to be done. I have two tropicaire humidifiers running on a 55-90g air pump. So I have a constant inflow of humid air, which pushes older air out the openings. Keeping the inflow humid helps prevent the humidity from decreasing from just opening the top more or otherwise blowing in house air. Other ideas I toyed with before that were creating an inflow and outflow (using computer fans) on two opposite corners of the tank. Since normal house humidity is around 35-55% in our area, opening the tank up too much isn't a good solution for me.Final ThoughtsI think that the hermit crabs will grow hardier with regular fluctuations. I think that constant conditions make them too accustomed to it, and it makes it more difficult to deal with changes. If they're used to regular fluctuations, then an hour or day or two in a different condition might not induce stress in them as bad. If you have a large amount of substrate and don't deep clean or change very often, constant conditions allow bacteria and microbes that thrive in that condition to grow unabated. But if you have regular fluctuations, it keeps things in check, so you don't end up with an unbalanced ecosystem in your substrate.

Thanks for all the information! You're the gold standard of crab care.

@JMT, I allow my enclosure to self adjust during the day and night by giving the right amount of air circulation, heat and humidity I usually get a fluctuation of five percent on my humidity from day time to night time and around five to ten degrees play on the temp between day and night with micro swings in between.I've found that not trying to keep the temp or humidity at an exact level has helped my cast to better deal with short lapses of heat or humidity loss.

oh crud, i have been stable with 77 at night and 81day temps but otherwise stable for oh about 6 months humidity is high 70, always...should i shoot for more fluctiations?

Sonny,Like everything else with crab care, there's levels of care, from basic to advanced.providing fluctuations specifically (by using timers and multiple sources of heat and humidity) is quite advanced, and can provide improvements. But it doesn't mean that not having these isn't good as well.Compared to petstore dry and cold condition and wire cages, hitting the 75-80 temp & humidity is already a crabby dream, constant or not, and already gets them 95% of the way there.But there's always more ideas, more possible things to do. Even I haven't accomplished them all yet. Every couple years I try to see what else I can try or do to the 'tat.That said, the point of this post was actually just that you don't necessarily have to "overly" worry about having constant conditions. For a while a few years ago it was regularly passed around that conditions had to be completely stable at the perfect temp & humidity (i.e the 75 & 75). And a lot of crabbers would fret and struggle at not being able to keep that constant value. But I think not being able to keep it constant isn't necessarily a bad thing, as long as the swings are within their normal habitat range, and they don't sit too long in either a too-low or too-high condition.

thanks for the info. i am always willing to try and better the life of my crabs and i truly like learning,thinking , plexing over ideas. i have "peak" time .for me that is 10:00 to 3:00 where the sun or uvb now is on ,it used to be uv heat,but i took from kuza and got the uvb , 2 do bask and this changes the daily heat from night ,only 3 degrees. i had assumed with the iso and adjusting new crabs to "perfect" conditions that it should never change. this does really help one just enjoy, thanks again love to read all your stuff.