Purple Thai Devil Crab - eggs???     3/18/18
Hey there! I work at an LFS, and a while back due to a clerical error we received a big shipment of Thai Devil Crabs instead of micro crabs. Mind you, not vampire or red claw crabs - these are the big, beefy purple ones.
<Oh my! One of the so-called Soapdish Crabs in the trade, which need to be packed individually, sometimes even in soap-dishes, to stop them killing each other!>
Just a little bit not what we had in mind!
<I'll say. Interesting pets, but very much one-to-a-tank creatures.>
Anyways, we had to scramble to figure out an appropriate habitat for them, and you guys were really useful in finding information for these fairly uncommon critters. I think I've got my coworkers turned on to using your site for reliably accurate info.
<Ah, that's nice; thanks for these kind words!>
Out of our original 12 crabs, we've sold down to 3.
<Not bad!>
We've had them housed this whole time in a brackish setup maintained at 0.01 salinity, and it's been working out well. As their numbers have dwindled they get happier and more active every day.
<Sounds good. They may be aggressive towards one another, and are certainly opportunistic cannibals, so keeping these, or indeed most other crabs, in groups tends to end badly. Since a singleton only really needs something like a 5 gallon tank, they're not super-demanding in terms of requirements, but they're definitely not social or community animals.>
Due to a maintenance issue with the brackish setup, we moved them temporarily to an empty freshwater tank we deemed suitable for them, intending on moving them back in a few days. Within 24 hours, the biggest crab started practically throwing little yellow eggs across the tank.
<Nice!>
We've vacuumed some of them up to keep them safe from the other crabs, but she just keeps laying them! I can find virtually NO information about this.
Everywhere I've turned I'm finding advice regarding vampire crabs or red devil crabs, but nothing about Thai Devils.
<Right now, I don't think anyone even knows which species of crab this is.
That's the main problem here. They may be Geosesarma species, like the Vampire Crabs. In this species the females carry the eggs until they're ready to hatch, at which point she dumps them in a freshwater pool, where
the tiny crabs hatch out and make their own way into the leaf-litter to hide away from predators. What might astonish you is that these Vampire Crab species were discovered and sold in the aquarium trade for many years
before scientists had a chance to identify them! Anyway, the Vampire Crabs were then described as Geosesarma dennerle (the purple species) and Geosesarma hagen (the red species), which you might recognise as names of German manufacturers of aquarium equipment. The Thai Purple Devil Crab remains a mystery to me, at least. Besides Geosesarma species, there's a good chance it could be one of the rice field crabs, Parathelphusa species, or else a Syntripsa species like the Syntripsa matanensis, or else a land crab Cardisoma species such as Cardisoma carnifex -- all of these have purple colours on them, and do turn up in the trade. You might care to look up photos of these species and genera, and see if anything looks particularly close to your chap.>
I guess I'm wondering if any of you knew if this was normal or healthy?
<It's a good sign.>
Do these crabs scatter eggs, and is their egg laying stimulated by fresh water, or do you suppose she flung them out due to stress from the move?
<Could be either, really. Changes in their environment are often 'spawning triggers' for many animals, indicating, for example, the rainy season.
Since these crabs are semi terrestrial, they're well able to walk between freshwater pools, dry land, and slightly brackish pools, and it's probably worth offering all three habitats, initially at least, to see which they prefer. Given we simply don't know what species the Purple Thai Devil Crab might be, it's really hard to state precisely which spawning or living conditions are optimal.>
If this is healthy or normal, what in the way of special care might the eggs require if they're fertile?
<If they're fertilised, the eggs will hatch and either larvae or mini crabs will emerge. If the former, brackish or even marine conditions will be most likely, but the mini crabs just get on with life from the get-go, and simply need to be kept away from potential predators, which includes one another! A soft marshy habitat with lots of moss or leaf litter is probably ideal, but you might equally try a shallow tank with warm water, a sponge filter, and various bits of stone or wood for hiding places and to allow them to climb out of the water.>
Would they need a salinity level like their parents, or do the larvae start out in fresh water and move on down to brackish later?
<Difficult to say. The mini crab type just get on with whatever's around them, so salinity isn't an issue. They'll probably spend more time on land anyway, only dipping in the water to refresh their gills. The larval type
are much more difficult to rear, and probably not worth fussing over. But to cut a long story short, what usually happens here is the larvae drift down to sea, or at least some friendly mangrove, develop there, and then when they have metamorphosed they start working their way back into rivers or dry land, as the case may be.>
I know this was a bit wordy, feel free to edit it down. There's just not much I can find online, and while I've been searching for hours the eggs are still sitting in a bucket of fresh water. I hope they're okay!
<So do I! I'd like to see a photo or two; might help to ID these crabs.>
Thanks for listening,
-Meg
<Most welcome. Neale.>