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.>

Purple Matano Crab Breeding      7/26/17
Hello Wet Web Media Crew, I wanted to follow up and see about the answers to my questions. Are you guys stumped like I am, or are you trying to do some research of your own to better answer my questions?
<First I've seen of this question, to be honest!>
This is the first time I've waited so long for a response (having been a week today), but I know these are advanced questions that I haven't been able to find an easy answer to. That being said, take as much time as you
need, just let me know what's going on, please. I might redesign the system I'm planning to house them in if certain factors won't work out or aren't necessary.
<Understood.>
*Details of the planned system* I'd like to set up a system for these crabs using 6 or 7 ten gallon aquariums that will be flow-through (since I'm not sure what determines their sex), but am concerned with issues with such a
system. For example, seeing as I plan to put a thin layer (half inch, maybe) of pool filter sand, a structure of rocks along the back and coming forward about 1/3 of the length of the tank (the tanks will be situated to provide maximum possible number of tanks with the "ends" [side panels] in the front and back instead of how is typically thought of), and a nice piece of driftwood front and center, I want to make sure the crabs will be happy and not have any water quality issues. The other key factor of this is the DIY PVC overflow with the water level only being a third to half way to the top (with a lid) to allow the crabs to exit the water if desired.
With such a low water level, I'm concerned with the feasibility of pumping water to the first tank and having an operating DIY PVC overflow to transfer that water to each of the other tanks as per the King of DIY's video on the subject.
<I agree with your concerns here. Even strictly freshwater crabs run the risk of clambering out of a tank if they can -- in the wild even saltwater crabs will leave rock pools in search of new homes if they feel confined or
stressed. So while your basic idea is fine, I'd be working around the idea of either (a) a single tank per crab with a fully enclosed filtration system; or (b) a large tank divided up with egg crate or cichlid tank separator to keep individual crabs separate but allowing for a free flow of water between the compartments. This latter is how I kept multiple Mantis Shrimps and works extremely well if done right.>
I also plan to have the seventh 10 gallon (or a plastic tote of larger volume) be the filtration unit for this rack. The overflows will transfer water from the front of one tank to the back of the next, with the water exiting and forming a sort of waterfall on the stack of river stones I plan to purchase and place. The pump will use a similar methodology, except pumping water from the back of the filtration unit enclosure to the back of the first 10 gallon in the series, as the final overflow (on the sixth aquarium) will simply overflow directly across (or down, if I keep the filtration below the rack) to save on resources.
<You *will* need to find a way to keep the crabs from escaping.>
*Concerns I'm facing, and some of why I asked the questions I did last week* If the crabs' sex isn't based on a factor such as conspecifics (other crabs of the same species) that are in the same area and their sex, but is
instead determined by temperature, genetics, or other such factors from a young age or during the prime of development, such a flow-through system is entirely unnecessary. I'm providing the flow-through since the sex might be determined by conspecifics in the same "area" (or rack, as the case may be in my tanks). Not sure what would happen with a crab that is completely isolated as far as developing into a male or a female, but that's not
something I want to test with these parental generation crabs.
<Have these crabs been bred in captivity? I'm finding very little about Syntripsa spp. reproduction. Freshwater crabs have wildly varying modes of reproduction, from the basic model (march to the sea/river, release planktonic eggs and hope for the best) through to species that brood their eggs like crayfish do, releasing miniature versions of themselves only when the baby crabs are developed enough to fend for themselves. Without knowing about Syntripsa reproduction, I think it's really difficult to plan a tank specifically for their breeding. My guess would be (as lake dwellers) that they either brood eggs or release relatively large eggs that quickly hatch into baby crabs, rather than having a prolonged planktonic stage, but I really have no idea. This is something you'll have to research. Obviously rearing crabs with a planktonic phase is hard. Sexing crabs is fairly easy though -- females tend to be smaller and have a broad flap-like 'apron' under the body whereas the males have a much narrower equivalent structure.>
*Conclusion* These are all answers I hope to answer with every type of freshwater, brackish water, and saltwater crab I might keep in the future (though I may stick with just the freshwater ones since there's so many),
along with many other questions I'm coming up with as things progress. The distinct lack of scientific info and inquiry on many crabs we keep in the hobby are why I've chosen to do this and focus on truly freshwater crabs
for the time being, possibly expanding into brackish water and saltwater in the future as mentioned. I fall in love with the Purple Matano Crabs I currently keep on a nearly daily basis, so the passion is there. I was just hoping you could help me out with some of this seeing as I would like to keep them properly, breed them successfully (to study the crablets, as well as having a source of revenue in the future), and ultimately further our knowledge base of these wonderful creatures. That being said, I completely understand if you guys don't know or can't answer certain parts of my questions. It's been difficult to find much info on these questions, but that's why I ventured to ask you since you guys are expert biologists from my understanding and I just have my Bachelor's in Conservation Ecology (BS from Arizona State University, 2017).
<A good grounding for what you plan to do, no doubt. I'd be hitting Google Scholar, finding out about Syntripsa spp. in terms of reproduction; and where lacking, making comparisons with its close relatives among the Parathelphusidae.>
Thank you once again. I hope you can provide some insight into the challenging questions presented in my previous email that I've expanded on in this one to follow up.
Best Regards, Jacob
<Good luck! Neale.>
Re: Purple Matano Crab Breeding      7/26/17
Neale, From my understanding, true freshwater crabs are those that have the suppressed larval stage.
<Seems reasonable, but recall that many freshwater shrimps have a marine larval stage. I think you would need to confirm the ecology of your chose Syntripsa species before planning on breeding them.>
Such crabs ... as you mentioned ... have fully developed crablets hatch out of the eggs. Others that march to the sea may be classified in a family of land crabs while not being truly freshwater since they still need some
fashion of saltwater to reproduce. I didn't know about the releasing of eggs that hatch into crablets, though! I thought they all brooded the eggs like crayfish.
<Since freshwater crabs evolved multiple times in many families, I'm sure there's great variation. I simply don't know enough about this group to be sure -- but as a zoologist, I'm minded to be skeptical of any hard-and-fast
rules.>
I have tried the egg crate method of separation for these crabs in my 100 gallon. Unfortunately, as I was introducing them into the tank one climbed through.
<Try tank dividers then; PennPlax make quite nice, easy to install ones. If possible, combine with undergravel filtration because these do restrict water flow, which undergravel filters bypass rather well.>
Several then followed suit (all except the largest could climb through)...
I documented it in one of my YouTube videos. Once they get to breeding size I might revisit it, but until then I'll likely just try to have separate filtration with a mattenburg filter just behind the start of the waterfall.
Should be interesting to find out what the two juveniles develop into.
<Understood.>
I hope I can figure out at what point to introduce the two crabs together (male and female), as well as the technique I 'should' use so I don't end up accidently killing the only female I know I have so far.
<I fear trial and error! But more realistically, large tanks might help by reducing stocking density, as well as numerous hiding places so that females can hide away when required. Ideally, if males/females very different in size, burrows only the female can fit in. Much like breeding aggressive cichlids, really.>
Would you recommend me looking into how others breed brackish and saltwater crabs for such info on the technique of intensively breeding crabs?
<Worth a shot! Crab farming is a thing, so you will find info online about Mud Crabs (Scylla serrata) and other species farmed in tropical Asia and Australia.>
Do you know anyone I can contact specifically, or do you possibly have advice you can give me from your own experiences?
<I know of none personally, and I would imagine that with these new aquarium species, any aquarist who knows about breeding them would be extremely secretive about it! But you could certainly try the usual forums like Planet Inverts and Pet Shrimps.>
These and related crabs have little info at all that has been studied scientifically using the scientific method, and likewise I don't think such crabs have been bred in captivity.
<Nor do I.>
I'll definitely do more research into these crabs, though. Thanks again!
Jacob
<Cheers, Neale.>

Parathelphusa reproduction      10/9/15
Hello,
<Hello Jacob,>
Do you by any chance know how many young one can expect when breeding Parathelphusa spp crabs?
<A small number by crab standards... suppressed larval stages, produce miniature 'crablings' instead... so likely comparable to crayfish and the like, dozens of juveniles rather than thousands of eggs.>
I'm going to be breeding the panther crab specifically (Parathelphusa pantherina), but if you only know the brood size of a related species in the same genus that'll be fine.
<Hmm... no specific breeding data on this species that I'm aware of in the aquarium literature... would suggest Google Scholar and use of relevant search terms... Parathelphusa for example.>
I've been looking for this info for quite a while, and would greatly appreciate the help. Thank you!
<With most of these crabs, the problem/challenge is stopping the adults killing each other. Parathelphusa are supposedly less bad in this regard than others, but still... wouldn't bet the house on it. Good luck, Neale.>