FAQs on Freshwater
Related Articles: Fresh to Brackish
Crustaceans, Invertebrates for
Freshwater Aquariums by Neale Monks, Forget Crawfish Pie,
Let's Make a Crawfish Tank! By Gage Harford
Related FAQs: Freshwater
Crabs 1, Freshwater Crabs 2, &
FAQs on: FW Crab Identification,
FW Crab Behavior, FW Crab Compatibility, FW Crab Selection, FW Crab Systems, FW
Crab Feeding, FW Crab Disease,
to Brackish Water Crabs,
FW Crustaceans 1, FW Crustaceans 2,
Terrestrial Hermit Crabs, &
Marine: Hermit ID,
Hermit Behavior, Hermit Compatibility, Hermit Selection, Hermit Systems, Hermit Feeding, Hermit Reproduction, Hermit Disease/Health, &
2, Crayfish ID, Crayfish Behavior, Crayfish Compatibility, Crayfish Selection, Crayfish Systems, Crayfish Feeding, Crayfish Disease, Crayfish Reproduction,
Purple Thai Devil Crab - eggs???
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
<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.
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
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
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
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
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,
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
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
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
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,
<Most welcome. Neale.>
Purple Matano Crab Breeding
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
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.
*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
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
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,
<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
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
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
<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
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
Should be interesting to find out what the two juveniles develop into.
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
<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!
Parathelphusa reproduction 10/9/15
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.>
Brown back crab
Hi I have a female brown back that was pregnant and over the
resent months I have seen a couple of babies but I am confused as they seem to
be of different sizes. And just recently I saw another baby that only looks like
it was just born. I did have three males when I first got them plus the female
but the other males died not long after she became pregnant. They
died from different causes. I am just wondering why the young crabs seem
to be hatching at different times.
<Holthuisana spp. produce eggs that the female carries around.
After hatching out, the juvenile crabs are essentially self sufficient, feeding
on the same sorts of soft fruit, detritus and carrion as the adults. But they're
also highly vulnerable to predation, including from the parents.
Feeding adequately may also be a problem, especially if their diet lacks iodine
(most easily supplied using marine aquarium vitamin supplements at half dose, or
specialist crustacean foods containing iodine). In the wild these juvenile crabs
predominantly inhabit wet leaf litter and do poorly kept permanently submerged
(as I'm sure you know, these crabs are more
amphibious than aquatic, and need a pet frog-style set-up with a dry land area
as well as a shallow water). If you build the dry land area to include some
coconut fibre or moss, it will be easier for the juvenile crabs to stay out of
harm's way, especially when moulting. Various rocks and bogwood roots can be
used to create further hiding places. It is important though the land area is
fairly warm (18-22 C is fine) and humid. Warming the water with a regular
aquarium heater while using a pane of glass (or acrylic) that almost (but not
quite) covers the top of the tank to keep the warm, damp air inside the tank
will work nicely at minimal expense. Allow a 1 cm gap at each end for fresh air
to get in though. Provided you make these tweaks, and provide a calcium-rich
diet, you will provide the best environment for your adult and juvenile crabs
(these crabs are very hardy, and the fact several have died already suggests
you're doing something wrong with their maintenance). There's probably a lot to
be said to moving juveniles to a separate terrarium built along the same lines
if you want to
produce large numbers of them. Hope this helps, Neale.>
Re: Brown back crab
Thank you so much for replying.
I am interested though on why I have seen over the past 6 months a big variety
in sizes of these young crabs I have three that are around the size of a fifty
cent coin and then have spotted a couple that are only the size of a five cent
coin and when I was looking at the tank only the other day I spotted two that
are extremely tinny and look like new borns, even though
we only had one female mature crab that was pregnant. I am confused by the
<May be multiple broods per mating; may be varying amounts of food/calcium
available so varying growth rates within broods; may simply be genetic/sexual
differences... hard to say. Some crustaceans are parthenogenic, but I don't
think these crabs are.>
reproduction question 12/15/14
I hope you can help me?
I have a tropical fish tank, with approximately 15 fish, along with one
crab . ( which I believe is a red Thai crab) .
<Perisesarma bidens; a common species in the hobby. Actually a brackish
species, but lives a few months to a year in freshwater without
problems. Not a community animal though, which I hope your
retailer explained. Needs land, brackish water, iodine drops for
successful moults, sometimes attacks fish, highly territorial.>
I have recently noticed, a baby crab! I'm very confused though as we
have only ever had the one crab. Nothing has been added to the tank for
over 6 months, so I don't understand how this crab has reproduced alone?
If you're able to respond with any suggestions I would be very great
<Crabs like these don't have babies. They produce eggs that they send
off into the sea (they live on seashores and mangrove swamps) where the
larval crab can develop in the plankton. Eventually the larvae
metamorphose into miniature adults, crawl out onto a beach somewhere,
and grow up through a series of moults. So a baby Perisesarma bidens
that was "born" in your
aquarium is not probable. On the other hand, there are tiny Thai Micro
Crabs (Limnopilos naiyanetri) in the trade, and you might have got one
with some plants or something like that. It's also true that crayfish
produce miniature baby crayfish, and these can survive in communities.
Furthermore, parthenogenesis is known from some crayfish, meaning a
female can produce youngsters, effectively clones, without a male being
Re: Crab reproduction question 12/15/14
Thank you for your quick response.
I may have the breed of crab wrong, we were never actually told what
breed it is, I've just looked on Google images and this is the closest
match I could find.
<Indeed. Red Claw Crabs are the commonest "freshwater" crabs in the
hobby (despite not being freshwater animals... hmm...). They are cheap
and cheerful, though as I say, don't breed in freshwater.>
The plants in the tank are not living, so came from the shop in a sealed
plastic box. There is literally nothing else that has been added in the
last six months....?
<Curious. Are you sure it's a baby crab then? Neale.>
Re: Crab reproduction question
Yes, definitely a baby crab. Myself and my partner both saw it on
Saturday, although we haven't seen it since and unfortunately didn't
think to take a photograph...
<Well, there are crabs that have "direct development",
which means the females hold large eggs under their tails from which
emerge fully-formed young. These are in the family Potamonidae. But the
family Sesarmidae, from which come the Red Claw Crabs, do not have
direct development and instead produce planktonic offspring of the type
described earlier. It's unlikely these would survive in an aquarium. So
that's the context. Without a photo of the "baby crab" it's hard to say
what it is. Cheers, Neale.>
Gecarcinus quadratus; repro. 6/9/14
Happy spring! Well, Olive the Halloween Moon Crab has mated 'again'.
Last year she had a bunch of eggs on her bottom...same thing this year.
Nothing came of it last year, but we had just a fountain for a water
source...since then we have sectioned off 1/3 of a 20 long aquarium for
a filtered water feature. They love it, but I don't think it will help
with actually having the eggs survive into baby crabs. I just don't want
to her to become too stressed out and die because of this. Popeye (her
buddy) is still in the tank with her. She has dug out an extensive
burrow in front of the aquarium in the sand next to the water feature
(we taped a piece of black paper to the front of the aquarium so she
would stop scooting away every time some walks into the room) and he is
staying in the burrows they have dug out in the coco peat section of the
tank. Any advice? Thanks. Heather
<I'm assuming this/these crabs are Gecarcinus quadratus, which release
eggs into the sea. So unless you introduce her to a vivarium with a
container of seawater, then remove the eggs to a marine aquarium
designed to support sufficient plankton (a major chore, believe me) then
breeding isn't going to happen. Give you've kept this/these crabs going
for a year now, probably best to carry on with what you're doing.
Halloween Moon Crab, repro. 4/24/13
My Halloween moon crabs mated. The female (Olive) is carrying the
eggs on her belly. I live in Northeast Ohio, both crabs live in a
20 long enclosure with coco peat, brackish water fountain and plenty of
places to hide.
What should I do???
<...Mmm, depends... likely nothing... Are you interested in trying to
raise the young? I'd be reading re Gecarcinus quadratus reproduction,
and if so, starting to rear food organisms for the young. Bob Fenner>
Re: Halloween Moon Crab 4/24/13
I'm not sure if I should set up a different enclosure for her, or for
<For the female if you want to try raising the young... this is quite
involved. Better to rear in a specialized hatcher...>
I don't know how delicate she is so I am not sure if she should be
<Once the shell re-hardens (a few days) easy to handle>
Her water fountain is an 8 inch square, should I increase her water
availability. I don't think there is much of a chance for the baby
<There is not... w/o their removal, raising through stages, feeding
but I would really hate to lose Olive from this. I was thinking of
setting up a sandbox with a water table, along with a sizeable water
area (non filtered), and putting some sort of coral food into the water.
I am not sure yet, I have some research to do. I just found out
she was with eggs this morning. My main interest is to have her
drop the eggs. From
what I understand, these crabs live in rainforests and migrate to the
ocean to mate/release eggs. I did not think they could mate in
<Oh, they can/do. BobF>
Re: Halloween Moon Crab 4/24/13
I think I am going to just take out the rock structure in her fountain
and see if she drops the eggs in the base of it...if/when I see her
<You need to be raising food (for the third time); like weeks ago>
I will take the fountain base out and place it in another enclosure.
I am taking this one step at a time. I added some white sand to
the fountain base, she will drag it out of the water if she goes in
there, so it is a way for me to keep track of her going into the water.
My main concern is for Olive, as long as she can survive this, I will be
satisfied. I hope that she feels comfortable enough to drop the
eggs. I partially covered the enclosure with paper, to give her
some privacy. She seems to be positioned at the opening of her
burrow, as if she wants to come out, but is too shy. She normally
never comes out with all the kids around, so that unto itself is
If/when she drops the eggs in the fountain base, I will move it to a
separate enclosure, add some more brackish water, plankton supplement
and see what happens.
I will keep you posted.
Heather and Alexander
<Ah, welcome. B>
O' Freshwater Crab Questions - 05/02/2004 What is the condition
in which the female crab will lay her eggs? <This is very, very
dependent upon species; some will need brackish or marine conditions
before they will mate, and a few species do not; temperature and pH may
play a factor in meeting the animals' breeding needs.> Do the
eggs have to separated from all other fish? <Crabs will carry their
eggs in a sort of a trapdoor hinged pouch under their
bellies. The eggs should not be loose or separate from the
female. However, it would probably be a good idea to isolate
a carrying female, to protect the eggs and hatching young from
predation - from other crabs as well as the fish. Wishing
you well, -Sabrina>