Crab Molting Issues

[Whiskey Fox]

Hello, i have an unusual pet which is a Red Rock Crab (Cancer productus) and just aquatic crabs in general are very hard to find good information about other than cooking.

I've had him for about 9 months now. I keep the water temperature and salinity matched to his natural home (Puget Sound) and have him connected to a Fluval canister filter. I also use water conditioner, have a cuttle bone for calcium, and alternate between feeding him algae pellets, fish meat, and wild prawns from the Puget Sound.

For the last half year, he has lost a lot of his red color, turned kind of a dull purple, extremities turned white, eyes foggy, and some specific spots of minor shell degradation. He has also become much more docile; there was one point over the winter that he didn't move for two months, but later came back to slightly higher activity.

My worry for a long time now is that he is experiencing a failed molt. I'm going to be testing for iodide but am wondering what other things may be contributing to this issue. I did previously have a small Graceful Crab who molted seemlessly and without me even knowing It was going to happen. I will say the difference in my Red Rock's activity did change when he was moved from his 40-gal with organic matter from the Sound into a sterile 10-gal (we had a major accident). He will be returning to a 40-gal soon after spending 8 months in the 10-gal.

Any advice or guesses are much appreciated! Looking for any other water tests to perform.

 

[ISpeakForTheSeas]

Welcome to Reef2Reef and sorry for your trouble!

How wide is the crab's carapace (not including the legs)?


Molting issues can be caused by a number of things, but basically it boils down to either stress, diet, or inadequate water quality. Some crustaceans also have issues with lighting sometimes, so that may be something to look into as well.

Anyway, I'll help get you started with checking for potential causes of any issues, but - just to put this out there - moving a presumably fairly large crab from an established (and presumably cycled) aquarium to a small, new aquarium (you didn't mention if it's cycled or not) is a recipe for high stress, and probably poor water quality as well if the crab is large enough.


So to start with the water quality:

I also use water conditioner,

What water conditioner do you use and what do you use it for specifically?

What water are you using (tap water, sea water, RO/DI, etc.)?


With crustaceans, some parameters you may want to check include:

-Calcium
-pH
-Nitrate
-Phosphate
-Alkalinity would probably be good to test as well.

With this specific situation, I also would suggest testing:

-Ammonia
-Copper

If you have reason to believe your water may have too high levels of contaminants or something like magnesium (this one is unlikely), then you may want to check for those too.


About your 10 gallon tank:

-Was the tank cycled?
-How many gallons is the canister filter recommended for?
-Do you have any other filtration (other filters, a refugium/cryptic refugium, algae scrubber, etc.) on the tank?
-Do you have any rock in the tank, or just sand?
-Does the tank have a sump?
-Can we get a picture of the full tank?
-What lighting does the tank get (sunlight, light fixtures, etc.)?


For the diet:

-How much are you feeding the crab each feeding?
-How often are you feeding the crab?
-What specific pellets, fish, and prawns are you feeding? How much of each are you feeding and how often?

 

[Whiskey Fox]

Welcome to Reef2Reef and sorry for your trouble!

How wide is the crab's carapace (not including the legs)?


Molting issues can be caused by a number of things, but basically it boils down to either stress, diet, or inadequate water quality. Some crustaceans also have issues with lighting sometimes, so that may be something to look into as well.

Anyway, I'll help get you started with checking for potential causes of any issues, but - just to put this out there - moving a presumably fairly large crab from an established (and presumably cycled) aquarium to a small, new aquarium (you didn't mention if it's cycled or not) is a recipe for high stress, and probably poor water quality as well if the crab is large enough.


So to start with the water quality:

What water conditioner do you use and what do you use it for specifically?

What water are you using (tap water, sea water, RO/DI, etc.)?


With crustaceans, some parameters you may want to check include:

-Calcium
-pH
-Nitrate
-Phosphate
-Alkalinity would probably be good to test as well.

With this specific situation, I also would suggest testing:

-Ammonia
-Copper

If you have reason to believe your water may have too high levels of contaminants or something like magnesium (this one is unlikely), then you may want to check for those too.


About your 10 gallon tank:

-Was the tank cycled?
-How many gallons is the canister filter recommended for?
-Do you have any other filtration (other filters, a refugium/cryptic refugium, algae scrubber, etc.) on the tank?
-Do you have any rock in the tank, or just sand?
-Does the tank have a sump?
-Can we get a picture of the full tank?
-What lighting does the tank get (sunlight, light fixtures, etc.)?


For the diet:

-How much are you feeding the crab each feeding?
-How often are you feeding the crab?
-What specific pellets, fish, and prawns are you feeding? How much of each are you feeding and how often?

Thanks so much for the reply! I wasn't expecting it to be so in-depth. I haven't been able to reply because i've been moving for the last several days (including moving the crab again) and it has been overwhelming. He was so stressed from the move that he plucked one of his claws off, which is something i've been kind of anticipating to happen. I'm definitely on high-alert with him now.

Sebastian is 5" in carapace width. You mentioned lighting and i will admit that his lighting has been fairly irregular. Lately i will have the LED lights on at night and then have the light off and the aquarium covered so it's moderately dark during the day, but not completely.

I should mention that during the move 8 months ago, he was inside the 40 gallon when we had an accident, resulting in the tank slamming to the concrete and shattering (and cutting my lateral artery in the process, sending me to the hospital). Sebastian was rescued by an EMT who brought him up to my room and put him in a room-temp bucket of salt water (removed from his tank earlier for moving). He would spend about 3 weeks in that Home Depot bucket (with the chiller and a small filter), he actually managed to escape the bucket and i found him on my carpet once. The reason for this situation is my left hand was completely unusable. He then went to the 10-gallon for 7 months and remains there today (only because nobody will help me with moving in my new 40-gallon lol). Then i moved him again a couple days ago where he spent about 1.5 hours in a Home Depot bucket again with no aeration and the water allowed to warm to 10°F above normal for him before i had his tank setup (again, nobody helped me set up his new tank in advance so i had to just move his current tank and set it up quickly). This is when he dropped his claw. So he has been put under a lot of stress just from all this drama. I'll get back with the other answers later (i'm on a short work break).

 

[Whiskey Fox]

Welcome to Reef2Reef and sorry for your trouble!

How wide is the crab's carapace (not including the legs)?


Molting issues can be caused by a number of things, but basically it boils down to either stress, diet, or inadequate water quality. Some crustaceans also have issues with lighting sometimes, so that may be something to look into as well.

Anyway, I'll help get you started with checking for potential causes of any issues, but - just to put this out there - moving a presumably fairly large crab from an established (and presumably cycled) aquarium to a small, new aquarium (you didn't mention if it's cycled or not) is a recipe for high stress, and probably poor water quality as well if the crab is large enough.


So to start with the water quality:

What water conditioner do you use and what do you use it for specifically?

What water are you using (tap water, sea water, RO/DI, etc.)?


With crustaceans, some parameters you may want to check include:

-Calcium
-pH
-Nitrate
-Phosphate
-Alkalinity would probably be good to test as well.

With this specific situation, I also would suggest testing:

-Ammonia
-Copper

If you have reason to believe your water may have too high levels of contaminants or something like magnesium (this one is unlikely), then you may want to check for those too.


About your 10 gallon tank:

-Was the tank cycled?
-How many gallons is the canister filter recommended for?
-Do you have any other filtration (other filters, a refugium/cryptic refugium, algae scrubber, etc.) on the tank?
-Do you have any rock in the tank, or just sand?
-Does the tank have a sump?
-Can we get a picture of the full tank?
-What lighting does the tank get (sunlight, light fixtures, etc.)?


For the diet:

-How much are you feeding the crab each feeding?
-How often are you feeding the crab?
-What specific pellets, fish, and prawns are you feeding? How much of each are you feeding and how often?

To continue, he was originally on a hang-over-the-edge style filter in the 40 gal, then on a little internal 10-gal filter. About two months ago, i upgraded him to a Fluval 207 with 45 gal capacity (while he's in the 10-gal). I added an inline UV filter that was only used until no longer needed.

Mostly i have been feeding him Tetra PlecoWafers, about once per day, and giving him a piece of fish or shrimp once a week or so. I think it is erring on the low end of feeding frequency but it's because the 10-gal gets dirty very fast and he is not always hungry, resulting in wafers sitting around making a mess. He used to get extremely aggressive when he smelled meat, now he is kind of lethargic but still interested. I fed him tilapia and spotted prawns (from the Puget Sound). Back in the day, he actually managed to kill and eat a live one that i had coexisting with him.

My previous crab (Graceful Crab) which molted just fine had a diet of mostly Crab Cuisine, which are too small for this bigger crab.

I use Aqueon water conditioner in tap water. Not sure of any specifics beyond that, i'm pretty novice.

If i could critique my own habits, water changes would be an area i need to improve. The ammonia levels are probably fairly high, especially while i was injured (basically no cleaning of the water at all for two months).

There is nothing but sand from Petco in the current tank (for 8 months). In his previous 40-gal home, he had rocks from the ocean with various other types of critters (sea anemone, eel grass, clams, etc) which he lived with in the Sound.

I'm attaching photos of his temporary, 10-gal tank.

 

[ISpeakForTheSeas]

He was so stressed from the move that he plucked one of his claws off, which is something i've been kind of anticipating to happen. I'm definitely on high-alert with him now.

Yeah, that's definitely not a good sign.

You mentioned lighting and i will admit that his lighting has been fairly irregular. Lately i will have the LED lights on at night and then have the light off and the aquarium covered so it's moderately dark during the day, but not completely.

That might be adding to the stress.

I should mention that during the move 8 months ago, he was inside the 40 gallon when we had an accident, resulting in the tank slamming to the concrete and shattering (and cutting my lateral artery in the process, sending me to the hospital). Sebastian was rescued by an EMT who brought him up to my room and put him in a room-temp bucket of salt water (removed from his tank earlier for moving). He would spend about 3 weeks in that Home Depot bucket (with the chiller and a small filter), he actually managed to escape the bucket and i found him on my carpet once. The reason for this situation is my left hand was completely unusable. He then went to the 10-gallon for 7 months and remains there today (only because nobody will help me with moving in my new 40-gallon lol). Then i moved him again a couple days ago where he spent about 1.5 hours in a Home Depot bucket again with no aeration and the water allowed to warm to 10°F above normal for him before i had his tank setup (again, nobody helped me set up his new tank in advance so i had to just move his current tank and set it up quickly). This is when he dropped his claw. So he has been put under a lot of stress just from all this drama.

Wow - glad you're alright; that would definitely be highly stressful for the crab.

Sebastian is 5" in carapace width.

Yeah, a 10 gallon is definitely too small for a crab that large.

If i could critique my own habits, water changes would be an area i need to improve. The ammonia levels are probably fairly high, especially while i was injured (basically no cleaning of the water at all for two months).

Yeah, with a 5" crab in a 10 gallon tank, I'd definitely get some ammonia tests and see how it's doing ammonia-wise; too high of ammonia can kill critters (slowly if only somewhat too high, and quickly if much too high).

I use Aqueon water conditioner in tap water. Not sure of any specifics beyond that, i'm pretty novice.

So, the water conditioner is designed to remove chlorine/chloramines from the water - that's good, but it doesn't prevent other issues that tap water can potentially have (such as excess metals like copper, which are toxic to inverts).

Seattle's water quality report is pretty lackluster compared to some I've seen, but it does list it's copper levels, so we have a decent example to work with here: 90% of houses had copper levels at or below 0.12 ppm.

Seems like a small amount, right? However, when you look at the copper level tolerances of various inverts (including, reportedly, corals), you find:

0.02 ppm can kill some inverts (reportedly including corals) - in fact, 0.002 ppm (A.K.A. 2 ppb) is enough to kill certain inverts.

Unfortunately, Seattle doesn't list the minimum levels of copper found, but it still means there's a chance that your crab could be getting slowly poisoned by the tap water - I'm not saying that's the case here (particularly with your previous crab having done fine, though different species have different tolerance levels - sometimes drastically different), but it's a possibility: this is why I personally always recommend RO/DI water.


Anyway, moving on to address a couple of other points that may help:

There is nothing but sand from Petco in the current tank (for 8 months). In his previous 40-gal home, he had rocks from the ocean with various other types of critters (sea anemone, eel grass, clams, etc) which he lived with in the Sound.

I would definitely add something to the tank to give the crab a place to hide, even if it's just a plastic overhang to hide under or a large PVC tube it can hide in.

In addition to decreasing stress by giving the crab a way to seek shelter, this also provides more surface area for more nitrifying bacteria, which could help with any excess ammonia levels.

And, finally:

Mostly i have been feeding him Tetra PlecoWafers, about once per day, and giving him a piece of fish or shrimp once a week or so. I think it is erring on the low end of feeding frequency but it's because the 10-gal gets dirty very fast and he is not always hungry, resulting in wafers sitting around making a mess. He used to get extremely aggressive when he smelled meat, now he is kind of lethargic but still interested. I fed him tilapia and spotted prawns (from the Puget Sound). Back in the day, he actually managed to kill and eat a live one that i had coexisting with him.

My previous crab (Graceful Crab) which molted just fine had a diet of mostly Crab Cuisine, which are too small for this bigger crab.

I suspect the Tetra Pleco Wafers just aren't cutting it as a diet staple, especially if only supplemented once a week with meaty foods; it's low in protein, low in fat, likely high in carbs, and essentially a vegan/vegetarian diet for an omnivorous (primarily carnivorous) crab.

Similarly, tilapia is also pretty low in protein and fat.

For a staple diet, I'd suggest something like Mytilus sp. mussels as the main feed, supplemented probably every daily or every other day with a smaller portion of NLS Marine Fish Pellets. I'd also suggest supplementing two or three times a week with a good quality fish (such as salmon), shrimp or other crustaceans (like mysis), and something like fresh urchins or snails if possible.

That said, the mussels would be by far the most important part, and the others would honestly just be nice additions to try and add variety with to ensure a well-balanced nutrition and optimal long-term health for the crab.

As an alternative to the NLS pellets, you can use macroalgae like Nori, Ulva, Halymenia, etc. (possibly even some kind of kelp) - the crab might eat it by itself, or you could try things like wrapping it around the meaty food, making a gelatin feed with it, etc.

 

[Whiskey Fox]

Yeah, that's definitely not a good sign.

That might be adding to the stress.

Wow - glad you're alright; that would definitely be highly stressful for the crab.

Yeah, a 10 gallon is definitely too small for a crab that large.

Yeah, with a 5" crab in a 10 gallon tank, I'd definitely get some ammonia tests and see how it's doing ammonia-wise; too high of ammonia can kill critters (slowly if only somewhat too high, and quickly if much too high).

So, the water conditioner is designed to remove chlorine/chloramines from the water - that's good, but it doesn't prevent other issues that tap water can potentially have (such as excess metals like copper, which are toxic to inverts).

Seattle's water quality report is pretty lackluster compared to some I've seen, but it does list it's copper levels, so we have a decent example to work with here: 90% of houses had copper levels at or below 0.12 ppm.

Seems like a small amount, right? However, when you look at the copper level tolerances of various inverts (including, reportedly, corals), you find:

Unfortunately, Seattle doesn't list the minimum levels of copper found, but it still means there's a chance that your crab could be getting slowly poisoned by the tap water - I'm not saying that's the case here (particularly with your previous crab having done fine, though different species have different tolerance levels - sometimes drastically different), but it's a possibility: this is why I personally always recommend RO/DI water.


Anyway, moving on to address a couple of other points that may help:

I would definitely add something to the tank to give the crab a place to hide, even if it's just a plastic overhang to hide under or a large PVC tube it can hide in.

In addition to decreasing stress by giving the crab a way to seek shelter, this also provides more surface area for more nitrifying bacteria, which could help with any excess ammonia levels.

And, finally:

I suspect the Tetra Pleco Wafers just aren't cutting it as a diet staple, especially if only supplemented once a week with meaty foods; it's low in protein, low in fat, likely high in carbs, and essentially a vegan/vegetarian diet for an omnivorous (primarily carnivorous) crab.

Similarly, tilapia is also pretty low in protein and fat.

For a staple diet, I'd suggest something like Mytilus sp. mussels as the main feed, supplemented probably every daily or every other day with a smaller portion of NLS Marine Fish Pellets. I'd also suggest supplementing two or three times a week with a good quality fish (such as salmon), shrimp or other crustaceans (like mysis), and something like fresh urchins or snails if possible.

That said, the mussels would be by far the most important part, and the others would honestly just be nice additions to try and add variety with to ensure a well-balanced nutrition and optimal long-term health for the crab.

As an alternative to the NLS pellets, you can use macroalgae like Nori, Ulva, Halymenia, etc. (possibly even some kind of kelp) - the crab might eat it by itself, or you could try things like wrapping it around the meaty food, making a gelatin feed with it, etc.

Again, i'm amazed at what a wealth of knowledge you are. I just did an emergency 50% water change using distilled water (in the interest of time, but i will look into an RODI system). I covered the aquarium to make it very dark.

I also got an assorted mix of seafood at the store (with mussels and shrimp) as emergency food. He ate one large muscle and two small claims and is the most animated i've seen him in a while.

Later on when i get a break, i'll go to the beach and collect some wild muscles and have them in a small 5-gal holding tank until it's time to feed.

I'll be looking into the RODI systems some more but off the top of your head, is there a particular budget-friendly system that you would recommend? I would say my budget will allow for up to $150 approximately.

Also thank you for doing such a thorough investigation of the Seattle water supply

 

[ISpeakForTheSeas]

I'll be looking into the RODI systems some more but off the top of your head, is there a particular budget-friendly system that you would recommend? I would say my budget will allow for up to $150 approximately.

The cheapest I know which I've heard good things about is the Aquatic Life RO Buddy - it's a four stage filter with a mixed bed DI; 50 gallons per day; $70.

There are other options out there for a higher budget with more stages (heavier duty filtration), but I don't remember them off the top of my head.

Also thank you for doing such a thorough investigation of the Seattle water supply

Once you learn how to read the water quality reports, it becomes pretty easy.

 

[Whiskey Fox]

The cheapest I know which I've heard good things about is the Aquatic Life RO Buddy - it's a four stage filter with a mixed bed DI; 50 gallons per day; $70.

There are other options out there for a higher budget with more stages (heavier duty filtration), but I don't remember them off the top of my head.

Once you learn how to read the water quality reports, it becomes pretty easy.

I just realized i said "muscles" instead of "mussels" spending too much time in the gym.

 

[ISpeakForTheSeas]

I just realized i said "muscles" instead of "mussels" spending too much time in the gym.

Haha, don't worry about it - typos happen all the time.

 

[ihateSpirorbids]

How's your crab doing? Did the iodide testing reveal anything on how much to dose?

 

[Randy Holmes-Farley]

How's your crab doing? Did the iodide testing reveal anything on how much to dose?

FWIW, I don’t think there’s any evidence that crabs or shrimp need iodine in the water for proper molting. It seems to just be something said over and over with no underlying evidence.

 

[Whiskey Fox]

How's your crab doing? Did the iodide testing reveal anything on how much to dose?

I ended up purchasing a RODI system like @ISpeakForTheSeas suggested and got my crab onto a strict mussel diet (occasionally supplemented by salmon). He's also been spending some time in a 60-gallon with a large, flat rock that he loves to burrow under. As soon as i made the changes, he went from being lethargic to his old, very active self.

Unfortunately, i mis-handled him and he ended up dropping his last claw out of defense, making him a clawless crab now. But despite being clawless for the tjme being, he seems to be much better off. Maybe in 3 years he will have his claws restored to normal.

In the meantime, i'm planning on getting a pet Dungeness to be my main showcase crab, since my red rock crab looks goofy now.

 

[ISpeakForTheSeas]

I ended up purchasing a RODI system like @ISpeakForTheSeas suggested and got my crab onto a strict mussel diet (occasionally supplemented by salmon). He's also been spending some time in a 60-gallon with a large, flat rock that he loves to burrow under. As soon as i made the changes, he went from being lethargic to his old, very active self.

Unfortunately, i mis-handled him and he ended up dropping his last claw out of defense, making him a clawless crab now. But despite being clawless for the tjme being, he seems to be much better off. Maybe in 3 years he will have his claws restored to normal.

In the meantime, i'm planning on getting a pet Dungeness to be my main showcase crab, since my red rock crab looks goofy now.

Glad to hear the crab is doing better (minus the claw)!

 

[ihateSpirorbids]

do you dose calcium in the tank for the crab? If so what's the general guideline?

 

[Randy Holmes-Farley]

do you dose calcium in the tank for the crab? If so what's the general guideline?

All reef tanks should ensure normal levels of calcium and alkalinity by testing.

 

[Byrtie21]

Hey, I'm looking into getting a small red claw crab or two. I would like to be able to set up the tank to the perfect conditions for the critter(s). Is there any way I can get a list of must-have items such as filtration options, lighting necessities, substrate, diet options, etc? I don't want to jump into this without any preparation and you seem like you're the right person for all this information. I can't seem to find enough details from other sources to know what I get is the right setup. I'm really only looking for a 10gal setup, so I would be getting little crab(s).

 

[ISpeakForTheSeas]

Hey, I'm looking into getting a small red claw crab or two. I would like to be able to set up the tank to the perfect conditions for the critter(s). Is there any way I can get a list of must-have items such as filtration options, lighting necessities, substrate, diet options, etc? I don't want to jump into this without any preparation and you seem like you're the right person for all this information. I can't seem to find enough details from other sources to know what I get is the right setup. I'm really only looking for a 10gal setup, so I would be getting little crab(s).

Welcome to Reef2Reef!

Red Claw Crabs (Perisesarma bidens) are a brackish species and out of my typical wheelhouse, but I'm happy to help if I can.

A very important note to start: these crabs are noted as being highly aggressive, and they are known to kill and cannibalize each other in aquariums. The males are noted as being more likely to actually fight. Generally, I would recommend either only keeping one or keeping either a male/female pair or a female/female pair, but these crabs are also noted as being difficult to sex accurately, so that a pair may be tough here.

These crabs live in mangrove sand/mudflat areas; they're known burrowers, but they also reportedly like to have natural hides (overhangs, crevices they can climb into, mangrove roots to hide in, etc.),* so a fine sand bottom with plenty of naturalistic hiding places is a good start. I have also heard that they need land available,** and this is, to my knowledge, supported by the respiratory system anatomy,*** so I would be sure to include a way for them to get up out of the water into similar hides if they want (ensuring that they have an easy way to get back in the water no matter where they go in the tank). Something like a mudskipper aquarium may be appropriate inspiration for the tank setup.

For the brackish water, I'd probably aim for around 18 ppt/1.0135 sg for the salt levels, but I don't know what would be ideal for the species, and it may change depending on their life stage (to say nothing of how much salinity levels change in the wild in mangrove forests).

P. bidens has had it's diet analyzed via gut contents (unfortunately, these did not go into specifics on what true plants these ate) and some very, very basic experiments (I take issue with at least one or two of these, but that's honestly irrelevant). Anyway, they primarily eat "plant material" and "inorganic detritus" (making up ~85% of the crab's diet, with the makeup being ~65% and 20% respectively), with a little bit of macroalgae, microalgae, "animal material," and "unidentified material" in the wild,*** so something mimicking that may be wise. However, the one experiment I came across (also describe in the *** source) that tried to offer them a decent imitation of their natural diet, including leaf litter and other plant parts from various species of mangrove plants (including the Tall-stilt Mangrove, Rhizophora apiculata; the Loop-root Mangrove, Rhizophora mucronata; and the White/Grey Mangrove, Avicennia marina) noted that the crabs reacted very quickly and nervously to any sort of mechanical disturbance in the aquaria they were kept in - so while they likely eat these sorts of things in the wild, it seems the crabs may have been too nervous/not adapted enough to eat them in captivity. Some other mangrove species that may be worth trying as well include: Kandelia obovata, Ceriops tagal, Avicennia alba, Avicennia officinalis, Avicennia rumphiana, and Bruguiera gymnorhiza.

If I had access to all of the above but could only pick one for this species, I would go with Kandelia obovata, as while I can't confirm that they eat the leaves, these crabs have been kept in tanks for breeding purposes successfully with the leaves offered as their food item (so I have strong reason to suspect they feed on them).****

Assuming you don't have access to a relatively large number of mangrove leaves, twigs, etc. from, it may be worthwhile to try and find a more readily available alternative, but I'm not sure how suitable the standard alternative feeds like algae pellets or lettuce might be for these critters. Maybe @Jay Hemdal would have some insight there or into appropriate alternatives?

As far as equipment goes, I don't think these would be too demanding:

-Flow/aeration like from a powerhead, gyre, pump, and/or air stone.

-Lighting may not even be necessary for the crab (lighting is important for some critters, but not so much for others), but if you're wanting either to imitate their natural habitat better, stimulate spawning (these crabs rely on light cues for spawning),**** or grow something that's photosynthetic (like plants, algae, coral, etc.), then I would pick up some some lights. What you want to grow with the light changes what I would specifically recommend, but the quotes below can help you get started, and I'm happy to answer questions if I can (a few general rules: more blue and less red for corals; with true plants, blue is for growth and red is for flowering; if the lights can grow other things, they can grow algae):

2 ) The color as our eyes see it (called Color Temperature and presented in Kelvin) isn't actually important except for aesthetics (so if we like how it looks or not). "White" light is actually a mixture of at least three different colors of light (a bluish light, a greenish/yellowish light, and a reddish light; these can mixed in different wavelengths and quantities to achieve the same "white" look).

3 ) What photosynthetic organisms - like corals and plants - care about are the spectrum/wavelengths of the light (so what specific kinds of light are produced by a fixture) and Photosynthetic Photon Flux Density (PPFD; generally, people refer to PPFD as PAR, or Photosynthetically Active Radiation; this is basically a measure of how much light photosynthetic organisms can "feed on" a fixture is producing).



To simplify the above; corals care about what kind of light (the wavelengths) they get and the strength of the light (the PAR values) they get.

So, to see if a light will work, you want to look at the Spectral Power Distribution Chart/Graph (shows basically how much of each color/wavelength of light is produced by the fixture) and the PPFD Chart (basically a map of the PAR output of the fixture).

"White" light with a lot of blue and a little red with some amount of green/yellow will grow corals fine as long as it's strong enough (i.e. as long as it has enough PAR). Just blue light will also grow corals fine as long as it produces enough PAR too.



As a note, Watts are not a measure of PAR, and some lights with high wattage won't work well - again, PPFD/PAR is thing to look for.

Also, one more thing to consider is the IP rating of a light if it's going to be exposed to water, especially saltwater, even if just through hanging over an open/partially open tank (evaporation is a thing to keep in mind). A higher IP rating is better; standard reef lighting options have good enough IP for the job, but some other lights (such as for plants or other kinds of pets) may not.

A quick rundown on lights:

-Lumens are a measurement of how bright light looks to us, not how useful it is to photosynthetic organisms (like plants, corals, algae, etc.). Some lights look very bright to us, but are useless for photosynthetic organisms.

-Kelvin is what color we perceive the light as being (not what colors are actually produced).

-Watts are how much electricity the lights use (some people use this as a measurement of the strength of the light, but it’s not always a good indicator of that; to measure the strength of the light, you want to look at PAR - the higher the number, the stronger the light). [Edit: the higher the PAR, the stronger light is for photosynthetic organisms; as a note, the higher wattage, the higher the cost to run the light.]

-The “nm” numbers describe the wavelengths of the light produced by the light; in other words, it tells you what kind of light is being produced. This is important, as photosynthetic organisms prefer certain kinds of light. As mentioned, light that produces wavelengths that excite Chlorophyll A are generally going to be the a really solid choice (the excitement peaks are ~440nm [blue] and ~660nm [red]); full spectrum (including those red and blue peaks with some yellow and green mixed in) are the best for photosynthesis, but - as mentioned - may not be the most cost effective.

TLDR; you want a light with high PAR values that produces ~440 and ~660nm light (either “blurple” or full spectrum”).

(Quick technical note about PAR and PPFD for anyone who cares: the "PAR" values people talk about are technically the PPFD values:)

-PPFD is Photosynthetic Photon Flux Density

-µmol/m2/s is the measurement of how many micromoles of light are hitting a square meter in a second (it's the standard measurement used for PPFD)

-PAR is Photosynthetically Active Radiation; technically speaking, this isn't a measurement of the light's strength, it's a range of light wavelengths (400-700nm)

-PAR is measured in µmol/m2/s; the number just excludes light with wavelengths outside of the PAR spectrum (so it doesn't count light wavelengths outside of the 400-700nm range)

Some people prefer metal halide lights, others LEDS, others T5's - they all work when done correctly. As a general rule, either 12 hours of light and 12 hours of dark (which these crabs have been kept with for spawning before)**** or 14 hours of light and 10 hours of dark are recommended.

-Filtration is honestly more of a personal choice than most things in the hobby - the point of filtration is basically just to remove excess nutrients (for our purposes, primarily nitrate and phosphate) from the water. Mechanical filtration (like canister filters, HOB filters, skimmers, etc.) can be used for this, as can biological filtration (live rocks/sand, refugia/cryptic refugia, algae reactors, etc.) or a combination of the two. I personally am biased toward biological filtration methods as they're friendlier to pelagic larvae (and aquaculture is a big interest of mine), so I'm not much help as far as specific equipment goes for filtration purposes, but there's plenty of info here on R2R to help with that, and you can always start your own thread asking for help with it too.


Other considerations:

-You'll need temperature regulation, so most likely at least a heater (and possibly a chiller too, depending on your local climate); a controller (like an Inkbird controller or similar) is generally recommended as well in case of heater malfunctions (lots of tanks have been wrecked by heater malfunctions roasting them). Without knowing the specific critter's ideal temperature range, 77F-81F is generally a safe range to aim for, and I know these crabs have been kept at 77F before in breeding tanks successfully,**** so I'd recommend that range here.

-I always personally recommend an RO/DI system (it isn't always needed, but it ensures clean water for the tank; you can buy distilled water or similar instead, but it generally is less cost effective long-term; I don't recommend getting water from your LFS - I've heard too many stories of people having issues with LFS water).

-Depending on your circumstances, you may need to consider how to keep the tank running in an extended power outage (things like generators, battery backups/battery powered backups, etc.).

-Water parameters like mentioned in one of my earlier posts:
With crustaceans, some parameters you may want to check include:

-Calcium
-pH
-Nitrate
-Phosphate
-Alkalinity would probably be good to test as well.

*Source:

https://www.researchgate.net/publication/331650418_Feeding_ecology_of_tree-climbing_mangrove_sesarmid_crabs_from_Luzon_Philippines

**Source:

FAQs on Freshwater Crab Reproduction

***Source (I had to request this one from the source; it was free and easy to access, but it makes it difficult to share easily):
THE POPULATION DYNAMICS AND FEEDING ECOLOGY OF THE MANGROVE CRABS METOPOGRAPSUS FRONTALIS (GRAPSIDAE) AND PERISESARMA BIDENS (SESARMIDAE), IN HONG KONG

The author is Poon Yiu Nam David.
****Source:

https://zoolstud.sinica.edu.tw/Journals/48.5/607.pdf

 

[Jay Hemdal]

Welcome to Reef2Reef!

Red Claw Crabs (Perisesarma bidens) are a brackish species and out of my typical wheelhouse, but I'm happy to help if I can.

A very important note to start: these crabs are noted as being highly aggressive, and they are known to kill and cannibalize each other in aquariums. The males are noted as being more likely to actually fight. Generally, I would recommend either only keeping one or keeping either a male/female pair or a female/female pair, but these crabs are also noted as being difficult to sex accurately, so that a pair may be tough here.

These crabs live in mangrove sand/mudflat areas; they're known burrowers, but they also reportedly like to have natural hides (overhangs, crevices they can climb into, mangrove roots to hide in, etc.),* so a fine sand bottom with plenty of naturalistic hiding places is a good start. I have also heard that they need land available,** and this is, to my knowledge, supported by the respiratory system anatomy,*** so I would be sure to include a way for them to get up out of the water into similar hides if they want (ensuring that they have an easy way to get back in the water no matter where they go in the tank). Something like a mudskipper aquarium may be appropriate inspiration for the tank setup.

For the brackish water, I'd probably aim for around 18 ppt/1.0135 sg for the salt levels, but I don't know what would be ideal for the species, and it may change depending on their life stage (to say nothing of how much salinity levels change in the wild in mangrove forests).

P. bidens has had it's diet analyzed via gut contents (unfortunately, these did not go into specifics on what true plants these ate) and some very, very basic experiments (I take issue with at least one or two of these, but that's honestly irrelevant). Anyway, they primarily eat "plant material" and "inorganic detritus" (making up ~85% of the crab's diet, with the makeup being ~65% and 20% respectively), with a little bit of macroalgae, microalgae, "animal material," and "unidentified material" in the wild,*** so something mimicking that may be wise. However, the one experiment I came across (also describe in the *** source) that tried to offer them a decent imitation of their natural diet, including leaf litter and other plant parts from various species of mangrove plants (including the Tall-stilt Mangrove, Rhizophora apiculata; the Loop-root Mangrove, Rhizophora mucronata; and the White/Grey Mangrove, Avicennia marina) noted that the crabs reacted very quickly and nervously to any sort of mechanical disturbance in the aquaria they were kept in - so while they likely eat these sorts of things in the wild, it seems the crabs may have been too nervous/not adapted enough to eat them in captivity. Some other mangrove species that may be worth trying as well include: Kandelia obovata, Ceriops tagal, Avicennia alba, Avicennia officinalis, Avicennia rumphiana, and Bruguiera gymnorhiza.

If I had access to all of the above but could only pick one for this species, I would go with Kandelia obovata, as while I can't confirm that they eat the leaves, these crabs have been kept in tanks for breeding purposes successfully with the leaves offered as their food item (so I have strong reason to suspect they feed on them).****

Assuming you don't have access to a relatively large number of mangrove leaves, twigs, etc. from, it may be worthwhile to try and find a more readily available alternative, but I'm not sure how suitable the standard alternative feeds like algae pellets or lettuce might be for these critters. Maybe @Jay Hemdal would have some insight there or into appropriate alternatives?

As far as equipment goes, I don't think these would be too demanding:

-Flow/aeration like from a powerhead, gyre, pump, and/or air stone.

-Lighting may not even be necessary for the crab (lighting is important for some critters, but not so much for others), but if you're wanting either to imitate their natural habitat better, stimulate spawning (these crabs rely on light cues for spawning),**** or grow something that's photosynthetic (like plants, algae, coral, etc.), then I would pick up some some lights. What you want to grow with the light changes what I would specifically recommend, but the quotes below can help you get started, and I'm happy to answer questions if I can (a few general rules: more blue and less red for corals; with true plants, blue is for growth and red is for flowering; if the lights can grow other things, they can grow algae):


(Quick technical note about PAR and PPFD for anyone who cares: the "PAR" values people talk about are technically the PPFD values:)

Some people prefer metal halide lights, others LEDS, others T5's - they all work when done correctly. As a general rule, either 12 hours of light and 12 hours of dark (which these crabs have been kept with for spawning before)**** or 14 hours of light and 10 hours of dark are recommended.

-Filtration is honestly more of a personal choice than most things in the hobby - the point of filtration is basically just to remove excess nutrients (for our purposes, primarily nitrate and phosphate) from the water. Mechanical filtration (like canister filters, HOB filters, skimmers, etc.) can be used for this, as can biological filtration (live rocks/sand, refugia/cryptic refugia, algae reactors, etc.) or a combination of the two. I personally am biased toward biological filtration methods as they're friendlier to pelagic larvae (and aquaculture is a big interest of mine), so I'm not much help as far as specific equipment goes for filtration purposes, but there's plenty of info here on R2R to help with that, and you can always start your own thread asking for help with it too.


Other considerations:

-You'll need temperature regulation, so most likely at least a heater (and possibly a chiller too, depending on your local climate); a controller (like an Inkbird controller or similar) is generally recommended as well in case of heater malfunctions (lots of tanks have been wrecked by heater malfunctions roasting them). Without knowing the specific critter's ideal temperature range, 77F-81F is generally a safe range to aim for, and I know these crabs have been kept at 77F before in breeding tanks successfully,**** so I'd recommend that range here.

-I always personally recommend an RO/DI system (it isn't always needed, but it ensures clean water for the tank; you can buy distilled water or similar instead, but it generally is less cost effective long-term; I don't recommend getting water from your LFS - I've heard too many stories of people having issues with LFS water).

-Depending on your circumstances, you may need to consider how to keep the tank running in an extended power outage (things like generators, battery backups/battery powered backups, etc.).

-Water parameters like mentioned in one of my earlier posts:
With crustaceans, some parameters you may want to check include:

-Calcium
-pH
-Nitrate
-Phosphate
-Alkalinity would probably be good to test as well.

*Source:

https://www.researchgate.net/publication/331650418_Feeding_ecology_of_tree-climbing_mangrove_sesarmid_crabs_from_Luzon_Philippines

**Source:

FAQs on Freshwater Crab Reproduction

***Source (I had to request this one from the source; it was free and easy to access, but it makes it difficult to share easily):
THE POPULATION DYNAMICS AND FEEDING ECOLOGY OF THE MANGROVE CRABS METOPOGRAPSUS FRONTALIS (GRAPSIDAE) AND PERISESARMA BIDENS (SESARMIDAE), IN HONG KONG

The author is Poon Yiu Nam David.
****Source:

https://zoolstud.sinica.edu.tw/Journals/48.5/607.pdf

I think you've given a great overview! I've never kept that species, so I'll sit on my hands here (grin).