How much food do crabs need? Turning filter compartment into crab box.

Tired · Oct 26, 2023

I have a hefty gorilla crab currently living in a 7gal with about 10 pounds of live rock. It's grown from a bit over an inch across to a 1" carapace and over a 3" legspan on pretty minimal feeding. I want to break down the tank, so I'm planning to move it into the (large) back compartment of my reef tank, with a lid so it can't climb out. Both so it has a home, and so it can eat any not-fully-soaked food that gets back there and would otherwise just rot.

How much food does a crab this size need? I'm going to put some rock back there for it to hide in, but I doubt it'll find enough stuff on there to eat. It's a spine-back hairy crab, and this is around maximum size for them, at that 1" carapace width and some nasty claws. They're a gorilla-type crab, and my understanding of their diet is "anything that is or has recently been alive and can be subdued", though this one has been ignoring a couple of stony corals and some clams on the rock. It'd mostly be getting mysis from the fish food, and I can supplement with either normal pellets or veggie pellets if needed.

Has anyone experimented with making anything like a refugium in a back filtration compartment? I'm considering putting a bright light over the compartment (with plenty of shade for the crab) to try and grow algae and such, in hopes that some copepods will make it through the pump and in order to provide the crab with veggies.

blaxsun · Oct 26, 2023

Pics!

Tired · Oct 27, 2023

The three prior molts, and the lad himself! Tweezers in both pics for size comparison. He's very cool, and he's now gotten large enough that he's no longer afraid of my hand outside the tank. He moves towards it instead to see if he can eat it.
I picked him up with my hand when he was a tiny baby I'd just caught off my live rock. I will not be picking him up now. That larger claw is as thick as my pinky finger- that'd hurt! Plus he's got sharp spines.

ISpeakForTheSeas · Oct 29, 2023

I know for a lot of organisms that ~5% of the specimen's weight per day is the recommended amount of feed, but I don't know for crabs specifically. I do know that a lot of crabs are very resilient critters that will do well even under relatively poor conditions though, so I expect that it would keep growing pretty well even if underfed.

That said, if you're wanting optimal health and growth/lifespan for the crab, then I would supplement its feed (primarily with meaty pellets, but also with a few veggie pellets). If you want to figure out an approximate number of pellets to feed daily:

ISpeakForTheSeas said:
I believe they're essentially saying to feed it a bunch of pellets once to see how many it will eat in one feeding - that should give you an approximate, soft-maximum number of pellets that the clown will eat each feeding. From there, watch during each of the next few feedings to see how many of that maximum number of pellets the fish is eating; if it eats less than the approximate maximum number every time, then you can subtract from the number of pellets you're feeding. For example, if you feed the clown and it eats 6 pellets the first feeding, but it only eats 4 of the 6 you offer it during the next two feedings, you should be safe to lower the number of pellets you're feeding. On the other hand, if the clown eats all 6 of the pellets offered at second and third feedings, then you should probably try offering it some more pellets to see if it needs more food/a higher soft-maximum.

Personally, I'd make sure to feed an extra pellet or two at every feeding with this approach (at least while the clown is growing/breeding) to monitor if the amount of feed they need changes so that I wouldn't end up unintentionally starving the poor thing as it grows.

vetteguy53081 · Oct 29, 2023

Tired said:
The three prior molts, and the lad himself! Tweezers in both pics for size comparison. He's very cool, and he's now gotten large enough that he's no longer afraid of my hand outside the tank. He moves towards it instead to see if he can eat it.
I picked him up with my hand when he was a tiny baby I'd just caught off my live rock. I will not be picking him up now. That larger claw is as thick as my pinky finger- that'd hurt! Plus he's got sharp spines.

These are scavengers and will eat constantly especially at night

Ordovician_Reef · Oct 29, 2023

Tired said:
I have a hefty gorilla crab currently living in a 7gal with about 10 pounds of live rock. It's grown from a bit over an inch across to a 1" carapace and over a 3" legspan on pretty minimal feeding. I want to break down the tank, so I'm planning to move it into the (large) back compartment of my reef tank, with a lid so it can't climb out. Both so it has a home, and so it can eat any not-fully-soaked food that gets back there and would otherwise just rot.

How much food does a crab this size need? I'm going to put some rock back there for it to hide in, but I doubt it'll find enough stuff on there to eat. It's a spine-back hairy crab, and this is around maximum size for them, at that 1" carapace width and some nasty claws. They're a gorilla-type crab, and my understanding of their diet is "anything that is or has recently been alive and can be subdued", though this one has been ignoring a couple of stony corals and some clams on the rock. It'd mostly be getting mysis from the fish food, and I can supplement with either normal pellets or veggie pellets if needed.

Has anyone experimented with making anything like a refugium in a back filtration compartment? I'm considering putting a bright light over the compartment (with plenty of shade for the crab) to try and grow algae and such, in hopes that some copepods will make it through the pump and in order to provide the crab with veggies.

I have this exact crab and throw him some crab cuisine every day. I think he is also eating the smaller ones I banish to the sump as well. I put in 300+ lbs of TBS rock so you can imagine how many crabs I have.

Tired · Oct 29, 2023

Oh, I guarantee you he's eating your smaller crabs. I had some gorilla-type crabs in one half of a split breeder box at one point, and half a dozen small decorator crabs in the other half. Until some of the gorillas got over, and suddenly I didn't have any more of those decorators.

I guess a question people might know the answer to is: how much does an emerald crab need to eat? If I had an emerald crab in a bare QT tank, what would it need to stay healthy? I imagine it scales up reasonably well for different crab sizes.
I don't need optimal growth, but I do want him healthy. I've decided to care for him rather than euthanizing him like all the other pest crabs (to save them the trouble of eating each other), so I should care for him properly.

AC1211 · Oct 29, 2023

2 to 3 small pellets is what I would give an emerald crab. Assuming I can confirm the crab is eating them. The difference is I would assume an emerald crab would pick off some other food. So in reality maybe consider 3 to 4 pellets. Then watch how it does. I doubt they truly need that much.