Emerald looks Good For it

[Miquel]

Was just checking the tank and noticed one of my new zoanthis frags has been torn up with a few heads missing apparently eaten. Am doubtful it was the banded shrimp so looks like Ill be giving my new emerald crab his walking papers

[Ninong]

Hi Miquel,

Too bad you didn't do a search on this particular reefkeeping bulletin board first. We have been warning about emerald crabs for the past eight years.

Are Emerald Crabs reef-aquarium safe?

In their natural environment, I believe emerald crabs are omnivorous as juveniles but become primarily carnivorous as adults: "The emerald crab, Mithrax sculptus occurs among the branches of the coral Porites furcata and feeds on the polyps at night. It has also been observed to feed on organisms attached to seagrasses (Colin, 1978)." [It has since been renamed Mithraculus sculptus.]

As with any of the marine animals that we keep in our tanks, their natural behavior, including feeding habits, may change to adapt to the conditions they find themselves in.

This question comes up so often that six years ago I compiled a list of anecdotal reports about problems with emerald crabs in this post. Six years ago the membership on Reef Central was only a fraction what it is today. I have read many, many more reports of reefkeepers reporting problems with emerald crabs in their systems.

I think it is correct to say that some reefkeepers have found emerald crabs to be "fairly reef safe," but it is probably a stretch to say that they are "fairly reef safe" under most conditions. In their natural habitat, they tend to be primarily herbivorous as juveniles but their diet changes to primarily protein based as they mature. They are omnivorous but they change from mostly herbivorous omnivores when young to mostly carnivorous omnivores as adults, according to various scientific papers that I have read.

Emerald crabs (Mithraculus sculptus) are sort of in the same category as the notorious Atlantic green brittle star (Ophiarachna incrassata) in that every time someone posts something bad about them, someone else will post that their experience was trouble-free. I suggest you read the reports summarized here and be guided accordingly. I could add at least 100 more similiar reports if I did a current search on all the boards. That was just a quick search on one board several years ago.

[Miquel]

Actually I did do some research here which is why Ive been keeping a close eye on himand my tank corals. Decided to introduce him in order to clean up some bubble algae and keep him only as long as he behaved. Im looking at him like a contractor doing work on my home. Bring him in, let him do the job, then send him on his way. Will be removing him today. Up until now hes been a model citizen. Too bad

[Ninong]

Emerald crabs will usually eat bubble algae but then there is the problem of getting them out of the tank later on. If the tank is large enough, I think a foxface rabbitfish is a better solution. I wouldn't recommend a foxface for anything smaller than a 135-gal tank.

I bought a juvenile foxface (Siganus vulpinus) for my 120-gal tank when it was only about 2.5-3" TL. Within three years it was 7" TL and the 120-gal tank was really too small for it. I gave it to someone with a 300-gal tank.

I never had any visible bubble algae. On a couple of occasions, when rearranging a couple of pieces of the live rock near the top, I discovered a few small bubble algae that apparently were impossible for my foxface to reach but exposed to enough light to survive. I didn't remove them. I simply made them more accessible to the foxface and within a matter of hours they were all gone. I wouldn't recommend a foxface for anyone with a tank smaller than 135 gallons because they go spastic in smaller tanks after they grow up. And they cannot develop normally in smaller tanks. In fact, a better minimum size tank for them is probably a 7' long 210-gal size. I don't know why so many online sites recommend small tank sizes for these fish.

[Miquel]

Well emerald crab has been evicted and moved back to the LFS.

Fortunately removing him took me all of about 30 seconds and I didnt have to move or remove any rock to do it. Thought I would share with you how I got him out.

Picked up one of those gadgets from the LFS for grabbing things in your tank without getting your hands wet. You know, what I mean, basically a twenty inch rod with a claw on one end and a trigger to open and close the jaws on the other end. I took a piece of frozen cooked shrimp, the tail end with the skin still on to make it a bit tougher and thus easier to grip, grabbed it with the claws of the tool, then waved it in front of the rock crevice where the emerald was hanging out. Once he got a whiff of it he immediately came out for what he though would be a free meal. Little did he realize. I kept inching the bait back just beyond his reach untill he was well exposed and I let him grab the bait. Then I simply released the shrimp from the jaws and turned them on the crab. As I said, the whole process, once I had the jaws and shrimp of course, took all of about thirty seconds.

The guy at the LFS argued with me that no way emeralds would eat coral as they were strictly algae eaters and that this was highly unusual. Rather than argue I offerred to return the crab for no credit provided he let me release him into his reef show tank. He declined and gave me a credit.

[TimH07]

I offerred to return the crab for no credit provided he let me release him into his reef show tank. He declined and gave me a credit.

;;

[kronos5]

Good One

[Seige]

ROFL That's hilarious. No idea how you managed to grab the crab quick enough. I'd have ended up grabbing the LR...

[metalhead]

That's awesome... I'll definitely remeber that one!!!

Good job!