birdsnest crab eating the coral?

[CTaylor]

Hi,

I have a birdsnest for the last 8 mo. It's grown from frag to colony. Now I see a dead spot in the lower middle, not at the base, but close, not at the tips. It seems to be growing from that area, not new areas.

**There is a tiny "acro crab" right next to the area. I dont know if I call it a birds nest crab or not, but it's one of those tiny white crabs that got on the coral sometime after I got it as a frag. **I'm thinking it's 'him' eating the coral (??). It makes sense, as I dont feed the crab, and I never seen him leave the coral. So if food isn't floating by him, would he eat the coral??? Is my coral destined to die now? What will likely happen? Do I need to remove the crab? I really crabby! << Not so much like if he's going to destroy the BN :-/
**My other acros in tank are doing great. Levels are fine as well.

TY!

[Uncle99]

I removed every crab 5 years ago now because of unknown deaths and coral losses
Since then, I have had no problems.

[mcgyvr]

There are certainly crabs that will eat corals.. like this..
https://www.melevsreef.com/critter/gall-crab

and certainly crabs that live on the corals that usually don't cause harm.. BUT.. a crabs gotta eat for survival.. Even your best friend looks like a tasty meal when you are starving to death..

[CTaylor]

Uncle and mcg,
TY ...
I'm going to likely to take him out and maybe put in a breeding trap for now.
**Yes Mcg, that looks like a brown fuzzy little monti/acro crab that i found ate coral very very fast.. that guy was pulled out asap. The one now is going more slowly.

[CTaylor]

Here are 2 pics of the crab (obviously removed). And a pic of the birdsnest, showing dead area right in the middle, right where crabby hung out.

Can it grow back over the skeleton? Maybe there is live tissue "in" the skeleton that will have it re-grow? **Is this coral going to die now b/c it was partially eaten? (assuming it was eaten).

BTW this was easiest the largest acro/sylo/birdnest crab I have seen in my history of reef tanks

TY!

[ThRoewer]

Originally Posted by CTaylor

Here are 2 pics of the crab (obviously removed). And a pic of the birdsnest, showing dead area right in the middle, right where crabby hung out.

Can it grow back over the skeleton? Maybe there is live tissue "in" the skeleton that will have it re-grow? **Is this coral going to die now b/c it was partially eaten? (assuming it was eaten).

BTW this was easiest the largest acro/sylo/birdnest crab I have seen in my history of reef tanks

TY!

That's a Trapezia crab, most likely Trapezia guttata or a similar species.
Never had issues with this crab or other Trapezia other than them disappearing on me. I would think the issues with the coral came from something else as these crabs are known to protect their host corals.

Do not kill or flush it!
These crabs are really hard to come by, yet highly desirable for a reef tank with SPS like Acropora, Pocillopora, Seriatopora,...
If you don't want it, give it to another reefer.
Also know that these crabs cannot survive without a coral for more than a couple of days. They need a special mucous the corals produce to attract them.

[CTaylor]

Thanks for that info Thro, as always
Perhaps there hasnt been long enough studies on each species and strain of those crabs to see if/when they eat more than coral mucus, as in if they eat coral tissue in certain circumstance, likke when then get large enough and mucus isn't enough. It might not be the crab, but I'm willing to bet it was the crab.

I'm finding that bnest do not easily regrow over their dead skeleton areas.. so unofrtunatly I'll likely frag it so algae doesnt overtake that area and infest the coral. At least I learned (i think ) some from this.

[ThRoewer]

I'm assuming the damage is due to the coral already struggling to survive and not because the crab is generally harmful.

BTW, I never had much success with Birdsnest corals and kind of given up on them.

[CTaylor]

See Uncle99's post. I just do not know how researchers can know unless they track a fair number of these crabs over a long period of time if they do or not ever eat SPS. It might not be something anyone thought to do over long period of time, so all is done is assume.

My bnest went from 3 branches of a frag (1") to pretty much a colony, it's been doing really well. Even when my tank was going through phases (for reasons due to alk fluxes -- bad test kits) where acros died.
obviousy anythign is possible.. but the crab is in there, a big one in comparison to other in my other SPS, all other (knock on wood lol) are doing well.
ONe thing possible, one thing that might have changed is that it sometimes gets pushed over my really large Mex turbos (which ate all my bryopsis -- that's another thread :-D ). and I've had to put bn back . I think i put it 180degrees turned from how it was last time I had to put it back. The change in lighting could have done this I'm thinking also. Really hard to tell Everything else I can test for is constant over the past. It might just stay a mystery

[ThRoewer]

There is no assume. There has been quite a bit research done on these crabs and their relation to corals and long time observations are were certainly part of it. The corals actually secrete a special mucous to attract and feed these crabs in exchange for protection from parasites and predators. That they are beneficial to corals is a fact and not just a theory.

Of course, a crab that is too large for a coral may stress it, especially if the coral is already stressed by fluctuating water parameters. In my tanks Birdsnest corals were always the ones to have problems first, long before Acropora or other supposedly more delicate corals.

[CTaylor]

ok ! .. so researchers have spent the time to investigate the little guys lol. Just odd that they can get all they need just from the mucus. Maybe (?) if the coral is not big enough to support the crab that it hosts, perhaps then the crab may eat polyps (?). OR maybe the crab is sitting in the same area too often because not much territory for it to exploere relative to it's larger size, and that impacted the amount of light that area was getting (?).

Interesting how my acros would (did when they did) die off and the bn would continue. I atttached a pic of when the bnext was nov'18 to right now today.

Though Im thinking more overall it may have the dead areas due to how it was oriented when I last put it back in its place. I just need to figure out if I need to frag it since the dead area will not grow over.

[clevername]

Originally Posted by ThRoewer

I'm assuming the damage is due to the coral already struggling to survive and not because the crab is generally harmful.

BTW, I never had much success with Birdsnest corals and kind of given up on them.

wondering myself how can a crab cause a larger coral to die...

[CTaylor]

Originally Posted by clevername2000

wondering myself how can a crab cause a larger coral to die...

the question, if it is one, is if the crab can/will/does eat it, or at least part of it. Yes, the coral is larger than the crab, but at least in my case the crab did not eat the entire colony (as you can see in the pic you cant even see the dead area, unless said crab is removed (again if it's even eating the coral, which is up for debate << at least for this specific crab species // I know the little brown fuzzy ones with blue eyes 100% eat sps << side note, but related).

[Daddi0]

I would rather have the crab in my reef than the birdsnest

[CTaylor]

Originally Posted by Daddi0

I would rather have the crab in my reef than the birdsnest

That's great news lol

[Uncle99]

With more than 4,000 species, it’s hard sometimes hard to identify conclusively which is which. LFS sell as necessities, they are not. They are all virtually scavengers and are unpredictable in the glass box.

In my post I do not mention anything about this crab is the culprit, my guess is likely not.

What I have learned over time is that once all crabs were removed, all this annoying and strange deaths stopped at the same time, or at least for me.

I have also learned that crabs are not a requirement for a healthy Reef and my fish are more comfortable without Jack the Ripper stalking at night.

If you love diversity then by all means, just keep in mind that with diversity, the rules of nature in a confined box can be skewed somewhat.

[CTaylor]

Originally Posted by Uncle99

With more than 4,000 species, it’s hard sometimes hard to identify conclusively which is which. LFS sell as necessities, they are not. They are all virtually scavengers and are unpredictable in the glass box.

In my post I do not mention anything about this crab is the culprit, my guess is likely not.

What I have learned over time is that once all crabs were removed, all this annoying and strange deaths stopped at the same time, or at least for me.

I have also learned that crabs are not a requirement for a healthy Reef and my fish are more comfortable without Jack the Ripper stalking at night.

If you love diversity then by all means, just keep in mind that with diversity, the rules of nature in a confined box can be skewed somewhat.

well said..
stil the crabs probably did it, not being politically correct < if for nothing else they might act differnt in an aquarium setting than in nature. For mine, I still can not see how my crab got enough food just eating the slime.. and there was very little if any food floating by him.

Though in nature, they seem to really spur the coral colonies to do better tahn without them. They seem to eat debris that can cause coral death if lingers on coral and algae growth.. and they fend off coral eaters like the spiny starfish.

They are also probably on much larger heads of coral than my 2-3" max bnest.

Yes i'm vascillating between who/what caused my BN to have the dead area ... swining back to crabby . Hopefully I can grow it large enough. Or I'll probably have to frag it and grow the frags large enough to IMO support safely a commensal crab.

[ThRoewer]

These are not commensal crabs but actual symbionts. In the wild the corals with crabs are healthier and have a higher chance to survive attacks by predators large (crown of thorns) and small (parasitic flatworms and pods).
In a reef tank they may or may not be beneficial to the health of the coral. It all depends on the coral-crab size relation and if there are potential external threats (parasites, other crabs, fish,...) to the coral in the tank.
These days these crabs don't come with corals by accident so I assume you added it intentionally. If you don't want it anymore, just bring it back to the store so another reefer who is looking for these can give it a new home.