Need help with mystery noise
 

About 5 months ago, I started noticing a strange noise coming from my reef. At first it only happened after the lights went out, but now it sometimes happens during the day as well.

It sounds like a series of sharp, fast knocks, with a resonating quality that makes them sound like a knock mixed with a super sharp frog's croak. Usually 4-6 in a row. Sometimes it's really loud, as though a small hammer is banging on the glass, other times it's quite muffled. You can hear it 20 feet from the aquarium.

I've not added any new live rock in a year, so whatever is making the noise must have pretty much been a larvae when it arrived.

It happens at least once a day, but sometimes 2 or 3 times a day. A couple days ago I was scraping the sides of the glass when it started - it sounded like it was coming from 2 inches in front of my face, but I couldn't see anything. In fact I've never seen anything, it always sounds like it's coming from behind the rockwork (the side I can't see).

I've narrowed it down to either a pistol shrimp or a mantis shrimp. I have lots of hermit crabs, and at least 3 other species of crab that hitch-hiked in, as well as several cleaner shrimp. None of them are disappearing so it makes me think it's not a mantis shrimp? I've never heard a pistol shrimp or a mantis shrimp before though. Any advice?

I have had the same thing in my tank for almost 5 years.

From what I understand.... and Mantis experts please feel free to correct me... but what I understand is that a Mantis would probably be out and about at some time. They are a predator so it only makes sense that they show themselves at some point... particularly if the tank is being fed.

My pistol shrimp makes most of his noise at night when lights go out. Its a fairly loud snap ( think tank cracking). Mines paired up with a Gobi but I don't see him or the gobi much (new tank and tank mates). If you have a pistol pair him up with a gobi and maybe you'll see him at a later time.

Is it a single noise? Or is it a sequence of cracks? When it's really loud it sounds like the tank is about to bust open, but it's always a series of them. The first one is usually slightly quieter than the rest as well, almost like the thing that's making the noise is 'warming up'.

DO you ever notice any piles of sand where a burrow has been dug? Either way, I'd say it's gotta be a pistol. The piles of sand would somewhat verify that (IMO). One of mine fires rapidly several times... the other is an occasional single crack.

Sometimes it just one, and some times its a lots. I think it depends on what he is doing? If he is out looking for food then usually its multiple snaps. He stuns what ever he is going to eat, and then eats it. But if its in a shell like a snail or hermit crab be breaks open the shell with his claw. I have found broken shells arouond the old tank. If he is making his home (under the rock work) he will push out all the sand in to piles by his entrance to his home, and he will drag little bits of rock and empty snail shells to make a solid entrance way for him. And then its usually just 1 snap.
Before I moved into the new tank I use to see my shrimp and gobi all the time. It was neat to see, if you saw the gobi you saw the shrimpa few mins later and the gobi would just watch him to make sure it was safe for him.

What ever is in mine has NEVER shown itself. I kid you not... it came on a rock in my original 20 gallon. I have since moved the rocks 3 times (tank leak, moved, upgrade to larger tank) and he comes along for the ride.

I never have broken shells and as for dead hermits or snails... nothing out of the ordinary. Everyone loses a snail here or there... and in almost 5 years I have only added extra snails since upgrading.

I am wondering if maybe he is a super small species or something? HUGE sound from the cracking though. It used to worry me, but now I could care less.

Anyone know how to find these buggers? I have no leads to a burrow or anything.

I am interested to find him as it seems this topic comes up quite frequently.

When mine was in my biocube I saw him regulary, but now that I moved into the 150 G I havent seen him at all. I dont even know where is burrow is! I know he is still alive as I hear him clicking. I also did not see the gobi for almost a month when I added new fish to the cube and its been almost another month with out see him in the 150 G, I saw him for the first time in almost 2 months at the start of the week.
If its a pistol shrimp you will not see him unless he is paired up with a gobi. But there is no garrinty you will see him after he is paired up with a gobi, they could only show up behind the rock work where you wont see them.

As well if the gobi is scared in the new tank you wont see them. When mine were in the cube if the gobi got spooked they both went back into the burrow and waited till the gobi thought it was safe again. He is the eyes for the two of them. Its a strange marriage.

my tank makes knocking noises too and it is typically always a large hermit crab banging his shell on the glass.