Cleaner shrimps vanished

First one and a few weeks later the second cleaner shrimp disapeard. Just vanished, gone, AWOL....Not a trace left. These were mature, 2" long body, shrimps. They showed no signs of distress and were molting regularly. I only have a few small fish and one small hermit crab and neither has ever bothered the shrimp.
Any ideas what could have happened? How old do cleaner shrimp get?

 

What fish types do you have?

 

Fish in my tank: 2 small Chromis, 1 yellowtail damsel, 1 small 6-line Wrasse, 1 Royal Gamma, 1 Maroon Clown.
I have not observed the fish bothering the shrimp - but then again I am not observing 24/7.

 

I have had clowns turn on shrimp after being in the same tank for years, just one possibility.

Any serpent stars, that would be my first usual suspect?

Next I would check the parameters, are you doing routine water changes?

 

+1

I've also read a few threads about royal grammas killing and eating shrimp.

 

I have some brittle stars but the ones I have see are small unelss there is a larger one I have not seen during the day. The maroon clown is about 2.75" and probably big enough to kill a good size shrimp. What puzzles me that there is no trace of any shell, legs or other hard body parts left over from "the kill".
The water parameters are normal and everything else is doing fine and I do change water regularly (a must for a small 20 Gallon tank).
I have not seen the Royal Gamma to go near the shrimp when they were still around although he too is large enough to tackle a shrimp.

 

6 fish in a 20 gallon tank? theres your answer to the missing/dead shrimp

 

I had 6 small fish in this tank for a long time. It may be the max but it is not too many. I have never seen any of my fish even attempt to go after the shrimp (they get plenty fed) and I had the shrimp for a long time too. And they were as big or bigger than the fish. I don't think this is it. Something else happened.
The shrimp must have been killed or died during the night (when the fish are nor active). I may have to observe the tank during the night to see if I have a large brittle star.

 

1: How old is the tank?
2: Do you have problems with Snails and Hermits disappearing?
3: Where did you get your LR?
4:How deep is your sand bed?

 

Just seems odd that they would both die of old age around the same time.