Hermit crabs hiding or dying?

[GilliganReef]

Ok my 40g tanks been running for 11 weeks now. About 3 weeks into my new 40 I have added my red hermit crabs. In the last two weeks I have added 10 pounds of premium live rock, covered in coralline, purple and pink. Before I added the LR I had 9-11 hermits I could only find and count 9 but I know I have purchased and or transferred from old tank 11 hermits. Now i have more LR and caves and hiding spots. Question is are they hiding and being hard to find or dead? Now I can only count 4 and I have look all over my tank. Went out at night and search.

40g breeder, 75lbs LS, 38lbs LR, aqua lunar 4 t5 h/o lighting, 20g sump, coral life super needle skimmer 65g.

Salinity 1.023
Ph 8.2
Nitrite 0
Ammonia 0
Nitrate 5pmm>
Calcium 520
Alk 7 dkh

The parameters have been the same since I have had the crabs in the tank. I do 10% water change weekly with a 20% every two weeks.

Anybody have a clue what's going on?

 

[Sugar Magnolia]

Mine tend to spend their day hiding in the rockwork. Lazy bums!

 

[PCygni]

Most of mine come out of the rocks when I feed the fish. Don't recall ever being able to count all my crabs at one time though. I know a few have died. I think the bigger ones prey on the smaller.