Starfish dropped a knob - help!

[reddustee]

I have a red-knobbed sea star and it has been rather active (and is still moving around) in my tank, which is only about 2 weeks old. I have live rock and live sand, a little crab (about 1cm diameter body), 3 featherdusters, 1 cardinal fish, 2 small little blue fish which i have no idea what species they are) and a few very tiny snails (2-3mm in size, that is if they have not been eaten up yet).

Today i noticed that one of the 5 central knobs dropped off. What's happening? Is it sick? Did it get into a fight with the litte crab? Is it reproducing? There also seems to be tiny black spots on it.

What should I do? I don't have any water condition testers.

I have attached 2 pics (sorry if it's blur), one shows the little knob that has dropped off, and the other the starfish after losing it's knob.

I'm very new to this hobby and am very keen to learn more about what i'm doing with my tank. Please offer some advice.

Thanks.

 

[mr.reef24]

how long have you had the starfish? what kinda crab is it and what do the snail's look like in the system?

mr.reef24

P.S. Any fish in the system?

 

[reddustee]

I've had the starfish for about 1 week. Here's a clearer picture of it without the knob. BTW, I can't locate that borken off knob in my tank anymore... not sure where it's gone to.

I've only got 3 other fishes in the tank (also since about 1 week ago, as my tank is still new) -2 blue ones and 1 cardinal (pictures attached).

As for the crab, I can't be sure what species, since it's hiding most of the time and only appears when it's dark (it came with the live rock), and so I also can't get a picture of it. It's brown and seems to be a bit furry. It's pretty tiny, even with it's leg spread out, it's about 2.5cm wide. Looks a bit like some of these:
Schizophrys aspera or Mithrax sculptus or Composcia retusa, based on pictures from this link:
http://www.wetwebmedia.com/Arthropoda/CrustaceanPIX/SWCrabs/SWCrabs2.htm

No idea about the snail, hardly see them around. again, it's tiny, less than 5mm in size.