Help - Dangerous critter?

I have had no luck or helpful advice in identifying a live rock hitch hiker in my marine aquarium, the kicker is..it's still alive and doing well despite these images. These seem to be the "shedding" it leaves behind 2 times in 1 month.





My 55gal. reef tank has been going for almost 4 weeks, when in to week #2 we see a nearly 2-3 inch long critter start rapidly floating all about under the moonlights. While attempting to net it, it became lifeless and it's body lay on the sandbed. We did'nt even touch it yet with the net just turned the lights on and it just seemingly died. Even stranger still the thing started inching it's way to a nearby rock then within a few moments stopped. Almost as if it had exited it's body. We knew something was strange about it and when removing it, it almost looked like a ghost shrimp. Now almost into week #4 we wake up to the exact same body shell laying ontop of a rock lifeless. After looking at it closer it appears to have 8 appendages or 6 legs and 2 antenna? I don't know, but it does'nt resemble any of the spiders I've researched. Whatever this thing is, it seems to shed itself instantly whenever it feels threatened?

The night I saw it in it's "live" form it was rather large at approx. 2 1/2 to 3 inches and virtually completely transparent. At first glance I thought maybe I had a jelly fish. But as it nestled under a rock it had what almost appeared to be antenna's..I've looked up lots of hitch hikers but cannot indentify this, if my tank or myself is in any danger I would sure like to know. I have had a few hermit crabs die or disappear but cannot say that is'nt from just being a new tank. Can anyone out there help me?

 

looks like an arrow crab maybe decorator arrow crab

 

Im pretty sure its some kind of worm, just chuck it down the toilet

 

I think you should consider the possibility that you have an alien in your tank,,,,,just saying !

Crab molts generally look just like the crab, so I am confused by your strange looking molt. But it does have the appearance of a spider crab. Once a crab molts his skin/shell he will retreat until his new shell hardens, it could be possible that you saw him after the molt and in his most vulnerable stage with a soft shell (I suppose it could appear transparent). Try to get some pictures of the live critter for us. I am sure someone on this site will know. but for the time being, I am going with the spider crab

 

Pycnogids (sea spiders) molt when they grow. They come in all shapes and sizes. That would be my best guess.

 

Thanks everyone for the advice. The pictures just do not do this thing justice, you are right, I need to capture it in the tank, which I will try hard to do. Sadly I have only seen this thing 3 times in 1 month. 2 of which seem to be "moltings".

Additionally, it "is" transparent, or see-thru whiteish. Swam in an parachute manner which made me think jellyfish. Whatever it is, how can I drive it out of the rock? I know it may not be harmless but it is unwelcomed lol. Another hermit ended up dead (that's 4 in 1 week) and I admit this could be from my new system however at this point I just want it out of my tank, any suggestions?

The live rock was from a combination package and truely was magnificent, of course it has experienced a lot of die off during curing and following thier directions we sat it in a higher salt content to drive away any unecessary critters, so I am baffled as to how to get it out of there, any and all info would be mega appreciated.

 

To get a better look, I resubmerged it in water, so I could spred it out abit from being so stiff and sure enough I now believe this is what it is also, but I do not believe he is the only one, as the one we saw was 3 times that size initially. Either way, if it has friends or not, it still lives in there and I need to remove it, how could I go about this, especially if there is more than one?

 

BTW, pardon my rudeness. Welcome to 3Reef!!!

Capturing this guy could be challenging. Some sea spiders eat worms. Others eat crustaceans or bi-valves. Some are herbivores. Some are omnivores and eat everything.

Frankly, if it's a species that stays small like it is now, I wouldn't worry about it too much. You might want to contact the retailer of your live rock and see what species inhabit where they collected their rock from.

 

Thank you for the warm welcome. I have tried other forums and I really like it here already

We actually just found it in the tank! It is at the very corner in the back half buried in sand? It is definitely alive and not another molt. It seems to have antenna though, I am completely puzzled. Will try to capture a nice shot when able and update. We don't have the proper tools yet to go digging behind the rock. Going to watch him awhile and see how it acts, moves, and most importantly what it's eating. Thanks again for the insight.

 

Well, we went and tore down some of the aquascape and got it. Much to our dismay it is what looks like the head of another molt or ghost shrimp, the saga continues ...