What is this crab?

[Lynnzer]

Just dragged this out of my tank. I have no idea how it got in there and I'd like to know what species it is to get an idea if it's a danger to my reef tank. It's about the size of a 10p coin over the shell width, ie about 20mm. It has sort of light spot marks formed into a curled V shape over the shell. It has proper equally sized nippers. These aren't overly long either which seems to suggest it's a close at hand feeder. So, what the heck is it?

 

[Slaphppy7]

I bet @Donya would know...she was just here a while ago, but seems to have logged out

 

[Donya]

Keep him out of the tank! He can live in a bucket for a day. At a glance my first thought was european green crab but I really need to see the claws to be sure of that without turning to books to remind myself of some of the key features. For claws, it's specifically the shape of the ends side view. Other than that I would definitely need to consult some references to be sure just from carapace since it's done by spine count and where the dimples are and there are a number of very similar looking things - I can likely do that later today just swamped at the moment. It's a good carapace shot btw so ID from that alone should be possible. At a glance though I don't get warm fuzzy feelings about that crab living in a reef. Back legs look moderately flattened which is typical of swimming crabs even when there are no paddles.

General guessing guidelines for crab temperment: if the claws have pronounced points in the curved inner area of the claw and the tips are points, it will risk eating your fish. If it has one end as a pin and the other a cup/spoon or two spoons it will eat encrusting animals but may otherwise be "safe" depending on max size (mythrax is in this category - this is not one of those though). If the inside of the claws is knobbly it's a generalist that can crush things and harder to predict.

 

[Donya]

Actually just realized I can see claw profile from the reflection - ha! Looks like that 3rd category which adds weight to the green crab guess, also given it has 5 points each side of the right shape which is characteristic. Color is highly variable in the species and they are mean little things. Fun for a species tank but also escape artists. They are native to the UK coast I believe so somewhere in the chain of various things you have in your tank some local seawater was likely involved, whether added by you or a shop to their tanks, etc. They can come in as larvae and go unnoticed for some time.

 

[emeraldking]

Wonder how it got there? Did you put newly a new stone or something in there that may have contained this crab?

 

[Essjay]

In previous posts, the OP has mentioned collecting sand, rocks and sea water from the nearby coast. So if this is ineded a crab native to the UK coast, that could be the source.

I set up a small tank just to condition rocks ands sand before I transfer everything into another larger tank.
As I just scooped up seawater and live rocks and sand direct from the beach, and a couple of small bits of seaweed, oh and a crab, nothing needed heat.

On the beneficial bacteria front, I collected more sand from in between high and low tide marks yesterday, along with a small crab and more fresh sea water. I'll be using those in another tub to plonk some more rock in.

 

[Donya]

In previous posts, the OP has mentioned collecting sand, rocks and sea water from the nearby coast. So if this is ineded a crab native to the UK coast, that could be the source.

Ah yep, that will be the source then and it will be a green crab. There may be other native surprises that show up over time too.

 

[Lynnzer]

The tank was all cleaned out that was used to condition the rocks and sand from the beach. How in hell it got through that I'll never know.
Just for the record though, the description of green crabs being escape artists hits the nail on the head. The one I put in the tank when I was conditioning the rocks, made its escape into my bedroom next door. Just about to get into bed and it scuttles out from underneath.
So, it's about to become a meal for my anemones.

 

[Donya]

How in hell it got through that I'll never know.

Green crabs are pretty much hard as nails compared to many other Crustaceans from the tropical hobby. Their hardiness makes them a real pest where they are invasive. They live coastally here and get into lower salinity estuaries and tidepools and such, which means they can survive some really stinky, stagnant water and also apparently don't mind a bit of a fw dose when it rains heavily. Typical cycling conditions are nothing to those guys.

So, it's about to become a meal for my anemones.

Be careful unless the anemone is truly enormous...crabs can do a lot of damage to anemones before succumbing to the sting (if even affected at all; again dependent on anemone type & size).

 

[Lynnzer]

Be careful unless the anemone is truly enormous...crabs can do a lot of damage to anemones before succumbing to the sting (if even affected at all; again dependent on anemone type & size).

Quick crunch to the head, removal of legs and shell. Anemones loved it.