Is my Arrow Crab normal? What is this?

[luconi]

It has a bottom that is like something on a hinge that opens and closes. It has had this for a while and seems fine. Is the opening hinge bottom thing normal for it?

 

[LeslieH]

Crabs have abdomens just like shrimp. In shrimp the abdomen is large (that's the meaty part we eat) but in crabs it's reduced to just a flap that curls under the body. In males the abdomen is shaped like the Eiffel Tower while in females it's broad & rounded. The female carries her eggs inside this tucked in abdomen; when full the abdomen bulges out & it flaps so fresh water washes over the eggs to oxygenate & clean them. So you might have a very pregnant female there.

The other possibility is that the crab is carrying a parasitic barnacle called Sacculina. The juvenile barnacle attaches itself to a crab. It never develops the hard outer plates; instead it burrows into the body of the crab. Some of it bulges out in the abdomen area & looks pretty much just like a bunch of eggs. If the mass inside the abdomen is solid then it may be a Sacculina. If it looks like a bunch of tiny grapes then it's eggs.

How to sex a crab: http://www.dfg.ca.gov/delta/mittencrab/crab_sexing.asp
Sacculina: http://www.theseashore.org.uk/theseashore/SpeciesPages/Saccullina.html
female with eggs: http://www.dfw.state.or.us/MRP/shellfish/commercial/largeimages/dungeness-with-eggs_ODFW.jpg

 

[luconi]

Crabs have abdomens just like shrimp. In shrimp the abdomen is large (that's the meaty part we eat) but in crabs it's reduced to just a flap that curls under the body. In males the abdomen is shaped like the Eiffel Tower while in females it's broad & rounded. The female carries her eggs inside this tucked in abdomen; when full the abdomen bulges out & it flaps so fresh water washes over the eggs to oxygenate & clean them. So you might have a very pregnant female there.

The other possibility is that the crab is carrying a parasitic barnacle called Sacculina. The juvenile barnacle attaches itself to a crab. It never develops the hard outer plates; instead it burrows into the body of the crab. Some of it bulges out in the abdomen area & looks pretty much just like a bunch of eggs. If the mass inside the abdomen is solid then it may be a Sacculina. If it looks like a bunch of tiny grapes then it's eggs.

How to sex a crab: http://www.dfg.ca.gov/delta/mittencrab/crab_sexing.asp
Sacculina: http://www.theseashore.org.uk/theseashore/SpeciesPages/Saccullina.html
female with eggs: http://www.dfw.state.or.us/MRP/shellfish/commercial/largeimages/dungeness-with-eggs_ODFW.jpg

Well when I llok inside it doesn't seem like anything is really in there at all. It just looks like a flap that opens and closes but has nothing in it. I'll probably check closer next time though to make sure

 

[LeslieH]

It may have just released the eggs then.