Shade
Monster Junkie
The incompleteness is so the reader can use their imagination to fill it in.
Do you have anything specific you'd like to complete it with?
Just a slight reworking to make it a complete sentence.

This scuttling mass is composed of saucer-sized crabs, climbing over each other in their eagerness to feed.
...or...
A scuttling mass of saucer-sized crabs climb over each other in their eagerness to feed.