Is this a run-on sentence?
Coconut crabs are the largest terrestrial arthropods on the planet they can weigh up to nine pounds and grow to be up to three feet across.

 - yes
 - no
This is a run-on sentence. It is formed from two sentences run together, joined without punctuation.
Coconut crabs are the largest terrestrial arthropods on the planet they can weigh up to nine pounds and grow to be up to three feet across.
Here is one way to fix the run-on sentence:
Coconut crabs are the largest terrestrial arthropods on the planet; they can weigh up to nine pounds and grow to be up to three feet across.