pycnogonid vs sea spider

pycnogonid and sea spider both are nouns.

Word	Noun	Adjective	Verb	Adverb
pycnogonid	Yes	No	No	No
sea spider	Yes	No	No	No

As nouns, pycnogonid and sea spider are synonyms defined as:

pycnogonid and sea spider: any of various small spiderlike marine arthropods having small thin bodies and long slender legs

pycnogonid (noun)	sea spider (noun)
any of various small spiderlike marine arthropods having small thin bodies and long slender legs	any of various small spiderlike marine arthropods having small thin bodies and long slender legs