The most common and most likely answer to this crossword clue is the 8 letter word TUNICATE
I've seen this crossword clue in The Guardian
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The straight part
We know that "marine invertebrate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marine invertebrate = tunicate.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TUNICATE mean?
TUNICATE noun- Any of very many chordate marine animals, of the subphyla Tunicata or Urochordata, including the sea squirts.
- Of or pertaining to these animals.
- Enclosed in a tunic or mantle; covered or coated with layers.
- Having each joint buried in the preceding funnel-shaped one, as in certain antennae of insects.