squid

Yeah ika would probably be written in hiragana when in doubt.
However, although katana is reserved for words originating from
languages other than japanese (as previously stated), it is sometimes
used to emphasis a word (like italic writting in english), and is
considered quite trendy.
my dictionary gives me both, along with the kanji. hiragana/katana
is also used when the kanji is not considered common, or is rarely used.
finally, ika has several meanings in japanese, and I think the katana
writting for ika is considered to mean squid (since the other "ika"
are always written in kanji (or hiragana if you can't remember).

another example of this is ebi (shrimp) which is written in Katakana
in restaurants.

Hi.

> languages other than japanese (as previously stated), it is sometimes 
> used to emphasis a word (like italic writting in english), and is 
> considered quite trendy.

You are most likely to see this when Japanese is written horizontally.
When written vertically, other typographical conventions such as
putting dots next to the word or putting a line next to the word are
available.