It's not clear from your question what you already know. If you have the 4 checked letters, the Chambers WordWizard only gives two
options - only one will work with the theme.
Yes -the word I have in the grid it is an alternate spelling for the shellfish word - as puzzled272 says it works in Chambers Word Wizard, and when treated leads to a suitable thematic word.
Are we talking at cross purposes? :)