crayfish: tiny lobster-like crustaceans usually boiled briefly
shellfish: meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)
retrovirus: any of a group of viruses that contain two single-strand linear RNA molecules per virion and reverse transcriptase (RNA to DNA); the virus transcribes its RNA into a cDNA provirus that is then incorporated into the host cell