Is it scientifically correct to say that crustaceans are insects of the sea, or is it just a myth/joke used to gross people out and put them off seafood?

 It's not entirely accurate to call crustaceans "insects of the sea" from a scientific standpoint. Here's the breakdown:

• Both are arthropods: Crustaceans and insects are indeed related. They share a common ancestor within the larger group called arthropods. This means they both have some similar features, like segmented bodies, jointed legs, and external skeletons.
• Crustaceans predate insects: Interestingly, crustaceans actually appeared on Earth before insects did.
• Separate groups: However, despite the similarities, crustaceans and insects are classified under different groups within the arthropod family tree. Insects belong to the group Hexapoda, while crustaceans have their own group, Crustacea.

So, while crustaceans and insects are close cousins in the animal kingdom, they're not the same thing.

However, the term "insects of the sea" might be used as a casual way to describe crustaceans because of the shared features and ecological role. In the ocean, crustaceans fill a similar niche that insects do on land – being abundant and diverse decomposers and scavengers.