Mussel

"So, when Gobber blew the horn and moved out of sight to find a comfortable rock to sit on and eat his mussel-and-tomato sandwich, Snotlout pushed Hiccup out of the way and took charge."

- How to Train Your Dragon

Mussels are mentioned very minimally in the How to Train Your Dragon Book Series.

Description
Mussels are any number of species in the Class Bivalva and are mostly grouped into the Family Mytilidae, though some species from other Families may be referred to as mussels. Mussels are typically characterized by dark-colored asymmetrical shells. The shells are usually longer than they are wide. Mussels are also sessile creatures, meaning they stay fixed in one location. There are also a few species referred to as mussels that occur in freshwater, but do not belong to the Family Mytilidae. Mussels are generally filter-feeders, consuming a lot of Plankton.

Function
The primary function in the Book series and in reality is for food. Many cultures around the world consume mussels, and a wide range of dishes exist to prepare them.

Mussels have also been used for environmental clean-up of ocean water. Mussels eat phytoplankton which absorb phosphorus and nitrogen, which is in excess from agricultural run-off and can damage the health of the waterways. The mussels are then harvested and either eaten or used elsewhere, thus removing the phosphorous and nitrogen.

How to Train Your Dragon
Mussels are mentioned in this book in a sandwich which Gobber eats.