A kind of mussel · also called common mussel

3 views: its portrait, and how it looks in the comparison photos.
About
A medium-sized edible shellfish whose whole shells pack loosely with plenty of open space between them.
Whole blue mussels have a bulk density of about 44 pounds per cubic foot (0.7 grams per milliliter) when loosely packed. Because their hard, curved shells do not fit tightly together, a container of whole mussels includes a large amount of empty air space. This makes a basket of whole mussels significantly lighter than the same volume of water or solid seafood meat.
From Wikipedia
The blue mussel, also known as the common mussel, is a medium-sized edible marine bivalve mollusc in the family Mytilidae, the only extant family in the order Mytilida, known as "true mussels". Blue mussels are subject to commercial use and intensive aquaculture.
Read more on Wikipedia →Text from Wikipedia’s “Blue mussel” article, CC BY-SA 4.0.
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One cup weighs
5.84 oz (166 g)
Quick answer
One US cup of whole blue mussels weighs about 5.9 ounces (166 grams). That is roughly the same weight as a cup of packed brown sugar.
Typical figures, the same ones every comparison on the site uses; real ones vary.
More like whole blue mussels
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Next to anything: a person, a car, another mussel.