Ostracods are a name given to a large family of crustaceans – coming in at a whopping 8,000 recognized species, making them the most successful group of crustaceans (that we know of, any way.) The largest member of this family reaches 32 mm (1.25 inches or 3.2 cm.)
Ostracods are mostly head and a few appendages. They have a two part hinged carapace that’s capable of shutting in their delicious (to fish) body and making them an unwanted snack for most smaller fish, but something akin to really crunchy popcorn for larger species. This shell is what gives them their common name of shell shrimp, seed shrimp, or mussel shrimp.
On first glance you can easily mistake them for daphnia (aka water fleas) or even cyclops shrimp.