Mermaid's Purse
The Mermaid's Purse is not an invertebrate at all - it is a fish egg capsule. The egg capsule of a skate or ray, to be precise. The "horns" are to secure the capsule in seaweed. You may find one with a baby skate growing inside, but most are empty. They are included here with bivalves because they are often found scattered around the bottom like shells, up to 3" long. They certainly don't look like fish!
All of these bivalves are filter feeders. The other major group of mollusks is the Gastropods or snails. Squid and octopus ( cephalopods ) are mollusks too, but they don't behave like other mollusks, so I included them elsewhere. Teredos are bivalves that have evolved a worm-like wood-boring habit.
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Skates and rays are related to sharks. Unlike flounders, these fish are flattened and lie on their bellies. Skates are harmless. Stingrays have one or more dangerous barbed stingers in the tail and will use them if molested. Finally, the Torpedo is an electric fish and can generate enough voltage to be dangerous. Some skates give birth to live young, while others lay their eggs encased in a horned Mermaid's Purse.
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