Slide Bivalve #2

Sunray Venus Shell © Diane Furrow

SEASHELLS – BIVALVES

Mollusks that have two shells are known as bivalves. These creatures have taken the shell for protection adaptation to the limit – they are completely covered with shell. The soft body inside has a heart, a foot, gills, muscles, nerves and digestive and circulatory systems. Did you notice any thing missing? Bivalves do not have a head as we think of it.

 

Even though their bodies are covered with shell, there are many predators that use them as a food source. Starfish and octopus are famous for their abilities to open tightly closed shells. Rays and other fish, as well as some turtles and birds have modified teeth (or bills) to crush the shell or cut the adductor muscle. We steam them to open the shell or cut their adductor muscle to reach the sweet meat inside.

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Cockle Shell © Diane Furrow

For the burrowers (most are known as clams), the foot is large and used for locomotion and digging. It is the foot we eat when we eat clams. Shorebirds are after the same thing when you see them in the swash line looking for coquina clams. Located near the foot is a sense organ called a statocyst that is used to sense orientation in the environment – like up from down. Most clams have developed tubes called siphons which act as snorkels to channel water to the gills. Then there are the bivalves that live unattached on the bottom with well developed tentacles and ocelli to detect danger in the environment like scallops. Scallops are the swiftest bivalves – they clap their shells together to create propulsion and avoid predators such as starfish.

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Scallop © UF/IFAS

Reproduction is external in this group. The gametes are released into the water at the same time in a mass spawning. The fertilized egg quickly develops into planktonic larva, known as veliger. Some species have long lived veliger stages like oysters. The dispersal of oyster veliger can travel as far as 800 miles! The larval stage ends when they attach to a surface and are called spat.

 

Many species such as oysters and mussels, do not move at all as adults. They have no need for a foot once they settle down. These are sessile animals.  Bivalves are filter feeders, removing organic particles and phytoplankton resulting in increased water clarity. A decline in bivalves has triggered the loss of both habitat and species in the Gulf region. Restoration efforts (particularly with oysters) are as much for the enhancement of the water and species diversity as it is for the commercial value.

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Oyster Reef Restoration © Carrie Stevenson

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