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Bivalves are aquatic animals much lower on the evolutionary scale from humans, but a definite link exists along the way to our complex selves. Bivalve anatomy as well as their reproductive behavior provide a link between more simplistic animals and more complex, like humans. We will explore a little about bivalves here.

What Are Bivalves?

Bivalves, or Bivalvia, refer to a class of animals that live in freshwater and saltwater. They belong to the phylum Mollusca, which are more commonly called ‘mollusks’.

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The name bivalve comes from Latin as most biological classifications do. ‘Bi’ means two and ‘valve’ is really ‘valvae’, which means ‘leaves of a door’. So essentially, the bivalves are those with a two sided shell that will open and close via a hinge, like a door.The first thing that might come to mind when you hear the word ‘bivalve’ is the common clam, oyster, or mussel. These are excellent examples of this class of animal as well as some of the most common.

Anatomical Features

The first characteristic of bivalves is two shells that open to reveal the animal itself.

Inside of the shell is a lining of soft tissue called the mantle. This is the real ‘meat’ of the animal and the part that one would eat if eating a bivalve such as a clam. At the front and rear of the animal are anterior and posterior adductor muscles, respectively.

These are responsible for holding the shell closed, similar to a human’s adductor muscles in the legs. In humans, these allow you to bring your legs together.

Looking closely at the left side picture you can see two small ovals, one towards the front and one towards the back. This is where the adductor muscles attach.
Here we see a pair of siphons sticking out from a clam.

Siphon

The digestive system of a bivalve is fairly simple, similar to a worm’s, a far cry from the human digestive system. An open circulatory system, heart, and nerves round out the main bulk of the mantle of the animal.

Again, this is all enclosed inside of the shell.Next let’s focus on the reproductive system since the next section will cover bivalve reproduction. The gonads of bivalves are very close to the intestinal tract of the animal.

Some are also hermaphroditic, possessing both male and female gonads, which is common in the more basic classes of animal. Most often, we see separate sexes in bivalves.

Reproduction

Even though bivalves make up a large class, they exhibit similar reproductive behaviors. Freshwater bivalves will ‘free spawn’ by releasing their sperm and eggs into the water. The eggs become fertilized in the water and develop as plankton, eventually growing into the animal itself. Many other bivalves will actually reproduce in some ways similar to humans.

The male releases sperm into the water that the female then takes in while taking in water to eat. The sperm fertilize the female’s eggs and larvae begin to form and grow. This happens inside of the female, until they reach a point where they are ready to be released.

Once the larvae are released, they attach to the gills of the fish as they swim through. This causes them to act similarly to parasites, utilizing the fish to provide protection. In some species, the fish are actually lured in by the bivalve, thinking it is a food source. When the fish attacks, it causes the sacs holding larvae to break open, causing them to spill out into the water and attach to the fish. The fish are unharmed and simply act as an unsuspecting host for the bivalve in question.In saltwater bivalves, fertilization and reproduction usually occur in the water outside of the animal. Sperm and eggs are released into the water, where the sperm can then fertilize the egg.

Many bivalves release hundreds of sperm and eggs into the water in mass spawning events. Others will release a few when needed. You will notice that this is similar to the behavior of some of the freshwater species of bivalve as well.

Lesson Summary

Bivalves are a class designation of animals. They typically have two shells joined by a hinged ligament that also includes interlocking teeth to keep the shell halves lined up. The mantle of a bivalve is the meaty portion that includes the circulatory, respiratory, digestive, and reproductive systems of the animal. Toward the front and rear of the animal exist adductor muscles that help snap the two halves together.

Some bivalves are hermaphrodites, but most have defined sexes. Males typically release sperm into the water (whether freshwater or saltwater) and females release their eggs. The eggs become fertilized, grow into larvae, and then finally an adult. In certain freshwater species, the female takes in the sperm, fertilizing the egg inside of her.

As it reaches larval stage, the female releases them into the water where they attach to the gills of fish, until reaching maturity. In some cases, the females will actually lure the fish in, allowing them to attack, which releases the larvae.

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