43 Veligers and Polyps-High

Veligers and Polyps FOCUS Question How do ocean invertebrates develop?

Life in the Sea When you think of animals, what do you picture? Dogs, cats, fish, and birds? These animals all have backbones. They are called vertebrates. But most animals on Earth are invertebrates. These animals do not have backbones. They include everything from insects to snails to octopuses. Many invertebrates live in the sea. Two examples are bivalves and sea jellies. These animals have strange and fascinating life cycles!

Bivalves If you eat seafood, you have probably seen bivalves. Clams, oysters, mussels, and scallops are all bivalves. An adult bivalve has a two-part shell. The shell protects the animal's soft body. Most of the time, the bivalve keeps its shell slightly open. A tube on its body sticks out of the shell. This tube, called a siphon, pulls water inside the shell and across the animal's gills. The siphon also pulls in tiny bits of food and moves them into the mouth. Many adult bivalves attach themselves to rocks or coral. Others bury themselves in soft sand or mud. Word Wise The prefix bi- means "two." Valve means "door." Bivalves can close their two shells like a pair of doors.

Baby Bivalves Young bivalves live different lives than adult bivalves. Bivalves hatch from eggs laid in the water. A young bivalve starts as a larva called a veliger. The veliger swims freely in the water. Veligers are tiny. They have only a single shell. They also have a body part called a velum (see photo). The veliger flaps its velum to swim and grab food such as tiny plants. As the veliger grows, its shell gets bigger and heavier. Eventually, the veliger sinks toward the bottom. It develops a strong foot that helps it crawl along the seafloor.

Changing Shape Once the veliger is on the seafloor, it goes through metamorphosis. It changes shape as it becomes an adult. The veliger shell grows into two halves that can open and close. Most adult bivalves become sessile, or stationary. They stay in one spot for the rest of their lives. Clams use their foot to dig a hole in the seafloor. Mussels and oysters make a powerful cement to attach themselves to a hard surface. They stick to such things as rocks, coral, or wood. Do You Know? Not all adult bivalves stay in one spot. Scallops can swim, even as adults! They use strong muscles to "flap" their shells together.

Aquaculture In many places, people farm and harvest bivalves. Several kinds of bivalves are raised as food sources. Oysters are raised for food and for the beautiful pearls they make, which can be used in jewelry. Ocean farming is known as aquaculture. Aquaculture involves several steps: 1. Farmers collect adult bivalves from the ocean. 2. Bivalves lay eggs in tanks of warm water. The veligers hatch. 3. The farmers feed and care for the veligers until they are ready for metamorphosis. 4. The farmers set up ropes, nets, or wood in a shallow seashore area or in the mouth of a river. 5. They release the veligers in this area. 6. The veligers attach to the structures set up by the farmers. There, they change into adult bivalves. 7. Bivalves are collected and sold to stores and restaurants.