Unionida

These animals are freshwater mussels.

They live in rivers and streams. They have two shells. The inside of the shell is shiny.

This shiny part looks like a pearl.

Mussels have a tiny foot. They use it to move. They also use it to bury themselves. This helps them stay safe in the mud.

Mussels have a special way to grow. They start as tiny babies. These babies must hitch a ride on a fish. They stick to the fish for a while. Then they fall off to grow. This helps them move to new places. They are very good at living in fresh water.

Unionida are a group of freshwater mussels.

They live in rivers, streams, and lakes. They do not live in the ocean. Most of these mussels have two shells. These shells have a shiny inside called nacre.

This shiny part looks like a pearl.

Mussels have a muscular foot. They use this foot to move slowly. They also use it to bury themselves in mud. To eat, they pump water through their bodies. This lets them get oxygen and food from the water.

These mussels have a very strange life cycle. They start as tiny larvae called glochidia.

These tiny babies use hooks to attach to a fish. They stay on the fish for weeks or months. This helps the mussels move upstream with the fish. Eventually, they fall off to grow into young mussels.

People once caught many mussels to make buttons. They also looked for pearls. Because of this, many species are now in trouble. Some are endangered or even extinct.

We must protect their clean water homes.

Unionida are a special group of living things called freshwater mussels.

These animals are bivalves, which means they have two shells joined by a hinge. Most of these mussels live in clean, fast-flowing rivers and streams. They do not live in the salty ocean. Instead, they stay in freshwater like creeks and lakes. These mussels are very important parts of their watery homes. They can even live for a very long time.

To live, these mussels use a clever way of working. They use a muscular foot to move slowly through the mud. They bury themselves in the bottom of the river. To eat, they pump water through their bodies. This helps them get oxygen and tiny bits of food. The shells have a shiny, pearly inside called nacre.

This nacre is made of organic matter. It can sometimes crack if the shell dries out. Their shells are often long and have pretty patterns on the outside.

The way these mussels grow is very unusual. They start life as tiny larvae called glochidia.

These tiny babies have little hooks on them. They use these hooks to grab onto a fish. The larvae stay on the fish for weeks or even months. This helps the mussels travel upstream with the fish. Some mussels even release sticky packets to help them stay put. Once they are ready, they fall off to grow into young mussels.

Humans have a long history with these animals. In the mid-1800s, people went through a "pearl rush."

People searched rivers for beautiful freshwater pearls to sell. Later, people used the shiny nacre to make buttons. This was a huge industry in the late 1800s. People even used a method called "pollywogging" to find them. This involved shuffling feet in the mud to feel for shells.

Today, many mussels face hard jobs to stay alive. Because so many were taken for buttons and pearls, many species are in trouble. In North America, about 70% of Unionida are extinct, endangered, or threatened.

Some species like the butterfly or the mapleleaf are well known. Many of these mussels are found in states like Alabama and Tennessee. We can learn a lot by looking at how they help keep our rivers healthy. Protecting their clean water is a big job for everyone.

Unionida is a monophyletic order of aquatic bivalve molluscs known as freshwater mussels.

This group includes most of the larger freshwater mussels, such as the freshwater pearl mussels. They are characterized by nacreous shells, which means their interiors are lined with a pearly substance. These shells are high in organic matter and may crack if they dry out. A defining trait of the order is a larval stage that is temporarily parasitic on fish. Unlike other bivalve orders, Unionida has no marine species, though one species, Glebula rotundata, can tolerate brackish water.

The physical structure of these mussels allows them to survive in specific environments. Their shells are typically elongate and equivalve, meaning the two valves are of roughly equal size. The interior features a pearly nacreous layer, radial sculpture, and an entire pallial line. To move, they use a muscular foot to slowly bury themselves in the substrate. They live in clean, fast-flowing freshwater like rivers, streams, and creeks. These mussels pump water through an incurrent aperture to obtain oxygen and filter food. Because their siphons are short, they cannot live deeply buried in sediment.

Reproduction in Unionida involves a complex and unique life cycle. Most species have separate sexes, though some, like Elliptio complanata, are hermaphroditic. During mating, sperm is ejected from the male's excurrent aperture into the female's incurrent aperture. Fertilized eggs move to the gills, called marsupia, to become glochidia. These are the first larval stage of the mussel. Mature glochidia are released and attach to a host fish using tiny hooks.

Some species use specialized methods to ensure their larvae find a host. Many release glochidia in mucilaginous packets known as conglutinates. These packets have a sticky filament to prevent them from being washed away. Some mussels produce a super-conglutinate that looks like a fish egg or an aquatic fly larva. This version has a dark eyespot to make it look appetizing to fish. When a fish eats the super-conglutinate, it breaks apart and releases the glochidia. Once attached, the larvae stay on the fish for weeks or months before falling off as juveniles.

The evolutionary history of Unionida is a subject of scientific study. Some fossils from the Carboniferous and Permian periods were suggested to be unionids. However, many authors disagree because those fossils lack the internal nacreous layer. This layer is found in the last common ancestor of all unionids and their marine relatives, the Trigoniida. The oldest unambiguous unionids come from the Triassic period. These are found in the Chinle Formation and Newark Supergroup of North America. Possible older examples might exist in the Middle Triassic of Tanzania and Zambia.

Humans have significantly impacted Unionida through various commercial industries. In the mid-1800s, a "pearl rush" occurred in North America. People used a method called "pollywogging," where they shuffled their feet in mud to find pearls. This was similar to the California gold rush. Later, in the late 19th and early 20th centuries, the nacre was used for button manufacturing. This heavy exploitation put many species close to extinction.

Today, many species face serious conservation issues due to habitat degradation and past over-exploitation. In North America, about 70% of Unionida are extinct, endangered, threatened, or of special concern. This includes 21 extinct species and 77 endangered species. Many species, such as the butterfly, mapleleaf, and washboard, were historically harvested. While natural pearls are often irregular, shaped like slugs or wings, the Japanese industry developed pearl culturing. This industry uses nacre from exported North American shells to create bead nuclei for round pearls.

Despite these challenges, the Unionida remain a vital part of freshwater ecosystems. They are found on six continents, with approximately 900 species worldwide. About 300 of these species are endemic to North America. Their ability to move upstream by hitchhiking on fish is a remarkable survival strategy. Understanding their life cycles and history helps us protect the rivers they inhabit. Many states, including Alabama and Tennessee, remain important areas for mussel production and study.