Myxozoa

Tiny animals live in the water. They are some of the smallest animals ever.

These animals live inside other living things. They often live inside fish or worms. This is how they get what they need to live.

They have a special way to move. They use tiny parts to float in the water.

When they touch a fish, they can go inside. They can live in a fish for a very long time. Some stay for the whole life of the fish.

These small creatures live in both salt water and fresh water. They are very interesting to study.

Myxozoans are tiny animals that live in water. They are some of the smallest animals ever known. They are also parasites. This means they live inside other living things to survive. Most of them live inside a fish and a worm.

These animals used to be like jellyfish. They could swim on their own. Over a long time, they changed. They became very small. They even lost parts like muscles and a gut.

Myxozoans use spores to move from one host to another. A spore is a tiny part used for making new life. Some spores have special parts called polar capsules. These capsules work like tiny anchors. When a spore touches a host, the capsule fires a thread. This thread helps the spore stick and get inside.

Some myxozoans can make fish sick. This is a big problem for people who farm fish. Two diseases, whirling disease and kidney disease, affect salmon. These tiny animals are still a big mystery to science.

Myxozoans are a class of tiny aquatic animals. They belong to a group called cnidarians. This is the same group that includes jellyfish. Most myxozoans are obligate parasites. This means they must live inside another living thing to survive. They are some of the smallest animals ever found. Scientists have described over 2,180 different species. Some experts think there may be 30,000 more waiting to be found.

These animals have a very interesting way they work. They often use two different hosts to complete their life cycle. One host is usually a fish. The second host is often an annelid worm or a bryozoan. The process starts with a tiny spore. This spore has special cells called polar capsules. These capsules hold coiled filaments that can fire out. When a spore touches a host, these filaments act like anchors. They help the spore stick to the host and get inside.

In the past, these animals looked very different. They evolved from free-swimming creatures that were like jellyfish. As they became microscopic parasites, they changed a lot. They lost many genes used for moving or communicating between cells. They even lost things like muscles and a gut. Some species even lost the ability to breathe using oxygen. One species, Henneguya salminicola, was found in 2020 to lack a mitochondrial genome. This means it cannot perform aerobic respiration.

Finding out where myxozoans belong was a hard job for scientists. At first, people thought they were protozoans. Later, DNA testing showed they were animals. There was some confusion because of their genes. Scientists found that some samples were contaminated with host tissue. This led to wrong ideas about their family tree. Careful study of 50 genes showed they are modified cnidarians. Their closest relatives are the Polypodiozoa. These groups shared an ancestor about 600 million years ago.

While many myxozoans live quietly in hosts, some cause problems. They can be pathogens, which are things that cause disease. This is a big concern for the commercial fish industry. This happens often in aquaculture, where people farm fish. Some myxozoans cause whirling disease in salmon. Another one causes proliferative kidney disease. There is also a disease called "hamburger disease" in catfish. These tiny animals show how much impact small things can have on our world.

Myxozoa are a class of aquatic animals that belong to the phylum Cnidaria. This phylum includes familiar creatures like jellyfish and corals. Myxozoans are unique because they are all obligate parasites. This means they must live inside a host organism to survive and complete their life cycles. They are notable for being some of the smallest animals ever discovered. Scientists have described over 2,180 species so far. However, some estimates suggest there may be at least 30,000 undiscovered species still out there.

The biology of a myxozoan is highly specialized due to their parasitic lifestyle. They are highly derived cnidarians, meaning they have evolved significantly from their ancestors. They likely evolved from free-swimming, self-sufficient jellyfish-like creatures. As they became microscopic parasites, they underwent dramatic evolutionary changes. They lost many genes responsible for multicellular development and cell-to-cell communication. They also lost many physical structures, such as a nervous system, a gut, and cilia. Some species have even lost the ability to perform aerobic respiration, which is the process of using oxygen to create energy. One species, Henneguya salminicola, was found in 2020 to lack a mitochondrial genome.

Most myxozoans follow a complex, two-host life cycle. The process typically involves an intermediate host and a definitive host. In many documented cases, the intermediate host is a fish. The definitive host is usually an annelid worm or a bryozoan. The infection begins with a spore, such as a myxosporean spore. These spores are very small, usually ranging from 10 μm to 20 μm in size. However, malacosporean spores can grow much larger, reaching up to 2 mm. The spore contains specialized capsulogenic cells. These cells hold polar capsules, which contain coiled polar filaments. When the spore is ingested by an annelid, these filaments extrude to anchor the spore to the gut epithelium. Once anchored, the shell valves open so the sporoplasms can penetrate the tissue.

After the initial infection, the parasite undergoes reproduction and development within the host's gut tissue. This process eventually produces actinosporean spore stages, also called actinospores, within a structure called a pansporocyst. When these mature actinospores are released, they float in the water column. If an actinospore contacts the skin or gills of a fish, the sporoplasms penetrate the epithelium. The parasite then develops into the myxosporean stage. During this stage, they exhibit a "cell-in-cell" state. This is where secondary daughter cells develop inside the primary mother cells. The parasites may migrate through the host's nervous or circulatory systems to reach their final site of infection.

Identifying the exact place of myxozoans on the tree of life was a major scientific challenge. Originally, researchers thought they were protozoans, which are single-celled organisms. They were placed in a group called Sporozoa. As DNA sequencing technology improved, scientists realized they were actually animals. There was significant confusion regarding their relationship to the Bilateria, a group of animals with bilateral symmetry. This confusion happened because some researchers accidentally used samples contaminated with host tissue. This led to false results in gene testing. Eventually, careful study of 50 coding genes proved they are modified members of the phylum Cnidaria. Their closest relatives are the Polypodiozoa, and they shared a common ancestor about 600 million years ago.

While many myxozoans live in harmony with their hosts, some act as pathogens. Pathogens are organisms that cause disease. This is a major concern for the commercial fish industry, particularly in aquaculture. Aquaculture is the farming of fish in controlled environments. When new species are brought together, fish may be exposed to myxosporeans they have no immunity against. For example, M. cerebralis causes whirling disease in salmon. Another parasite, T. bryosalmonae, causes proliferative kidney disease (PKD) in salmon. Other notable diseases include enteromyxosis in marine sparids and proliferative gill disease, also called "hamburger disease," in catfish. These infections can have severe economic impacts on wild and farmed fish stocks.

Despite their tiny size, myxozoans show incredible biological diversity. They can live in both freshwater and marine habitats. Their anatomy is incredibly reduced, yet they remain functional. Because most lack muscles, they use other methods to move inside a host. These methods include using filopodia, spore valve contractions, or amoeboid movements. They do not undergo traditional embryogenesis and have lost true gametes. Instead, they reproduce through multicellular spores. This combination of extreme reduction and complex life cycles makes them one of the most fascinating groups of animals in the aquatic world.