Dicyemida

Tiny bugs live inside some sea life.

These bugs are very small. You need a microscope to see them. They live in the parts that clean the host animal.

These bugs have different shapes on their heads. Some heads look like cones. Others look like flat disks.

Shapes help them find a place to stay. Cone shapes fit in small folds. Flat shapes stay on smooth spots. This helps many bugs live together. They do not fight for space. It is a clever way to live.

Dicyemids are tiny parasites. They live inside the kidneys of cephalopods. Cephalopods are animals like octopuses. These bugs are so small you need a microscope to see them.

Each adult bug has the same number of cells. This is called eutely. They have a very simple body. They lack a heart or a brain. A single center cell is covered by a jacket of tiny cells. These outer cells have hairs called cilia. These hairs help the bug move.

The front of the bug is called a calotte. This part helps them stick to their host. Calottes come in different shapes. Some look like cones. Others look like flat disks.

Shape helps them find a home. Cone shapes fit in folds. Disk shapes stay on smooth spots. This helps many species live in one host. They do not fight for space.

Dicyemids have two life stages. They can make more bugs without a partner. This is asexual reproduction. Later, they use sexual reproduction. They make larvae that swim like they have headlights. These larvae leave the host when it goes to the bathroom.

Dicyemids are tiny parasites that live inside cephalopods. Cephalopods are animals like octopuses. These creatures live in the renal appendages, which are parts of the host's kidneys.

Because they are so small, you need a light microscope to see them. They are very simple living things. They do not have a heart, a brain, or a stomach. They also lack systems for breathing or moving waste.

A dicyemid has a very basic body structure. It has one central cell called an axial cell. This center cell is wrapped in a jacket of twenty to thirty cells. These outer cells have tiny hairs called cilia. The front part of the body is called a calotte. This calotte helps the parasite stick to the host's kidney folds.

Different species have different calotte shapes. Some are shaped like cones, while others look like disks or caps.

Scientists have studied these tiny animals for a long time. In 1938, researchers found two types in Japan. They were named D. misakiense and D. japonicum. At first, scientists thought they were the same species. They looked very similar to the naked eye. Later, they saw the calotte shapes were different. Now, experts still argue about how these two species are related.

There are many interesting facts about how dicyemids grow. Each adult of a species has the exact same number of cells. This special trait is called eutely. Shape helps different species live together without fighting. Conical calottes fit into the folds of the kidneys. Rounded calottes attach to smooth surfaces instead. This allows many species to share one host.

They use different spots so they do not compete for space.

The life of a dicyemid has two main stages. In young hosts, they use asexual reproduction. They create larvae called nematogens that grow into more adults. In mature hosts, they switch to sexual reproduction. They form rhombogens which have special parts called infusorigens. These produce larvae that swim with rings like headlights.

These larvae leave the host when it eliminates urine. We still do not know exactly how they infect a new host.

Dicyemida, also known as Rhombozoa, are a phylum of tiny parasites. They live inside the renal appendages of cephalopods, which are animals like octopuses. These organisms are extremely small. You can only see them using a light microscope.

Scientists are still debating how to classify them. Traditionally, they were grouped with Orthonectida in the phylum Mesozoa. Recent molecular evidence from 2017 seems to support this grouping. However, other molecular studies suggest they might be closer to roundworms. Other evidence even points to them being derived from the Lophotrochozoa. This ongoing debate shows how much we are still learning about their history.

The body of a dicyemid is very simple. They lack respiratory, circulatory, excretory, digestive, and nervous systems. The structure consists of a single axial cell at the center. This central cell is surrounded by a jacket of twenty to thirty ciliated cells. The front part of the organism is called a calotte. This calotte helps the parasite attach to the surface of the host's renal appendages.

One interesting trait is called eutely. This means every adult of a specific species has the exact same number of cells. This constant cell count is a useful way for scientists to identify different species.

Calotte shape is a vital part of how these parasites live. Calottes can be conical, disk-shaped, or cap-shaped. The shape determines exactly where a dicyemid can attach. For example, species with conical calottes fit best within the folds of the kidneys. Species with rounded calottes, like disks or caps, attach to smooth kidney surfaces. This is a form of extreme segregation of habitats. Because they occupy different ecological niches, multiple species can live in one host without competing for resources.

To date, no two different species with the same calotte shape have been found in the same host.

Finding a single species of dicyemid in a host is actually quite rare. Usually, a host will contain many different species at once. If two species have very similar calotte shapes, one will often dominate the other. This shows that one species has adapted better to that specific environment. In studies of octopuses, researchers found that species with similar shapes rarely coexist. This suggests they do compete strongly for habitat. In Japan, researchers found two types, D. misakiense and D. japonicum, living together.

In 1938, scientists thought they were the same species because they looked so similar. They only realized they were different once they observed the different calotte shapes.

The life cycle of a dicyemid changes based on the host. They exist in both asexual and sexual forms. In juvenile or immature hosts, asexual reproduction is the main method. These asexual forms are called nematogens. They produce vermiform larvae within the axial cell. These larvae develop directly into more nematogens. This process allows the parasites to fill the kidneys of young cephalopods quickly. This density-responsive cycle is similar to how some trematodes reproduce in snails.

As the infection ages or the density increases, the cycle shifts. The larvae mature into rhombogens, which are the sexual life stage. Rhombogens contain hermaphroditic gonads located inside the axial cell. These are more accurately called infusorigens. These organs self-fertilize to produce infusoriform larvae. These larvae have a very distinct look. They swim using ciliated rings that look like headlights.

It is believed these larvae are released when the host eliminates urine. This is likely how they disperse to find new hosts.

While we understand much about their shapes, many mysteries remain. We still do not know the exact mechanism of how they infect a new host. We also do not know what effects they have on their hosts. Dicyemids are most abundant in temperate benthic environments. While they appear in the tropics, infection rates there are much lower. They are also not found in truly oceanic cephalopods. Those hosts instead carry a different group called ciliate fauna. Most dicyemid species are only found in a few closely related host species.