Hydroid jellyfish belong to the class Hydrozoa and Coelenterata. They live in water. They are closely related to polyps, but their structure is slightly more complex. This type of jellyfish differs from others in that it can live forever, as hydroids can regenerate from an adult organism to a juvenile one.
Jellyfish don't have a mouth, but they do have a proboscis. They can trigger their rebirth mechanism at any time. Fernando Boero reported the rebirth of jellyfish; he was conducting experiments on them while studying hydroids. He placed some of them in an aquarium, but unfortunately, the experiment was disrupted, causing the water to dry out. Fernando discovered that the jellyfish hadn't died, but had merely shed their tentacles, transforming into larvae.
Food resources and the process of eating
Plankton, Artemia
The hydroid jellyfish's main food resource is plankton. Their main diet is Artemia, such as jellyfish are considered predatorsThe tentacles located at the edge of the umbrella-shaped body serve as food-gathering tools. The digestive system of these jellyfish is called gastrovascular. Jellyfish catch prey by passively moving their tentacles through the water, trapping plankton, after which they begin actively swimming. The nervous system of these jellyfish consists of cellular networks that form two rings: the outer one, responsible for sensation, and the inner one, responsible for movement.
One of the hydroid jellyfish have light-sensitive eyes, which are located in the center of the tentacle. Hydras are predators by nature, feeding on ciliates, planktonic crustaceans, and even small fish. They lie in wait for prey, clinging to an aquatic plant and spreading their tentacles wide. When at least one tentacle reaches the prey, all the remaining tentacles completely envelop it. The hydra quickly devours its prey whole; when sated, its tentacles contract.
Types of hydroid jellyfish
Trachylids move by displacing water from the tube, and they also have a ring-shaped outgrowth located on the inner side, which gives them greater speed during movement.- The leptolids species move slowly underwater because the intensity from the umbrella is slower.
- There is also a species called the medusoid hydrocoral, which bears virtually no resemblance to common jellyfish.
- Another jellyfish, the siphonophore, has a unique and distinctive structure. It is a colony, each playing a role in the functioning of the entire organism. Externally, it resembles a huge bladder that floats like a ship. It contains glands that produce gas, which helps it ascend. When the siphonophore descends, it must relax its adductor, a muscular organ. The siphonophore has a bladder, and beneath it are other jellyfish, shaped like swimming bells, followed by gastrozoa, and then gonophores, which serve to procreate within the genus.
- At one time, Velella and Porpita belonged to the genus Siphonophora. Velella was also called "sailfish." But later, Porpita and Velella were considered not a colony, but large floating polyps. Sailfish live in the ocean, unafraid of water, as they cannot drown.
Trachylids move by displacing water from the tube, and they also have a ring-shaped outgrowth located on the inner side, which gives them greater speed during movement.Reproduction
Reproduction in hydroid jellyfish is more often external than internal. Mature reproductive cells migrate outward, after which a blastula is formed Some cells end up inside, forming the endoderm. After some time, several cells degenerate to form a cavity. The egg then develops into a larva, a planula, and then into a hydropolyp, which gives rise to other polyps, including small medusae. These small medusae eventually grow and begin independent development.
Hydra is one of the most convenient objects for conducting experiments with which scientists study regeneration in animalsWhen cut in half, Hydra regenerates the missing parts after a period of time. This species is also easy to operate without anesthesia and requires no special instruments. Hydra has the ability to regenerate not only from a single half, but even from the smallest pieces, multiple polyps can be regenerated.
Hydra habitats
Hydroid jellyfish are not always found, but rather in large aggregations carried by currents. The class of benthic jellyfish includes the polyp stages that lead a sedentary life, with the exception of class of planktonic hydroid polypsHydrozoans are also capable of grouping together into large groups, aided by the wind, but hydrozoan polyps appear to be a single entity when clustered together. If the jellyfish and polyp are hungry, their movements will be directed solely toward foraging, but once they are full, their tentacles will begin to contract and retract toward the body.
Habitat zones
Jellyfish move depending on whether they're hungry or not. Generally, all species occupy a specific habitat, which can be a lake or an ocean. They don't intentionally invade new territories. Some prefer to live in warmthWhile others, on the contrary, thrive in colder waters. They can also be found both in the deep waters and at the surface of the water. Hydroid jellyfish can live in the intertidal zone and are not afraid of the surf. Most of these jellyfish have a polyp protected from impact by a skeletal cup (theca). The theca is thicker than that of other species that live deeper, where waves are much less noticeable.
At greater depths, there lives a special kind of hydroid that is unlike the littoral ones. At such depths colonies are found, having a form such as:
- tree,
- Christmas tree,
- feather,
- There are also types of colonies that look like a ruff.
These species grow from 15 to 20 cm and cover the entire seabed with a dense forest. Some species, such as the sea spider, live in these forests and feed on hydropolyps.
Hydra can very rarely inhabit less saline waters, such as the Gulf of Finland, where the salinity of the habitat should not exceed 0.5%. Hydroid jellyfish often live close to the shore and in brighter areas. This type of jellyfish is not prone to mobility; they are most often attached to a plant branch or stoneOne of the hydroid jellyfish's favorite positions is to be upside down with some of its tentacles down.
Jellyfish species dangerous to humans
But not all of them are safe for human life. One of the most beautiful species called Portuguese man-of-war The bell, while present and attractive, can cause harm to a person.
The Portuguese man-of-war, found in Australia and along the coasts of the Indian and Pacific Oceans, and even the Mediterranean, is one of the largest species of hydrozoans. The man-of-war's bladder can reach a length of 15 to 20 cm. However, its tentacles can be much more dangerous, as they can extend up to 30 meters deep. The man-of-war can leave stings on its victims. An encounter with the Portuguese man-of-war is especially harmful to people with weakened immune systems and those prone to allergies.
However, most hydrozoan jellyfish are harmless to humans, unlike scyphozoans. There is a so-called white algae from the genus Polyp, which was once used as decorative jewelry. Some hydrozoan species serve as laboratory animals—polyps from the class Hydra—and are even used in schools around the world.
