HYDROZOA: CHARACTERISTICS, HABITAT, REPRODUCTION, FEEDING - BIOLOGY - 2025

The hydroids (Hydrozoa) are animals belonging to an order of the Cnidaria phylum covering almost all colonial organisms. These can be fixed to the substrate, or moving freely through the waters.

Hydrozoans were described for the first time in the year 1843 and since then a total of approximately 3000 species have been discovered, which are distributed in the five orders that make up the group.

Specimens of the order Hydrozoa. Source: Fred Hsu (Wikipedia: User: Fredhsu on en.wikipedia)

This is a fairly diverse and varied group in terms of shapes and colors, finding organisms so small that they only span a few centimeters, up to large colonies several meters long.

Likewise, as with most of the members of the phylum cnidaria, hydrozoans have stinging cells (cnidocytes) that synthesize and secrete a toxic substance that they use both to defend themselves and to get their prey.

Taxonomy

The taxonomic classification of hydrozoa is as follows:

- Domain: Eukarya.

- Animalia Kingdom.

- Phylum: Cnidaria.

- Subphylum: Medusozoa.

- Class: Hydrozoa.

characteristics

Taking into account that hydrozoans belong to the Eukarya domain, all the cells that make them up are eukaryotic. This means that its DNA is enclosed in an organelle called the cell nucleus, delimited by the nuclear membrane.

Similarly, during their development process, their cells undergo a process of differentiation and specialization that allows them to perform specific functions such as nutrition, reproduction and production. Due to this, it can be said that hydrozoans are multicellular organisms.

Likewise, they are characterized by having the shape of polyps and jellyfish, although there are species that only have polyps and others that only have the shape of jellyfish.

One of the most outstanding characteristics of this group of living beings is that they are not a single individual, but in most cases they are made up of several individuals. In this way they constitute a colony, within which each one fulfills specific and detailed functions.

Regarding their biological cycle, in hydrozoans two types of cycles can be seen: metagenetic, which is the one with alternation of generations (polyp and jellyfish); and the hypogenetic in which only one form is observed (polyp or jellyfish).

Morphology

The size of the organisms that belong to this class is varied. Polyps that barely reach 500 microns and jellyfish with a diameter of 40 cm have been described, while large colonies of polyps can reach 30 meters in size.

In order to study and understand the morphology of hydrozoans, it is necessary to study the polyp and the jellyfish separately.

-Polyp

They are known by the name of hydropolyps, and in most cases they establish large colonies. Only a few species like the hydra are solitary.

As with the polyps of other species of cninadria, the polyps of hydrozoans are fixed to the substrate through a structure known as hydrorriza, which closely resembles the roots of land plants. From that first polyp that is attached to the substrate, the other polyps that make up the colony develop.

Colony of hydrozoans. Source: Rob Growler

Structurally they are made up of several layers: ectodermis (epidermis), gastrodermis and mesoglea. The set of these three layers is known by the name of cenosarco. In turn, the cenosarch is protected by a kind of exoskeleton made up of chitin, which is called the perisarch.

It is important to note that the portion of the perisarcus that covers the polyp in its entirety is called teak. There are three types of teak, depending on the part of the polyp it covers: gonoteca (covers the gonozoids), hydrotheca (covers the gastrozoids) and dactiloteca (covers the dactylozoids).

They also have a common cavity that is known as the gastrovascular cavity. These structures are unique to each polyp. However, each polyp is connected to another through them.

The gastrovascular cavity is a space that connects all the polyps in the colony. In it the final part of the digestive process takes place.

In reality, colonies are made up of organisms called hydroids, each with a specific morphology, as well as specific functions. The types of hydroids that are observed in these colonies are: dactylozoids, gonozoids and gastrozoids.

Dactylozoids

These hydroids have the function of defending the body. They do not have specialized structures such as tentacles or mouths. They are full of cnidocytes, cells that synthesize stinging substances that, in contact with other animals, can cause serious damage.

The dactylozoids are located very close to the gastrozoids, since they are in charge of neutralizing possible prey so that they can be ingested.

Gonozoids

Gonozoids are hydroids that are solely responsible for the reproductive process of the polyp. Although their shape can be varied, they are most often bottle-shaped and surrounded by a protective covering called the gonoteca. They also have a structure called blastostyle, which, together with the gonoteca, makes up what is known as gonangium.

The type of reproduction that is observed in gonozides is asexual, being represented by budding.

Gastrozoids

They are hydroids specialized in the digestion of nutrients. Structurally they are made up of a large number of tentacles that contain stinging cnidocytes.

They also present a rudimentary gastrovascular cavity, in which digestive enzymes are synthesized that contribute to the initial digestion of the prey that are ingested.

Of all the hydroids that make up the hydropolyps, the gastrozoids are the ones that are found in the greatest quantity, but they are also the ones with the least degree of specialization.

-Jellyfish

It is the least predominant phase in the life cycle of hydrozoans. These are generally small jellyfish, reaching up to 6cm in diameter. They are the smallest jellyfish of the phylum Cnidaria.

They have the characteristic mushroom shape of some jellyfish. Like the cubomeduzas, this type of jellyfish also has a tissue extension on the lower edge of its umbrella known as the veil. This structure is important as it plays a key role in the animal's movement process.

Likewise, in the sub-bum part they present an extension that is called the manubrium. At the end of this manubrium there is a hole or opening known as the mouth.

Jellyfish of the order Hydrozoa. Source: Dennis Wet

The mouth opens into the gastrovascular cavity or stomach. A series of ducts (4 in total) arise from the stomach, which are called rays. These communicate with a ring that runs along the edge of the umbrella.

Digestive system

The digestive system of these jellyfish is quite rudimentary. It is made up of simply the mouth and a primitive esophagus that empties into the stomach. There digestive enzymes are synthesized that help digest the captured prey. The mouth also fulfills the function of releasing wastes from digestion.

Nervous system

The nervous system of hydromedusae is simple. It is made up of two nerve plexuses, one subumollar and the other exumulus (subgastral). Of both plexuses, the subumulalar is the most developed and actively participates in the movement and displacement of the jellyfish.

Reproductive system

It is made up of the gonads, which is where gametes are produced. The gonads present an ectodermal location, specifically on the manubrium or on the radial canals.

Habitat and reproduction

Hydromedusae are widely distributed throughout the globe. They are almost entirely marine animals. However, it is possible to find them in freshwater ecosystems.

The polyps are fixed to the substrate, while the jellyfish are free, moving calmly in the waters; and can be found both near the surface and at depth.

When it comes to temperature, hydrozoans prefer warm, tropical waters. However, some specialists have even described species that are regular inhabitants of the Arctic region.

Feeding

The digestive process is going to be different in polyps and in jellyfish. In the latter, the prey are paralyzed and neutralized with the toxin secreted by the cnidocytes. Once this is done, they enter the mouth and are passed to the stomach, where they undergo the action of the digestive enzymes that are secreted there. Subsequently, the nutrients are absorbed and what remains, the waste, are released through the mouth to the outside.

On the other hand, the digestive process in polyps occurs in two stages. In the first, extracellular predigestion, the tentacles of the gastrozoids capture the prey and introduce it into the gastrovascular cavity. There it is subjected to digestive enzymes, forming a kind of mush.

This pap is subsequently distributed to the common gastrovascular cavity of the colony of polyps. There intracellular digestion takes place, where nutrients are finally absorbed and waste is released into the environment.

Reproduction

In hydrozoans, the two types of reproduction are observed: asexual and sexual. Asexual reproduction is observed in polyps, whereas jellyfish reproduce sexually.

Asexual reproduction

Asexual reproduction in polyps occurs through budding. According to this process, a kind of buds or gonophores are formed on the surface of the gonozoids, which undergo a process of cellular differentiation until a new polyp is formed. This formed polyp can detach from the colony or remain attached to it.

It is important to note that jellyfish may also form from the buds that grow on the polyps.

Sexual reproduction

This type of sexual reproduction occurs only in the jellyfish phase. Sexual reproduction involves the union of two gametes, male and female.

Depending on the species, fertilization can be external or internal. The most common is external fertilization. What happens here is that the gametes are released into the sea and are found there. The egg is fertilized by the sperm.

As a product of this fertilization, a planula larva (flat) is formed that has the ability to swim freely until it achieves a suitable place in the substrate and is fixed. There you will experience a process of tissue differentiation, first creating the various layers that make up the polyp, until finally giving way to the gastrovascular cavity of this and other structures.

Classification

The hydrozoa class is classified into five orders: Hydroida, Actinulida, Chondrophora, Trachylina, and Siphonophora.

Hydroide

It is the order that presents the largest number of species. It is characterized by presenting both forms: polyp and jellyfish. Likewise, these animals tend to form colonies whose polyps can all be the same or different shapes.

Actinulide

It can be said that they are the simplest hydrozoa out there. They are not medusa-shaped, but only polyp. They do not establish colonies and their solitary polyps have a surface with cilia.

Chondrophora

They also do not have the shape of a jellyfish. They form floating colonial organisms that slide freely thanks to the action of ocean currents. It is important to highlight that, despite the fact that they are floating colonies, they do not present a pneumatophore.

Trachylina

It is not in the form of a polyp, but only a jellyfish. It is perhaps the most primitive class within the hydrozoans. The jellyfish develops directly from an actinula. In addition, its life cycle includes a multicellular phase and a unicellular phase.

Siphonophora

Individuals of this order form colonies that move freely through the sea. A characteristic of this order is that they present a structure called a pneumatophore, which is nothing more than a kind of bell filled with gases that remains above sea level. This pneumatophore is responsible for the movement of the organism, thanks to the action of the wind.

Physalia physalis, representative specimen of the hydrozoans. Source: Rhalah

References

1. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
2. Cartwright, P. (2010). Character evolution in Hydrozoa (Phylum cnidaria). Integrative and Comparative Biology. 50 (3) 456-472
3. Gravili, C., Pagés, F. and Gili, J. (2006). An introduction to hydrozoa. Taken from: researchgate.com
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
5. Mills, C., Marques, A., Esteves, A. and Calder, D. (2007). Hydrozoa: Polyps, Hydromedusae and Siphonophora. Taken from: researchgate.com
6. Ruppert, EE & Barnes, RD, 1994. Invertebrate zoology (6th ed.). Fort Worth, USA: Saunders College Publishing.