Hydra Biology | A Glance into Digestive System and Reproduction

Hydra is a freshwater organism belonging to the phylum Cnidaria. It is a multi-cellular organism, yet it is not highly evolved in anatomy like other animals.

hydra biology

Hydra Biology

This can be studied in terms of

  1. Nutrition.
  2. Digestion
  3. Egestion
  4. Respiration
  5. Excretion
  6. Nervous system and
  7. Reproduction.


1. Food

  • Hydra is exclusively carnivorous.
  • They devour on small aquatic animals, larvae of insects like crustaceans, and some annelid worms.
  • Hydra may swallow preys larger than their entire body sizes, such as young fish and tadpoles.

2. Ingestion

  • A hungry hydra first waits for its food to approach it.
  • It normally rests its basal disc attached to some substratum, while its tentacles, which are full of nematocysts, extend out in the water, controlling a considerable amount of hunting territory. 
  • When a small prey animal passes across the hydra touching the tentacles, which activates dozens of nematocysts that get discharged on the prey at once.
Nematocyst in hydra
Credit: National Oceanic and Atmospheric Administration
  • The volvents coil around the bristles and appendages around the prey, while the gulinants fastens to its surface, thus holding it fast.
  • The process results in a puncture on the victim’s surface and injects hypnotoxin, which paralyzes the prey.
  • The tentacles now hold the prey and contract and bend rhythmically and move it to the mucus-lined mouth, which opens widely to swallow the whole organism.
  • Contractions of hypostome and body wall undergo peristaltic movement through which food is taken inside the gastrovascular cavity where digestion occurs.

3. Digestion

The digestion in hydra is both intracellular and extracellular and occurs in two phases as

Extracellular digestion

  • The prey that gets inside the hydra’s gastrovascular cavity is first killed by the action of digestive juices secreted by the gland cells of the gastrodermis.
  • The churning, contractions, and lashing movement of the flagella of nutritive-muscle cells thoroughly mix up the digestive juices with food that gets broken into small chunks.
  • This type of digestion, which occurs in the cavity outside the gastrodermis cells, is called extracellular digestion.

Intracellular digestion

  • The disintegrated smaller chunks of food are now engulfed by nutritive-muscle cells by means of pseudopodia.
  • The chunks get digested within the food vacuoles.
  • This process is called intracellular digestion.
  • The food vacuoles undergo both acidic and alkaline phases, and the digestion of proteins is done by proteolytic enzymes.

The retention of intracellular digestion is probably due to its aquatic life nature.

4. Absorption

  • The absorption of soluble food matter is done by the diffusion process from cell to cell.
  • Some of the gastrodermis cells containing food vacuoles may become detached and move freely to distribute the digested food to all parts.
  • Thus, the gastrovascular cavity serves both digestion and circulation.
  • Hydra can digest proteins, fats, and some carbohydrates but not starch.

The reserve food of hydra is glycogen and fats, which are stored in gastrodermis cells.

5. Egestion

  • The indigestible residues, such as the exoskeleton of the crustacea, are egested through the mouth as there is no anus.
  • Egestion occurs by a sudden spurt due to muscular contraction of the body so that the debris is thrown at a distance.

6. Respiration, excretion, and osmoregulation

  • Hydra does not have the blood, blood vessels, organs for excretion and respiration.
  • These functions are carried by the cells individually.
  • Due to the thinness in the walls of the cells and gastrovascular cavity presence, most cells of the body remain freely exposed to the surrounding water.
  • As a result of this, cell diffusion facilitates the exchange of oxygen and carbon dioxide and the excretion of nitrogenous waste.

7. Nervous system

  • Hydra possesses a very primitive nervous system.
  • It consists of bipolar and multipolar nerve cells or neurons lying immediately above the muscle process.
  • This, as a result, forms the nerve net or nerve plexus.
  • Neighboring cells are not fused, but their neurites form synaptic junctions.
  • Such nerve nets are called synaptic nerve nets.
  • Nerve cells are found in large numbers around the mouth of the hydra.
  • Hydra does not possess a blood and circulatory system. Instead, the water enters into their body and carries away the waste.

8. Reproduction in Hydra

  • This mostly occurs by a process called budding.
  • However, Hydra has efficient regeneration properties.
  • If a hydra is cut into a number of pieces, each piece can grow into a new hydra.

By Budding

hydra budding
Budding in hydra steps.
  • A new organism is formed from an outgrowth over the parent body.
  • This outgrowth is called bud, which occurs due to the repeated cell division at a particular site on the parent body.
  • The new organism, once fully developed, gets detached from the parent body and starts its life as a new and independent organism.

This process occurs during the summer months when the hydra is well-fed.

It is healthy, and it undergoes asexual budding a the usual method of reproduction for them.

  • On the hydra plant, near the basal part of the body, a bulging appears, which is a result of repeated multiplications of the epidermal interstitial cells.
  • This repeated multiplication gives rise to buds with its wall consisting of the epidermis and gastrodermis and has an inner lumen with continuation with the parent’s gastrovascular cavity.
  • The bud or outgrowth enlarges gradually and nurtures in due time.
  • The bud that enlarges develops a mouth and a circlet of tentacles at the free end of its body.
  • These tentacles help them to capture their food.
  • When the bud or outgrowth gets fully grown, the bud constricts at the base and gets finally separated from their parent body.
  • The separated small hydra feeds itself and grows further in independent hydra.
  • Occasionally, it is seen that several buds occur at the same time on a single parent, and these, in turn, may develop into secondary buds or outgrowth. As a result of these secondary buds, a colony of hydra develops, which temporarily resembles a colonial hydroid.

In this way, a hydra reproduces itself asexually into its offspring that are identical to them.

The siblings consist of the same genetic attribute as the parent.

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