Difference Between Hydra and Obelia

Main Difference – Hydra vs Obelia

Hydra and Obelia are two organisms that belong to the class Hydrozoa. Hydrozoa belongs to the phylum Cnidaria, which contains radially symmetric, diploblastic organisms. The two body forms of Cnidarians are polyp and medusa. The primary body form of the Hydrozoa is the polyp. Hydra is a simple freshwater animal while Obelia lives either in marine or fresh water. The main difference between Hydra and Obelia is that the main body form of Hydra is the polyp whereas Obelia comprises both polyps and medusae in its life cycle. Therefore, Hydra is sessile throughout its life but, Obelia is both sessile and mobile. The polyp stage of both Hydra and Obelia asexually reproduce by budding. The sexual reproduction of Obelia occurs in the medusa.

Key Areas Covered

1. What is Hydra
      – Definition, Characteristics
2. What is Obelia
      – Definition, Characteristics
3. What are the Similarities Between Hydra and Obelia
      – Outline of Common Features
4. What is the Difference Between Hydra and Obelia
      – Comparison of Key Differences

Key Terms: Blastostyle, Budding, Hydra, Hydranths, Hydrozoa, Medusa, Obelia, Polyp, Tentacles

What is Hydra

Hydra is a minute freshwater coelenterate that consists of a stalk-like tubular body along with a ring of tentacles around the mouth. Hydra exclusively lives in freshwater habitats. A completely grown Hydra is about 30 mm long. Hydra is attached to rocks or submerged plants in the water. The body of these animals is a hollow cylinder with one opening that serves as the mouth. The hollow interior serves as the digestive tract. The mouth is surrounded by 6-10 tentacles. Hydra moves water in and out of the body by hydraulic pressure created in the digestive tract. The wall of the body of Hydra is made up of two cell layers. Hence Hydra is a diploblastic animal. Some single-celled algae such as Chlorella live inside the cell lining of the digestive tract.

Figure 1: Green Hydra

The asexual reproduction of Hydra occurs by budding new individuals from the body wall. It occurs in warm seasons. Some Hydra species are unisexual and others are hermaphroditic. During cold seasons, Hydra begins to reproduce sexually as well. The eggs are produced on the body wall of female individuals, which are fertilized by the sperms released into the water by nearby male individuals. Several species of Hydra can be identified such as green Hydra (Hydra viridissima), brown Hydra (Hydra oligactis), slender hydra (Hydra attenuata), and colonial hydra (Cordylophora lacustris). A green Hydra is shown in figure 1.

What is Obelia

Obelia is a sedentary, colonial coelenterate that consists of upright branching stems, composed of minute cups. Obelia lives in both freshwater and marine habitats. It attaches to the surfaces of the rocks, molluscan shells, and seaweeds. Since Obelia forms a branched tree-like structure, it is commonly called the sea-fur. An Obelia is shown in figure 2.

Figure 2: Obelia

Each Obelia colony is attached to the surface by horizontal, thread-like roots called hydrorhiza. The vertical branching stem arises from the hydrorhiza, which is about 2.5 cm long. The stem is called hydrocaulus. Both hydrorhiza and hydrocaulus are hollow structures. The hydrocaulus bears polyps in small cup-like structures. The polyp contains a stem and a terminal head called hydranth. The three types of polyps that occur in Obelia are nutritive polyps, blastostyles, and medusae. The hydranths capture food such as small larvae in the water. The blastostyles are budding polyps that are formed during asexual reproduction. The medusae are hollow buds produced from the blastostyle during spring and summer. They are saucer-shaped structures, which can freely swim on the water surface. Sexual reproduction of Obelia occurs in medusae.

Similarities Between Hydra and Obelia

Both Hydra and Obelia are simple, invertebrate animals.
Both Hydra and Obelia belong to the class Hydrozoa.
Both Hydra and Obelia are diploblastic animals.
Both Hydra and Obelia live in aquatic habitats.
Both Hydra and Obelia exhibit radial symmetry.
Both Hydra and Obelia possess a polyp body form, which reproduces asexually by budding.
Both Hydra and Obelia are made up of hollow structures.

Definition

Hydra: Hydra is a minute freshwater coelenterate that consists of a stalk-like tubular body along with a ring of tentacles around the mouth.

Obelia: Obelia is a sedentary, colonial coelenterate that consists of upright branching stems, composing of minute cups.

Habitat

Hydra: Hydra exclusively lives in freshwater habitats.

Obelia: Obelia lives in both fresh and marine water habitats.

Colonial

Hydra: Only few Hydra species are colonial.

Obelia: Obelia is a colonial animal.

Structure

Hydra: Hydra contains a stalk-like tubular boy.

Obelia: Obelia contains branching stems containing minute cups in which the polyps sit.

Size

Hydra: Hydra is small (30 mm long).

Obelia: Obelia are longer than Hydra (25 cm long).

Medusa

Hydra: Hydra lacks a medusa body form.

Obelia: Obelia consists of a medusa form.

Sessile/Mobile

Hydra: Hydra are sessile animals.

Obelia: Obelia are both sessile and mobile during their life cycle.

Sexual Reproduction

Hydra: Hydra sexually reproduces by forming gametes.

Obelia: The sexual reproduction of Obelia occurs in the medusae.

Diversity

Hydra: Only few Hydra species occur in freshwater.

Obelia: Obelia is more diversified than Hydra.

Conclusion

Hydra and Obelia are two organisms that belong to the class Hydrozoa. Both Hydra and Obelia are aquatic simple animals that feed on small larvae and planktons dissolved in the water. Only polyp stage can be identified in Hydra while Obelia possesses both polyp and the medusa stages. Both Hydra and Obelia asexually reproduce by budding. The sexual reproduction of Obelia occurs in the medusa stage. The main difference between Hydra and Obelia is the types of body forms present in each organism.

Reference:

1.“Hydra.” Information on Hydra, Available here. Accessed 24 Sept. 2017.
2. “Obelia – Habitat, Structure, and Diagram”. Biology Discussion. Com, Available here. Available here. Accessed 24 Sept. 2017.

Image Courtesy:

1. “Mikrofoto.de-Hydra 15” By Frank Fox – Mikro-foto, (CC BY-SA 3.0 de) Available here
2. “Obelia-Hydrozoa-at40x” (Public Domain) via Commons Wikimedia