Difference Between Hydra and Obelia

Hydra vs Obelia

Though Hydra and Obelia are both cnidarians found in the Class Hydrozoa, there are some fundamental differences between Hydra and Obelia. Class Hydrozoa contains about 3,700 species. All these species are exclusively aquatic. However, most of them live in marine habitats but a few species live in freshwater habitats. The common features of Hydra and Obelia are the presence of tissue level of organization, radial symmetry, mesoglea, tentacles around the mouth, single opening, which acts as both mouth and gut, and the absence of head and segmentation. The most unique feature of these creatures is the presence of special stinging cells called cnidocytes, which contains nematocytes that are used to capture prey and for defensive actions. This article will outline the difference between Hydra and Obelia.

What is Hydra?

Hydra is a solitary, predatory species found in freshwater habitats with the body size of few millimeters long. It has no medusa stage unlike the other hydrozoans. It has a cuplike basal disk that helps to attach to substrates like rocks, aquatic plants, or detritus. Both sexual reproduction and asexual reproduction methods can be seen in Hydra. Unlike the other cnidarians, Hydra has a remarkable power of regeneration. Under optimum environmental conditions, these creatures used to reproduce asexually by budding. If the water is immobile, they differentiate into male and female and reproduce sexually by producing gametes.

Hydra vs Obelia - Image of Hydra

What is Obelia?

Obelia is a marine cnidarian who lives on a branched framework as individual polyps. These creatures are found in all the marine habitats except high-arctic and Antarctic seas. However, these creatures do not live in deep sea. Obelia has a very simple body structure with two true tissue layers; epidermis and gastrodermis. A jelly like layer called mesoglea is found in between these two tissue layers. They have incomplete digestive system with one opening where both ingestion of food and expel of waste is taken place. Obelia has a simple nerve net with no brain or ganglia. The life cycle of Obelia has two stages; motile medusa and sessile polyp. During their life cycle, polyps reproduce asexually by budding and medusa reproduces sexually by produce gametes through meiosis. Both polyp and medusa forms are diploid, whereas their gametes are haploid. In the polyp form, mouth is situated at the top of the body, surrounded by tentacles while, in the medusa stage, the mouth is situated at the distal end of the body.

Difference Between Hydra and Obelia - Image of Obelia

Medusa form

What is the difference between Hydra and Obelia?

• Hydra is a solitary species and lives attached to substrates, whereas Obelia is a colonial species and lives as polyps in an interconnected branching network.

• Hydra live in freshwater habitats, whereas Obelia is exclusively marine.

• Hydra does not have medusa form in their life cycle, whereas Obelia has both forms; polyp and medusa.

• Unlike Obelia, Hydra has great regenerative powers.


Images Courtesy:

  1. Hydra via  Wikicommons (Public Domain)
  2. Obelia by Doc. RNDr. Josef Reischig, CSc. (CC BY-SA 3.0)