Cnidaria

Definition

Cnidaria is a phylum of simple, aquatic animals known for their unique stinging cells called cnidocytes. These organisms, which include jellyfish, corals, and sea anemones, exhibit a range of body forms and life cycles, showcasing both polyp and medusa stages. Cnidarians are key players in marine ecosystems, contributing to biodiversity and forming essential habitats.

5 Must Know Facts For Your Next Test

1. Cnidarians are classified into four major classes: Hydrozoa (hydras), Scyphozoa (true jellyfish), Anthozoa (corals and anemones), and Cubozoa (box jellyfish).
2. They exhibit radial symmetry, meaning their body parts are arranged around a central axis, allowing them to respond equally from all directions.
3. Cnidarians are primarily carnivorous, using their stinging cells to capture prey such as small fish and plankton.
4. Coral reefs, formed by the calcium carbonate skeletons of corals, provide crucial habitats for many marine species and are vital for marine biodiversity.
5. Cnidarians have a simple body plan with two tissue layers: an outer epidermis and an inner gastrodermis, separated by a jelly-like substance called mesoglea.

Review Questions

• How do cnidocytes contribute to the survival and feeding strategies of cnidarians?
  • Cnidocytes contain nematocysts that release toxins when triggered, allowing cnidarians to capture prey efficiently and defend against predators. This adaptation enables them to thrive in diverse aquatic environments where they can immobilize or kill small animals for food. The ability to use these specialized cells for both hunting and protection is essential for their survival.
• Compare the polyp and medusa forms of cnidarians in terms of structure and function.
  • Polyps are sessile and cylindrical with their mouth facing upwards, often attached to surfaces like rocks or coral. They typically reproduce asexually through budding. Medusae, on the other hand, are free-floating with a bell-shaped body and tentacles hanging downwards. They usually reproduce sexually, releasing gametes into the water. Both forms allow cnidarians to occupy different ecological niches throughout their life cycle.
• Evaluate the ecological significance of cnidarians within marine ecosystems and the potential impact of their decline due to environmental changes.
  • Cnidarians play a crucial role in marine ecosystems as predators and prey, contributing to the balance of food webs. Coral reefs created by cnidarians provide habitat for countless marine species, promoting biodiversity. A decline in cnidarian populations due to factors like climate change and ocean acidification could lead to habitat loss for many marine organisms, disrupting entire ecosystems and threatening the stability of marine environments.

Related terms

Cnidocyte: A specialized cell found in cnidarians that contains a stinging structure called a nematocyst, used for capturing prey and defense.

Polyp:

One of the two main body forms of cnidarians, characterized by a cylindrical shape and typically attached to a substrate.

Medusa: The free-swimming body form of cnidarians, resembling an umbrella shape, often seen in jellyfish.