Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 33.2 Cnidarians Are an Ancient Phylum of Eumetazoans
Cnidarians are among the oldest lineages of true animals (eumetazoans), having diverged around 680 million years ago. Despite their simple diploblastic, radial body plan, they exhibit specialized tissues and predatory adaptations that have enabled them to thrive in marine and freshwater ecosystems.
Basic Body Plan and Forms
- Cnidarians include hydras, jellies, sea anemones, and corals.
- Body is a sac-like structure with a gastrovascular cavity:
- Central digestive compartment
- Single opening serves as both mouth and anus
- Two main body forms:
- Polyp: cylindrical, sessile (e.g., hydras, anemones)
- Medusa: free-swimming, bell-shaped (e.g., jellies)
- Some species alternate between both forms.
Feeding and Defense
- Carnivorous predators that use tentacles to capture prey.
- Prey is digested in the gastrovascular cavity.
- Tentacles contain cnidocytes—specialized stinging cells:
- Contain capsule-like cnidae
- Nematocysts eject a stinging thread to immobilize prey
Muscles and Nervous System
- Epidermis and gastrodermis have contractile cells.
- A jelly-like mesoglea lies between these layers.
- Hydrostatic skeleton enables movement.
- No brain, but have a noncentralized nerve net for coordination.
Major Clades: Medusozoa and Anthozoa
1. Medusozoa
- Includes scyphozoans (jellies), cubozoans (box jellies), and hydrozoans.
- Most produce a medusa stage:
- Hydrozoans: alternate between polyp and medusa (e.g., Obelia)
- Scyphozoans and cubozoans: medusa-dominant
- Cubozoans have box-shaped medusae with highly toxic cnidocytes (e.g., Chironex fleckeri)
2. Anthozoa
- Includes sea anemones and corals—only exist in polyp form.
- Corals:
- Form symbioses with algae and secrete calcium carbonate exoskeletons
- Build coral reefs—major biodiversity centers
- Coral reefs face threats from climate change, acidification, pollution, and overharvesting.
Reproduction: Obelia Life Cycle Example
- Asexual polyp stage: reproduces by budding to form colonies
- Some polyps specialize in feeding, others in reproduction
- Sexual medusa stage: releases eggs and sperm into the water
- Fertilized egg becomes a larva, which settles and becomes a new polyp
In a Nutshell
Cnidarians are simple yet highly adapted predators with radial symmetry, true tissues, and specialized stinging cells. Their two body forms—polyp and medusa—and life cycle flexibility help them succeed in a wide range of habitats. Coral-building anthozoans support entire ecosystems, making cnidarians crucial players in marine biodiversity.