Phylum Cnidaria Unit 3.2

Phylum Cnidaria Phylum Cnidaria is one of the oldest groups in this clade. The fossil record shows that members of this phylum appeared 700 MYA

Phylum Cnidaria Cnidarians have: Radial symmetry Gastrovascular cavity – extracellular digestion Tissues – both endoderm and ectoderm Cnidocytes – specialized cells with stinging organelles called nematocysts.

Phylum Cnidaria Cnidarians have diversified into a wide range of both sessile and floating forms including jellies, corals, and hydras. Polymorphism – some species exist as both polyps and medusae during their life cycles.

Phylum Cnidaria The basic body plan of a cnidarian is a sac with a central digestive compartment, the gastrovascular cavity. A single opening functions as both mouth and anus.

Body Wall Cnidarians have an outer tissue layer, the epidermis, derived from ectoderm, and an inner gastrodermis, derived from endoderm, with jellylike mesoglea in between.

Reproduction Polyps can reproduce asexually by budding, fission, or pedal laceration.

Reproduction Cnidarians, typically medusae, can also reproduce sexually. A zygote usually develops into a motile planula larva. Some species only exist as polyps, others only as medusae, others alternate between the two.

Feeding Cnidarians are carnivores that use tentacles to capture prey. The tentacles are armed with cnidocytes – unique cells that function in defense and the capture of prey. Nematocysts contain toxins used for prey capture and defense.

Feeding Nematocysts are used to inject a toxin. Variable in form. May be used for prey capture or defense. https://youtu.be/Pu_ijC8HFRU

Feeding Extracellular digestion begins in the gastrovascular cavity, but is completed within the cells of the gastrodermis. Some cnidarians supplement their diet with nutrients collected from algal symbionts (zooxanthellae).

Nerve Net Cnidarians have a simple nervous system. Nerve cells form two interconnected nerve nets in the epidermis and gastrodermis. They have no concentrated grouping of nerve cells, therefore they have no central nervous system. A central nervous system does not provide advantage for radially symmetrical animals where stimuli approach from all sides.

Classification The phylum Cnidaria is divided into four major classes: Class Hydrozoa Class Scyphozoa Class Cubozoa Class Anthozoa

Class Hydrozoa Hydrozoans are variable in form. Mostly marine. Usually colonial, sometimes solitary. http://youtu.be/MpSPdeTN74I

Class Hydrozoa – Typical Life Cycle Most hydrozoans alternate between polyp and medusa forms. Some have only polyps. Some have only medusae.

Class Hydrozoa - Polyps & Medusae Typical hydroid polyps have a base, a stalk, and one or more individual polyp animals Hydroid medusae are usually smaller than the “true jellyfish”, and they have a Velum, an inward projection of the bell.

Class Hydrozoa Examples Portuguese Man o’war This colony can be extremely dangerous Obelia Exhibit both polyp and medusa Hydra Exhibit only polyp stage and reproduce with buds

Class Cubozoa Cubozoans exhibit a unique medusa body form. The bell is square shaped with one or more tentacles extended from each corner. They grow to between 15-25 cm. http://youtu.be/MpSPdeTN74I

This is the most venomous group of all the Cnidarians Predators in the Ocean They look like your basic jellyfish, but they can swim pretty fast, maneuver around things, and see fairly well. Their eyes are surprisingly complex but it is still unclear how the images created by these lenses are interpreted by the animal since they do not have a brain. This is the most venomous group of all the Cnidarians

Class Hydrozoa Examples Portuguese Man o’war This colony can be extremely dangerous Obelia Exhibit both polyp and medusa Hydra Exhibit only polyp stage and reproduce with buds

Class Scyphozoa In the class Scyphozoa, jellies (medusae) are the prevalent form of the life cycle. No velum present.

Class Scyphozoa Tentacles around the periphery of the bell contain nematocysts used to paralyze prey animals. In the center are four frilly oral arms used to capture and ingest prey. http://youtu.be/aJUuotjE3u8

Class Scyphozoa – Typical Life Cycle Planula larvae develop into a polyp-like form. Saucer-like buds called ephyrae are produced by strobilation.

Class Scyphozoa Examples

Class Anthozoa Class Anthozoa includes the corals and sea anemones which occur only as polyps – no medusa stage. All marine Solitary or colonial

Sea Anemones Found in coastal waters all around the world. Attach to rocks using their pedal disc. Feed on fish or any other food of suitable size.

Mutualisms Sea anemones sometimes harbor zooxanthellae (photosynthetic protists) like hard corals do. Some crabs will decorate their shells with anemones.

Mutualisms Some damselfish (anemone fishes) form associations with large anemones. Fish gains protection from living in the anemone. The fish may help ventilate the anemone, or keep it free of sediment.

Corals These are the true or stony corals shown in the picture. They are like tiny anemones living in a calcareous cups.

Polyps may be retracted into the skeleton. Often retracted during the day. The polyps expand for feeding.

Coral Reefs Coral reefs are one of the most productive and diverse ecosystems on Earth. Found in shallow tropical seas. They require warm water and sunlight for symbiotic zooxanthellae.

Coral Reefs Symbiosis between zooxanthellae and corals is threatened by global warming. Warmer water damages the photosynthetic mechanism in zooxanthellae. Coral tissues turn white and brittle, this is called coral bleaching. Zooxanthellae die or are expelled by corals.

Phylogeny Cnidarians may have evolved from a radially symmetrical planula-like ancestor. Trichocysts and toxicysts found in some ciliates may be precursors to nematocysts.