Radiate Animals 2e342b

Radiate Animals Chapter 13

Radiate Animals 

Phylum Cnidaria & Phylum Ctenophora Radial Symmetry  Diploblastic – 2 embryonic tissue layers 

Phylum Cnidaria 

All animals except sponges belong to the clade Eumetazoa, the animals with true tissues.

Phylum Cnidaria 

Phylum Cnidaria is one of the oldest groups in this clade. 

Fossil history back 700 MY.

Phylum Cnidaria 

Cnidarians have: Radial symmetry  Gastrovascular cavity – extracellular digestion  Tissues – derived from two embryonic germ layers  Cnidocytes – special 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.

Phylum Cnidaria 

In colonial forms that share a gastrovascular cavity, polyps may be specialized for feeding, reproduction, or defense.

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.

Body Wall 

The epidermal layer contains several types of cells organized into tissues.

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. 

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 diffuse nervous system. Nerve cells forming two interconnected nerve nets in the epidermis and gastrodermis.  No concentrated grouping of nerve cells forming a central nervous system. 

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No 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

Classification 

A fifth class, Staurozoa, has been proposed. 

No medusae in life cycle but polyp topped by medusa-like region.

Phylum Cnidaria 

Class Staurozoa 

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Commonly called stauromedusans

No medusa stage. Solitary polyp body that is stalked. Uses adhesive disk to attach to seaweeds, and objects on sea bottom. Polyp top resembles a medusa with eight extensions (“arms”) ending in tentacle clusters surrounding mouth. Reproduce sexually.

Class Hydrozoa 

Hydrozoans are variable in form.  Mostly marine.  Usually colonial, sometimes solitary.

Class Hydrozoa – Typical Life Cycle 

Most hydrozoans alternate between polyp and medusa forms.  

Some have only polyps. Some have only medusae.

Class Hydrozoa - Polyps 

Typical hydroids have a base, a stalk, and one or more terminal zooids (individual polyp animals). 

Thecate – presence of a protective cup around the polyp. 

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Obelia

Athecate – no such protection. 

Ectopleura

Class Hydrozoa - Medusae 

Hydroid medusae are usually smaller than those in the class Scyphozoa. 

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Gastrovascular cavity is continuous from mouth to tentacles and is lined by gastrodermis. Velum, inward projection of the bell, is present.

Specialized organs:  

Statocysts – equilibrium Ocelli – light sensitive

Class Hydrozoa 

of the order Siphonophora, such as the Portuguese man-ofwar, are actually colonies of polyp individuals. 

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One polyp may be gas filled and used as a float. Feeding polyps each with one long tentacle Reproductive polyps

Class Hydrozoa 

Hydrocorals resemble true corals. 

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Calcareous skeleton Fire coral

Class Scyphozoa 

In the class Scyphozoa, jellies (medusae) are the prevalent form of the life cycle.  

No velum present. Rhopalium – sense organ containing statocysts and sometimes ocelli.

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.

Class Scyphozoa – Typical Life Cycle 

Planula larvae develop into a polyp-like form.  Saucer-like buds called ephyrae are produced by strobilation.

Class Cubozoa 

In the class Cubozoa, which includes box jellies and sea wasps, the medusa is boxshaped and has complex eyes.  Polyps are tiny and develop directly into medusae.

Class Cubozoa

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 – Order Actinaria 

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.

Sea Anemones – Order Actinaria 

Sea anemones usually move by gliding slowly along on their pedal discs.  When a predator approaches, most withdraw.  Stomphia detaches its disc and “swims” away.

Sea Anemones – Order Actinaria 

Tentacles arranged around the central mouth.  The gastrovascular cavity is divided into six radial chambers. 

Increases the surface area of the gastrodermis.

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. 

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Fish gains protection from living in the anemone. The fish may help ventilate the anemone, or keep it free of sediment.

Hexacorallian Corals 

Hexacorallian corals (Order Scleractinia) are the true or stony corals.  Like tiny anemones living in calcareous cups.

Hexacorallian Corals 

Hexacorallians (Subclass Hexacorallia) have a gastrovascular cavity subdivided by septa in multiples of six.  The calcium carbonate skeleton is secreted below living tissue.

Hexacorallian Corals 

Polyps may be retracted into the skeleton. 

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Often retracted during the day.

The polyps expand for feeding.

Tube Anemones and Thorny Corals 

of subclass Ceriantipatharia.  Have coupled but unpaired septa.  Tube anemones 

Solitary and live in soft sediments.

Tube Anemones and Thorny Corals 

Thorny or black corals 

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Colonial and attach to firm substrata.

Both groups have few species and live in warmer seas.

Octocorallian Corals 

of the Subclass Octocorallia have eight pinnate tentacles and eight septa.

Octocorallian Corals 

Sea fans, sea pens & sea pansies belong to this group.  Often beautifully colored.

Phylum Cnidaria Cnidaria Medusozoa Anthozoa

Staurozoa

Gut with septal filament s Siphonoglyph Anthozo an pharynx Hexaradial and octaradial symmetry

Scyphozoa

Strobilation

Hydrozoa

Cubozoa

Complex eyes

Trachylinelike hydrozoa

Hydroids

Man-of-war

Other hydrozoa

Velarium Boxlike medusa body

Creeping planula without cilia

Rhopalium Polyp reduced or lost

Polyp lost

Velum in medusae Medusae produced by lateral Medusoid body formbudding and entocodon Motor nerve net

Primary polyp tentacles hollow Mouth surrounded by solid tentacles Planula larva Cnidocytes Radial, polypoid body form

Coral Reefs 

Coral reefs are one of the most productive and diverse ecosystems on Earth.  Found in shallow tropical seas. 

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They require warm water. Sunlight required for symbiotic zooxanthellae.

Coral Reefs 

The main structure of the reef is calcium carbonate secreted by hermatypic (reefbuilding) corals and coralline algae.

Coral Reefs 

Octocorallian corals and some hydrozoans (those known as fire coral) also contribute calcium carbonate.

Types of Reefs 

Fringing reefs are close to a landmass with no lagoon or a narrow lagoon.

Types of Reefs 

Barrier reefs run parallel to shore and have a wide, deep lagoon.

Types of Reefs 

Atolls are reefs that encircle a lagoon, but not an island.

Types of Reefs 

Patch reefs are scattered throughout a lagoon.

Reef Zones 

The fore reef slope, or reef front, is the side that faces the sea. 

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Slopes into deeper water.

The reef crest is the shallow or even slightly emergent top of the reef. The reef flat is the shallow back reef area that slopes into the lagoon.

Coral Reefs 

Nutrients from fertilizer and sewage threaten coral reefs with excessive algal growth.  Coral reefs in many areas are threatened by factors mostly of human origin.  Higher atmospheric concentrations of carbon dioxide (from burning hydrocarbon fuels) tends to acidify ocean water, which makes precipitation of CaCO3 by corals more difficult metabolically. http://www.youtube.com/watch?v=QAaRJKKTOys

Coral Reefs 

Symbiosis between zooxanthellae and corals is threatened by global warming. 

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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.

Phylum Ctenophora 

Phylum Ctenophora are the comb jellies.  

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No nematocysts. Tissue level of organization, like cnidarians. Mostly free swimming.

Phylum Ctenophora 

They use their ciliated comb plates for swimming. 

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Not strong swimmers.

Ctenophores are bioluminescent.  Statocyst – sense organ

Phylogeny 

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