Cnidaria and Ctenophora Chapter 7, Zoology

Phylum Cnidaria Pronounced (ny-dar'e-a). Name means “like or connected with nettles”. Approximately 9,000 species. All aquatic; mostly marine, a few freshwater. All have radial symmetry. All have two germ layers (diploblastic); the ectoderm and endoderm. Polymorphism, two body types; polyp and medusa.

Phylum Cnidaria It's name is derived from the many stinging cells that they bear called Cnidocytes. Cnidocytes usually contain stinging organelles called nematocysts. The nematocysts function as weapons that deliver powerful toxins to kill, paralyze, or injure their prey or enemies.

Cnidocytes and Nematocysts

Ecology of Cnidaria Corals play an important role in the ecology of coral reefs by building reefs and providing a habitat for other marine organisms. Found mostly in shallow, warm equatorial/tropical marine habitats. Hydra and Sea Anenomes often live symbiotically on mollusc and crab shells. Floating medusae can be found in open ocean (pelagic) zones and open lake (limnetic) zones.

Dimorphism (Polymorphism) Cnidarians exhibit polymorphism, meaning they have two different body types or plans. The sedentary or sessile type that lives attached to the ground are called Polyps. The mobile type that floats is called a Medusa.

Description of Polyp Body Form Cylindrical or tube-shaped body. Mouth is surrounded by tentacles. Mouth leads to the gastrovascular cavity. The aboral end is attached to the ground. Anemone in the Polyp Body Form

Asexual Reproduction in Polyps There are three different ways a polyp can reproduce asexually. Budding – A round ball of tissue forms on the side of the polyp and develops into an adult polyp with a functional mouth and tentacles. Fission – When an adult polyp splits in half and develops into two identical adult clones. Pedal Laceration – When tissue is torn from the base of the polyp and develops into a new polyp.

Polyps Budding

Budding Colonies If buds stay attached to the polyps, a cnidarian colony can form. Budding colonies can share food through a common gastrovascular cavity. Different polyps in a colony that specialize and perform specific tasks are called zooids. Gastrozooids aid in feeding. Dactylozooids aid in defense. Gonozooids aid in reproduction.

The Portugese Man-of-War Example of colonies of zooids in a common cnidarian

Description of Medusa Medusa are free-swimming or floating. Medusa are bell-shaped or umbrella-shaped. The mouth of the medusa (oral side) faces downward and is surrounded by tentacles. Both the polyp and medusa have a jelly-like fluid in the center of their bodies called mesoglea. The mesoglea is much thicker in medusa making them buoyant, giving them the name jellyfishes.

Medusa

Cnidarian Movement Colonial polyps are stationary Solitary polyps can move slowly along the surface of the ground by secreting mucus and sliding along their base. Medusa can move freely by contracting their body and by excreting water out their mouth.

Cnidarian Life Cycles Life cycles of cnidarians can be complex, but in general life begins in the form of a larva called a planula. The planula settles and develops into a an adult polyp. Depending on the type of cnidarian, the polyp can remain a polyp for life (like sea anemones and corals) or the polyp may develop into a medusa (like a jellyfish). Eventually becoming sexually mature and releasing gametes (sex cells) that will unite to form a zygote.

Typical Cnidarian Life Cycle

Typical Cnidarian Life Cycle

Cnidarian Digestion The tentacles draw the prey into the mouth. Digestion begins in the gastrovascular cavity. Digestive enzymes are secreted in the gastrovascular cavity and the food is broken down into smaller pieces. After the food is broken down, it gets absorbed by the cells lining the gastrovascular cavity. Eventually waste is expelled out the mouth.

Cnidarian Cells and Tissues The body of a cnidarian consists of three parts The outer epidermis (derived from ectoderm) The inner gastrodermis (derived from the endoderm) And the middle mesoglea

Cnidarian Cells Cnidarians posses a variety of cells in the epidermis and gastrodermis. Nutritive-muscular cells – provide support and structure with a hydrostatic skeleton; additionally they also function as digestive cells (found in the gastrodermis). Gland cells – secrete digestive enzymes (gastrodermis). Epitheliomuscular cells – provide a protective covering and muscular movement. Contracts tentacles. (epidermis). Interstitial cells – stem cells that can develop into a variety of other cells including cnidocytes, sex cells, buds, nerve cells etc. (found in the epidermis).

Cnidarian Cells Cnidocytes – These cells are the defense mechanism for the cnidarians. The cnidocytes contain little spines attached to a coil called nematocysts that function as weapons and can injure or paralyze their prey and enemies. There are three types of nematocysts; Those that penetrate and inject poison (penetrants), those that entangle prey (volvents), and those that secrete an adhesive substance (glutinants)

Nematocysts

Cnidarian Nervous System No central nervous system in Cnidarians. All of the sensory and nerve cells are connected via a nerve net which is a complex network of cells that are able to communicate and respond to each other.

Cnidarian Senses Some Cnidarians also have some basic sensory cells that are connected to their nerve net. Statocysts provide equilibrium and balance. Ocelli are photosensitive and detect light.

Class Hydrazoa (hi-dro-zo'a) Name means “water serpent animal”. Most Hydrazoa live in marine environments and live in colonies. Occasionally Hydrazoa live in freshwater and are solitary. Most commonly polyps reproduce asexually and medusa reproduce sexually Examples of genera include; Hydra, Obelia, Physalia,

Hydra and Obelia Hydra are freshwater cnidarians that are solitary and exist in the polyp body form Obelia often live in colonies and exist in both the polyp and medusa body form Solitary Hydra on left and Obelia colony on the right

Physalia (Portugese Man-of-War) Exist as both medusa and polyp forms. These Hydrazoans form floating colonies. The nematocysts secrete a powerful neurotoxin that can inflict painful stings. This portugese man-of-war is actually a colony of hydrazoans acting as one unit.

Class Scyphozoa (si-fo-zo'a) Name means “cup animal” Includes most of the large common jellyfish Solitary animals All scyphozoans are marine Polyp stage is sometimes absent Scyphozoans have a bell-shaped medusa filled with an enlarged mesoglea and they lack a velum. The velum is a circular membrane surrounding the cap of a medusa that aids in swimming. Examples of Scyphozoans genera include; Aurelia, Cyanea, Cassiopeia, and Rhizostoma

Aurelia and Cyanea Aurelia on top “moon jellyfish”. Feeds on plankton caught in mucus on its umbrella Cyanea capillata on bottom. Called “Sea blubber” by fishermen. Aka Giant jellyfish or Lion's mane jellyfish) with a bell diameter of 2 meters and tentacles that can reach up to 60 or 70 meters long.

Class Cubozoa (ku'bo-zo'a) Name means a “cube-shaped animal”. Solitary animals. Very little is known about the life cycle of cubozoans. Medusa are the primary body form. The bell-shaped medusa have a square or cube-shaped cross section. Examples of cubozoa include; Carybdea, Tripedalia, Chironex, and Chiropsalmus

Class Cubozoa Box Jellyfish (Carybdea marsupialis)

The Sea Wasp Box Jellyfish (Chironex fleckeri) Stings are very painful, dangerous and sometimes fatal. Found in the waters of Australia.

Class Anthozoa (an-tho-zo'a) Name means “flower animal”. All polyps, no medusa. All marine, some colonial, some solitary. Found in deep, shallow, tropical and polar seas. Three different subclasses. Zoantharia – Sea anemones and hard corals Ceriantipatharia – Tube anemones and thorny corals Octocorallia – Soft and horny corals such as sea pens, sea fans, and sea pansies. Corals with an octomerous arrangement.

Class Anthozoa Subclass Zoantharia Sea Anemones and Hard Corals

Class Anthozoa Subclass Ceriantipatharia Tube Anemones and Thorny Corals

Class Anthozoa Subclass Octocorallia Soft and horny corals Octocorallians are octomerous meaning they usually have an arrangement of eight tentacles arranged around the mouth.

Phylum Ctenophora (te-nof'o-ra) Name means comb bearing. Ctenophorans are called “comb jellies”. All marine, less than 100 species known. Live mostly in warm, tropical waters. Eight rows of comblike plates for locomotion. Radial symmetry like Cnidarians. Most Ctenophores do not have nematocysts like Cnidarians. Instead they use sticky glue-like cells called colloblasts to capture prey. Comb jellies are also bioluminescent.

Ctenophora (Comb Jellies)