Phylum Cnidaria

General Information: Cnidaria means ‘a stinging thread’ Examples: sea anemones, corals, jellyfish, freshwater Hydra, and the Portuguese man-of-war Habitat: marine and fresh water

Sea Anemone, Clover Point, Victoria B.C.                                                                                     Soft Coral Sea Anemone, Clover Point, Victoria B.C.

Portuguese Man Of War Brain Coral

Corals are an important part of BC marine ecosystems! In BC, deep water corals such as the red tree or “Gorgonian coral” provide shelter for juvenile rockfish such as the yelloweye rockfish or sharpchin rockfish

Characteristics: All cnidarians have a basic radial symmetry and possess only 2 layers of living tissues. 2 tissue layers: ectoderm and endoderm Contain a mesoglea Some possess an exoskeleton (for protection) made up of CaCO3 (e.g. corals)

Characteristics Have 2 body forms: a. Polyp (sessile) – e.g. hydra, coral, or sea anemone b. Medusa (free swimming) – bell shaped – e.g. jellyfish

Some cnidarians contain only one body shape during their life cycle, while others contain both during their life cycle When a cnidarian exhibits the 2 body forms in its life cycle, it is called polymorphism (‘many shapes’) The evolution of a primitive nervous system sets these organisms apart from the sponges – called a nerve net They contain nematocysts found on the tentacles used for capturing prey and for protection

Mesoglea is sandwiched between ectoderm and endoderm There are amoebocytes in the mesoglea of cnidarians, much like in sponges Cnidarians possess a simple digestive system – the gastrovascular cavity – where digestion and circulation of nutrients occur http://shapeoflife.org/video/cnidarians-life-move

Sea Anemones – Class Anthozoa: No medusa stage They are all marine Polyp body form exclusively Reproduce asexually through longitudinal or transverse fission (like binary fission, but on a much larger scale) Are primarily carnivorous – capture food using nematocyst-studded tentacles and transfer it to a central mouth opening There are around 6000 species of sea anemones

Coral – Class Anthozoa Colonial form of Class Anthozoa Can have soft and hard corals Hard corals secrete external CaCO3­ skeletons and are called ‘stony corals’ Stony corals make up coral reefs (not all make reefs though) Coral add about 10 kg of additional CaCO3 per m2 of reef yearly Soft corals do not have hard external skeletons

The Anatomy of A Coral Many corals have symbiotic green algae called zooxanthellae living in their polyps Warmer ocean temperatures can cause these algae to die, resulting in a phenomenon known as “coral bleaching”

Jelly Fish – Class Scyphozoa Only a few hundred species – nearly all are marine The body is in the form of an inverted cup, with nematocyst-studded tentacles extending downward from the cup. Some jellyfish have symbiotic unicellular algae that give them nutrients The life cycle of jelly fish involves the two different body forms: polyp and medusa

A Typical Jellyfish Life Cycle

A Typical Cnidarian - Hydra Structure: Polyp body form Locomotion: Sessile; however, some can somersault and glide along while secreting mucus

Ingestion, Digestion, Elimination Food is captured by nematocysts in the tentacles by stinging or stunning prey Tentacles push food through mouth into gastrovascular cavity Flagella line the endoderm layer and create a current The endoderm cells secrete enzymes to break down food Food is taken into cells and nutrients are circulated by amoeboid cells Wastes are eliminated out through the mouth

The Nematocyst Nematocysts vary in shape, but all are adapted for stinging or stunning prey In the sea anemone on the right, nematocyst aggregations are visible as the dark pink lines

Respiration O2 enters the cells by diffusion CO2 leaves the cells by diffusion

Nervous System Have a nerve net within the mesoglea When stimulated, the nerve net will respond by contracting / relaxing the muscles Have sensory cells that are sensitive to touch, light, chemicals, and balance

Muscular System Latitudinal and circular muscle fibres are found in the endo / ectoderm layers

Excretion Nitrogenous wastes leave the cells by diffusion Wastes enter the gastrovasuclar cavity and leave via the mouth

Reproductive System A. Asexual Budding (bud breaks away) Regeneration (re-growth of missing parts) A Hydra Budding

Reproductive System B. Sexual Some are hermaphrodites Ovaries and testes grow on the side of adults Ovary produces an egg and remains attached to the adult Testes produce the sperm which swim to the egg, resulting in a zygote Zygote develops into a ball of cells called the blastula, which develops into a ciliated larva called a planula, which can swim to another location, settle, and develop into a polyp adult

Ecological Importance of Cnidarians Are extremely important in the ecology of tropical and temperate oceans in the form of coral (eg Great Barrier Reef) Corals provide habitats and food for many species of fish and invertebrates Reefs protect the land from ocean waves Cnidarians form symbiotic relationships with other organisms living among the tentacles (clown fish and sea anemones)

The Importance of Deepwater Corals Throughout the world, many deepwater coral reefs are being destroyed by trawlers, removing vital nursery grounds for many fish species

Cnidarians and Medicine Used in medical research for their chemicals (cancer therapy) Stony corals are now being used in certain surgical procedures. Because they have many pores for blood vessels to grow through, small pieces of coral are now being used in bone grafts, particularly in face and jaw reconstruction, and in arm and leg surgery These pieces are eventually replaced by normal bone tissue

Evolutionary Significance Cnidarians are the first animals to have organized true cell tissues They are therefore the first true multicellular organisms to appear They are the first organisms to have muscle tissue and nervous tissue Cnidarians, having radial symmetry are thought to share a common ancestor with all other multicellular animals