Phylum Cnidaria

General Characteristics They are radially symmetrical; oral end terminates in a mouth surrounded by tentacles. They have 2 tissue layers Outer layer of cells - the epidermis Inner gastrodermis, which lines the gut cavity or gastrovascular cavity (gastrodermis secretes digestive juices into the gastrovascular cavity) In between these tissue layers is a noncellular jelly-like material called mesoglea

Cnidarian Body Plans (Dimorphic) Polyp form Tubular body, with the mouth directed upward. Around the mouth are a whorl of feeding tentacles. Only have a small amount of mesoglea Sessile Medusa form Bell-shaped or umbrella shaped body, with the mouth is directed downward. Small tentacles, directed downward. Possess a large amount of mesoglea Motile, move by weak contractions of body

Forms of Cnidarians

Movement The cnidarian body is capable of some kind of coordinated movement Both the epidermis and the gastrodermis possess nerve cells arranged in a loose network - nerve net (plexus), which innervate primitively developed muscle fibers that extend from the epidermal and gastrodermal cells Stimulus in one part will spread across the whole body via the network

Decentralized Nervous System

Nutrition Cnidarians are carnivores with hydras and corals consuming plankton and some of the sea anenomes consuming small fishes They use they tentacles to capture prey and direct it toward the mouth so that it can be digested in the gastrovascular cavity via secretions from gland cells (extracellular digestion); some food is phagocytized by special cells and digestion occurs intracellularly The gastrovascular cavity exists as 1 opening for food intake and the elimination of waste There is no system of internal transport, gas exchange or excretion; all these processes take place via diffusion

Stinging Organelles Prey capture is enhanced by use of specialized stinging cells called cnidocytes located in the outer epidermis. Each cnidocyte has a modified cilium - cnidocil, and is armed with a stinging structure called a nematocyst. The undischarged nematocyst is composed of a long coiled thread When triggered to release, either by touch or chemosensation, the nematocyst is released from the cnidocyte and the coiled thread is everted Some nematocysts function to entangle the prey; others harpoon prey and inject a paralyzing toxin

Everted nematocyst Nematocyst Firing

Reproduction One of the most amazing adaptations is the ability of some cnidarians to regenerate lost parts or even a complete body Asexual reproduction is common with new individuals being produced by budding Sea anenomes engage in a form of asexual reproduction called pedal laceration Cnidariand are dioecious (male and females) Fertilization is external, with the zygote becoming a elongated, ciliated, radially symmetrical larva - planula larva Planula larva

Cnidarian Dimorphic Life Cycle Fig. 13.9 Colonial hydrozoan

Cnidarian Taxonomy Phylum Cnidaria Class Hydrozoa (Hydras) Class Scyphozoa (Jellyfish) Class Cubozoa (Box Jellyfish) Class Anthozoa (Anemones and Corals)

Phylogeny of Cnidaria Cladogram based on molecular systematic data

Class Hydrozoa Includes the solitary freshwater hydra; most are colonial and marine Typical life cycle includes both asexual polyps and sexual medusa stages; however, freshwater hydras and some marine hydroids do not have a medusa stage Solitary Hydras Freshwater hydras are found in ponds and streams occurring on the underside of vegetation Most possess a pedal disc, mouth, hypostome surrounded by 6-10 tenetacles Mouth opens to the gastrovascular cavity The life cycle is simple: eggs and sperm are shed into the water and form fertilized eggs; planula is by passed with eggs hatching into young hydras Asexual reproduction via budding

Colonial Hydrozoans - e.g., Obelia Class Hydrozoa cont. Colonial Hydrozoans - e.g., Obelia Possess a skeleton of chiton that is secreted by the epidermis All polyps in the colony are usually interconnected Two different kinds of individuals that comprise the colony: feeding polyps or gastrozooids (C) and reproductive polyps or gonozooids (B)

Gonozooids release free swimming medusae Class Hydrozoa cont. Life Cycle of Obelia Gonozooids release free swimming medusae Zygotes become planula larvae, which eventually settle to become polyp colonies The medusae of hydroids are smaller than those of jellyfishes (C. Scyphozoa) Also, the margin of the bell projects inward forming a shelf-like velum

Other Hydrozoans Portuguese man-of-war: Class Hydrozoa cont. Other Hydrozoans Portuguese man-of-war: Single gas-filled float with tentacles Tentacles house the polyps and modified medusae of the colony

Fire Corals NOT TRUE CORAL TRUE CORAL Cnidocytes cause severe pain Hydrozoan colony TRUE CORAL Anthozoan colony Cnidocytes cause severe pain Fig. 13.15

Dimorphic Life Cycle and Reproductive Modes Asexual Budding Medusa buds Polyp buds Sexual Gonadal tissue Gametes Fertilization, embryogenesis Planula larvae Fig. 13.Fig. 13

Scyphozoan Characteristics Jellyfish Medusa is dominant body form Go through small polyp stage during life cycle Stinging tentacles May live close to shore or the open ocean

Parts of a Jellyfish 24 24

Class Scyphozoa Jellyfish The medusae are large and contain massive amounts of mesoglea The differ from the hydrozoan medusa in that the lack a velum Possess four gastric pouches lined with nematocysts; these are connected with the mouth an the gastrovascular system

Parts of a Jellyfish 26 26

Scyphozoan Life Cycle - Aurelia Gametes develop in gastrodermis of gastric pouches; eggs and sperm are shed through mouth Fertilized eggs develop into a planula larva; settles on substrate and develops into a polyp - scyphistoma Scyphistoma produces a series of polyps by budding - strobila The polyps undergo differentiation and are released from the strobila as free swimming ephyra Ephyra matures into an adult jellyfish

Life Cycle of Aurelia

Aurelia aurita (Moon jelly) Fig. 13.19

CNIDARIA Class Scyphozoa Sea thimble jellyfish (Linuche unguiculata), Honduras Tiny jellyfish (< 1 inch) that swarm in the spring Adults and larvae (“sea lice”) may cause a severe skin reaction in humans Photo Copyright © Diane R. Nelson

CNIDARIA Class Scyphozoa Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display CNIDARIA Class Scyphozoa Jellyfish (Mastigias sp., family Rhizostomeae) from Jellyfish Lake in Palau, western Pacific Have lost their ability to sting and depend on zooxanthellae for nutrition Photo Copyright © Diane R. Nelson

CNIDARIA Class Scyphozoa Lion’s Mane Jelly (Cyanea capillata) Bell – 6 ft Tentacles – 120 ft

CLASS CUBOZA The medusa form of a box jellyfish has a squarish, box-like bell. From each of the four lower corners of this hangs a short pedalium or stalk which bears one or more long, slender, hollow tentacles

underside of the bell is a mobile appendage called the manubrium which somewhat resembles an elephant's trunk gastrovascular cavity. It is divided by four equidistant septa into a central stomach and four gastric pockets. The eight gonads are located in pairs on either side of the four septa. The margins of the septa bear bundles of small gastric filaments which house nematocysts and digestive glands and help to subdue prey

The box jellyfish's nervous system is more developed than that of many other jellyfish. Notably, they possess a nerve ring around the base of the bell that coordinates their pulsing movements box jellyfish are unique in the possession of true eyes, complete with retinas, corneas and lenses. Their eyes are set in clusters called rhopalia, located in pockets halfway up the outer, flat surfaces of the bell.

Class Anthozoa No medusae, polyp only Mouth has a pharynx Gastrovascular cavity is divided into sections Mesoglea contains amoeboid cells Sexual and asexual reproduction

Body structure of Anthozoans Pedal disk Oral disk Siphonoglyph – moves water into gastrovascular cavity to maintain hydrostatic pressure. Acontia – prevents live prey from damaging gastrovascular cavity.

Class Anthozoan Characteristics. Sea Anemones Brightly colored & Resemble flowers Solitary polyps Feed on invertebrates & fish Corals Most are colonial Build limestone case Live as polyps in their case

Class Anthozoa Exclusively marine; there is no medusa stage At one or both ends of the mouth is a ciliated groove called the siphonoglyph; generates a water current and brings food to the gastrovascular cavity Possess a well developed pharynx The gastrovascular cavity is large and petitioned by septa or mesenteries; increase surface area for digestion or support Edges of the septa usually have threadlike acontia threads, equipped with nematocysts and gland cells

copyright cmassengale Sea Anemones & Coral Exist only as Polyps Clownfish live in anemone tentacles Corals build a limestone case which build up & form reefs ANEMONE copyright cmassengale CORAL 41

Solitary anthozoans include sea anemones Class Anthozoa cont. Solitary anthozoans include sea anemones Most anthozoans are colonial (e.g. corals) and secrete external skeletons composed of calcium carbonate. Corals obtain much of their energy from microscopic photosynthetic green algae (zooxanthellae) or dinoflagellates that live symbiotically inside the cells of the coral

Distribution of Coral Reefs copyright cmassengale 43