1. CHAPTER 13 – The Cnidarians (Radiate Animals)

2. Phylum Cnidaria Hydra, jellyfish, coral, & sea anemones

3. Jellyfish can be funny….

5. General Characteristics Radial symmetry Their body is arranged around an oral-aboral axis Oral end terminates in a mouth that is surrounded by tentacles Good for sessile; sedentary lifestyle Reach the tissue level of organization Cells are organized into groups of tissues that work together Contains 2 tissue layers (diploblastic) including: epidermis (gives rise to the outer body wall) and gastrodermis(gut)

6. General Characteristics Acoelomated organism- they contain NO body cavity All are carnivores Over 9,000 species are in the phylum No system for circulation, respiration or excretion-> occur by diffusion Some cnidarians can regenerate lost parts or even a complete body Most cnidarians are dioecious (2 forms: male and female) All named for the presence of cells called Cnidocytes: cells that contain the stinging organelle called nematocysts Found only in cnidarians

8. General Characteristics Diploblastic: contain 2 well-defined germ layers: ectoderm (epidermis) and endoderm(gastrodermis) Examples include: Hydras, jellyfish, sea anemone, corals, Portuguese man of war, box jelly fish Movement: mostly sessile, some move or “swim” slowly All contain some type of Gastrovascular cavity with one opening for food intake and elimination of waste

10. General Characteristics Both the outer (epidermis) and inner ( gastrodermis) contains nerve cells that are arranged in a loose network called a Nerve Net Innervates their primitive muscles that extend form the epidermal and gastrodermal cells Stimulus in one part will spread across the whole body by the network

11. General Characteristics Location Found mostly in marine habitats (some freshwater) Most abundant in warm (tropical), shallow, marine waters Colonial organisms can be found attached to rocks, animals, or wharves

12. Ecological Role Predator/Prey Relationships Neurotoxins in medical research Coral- jewelry, reef building Symbiosis with other organisms Coral reefs- habitat for many Great biodiversity Protect the coastline

13. Symbiotic Relationships Mutualism Hydroids and sea anemones can live on hermit crab shells Algal cells live in the tissue of cnidarians Clown fish and sea anemone Portuguese Man of War and the Nomeus fish

17. Cnidarian Body Plans Dimorphism Existence of 2 morphological types within the same species Allows the organism to obtain food from different locations All cnidarians fit into one of the following types: *polyp *medusa

18. Cnidarian Body Plans Polyp (hydroid): Fits a sessile lifestyle Can be found singly or in colonies Structure: tubular body with a mouth at one end (directed upward) surrounded by tentacles The aboral end is attached to a substrate

19. Polyp Structure

20. Cnidarian Body Plans Medusa Jellyfish form Fits the swimming/floating lifestyle Mobile, move by weak contractions of body Bell or umbrella shape Mouth is usually directed downward and centered on the concave side and tentacles extend from the rim “mouth down” version of the polyp Contains more mesoglea (middle jelly-like layer) than polyp stage

21. Polyp vs. Medusa

23. Cnidocyte/Nematocyst Cnidarians are named after the presence of cnidocytes: the stinging cell of cnidarians that produces the nematocyst Equivalent to the container(gun) that contains the stinging organelle (bullet) Nematocyst: the stinging organelle contained within the cnidocyte Most characteristic structure Helps with taxonomic classification Over 20 different types of nematocysts

24. Structure of the Nematocyst Enclosed within the cnidocyte (made of chitin) All cnidocytes (except for Anthozoa class) have a cnidocil- modified cilium that triggers the nematocyst to eject Tactile or chemical stimulation causes the nematocyst to discharge(prey swimming) After discharge the cnidocyte is absorbed and a new one replaces it

25. Structure of the Nematocyst Operculum: the covering (lid) that encloses the nematocyst inside the cnidocyte Triggered to open through a change in pressure Effects (types) of the nematocysts Barbs: (not found in all) Penetrants: inject poisons Volvents: Long tentacle (string like); entangles prey Glutinants: secretes an adhesive

27. Steps of release for a nematocyst Stimulation of the cnidocil The operculum opens When the operculum opens there is a change in pressure that forces the nematocyst out The nematocyst is ejected inside out Poisons may be injected when it penetrates the prey

28. Cnidocyte: Before and After Discharge

30. Nerve Net The simple nervous system of cnidarians Variation of the nerve net is found between the classes Cnidarians have neurotransmitters on both sides of their synapses (junction between 2 nerve cells) this functions to transmits impulses in both directions Located in the gastrodermis and epidermis Forms 2 interconnected nerve nets

31. Nerve Net They do form a simple neuromuscular system- their sensory nerve net plus they contain contractile fibers in their body wall to coordinate muscular contractions They do not have: Myelin covering their axons Central nervous system

32. Nerve Net

33. 4 Classes of Cnidarians Class Hydrozoa (Hydra, PMOW, Obelia) Class Scyphozoa (common jellyfish) Class Cubozoa (box jelly) Class Anthozoa (sea anemone and coral)

34. Class Hydrozoa General Characteristics: Mostly marine (except freshwater hydra) Most are colonial Most have a sexual (medusa) and asexual (polyp) lifecycle Examples we will discuss include: Freshwater hydra- only has a polyp stage Obelia PMOW

35. Freshwater Hydra Belong to class Hydrozoa Are solitary animals (not colonial) Found in freshwater environments attached to anything…rocks, aquatic leaves… Only found in the polyp stage (no medusa stage) Feed mainly on plankton

37. Body Plan of the Hydra Cylindrical tube with a point of attachment at the bottom and mouth at the top of the tube surrounded by tentacles Contains the following parts Basal disc Located at the bottom of the tube body Serves as the site for attachment Secretes substances for adhesion Can produce gas bubble at the end of the basal disc for floating

39. Body Plan of the Hydra Hypostome Cone shaped elevation Mouth is at the center Surrounded by tentacles Mouth Surrounded by tentacles Ingests food Gastrovascular cavity Open cavity that is surrounded by the body wall Site of extracellular digestion

40. Body plan of the Hydra

41. Body wall of the Hydra Surrounds the GVC Consists of 3 layers Epidermis (outer layer) Gastrodermis (inner layer) Mesoglea (middle layer)

42. Epidermis of the Hydra Outer layer that serves as protection Composed of several cell types Epitheliomuscular cells Interstitial cells Gland cells Nerve cells Sensory cells Cnidocytes

43. Epitheliomuscular Cells Composes the majority of the epidermis Functions to protect the hydra and to produce muscular contractions

44. Interstitial Cells Located at the base of the epitheliomuscular cells Functions as stem cells to turn into almost any other cell type (cnidoblasts, sex cells, buds, nerve cells) ***What type of sponge cell does this sound like?

45. Gland Cells Located on the basal disc and around the mouth Functions to secrete adhesive or lubrication

46. Cnidocytes Contain nematocysts Function to protect the organism and to trap and kill food

47. Sensory Cells Located around the mouth, tentacles, or basal disc Allows the organism to be stimulated by the environment and to be aware of its surroundings Structure: one end had flagella for stimulation and the other end goes to the nerve cells

48. Nerve Cells Connects with sensory cells, other nerve cells, cnidocytes, or muscular cells Coordinates all of the activities

49. Body wall of the Hydra

50. Gastrodermis of the Hydra Lines and directly touches the GVC The following cell types are found in the gastrodermis: Nutritive-muscular cells Interstitial cells (performs the same function as in the epidermis- they act as stem cells) Gland cells

51. Nutritive-muscular cells Contain cilia to create a flow of food, water and nutrients inside the GVC Tall cells that contain food vacuoles Absorbs, digests and circulates food and fluids Site of intracellular digestion

52. Gland cells Gland cells in the gastrodermis are located around the hypostome and inside the column Function: Hypostome gland cells secrete lubricant and digestive enzymes into the GVC Inside the GVC the gland cells secrete digestive enzymes

53. Gastro-vascular cavity Open space inside the cnidarian Filled with water to act as a hydrostatic skeleton (provides support) Lined with gland cells to help with digestion Extracellular digestion occurs in the GVC

55. Mesoglea Location: in between the epidermis and gastrodermis Structure: non cellular, gelatinous material composed mainly of water Function: support and flexibility Mesoglea is thickest in the stalk (strength) Mesoglea is thinnest in the tentacles (flexibility)

56. Locomotion of Hydra The Hydra can move in 3 ways: Glide on basal disc (bottom of the organism) Bend over and attach tentacles to the surface Gas bubble

57. Feeding and Digestion: Hydra Hydras eat: crustaceans, insect larvae, annelid worms Feeding process: Tentacles trap prey with the help of the nematocyst (encased in the cnidocyte) Mouth widens (with the help of gland cells) to engulf prey Extracellular digestion takes place in the GVC with the help of gland cells secreting digestive enzymes Food particles go into nutritive-muscular cells for further intracellular digestion

60. Glutathione A chemical activator that triggers the feeding process of certain cnidarians Cause the tentacles to move more Causes the hydra to be more alert Causes the gland cells to release lubricant and widen the mouth Prepare for food intake

61. Hydra Reproduction Reproduce both sexually and asexually Sexual Reproduction Form temporary gonads in the fall Eggs and sperm and are shed into the water and form fertilized eggs Hydra will hatch in the spring (favorable environment) Asexual Reproduction Budding: starts as a growth on the side of the “parent” hydra Eventually will detach from parent and live on its own

62. Hydra Budding

63. Obelia- Hydroid colony 2 nd example of Hydrozoan class Has a polyp and medusa stage in its life cycle All polyps in the colony are usually interconnected Contains a base, stalk and terminal zooids Zooid: general term for an individual polyp animal in a colonial cnidaria

64. Obelia Structure Hydrorhiza: (root-like) base that attaches the colonial polyp to a substrate Hydrocauli: (stem) part that extends from the hydrorhiza and the individual zooids Zooid: attached to the hydrocaulus; individual polyp animals

65. Obelia Structure

66. Types of tissue found in the Obelia Coenosarc: the inner, living, cellular part of the Obelia Perisarc: the outer, protective, non-living, covering of the Obelia Made of chitin

67. Obelia Structure

68. Types of Zooids Gastrozooids Also called hydranths Feeding animals of the colony All have tentacles with cnidocytes Feed on crustaceans, worms, and larvae If one polyp eats it provides nutrients for the entire colony Digestion and circulation of food is aided by ciliated nutritive muscular cells and body wall contractions

69. Types of Zooids Gonozooids Reproductive polyp that release medusa buds Obelia reproduces both sexually and asexually. Sexual Reproduction: (most common pathway) * young medusa leaves the colony * medusa buds mature and shed gametes (sperm and egg) * fertilization occurs * Zygote created turns into a Planula (free swimming larvae stage) that swims and attaches to form new colony

70. Obelia Reproduction Asexual Reproduction: Occurs by budding-> increases the size of the colony Occurs but outgrowth of the body wall Creates new gastro- and gono- zooids

72. Obelia Reproduction

74. Medusa structure of Obelia “Jellyfish” form Contains: Velum: membrane under the surface if the umbrella of the medusa that projects inward Contains tentacles with cnidocytes Manubrium: tissue projecting from the oral side of the medusa; surrounds the mouth

75. Internal medusa structure of Obelia Entire system is lined by the gastrodermis Gastrovascular cavity is continuous from the mouth to the tentacles Pathway of food flow: (disperses food w/o circulatory system) Mouth-> opens to the manubrium projecting form the oral side of the mouth-> leads to the stomach and the 4 radial canals-> connects to the ring around the margin of the bell-> connects the tentacles

76. Movement of Obelia medusa Aboral side first Characterized by weak jet propulsion Muscular pulsations fill bell with water and empty water to propel medusa

77. Obelia: Medusa Stage

78. Nerve Net of Obelia Medusa Concentrated in 2 nerve rings at the base of velum Composed of Statocysts: functions in equilibrium Ocelli: light sensitivity

79. Nerve Net

80. Physalia physalis Portuguese Man of War (PMOW) 3 rd example of Hydrozoan class Located in warmer, tropical waters Considered to be a polymorphic swimming community that is composed of several types of animals that swim and act as 1 colony Nematocysts are located all over the tentacles Some are very dangerous-> secrete a neurotoxin

81. Structure of PMOW Pneumatophore Rainbow colored float Filled with gas Carries future generations and acts as a nursing center Can briefly deflate for protection

82. Types of individuals in a colony Gastrozooids: feeding zooids that ingest the food Dactylzooids: long fishing tentacles that sting, capture the prey and bring it to the mouth On average are 30 feet long (can be as long as 165 feet underwater) Covered in nematocysts Gonozooids: sacs with ovaries and testes Function in reproduction

83. Portuguese Man 0’ War Tentacles of Physalia physalis

88. Review of Cnidarian Class Hydrozoa Most are marine (except freshwater hydra) Most are colonial (except freshwater hydra) Most have a polyp and medusa stage (except freshwater hydra-> polyp only) Examples are the Hydra, PMOW and the Obelia

89. Class Scyphozoa: True Jellyfish

90. Class Scyphozoa: General Characteristics Class includes the larger jelly fish Medusa stage is dominant; most of lifecycle spent in this form Located floating in the open sea (one order sessile) Aggressive predators

91. Class Scyphozoa: Structure Bells vary in shape and size (depth)- made mainly of mesoglea (95-96% water) No velum (like hydrozoan medusa) Vary in number and length of tentacles Margin of the bell is usually “scalloped” and divided into 8 indentations Mouth is beneath the umbrella on the “sub-umbrella” side Has a manubrium that forms four (or multiples of 4) oral arms to help capture and ingest prey

92. Class Scyphozoa: Structure Tentacles and manubrium contain cnidocytes with nematocysts (maybe found all over body as well) Each “indent” contains a rhopalium- a sense organ that contains a statocyst (equilibrium) and eyespots (ocelli)

93. Class Scyphozoa: Nervous System and Movement Movement: rhythmical pulsations of the umbrella The main purpose of the constant movement is to ensure that water and food is forced into the mouth since many jellyfish feed on small plankton animals Controlled by the nerve net The sub-umbrella nerve net controls bell pulsations

94. Class Scyphozoa: Pathway of Food Mouth (sub umbrella surface)-> leads to the manubrium that forms the 4 oral arms Oral arms each are lined with cnidocytes and contain a ciliated groove that functions to encourage water flow towards the mouth and to capture and ingest prey The gastrodermis is lined with nutritive muscular cells that bring in food and water and keeps the current and circulation moving Once food enters the GVC (stomach) there are 4 gastric pouches extending off the GVC -> the gastric pouches are lined with nematocysts Extending from the gastric pouches are radial canals that lead to a ring canal this allows for distribution of food around the bell and down the tentacles

96. Class Scyphozoa: Aurelia aurita Found in waters off the east and west coast Feed on small plankton animals AKA: Moon Jelly

97. Scyphozoan Lifecycle: Aurelia Sexes are separate Gonads are located and gametes develop in the gastric pouches Fertilization is internal Sperm is carried by ciliary currents into the mouth of the female to fertilize the egg in the gastric pouches The zygote can develop in the water or stored in the folds of the oral arms

98. Scyphozoan Lifecycle: Aurelia The zygote will develop into a ciliated planula larvae to disperse Eventually the ciliated larvae will and attach to a surface and develop into a scyphistoma: hydra like polyp form This will begin the process of strobilation: the schyphistoma of the Aurelia forms a series of saucer like buds called ephyrae (immature medusa) by budding The polyp is now called a strobila (polyp containing many ephyrae) Eventually the ephyrae in the strobila will differentiate and be released from the strobila as swimming ephyra Ephyra will mature into an adult jellyfish

99. Scyphozoan Life Cycle

100. Other Scyphozoan Examples 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

101. Other Scyphozoan Examples Upside down jellyfish (Cassiopea xamachana) from Bermuda; found in warm coastal waters Form a symbiotic relationship with zooxanthellae (algae)-> algae gets CO2 and protection and jelly gets food produced by photosynthesis Can also eat zooplankton Oral arms are fused together to form hundreds of tiny mouth openings Usually found with its umbrella side attached to a surface or pulsating upside down and its tentacle side exposed to the sunlight “sunbathing”-> major role in the symbiotic relationship

102. Upside down jellyfish - Cassiopea xamachana

103. Upside down Jelly fish

104. Lions Mane jelly fish

105. Giant Jelly off Coast of Japan

106. Lion’s mane jellyfish eating

107. Jellyfish

108. Lion’s mane

109. Jellyfish

110. Sea Nettle

112. Beached Jelly fish

114. Class Cubozoa: General Characteristics Strong, fast swimmers (not just drifters) living in coastal waters off Northern Australia, throughout the Indio-Pacific, and Hawaii Once was part of Scyphozoan class In transverse section the bell looks square with a group of tentacles (about 15) found at each corner Tentacles can reach 10 feet long Each tentacle can contain 5,000-500,000 cnidocytes Cnidocytes are triggered by chemical on the outer layer of its prey The base of each tentacle is flattened into a tough blade called a pedallium- used for swimming, forcing prey into bell and acting like a whip Umbrella is not scalloped and the sub umbrella edge turns inward to form a velarium (increases swimming efficiency) Feed on fish and shrimp

115. Class Cubozoa: General Characteristics Reproductive polyps do not undergo strobilation-> the entire polyp becomes the juvenile medusa The venom is considered to be the most deadly and painful in the world Attacks heart, nervous and skin cells-> leads to shock, scaring, intense pain and death Best eyes of any cnidarian Grouped into clusters of 6 on the four sides of their bell Each cluster contains a pair of eyes with a lens, retina. iris, and cornea (not sure how they process what they see w/o CNS) Can avoid obstacles and washing up on beaches

116. Cubozoa Structure

119. Class Anthozoa Known as the flower animals Only found in the polyp stage- no medusa stage Located in deep or shallow marine waters Vary in size May live solitary (sea anemone) or colonial (corals) lifestyle

120. Class Anthozoa- Typical Polyp Form At one or both ends of the mouth is a ciliated groove called the siphonoglyph; generates a water current and brings food to the pharynx and GVC Possess a well developed pharynx lined with cilia The gastrovascular cavity is large and petitioned by complete and incomplete septa or mesenteries (number depends on class); increase surface area for digestion or support Contain septal perforations: openings between chambers to allow for water and food circulation

121. Class Anthozoa Functions of the water currents: Allows water and oxygen to be carried into the animals Allows waste products to leave Maintain hydrostatic pressure

122. Class Anthozoa- Typical Polyp Form

123. Class Anthozoa There are 3 sub-classes of Anthozoa Hexacorallia: sea anemones and hard coral Symmetry based on the number 6 Tube anemones and thorny corals (used to be placed in separate sub-class) Octocorallia: soft and horny corals Sea fan, sea pen, sea pansies Symmetry based on the number 8

124. Sub-class Hexacorallia: Sea Anemone Example #1 is the sea anemone Solitary; found attached to shells, rocks, timber Carnivorous; feed on living or dead animals Larger than hydrozoan polyps Located in warm, marine, coastal areas Contain an oral disc around mouth with tentacles and a pedal disc on inferior surface for attachment

125. Sub-class Hexacorallia: Sea Anemone Muscular system is well developed Contains longitudinal and circular muscles Movements: Can glide on pedal disc Can expand and stretch to capture food Can contract and withdraw tentacles Some can swim to escape enemy(sea star)-> reaction is triggered by touch or chemicals released by the predator

127. Sub-class Hexacorallia: Sea Anemone Symbiotic relationship (mutualistic): Some contain mutualistic relationship with symbiotic algae (zooxanthellae) Some found attached to hermit crab shells Some fish have a mucus on their skin that causes the sea anemone’s cnidocytes to not discharge

130. Sub-class Hexacorallia: Sea Anemone Sexual Reproduction Most have separate sexes Gonads are found in the margin of the septa Fertilization can be internal or external Zygotes develop into ciliated planula larvae

131. Sub-class Hexacorallia: Sea Anemone Asexual Reproduction occurs in 4 ways Budding Transverse fission Longitudinal fission Pedal laceration: piece of pedal breaks off and regenerates

132. Class Anthozoa: Sea Anemones

136. Class Anthozoa: Hexacorallian Corals AKA true or stony corals Can be solitary or compound Miniature sea anemone that secrete and live in a calcareous cup to serve as protection Do not have a siphonoglyph Most are the main coral reef builders Instead of a pedal disc, they secrete a skeletal cup with a sclerosepta projecting up into the polyp

138. Class Anthozoa: Hexacorallian Corals In colonial corals, the skeleton may become massive and build up many layers over time In most sexes are separate; fertilization is external Forms a ciliated planula larvae stage The living coral will repeatedly reproduce asexually by budding The individuals will be interconnected forming colonies The living polyps are interconnected by coenosarc: horizontal sheets of living tissue that extends over the outer surface of the skeleton (completely covering it) The living sheets of tissue contain extensions of the GVC so that food and water are constantly circulating between the members

140. Class Anthozoa: Hexacorallian Corals Two main types of Stony Coral: Zooxanthellate (hermatypic) Contain symbiotic algae (zooxanthellae) Provide food and oxygen for corals by photosynthesis Growth occurs 3x faster with this relationship Only found in shallow, well-lit, warm areas Most feed on plankton; some can eat invertebrates and small fish Most are nocturnal; retreating into skeletons during the day Non-zooxanthellate (non-hermatypic) Non reef builders- can be found in colder, darker areas

141. Zooxanthellae Green algae Forms part of the mutualistic relationship with hermatypic corals

142. Class Anthozoa: Hexacorallian Corals

143. Colt Coral

144. Elkhorn Coral

145. Flower Corals

147. Lamellina Coral

148. Class Anthozoa: Sub-class Octocorallian Contain octomerous symmetry (anatomy based on 8) Live a colonial lifestyle Consist of tiny polyps living in soft matrix Soft corals; contain no stony skeleton GVC of polyps work together through a system of interconnected gastrodermal tubes called solenia that run through the mesoglea; water and food can run freely between colony Skeleton (coenenchyme) is within their internal tissue and consists of limy spicules, fused spicules or proteins for support

149. Class Anthozoa: Sub-class Octocorallian Variations in skeletal patterns lead to a variety of different corals Contain no symbiotic algae Examples: sea pansies and others sea pen Tentacles give a feather like appearance containing many branches

150. Body of an Octocorallian Coral

153. Cabbage Coral

154. Feather Coral

155. Gorgonian Fan Coral

156. Subergorgia Coral

157. Soft Coral

158. Sea Pen Marine animals found in deeper tropical and temperate waters Strengthened by an axial rod and spicules both made of calcium carbonate Colonial animals with multiple polyps (each look like a miniature sea anemone) Polyps are specialized to specific functions A single polyp develops into the erect stalk and loses its tentacles forming the base The other polyps branch out forming water intake structures, feeding structures, and reproductive structures When touched they emit a bright greenish light- bioluminescence

159. Sea Pen

161. Sea Fan

162. Sea Plume

163. Coral Reefs Large formations of calcium carbonate laid down by organisms over thousands of years Found in marine, shallow, tropical waters Living plants and animals are confined to the top layers of the reef (above calcium carbonate deposits), where they add to the deposits Formed mainly by hermatypic corals and coralline algae Habitats support great animal diversity for a large number of corals and fish

164. Benefits of Coral Reefs Home for ¼ of all marine fish Benefit coastal regions by acting as a buffer from strong waves and storms Provide jobs and food for costal areas Advances in modern medicine

165. How are Coral Reefs formed? Most formed by the symbiotic relationship between polyp animal and zooxanthellae algae Algae supplies the polyp with oxygen and food Polyp shelters the algae and provides it with carbon, nitrates, and phosphates for photosynthesis Must be located in shallow, warm waters- need sunlight for photosynthesis Coral polyps use the energy supplied by their algae to produce calcium carbonate or limestone to produce their protective cup

166. How do Coral Reefs grow? Continually add on to their calcium carbonate base- > grow up and out Reproducing Asexually by budding Sexually- external fertilization

167. Changes in Coral Reefs Coral reefs change throughout time They begin with a new tropical island (produced by an oceanic hot spot or a plate boundary) They gradually change through thousands of years from a fringing reef, to a barrier reef, to an atoll, and an extinct reef.

168. Types of Coral Reefs Fringing Reef Barrier Reef Atolls Patch or Bank Reef

169. Fringing Reef Forms directly attached to a shore of most new tropical islands Little to no lagoon separating the reef from the shoreline

170. Fringing Reef

171. Barrier Reef Reef separated from the main land or island shore by a deep channel or lagoon Forms as the island begins to sink into the Earth’s crust due to the absence of volcanize island building forces, the added weight of the coral reef, and erosion of the shoreline As the island sinks the reef continues to grow upward

172. Barrier Reef

173. Atoll Forms when the oceanic island sinks below the surface of the island but the coral reef continues to grow upward Usually circular in shape that continues all the way around the lagoon without an island in the center

174. Atoll