28.2 Phylum Cnidaria
3 min read•june 14, 2024
Cnidarians, including and corals, represent a significant leap in animal complexity. These creatures boast true tissues, a radially symmetrical body plan, and specialized cells called . Their unique features set them apart from simpler organisms like sponges.
Cnidarians come in two main body forms: the sessile and the free-swimming . They're divided into four classes, each with distinct characteristics. These animals have adapted to various aquatic environments, showcasing remarkable abilities and diverse reproductive strategies.
Phylum Cnidaria
Organization of Porifera vs Cnidaria
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- Porifera (sponges)
- Asymmetrical or radially symmetrical body plan
- Lack true tissues and organs, instead have loosely organized cells
- Possess a water canal system with choanocytes that filter feed by creating water currents (Leuconia)
- Skeleton composed of spicules made of calcium carbonate or silica, or spongin protein fibers (bath sponge)
- Cnidaria (jellyfish, corals, and sea anemones)
- Radially symmetrical body plan with oral and aboral ends
- Possess true tissues organized into two layers (), an and
- Epidermis (outer layer) provides protection and contains cnidocytes
- Gastrodermis (inner layer) lines the aids in digestion and absorption
- (non-cellular layer) separates epidermis and gastrodermis, providing support (jellyfish)
- Gastrovascular cavity () serves as a digestive tract, distributing nutrients throughout the body
- Specialized cells called cnidocytes contain used for defense and prey capture (jellyfish )
Tissue development in Cnidaria
- Cnidarians are the first animals to develop true tissues, a key step in increasing animal complexity
- Epidermis and gastrodermis are organized into functional layers with specific roles
- Cells within each layer are connected by cell junctions, allowing communication and coordination
- Tissue organization enables specialization and division of labor among cells
- Epidermal cells protect the animal from the environment and contain cnidocytes for defense ()
- Gastrodermal cells secrete digestive enzymes and absorb nutrients from the gastrovascular cavity
- Presence of a nervous system with nerve nets allows for coordination of body movements and responses to stimuli
- Nerve nets consist of interconnected neurons that transmit signals throughout the body (Hydra)
- Development of tissues in cnidarians represents a significant increase in complexity compared to sponges
Polyp vs medusa body forms
- Polyp form
- Sessile lifestyle, attached to a substrate
- Cylindrical body with a mouth surrounded by tentacles used for feeding and defense
- Mouth opens into a gastrovascular cavity that serves as a digestive tract
- Examples include corals, sea anemones, and hydras
- Medusa form
- Free-swimming lifestyle, moving through the water column
- Bell-shaped body with tentacles hanging from the margin, used for swimming and prey capture
- Mouth located on the underside of the bell, leading to the gastrovascular cavity
- Examples include jellyfish like moon jellies and lion's mane jellies
- Some cnidarians exhibit both polyp and medusa forms in their life cycle, a phenomenon called (Obelia)
Characteristics of cnidarian classes
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- Exhibit both polyp and medusa forms in their life cycle
- Can be colonial or solitary
- Examples include Hydra, Obelia, and Portuguese man-of-war (Physalia)
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- Predominantly exist in the medusa form
- Have a complex life cycle that includes a , polyp, and medusa stage
- Examples include moon jellyfish (Aurelia) and lion's mane jellyfish (Cyanea)
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- Exclusively exist in the medusa form, lacking a polyp stage
- Possess a box-shaped bell with complex eyes and potent venom
- Examples include box jellyfish (Chironex) and sea wasps (Carukia)
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- Exclusively exist in the polyp form, lacking a medusa stage
- Includes both solitary and colonial species
- Examples include corals (Acropora), sea anemones (Nematostella), and sea pens (Pennatula)
Adaptations and Reproduction
- Cnidarians possess remarkable regeneration abilities, allowing them to regrow lost body parts
- Many species exhibit , with individuals capable of producing both male and female gametes
- Tentacles play a crucial role in prey capture, defense, and environmental sensing
Key Terms to Review (38)
Anthozoa: Anthozoa is a class within the phylum Cnidaria that includes marine invertebrates such as corals and sea anemones. These organisms are characterized by their polyp form, which is typically sessile, and they possess a unique body structure featuring a central mouth surrounded by tentacles equipped with specialized cells called cnidocytes. Anthozoans play vital roles in marine ecosystems, particularly as foundational species in coral reef habitats.
Budding: Budding is a form of asexual reproduction where a new organism develops from an outgrowth or bud due to cell division at one particular site. This process can be seen in various organisms, including viruses and certain animals like hydras.
Budding: Budding is a form of asexual reproduction where a new organism develops from an outgrowth or bud on the parent organism. This process allows for rapid population increase and is a key feature in some animals, particularly in specific phyla, illustrating how organisms can reproduce without the need for gametes.
Cnidoblasts: Cnidoblasts are specialized cells found in members of the phylum Cnidaria, responsible for producing stinging structures known as nematocysts. These cells play a crucial role in the feeding and defense mechanisms of organisms such as jellyfish, sea anemones, and corals, allowing them to capture prey and deter predators. The ability of cnidoblasts to discharge nematocysts is a defining characteristic of Cnidarians, contributing significantly to their ecological success.
Cnidocyte: A cnidocyte is a specialized cell found in members of the phylum Cnidaria, such as jellyfish, corals, and sea anemones. It contains a stinging organelle called a nematocyst used for prey capture and defense.
Cnidocytes: Cnidocytes are specialized cells found in members of the Phylum Cnidaria that serve as the primary means of capturing prey and defending against predators. These unique cells contain organelles called nematocysts, which are capable of injecting toxins into other organisms upon contact, facilitating both predation and protection. Cnidocytes play a crucial role in the ecological success of cnidarians like jellyfish, corals, and sea anemones.
Coelenteron: The coelenteron is the central digestive cavity found in members of the Phylum Cnidaria, serving as a site for digestion and nutrient absorption. This cavity is also known as the gastrovascular cavity and plays a crucial role in the overall physiology of cnidarians, facilitating both digestion and distribution of nutrients throughout the organism. The structure allows for a single opening that functions as both mouth and anus, making it integral to the unique body plan of these organisms.
Coral: Coral refers to marine invertebrates within the class Anthozoa, primarily found in tropical oceans, that live in colonies made up of numerous identical polyps. These polyps secrete calcium carbonate, forming a hard exoskeleton that contributes to the structure of coral reefs, which are vital ecosystems supporting diverse marine life. Corals are also symbiotic organisms, hosting algae called zooxanthellae, which provide them with energy through photosynthesis, highlighting their importance in marine ecology.
Coral reef: A coral reef is a diverse underwater ecosystem made up of colonies of coral polyps that form large structures in shallow tropical and subtropical waters. These reefs are crucial for marine biodiversity, serving as habitats for countless species, and play an essential role in the health of oceanic environments.
Cubozoa: Cubozoa is a class within the phylum Cnidaria, known for its unique cube-shaped medusa form and powerful stinging cells called nematocysts. These organisms are primarily marine and include well-known species such as the box jellyfish, which are famous for their complex eyes and potent venom. Cubozoans exhibit fascinating behaviors, including rapid swimming and an ability to capture prey efficiently.
Dimorphic: Dimorphic organisms exhibit two distinct forms within their life cycle, often adapted to different environments or functions. This feature is common in many species for survival and reproduction.
Diploblastic: Diploblastic refers to organisms that develop from two primary germ layers: the ectoderm and the endoderm. This characteristic is significant in understanding the basic structural organization of certain animals, particularly within specific phyla, as it influences their body plan and the development of tissues and organs. The presence of only these two layers indicates a simpler level of organization compared to triploblastic organisms, which have a third layer called the mesoderm.
Epidermis: The epidermis is the outermost layer of cells in both plants and animals, providing protection and serving various functions such as gas exchange and moisture retention. In plants, it plays a crucial role in regulating water loss and protecting against environmental stresses, while in animals, it acts as a barrier against pathogens and harmful substances.
Gastrodermis: The gastrodermis is the innermost tissue layer that lines the gastrovascular cavity of cnidarians, such as jellyfish and corals. It plays a critical role in digestion and nutrient absorption, utilizing specialized cells like gastrodermal cells and cnidocytes to facilitate these processes. This layer also contributes to the overall structure and function of the organism's body plan.
Gastrovascular cavity: The gastrovascular cavity is a central digestive compartment found in organisms such as cnidarians, which serves both as a site for digestion and as a means of nutrient distribution throughout the body. This cavity has a single opening that functions as both mouth and anus, allowing for the intake of food and the expulsion of waste. Its simple structure is crucial for the functioning of these organisms, enabling them to efficiently process food and circulate nutrients.
Hermaphroditism: Hermaphroditism is a reproductive strategy where an organism possesses both male and female reproductive organs. This allows the organism to produce both eggs and sperm, often enabling self-fertilization or mating with any individual of its species.
Hermaphroditism: Hermaphroditism is a reproductive strategy where an organism possesses both male and female reproductive organs, allowing it to produce both eggs and sperm. This adaptation increases reproductive flexibility and enhances the chances of successful mating, particularly in species with low population densities or where finding mates is challenging.
Hydrozoa: Hydrozoa is a class within the phylum Cnidaria that primarily consists of small, mostly marine organisms, including both solitary and colonial forms. This class is notable for its diverse life cycle, which typically includes both a polyp stage and a medusa stage, showcasing unique adaptations for survival in aquatic environments.
Jellyfish: Jellyfish are gelatinous, free-swimming marine animals belonging to the phylum Cnidaria. They are known for their umbrella-shaped bell and tentacles, which contain specialized cells called cnidocytes that can sting and capture prey. Jellyfish are not only fascinating due to their unique anatomy but also play important roles in marine ecosystems as both predators and prey.
Medusa: Medusa refers to a free-swimming, umbrella-shaped life stage of certain cnidarians, such as jellyfish. This form is characterized by its gelatinous body and tentacles that hang downwards, allowing it to float in water and capture prey with specialized stinging cells called cnidocytes. The medusa stage contrasts with the polyp stage, showcasing the complex life cycle of these organisms.
Mesoglea: Mesoglea is a gelatinous, non-cellular layer found between the two main tissue layers in animals of the Phylum Cnidaria, primarily in jellyfish, corals, and sea anemones. This layer provides structural support and buoyancy, playing a crucial role in the body architecture of these organisms. Its unique composition allows Cnidarians to maintain their shape and enables them to float in aquatic environments.
Metagenesis: Metagenesis refers to a life cycle that alternates between sexual and asexual reproduction, commonly seen in certain organisms. This process allows for increased genetic diversity through sexual reproduction while also enabling rapid population growth via asexual reproduction. In the context of Cnidaria, metagenesis is particularly significant as it relates to the transition between polyp and medusa forms, showcasing the adaptability and complexity of this phylum's life cycle.
Nematocyst discharge: Nematocyst discharge refers to the rapid release of specialized stinging cells called nematocysts found in organisms within the Phylum Cnidaria. These cells are used for capturing prey and defending against predators, releasing a harpoon-like structure that can inject toxins into the target. The mechanism is a remarkable example of how cnidarians have evolved to use their unique adaptations for survival in aquatic environments.
Nematocysts: Nematocysts are specialized stinging organelles found within the cnidocytes of cnidarians, such as jellyfish, corals, and sea anemones. These structures play a critical role in the feeding and defense mechanisms of these organisms, allowing them to capture prey and deter predators. The mechanism of nematocyst function involves the rapid discharge of a coiled tubule that can inject toxins into targeted organisms.
Nerve net: A nerve net is a decentralized and diffuse nervous system found in certain organisms, notably in the phylum Cnidaria. This network of interconnected nerve cells, or neurons, allows for simple and coordinated responses to environmental stimuli, facilitating basic motor functions and reflexes in animals like jellyfish and sea anemones. The structure of a nerve net is significant because it supports the unique body plans and lifestyles of these organisms, enabling them to function without a centralized brain.
Pedalia: Pedalia are the muscular, paddle-like structures located at the base of the tentacles in certain members of the class Cubozoa. They play a crucial role in locomotion and prey capture for these jellyfish.
Planula larva: The planula larva is a free-swimming, ciliated larval stage of cnidarians that develops from the fertilized egg and eventually settles to form a polyp. This stage is crucial for the life cycle of cnidarians, as it allows for dispersal and colonization of new habitats. The planula's ability to swim and its ciliated surface enable it to move through the water, increasing its chances of finding suitable substrate to settle and grow into the adult form.
Polyp: A polyp is a cylindrical, tubular body form found in certain organisms within the Phylum Cnidaria, such as jellyfish and sea anemones. This body form is characterized by a mouth surrounded by tentacles at one end and a basal disk that attaches to a substrate at the other end. Polyps are typically sessile and can reproduce asexually, contributing to the life cycle of cnidarians.
Radial symmetry: Radial symmetry is a body plan in which any longitudinal cut through the central axis results in identical halves. It is commonly found in organisms like jellyfish, sea anemones, and starfish.
Radial symmetry: Radial symmetry is a body plan in which organisms can be divided into similar halves along multiple planes through a central axis. This type of symmetry is common in aquatic animals and plays a significant role in their interaction with the environment, as it allows them to sense and respond to stimuli from all directions.
Regeneration: Regeneration is the biological process by which organisms replace or restore lost or damaged tissues, organs, or body parts. This ability varies widely among different species, showcasing remarkable adaptations in certain groups to survive injury or predation while also playing a role in growth and development.
Scyphozoa: Scyphozoa is a class within the phylum Cnidaria, primarily known for its jellyfish species. These organisms are characterized by their gelatinous bodies and often large, umbrella-shaped bell, which enables them to swim through the water by contracting and relaxing their bell. Scyphozoans are typically free-swimming in their medusa stage, and they play essential roles in marine ecosystems as both predators and prey.
Sea anemone: A sea anemone is a soft-bodied, sessile marine animal belonging to the phylum Cnidaria, characterized by its colorful, flower-like appearance and tentacles that contain specialized cells called cnidocytes. These creatures are closely related to corals and jellyfish and play essential roles in marine ecosystems as both predators and habitat providers. Sea anemones are known for their symbiotic relationships, particularly with clownfish, which provide mutual benefits to each other.
Siphonophores: Siphonophores are colonial marine organisms composed of specialized individual zooids that function together as a single entity. These members of the phylum Cnidaria exhibit a high degree of polymorphism and specialization among their zooid components.
Strobilation: Strobilation is a form of asexual reproduction that involves the sequential budding of segments called ephyrae from the bell of certain cnidarians, particularly in the class Scyphozoa, which includes true jellyfish. This process allows for the rapid production of multiple offspring, contributing to the life cycle of these organisms, where a medusa stage is predominant. Strobilation connects to the unique life cycles and reproductive strategies found within Phylum Cnidaria.
Symbiodinium: Symbiodinium is a genus of dinoflagellates that forms a mutualistic relationship with various marine organisms, especially corals. These tiny algae live within the tissues of their host and provide essential nutrients through photosynthesis, while receiving protection and access to light from the host. This symbiotic relationship is crucial for the health of coral reefs and contributes significantly to the energy dynamics of these ecosystems.
Tentacles: Tentacles are elongated, flexible appendages found in various organisms, particularly within the Phylum Cnidaria, where they serve crucial roles in feeding, locomotion, and sensory perception. In this phylum, tentacles are often armed with specialized cells called cnidocytes, which contain nematocysts that can deliver toxins to capture prey or deter predators. These structures significantly enhance the ability of cnidarians like jellyfish, sea anemones, and corals to interact with their environment.
Velum: The velum is a membranous structure found in certain cnidarian medusae, such as hydromedusae. It forms a rim around the inner margin of the bell and aids in locomotion.