Survey of the Animal Kingdom: Phyla Porifera and Cnidaria

General Introduction to the Animal Kingdom

• Conceptual Characteristics: Animals are defined as eukaryotic, multicellular, and ingestive-feeding heterotrophs.

• Heterotroph Definition: Organisms that derive their energy from organic molecules made by other organisms.

• Embryonic Development and Germ Layers:

  • Tissues arise from germ layers during embryonic development.

  • Diploblastic Animals: Organisms where tissues arise from two germ layers: an outer ectoderm and an inner endoderm. In sponges (Porifera), the space between these layers is a nonliving matrix called mesenchyme. In jellies (Cnidaria), this nonliving matrix is called mesoglea.

  • Triploblastic Animals: Organisms where tissues arise from three germ layers: the ectoderm, the endoderm, and a middle layer called the mesoderm, which is comprised of cells.

Overview of Phyla Porifera and Cnidaria

• Phylum Porifera (Sponges):

  • Examples: Barrel sponges, boring sponges, basket sponges, vase sponges.

  • Key Characteristics: Asymmetrical bodies that lack distinct tissues or organs. The body is saclike and consists of two layers breached by numerous pores. The internal cavity is lined with food-filtering cells known as choanocytes.

  • Habitat: Mostly marine, though approximately $150$ species live in freshwater.

  • Named Species: Approximately $5,150$ species.

• Phylum Cnidaria (Cnidarians):

  • Examples: Jellyfish, sea jellies, Hydra, corals, sea anemones.

  • Key Characteristics: Radially symmetrical, soft, gelatinous bodies with a digestive cavity that has a single opening. They possess tentacles equipped with stinging cells called cnidocytes. These cells shoot sharp harpoons called nematocysts.

  • Habitat: Almost entirely marine.

  • Named Species: Approximately $10,000$ species.

Detailed Study of Phylum Porifera

• Etymology and Basic Biology: "Porifera" means "pore bearer." These organisms use many pores to pull in water and filter food. They are sessile, meaning they are attached to a substrate.

• Structural Organization: Sponges are the simplest animal phyla. They lack true tissues and organs and are typically asymmetrical assemblages of cells.

• Cellular Independence: Cells are loosely assembled and self-sufficient. If individual cells are separated, they can survive.

• Commercial Use: While some species were historically used as bath sponges, most modern commercial bath sponges are synthetic.

• Named Examples:

  • Yellow tube sponge (Verongia).

  • Red encrusting sponge (Ophlitaspongia).

  • Grantia: A common, tubular, open-ended chamber surrounded by a thin, porous, folded wall of cells.

Sponge Anatomy and Water Flow Systems

• Feeding Mechanism: Sponges are filter feeders that remove food particles from seawater.

• Body Layers:

  • The outside of the body wall is lined by an epithelial layer of cells.

  • Spongocoel: The central cavity of the sponge.

• Cell types and Internal Surfaces:

  • Choanocytes (Collar Cells): Flagellated cells that line the spongocoel in simple sponges for filtering food. The beating of their flagella creates a current that draws water through pores within pore cells, into the spongocoel, and across the collars to trap food particles.

  • Osculum: A large hole through which filtered water exits the sponge.

• Systems of Structural Organization:

  • Ascon (Asconoid) System: The simplest type, featuring pores but no incurrent or flagellated canals.

  • Sycon (Syconoid) System: Features two types of canals. Incurrent canals are open to the outside. Flagellated canals open to the spongocoel. Water flows from incurrent canals, through pores, into flagellated canals (where food is absorbed), and into the spongocoel.

  • Leucon (Leuconoid) System: Features flagellated chambers where food is absorbed.

• Specific Water Flow Path in Grantia: External environment $\rightarrow$ incurrent canal $\rightarrow$ pore cells in the folded wall lined with choanocytes $\rightarrow$ flagellated canal $\rightarrow$ spongocoel $\rightarrow$ osculum.

Sponge Skeletal Structure and Specialized Cells

• Mesohyl (Mesenchyme): The nonliving matrix located between the outer and inner cellular layers.

• Skeleton: A simple skeleton found in the mesohyl made of crystalline structures called spicules.

• Classification by Spicule Composition:

  • Class Calcarea: Possesses calcium carbonate ($CaCO_3$) spicules.

  • Class Hexactinellida: Possesses $6$-rayed (hexactinal) siliceous spicules.

  • Class Demospongiae: May contain $1$-rayed or $4$-rayed siliceous spicules, spongin protein, or both. (A four class is sometimes suggested by taxonomists, but usually treated as a subclass of Demospongiae).

• Functional Cells within the Mesohyl:

  • Amoebocytes: Creeping, motile cells within the sponge walls. They perform digestion and can differentiate into other cell types as required.

  • Sclerocytes: Specialized cells derived from amoebocytes that produce spicules.

• Reproduction:

  • Asexual: Occurs via budding or the release of gemmules. Gemmules are stress-resistant aggregates of amoebocytes that can grow into a mature organism when favorable conditions return.

Phylum Cnidaria: General Characteristics

• Biology: Marine, carnivorous animals with specialized stinging cells (cnidocytes).

• Symmetry and Complexity: Radially symmetrical and more complex than sponges. Sensory organs are exposed to the environment in all directions around the body perimeter.

• Primary Body Forms:

  • Polyp: A body form with the mouth and tentacles pointed upward.

  • Medusa: A body form with the mouth and tentacles pointed downward.

• Body Wall Layers:

  • Ectoderm: The outside cellular layer.

  • Endoderm (Gastroderm): Lines the interior gastrovascular cavity.

  • Mesoglea: A gelatinous layer separating the ectoderm and endoderm.

• Tissues: Possess true nervous, muscular, and reproductive tissues, though organs are loosely organized.

• Feeding and Digestion: Tentacles capture prey and bring it to the mouth. Digestion occurs in the gastrovascular cavity and is both extracellular (via enzymes) and intracellular.

• Stinging Morphology:

  • Cnidocytes: Stinging cells found on tentacles.

  • Nematocysts: Small, barbed, harpoonlike structures within cnidocytes that discharge when triggered.

Class Hydrozoa

• General Features: The polyp stage typically dominates the life cycle, though both forms occur in most species. Amoeboid cells circulate in the mesoglea.

• Cell Functions: Ectodermal cells include cnidocytes and muscular contractile cells. Endodermal/glandular cells secrete enzymes for extracellular digestion in the gastrovascular cavity.

• Genus Hydra:

  • Size: Small (<1\,cm).

  • Habitat: Shallow, freshwater ponds.

  • Life Cycle: No medusa stage.

  • Anatomy: Use a basal disk to adhere to substrates or the water surface.

• Genus Obelia:

  • Features colonial polyps and free-swimming medusae.

  • Polymorphism: Polyps are specialized. Gastrozooids are for feeding; Gonozoids are for reproduction.

• Genus Physalia (Portuguese Man-of-War):

  • A floating colony of polymorphic polyps. One polyp forms a gas-filled float.

  • Tentacles can reach lengths of up to $15\,m$. The sting is lethal to small fish and painful (rarely lethal) to humans.

• Genus Gonionemus:

  • Features an atypically large medusa.

  • Movement: The velum and manubrium contain muscle fibers. The bell pulses for propulsion.

  • Gastrovascular Cavity: Radiates into ring canals connected by a circular canal at the perimeter.

  • Gonads: Tissues that produce gametes; they attach to the radial canals.

Class Scyphozoa (Sea Jellies)

• Life Cycle: The gelatinous medusa is the dominant stage. The polyp is reduced to a small larval stage.

• Anatomy: The mesoglea contains amoeboid cells. The gastrovascular cavity is divided into four radiating pouches. The gastrodermis contains cnidocytes.

• Major Examples:

  • Cyanea capillotta: One of the largest invertebrates, living in the North Sea; can exceed $2\,meters$ in diameter.

  • Aurelia: A typical jelly, measuring $2-6\,cm$ in diameter.

• Reproductive Life Cycle Stages: Adult medusa ($2n$) $\rightarrow$ Gametes (sperm/egg) $\rightarrow$ Fertilization $\rightarrow$ Zygote $\rightarrow$ Blastula $\rightarrow$ Planula larva $\rightarrow$ Scyphistoma $\rightarrow$ Budding polyp $\rightarrow$ Ephyrae.

Class Cubozoa (Box Jellies)

• Morphology: Box-shaped medusae with tentacles or groups of tentacles located at each corner of the "box."

• Ecology: Strong swimmers and voracious predators of fish in tropical and subtropical waters. Medusae range from a few centimeters to $25\,cm$ in height.

• Venom and Toxicity:

  • Chironex fleckeri: Native to Australia and Indonesia. Possibly the most dangerous jelly.

  • Tentacles can reach lengths of $3\,m$.

  • One individual contains enough venom to kill $60$ people. Death can occur within minutes of a severe sting.

Class Anthozoa (Anemones and Corals)

• Diversity: The largest class of cnidarians with over $6,000$ species.

• Organization: Solitary or colonial polyps. There is no medusa stage in the life cycle.

• Anatomy: The mouth leads to a tubular pharynx and a gastrovascular cavity with septate compartments. Septa distinguish anthozoan polyps from those in other classes. Gonads are gastrodermal.

• Sea Anemone (Metridium):

  • Sessile; attach to substrates via a flat, sticky basal disk. They can move slowly on a film of mucus.

  • Fragmentation: A form of asexual reproduction where pieces of the basal disk tear away and grow into new individuals.

  • Acontia: Threads of tissue along the edges of the septa that bear dense batteries of cnidocytes for defense and capturing prey.

• Corals:

  • Usually colonial and structurally similar to small anemones. Most secrete a calcium carbonate ($CaCO_3$) skeleton.

  • Environmental Threats:

    • Carbonic Acid: Human-produced $CO_2$ dissolves in ocean water ($CO_2 + H_2O \rightarrow H_2CO_3$), weakening calcareous skeletons.

    • Coral Bleaching: Rising temperatures cause polyps to expel symbiotic algae and die.

  • Named Examples: Stony Coral, Organ Pipe Coral.