Comprehensive Notes on Phylum Porifera

Phylum Porifera: The "Pore Bearers"

Etymology and Definition: Phylum Porifera is known as the "Pore Bearers," characterized by a body containing numerous pores and canals.
General Examples of Sponges:
- Red boring sponge.
- Encrusting sponge.
- Finger sponge.
- Variable sponge.
- Tube sponge (e.g., Yellow tube sponge).
- Coral head (associated habitat).

Learning Outcomes for Phylum Porifera

After studying this material, learners should be able to:
- List distinguishing characteristics of Phylum Porifera.
- Describe sponge anatomy and the function of major cell types.
- Define terminology: spicules, spongin, ostia (ostium), and oscula (osculum).
- Compare and contrast asconoid, syconoid, and leuconoid canal systems, including the flow of water.
- Describe feeding mechanisms and patterns of asexual and sexual reproduction.
- Identify the four classes of extant sponges and their key features.
- Contrast the body forms of Hexactinellida and Demospongiae.
- Explain the ecological and pharmacological importance of sponges.

Phylogeny of Animals

Phylogenetic Position: Porifera is situated at the base of the animal kingdom (Animalia).
Developmental Milestone: The blastula stage occurs during development.
Related Groups and Clades:
- Diploblasts: Sponges precede the divergence of Bilateria (Nephrozoa).
- Bilateria: Includes Protostomia and Deuterostomia.
- Protostomia: Divided into Lophotrochozoa (includes Annelida, which encompasses pogonophorans, echiurans, and sipunculans) and Ecdysozoa (includes Kinorhyncha and Arthropoda, the latter including pentastomids).
- Micrognathozoa, Gnathostomulida, and Acanthocephala: Associated lophotrochozoan lineages.
- Chaetognatha: Discussed alongside deuterostomes.
- Xenacoelomorpha: Basal bilaterian group.

General Characteristics of Phylum Porifera

Habitat: All sponges are aquatic, with the vast majority being marine.
Symmetry: Sponges exhibit either no symmetry (asymmetrical) or radial body symmetry.
Level of Organization: Sponges possess a Cell/Tissue level of organization.
- The multicellular body is composed of an aggregation of differentiated cell types with various functions.
- Whether sponges possess "true tissues" is a subject of active debate among scientists.
Anatomical Absence: They lack true organs or organ systems but are still capable of responding to stimuli.
Mobility: Adults are sessile and attached to a substrate.
Structural Features: The body is characterized by pores and canals.

Support Structures: Spongin and Spicules

Spongin: A fibrous protein that provides structural support and flexibility in many sponges.
Spicules: Mineralized skeletal elements that provide rigidity and defense.
- Siliceous Spicules: Found in Class Hexactinellida (6-rayed) and Class Demospongiae (not 6-rayed).
- Calcareous Spicules: Composed of calcium carbonate ( $CaCO_3$ ), characteristic of Class Calcarea.
Axial Filament: Present in siliceous spicules of some classes to organize structure.

Sponge Cell Types and Layers

The Pinacoderm (Outer Layer):
- Similar to epithelial tissue.
- Pinacocytes: Flat, epithelial-like cells that cover the outer surface and some inner surfaces.
- Myocytes: Modified pinacocytes that contain contractile filaments to help regulate water flow by changing pore size.
The Mesohyl (Middle Layer):
- A gelatinous extracellular matrix that acts as the "connective tissue."
- Contains spicules, various fibrils, and organic skeletal elements.
- Archaeocytes: Amoeboid cells that can move through the mesohyl; they are totipotent and can differentiate into other cell types (e.g., for digestion or gamete production).
- Sclerocytes: Cells that secrete spicules.
- Spongocytes: Cells that secrete spongin fibers.
- Collenocytes: Cells that secrete fibrillar collagen.
The Choanoderm (Inner Layer):
- Composed of Choanocytes (collar cells).
- Choanocyte Anatomy: Includes a flagellum surrounded by a collar of microvilli and microfibrils. Features include a nucleus, mitochondria, food vacuoles, and contractile vacuoles.
- Function: The beating of the flagellum creates water currents, and the collar traps food particles.

Canal Systems and Water Flow

Asconoid System (e.g., Leucosolenia):
- Simplest form; small and tube-shaped.
- Flow: Ostium $\rightarrow$ Spongocoel (lined with choanocytes) $\rightarrow$ Osculum.
Syconoid System (e.g., Sycon):
- Body wall is folded into canals.
- Flow: Dermal ostium $\rightarrow$ Incurrent canal $\rightarrow$ Prosopyle (opening) $\rightarrow$ Radial canal (lined with choanocytes) $\rightarrow$ Apopyle (opening) $\rightarrow$ Spongocoel (lined with pinacocytes) $\rightarrow$ Osculum.
Leuconoid System (e.g., Callyspongia):
- Most complex system, allowing for larger body sizes.
- Flow: Ostia $\rightarrow$ Incurrent canal $\rightarrow$ Prosopyles $\rightarrow$ Flagellated chambers (lined with choanocytes) $\rightarrow$ Apopyles $\rightarrow$ Excurrent canals $\rightarrow$ Oscula.

Biological Processes and Principles

Principle of Continuity:
- In sponges, the product of cross-sectional area ( $A$ ) and velocity ( $V$ ) is constant: $A \times V = \text{constant}$ .
- Increased total cross-sectional area leads to decreased velocity (inside flagellated chambers to allow for nutrient capture).
- Decreased cross-sectional area (at the osculum) leads to increased velocity to expel wastes far from the sponge.
Sponge Feeding:
- Water is pumped through the sponge (some large sponges pump up to $1500\,dm^3$ per day).
- Food particles are trapped by the choanocyte collar and taken in via endocytosis.
- Digestion occurs in food vacuoles; nutrients are transported by archaeocytes.
- Carnivorous Sponges: A subset of sponges (e.g., family Cladorhizide) lacks choanocytes and captures small crustaceans with hook-like spicules.

Reproduction and Development

Asexual Reproduction:
- Somatic embryogenesis: Reorganization of fragmented cells into a new organism.
- Fragmentation: Breaking into pieces followed by regeneration.
- External Budding: Formation of buds on the outer surface.
- Internal Budding (Gemmules): Dormant masses of encapsulated archaeocytes found in freshwater sponges to survive harsh conditions.
Sexual Reproduction:
- Most sponges are monoecious (hermaphroditic).
- Gametes are derived from choanocytes or archaeocytes.
- Sperm are released into the water (spawning); filtration brings sperm to eggs of another sponge for internal fertilization.
- Larval Stages:
  - Parenchymula: A solid-bodied ciliated larva found in demosponges.
  - Amphiblastula: Found in calcareous sponges like Sycon; undergoes Inversion (stomoblastula turns inside out) during development.

Taxonomical Classes within Phylum Porifera

Homoscleromorpha ( $\sim 1\%$ of species):
- Marine sponges, often encrusting or cushion-shaped.
- Found in dark ecosystems like caves.
- Skeleton is absent or composed of siliceous spicules without an axial filament.
- Characterized by a basement membrane underlying the pinacoderm.
Calcarea (Calcispongiae) ( $\sim 8\%$ of species):
- Spicules composed of calcium carbonate ( $CaCO_3$ ).
- Internal system of pores and canals.
Hexactinellida ( $\sim 8\%$ of species):
- Known as "Glass Sponges."
- Siliceous spicules with 6 rays organized around an axial filament.
- Body forms a syncytial trabecular reticulum (a single continuous syncytium).
- Found in deep-water reefs (e.g., Howe Sound, British Columbia) and used as shelter by various marine life like the Giant Pacific octopus.
Demospongiae ( $\sim 83\%$ of species):
- Most diverse class.
- Siliceous spicules (not 6-rayed) or a spongin network, or both.
- Includes boring sponges, finger sponges, and tube sponges.

Ecology and Pharmacology

Ecological Role: Sponges act as water filters and provide habitat/shelter for other marine organisms.
Pharmacological Importance:
- Sponges are the richest sources of pharmacologically active chemicals from marine organisms.
- Over $5300$ products known; more than $200$ new metabolites reported annually.
- Key Drugs:
  - ara-A: Antiviral drug used against herpes simplex encephalitis virus.
  - Manzamine A: Displays activity against malaria, tuberculosis, and HIV.
  - Lasonolides: Possess antifungal activity.
  - Psammaplin A: Possess antimicrobial activity.
Predators and Defenses:
- Predators: Hawksbill Sea Turtle, Angel Fish, Sea Slugs, and other mollusks.
- Defenses: Physical protection via spicules and chemical protection via allelochemicals (secondary metabolites).