Sponges & Cnidarians: Structure, Function, and Diversity (Chapter 30 Study Notes)

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Sponges & Cnidarians (Chapter 30)

Introduction

This chapter covers two major animal phyla: Porifera (sponges) and Cnidaria (jellyfish, corals, and related animals). These groups represent early branches in animal evolution and illustrate fundamental differences in body structure, development, and ecological roles.

Porifera (Sponges)

General Characteristics

Phylum Porifera includes sponges, which are simple, multicellular animals.
Sponges are asymmetrical and lack true tissues and organs.
Most species are marine, though some live in freshwater.
Adults are sessile (attached to a surface), while larvae are free-swimming.

Cellular Organization

Sponges have several specialized cell types but do not form true tissues.
The body wall consists of two layers of cells separated by a gelatinous region called the mesohyl.
Choanocytes (collar cells) line the inner layer and are responsible for water flow and food capture.
Amoebocytes move through the mesohyl, digest food, and distribute nutrients.
The outer layer is made of epithelial cells.

Body Plan and Structure

Sponges have a porous body with channels and chambers for water flow.
Water enters through porocytes (pore cells), flows into the spongocoel (central cavity), and exits via the osculum.
Sponges lack a true body cavity (coelom).

Nutrition and Feeding

Sponges are filter feeders, capturing bacteria, protists, and detritus from water.
Feeding is intracellular: choanocytes trap food particles and amoebocytes digest and distribute nutrients.
Sponges can filter large volumes of water; for example, 100g of sponge can filter about 1,000kg of water daily.

Reproduction

Sponges can reproduce both sexually and asexually.
Most species are hermaphroditic (produce both eggs and sperm).
Fertilization is usually internal; sperm released into water is captured by another sponge.

Development

Sponges do not produce a gastrula stage like other animals and do not form embryonic tissue layers.
They are not classified as Eumetazoa (animals with true tissues).

Evolutionary Relationships

Sponges are closely related to choanoflagellates, a group of colonial protists.
Choanocytes in sponges are structurally similar to choanoflagellates, supporting their evolutionary link.

Summary Table: Sponge Characteristics

Feature	Porifera (Sponges)
Symmetry	None (asymmetrical)
Tissues	Absent
Body Cavity	Absent
Embryonic Layers	None
Feeding	Filter feeding (intracellular)
Reproduction	Sexual & asexual

Cnidaria

General Characteristics

Phylum Cnidaria includes jellyfish, corals, sea anemones, and hydras.
Most species are marine.
Cnidarians are radially symmetrical.
They are diploblastic (have two embryonic tissue layers: ectoderm and endoderm).
Cnidarians possess true tissues (Eumetazoa).

Body Structure

Two main body forms: polyp (sessile) and medusa (free-swimming).
Body wall consists of an epidermis (from ectoderm) and a gastrodermis (from endoderm), separated by a jelly-like mesoglea.
Central gastrovascular cavity functions in digestion and circulation.
Digestive system is incomplete: food and waste enter/exit through the same opening (mouth).
The gastrovascular cavity also acts as a hydrostatic skeleton.

Feeding and Defense

Cnidarians are carnivorous, capturing prey with specialized cells called cnidocytes.
Each cnidocyte contains a nematocyst, an organelle with a coiled, injectable thread that delivers a sting.
Triggering the cnidocyte releases the nematocyst to immobilize prey.

Corals and Coral Reefs

Corals are colonial cnidarians that build calcium carbonate skeletons, forming coral reefs.
Coral reefs are among the most diverse ecosystems, supporting about 25% of marine species.
Reefs provide food, tourism, and coastal protection for millions of people.
Corals have symbiotic relationships with photosynthetic dinoflagellates (zooxanthellae), which provide energy.
Coral reefs are threatened by bleaching (loss of symbionts due to heat stress), climate change, and pollution.

Coral Bleaching and Damage

Degree Heating Weeks (DHW) is a measure of heat stress on corals.
Bleaching can occur at DHW > 4 within 12 weeks; severe bleaching at DHW > 8.
Coral death is likely at high DHW; lower DHW may allow recovery.
Recent years have seen increased bleaching events, but some reefs show resilience.

Reproduction in Cnidarians

Many cnidarians reproduce sexually by releasing eggs and sperm into the water (external fertilization).
Mass spawning events are synchronized by environmental cues (e.g., full moon, weak tidal currents).
Sexual reproduction increases genetic diversity and dispersal.

Jellyfish Blooms and "Apocalypse"

Jellyfish populations can fluctuate dramatically (booms and busts).
Media reports of a "jellyfish apocalypse" may be exaggerated; scientific data do not confirm a global increase.
Jellyfish blooms can impact fisheries, tourism, and ecosystem balance.

Summary Table: Cnidarian Characteristics

Feature	Cnidaria
Symmetry	Radial
Tissues	Present (diploblastic)
Body Cavity	Absent (gastrovascular cavity)
Embryonic Layers	2 (ectoderm, endoderm)
Feeding	Carnivorous (cnidocytes, nematocysts)
Reproduction	Sexual & asexual

Comparative Evolutionary Context

Animal Phylogeny Overview

Sponges (Porifera) are the earliest diverging animal lineage, lacking true tissues.
Cnidarians are the first animals with true tissues and radial symmetry.
Bilaterians (e.g., worms, mollusks, arthropods) evolved later, with bilateral symmetry, three tissue layers (triploblasty), and often a coelom.

Key Evolutionary Features (from diagram)

Group	Key Features
Porifera	Multicellularity, no tissues
Cnidaria	Diploblasty, radial symmetry, true tissues
Bilateria	Triploblasty, bilateral symmetry, coelom, central nervous system

Key Terms and Definitions

Porifera: Phylum of sponges; simple, multicellular animals without true tissues.
Cnidaria: Phylum including jellyfish, corals, and sea anemones; animals with radial symmetry and true tissues.
Choanocyte: Flagellated cell in sponges responsible for water flow and food capture.
Amoebocyte: Mobile cell in sponges that digests and distributes nutrients.
Mesohyl: Gelatinous matrix within a sponge, between cell layers.
Polyp: Sessile body form of cnidarians.
Medusa: Free-swimming body form of cnidarians.
Cnidocyte: Specialized cell in cnidarians containing a nematocyst.
Nematocyst: Stinging organelle within a cnidocyte.
Diploblastic: Having two embryonic tissue layers.
Triploblastic: Having three embryonic tissue layers.
Gastrovascular cavity: Central digestive compartment in cnidarians.
Degree Heating Weeks (DHW): Measure of heat stress on coral reefs.

Formulas and Equations

Degree Heating Weeks (DHW): where is the weekly mean temperature, is the long-term average, and only weeks with are counted.

Summary Comparison Table: Sponges vs. Cnidarians

Feature	Sponges (Porifera)	Cnidarians
Symmetry	None	Radial
Tissues	Absent	Present
Body Cavity	Absent	Absent (gastrovascular cavity)
Embryonic Layers	None	2 (diploblastic)
Feeding	Filter feeding	Carnivorous (stinging cells)
Reproduction	Sexual & asexual	Sexual & asexual

Conclusion

Sponges and cnidarians illustrate the diversity of animal body plans and evolutionary innovations. Sponges represent the simplest multicellular animals, while cnidarians introduce true tissues and specialized cells for feeding and defense. Understanding these groups provides insight into the origins and adaptations of animal life.

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard biology textbooks.