Animals: Sponges and Cnidarians – Structure, Function, and Ecological Roles

Animals: Sponges and Cnidarians

Introduction

This study guide covers the basic biology of sponges (Phylum Porifera) and cnidarians (Phylum Cnidaria), focusing on their characteristics, methods of energy acquisition, reproduction, and ecological roles. These groups represent some of the earliest branches in animal evolution and are foundational for understanding animal diversity.

Phylum Porifera (Sponges)

Basic Characteristics

• Cellular Organization: Sponges have few cell types and lack true tissues or organs. Their bodies are organized at the cellular level.

• Body Plan: Asymmetrical, meaning they do not have a defined shape or symmetry.

• Embryonic Development: Monoblastic, with only one germ layer formed during development.

• Nervous System: Absent; sponges do not possess neurons or any form of nervous tissue.

• Movement: Sessile organisms; they are fixed in place and do not move.

Example: Common sponges include Spongilla and Euspongia.

Nutrition

• Feeding Strategy: Sponges are primary consumers, feeding on prokaryotes and protists, and also act as decomposers/detritivores by consuming organic matter.

• Feeding Mechanism: Water is drawn through pores into the sponge's body, where specialized cells called choanocytes capture and digest food particles.

Example: The sponge's filter-feeding system efficiently removes bacteria and organic debris from water.

Reproduction and Development

• Asexual Reproduction: Commonly occurs via fragmentation, where pieces of the sponge break off and grow into new individuals through mitosis.

• Sexual Reproduction: Sponges release sperm into the water; fertilization occurs when sperm is taken in by another sponge, leading to the development of a new organism.

Example: Gemmules are a form of asexual reproductive structure in freshwater sponges.

Ecosystems and Ecological Impact

• Habitat: Sponges are restricted to aquatic ecosystems, both marine and freshwater.

• Ecological Role: As decomposers, sponges influence biological production and biodiversity by increasing rates of nutrient cycling.

Example: Sponge beds can enhance water quality and provide habitat for other aquatic organisms.

Phylum Cnidaria (Cnidarians)

Basic Characteristics

• Tissues and Organs: Cnidarians possess true tissues and organs, unlike sponges.

• Body Plan: Radial symmetry, meaning body parts are arranged around a central axis.

• Embryonic Development: Diploblastic, with two germ layers (ectoderm and endoderm).

• Nervous System: A simple nerve net, allowing basic responses to stimuli.

• Movement: Limited movement via muscle cell contractions; some are sessile, others are free-swimming.

Example: Jellyfish (Aurelia), sea anemones (Actinia), and corals (Acropora).

Nutrition

• Feeding Strategy: Cnidarians are secondary consumers, acting as predators.

• Feeding Mechanism: Use specialized stinging cells called cnidocytes to capture and immobilize prey.

Example: The Australian box jellyfish is among the most venomous animals, using its tentacles to deliver potent toxins.

Reproduction and Development

• Asexual Reproduction: Occurs via budding, where new individuals form from the parent organism.

• Sexual Reproduction: Involves multiple life cycle stages, including egg, larva (planula), and adult forms.

Example: Many cnidarians alternate between polyp and medusa stages in their life cycle.

Ecosystems and Ecological Impact

• Habitat: Cnidarians are restricted to aquatic ecosystems, primarily marine environments.

• Ecological Role: Corals act as ecosystem engineers and keystone species, increasing biological production and biodiversity by forming coral reefs.

Example: Coral reefs support thousands of marine species and protect coastlines from erosion.

Animal Phylogeny: Placement of Sponges and Cnidarians

Evolutionary Relationships

• Porifera: Represent one of the earliest diverging animal lineages, characterized by multicellularity but lacking true tissues.

• Cnidaria: Diverged after Porifera, with the evolution of true tissues and radial symmetry.

• Bilaterians: More complex animals with bilateral symmetry and three germ layers.

Additional info: Phylogenetic trees show Porifera and Cnidaria as basal animal groups, preceding the evolution of more complex bilaterian animals.

Phylum	Symmetry	Germ Layers	Nervous System	Movement	Feeding Strategy
Porifera	Asymmetrical	Monoblastic (1 layer)	None	Sessile	Filter-feeding, decomposer
Cnidaria	Radial	Diploblastic (2 layers)	Nerve net	Limited (muscle contractions)	Predatory (cnidocytes)

Human Impacts on Coral Reefs

Effects of Human Activities

• Pollution and Climate Change: Human-generated pollution and rising global temperatures are causing declines in coral abundance and health.

• Consequences for Humans: Loss of coral reefs can reduce biodiversity, decrease fishery yields, and diminish coastal protection, negatively impacting human well-being.

Example: Coral bleaching events, driven by increased water temperatures, threaten the survival of reef ecosystems and the livelihoods of people who depend on them.