Species Information: Approximately 15,000 living species; primarily aquatic and sessile (benthic).
Prevalence: Sponges are very common in various aquatic environments.
General Characteristics
Symmetry: Often exhibit no symmetry.
Historical Classification: Aristotle classified sponges as plants; distinct from other metazoans, indicating a primitive lineage in the animal kingdom.
Cellular Organization
Grade of Organization: Multicellular, but lacking a basal membrane.
Unique Cell Properties: Cells can be separated and re-assemble into a sponge, indicating mutual dependence for survival, distinguishing them from protozoa.
Body Structure
General Organization: An assemblage of cells embedded in an extracellular matrix, supported by minute spicules (skeleton).
Lack of Systems: Do not possess true organs or tissues, including:
Digestive system
Circulatory system
Respiratory system
Excretory system
Muscular and nervous systems
Sponge Body Plan
Key Features:
Spongocoel/Atrium: Internal space for water flow.
Ostia: Incurrent pores (sing. ostium) that allow water entry.
Water Flow: Water flows in through ostia and out through oscula, maintaining unidirectional flow.
Sponge Cell Types
Amoebocyte: Functions in digestion, nutrient transport, excretion, reproduction, and secretion of spicules and spongin.
Choanocyte: Found in the choanoderm, generates water flow and captures food particles via phagocytosis.
Pinacocyte: Provides structural support and assists in feeding.
Mesohyl: An extracellular gel in which sponge cells reside.
Feeding Mechanism
Filter-Feeding: Sponges filter feed on very small particles, primarily bacteria, using phagocytosis.
Microorganisms: Sponge-associated microorganisms can constitute up to 40% of the sponge's volume, contributing to metabolism through processes like photosynthesis and nitrogen fixation.
Types of Filtration
Primary Filter: Absorption of large particles (2-5 µm) by pinacocytes and transfer to amoebocytes.
Secondary Filter: Capture of smaller particles (<1.5 µm) by choanocytes, also transferred to amoebocytes.
Excretion and Gas Exchange
Developed systems for cellular diffusion rather than through specialized organs. Waste is expelled through exocytosis and gas exchange occurs at the cellular level due to constant proximity to water.
Nervous System and Locomotion
Limited movement ability with no true muscles or nervous system. Some sponges can contract myocytes, muscle-like cells, to control the osculum's opening.
Skeletal Support
Composition: Internal skeleton comprises spicules (calcium carbonate or silica) and fibers (spongin), secreted by specialized cells.
Reproduction
Sexual: Most sponges are hermaphrodites with external fertilization.
Asexual: Reproduction occurs through fragmentation, gemmules (in freshwater), and budding.
Developmental Stages
Sponges exhibit a larval stage that is free-swimming and responds to environmental stimuli. Upon settlement, they undergo metamorphosis to form adult structures.
Grades of Organization
Types:
Asconoid: Simplest form with a single atrium and osculum.
Syconoid: More complex, folds in the body wall create choanocyte chambers.
Leuconoid: Most complex structure, extensive branching of canals and multiple oscula.
Classification by Spicules
Classes:
Calcarea: Calcium carbonate spicules; can be asconoid, syconoid, or leuconoid.
Demospongiae: Mostly leuconoid with spongin and/or silica spicules; most diverse group.
Hexactinellida: Glass sponges with silica spicules that have six rays.
Ecology and Environmental Impact
Habitat preferences range from intertidal regions to deep-sea environments. Sponges play significant ecological roles and have varied growth forms leading to biodiversity.
Threats: Habitat destruction and sedimentation can affect sponge populations.
Chemical Defense Mechanisms
Sponges produce toxins as a defense against predators, and these compounds are being researched for pharmaceutical applications, including anti-inflammatory and anti-viral effects.