6.1 Diversity and biology of marine sponges

Sponges are fascinating marine creatures with unique anatomy and physiology. These filter-feeding organisms lack true tissues but have specialized cells that perform vital functions. Their body structure includes a gelatinous matrix, outer and inner cell layers, and a skeleton of spicules or spongin fibers.

Sponges play crucial ecological roles in marine ecosystems. They provide habitat for other organisms, filter large volumes of water, and contribute to nutrient cycling. Some species even help maintain reef balance through bioerosion. Sponges also produce valuable secondary metabolites with potential pharmaceutical applications.

Sponge Anatomy and Physiology

Anatomy and physiology of marine sponges

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• Asymmetrical, sessile, multicellular organisms that lack true tissues and organs but are composed of specialized cells with specific functions
• Body structure consists of:
  • Mesohyl gelatinous matrix that fills the space between the external and internal cell layers
  • Pinacoderm outer cell layer made up of flattened pinacocyte cells that cover the exterior surface
  • Choanoderm inner cell layer composed of flagellated choanocyte (collar) cells lining the interior canals and chambers that beat flagella to create water currents for filter feeding
• Skeleton composed of spicules (siliceous or calcareous) that provide structural support and defense against predators and/or spongin flexible protein fibers that form the sponge skeleton
• Canal system includes:
  • Ostia tiny pores on the surface that allow water to enter the sponge
  • Incurrent canals that transport water from the ostia to the choanocyte chambers
  • Choanocyte chambers spaces lined with choanocytes that filter food particles from the water
  • Excurrent canals that transport filtered water out of the sponge
  • Oscula large openings where filtered water exits the sponge

Feeding and reproduction of sponges

• Filter feeders that obtain food by filtering water through their bodies using choanocytes to create water currents and trap food particles like bacteria, phytoplankton, and dissolved organic matter which are then digested by pinacocytes and amoebocytes and nutrients distributed
• Reproduce asexually through:
  1. Budding where new individuals grow from the parent sponge and detach
  2. Gemmules resistant structures containing totipotent cells that survive unfavorable conditions and develop into new sponges
  3. Regeneration where sponges can regrow from small fragments
• Reproduce sexually as hermaphroditic organisms that produce both sperm and eggs
  • Oviparous eggs are fertilized internally and larvae are released into the water column
  • Viviparous eggs are fertilized and develop into larvae within the parent sponge
  • Larval stage planktonic larvae disperse before settling and metamorphosing into adult sponges

Ecological roles of marine sponges

• Provide habitat and shelter for various organisms like crustaceans, worms, and echinoderms and contribute to the formation of coral reefs and other benthic communities
• Filter large volumes of water, removing particulate matter and dissolved organic carbon, playing a crucial role in coupling benthic and pelagic environments and contributing to nutrient cycling by converting suspended particles into food for other organisms
• Some species bore into coral skeletons and calcareous substrates through bioerosion, helping maintain the balance between reef growth and erosion
• Host diverse microbial communities within their tissues in symbiotic relationships that contribute to sponge metabolism and chemical defense
• Produce a wide array of secondary metabolites with biotechnological potential for pharmaceutical and biomedical applications with antiviral, antibacterial, antifungal, and anticancer properties

Classes of marine sponges

• Calcarea (calcareous sponges) have a skeleton composed of calcium carbonate spicules, are exclusively marine, have simple leuconoid or syconoid canal systems (Sycon, Leucosolenia)
• Hexactinellida (glass sponges) have a skeleton composed of siliceous spicules with a triaxonic (six-rayed) symmetry, are mainly deep-sea inhabitants, have syncytial tissue organization (Euplectella/Venus' flower basket, Aphrocallistes)
• Demospongiae (demosponges) are the largest and most diverse class of sponges with a skeleton composed of siliceous spicules and/or spongin fibers, have a leuconoid canal system, and diverse morphologies and adaptations (Aplysina/Yellow tube sponge, Cliona/Boring sponge, Xestospongia/Giant barrel sponge)
• Homoscleromorpha have a skeleton composed of siliceous spicules with a tetraxonic (four-rayed) symmetry, a simple canal system, and cellular organization similar to other metazoans (Oscarella, Plakina)