Porifera

Life Cycle with Larval Stage

• Leads to reproductive adaptations:
  • Synchrony: gametes are produced and released at the same time, controlled by environmental signals
  • Proximity: gametes are released near each other
    • Hermaphroditism: common in low population levels or benthic adults
    • Internal fertilization: leads to modifications of the reproductive tract, lower number of gametes produced than external fertilization

Direct Development

• Common in freshwater animals due to hazards of current and turbidity

Phylum Porifera

Introduction

• Poriferans or sponges are the lowest among metazoa, with a cellular level of organization
• They form only tissues and never organs
• Ideal study of animals should start with the simplest, i.e., the porifera

History

• Debate about whether sponges are animals until the detection of water currents and contractile movements of the oscula in 1765
• Grouped with anthozoans (corals and sea anemones) until 1825, when morphology and physiology were adequately understood
• Separated and given the name Porifera in 1836, and placed in a separate branch of metazoans, the parazoa, in 1875

Definition

• Asymmetrical or radially symmetrical animals with a cellular grade of construction and no organs
• Sessile animals with a body permeated with pores, canals, and chambers through which water currents pass

General Morphology and Physiology

• Most sponges are brightly colored, with coloration suggested to protect from solar radiation and as warning coloration
• Shape: most are irregularly shaped with branching, encrusting, and massive growth forms, but a few have definite shapes like vase-like cylinders, globes, or peculiar branching patterns
• Symmetry: irregularly shaped sponges are asymmetrical, while vase-like or globular forms are radial
• Size: range from 1-2 mm in height to forms that may reach several meters in diameter

Histology

• Sponges are dipoblastic animals, with pinacoderm and mesohyl differing from the usual epidermis and mesenchyme of other metazoans
• Pinacoderm:
  • Pinacocytes: flattened, polygonal, uninucleate cells that are contractile and form the outermost layer
  • Endopinacocytes: pinacocytes that line internal cavities like the walls of incurrent canals and the spongocoel
  • Porocytes: tubular cells extending from the pinacoderm to the spongocoel, pierced by a central canal that acts as the incurrent pore
• Mesohyl:
  • Collencytes: with slender branching pseudopodia that may unite to form a network, secreting dispersed collagen fibers
  • Chromocytes: with pigment bodies
  • Thesocytes: with food reserves/vacuoles
  • Scleroblasts: secreting the skeletal components of the sponge, depending on the skeletal component
  • Archeocytes: with blunt pseudopodia, large nuclei, and a conspicuous nucleolus, undifferentiated and totipotent
  • Gland cells: attached to the surface of the sponge and secreting slime
  • Desmacytes/fiber cells: long, slender cells found in layers
  • Myocytes: fusiform, contractile cells usually arranged in a circular fashion to form a sphincter around the osculum or other opening

Choanoderm

• Choanocytes/collar cells: round or oval cells whose base rests on the mesohyl, with a flagellum and a transparent, contractile collar, responsible for moving water through the sponge and obtaining food

Skeleton

• Sponge skeleton may be spicules, spongin fibers, or basal encasements
• Spicules/sclerites: minute to microscopic bodies with a crystalline appearance, consisting of simple spines or spines radiating from a point
  • Chemical composition: possessing an axis of organic material around which are deposited inorganic substances
  • Size: divided into two general sizes: microscleres and megascleres
  • Types: monaxons, tetraxons, triaxons, polyaxons, and spheres