Introduction to Invertebrates: Sponges

Learning Objectives

  • Describe the organizational features of the simplest multicellular organisms.

  • Explain the various body forms and bodily functions of sponges.

Invertebrates Overview

  • Invertebrates account for approximately 97% of all animal species.

  • The simplest invertebrates are sponges (Phylum Porifera).

    • About 5,000 described species with likely many more undiscovered.

  • Unlike other animals, sponges do not possess true tissues and are not embryologically homologous to them.

    • They do not form a true gastrula during development.

    • Instead, specific cell types function analogously to tissues.

Sponge Morphology

  • Sponges have an irregular cylindrical shape with a central cavity known as the spongocoel.

  • Water flows into the sponge through openings called Ostia and exits through the osculum.

  • Sponges can be categorized as simple or complex, depending on the presence of multiple chambers.

Functional Tissues of Sponges

  • Pinacocytes: These are the epidermal cells that cover the body surface.

  • Porocytes: These cells form the ostia (pores) through which water enters.

  • Mesohyl: A jelly-like substance that contains various cell types:

    • Amoebocytes: Stem cells responsible for nutrient delivery and diversity of functions, including producing eggs.

    • Sclerocytes: Cells that create skeletal support structures called spicules.

  • Choanocytes: Collar cells with flagella that help with water flow and can differentiate into sperm for reproduction.

Sponge Phylogenetics

  • Sponges are classified based on their types of spicules into four classes:

    • Calcarea: Characterized by CaCO3 spicules.

    • Hexactinellida: Glass sponges with silicon-based spicules.

    • Demospongiae: Composed mainly of spongin; may lack true spicules.

    • Sclerospongiae: Have multilayered CaCO3 skeletons.

Physiological Processes of Sponges

Digestion

  • Food is trapped by choanocytes and digested via phagocytosis.

  • Pinacocytes can handle larger particles, along with amoebocytes assisting in digestion.

Gas Exchange, Circulation, and Excretion

  • These processes occur primarily through diffusion due to the simple body structure of sponges.

Reproduction

  • Sponges can reproduce both sexually and asexually:

    • Asexual Reproduction:

    • Budding: A new individual forms as an outgrowth from the parent and detaches when mature.

    • Fragmentation: A piece of the sponge breaks off and settles on a new substrate to grow.

    • Gemmules: Special structures formed in some freshwater sponges that can survive adverse conditions.

    • Sexual Reproduction: Sponges are generally monecious (having both male and female reproductive organs), with eggs produced by amoebocytes and sperm from choanocytes.