28.1 Phylum Porifera

Phylum Porifera

Sponges are the simplest animals, yet their filter-feeding design has kept them thriving in aquatic environments for over 500 million years. They lack true tissues and organs entirely. Instead, they rely on specialized cells and a water current system to feed, exchange gases, and remove waste. Understanding sponges gives you a baseline for comparing all other animal body plans.

Structure and Function of Sponges

Sponges are asymmetrical, sessile (permanently attached), aquatic invertebrates. Their body wall has three layers:

• Pinacoderm — the outer layer of flattened cells that covers the sponge's surface
• Choanoderm — the inner layer, composed of flagellated cells called choanocytes
• Mesohyl — a jelly-like matrix between the two layers that contains amoebocytes and skeletal elements (spicules or spongin fibers)

The water current system is central to how sponges survive. Water enters through small pores called ostia, flows into the interior cavity called the spongocoel, and exits through a single large opening called the osculum. Choanocytes drive this flow by beating their flagella, pulling water inward. This constant current delivers food and oxygen while carrying away waste.

Feeding and Digestion in Sponges

Sponges are filter feeders, meaning they extract tiny food particles (bacteria, algae, organic debris) from the water passing through their bodies. There's no gut, no mouth, and no digestive system. All digestion happens inside individual cells.

Here's how the process works:

1. Choanocytes beat their flagella, drawing water through the ostia.
2. Food particles in the water stick to the collar of the choanocytes and are engulfed by phagocytosis.
3. Choanocytes pass some nutrients to amoebocytes in the mesohyl.
4. Amoebocytes digest and transport nutrients to cells throughout the sponge body.

This is called intracellular digestion because digestion occurs entirely within cells rather than in a body cavity.

Cell Types and Roles in Sponges

Sponges compensate for their lack of tissues by having several specialized cell types:

• Pinacocytes — flat cells forming the outer surface (pinacoderm). They provide protection, similar to skin cells.
• Choanocytes (collar cells) — flagellated cells lining the inner chambers. They generate water currents and capture food particles via phagocytosis.
• Amoebocytes (archaeocytes) — mobile cells in the mesohyl with multiple roles:
  • Transport and digest nutrients
  • Produce skeletal elements (spicules or spongin)
  • Differentiate into other cell types as needed
• Porocytes — tubular cells that form the ostia, controlling water entry into the sponge.
• Sclerocytes — secrete spicules, which are hard skeletal elements made of silica (as in glass sponges) or calcium carbonate.
• Spongocytes — secrete spongin fibers, flexible protein fibers that form the skeleton in sponges like bath sponges.

The ability of amoebocytes to become other cell types is a key concept. Sponge cells exhibit totipotency, meaning any cell can potentially transform into any other cell type the sponge needs.

Reproduction and Development in Sponges

Sponges reproduce both sexually and asexually.

Asexual reproduction happens in two main ways:

• Budding — a small outgrowth develops on the parent sponge, eventually detaching as a new individual
• Fragmentation — a piece of the sponge breaks off and regenerates into a complete new sponge (this works because of totipotency)

Sexual reproduction follows these steps:

1. Specialized cells (often derived from choanocytes or amoebocytes) produce sperm and eggs. Most sponges are hermaphrodites but typically don't self-fertilize.
2. Sperm are released into the water and carried to another sponge, where they're captured by choanocytes and transferred to eggs internally. (Note: fertilization in sponges is usually internal, not external, despite the sperm traveling through the water column.)
3. A free-swimming larva develops and is released.
4. The larva eventually settles on a substrate and metamorphoses into a sessile adult sponge.

Body Plan and Organization

Sponges have no true tissues or organs, but they do show variation in how their water canal systems are arranged. The three body plans increase in complexity and efficiency:

• Asconoid — the simplest form. Water flows directly through the ostia into a single spongocoel lined with choanocytes, then out the osculum. Found only in small sponges because the surface-area-to-volume ratio limits efficiency.
• Syconoid — the body wall is folded, creating finger-like canals lined with choanocytes. This increases the surface area for feeding compared to the asconoid plan.
• Leuconoid — the most complex and most common plan. Multiple chambers lined with choanocytes are connected by a network of canals. This design supports much larger body sizes because it maximizes filtering efficiency.