Sponges have holes because their entire survival depends on moving water through their bodies. Unlike almost every other animal, sponges have no mouth, no stomach, no lungs, and no heart. Instead, they use a system of pores and internal channels to pull in seawater, extract food and oxygen from it, and flush out waste. Every hole in a sponge’s body is part of this system. A single sponge weighing one kilogram can pump up to 24,000 liters of water through itself in a single day.
Two Types of Holes, Two Different Jobs
The small holes covering a sponge’s outer surface are called ostia. These are the entry points where seawater flows in. Some sponges form these tiny pores using specialized tube-shaped cells that act as valves, opening and closing to regulate how much water enters the body. The number of ostia on a sponge can range from hundreds to millions depending on the species and body size.
At the top of the sponge sits a much larger opening called the osculum. This is the exit. Water that has been filtered inside the sponge gets expelled through this opening in a steady stream. If a sponge is a colony of multiple units fused together, it may have several oscula, but the principle is the same: small holes in, big hole out.
How Sponges Move Water Without Muscles
The engine behind this water flow is a type of cell called a choanocyte, or collar cell. Choanocytes line the interior chambers of the sponge, and each one has a tiny whip-like structure called a flagellum. These flagella beat in coordinated waves, creating a current that draws water in through the ostia and pushes it toward the osculum.
The mechanics are surprisingly sophisticated. Choanocytes are arranged in spherical chambers, with their flagella all pointing inward. At first glance, this setup seems like it shouldn’t work, since some flagella inevitably beat against the direction of flow. But research published in the Proceedings of the National Academy of Sciences found that those “opposing” flagella actually serve a purpose: they raise the pressure inside the chamber, which increases the overall flow rate and pumping efficiency. Specialized cone-shaped cells at the chamber exits also prevent water from flowing backward, keeping the current moving in one direction.
Feeding Through Filtration
The water flowing through a sponge carries its food. Sponges are filter feeders that capture bacteria, microscopic algae, and other tiny particles suspended in seawater. As water passes through the choanocyte chambers, each collar cell is surrounded by a fine mesh of finger-like projections. Particles that hit this mesh get trapped and absorbed directly into the cell, where they’re digested internally.
This system is remarkably efficient. Studies on certain sponge species show they capture small algae and high-activity bacteria with near 100% efficiency. Even harder-to-catch particles like low-activity bacteria are retained at roughly 60% efficiency. For an animal with no digestive system whatsoever, that’s an impressive rate of food capture.
Breathing and Waste Removal
The holes in a sponge don’t just serve feeding. They’re also how the animal breathes and eliminates waste. Sponges have no respiratory organs and no circulatory system. Instead, every cell in the sponge’s body absorbs oxygen directly from the water passing by and releases carbon dioxide back into it through simple diffusion. Metabolic waste, primarily nitrogen-containing compounds from protein breakdown, gets dumped into the water the same way. The constant flow of fresh seawater through the sponge’s pores makes this possible. Without the holes and the current they create, the sponge’s cells would quickly suffocate in stagnant water.
Reproduction Uses the Same Plumbing
Sponges also rely on their hole system to reproduce. When a sponge releases sperm cells, they exit through the osculum and drift through the ocean. If those sperm reach another sponge, they get pulled in through the ostia along with the regular water flow. Once inside, the sperm cells encounter egg cells, and fertilization happens internally. The sponge essentially hijacks its own feeding current to bring in genetic material from other individuals, no specialized reproductive organs needed.
Not All Sponges Are Built the Same
Sponges come in three basic body plans, each with a progressively more complex system of internal holes and channels.
- Asconoid sponges are the simplest. They’re shaped like a hollow tube with pores in the walls. Water enters through the pores, flows into the central cavity, and exits through a single osculum at the top. These sponges tend to be small because the simple design limits how much water they can process.
- Syconoid sponges have thicker walls with longer, canal-like pores. The collar cells line these canals rather than the central cavity, giving the sponge more surface area for filtering. This allows syconoid sponges to grow somewhat larger.
- Leuconoid sponges are the most complex and the most common. Their bodies contain an elaborate network of chambers and branching channels, each lined with collar cells. This design maximizes filtering surface area and allows leuconoid sponges to grow to large sizes, sometimes several feet across.
The progression from asconoid to leuconoid is essentially a story of adding more holes and more internal complexity. A simple tube works for a tiny sponge, but scaling up requires a branching network of channels to maintain enough water flow to keep every cell fed and oxygenated. The larger and more complex the sponge, the more elaborate its internal hole system becomes.
Why This Design Works So Well
Sponges are among the oldest animal groups on Earth, with fossils dating back over 500 million years. Their hole-based body plan has persisted because it solves multiple survival problems with a single elegant system. One continuous flow of water handles feeding, gas exchange, waste removal, and reproduction all at once. There’s no need for separate organs dedicated to each function. The trade-off is that sponges are stuck in place, entirely dependent on the water around them. But in nutrient-rich marine environments, that’s a trade-off that has paid off for hundreds of millions of years.