Question

Explain how digestion occurs in sponges.

Short answer

Digestion in sponges occurs through a process involving the structure and organization of the sponge, the role of choanocytes, ingestion of food particles, intracellular digestion, and egestion of waste products. Sponges are filter feeders that draw in water through their pores and capture food particles using specialized flagellated cells called choanocytes. These cells ingest the food particles into food vacuoles, where intracellular digestion takes place, and the nutrients are absorbed and transported to other cells within the sponge. Waste products are then released by the choanocytes into the surrounding water and flushed out of the sponge through the osculum.

Step-by-step solution

1. Structure and organization of a sponge

Sponges (Phylum Porifera) are simple, sessile marine animals that lack true tissues and organs. They have a porous structure made up of a few cell types, primarily including choanocytes, which play a significant role in the digestive process. The body of a sponge consists of an outer dermal layer, an inner layer called the choanoderm, and a central cavity called a spongocoel. The pores or ostia on the surface lead to a system of canals, which are lined with choanocytes.

2. Role of choanocytes in digestion

Choanocytes, also known as collar cells, are specialized flagellated cells that line the inner canals and chambers of a sponge's body. They have a thin collar surrounding a single flagellum, which creates water currents when the flagella beat. Choanocytes are responsible for filtering food particles from the water and initiate the process of digestion.

3. Ingestion of food particles

Sponges are filter feeders, which means they obtain their nutrients by filtering water through their porous bodies. The beating of flagella from choanocytes creates a steady flow of water through the sponge's canal system, entering into the ostia and exiting through the osculum. As the water passes through the canals, tiny food particles such as bacteria, plankton, and organic matter are trapped by the collar of choanocytes and ingested into the cell.

4. Intracellular digestion process

Once the food particles are ingested, they are enclosed within a food vacuole inside the choanocyte. The intracellular digestion of food particles occurs within these food vacuoles, where enzymes are secreted to break down the nutrients. The digested nutrients are then absorbed by the choanocyte and transported to other cells within the sponge's body.

5. Egestion of waste products

Waste products from digestion, such as carbon dioxide and other excretory materials, are released by the choanocytes into the surrounding water. This process of eliminating waste products is called egestion. The water current generated by the choanocytes continuously flushes waste products out of the sponge through the osculum, maintaining a clean environment and providing a continuous supply of nutrients for the sponge.

Key concepts

Choanocytes

Choanocytes are fascinating cells that play a crucial role in sponge digestion. These specialized cells are sometimes referred to as collar cells due to their unique structure; each has a collar made of microvilli surrounding a central flagellum. The movement of the flagellum is essential as it generates water currents inside the sponge. This current is what brings water, and the food particles within it, into the sponge's body.

The choanocytes line the internal chambers and canals of the sponge and are responsible for capturing food particles from the water. As water flows over the collar, particles such as bacteria and plankton get trapped. This method of capturing food is extremely effective in nutrient-poor environments where sponges often reside.

Filter Feeding

Filter feeding is a clever and efficient way for sponges to obtain food. It involves straining food particles from the water and is central to how sponges live and grow. The process begins when choanocytes create a flow of water into the sponge's body through tiny pores known as ostia.

As the water moves through, it passes over the choanocytes where food particles are collected. This system allows sponges to continuously gather food without actively hunting. This passive feeding strategy suits their sessile lifestyle, as they are anchored in one spot in the ocean. Thanks to filter feeding, sponges can thrive in a wide range of marine environments.

Intracellular Digestion

Once the food particles are trapped by the choanocytes, they are ingested through a process called intracellular digestion. This means that the digestion happens inside the cells.

The food particles get engulfed in vacuoles within the choanocytes where digestive enzymes are then released. These enzymes break down food into simpler substances that can be absorbed and used by the sponge's cells.

Intracellular digestion is efficient for sponges since they lack a complex digestive system. This method helps in directly distributing nutrients to where they are needed most within the sponge's body.

Sponges

Sponges are ancient and simple creatures belonging to the phylum Porifera. They lack the complex structures found in many other animals, such as true tissues and organs, but are incredibly successful in marine environments. Their bodies are permeated by a porous system that allows water to flow through them, which is key to their survival.

Despite their simplicity, sponges come in a variety of shapes and sizes and can be found from shallow coastal waters to the deep sea. Their survival depends largely on the efficiency of their cellular processes, including digestion by choanocytes. This simplicity provides a unique perspective on the basic biological processes necessary for life.

Porifera

Porifera, commonly known as sponges, is one of the earliest branches in the animal kingdom. They are considered a foundational phylum because they represent one of the earliest forms of multicellular organization. This phylum consists of a vast diversity of species adapted to different aquatic environments.

Members of the Porifera are primarily marine, although there are a few freshwater species as well. They demonstrate a fascinating approach to survival, relying on the movement of water through their bodies to obtain food, oxygen, and remove waste. Studying Porifera provides insight into the evolution of biological systems and how simple organisms function in complex ecosystems.