Sponges (Phylum Porifera) are among the most ancient multicellular animals, defined by their porous structure. While they resemble stationary underwater growths, the common perception that sponges are entirely immobile is nuanced. Their life cycle and internal physiology involve distinct forms of movement. Understanding sponge movement requires looking beyond the adult form to the microscopic and larval stages.
The Sessile Nature of Adult Sponges
The majority of adult sponges are classified as sessile, meaning they remain fixed in one place for their entire lives. They attach permanently to a substrate, such as rocks, coral, or shells, using specialized cells at their base called basipinacocytes. This attachment eliminates the possibility of whole-organism locomotion across the seafloor.
The structural rigidity that prevents movement comes from their internal skeleton, which is made up of microscopic, needle-like structures called spicules, or a protein network known as spongin. This skeletal framework provides support and helps the organism maintain its shape. Their entire lifestyle revolves around filter-feeding, where water is drawn in through tiny pores called ostia and expelled through a larger opening, the osculum. This constant flow brings in food particles, removing the need for the sponge to move to find nourishment.
Movement During the Motile Larval Stage
Although the adult is stationary, a significant exception is found in the larval stage. After sexual reproduction, the sponge embryo develops into a tiny, free-swimming larva, which is the organism’s primary means of dispersal. This motile stage is crucial for the species to colonize new habitats and avoid competition with the parent sponge.
The two main types of sponge larvae are the parenchymula and the amphiblastula, both of which are non-feeding. The parenchymula larva is often covered entirely in hair-like structures called cilia, which beat rhythmically to propel the larva through the water. The amphiblastula larva, common in calcified sponges, has distinct regions of ciliated and non-ciliated cells, with the ciliated half driving movement.
These larvae drift or swim for a limited time, usually a few days, before actively searching for an appropriate substrate on which to settle. Once a suitable location is found, the larva attaches and undergoes a dramatic metamorphosis, transforming into the sessile adult form. This free-swimming larval stage ensures genetic exchange and the wide distribution of the species.
Internal Cell Dynamics and Body Contraction
Movement in the adult sponge exists on a localized and microscopic scale, involving the action of individual cells and the entire body’s subtle contractions. Within the sponge’s middle layer, the mesohyl, specialized cells known as archaeocytes and amoebocytes are constantly migrating. These cells use amoeba-like movement to transport nutrients, distribute oxygen, and participate in tissue repair and regeneration.
The entire sponge body is also capable of slow, coordinated contractions that are observable over time. These contractions are not used for locomotion but rather for physiological maintenance. For example, sponges can contract their whole body or close their oscula and ostia in response to high sediment loads or other environmental stimuli. These contractions, which can take place over a period of minutes or hours, help to purge the water canal system of accumulated waste or debris. The ability to regulate the size of the openings and the overall body volume demonstrates a form of action that, while not moving the organism from one location to another, is a dynamic and controlled internal movement.