Sponges (phylum Porifera) are among the most primitive multicellular animals, with a fossil record stretching back hundreds of millions of years. These sessile organisms live anchored to the seabed in nearly all aquatic environments, filtering water to sustain themselves and playing a significant role in marine ecosystems. Lacking a nervous system, a brain, or even tissues and organs, the way a sponge perceives its surroundings poses a biological puzzle. Recognizing and reacting to environmental changes without the typical machinery of sensation reveals ancient, distributed methods of perception.
The Lack of Traditional Sensory Organs
Sponges fundamentally differ from most other animals because they lack true tissues, meaning they do not form complex organs like eyes, ears, or a centralized brain. The entire body is organized at the cellular level, with the various cell types functioning mostly independently or in loose coordination. The outer layer is composed of pinacocytes, which form a protective boundary, while the interior contains choanocytes (collar cells) that use flagella to create currents for filter feeding. The absence of a nervous system means sponges do not have specialized neurons to rapidly transmit electrical signals. Instead, the ability to sense and respond to stimuli is distributed among individual cells, resulting in a much slower process where responses often take seconds to hours to complete.
Detecting Dissolved Chemicals
Chemoreception is a primary mode of sensing for a sponge, governing its fundamental processes of feeding and reproduction. The water flowing through the sponge’s canal system carries molecular information that certain cells are equipped to detect. This allows the sponge to identify the presence of food particles, such as phytoplankton and bacteria, in the surrounding water. Specialized sensory cells detect specific molecular gradients; for instance, the amino acid L-glutamate triggers a coordinated whole-body contraction in some freshwater sponges. Chemical sensing is also essential for reproduction, used to detect gametes released by other sponges for internal fertilization. By detecting the chemical profile of the water, sponges can regulate the pumping rate of the choanocytes to optimize filtration or close the osculum to expel waste.
Awareness of Light and Physical Movement
Sponges are photosensitive, meaning they can detect and respond to light, even without possessing any form of eye or light-focusing structure. Their awareness of light is primarily limited to detecting changes in intensity rather than forming an image. Cells containing light-sensitive pigments, such as cryptochrome, act as rudimentary photoreceptors. In the larvae of some species, a ring of pigmented cells guides the organism in a process called phototaxis, helping them find a suitable place to settle. In adult sponges, light detection can influence the opening and closing of the osculum, regulating their exposure and potentially controlling the activity of photosynthetic symbionts.
Sponges also exhibit mechanoreception, the ability to sense physical forces like touch or water flow. Specialized non-motile cilia, often found lining the osculum, function as flow sensors. These cilia are oriented perpendicular to the water flow, making them highly sensitive to changes in pressure or current speed. The detection of physical stimuli, such as a strong current or the accumulation of unwanted sediment, triggers a protective response. This sensory input is thought to initiate a sudden, wave-like contraction of the body, sometimes called a “sneeze,” which helps the sponge expel accumulated debris and maintain filter-feeding efficiency.
How Sponges Respond to Sensory Input
The lack of a nervous system requires sponges to use alternative, slower mechanisms to transmit information and coordinate a response. The sensory signal detected by one cell must be passed to neighboring cells, often through chemical signaling pathways or direct physical contact. Calcium signaling is one proposed mechanism, where a wave of increased intracellular calcium concentration slowly propagates through the cell network. The ultimate physical response involves the action of contractile cells, particularly pinacocytes, which contain contractile modules. When stimulated, these cells slowly shorten, leading to a visible change in the sponge’s shape. This slow coordination is most evident in regulating water flow, where a stimulus causes the closing of the ostia or the osculum to temporarily stop filtering water and protect the organism.