Biological Sensory Systems: Photoreception and Electroreception

Categories of Sensory Stimuli

Organisms interact with their environment through three primary types of stimulus detection. Chemical stimuli involve organisms detecting and responding to chemical signals that bind specifically to receptors located on sensory cells. Mechanical stimuli occur when organisms detect and respond to the physical deformation or shape change of sensory cells. Electromagnetic stimuli involve the detection of and response to changes in electromagnetic waves; this category specifically includes photoreception, which is the sensing of light, and electroreception, which is the sensing of electrical charges.

Photoreception in Fungi and Plants

In fungi, light signaling is a critical regulatory mechanism that affects various stages of fungal growth. It regulates growth and development, reproduction (including both sexual and asexual modes), pigment production, and metabolic production. Photoreception in these organisms allows them to respond to light hitting their sensory cells to coordinate these complex biological processes.

Plants utilize photoreception primarily through a process known as phototropism, which is the growth of plants in response to a light stimulus. This process is mediated by specialized proteins called phototropins that sense light. When light is detected, the plant hormone auxin is relocated to the "shady side" of the plant, away from the light source. High concentrations of auxin lead to cell expansion by making the cell wall acidic and breaking the bonds between cellulose fibers. Consequently, the cells on the shady side expand more than those on the lit side, causing the plant to bend toward the light source.

Photoreception Across Animal Lineages

Animals use photoreceptor cells containing light-sensitive proteins called opsins. Even animals without complex eyes, such as corals and anemones, possess opsins to help regulate their circadian rhythms, which are approximately $24\,hour$ cycles. This regulation is vital for spawning and for maintaining photosynthetic symbionts. Sponges similarly lack eyes, but certain sponge larvae utilize a blue-light sensitive protein known as cryptochrome to sense their environment.

Simple eyes, often described as cups of light/dark photoreceptors situated under a layer of pigment cells, are found in various invertebrates. Cnidarians, which exhibit radial symmetry, often possess rhopalia. Most jellyfish have $8$ of these sensory organs arranged around the bell, which include simple eyes. Many invertebrates with bilateral symmetry feature simple eye spots at the anterior end as a component of cephalization.

Complex Image-Forming Eyes and Plant Mimicry

Image-forming eyes use a lens to focus light and have evolved multiple times independently across different lineages. Arthropods possess compound eyes made of multiple units called ommatidia. Each ommatidium has a narrow visual range, and the collection of these units results in a low-resolution, pixelated image. These structures involve a retina with photoreceptors, nerve cells, and an optic nerve composed of nerve cell axons.

Molluscan eye morphology and complexity are closely tied to their specific lineage. In mollusks, the neuron bodies generally lie underneath the light-sensitive layer. In contrast, vertebrate eyes are analogous to cephalopod eyes but feature a different structural arrangement where neuron bodies lie on top of the light-sensitive layer. This orientation results in a biological "blind spot" or a region with no receptors where the optic nerve exits the eye.

Whether plants can "see" is an active area of observation, specifically regarding phenotypic plasticity in Boquila trifoliolata, a woody rainforest vine. This plant's leaves can mimic the leaves of surrounding host plants. Proposed mechanisms for this behavior include chemical cues from hosts triggering mimicry, horizontal gene transfer, or the presence of plant photoreceptors capable of responding to shapes of light and dark patterns. To test this, researchers grew B. trifoliolata with plastic plants to see if the leaves would respond to non-biological shapes. The experiment compared control leaves with nothing to mimic against experimental leaves grown next to a plastic vine to determine the extent of hormonal communication in response to light.

Electroreception in Fungi and Aerial Environments

Electroreception is defined as the ability of organisms to detect and respond to changes in voltage. In the context of soil interactions, sometimes referred to as the "Wood Wide Web," some fungi show changes in membrane potential when exposed to plant root chemicals. Fungi utilize chemoreception, and action potentials are part of their physiological response. Electrical currents and signals have been measured at the interface where fungi interact with plant roots, raising the open question of whether fungi possess a form of electroreception.

Research is also exploring aerial electroreception, a relatively new field. One research question asks if flowers can detect changes in charge from approaching pollinators. It has been observed that flower mites are attracted to electric charges, which may help them detect and hitch a ride on hummingbirds visiting the flowers. While electroreception is well-studied in aquatic organisms, less is known about how electricity is sensed in the air.

Passive and Active Electroreception in Vertebrates

Passive electroreception is the ability to detect external electric fields, such as those generated by other living organisms. This is likely an ancestral trait for vertebrates that was lost when they moved to land. Aquatic vertebrates use specialized structures called Ampullae of Lorenzini to scan for weak electrical signals. These structures consist of a skin surface opening to a gel-filled canal. Sensory cells detect the difference in charge between the top and bottom of the canal, and axons transmit this information as action potentials (AP) to the Central Nervous System (CNS).

Active electroreception involves the ability to produce electric fields via an electric organ and detect resulting voltage changes. This ability evolved independently in multiple lineages. The electric organ is typically constructed from modified muscle tissue. The uses for active electroreception include electrolocation (interpreting the environment based on electrical properties), electrocommunication (recognizing conspecifics or maintaining group cohesion), and the location or stunning of prey.

Secondary Evolution of Electroreception in Mammals

Passive electroreception has secondarily evolved in two distinct groups of mammals, meaning it was lost in their ancestors and then re-developed using modified structures from different systems. Monotremes, such as the platypus and echidna, use free nerve endings associated with mucus glands in the snout for prey detection. Dolphins, specifically observed in the Guiana dolphin, have pits on their snout modified from ancestral whiskers that serve the same purpose of detecting prey through electrical signals.