Lampreys are ancient jawless fish, diverging from other vertebrate lineages over 500 million years ago. These aquatic creatures, often resembling eels, possess a distinctive evolutionary history reflected in their visual system. Studying lamprey eyes offers a unique window into the early development of vertebrate vision.
A Transformation in Vision
The visual capabilities of a lamprey undergo a transformation throughout its life cycle. As larvae, known as ammocoetes, these creatures spend several years burrowed in the sediments of freshwater streams, feeding by filtering organic matter. During this larval phase, their eyes are rudimentary eyespots, covered by non-transparent skin, rendering them blind for image formation.
The larval retina remains undifferentiated, with a small area near the optic nerve head showing developed photoreceptors and synaptic connections. This limited visual apparatus is sufficient for basic light detection, allowing the burrowing larvae to exhibit negative phototaxis, moving away from light. After three to seven years, the ammocoete undergoes metamorphosis, during which its body structure changes significantly.
This metamorphosis involves the development of the rudimentary eyes into functional camera-type eyes. The transformation equips the lamprey for its adult life, transitioning from a sedentary, sightless filter-feeder to a free-swimming, visually-guided predator or non-feeding adult. The newly developed image-forming eyes are used for navigation, locating prey or mates, and responding to environmental cues in open water.
Unique Structure of the Adult Eye
The adult lamprey possesses a camera-type eye, sharing similarities with the eyes of other vertebrates, including a cornea, a lens, a pigmented epithelium, and a layered retina. Despite these shared features, the lamprey eye exhibits primitive characteristics. For instance, the cornea, the transparent outer layer, is continuous with the skin of the head, rather than being separate.
Unlike the eyes of many vertebrates, the lamprey’s lens has a fixed focus and lacks the muscles used for accommodation (changing focus). Instead, the lamprey may adjust its focus by flattening its cornea, pushing the lens closer to the retina. The lamprey eye also lacks intrinsic eye muscles for movements. The retina contains various cell types, including photoreceptors, bipolar, horizontal, amacrine, and ganglion cells. Some species possess up to five distinct photoreceptor types, including both rod-like and cone-like cells.
The Third Eye and Light Perception
Beyond its pair of lateral, image-forming eyes, the lamprey also possesses a unique light-sensing organ known as the pineal eye, often referred to as a “third eye.” Situated on the top of the lamprey’s head, this photoreceptor is not capable of forming images but is sensitive to changes in light intensity. Its primary function is detecting light and dark cycles in the environment.
The pineal eye plays a role in regulating the lamprey’s circadian rhythms, influencing daily activity and behavior. It also controls seasonal behaviors, such as migration to spawning grounds and the timing of reproduction. The pineal organ synthesizes melatonin, a hormone regulated by light, which helps synchronize biological processes with environmental light conditions. The combined input from the image-forming lateral eyes and the light-sensitive pineal eye allows the adult lamprey to navigate its surroundings, locate hosts or mates, and align its physiological and behavioral rhythms with light-dark cycles.