The notion that a jellyfish possesses eyes might seem unexpected, given their often-perceived simple biology. However, box jellyfish, belonging to the class Cubozoa, stand apart from other jellyfish due to their surprisingly complex visual capabilities. Unlike most jellyfish that merely sense light and dark, box jellyfish have developed a sophisticated visual system that allows them to interact with their environment in a more active manner.
The Remarkable Visual System of Box Jellyfish
Box jellyfish are distinct among cnidarians for their elaborate visual system, which includes a total of 24 eyes. These eyes are organized into four clusters, known as rhopalia, located on each side of their cube-shaped bell. Each rhopalium contains six eyes.
Eye Types and Structure
Within each rhopalium, there are two image-forming camera-type eyes: an upper lens eye and a lower lens eye. These eyes share structural similarities with vertebrate eyes, featuring a lens, retina, and cornea. Accompanying these are four simpler light-sensing structures: two pit eyes and two slit eyes. The pit and slit eyes detect light levels, while the lens eyes form images.
What Box Jellyfish Can See
The two camera-type eyes within each rhopalium provide box jellyfish with different visual information. The upper lens eye is consistently oriented to look upwards, allowing it to perceive objects above the water surface. This specialized upward gaze is crucial for detecting terrestrial cues such as mangrove canopies, even from distances of at least eight meters.
Field of View and Perception
Conversely, the lower lens eye is directed downwards, enabling the jellyfish to see objects beneath the water’s surface. This dual orientation provides a nearly 360-degree field of view of their surroundings. While their vision is not as sharp or detailed as human vision, they can perceive size, shape, and contrast. Research suggests that while some box jellyfish species may have a single opsin, indicating color-blindness, others might possess the ability to discern color, particularly in the blue-green spectrum.
How Vision Drives Box Jellyfish Behavior
The sophisticated visual system of box jellyfish is instrumental in driving their active lifestyle. Their ability to form images and detect objects allows them to engage in active hunting, pursuing small fish and crustaceans. This visual acuity helps them track potential meals and move purposefully towards them.
Navigation and Obstacle Avoidance
Vision plays a central role in their navigation and obstacle avoidance. Box jellyfish inhabit complex environments, such as mangrove swamps, where they must maneuver around roots and other submerged structures. The upper lens eyes, by continuously monitoring the overhead canopy, enable species like Tripedalia cystophora to remain within their preferred habitat near mangrove roots, where copepods, their primary food source, are abundant. Their visual capabilities also contribute to avoiding collisions and adjusting swimming speed and direction.