Location: Santa Barbara, California
Oceanic Region: Pacific Ocean

Box Jellyfish May Be Able to See

Figure 1 The cubozoan jellyfish Carybdea marsupialis. Rhopalia (arrows) bearing simple and complex eyes hang from each quadrant of the bell.

Cubozoans, often called the "killer box jellyfish," are named for their box-shaped bells. They are found in the Caribbean, the Indo-Pacific, and just recently along the southern Florida coastline. These jellyfish (Figure 1) are armed with four tentacles that dangle from the corners of the bell. Each tentacle, which may be up to 3 feet long, is packed with a powerful venom that can inflict painful stings to humans. A box jellyfish sting leaves an itching, burning welt that can last as long as 3 weeks. The most dangerous of the box jellies, the sea wasp, is found in the Indo-Pacific. Its sting kills more people each year than do shark attacks. The power of the cubozoan sting is often compounded because box jellies are known to swarm.

Cubozoans, members of the phylum Cnidaria, are gelatinous organisms composed of two body layers. They are capable of limited locomotion and are of interest to researchers because they possess complex eyes and no obvious brain or central processing structures. Their nervous system is a loose assembly of interconnected neurons arranged in a nerve-net pattern over the surface of the animal.

Swarming cubozoan jellyfish with complex eyes appear seasonally along the California coastline near Santa Barbara during late summer and fall into January. The jellyfish, Carybdea marsupialis, has a clear bell, except for some small brownish flecks, giving it a slightly peppered appearance (Figure 1). Bell size ranges from 2-4 centimeters in height, with extended tentacles close to 150 centimeters long. Carybdea is found in the shallow waters in the nearshore part of the kelp beds in areas known as sand channels. They are very patchy, however. When found, they are usually in densities of 1-2 over a 9-meter square. Densities of 30-50 per cubic meter have been reported in recent years. The jellyfish seem to stay in the sunny areas between the kelp beds over the clean sand. When they venture into the shadows of the kelp they reverse course and head back to the sunny areas. Shading them with a hand also results in their changing direction. They are positively phototactic and are captured easily using night lights. Their average water depth is 4 meters, and they are found within a meter of the bottom trailing their tentacles just above the sand. Here they feed on swarms of mysids and fish hatchlings.

Carybdea marsupialis has a sophisticated visual system. This cubozoan has four sensory structures, called rhopalia, one on each quadrant of the bell (Figure 1). Each rhopalium is a club-shaped structure suspended by a stalk from the bell; the rhopalium can twist and swing back and forth, resembling a pendulum of a clock. Each rhopalium has six eyes-four simple eyes and two complex eyes. The simple eyes, called ocelli, are capable of light and dark detection. The complex eyes, one small and one large, have a cornea, lens, and retina of ciliated photoreceptors. The large eye is directed laterally in toward the center of the bell, and the small complex eye is directed upward toward the apex of the bell.

Figure 2
A. Rhodopsin in the complex eye of the cubozoan jellyfish. B. Green opsin in the complex eye of the cubozoan jellyfish. C. Absence of red opsin in the complex eye of the cubozoan jellyfish. D. Absence of ultraviolet opsin in the tips of the photoreceptors of the complex eyes of the cubozoan jellyfish.

Figure 3 Ultraviolet opsin in the middle regions of certain photoreceptors in the complex eye of the cubozoan jellyfish.

The ability to see images with these eyes has been postulated. Electroretinograms suggest that the sensory cells of the retina have a photoreceptor function. Recent studies in my laboratory indicate that the ciliated photoreceptor cells of the jellyfish retina resemble the rod cells of vertebrate eyes. Further, the visual pigments required for phototransduction, rhodopsin, blue opsin, green opsin, and ultraviolet opsin, have been reported in the jellyfish retina (Figures 2 and 3). Red opsins, pigments that detect red light, are absent. Finally, a novel family of crystalline polypeptides makes up the spherical lens of the cubozoan jellyfish eye. The presence of a spherical lens improves resolution and illuminance by focusing light from a given direction to a single small region of the retina. The composition of the lens is heterogeneous. Such a composition is typical of graded index lenses, so named because the gradients in crystalline protein density create gradients in the refractive index of the lens. The refraction gradient focuses the light to one focal depth. Spherical graded lenses work extremely well in all directions and are relatively free of spherical aberrations.

The work on the cubozoan visual system suggests that jellyfish have the machinery to see images; these jellyfish may have developed unique image-forming eyes. In the absence of a brain, they may also have developed a unique method of processing the information. The eyes do connect into the neural network of the jellyfish, and studies suggest there are several mini-conglomerates of nerves that may function as small processing centers for vision.

What might these eyes be doing in the jellyfish? If they perceive images, they may be involved in feeding and/or reproductive behaviors. Certain cubozoans are know to chase small fish and seize them with their tentacles. Further, many cubozoans exhibit complex sexual behaviors in which the males chase the females, grasp them with a tentacle, and subsequently inject packets of sperm into them. Vision may be important to the jellyfish for such complex behaviors.

Jellyfish are over 600 million years old. The basic eye pattern of the cubozoan complex eye is similar to the vertebrate eye. Numerous questions arise as to the nature of the jellyfish eye. How have organisms without a brain engineered their photoreceptors and optical systems? What role does vision play in the ecology and behavior of cubozoans? Are their complex eyes able to form images of their surroundings? What roles might vision play in feeding and reproductive behaviors? Finally, human encounters with cubozoans are becoming more frequent. Box jellies, with their swarming ability and potent venoms, can deliver a powerful, sometimes deadly, sting to swimmers. A careful examination of the cubozoan eye and cubozoan behavior may well indicate that such swarming is correlated with vision.

Key Principles

1. Ecology
2. Vision
3. Human interactions with jellyfish
4. Behavior

Author

Vicki J. Martin
Department of Biology
Appalachian State University
Boone, North Carolina 28608
martinvj@appstate.edu