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Imagine you’re wading out into the ocean with a snorkeling mask on your face, the sun beating down on your back. As you slip under the Caribbean waters and open your eyes, eager to catch your first glimpse of tropical fish or colorful coral, you come face to face with a tiny, transparent mass of tentacles a few feet away. You’ve stumbled on a jellyfish, and while they’re beautiful, they can also be dangerous! You slowly wade backwards through the water, hoping that the jellyfish won’t spot you and make any aggressive moves in your direction.
Basic Jellyfish Anatomy
When talking about jellyfish anatomy, the conversation is actually quite brief. In comparison to the vast majority of animals that we are familiar with, jellyfish are incredibly simple. Jellyfish lack a brain, a heart or bones, as well as every other critical organ that we cherish as humans. Jellyfish are members of the Cnidaria phylum, and are not actually classified as fish. In fact, they are a type of plankton, and are related to the microscopic food source that forms the foundation of most marine food chains.
Not only are they very simple in their physical structure, they are also made up of roughly 98% water, and a short list of body parts. The epidermis protect the organs, the gastrodermis is the inner layer, and the mesoglea can be found between these two layers. Jellyfish also possess a mouth (orifice) that takes food in and excretes it out, as well as a gastrovascular cavity, a rudimentary stomach. The last element of their physical structure, and what most people associate with these cnidarians, is the tentacles. While these can vary widely in terms of length and number, they are arguably the most important body part of jellyfish. They are also the primary sensory organs for these basic creatures, including giving them the power of “sight”.
Do Jellyfish Have Eyes?
When we think of eyes, it is natural to picture them as structures similar to our own, but the variety of eye and sight-providing organs in nature is immense. There remains an intense debate over whether eyes developed once or multiple times in the evolutionary tree of life on Earth. The consensus is that some sort of light-detecting mechanism was present in an early ancestor of sighted animals, and the diversity of how that genetic programming developed remains a source of fascination for researchers.
In the case of jellyfish, which don’t possess a brain to process complicated visual stimuli, they must rely on a simplistic sensorial organ located in their tentacles. Yes, those stinging bits of a jellyfish that can leave lashes of red across your legs are also the only way that jellyfish can experience and move through the world. There are sensory organs at the ends of these tentacles that can detect light and different chemical traces (smells) in the water, while also helping to orient themselves spatially.
Unlike humans, the “eyes” of most jellyfish are not concentrated in a single organ; instead, the ability to see is facilitated by a network of nerves and proteins called opsins. Interestingly enough, not all jellyfish are created equal in terms of their vision capabilities. Some studies, in fact, have concluded that eyes developed numerous times in cnidarians, resulting in many different kinds of eyes, of varying complexity, in jellyfish. Box jellies, for example, have concentrated bundles of 6 eyes on each side of their “bell”. While the unifying lack of a brain in cnidarians makes the processing of visual information impossible, the varying levels of advancement in eyes is evidence of convergent evolution within different species of the same phylum.
A Final Word
While it may seem like jellyfish have eyes and are intentionally seeking out your shins out there in the ocean, they aren’t exactly adept hunters. Luck and the limited sensory information from rudimentary “eyes” and light-sensitive proteins are enough to help this vastly diverse and globally prolific phylum of marine life survive.