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Jellyfish do not have brains. They don’t have hearts either, but jellyfish do have a very basic set of nerves at the base of their tentacles. These nerves detect touch, temperature, salinity, etc.
The Earth has no shortage of weird animals, and it shouldn’t surprise you that the vast majority of these bizarre earthlings are found underwater, considering how much of the deep blue remains unexplored. This article, however, is about one particular water-dwelling creature—the Jellyfish—and its essential organs, or lack thereof.
What is a Jellyfish?
A jellyfish, contrary to what its name suggests, is not actually a fish. These animals are a kind of zooplankton, which translated from Greek, meaning animal (zoon) wanderer (planktos). Zooplankton (and the photosynthetic phytoplankton) are a diverse group of organisms that float in water and are mostly microscopic, but some are large like our jellyfish.
Jellyfish belong to the same phylum as sea anemones and corals. These organisms on the more ancient branches of the evolutionary tree have a relatively simple anatomy. In fact, jellyfish are composed of about 95% water! A good way of visualizing this is: if a jellyfish washed up on the beach, after a while, due to evaporation, it would basically disappear! This simplicity has allowed their body plan to be relatively simple too!
Perhaps most notable, they don’t have a brain or a heart or a stomach.
Jellyfish don’t have a brain, but they do have a rudimentary nervous system!
While jellyfish don’t have a brain, they do have a very basic set of nerves or a nerve net that extends out radially through the jellyfish. These nerves serve as its sensory organs, detecting touch, temperature, salinity etc., and the jellyfish reflexively respond to these stimuli. Their neurons are interspersed in the different body layers of the jellyfish.
However, don’t let their lack of a brain fool you into thinking that jellyfish have a simple, straightforward nervous system. There is a rich diversity of nervous systems that could whet the appetite of even the most curious mind. These creatures have sense organs that detect light, salinity, gravity, touch and temperature.
They sense light via light-sensitive cells along the ridge of their bells called rhopalia. Nerves in the rhopalia are aggregates, unlike the diffused nerves in the rest of the jellyfish. Within the rhopalia are eye-spots that detect light.
Statocysts let the jellyfish balance
Next to the eyespots, within the rhopalia, lies the sensory machinery that allows jellyfish to maintain their balance. The statocysts, the balancing organ, is composed of a sand grain structure in a particular order within the organ. When the jellyfish moves, the minerals press against sensory neurons on the opposite side of the direction of motion, due to inertial resistance. Based on this input, the jellyfish will autocorrect its course through reflexive movements.
The rhopalia are spectacularly visible in typical jellyfish (Scuphozoa), like the Moon jellyfish, and in box jellyfish, such as the Tripedalia cystophora.
Some jellyfish have eyes!
Box jellyfish have a remarkable 24 eyes in their 4 rhopalia, 6 in each. The eyes aren’t identical; there are four different types. Two of these, the upper lens eye and the lower lens eye, are much like our own eyes, boasting a cornea, lens, pupil and retina!
This results in some interesting behaviors. Tripedalia cystophora can look up at the water’s surface to find their favorite habitat, mangrove trees. Some jellyfish are even known to court their mates, such as box jellyfish Carybdea sivickis. In such a situation, the male jellyfish grabs the female with his tentacles and mates with her.
All of this isn’t possible with just a simple diffused nerve net. Some jellyfish do have a central nervous system of a sort, a nerve ring, as well as the rhopalial nervous system. This nerve ring runs along the bell, connecting the different rhopalia and sensory neurons.
And while the layout is simple, it still holds many mysteries. For one, scientists still don’t know how jellyfish coordinate information between their different rhopalia. We frequently associate those deliberate behaviors with “brains”, yet the jellyfish perform these tasks—mating, seeking shelter and migrating. How they do it remains a great mystery.
Jellyfish circulate nutrients and gases through diffusion
Before answering that question, we should understand exactly what a heart does. It basically pumps blood around the body so that the cells of an animal can get oxygen and nutrients from the blood and expel soluble wastes, like carbon dioxide. However, what is special about jellyfish anatomy is the thickness of the outer layer, known as the Ectoderm. It is only a few cells thick, so oxygen simply diffuses into the jellyfish body.
Jellyfish also have a very rudimentary digestive system, where simple nutrients merely diffuse into the rest of the body. Neither of these processes of oxygen and nutrient diffusion require a powerful pump like the heart for them to permeate throughout the organism’s body, due to the simplicity of jellyfish anatomy.
Nature’s first draft of a nervous system might be simple, but it still works. Considered among the first to even have a nervous system, the cnidarian’s nerve nets reveal the evolutionary origins that led to later, more convoluted nervous systems, like our own!
Jellyfish are genuinely fascinating creatures that manage to survive without many of the organs that we’ve come to think of as vital for life. Their bodies are well-suited to survive the conditions of the endless blue, drifting across the vast oceans, displaying their beauty for all to see—at a safe distance!