Lesson: Cnidaria

Karben Jelly Fish 6

Cnidaria?  Ok, so maybe I should’ve called this post “Jellies” but I figured I’d be formal and give these majestic creatures the respect they deserve.  Most of the common jelly fish you know and love belong to the phylum Cnidaria.  Jelly fish are amazing organisms with a seemingly simple life of floating through the ocean being dragged through currents.  My job is to show you how complex these “brainless” creatures truly are.  So let’s do this!

Karben Jellyfish

Jelly fish or Cnidaria are comprised of three cell layers:  the epidermis, mesoglea, and gastrodermis.  The gastrodermis is a highly permeable membrane which takes in nutrients and oxygen while waste is eliminated.  This layer is comprised of muscular nutritive cells and glandular cells.  While they absorb nutrients taken in through the mouth, they also receive and release reproductive cells.  Located between the epidermis and gastrodermis, the mesoglea is a jelly-like layer which comprises the largest volume of the jelly fish.  While this gelatinous layer is mostly water and structural proteins, it does contain muscle and nerve cells which help form a type of internal “skeletal structure” for cnidaria.  Along the outer membrane of the jelly fish, is the epidermis, a sensory, contractile layer which allows for movement and sensation.  These sensory or nerve cells form the “nerve net” which is thought to be the most primitive nervous system in a multicellular organism.

Karben Jelly Fish 2


This “nerve net” is comprised of multiple types of nerve cells which are responsible for balance in space (in other words what direction is up or down) called statocysts, light receptors known as ocelli, and many other nerve cells which can help distinguish the presence of nearby food sources, water composition, and animals in the near vicinity.  While we think jellies simply float around, aimlessly spinning through the water in random directions, their simple yet complex nerve net allows them to swiftly navigate through the waters and feed on prey.  Some jellies have complex eyes, complete with corneas, retinas, and lenses within a structure known as rhopalia however marine biologists still do not know how they interpret this complex visualized information given their primitive and simple nervous “system.”


The jelly fish tend to live in two major forms: polyps which are bottom dwelling and medusae, which are the jellies you know as the undulating bells with dangling tentacles.  As a medusa, jellies use nematocysts which are basically microscopic tethered harpoons kept under high pressure distributed along each tentacle.  When in contact with another organism, the nematocysts rupture like a jar-lid blowing open and the harpoon launches outward piercing the organism and releasing toxins which can sometimes be fatal to humans.  Many species of cnidarians hitchhike on the backs animals as their nematocysts tether the jelly to the animal with which it came in contact.

Karben Jelly Fish 3


Another defense mechanism used by jellies is that of camouflage and bioluminescence.  While the reason for this is still unknown, one theory exists that jellies use bioluminescence as a means to attract larger animals so that their predators become prey themselves.  Another theory suggests that when a predator comes into contact with a bioluminescent jelly, the flash of bioluminescence may frighten the predator.

Karben Jelly Fish 4

Another defense mechanism is that of light perception and camouflage.  Many jellies are clear to remain invisible in the dark ocean depths.  However, those jellies that live in the deep ocean where light is scarce, tend to produce red pigments which make them virtually invisible to other organisms.  Red light is absent in the deep ocean therefore many jellies will produce red pigment to ensure they do not refract any visible light.  Also, they will have deeply red-pigmented digestive linings to hide any bioluminescent prey from potential jelly fish predators.

Karben Jelly Fish 5

As you can see, jelly fish or cnidarians, are simple yet complex creatures with an evolutionary advantage over more complex multicellular organisms.  Their graceful behavior yet toxic abilities make them interesting animals who have survived millions of years of evolution.  Next time you are at the aquarium take note of their anatomy and why they look the way they do.

As always, enjoy!

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