The defining cnidarian feature is, of course, their possession of stinging cells, or cnidocytes . Characteristic of the epidermis, they are also sometimes found in the gastrodermis. Cnidocytes contain an explosive organelle, the cnidae. Upon proper stimulation, cnidae invert and release a slender, often barbed and toxic thread in the direction of prey or predator Three overall types of cnidae are found in cnidarians, nematocysts (in nematocytes), spirocysts (in spirocytes), and ptychocysts (in ptychocytes). All toxic cnidae are nematocysts whereas spirocysts are sticky, and the everted tubules of ptychocysts are used for constructing feltlike tubes. Most cnidae are nematocysts and these are present in all three major Cnidarian clades. Spirocysts and ptychocysts are found only in Anthozoa.
To observe cnidae (usually nematocysts) we steal a tentacle from one of our anemones and add pressure by placing it on a slide with a cover slip. Note in the movie below, the tentacle swims around and around because discharged nematocysts are holding the tentacle in a circle configuration. The pictures are of the nematocysts under the microscope. I think the students did as good a job as the film you have already observed on cnidocyte discharge.
There are three major subgroups of Cnidaria, with Anthozoans being the oldest and Hydrozoans the yongest, most diverse clade. Scyphozoans are the true jelly fish, with large medusae stages. The three clades are distinguished by life cycle differences and students generally would examine as many of these differences as they can in this lab.
Students work with Aiptasia pallida, or related species, has long tentacles and demonstrates partial retraction. Because they reproduce by budding, a few well-fed individuals will soon multiply to hundreds and cover the rocks like soft brown fuzz.
This anemone, contains symbiotic dinoflagellates or unicellular algae. It also is a voracious predator. Almost all Cnidarians are predatory and even coral which belongs to this group will take prey at night. Aptasia will lose its symbionts if stressed or keep in the dark. Since coral will do the same if stressed, scientists can use Aptasia as a model organism for studying coral bleaching. The life cycle involves only polyp, and in most species, males and females will simply release sperm and eggs into the water column. Fertilized eggs develop into planulae larvae that are pelagic until finding a suitable substrate. The larvae give rise to a polyp that can give rise, as in the case of coral, to a colony of polyps.
Although containing symbionts, Aiptasia has not given up any of its predatory habit. They will readily take any prey that happens to contact their tentacles as the anemone in the film below. Interestingly they also will lose their symbionts as true coral when stressed. Some years we have done experiments in 402 lab exploring the relationship between different stresses and symbiont loss using Aiptasia. Aiptasia survives after losing its symbionts, coral does not.
We usually do not get representatives of the true corals but closely related to them are the gorgonians.
Gorgonians are also known as octocorals, so named because the individual polyps have eight tentacles that they use to feed. Many gorgonians, sea pansies, fans or pens look superficially like hydroid colonies, but on close examination, you can see their more “sea anemone” or anthozoan morphology.
Below are video of a sea fan and a sea pansy we have in the laboratory this semester.
If you look closely at the branches of a soft coral (gorgonians are probably the most obvious), you may see the tiny tentacles of the individual polyps. Each of these tentacles may looks like it is feathered because it can bears numerous outfoldings.
Sea pansy---close up of polyps. Sea pansy polyps move very slowly. They feed on microscopic plankton in the water column.
A few pictures obtained by students for their journals last year of fully extended polyps.
Interestingly, sea pansies exhibit bio-flourescence. Visit the following website to view a presentation on sea pansy bio-flourescence produced by the lab that supplies our sea pansies. https://www.youtube.com/watch?v=lhMWrccI5Lc
Below is a picture of the flourescence observed by our students. It appeared green to the eye, but photographed blue under uv light in our laboratory. Each group of blue spots corresponds to an individual polyp.
This group is distinguished by a life cycle that involves a polyp and medusa stage. The medusa is the dominant stage. Below is a diagram of a typical life cycle of a scyphozoan. Review the videos provided in the "lecture" portion of the course.
We have an unusual Scyphozoan culture in our laboratory. We have been able to keep Cassiopea (upside-down jellyfish) in our lab rooms for extended periods of time. They will reproduce for us, but we cannot get the young medusae to mature in our laboratory probably becaue we cannot provide as much food as they need for rapid growth to adulthood.
By lying upside-down, the jelly exposes its symbiotic algae to the sun, allowing it to photosynthesize. The jelly lives off food the algae produce, as well any zooplankton it can capture. Below are movies of the upside down jellies in our laboratory.
Note the complexity of structure of the medusa. The region devoted to feeding, the edges of its eight oral arms, are fused and folded into elaborate frills containing hundreds of tiny mouth openings, a unique feature of this species. The mouth openings are connected by channels to its stomach. By pulsing its bell, it forces zooplankton into the nematocysts on its mouth openings. It also has paddle like extensions that also house cnidocysts that can produce strings of mucus. The function of the this mucus is considered primarily defensive, although there is some evidence that some strings may stay somewhat connected to the individual and so may act as ways to bring prey to the animal.
Diagram of mouth structure in upside down jellies.. ---------------------------------------Mucous strings of nematocysts.
Jellyfish gonads develop in the lining of the gut; thus, the sperm is released from the mouth. Depending on the species, the females carry their eggs in a brood pouch or in their stomach, so fertilization occurs in one of these locations. The polyp produced is a simple one compared to most Scyphozoans, and gives rise to one medusa.
The Hydrozoa are discussed on a separate page to allow for easy comparisons of their life cycle to Scyphozoans and loading of pages in general.