Still small – Cnidaria Part 1

This will be a short post, as I am in the middle of that annoying revision period for exams.  I haven’t written for several days due to writing a report on enzyme kinetics (far too abstract, and too much maths for my taste!), and am now revising for next weeks zoology exam.

There will be several posts about various species within Cnidaria, as this is a wide phylum which has a ton of amazing organisms in it.

This post will be about the general attributes life cycle of organisms in this phylum.  Later posts will focus on specific organisms within this phylum, such as sea anenomes, corals, sea pens, and colonial swimming organisms like the Portuguese Man’o War.

There are two distinct forms of organisms within Cnidaria, and many of the members use both forms throughout their life, whilst others are found in one formonly.

Polyps are the stationary (sessile) part of the life-cycle, and whilst in corals this is the adult stage, in jellyfish and other organisms which have a free-swimming adult stage, this is the juvenile part of their life cycle.

A Hydra in the polyp stage. Image from SARS centre for Marine Molecular Biology

Diagram of a Cniadarian Polyp.

Image from structure of cnidarians

These are effectively tube-like structures, with tentacles around the mouth to direct food into the gastrovascular cavity (Also called a blind gut, as it has no exit).  Many jellyfish polyps form symbiotic relationships with corals, and some species even attach to larger, mobile creatures, the image below shows a hermit crab covered in polyps.

Polyps from Hydractinia echinata covering a hermit crab shell. Image from

Whilst in this form, the organism reproduces asexually by a process known as budding.  This is where a lump of tissue forms on the side of the polyp, and develops its own mouth and tentacles.  Once this has been achieved, one of two things occurs.  The newly developed polyp can break off as a clone, or it can stay attached to the parent polyp, and then a colony is formed, often with one shared gastrovascular cavity.  Colonial budding allows for different polyps to perform different tasks.  In Hydrozoan polyps, some retain tentacles and become feeding polyps (hydranths) whereas others do not retain tentacles, and become reproductive polyps (gonangia).  There are variations on this stage of the life cycle, and these will be covered as we go into more specific organisms.

Life cycle of Aurelia aurita. The polyp phase is at the bottom. Image from Black Sea Education

As can be seen in the diagram, the polyps can keep budding new polyps, but if the conditions are favourable, then these polyps produce Ephyrae, which are immature medusa (medusa is the term for the free swimming form). These mature into adults, which reproduce sexually to produce the larva which makes new polyps on the sea floor, and the cycle begins again.

There are some jellyfish which are able to bud directly from themselves, but the vast majority bud from polyps.  Sexual reproduction is usually done by the mass release of sperm and eggs from the adults.  As can be seen from the diagram below, medusa are essentially upside down polyps, but able to swim, rather than being attached to the sea floor.

Why would this phylum have such distinct life cycles, instead of just reproducing to directly form immature medusa?

One reason is that this allows for the organisms to coexist in the same area of ocean without directly competing with each other for resources.  Polyps occupy the benthic (bottom of the ocean) zone, whilst medusa live in the pelagic zone (The open water zone).  The polyps are also afforded some protection from open ocean predators whilst in the juvenile form.  Finally, and for me, most important, it allows for the organism to not become adult until the conditions are right.  This is one cause of the blooms of jellyfish seen in coastal regions.  The asexual reproduction of polyp budding to new polyp can continue until the conditions are right for production of ephyra.  When these conditions arise, the polyps will switch to production of ephyra, and so vast quantities of medusa will appear at once.

As a final point of interest, there is one species of jellyfish Turritopis nutricula which is able to switch between polyp and medusa forms.  This involves some amazing cellular acrobatics, and means that theoretically, the organism is biologically immortal.   First, the umbrella of the jellyfish inverts itself, then the tentacles and the mesoglea (the jelly in jellyfish) gets absorbed into the body. Once this has been achieved, the end opposite the mouth attaches itself to the ocean floor, and begins producing polyps.  The process by which it does this is known as transdifferentiation whereby one type of cell turns into another.  This occurs very rarely in nature, and so is quite an interesting process.

Next time, we will start looking at some specifics of this phylum, and I can get further into showing you the really cool stuff that these organisms do.


3 thoughts on “Still small – Cnidaria Part 1

  1. Pingback: Cnidaria Part 2: « skepticalsquirrel

  2. Pingback: Still corals! « skepticalsquirrel

  3. Pingback: Recap « skepticalsquirrel

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