Chapter 30, Problem 1Q

Summary Introduction

To review:

The represented data are given in the graph and the purpose of reverse development in some cnidarians.

Introduction:

Cnidarians are marine, invertebrate, and radially symmetrical animals that belong to phylum Cnidaria. These animals do not have a complete gut instead have a blind sac called the gastrovascular cavity, which is connected to the single opening called the mouth. The cnidarians exhibit two distinct stages during its life cycle.

Explanation of Solution

The life cycle of all cnidarians involves the formation of two distinct body forms: one sessile (immobile) and other motile. The sessile stage is known as the polyp, which is cylindrical in structure and is attached to a substratum, while the motile stage is known as the medusa, which is umbrella or bell shaped and is free swimming in the sea. The polyp stage exhibits the asexual young stage that converts into the medusa stage representing the sexual mature phase of the organism.

Some cnidarians such as Turritopsis dohrnii shows the reversal of sexual mature phase into young asexual phase. This reversal process is referred to as the reduction. The medusa stage under reduction into its younger version is called as the reducing medusa. Such cnidarians are regarded as “immortal jellies.” Research has shown that this revert back process occurs when the organism faces stress conditions in its surrounding such as unavailability of food, injury, and unfavorable conditions to sexually reproduce.

The following graph represents the data generated when the Turritopsis dohrnii species were incubated under different concentrations of cesium chloride (CsCl) for 3 hours, as a part of a study of reversion phenomenon shown by this species.

From the graph, it is clear that at CsCl concentration of 14.5 mM (low), maximum number of reducing medusa stages were recovered as compared to the control where no stress is given and maximum number of normal medusa stages were seen. At intermediate CsCl concentrations (29 Mm and 58 mM), equal number of reducing medusa stages and polyp stages were seen. However, at very high CsCl concentration (116 mM), absence of reducing medusa was seen while maximum number of polyp stages were recovered. This represents the reversion of all the medusa stages into the polyp stage.

Conclusion

Therefore, it can be concluded that as the stress conditions increase in the environment of cnidaria, the revert back phenomenon is more as more number of medusa stages starts reverting back to the young polyp stages. This reversion process contributes to the immortality of these animals.