Sexual Reproduction in Freshwater Jellyfish
(Craspedacusta sowerbyi)

by Gen-yu Sasaki, Japan

Two embryos in the two-cell stage, and two unfertilized eggs.

Reproduction of jellyfish

In the previous article, I reported on these jellyfish, especially on their polyps and asexual reproduction. Now, I intend to write "another story" mentioned in it. That is, the sexual reproduction of the jellyfish.

I had reared the medusae for about one month from mid October 1999. They were all given by Miyakawa who is a veteran amateur on this species; two females were grown up from polyps in his aquarium tank, and the two wild males were collected by him in Nagoya city, central Japan.

However, as a matter of fact, female and male individuals of this species are usually not found at the same time in the same habitat. Some people think that this is due to the lack of complete investigations, but the sexual reproduction in nature may not be common.

Nevertheless, the sexual reproduction is very important. It helps to improve the genetic diversity of the population, and information about their development is valuable for the understanding of these creatures. I hope that this article makes you interested in medusae.

Eggs

Pictured left are the eggs of freshwater jellyfish, C. sowerbyi. They were spawned in 20th Oct. and they are about 0.1mm in diameter. They were sunk in the bottle in which I rear medusae, and I could pick them up in a watchglass.

I separated females and males and I tried artificial insemination. Various eggs were observed as seen in this picture. But these all seem unfertilized because I was not able to observe any cleavage this time.

The three eggs marked with a '*' were good, whereas the other eggs were bad and cannot develop, I think. I don't know the reason why there were differences in egg viability.

Cleavage!

On the other hand, I couldn't find any sperms of the medusae, or make any eggs fertile. Although Miyakawa successfully fertilized eggs artificially, I abandoned it and started to rear all medusae in the same bottle. This is not a good method to observe the start of cleavage. In fact, I couldn't observe the fertilization.

But luckily enough, near the midnight of the 27th Oct. 1999, I found two embryos that were in the two-cell stage of cleavage! Pictures were taken continuously until morning with intervals of about five minutes. I show some of them here.

23:54	00:46	01:53
02:38	03:28	16:08 (The left is unfertilized).

Two developing embryos can be seen in these pictures (except the last). The indicated times are when the photo was taken. Animation of the left and the right embryo are also available (each about 200KB GIF, 100x100 pixels). In these animations, disturbances in the arrangement of cells are seen especially when they divide.

1. (23:54) The embryos are in the two-cell stage with its distinctive shape. This shape is also reported in Hydra and Spirocodon. I can see a membrane in which the embryo is covered. (Look at the right embryo, and the top photo). This membrane confuses me, because I hear that there is no fertilization membrane in Hydrozoa, to which freshwater jellyfish belongs.
2. (00:46) This is in the four-cell stage. The four cells are arranged in two ways. Tetrahedron on the left and flat on the right.
3. (01:53) Probably in the eight-cell stage. The arrangement of the cells are not in good order.
4. (02:38) 16-cell stage maybe. I cannot count the number of cells.
5. (03:28) 32-cell stage? The shape of the left embryo is strange. But it recovered as a ball afterward.
6. (16:18) The cleavage is already finished. The left ball is an unfertilized egg and the right is the embryo. They are very similar. This embryo didn't swim at this moment.

Although eggs or planulae were seen several times, this is the only cleavage I observed.

Planulae

After the cleavage, the embryo began to swim. I observed a ball-shaped embryo swimming slowly in small circles. This swimming soon ceased. The embryos began to elongate and creep on the substrate. The term "planula" is applied to this swimming stage and creeping stage until they settle somewhere and become polyps.

Lefthand animation shows a creeping planula on the watchglass. The interval is ten minutes and total length is 80. The speed of the movement is variable according to the cases.

You may think that you saw this kind of animation somewhere else. Yes, I showed the movement of a "frustule", asexual larva, in the article "Freshwater Jellyfish". Frustules from polyps are very similar to planulae in their morphology and movement. Righthand picture is not a planula but a frustule.

But they are different in some points, I gather. At any rate, frustules are far larger than planulae as you see in the pictures above. Actual length of the frustule in this picture is about 0.95mm.

Tiny polyps from planulae

A tiny polyp from a planula (left) and a mature polyp (right).

Some planulae became tiny polyps in several days. They are very small. Enlarged image of a tiny polyp is also available (brightfield: 20KB). The magnification on the photographs of a creeping planula, a frustule, and these two polyps are all the same.

The mouth of a tiny polyp is about 0.1mm in its diameter! What do they feed on? Protozoa? I have recently successfully fed them with rotifers, something like Rotaria. An animation is also available (ca.25min., 18frames: 59KB GIF). I hope I can grow them up to mature polyps with these kind of creatures "generated naturally" in the container.

Acknowledgements:

Ryuta Miyakawa and other members of the "jfish-ML", Wim van Egmond and Micscape Editor, Dave Walker encouraged and helped me very much again. I express my gratitude to them.

References:

• Payne, F. 1926. Further studies on the life history of Craspedacusta ryderi, a freshwater hydromedusan. Biological Bulletin of the Marine Biological Laboratory (Woods Hole, Mass.) 50: 433-443.

Last update: 27-NOV-1999

Photographs taken on: 21, 27, 28-OCT, 08, 22-NOV-1999

Copyright © 1999 by Gen-yu Sasaki . All rights reserved. Unauthorized reproduction prohibited.

Microscopy UK Front Page
Micscape Magazine
Article Library

Published in the December 1999 edition of Micscape magazine.