Spore Cases of Clover Fern, Marsilea vestita, in 2001

This page was formerly titled Western Spadefoot Toad Eggs at the Main Vernal Pool Vs. Time In 2001, which accounts for the name of this file.

For eight years, I thought strings of ~20-40 yellow uniformly-spaced "seeds" on an ethereal medium were laid by the Western Spadefoot Toad.

The reasons were many:

Usually, with that much evidence, one's conclusions are pretty solid. But not this time.

Fortunately, Daniel Marion wrote me that he had seen similar "eggs" in the Madrona Marsh in Torrance, CA, and let them mature in the lab. Instead of turning into tadpoles, the "eggs" turned in clover fern, Marsilea vestita! I examined a sample of our "eggs" on 10 January 2008, and Daniel was exactly correct. These are the sporangia (spore cases) of the clover fern.

The following pictures were taken in the field immediately after removing some of the spore cases from the water:

The picture at left shows the transparent tissue (on the right of the yellow spore cases) that connects the spore cases. The spore cases are on top of a dandelion digger whose tip is 16 mm wide at its widest part. Inside the third spore case from the top, the seed-like megagametophytes (~0.8 mm long) are visible, which will develop into clover ferns. A better view of them is seen on the right picture, taken on top of a manila envelope.

A view under the microscope, shown below, also shows the zillions of little microspores (≤0.1 mm) that used to contain the sperm. The megagametophytes have the olive with a little pimento sticking out of it shape as described in A fern with alternation of generations - Marsilea.

Back in 2001 when I was trying to identify these spore cases, I couldn't find any pictures of spadefoot toad eggs. I now have found two pictures: Bruce Farnsworth's photo and in Field Guide to Amphibians and Reptiles of the San Diego Region, Jeffrey M. Lemm, California Natural History Guides, UC Press, 2006. Those eggs have a yolk in the center appearance that is quite different from the uniform appearance of these spore cases.

I should have taken a look at these out of the water before! Thanks much to Daniel Marion for setting me straight on the identification here.

Here is what apparently happens in the evolution of the spores. The clover fern produces a very-hard stalked sporangium case (sporocarp) that is 3-8 mm long and 3-7 mm wide. The sporangium case is very resistant to water, and so they do not all break open the moment the pool forms. As noted below, some sporocarps break open the first day the pool is formed, but others can break open as long as a month later.

When the sporocarp breaks open, the transparent tissue absorbs a lot of water, expanding into the ~linear string of spore cases observed in the water. Each of those spore cases is ~4 mm x 2 mm. Within a day, the sperm from the microspores have fertilized the megaspores that then become the megagametophytes which soon begin to produce their first fronds.

The rest of this page gives a photo gallery of the spore cases vs. time at the Main Pool in 2001, as seen in the water.

All the pix below were taken in a shallow Pool, with depth less than 3", except for Day 28, where the depth was 15". In fact, the rightmost set of spore cases was not covered by water when the Pool depth receded on day 5.

I have made no attempt to match the spore cases between the days, if any of these sets of spore cases are indeed the same.

In the table below, after the Day number, I give the total number of sets of spore cases that I observed from the entire boardwalk. The number of sets peaked at ~8 on Days 8-10, then gradually declined to ~4 sets on Day 13. When the Pool became much larger by Day 15, there were ~12 sets, with the 9 new ones deposited in the newly-formed shallower part of the Pool. The numbers declined quickly to ~5 on day 19 and 1-3 on days 26-28.

Day 1 (1 set of eggs)
Day 3 (5 sets of eggs)
Day 5 (6 sets of eggs)
Day 8 (8? sets of eggs)
Day 28 (3 sets of eggs)

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Copyright © 2001-2008 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Updated 15 January 2008.