Development: how big is big enough?

This picture is of a fertilized sand dollar egg. You can see the dark pigment dots and the clear circle around the egg, which is the fertilization membrane, and the light smears which are sperm still trying to get at the egg. However, the fertilization membrane prevents any other sperm from entering the egg.

I read an interesting paper about development and sea urchins. A little background: there are three different developmental modes in many different invertebrate species. One is nonfeeding, where the young develop into adults using only the energy (yolk) provided by the mother. Another is feeding where the young spend more time in the larval stage feeding and that’s where they get the energy to transform into the adult. The final mode is very rare and it is called facultative feeding. In this case the young don’t have to feed to transform, but they can to get extra energy. Unlike the nonfeeding larvae who can’t feed even if they wanted to, because they don’t have a complete digestive track.

As you can imagine egg sizes for these different mode vary. Since the nonfeeding modes rely only on what the mother supplies to get to a suitable spot and change into an adult, they tend to be very large. The feeding ones are smaller, because they only have to have enough energy to create a gut, then they can feed themselves till they become an adult. The facultative ones are in between (in general). It is thought that this mode is a very unstable one, and that the species that have it are on their way from feeding to nonfeeding modes or vice versa.

What these researchers tried to do was to take a facultative feeder and force it to become a feeder by reducing the amount of energy available to the young. They did that by taking 2-cell and 4-cell stage and breaking them apart, so that one treatment had ½ the energy and the other treatment had ¼ the energy.

2-cell stage 4-cell stage

They then raised some of each type with food and without food to see if the young with less energy were eating and growing faster than those with less energy who had no food. Of course, they found that young that had not been manipulated were larger that those that had been halved or ‘fourthed’. What was interesting was that none of the size-manipulated young were forced to feed to complete metamorphosis. All young transformed at the same time!
This means that the mothers gave up to 4 times the amount of energy needed to undergo metamorphosis to their young!

For more info see the original paper: Allen, J.D., C. Zakas, R.D. Podolsky. 2006. Effects of egg size reduction and larval feeding on juvenile quality for a species with faciltative-feeding development. Journal of Experimental Marine Biology and Ecology. 331:186.197