Enigmatic eggs

A comparison of egg size of two different Central American cichlid species. One inhabiting slow flowing courses, Archocentrus nigrofasciatus (to the left) and a reophilic species, Tomocichla tuba (to the right), which shows larger eggs. Photo by Ron Coleman.

Every cichlid starts out as an egg, and yet we know surprisingly little about cichlid eggs. We know they vary in size, in shape, and in color, and we know that parents of different species take care of them in radically different ways, from substrate spawning to mouthbrooding. But what does this variation mean? Is it important?

In the last few years I have become increasingly interested in this variation. In this essay I will review one of my own publications, done with Alison Galvani of Oxford University. The paper examines the relationship between egg size and offspring size, and has interesting implications for scientists and hobbyists alike.

Within the family Cichlidae, the size of eggs varies from the almost microscopic eggs of rams (Microgeophagus ramirezi) to the massive eggs of certain African mouthbrooders like Cyphotilapia frontosa. To cichlid breeders, the most striking result of these differences is apparent immediately: while a small ram may produce a hundred eggs or more, even the largest C. frontosa female can only produce a few dozen eggs. Why this should be has to do with how cichlids make eggs.

When a fish is going to breed it has a certain amount of energy and materials (proteins, fats, etc.) that it can spare from the resources it needs to survive and grow. These materials will be put into the eggs. There are a couple of ways the fish can proceed. Many mammals, for instance, divide the resources they have for reproduction into a set number of embryos, usually a small number. If there is more energy available, the mother makes a larger baby, but if there is a shortage of food, the mother makes a smaller baby.

Cichlids don't work that way. They make a certain-sized egg, regardless of how much energy they have to put into reproduction. The process is similar to making cookies, all of which have to fit into a cookie package. Each cookie has to be the same size or it won't fit correctly. While female cichlids do not vary the size of the egg, they can vary the number of eggs they produce. So, if one female is larger than another, or has had a lot to eat just before laying eggs, she doesn't make larger eggs, she just makes more of them. There may be very slight differences between the eggs, but in general they are all the same size.

The constancy of egg size holds true for all females within a species. It does not hold true between species, and this contrast intrigued me. It suggests that cichlid eggs are shaped to an exact size by each species. What factors could be so important that a convict cichlid (Archocentrus nigrofasciatum) egg is invariably 1.5 mm in diameter, while the egg of an angelfish (Pterophyllum scalare) is 1.3 mm in diameter?

The most logical starting point to answer this question was to look at the hatchlings. Certainly if you see a C. frontosa hatchling and compare it to a ram hatchling, there is no question: larger eggs make much larger hatchlings. But what about on a finer scale? Does the difference in size between the eggs of a rose-breast cichlid (Amphilophus longimanus) at 1.4 mm and a convict egg of 1.5 mm really matter? Or are these small differences caused by slight and unimportant variations in the amount of water the egg soaks up as it comes out of the female?

The answer is: yes, it does matter. To reach this conclusion, Alison Galvani and I bred a large number of neotropical cichlids over a period of roughly three years (Coleman and Galvani, 1998). We obtained data on 26 species, mostly from the "Cichlasoma" group. The eggs of these species ranged from 1.2 to 2.4 millimeters in diameter. By taking a sample of the eggs and a sample of the hatchlings on the very first day after hatching, then measuring both under a dissecting microscope to hundredths of a millimeter, a clear picture emerged. Small eggs produced distinctly smaller hatchlings than larger eggs, even over relatively tiny differences in egg size.

To investigate further, we looked at the eggs laid by different females of the same species and even within the clutch of a single female. We used convict cichlids for this. By picking out the largest eggs and the smallest eggs from these groups and hatching them, we showed that even the minute differences between them (a few tenths of a millimeter) were enough to produce important differences in the size of the hatchlings.

What does this mean? It means that egg size is no accident. We proposed that the female is in fact producing hatchlings of a very specific size. If we are to understand why we get lots of convicts but only a few frontosas from a spawning, we have to understand the advantages and disadvantages of being a large or smaI1 hatchling.

For example, can larger hatchlings feed on different things than smaller hatchlings? Or can larger hatchlings escape predators or swim in faster water than smaller hatchlings? How large do these advantages have to be to outweigh the loss in egg number suffered by females laying large eggs? When is it good to lay many small eggs? These are the questions I am currently researching.

As a final thought, consider that these small differences may explain an old puzzle about growing cichlids. People who raise batches of young in an aquarium soon notice, often within weeks, that the juveniles don't grow up evenly. Some quickly get ahead and rapidly outgrow the others, a phenomenon known as "growth depensation." Aquarists have often assumed this was the result of better genes in the largest juveniles. But our results suggest that those rapid-growers simply got a smidge more material to work with as an egg. They hatched into slightly larger wrigglers, and became slightly larger fry than their brothers and sisters. They got a headstart in feeding, and they continued to benefit from their extra size for weeks or even months after. One implication of this scenario is that attempting to breed "faster growing" cichlids by choosing the largest from a group of young will have little effect.

The size of the eggs also determines the size of the newly hatched fry. Photo by Ron Coleman.

There are many more mysteries about cichlid eggs. Resolving these mysteries requires a database of cichlid egg size. Surprisingly this does not exist in the scientific literature. To remedy this situation, I started the Cichlid Egg Project. My goal was to encourage cichlid hobbyists to donate small samples of eggs from the cichlids they breed. I measure the eggs and post the information on the Cichlid Research Home Page (http://cichlidresearch.com). Through the combined efforts of dozens of hobbyists from around the world, we now have top quality egg-size data on over 200 species of cichlids! Amazingly, there are still lots of other species regularly kept and bred by hobbyists from which I have no samples. The Project continues. If you are interested in getting involved, check out the website. Collectively, we can start to understand the many enigmas surrounding cichlid eggs.