If youngsters get cared for at all, the mother is usually involved. But in fish and a few other groups where eggs are not abandoned, fathers are the primary care providers. Males are sometimes such devoted dads that it takes longer for them to care for the young than it does for the females to produce the eggs. If potential fathers are in limited supply, stereotypical male and female roles get reversed, with males more interested in food and females more interested in sex.

Some male fish build nests. A male will accept the eggs of more than one female, because he only has to defend them and keep them clean. In fact, females often prefer to lay their eggs in nests that already have eggs. There are limits however—peacock blenny males will reject female offers if the cost of caring for so many eggs gets overwhelming. And male fathers are not entirely trustworthy, as they often eat some of the eggs.

Cardinalfish go one step further—a male carries the eggs of a female in his mouth until they hatch. Here the temptation to eat them is even greater. Small clutches of eggs are especially likely to wind up in the stomach of the father, because forgoing dinner for a week or two is only worth it if lots of babies result. What’s a suspicious mom to do? In at least one species, females produce some cheap yolkless eggs that they mix with the normal ones to fool the male into thinking he has a big clutch worth caring for.

Some ocean fathers even have specialized body parts to carry developing eggs. Sea spiders usually use their legs, whereas seahorses and pipefish have a patch or womblike pouch on the belly or under the tail. In both groups, elaborate courtship dances may precede the transfer of eggs from female to male. Seahorses and pipefish have more to worry about than their eggs, however—every year 20 million of the creatures wind up in aquariums or, even more sad, dried as curios or ground up for traditional medicine.

Editor's Note: This post is an excerpt from Nancy Knowlton's book, Citizens of the Sea: Wondrous Creatures from the Census of Marine Life.