When it comes to parenting, some animals will go the extra mile. Possums, for example, have their young clinging on as they trundle around. Kangaroos give birth to immature young that they keep in a pouch until it's bursting at the seams. Humans let 30 year old offspring move back in.
And fish? Do mouth brooding.
(A mouthful! Source)
The fish above has a mouthful of eggs. It's a cichlid. And some species of cichlid will go the extra...mouthful to protect their eggs. They are so worried about the predation of their eggs that the females lay eggs, circle around, and then immediately gobble them up, holding them in their mouths until the fry hatch, and often even longer, until they are big enough to make it on their own.
However, it turns out that the female fish are in such a rush to pick up their eggs, they sometimes don't even give them time to get fertilized! Holding a mouth of unfertilized eggs seems like kind of a waste. But the males have figured out not just how to get fertilization to happen...they got some oral sex out of the bargain.
Mrowka, W. "Oral Fertilization in a Mouthbrooding Cichlid Fish" Ethology, 1987.
The author of this study noticed how quickly the females were slurping up the eggs, and wondered if there was time for the eggs to be fertilized. He also noticed another behavior in the fish: the males have a spot on their anal fin. It looks like an egg, and it called the "egg spot" or "dummy egg".
He noticed that the female fish were attracted to this egg spot and NIBBLED at it, like it was an egg they were trying to gobble up. What was going on?
First, he decided to check whether eggs that got into mouths PRIOR to a male getting at them ended up getting fertilized. He took groups of spawning fish, and looked at three groups: eggs he removed right after that were laid, eggs removed right after the males made a movement to inseminate them, and eggs that were gobbled up in the female's mouth. All eggs were checked 48 hours later.
He showed that when the eggs were removed immediately after they were laid, before the male made a movement to inseminate, only 29.6% of the eggs ended up fertilized (I really wonder where the 29.8% is coming from? Free floating sperm?). If the eggs were removed AFTER the males made the movement to inseminate, 49.8% got fertilized. If the females scooped them up in their mouths and were checked 24 hours later, 94.9% of eggs were fertilized.
It's pretty easy to do the math and see that a LOT of eggs were getting fertilized. In a second experiment where the male and female got separated immediately after the female laid eggs, the scientist was able to show that a total of 63.7% of eggs got fertilized. Leaving 36.3% of eggs that didn't.
But in normal conditions, 94% of eggs got fertilized.
...what hanky panky have those fish been UP to?!
The author hypothesized that when the female nibbled the anal fin of the male (attracted to his "dummy egg"), the male was able to release sperm...into her mouth. Where it then fertilized the eggs there, making sure that as many eggs got fertilized as possible. That's right. Fish oral.
It's a neat way to work out a problem. The male makes sure he gets his sperm in, and the eggs end up getting fertilized in a safe environment where they don't have to face predation. And all it takes is a little oral. The biological world never ceases to amaze me.