I once had the privilege of observing the mating ritual of the octopus. It looked more like passionate hand holding as they intertwined a tentacle with one another. I wasn't really aware of what was going on until my dive buddy explained it to me using a few well chosen hand signals.
After doing some reading on the subject I learned that the male has a specialized tentacle which sends sperm to the mantle cavity of the female. One article claimed the female lays strings of 45,000 eggs and another claimed 150,000. Although the information I read didn't specify, I presume the egg count can differ from species to species. Either way it's quite a lot, though only a very small percentage actually survive to adulthood.
I saw part of an excellent documentary on the TLC channel that really took the time to show and explain the whole process. The strings of eggs were hanging from the ceiling of her lair as she cleaned and aerated them by blowing water through her siphon. She spends the rest of her time guarding them, never feeding herself, leaving only to fight off a potential predator that gets too near. The eggs can take anywhere from 1 to 6 months to hatch. There was some really great close up footage of the tiny little octopi emerging from their pearly, translucent, teardrop shaped eggs!
The down side for the mother is that once the eggs have hatched she is weak and vulnerable to predators when she finally emerges in search of a meal. I had read one article that claimed the female dies after the eggs hatch, but I have since heard that some octopus mothers in captivity do just fine afterwards. I can only hope that some mothers in the wild manage to survive as well.
I became aquainted with a woman, through my guest book, who works in a zoo in Missouri. I asked her about the life expectancy of the mother octopus in captivity. She said, "No, sorry, we have not had any luck keeping one alive after the hatching of the eggs and it seems that some of the females die after laying eggs. There was one Aquarium that has managed to keep their octopus alive through several egg laying cycles. How I'm not really sure, but we had hoped to do the same with ours. Another Aquarium was able to save one after a hatching from their female, but that was like one out of 4,000 and with a 24 hour keeper watch."
The really great thing about the documentary I mentioned before was the way it spent the time necessary to follow the life of one female octopus and all that was involved with the laying and hatching of her eggs. I missed the beginning of it and have been watching for it to be shown again ever since. It is well worth seeing. I believe it was part of their "Secret Life" series.