In my job as a Marine Science Educator, I often get asked questions that I don’t know the answer to. In an effort to increase my own knowledge and better remember the answers to these questions, I thought I’d post the questions and answers here.
A few days ago, we saw a tiny juvenile octopus during a bottom dredge. A student asked me, “How many babies does an octopus have?”, which got me wondering about how octopuses reproduce.
It’s worth backing up a bit and reviewing some octopus biology. Here in Bamfield, we have two species of octopus that are fairly common: the giant Pacific octopus (Enteroctopus dofleini) and the Pacific red octopus (Octopus rubescens). They can both be found in shallow waters, including the intertidal, though the giant Pacific octopus can go as deep as 1500 m. Octopuses are known for their intelligence so much so that in some countries, they are considered “honorary vertebrates”. Octopuses also have complex physiology and anatomy when compared to most other invertebrates, including well developed eyes. Giant Pacific octopuses have even been documented to be able to recognize individual humans (though the sample size for the study was small) (Anderson et al., 2010).
Reproduction in octopuses is surprising. Both males and females are relatively short lived, and die not long after reproduction. In the male, the third right arm is specialized for reproduction and is called a hectocotylus. He uses this arm to insert a spermatophore (sperm packet) into the female. Within a few weeks of copulation, the male dies.
The female octopus can store the sperm and fertilizes her eggs once her den is ready. She will seal herself inside her den using rocks, and once the eggs are fertilized, she will lay anywhere from 20 000 to 100 000 of them in strings on the upper side of the den. She then takes care of the developing eggs by blowing sea water over them to keep them oxygenated and removing any organisms (such as bacteria, algae or colonial animals) that may grow on them. After several months (the length of time varies depending on species), the eggs hatch. The female then leaves the den and since she has not fed during the entire incubation period, she soon dies. The larval octopuses then spend some time as plankton before settling on the bottom to develop into adults. The life span of the giant Pacific octopus is about three to five years.
I found it surprising that the life history of octopuses seems to fall somewhere between r-selection and K-selection, given the high investment the female puts into looking after the eggs. But then I read that r/K selection theory is actually considered out of date! I learned r/K theory during undergraduate work in the early 2000s so I wonder if current undergrads are still being taught it. Something for another time.
Anderson, RC, JA Mather, MQ Monette, and SRM Zimsen, 2010. Octopuses (Enteroctopus dofleini) recognize individual humans. JOURNAL OF APPLIED ANIMAL WELFARE SCIENCE. 13 (3), 261-272