Why Poop-Eating Vampire Squid Make Patient Parents
Squid and octopuses are famous for their "live fast, die young" strategy. At one-year-old or younger, they spawn masses of eggs and die immediately. But scientists have just discovered a striking exception, reported April 20 in the journal Current Biology.
Females of the bizarre species known as “vampire squid” can reproduce dozens of times and live up to eight years. This strategy is probably related to the vampire squid's slow metabolism and its habit of eating poop.
These shoebox-sized animals have fascinated biologists since their discovery in 1903, not because of any actual vampiric habits, but because of their puzzling place within the cephalopods—the group of animals that contains squids and octopuses.
Vampire squid are neither a squid nor an octopus, and they're tricky to study because they live hundreds of meters below the surface, in frigid water with very little oxygen.
In addition to eight webbed arms, they have two strange thread-like filaments, whose purpose—collecting waste for the vampire squid to eat—wasn't understood until 2012. A clear picture of the habits and evolution of these animals remains elusive.
Take a Rest Between Eggs
Henk-Jan Hoving, currently at the Helmholtz Centre for Ocean Research in Kiel, Germany, began his investigation of vampire squid while at the Monterey Bay Aquarium Research Institute. For the spawning study, he worked with specimens that had been collected by net off southern California and stored in jars at the Santa Barbara Museum of Natural History.
Out of 27 adult females, Hoving and his colleagues found that 20 had “resting ovaries” without any ripe or developing eggs inside. However, all had proof of previous spawning.
As in humans, developing eggs are surrounded by a group of cells called a follicle. After a mature egg is released, the follicle is slowly resorbed by the ovary. The resorption process in vampire squid is so slow, in fact, that the scientists could read each animal's reproductive history in its ovaries.
Counting 38 to 100 separate spawning events in the most advanced female, and estimating that at least a month elapsed between each event, Hoving and his co-authors concluded that adult female vampire squid spend three to eight years alternately spawning and resting.
This length of time is reminiscent of the deep-sea octopus who brooded her eggs for over four years. In both cases, the animals' actual lifespan must be longer than their reproductive period, which suggests truly venerable ages for members of a group whose most common representatives live for just a few months. These long life spans are related to a slow metabolism and the chill of the deep sea—around 2 to 7 degrees Celsius, or 35 to 44 Fahrenheit.
Limited Calories, But Limited Danger
A single spawning event is not actually a strict rule for octopuses and squid. A few species are known to spawn multiple batches of eggs, even as they continue to eat and grow. However, all species reach a continuous spawning phase at the end of their lives.
Once a female starts to lay, her body is in egg-production mode until she dies, her ovaries constantly producing. That's why the discovery of a “resting phase” in the ovaries of vampire squid was so surprising.
But this unexpected strategy makes sense in the context of a vampire squid's lifestyle. The mass spawnings of other cephalopods are fueled by a carnivorous diet of fish, crabs, shrimp and even fellow squids and octopuses.
By contrast, the fecal material and mucus that make up most vampire squid meals are not nearly as calorie-rich. The animals may be simply unable to muster enough energy to ripen all their eggs at once.
There's an advantage, however, to living in the food-poor, oxygen-poor depths of the ocean. Few large predators can survive there for long, so vampire squid are relatively safe—compared to their cousins, who are constantly on the run from fish, dolphins, whales, seabirds and each other.
When you face a high risk of being eaten on any given day, it's a good idea to get all your eggs out as quickly as possible. But vampire squid are free to engage in leisurely, repetitive spawning. It's the ultimate work-life balance: alternately popping out babies, then returning to business as usual.