Squid are free-swimming mollusks, and like their close relatives the octopods, they are intelligent, predatory, fast-growing and have a depressingly short life span usually lasting merely a year.
The long-finned squid (Loligo pealei), one of about forty species that occur worldwide, is most abundant in the Atlantic and is the animal you are most likely eating when you order calamari. It grows to a little over a foot in length and, like all squid, have ten tentacles surrounding the mouth. These are used to capture prey and transfer them into the beak-like mouth.
Loligo are solitary hunters at night and then gather in large schools during the day. In the spring these schools migrate into shallow water to perform a mating ritual. Although the mass of reproducing adults at first may seem haphazard, there is actually quite a bit of structure and hierarchy going on. Couples pair off based on size, with many of the smaller males getting left out at first.
Females hold their eggs inside the mantle (the main tube-like part of the body). Males fertilize these eggs by transferring sperm packets to the mantle cavity of the female using its fourth right arm, which is modified into a spoon-like organ called a hectocotylus. The sperm is carried in a two-inch long structure called a spermatophore. It looks kind of like a small plastic baseball bat with a little cap on the end. This keeps the gametes from being carried away by the ocean before they can reach the female’s mantle. Once the spermatophore is in place the cap pops off, releasing the contents and the eggs are fertilized.
The male then darts off to try to find another female to pair with. This leaves an opening for lesser males to sneak in and mate with the female as well. She may contain up to 50,000 eggs and the young may actually have three or four different fathers. Once all the eggs are fertilized she begins to cover batches of them in a gelatinous substance produced by a structure called a nidimental gland. In between the layers of gelatin surrounding each group of eggs, colonies of bacteria grow which, as a by-product, produce a fungicide that protects the eggs as they develop. Below is a female in the process of producing one of these sacs.
These egg sacs, each of which may contain as many as a two hundred embryos, are held by the arms as they form and are glued to a shell or pebble that is lying on the sand. Dozens of capsules are in turn cemented to the same substrate, resulting in an "egg mop" that looks like this:
The female continues to form these egg mops until all the embryos have been released. Then she dies. The males survive for a while longer, fertilizing the last of the females, before they die as well. You can watch a short video of this mating and egg-laying behavior here. You may notice in that video that the males (the ones doing the grasping) seem to get more aggressive as time goes on. They are at the end of their lives and don’t have much time to complete the mating. As it turns out, the mere sight of egg mops being produced increases male agonistic behavior, both in their pursuit of females as well as battles with other competing males. Agonism, or aggression, results in biting, tentacle fighting and rapid changes in the skin color.
It’s hard to imagine what the ocean bottom looks like once an entire school of squid has completed this mating frenzy, so watch a bat ray swimming over a vast crop of egg mops.
After all the adults have died, the egg sacs are left to develop on their own. The larvae are nourished by a yolk and take several weeks to hatch. When they do a baby squid emerges and, using the tiny siphon for jet propulsion, swim up towards the surface to begin feeding on plankton.
Fun Fact: Although fish will not feed on the egg mops, many species will hang around these egg mop fields waiting for the young to hatch.
Other diaries in this series can be found here.