Yesterday's post about squid had a most unsatisfying conclusion, so I feel compelled to mention two things: squidblog has a brief explanation of squid jet propulsion, and I've dug up another older paper on squid movement. Even better, it's about squid nuptial dances and mating.

Here, see? Pretty squid post coitus planting a string of fertilized eggs on the sea floor.

Photo of a large male escorting a female squid (Loligo vulgaris reynaudii) as she attaches an "egg finger" of about 100 eggs to an egg bed.

This work was motivated by growing demand in the South African squid fishery; it had been found that the squid congregate seasonally into dense clusters that were particularly easy to plunder, hoisting up tons of squid every day. As it turns out, they were gathering to mate and lay eggs, exactly the worst time to disrupt their activities if you're interested in maintaining a sustainable fishery. This study by Sauer et al. was intended to find out exactly what those lovely beasties were doing down there.

The cool thing is that they seem to be lekking. Lekking is a behavior best known in a few species of birds; all the males gather together in a small area, put on elaborate displays, and the females stroll among the males flaunting their stuff to pick and choose the most desirable. It's a competitive scheme in which many males put on a show and fail…and it's an excellent example of sexual selection in which female mate choice is paramount.

The squid do something similar. The males gather at dawn, and swim in large circles above the mating ground. Females join in later, and they all dart and dance together, and eventually pair off, mate, and lay eggs. Then they all do it again—the nuptial dances go on all day until an hour or two after sunset.

How do they know what the squid are doing? The investigators fitted a sample of squid with acoustic transmitters that pinged buoys, which triangulated the signals, calculated their positions, and transmitted the information to computers that recorded the minute-by-minute positions of the tagged squid. They produced detailed records of their location, like the day record below of a pair of males (red and blue-black) and a female (green), which culminated in the deposit of an egg mass at the large black dot.

Overhead view of a typical full day of tracks for a large male (blue). a female (green), and a sneaker male (red): full details of behaviors can be replayed on computer animations. The black dot identifies an egg mass confirmed by divers: video surveys also showed numerous squid and scattered egg fingers beneath the area of heavy tracks on the opposite side of the array. Ten days of similar records show that males and females begin at dawn making circlesof 50-150 m radius. high in the water column. around the sonobuoy array (A,B,C,D). Circling decreases by mid-morning. and in the afternoon females spend more time near bottom at individual egg patches. Sneaker males remain outside the main arena, low in the water column and near egg patches: direct observations show that they rush in to intercept pairs. Speeds average 18 cm s^-1 for females. 14 cm s^-1 for large males, and 17 cm s^-1 for small sneakers.

While some play the game of courtship, others take advantage of it. The dark lines are from a large male cruising the mating grounds, looking for potential mates, and basically flaunting his fitness in anticipation that he would be chosen. The red lines are for a "sneaker" male, one who has chosen a different strategy of darting in quickly to a receptive female and depositing a sperm packet in her, almost on the fly. While large males might indulge in prolonged, languid love-making of 16-20 seconds, the sneaker males would get the job done in under 6 seconds. (In case you've forgotten how squid mate, it involves the male scooping up some sperm with a tentacle, and sticking it in the female's buccal cavity).

Computerized data recording isn't all they did, though—divers were also on hand to film the action. Here's a ménage à trois caught in flagrante delicto. The sneaker male is the smaller fellow on the left who is sticking his tentacle into the tangle of the larger pair's coupling.

This enhanccd video image of a "sneaker" male (left) intercepting a temporary pair and attaching a sperm package to the female with no response by the escort (right) is direct evidence of sneaking in squid mating systems.

The observations support the idea that squid mating systems are impressively complex, comparable to those seen in birds. The authors also argue that such a system is also susceptible to long-term disruption by fishing methods—it's not just that large-scale fishing would deplete numbers directly and reduce the number of eggs laid, but it could also change the balance of mate choice, leading to unpredictable changes in the genetics of the population.

Sauer WHH, Roberts MJ, Lipinski MR, Smale MJ, Hanlon RT, Webber DM, O'Dor RK (1997) Choreography of the Squid’s "Nuptial Dance". Bio. Bull. 192: 203-207.