Males of the species Loligo bleekeri follow one of two mating strategies. Larger males mate with a female in the traditional way (for a squid), depositing packets of their sperm inside her oviduct. They then guard the female to make sure no other males mate with her. Small males, on the other hand, don't bother with the colorful displays that are required to woo and mate with a female. Instead, they dart up to an established squid couple and stick some sperm packets on the female's head.
This may seem futile, but the females of L. bleekeri happen to have a secondary sperm-storage organ near the mouth. The so-called "sneaker" males aim for this receptacle when adding their contribution to the mix. When a female eventually extrudes a string of eggs, it passes by both the internal and external sperm stores before landing on the sea bed. This gives all males involved a chance to father some bouncing baby squid.
A group of researchers in Japan and the United Kingdom, led by Yoko Iwata, collected female and male L. bleekeri squid, including both large and small males. They relieved the males of their sperm packets, called spermatophores, and studied their contents. What they saw was that squid sperm come in two distinct sizes. Large squid have smaller sperm, and small squid have larger sperm. (Before you ask, both varieties are larger than human sperm.) And the two sperm sizes are completely segregated between the two receptacles in a female.
It's the first time a single species has ever been discovered to have two separate sperm types. The squid themselves come in two distinct sizes, too; they seem to be built specifically for one mating strategy or the other. How they develop this way is unknown.
The different sizes of sperm don't seem calibrated for competition with each other, though. In swimming tests, large and small sperm were equally fast. They also were both able to fertilize eggs. The researchers speculate that the different sperm sizes have evolved due to the environments they're left in. Small squid leave their sperm out in the open, where they're more susceptible to being washed away by water. Other factors that differ between the two fertilization environments, "such as salinity, viscosity, pH and concentrations of gases and nutrients," may have influenced the evolution of large and small sperm. Each size of sperm seems to be optimized for the mating strategy it's used in, giving both sizes of squid a fighting chance at fertilization.
Squid are far from the only animals to employ spermatic gamesmanship. Unromantic males of various other species use a "sneaker" strategy. For example, in the European common frog Rana temporaria, a "pirate" male searches for freshly laid piles of eggs, then deposits his sperm directly onto them. The eggs have already been fertilized once before, as the female released them, by the male who was actually mating with her. But the pirate has a chance to fertilize any eggs in the batch that got missed the first time. To increase his odds, he sometimes crawls bodily into the egg mass before doing the deed.
Other species use a "copulatory plug," a sticky mass left in the female after mating to prevent other males' sperm from getting in. This is a popular strategy that shows up in species ranging from bumblebees to snakes to squirrels to monkeys. Males of the orb-weaver spider Nephila komaci, not messing around, break off their genitalia in the female to serve as a plug. The male black-winged damselfly has a penis shaped like a scrub brush, which he uses to scrub away his rivals' sperm. In an especially creative adaptation, some rodents produce hooked sperm that join together in transit to form (this is an actual scientific term) sperm trains.
Sperm competition is a high-stakes game; it's the difference between passing on your genes and not. Females, whose only directive is to find the fittest male to fertilize her eggs (if she gets any choice in the matter), don't face nearly the same evolutionary arms race. I'm sure that's some consolation to the female L. bleekeri, drifting through the ocean with sperm packets glued all over her head.
Iwata, Y., Shaw, P., Fujiwara, E., Shiba, K., Kakiuchi, Y., & Hirohashi, N. (2011). Why small males have big sperm: dimorphic squid sperm linked to alternative mating behaviours BMC Evolutionary Biology, 11 (1) DOI: 10.1186/1471-2148-11-236