Sepia apama spawns in dense aggregations where males outnumber females by roughly four to one. Large males compete for access by displaying and grappling, then guard a female closely, so brute contest favours size.
Smaller males take a different route, documented by Mark Norman, Julian Finn and Tom Tregenza. A sneaker male retracts the fourth pair of arms, which in males is longer and displayed, adopts the mottled skin pattern typical of a female, and alters his posture to resemble an egg-laying female. Guarding males frequently allow such an animal to approach, at which point the sneaker attempts to mate. Field observations recorded successful mating in a substantial proportion of attempts, and paternity work confirms sneakers father offspring.
The Spencer Gulf aggregation itself is the population's vulnerability. It is concentrated on a small area of reef near Whyalla, and numbers fell sharply in the 2000s before a fishing closure and subsequent recovery. Why the decline happened is still debated, with fishing pressure, salinity and temperature all proposed and none conclusively demonstrated.