Discovered by divers in the late 1990s, the upper Spencer Gulf population is the world's only known mass cuttlefish spawning aggregation, with hundreds of thousands of "S. apama" cuttlefish congregating on subtidal reefs around Point Lowly near Whyalla between May and August. While outside of the breeding season, the sex ratio is one to one, Spencer Gulf males outnumber females by up to 11 to one in the spawning aggregation. If this is due to fewer females taking part or to males breeding for a longer period of time than females is not known. With densities of one cuttlefish per square metre, covering about , the sheer numbers of "S. apama" make this breeding aggregation unique in the world. As the cuttlefish are oblivious to divers while spawning, they are now a major regional tourist attraction for divers from around the world. Professor Roger Hanlon of the Woods Hole Oceanographic Institution has called the breeding aggregation "the premier marine attraction on the planet."