Australia's Rock Stars

Natural History, April, 2000 by Simon Foale, Mark Norman

For the giant cuttlefish of southern Australia, Spencer Gulf is the place to spawn.

From April to July of most years, the northwest coast of Spencer Gulf in the state of South Australia is the setting for one of the most colorful and vibrant biological events: the spawning of Sepia apama, the giant cuttlefish. These animals, which measure up to three feet, can be found along the entire south coast of Australia. But the greatest concentration (as many as thousands per acre) is found only on the few rocky reefs between the industrial port of Whyalla and Point Lowly, ten miles northeast. This distinction may be related to the scarcity of rocky seafloor elsewhere in the gulf--a circumstance that forces the cuttlefish to congregate on the shallow reefs close to shore, where the undersides of large boulders afford the females suitable surfaces on which to lay their eggs.

The gripping choreography of cuttlefish combat, courting, and copulation can be observed within about fifteen feet of land, where the water is as shallow as six feet. Because sites for laying eggs may be limited, males often rove from boulder to boulder in search of a good spot that isn't already being used. Having found one, the males can then court females that come looking for a place to lay their eggs. Males compete for females by swimming alongside their opponents and trying to make themselves look as large and frightening as possible. They flare out their ornately decorated arms, which bear bannerlike webs, and also produce a mesmerizing pattern--an inverted V of black stripes in constant motion--on the side of the body facing their rival. When rival males are unevenly matched in size, the smaller one invariably swims away before too long. If they are evenly matched, however, the showdown can sometimes escalate into violence; most individuals sport the evidence of past brawls, such as bite marks, missing tentacles, and scratches and scars of varying severity. In any case, the winner gets the female.

Giant cuttlefish copulate head to head, with the male using the tip of an arm to transfer a spermatophore (sperm packet) to a pouch under the female's mouth. Many males first attempt to displace previous suitors' spermatophores by vigorously squirting jets of water into the female's pouch, using the funnel they normally employ for jet propulsion--in effect, hosing out their competitors' sperm. The females lay the spindle-shaped, marble-sized eggs one by one, and each is fertilized by being passed over the pouch of spermatophores under her mouth. The spermatophores that fertilize the eggs are the ones closest to the outside--that is, those from the most recent mating. This explains the practice of "mate guarding": after copulation, the male maintains a close watch over the female as she lays the eggs that have been fertilized by his sperm, and he vigorously tries to fend off the frequent, if not constant, challenges from numerous sources.

Every now and then, however, much smaller males manage--through stealth and disguise--to mate with females. By pulling in their arms and bannerlike webs and taking on a mottled coloration, they achieve a typically female appearance. These small males may even approach females that are guarded by large males. Seemingly cued to the aggressive visual displays of sizable competitors, a large guarding male will readily accept the approaches of an apparent female. Then, when the guard is busy fending off some other big brute, the small "sneaker" male makes his move to mate with the female. The small male's disguise appears to work even during mating; large males have been observed to tolerate copulations between a female they're guarding and a sneaker male dressed up as a female.

During the three-month breeding season, a female can lay several hundred eggs in a number of separate clutches. Before laying them on the undersides of boulders or in crevices away from currents and large predators--she uses her arms to draw the eggs out of their gill cavity and pass them under her mouth to fertilize them. With her head in a crevice and the rest of her body protruding into the open, the female may produce a pair of large Nack eyespots on the back of her mantle: possibly a deterrent to would-be predators. Egg laying is a hazardous business. Each egg is glued to the boulder individually, and eggs are frequently caught on the spines of urchins, which take shelter in large numbers on the undersides of boulders and are among the many predators known to feed on the eggs. When the female S. apama is not laying eggs, she and her guard can be seen close to their boulder, with the male usually hovering a little above his mate. Even then, challengers often attempt to get to the female by using their tentacles to "scoop" the male up and away from her. About five months after being laid, the eggs hatch into flee-swimming, miniature cuttlefish; by then, the adults will have long since dispersed or died.

Recent interest in fishing for Spencer Gulf's giant cuttlefish may threaten the spawning aggregations. For many years, a small number of giant cuttlefish (10 tons' worth a year) were harvested for use as bait, without apparent impact on the spawning population. In 1996, however, a new Asian market for cuttlefish was discovered, and fishing efforts exploded. More than 200 tons were taken in the 1997 season alone. After lobbying by local divers and conservationists, the South Australian state government placed a temporary ban (for 1998) on fishing for these animals in the gulf. Catches at the start of the 1999 season were so low that the fishery was quickly closed to protect remaining stock. In advance of the 2000 season, fisheries scientists released research findings that support permanent closure of the region to fishing. The fate of the giant cuttlefish now lies in the hands of politicians and other decision makers.