Australian Giant Cuttlefish (Sepia apama)

Author: Dr Robert Browne, Seadragon Foundation Inc.

Prelude: The document is an expanded excerpt for a “Conservation Guide” for the proposed BHP Billiton Desalination Plant at Port Bonython, Upper Spencer Gulf. The evolving “Conservation Guide” is available at (SFI 2007). http://www.seadragonfoundation.org/SFI%20Articles/Articles.htm

The Australian giant cuttlefish (Sepia apama), protected syngnathids, recreational and commercial fish, and unique ecosystems are of significance to the sustainable management of marine biodiversity in the Port Bonython area (SFI 2007). Port Bonython is located on Point Lowly. The Australian giant cuttlefish is a species of special consideration because of their large spawning aggregations at Point Lowly and at other sites in the region (Steer and Hall 2005; ABC 1, 2 provide news stories). The spawning dynamics of the Australian giant cuttlefish at Point Lowly are unsurpassed in sophisticated sexual mimicry (Hanlon et al. 2005). 

The shoreline reefs at Point Lowly and the surrounding areas support large and unique spawning aggregations of the Australian giant cuttlefish (Sepia apama) of high conservation and biological significance. The specific geographical source the aggregating cuttlefish, and their migration routes and means of navigation to the close inshore reefs around Point Lowly are unknown (Steer and Hall 2005). The Australian giant cuttlefish is a very large, widespread and abundant cuttlefish species. Estimates of maximum size vary of mantle length (mantle length is the body only excluding the head and tentacles) but a published value is 520 mm mantle length and a weight of 6.2 kgs (PIRSA 1,2,3., 2007; Steer and Hall 2005). Divers claim to see much larger Australian giant cuttlefish especially in the Solitary Islands, NSW (N. Skinner pers. comm.).

Australian giant cuttlefish range across the whole of southern Australia from northern New South Wales, south to the north coast of Tasmania, and west to Ningaloo, Western Australia. The east coast and southern Australian populations of Australian giant cuttlefish form one relatively continuous population (Kassahn et al. 2004). However, within this population there are 5 discernable groups with the breeding aggregation north of Whyalla representing a separate sub-population from the rest of SA, with the zone of overlap at about Wallaroo (B. Gillanders, pers. com).

Over most of their range, usually only two or three Australian giant cuttlefish are found mating and laying eggs in caves on reefs. However, every winter in the Whyalla area, South Australia, tens of thousands of giant Australian cuttlefish aggregate to spawn over the shallow inshore rocky reefs and artificial rocky structures close inshore in about 3-10 meters depth. The cuttlefish start arriving from Whyalla, and 20 km north east across False Bay to Point Lowly then north all along the coast to Black Point (B. Gillanders pers com), in early May and reach peak numbers by the start of June (Steer and Hall 2005).

Between 4 on average and up to 11 males compete for one female. Females can mate seventeen times a day with two to eight males. Successful fertilization did not differ between males paired or unpaired with females, or depend on size or sneaker males. Sneaker males are males that mate with females while the females are primarily consorted by another male. There are several strategies adopted by sneaker males to approach females with a consort, to approach while the consort is repelling another male, by using hidden stealth for example mating with female hidden under rock, and most unique to these cuttlefish mimicking the appearance and behavior of females (Hanlon et. al. 2005, Hall and Hanlon 2002).

The male cuttlefish mimic females by hiding their obviously specialized male arms, adopting the mottled skin pattern of females, and shape their other arms to look like a female laying eggs. The sneaker males can change from the male to female appearance ten times in five minutes. This strategy is successful about 50% of the time in achieving mating and two out of three times in achieving fertilization (Hanlon et. al. 2005, Hall and Hanlon 2002). Females used all their sperm as well as sperm from previous matings to fertilize eggs, and 70% of clutches have multiple sires. Therefore, the mating system of Australian giant cuttlefish has a high level of multiple mating and multiple paternity and that males of any size or status can obtain successful fertilizations (Naud et al. 2004; Hall and Hanlon 2002; Norman et. al. 1999).

In the Point Lowly area the spawning dynamics of the Australian giant cuttlefish, driven by the high operational sex ratio and the high densities in the aggregation, are unsurpassed at a global scale in sophisticated sexual mimicry. The operational sex ratio is exceptional because of the fluidity of change between one type of sneaking behavior and the sneak behavior of female mimicry (Hanlon et. al. 2005). The elaborate colour and pattern displays of the Australian giant cuttlefish are legendary (Hall and Hanlon 2002) and it is intuitive that the cuttlefish would have exceptional colour vision to match.

Cuttlefish can see clearly. They have a unique arrangement of an invertebrate’s visual system with a vertebrate like one. Cuttlefish have an active iris and a rectangular expanding and contracting pupil. They also change the position of their lens to focus like a camera lens system and this part of their visual system is excellent. However, in spite of the mechanical advantages their vision is restricted by a low number of photoreceptors. The skin colour of some fish groups (Ramachandran et al. 1996) and many cephalopods change in response to visual stimuli (Hanlon and Messenger 1988). Eyes need at least two colour pigments to see colour. However, there is only one color pigment in the eyes of the common cuttlefish (Sepia officinalis) (Bellingham et al. 1998). How cuttlefish mimic chromatically rich environments, found in shallow well lit waters is unexplained (Mathger et al. 2006).

Colour changes in cephalopods are controlled by nerves and muscles and can occur rapidly. Other animals slowly change their body colors mainly through hormones. Cuttlefish can match black and white backgrounds from birth and are good at mimicking contrasting backgrounds and quickly mimicking patterns contrasting in shade of greater than 15% difference in intensity (Mathger et al. 2006). For comparison owls can differentiate 1% contrast (Porciantti et al. 1989) and human 2% (Lythgae 1979, p279). However, backgrounds of different colour such blue and yellow of the same shade are recognized as uniform (Hanlan and Messenger 1996). Cuttlefish can see polarised light like many marine animals. This possibly enables cuttlefish not to be disorientated by flare caused by scatter from above.

After mating females lay about 5-39 eggs a day and 200 eggs in total, individually within protective casings, with the eggs attached to the underside of flat rocks and in tight hard to get at spaces among crevices in reefs (Hanlon et al. 2005). Australian giant cuttlefish appear to use artificial as well as natural substrata for deposition of their eggs. Anecdotal evidence suggests that cuttlefish will fix eggs to other man-made objects, such as corrugated iron thrown into the water in the vicinity (Steer and Hall 2005).

Cuttlefish spawn in the rock walls of the Whyalla Boat Harbour as well as on the more extensive artificial rock walls adjacent to the steelworks. The eggs hatch within three to five months, depending on the water temperature, with hatchlings looking similar to their adult form.  Hatchlings appear in early September (PIRSA 1). After spawning most adult cuttlefish then mysteriously disappear again by the end of August (PIRSA 2).

Australian giant cuttlefish are solitary animals when not breeding. Daytime activity cycles showed individualistic behavior. Australian giant cuttlefish live in dens, or hover under rocks and emerge during daylight for short food excursions. The cuttlefish enhances this conservative lifestyle with efficient foraging while exposure to predation is minimized (Aitkin et al. 2000).

A straight line between the One Steel Jetty at Whyalla to the east at Point Lowly encloses False Bay (a closure area for cuttlefish fishing) where tens of thousands of cuttlefish aggregate to mate and spawn. The closure area includes all waters enclosed by a line from the light house at Point Lowly to the southern end of the Port Bonython jetty, then in a south westerly direction to the eastern most point of the One Steel jetty, position latitude 33º 02” 12.63’ south and longitude 137º 36” 1.98’ east, near Whyalla, then to the high water mark at the base of the jetty, then following the high water mark along the shoreline in an easterly direction back to the point of commencement (Map grid GDA94). Australian giant cuttlefish also spawn at Black Point northward from Point Lowly across Fitzgerald Bay (Steer and Hall 2005).

In the mid-1990s, an overseas market was discovered for the Australian giant cuttlefish and export licenses were granted. In 1997, 26 boats took 262,000 kg: that is approximately 60,000 cuttlefish over a full breeding season and cuttlefish numbers dropped dramatically. The boats and commercial fishers were black with cuttlefish ink. Thousands of dollars a day could be made by harvesting the aggregated spawning cuttlefish. The Australian giant cuttlefish fishery in the area has since been regulated and a closure created in False Bay. Australian giant cuttlefish are also caught by recreational fishers in the Whyalla (Steer and Hall 2005).

Upper Spencer Gulf is home to an expanding marine-based ecotourism industry. The unique spawning aggregations of the charismatic Australian giant cuttlefish were devastated by excessive fishing. With greater protection from fishing the cuttlefish are now recovering. However, the minimum biological and practical requirement for eco-tourism should be recovery of the spawning aggregation to pre-fishery levels. A boat based and shore based diving industry exists in the area dependent on the Australian giant cuttlefish. At Point Lowly platforms are in place for divers to walk down to the water. From the platform, divers cover about twenty meters over flat rock and then swim into six meters of water. Water temperatures during the spawning season can be as cold as twelve degrees centigrade.

Suggestions from the public of how ecotourism could contribute to Australian giant cuttlefish conservation are; the establishment of spawning reefs including rock walls and breakwaters, monitoring of sites, and provision of exclusion zones to fishing for around all Australian giant cuttlefish aggregation reefs in the region. There are unprotected spawning aggregations to the north of Point Lowly. Artificial spawning sites need depth as Australian giant cuttlefish normally spawn in 3 -10 m of water (Steer and Hall 2005).

Australian giant cuttlefish spawn in the rock walls of the Whyalla Boat Harbour as well as the more extensive artificial rock walls adjacent to the steelworks. There appears to be potential to establish additional artificial habitat south of Whyalla (Steer and Hall 2005).  

References:

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