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of the Blairgowrie Yacht Squadron Pier,
a giant spider crab tumbles to the sea floor
where it will be at risk from predators.
spider crabs
most extraordinary natural
spectacles, a writhing mass of
spindly claws and legs rises from
the depths of Bass Strait into the
sheltered waters of Port Phillip
Bay, Melbourne.
Spider crabs converge like this at several locations around Australia – including in South Australia and Tasmania – but no aggregation is as predictable or easily accessible as that in Port Phillip Bay. The timing of this great assembly usually occurs between late May and June, and relates to lunar cycles when the water temperature is a fresh 11°C to 15°C.
Sprawling across 1930sq.km with a depth of up to 24m, Port Phillip Bay contains some of Victoria’s most important types of marine environments. Vast areas of undulating sand, thriving seagrass meadows and distinct areas of rocky reef create complex habitats that are home to a huge diversity of marine wildlife, including weedy sea dragons, brightly coloured nudibranchs, pot-bellied seahorses and giant cuttlefish. During its annual pilgrimage, the armoured brigade of giant spider crabs moves with purpose through these habitats scavenging what food it can, often leaving barren surfaces in its wake.
It commands attention as it enters the coastal waters fringing our second largest city. And word spreads through Melbourne’s scuba diving community as the aggregation arrives at easily accessible shore dives around Blairgowrie Yacht Squadron Pier and Rye Pier where the crabs seek shelter by climbing the jetty’s pylons. During the few weeks the aggregation is accessible, the spectacle is enjoyed by a range of curious onlookers, including filmmakers and photographers who travel from around Australia (and the world) to catch a glimpse of the crabs made famous by their starring role in high-profile documentaries, including Sir David Attenborough’s recent Blue Planet II series. Scuba divers have the benefit of being able to get closer to and linger longer with the crabs. But it is possible to see them from the water’s surface with only a wetsuit, mask and snorkel. They can also be seen in the water below the jetty at Rye Pier, or from the nearby beach when they make their way into the shallows.
The reason for the mass gathering is a curious biological event that benefits from the safety of large numbers. To grow, a crustacean must moult its tough external skeleton (its exoskeleton) throughout its life. As the time for a moult nears, a new exoskeleton forms beneath the old one. Once this is in place, the old one is discarded and the calcium absorbed from it is shifted to harden the new one. Moulting for the giant spider crab takes up to an hour.
hardens enough to be an effective deterrent to predators.
disperse back into deep water remains a mystery.
An early explanation that the annual aggregation was related to reproduction seems unlikely, because mating has only been observed occasionally among the thousands of moulting crabs. Once the crabs begin descending back into the depths, empty shells scatter the sea floor as a reminder of the recent spectacle. Exactly what happens when the crabs disperse back into deep water, however, remains a mystery.
DURING THE MARCH last year, a new threat to the natural event in Port Phillip Bay emerged. An unprecedented number of crab fishers flocked to Blairgowrie, deploying baited crab traps from the pier there.
Under Victorian fisheries legislation, it is illegal to catch more than “30 crabs or 1 litre of whole or parts of crabs” outside protected areas. A fisher can catch them in water deeper than 2m, by hand, spear or by using up to two bait traps or up to two hoop nets. But last year community observers of the aggregation raised concerns that restrictions were being flouted after people were seen swimming to collect freshly moulted crabs and loading them up in traps. With trap-loads of soft-shelled crab being hauled from the bay, fears were raised about the sustainability of this practice and its potential impact on future aggregations. In response, protesters began cutting nets and confronting fishers, calling for authorities to increase patrols and compliance activities.
The Spider Crab Alliance was formed to highlight community concerns and provide a platform for sharing these with governing bodies. Signatures on a petition calling for a halt to spider crab fishing during the migration and to lower bag limits have since continued to grow. The aim of the petition is to achieve change in a respectful and constructive manner for the best outcome possible for the crabs and for the thousands of local, national and global enthusiasts who travel from around the world to witness the annual event.
If the petition is successful, the alliance will ask the Victorian government to increase research activities into giant spider crab ecology, place a seasonal ban on collection at aggregation sites, reduce recreational bag limits, and increase compliance and education activities. With those measures in place, it’s hoped the aggregation can continue for future generations to witness.
FOR MORE INFO: facebook.com/spidercraballiance
Beauty on the brink
IT’S JUST AFTER lunchtime, the morning rains have finally cleared and the sun begins breaking through the clouds. Max Breckenridge, a captive release project officer with BirdLife Australia (BLA) is slowly walking along a dirt road that cuts through a patch of untouched spotted gum–ironbark forest in the Lower Hunter Valley in New South Wales.
Our new conservation superpower
There were expectations it would reveal much about the evolution of this continent’s largely unique mammal fauna, and it has. But to most scientists working in Australian wildlife conservation, it didn’t seem to have a lot of practical relevance.
Lifeblood of the nation
Below the modern Brewarrina Weir in north-western New South Wales, a 2km bend holds the remains of this ancient structure, a complex of dry-stone walls and a site of engineering brilliance. Displaying advanced knowledge of river hydrology and fish ecology, the traps were designed to catch Murray cod, golden and silver perch and other fish, but allow breeding stock to pass through.
When a city rises
THAT FEBRUARY 2011 earthquake, which struck during the city’s lunch hour, was the most destructive in a series. It was the one that broke so many of the Central Business District’s verticals and horizontals – its buildings and the roads, sewers, water and gas pipes. It was shallow and ferocious. Its peak vertical ground acceleration of 2.2G (more than twice the acceleration of gravity) momentarily lifted parts of Christchurch to the sort of face-distorting speeds astronauts experience when they ascend into space.
Grantham’s road to recovery
IT WAS 10 JANUARY 2011 and once again it was pouring with rain. Queensland had already endured its wettest spring and December on record, and yet still the rain kept coming. I was working at the time as a freelance photographer for Queensland’s main newspaper, The Courier Mail. And weary of capturing “wet weather photos”, I’d decided to stay dry inside my Toowoomba office.