Every autumn and winter, something extraordinary happens in Port Phillip Bay. Hundreds of thousands of giant spider crabs march in from deeper water, pile onto pier pylons and across the sandy seabed, and form the largest known crab aggregation on the planet. It is visible from pier decks, accessible to snorkellers, and for scuba divers it ranks among Melbourne's most unforgettable underwater spectacles.
The Giant Spider Crab
The giant spider crab (Leptomithrax gaimardii) is Victoria's largest spider crab species. Adults have a rounded, red-brown carapace up to 16 centimetres wide and a leg span that can reach 70 centimetres. For most of the year they are solitary and remarkably well camouflaged — algae, sponges, and sea squirts colonise their shells, making them nearly invisible against the reef. They are scavengers, feeding on a range of organic material on or near the seafloor, and are found at depths from the shallows down to over 800 metres.
Outside the aggregation season, you could dive a hundred times at a pier site and never notice one. That changes dramatically when the migration begins.
Why They Aggregate
The spider crab aggregation is driven by moulting — the process of shedding an old exoskeleton to allow the animal to grow. Like all crustaceans, spider crabs are encased in a rigid shell that cannot expand. To grow, they must periodically extract themselves entirely from their old shell and produce a new, larger one. The moulting process takes roughly 30 minutes to an hour per crab, during which the animal is soft, defenceless, and extremely vulnerable to predation.
A freshly moulted spider crab, with its soft new shell still hardening over the following 24 to 48 hours, is an easy meal for smooth stingrays, seals, dolphins, and seabirds. The solution is safety in numbers. By aggregating in the tens or hundreds of thousands, each individual crab dramatically reduces its chance of being the one that gets eaten — the same dilution principle that drives schooling fish and herding mammals.
Scientists believe the aggregation primarily represents a "puberty moult" — the moult that brings juvenile crabs into adulthood and sexual maturity. An earlier theory that the gathering was mainly for mating has been largely discounted; following mass moulting events, only occasional mating pairs have been observed. The crabs moult almost simultaneously within the aggregation, littering the seabed with thousands of ghostly empty shells.
When to See Them
The aggregation typically occurs from late May through mid-June, though the broader window extends from March to July. For a complete overview of what to expect underwater throughout the year, see our Melbourne diving seasonal calendar. The peak — when tens of thousands of crabs are piled high on pylons and carpeting the seabed — usually lasts only a few weeks at a given site.
The exact timing is genuinely unpredictable from year to year. It appears to be linked to the full moon cycle in May or June, but there is no reliable way to predict the precise week. The crabs walk in from deeper waters of Bass Strait and outer Port Phillip Bay, and they choose their own schedule.
The practical reality: the only way to know when the crabs have arrived is to monitor reports from other divers. The Spider Crabs Melbourne Facebook group is the essential resource — divers and snorkellers post real-time sighting updates from mid-May onwards. When the first photos appear showing carpets of crabs at a pier site, you have days to weeks before they disperse back to deeper water. Be ready to go at short notice.
Where to Find Them
The aggregation occurs along the southern Mornington Peninsula coast of Port Phillip Bay, in shallow water near pier structures. The crabs favour sandy bottom areas in 3 to 5 metres of water — exactly the kind of easy, sheltered sites that make Melbourne's pier diving so accessible.
Blairgowrie Pier
The most consistently reported location for spider crab aggregations. Blairgowrie's sheltered marina and pier offer calm conditions and easy shore entry. The crabs gather on the sandy bottom and climb the pier pylons, sometimes stacking six or seven deep. This is the site most frequently featured in news coverage and photography.
Rye Pier
The other primary aggregation site, with easy access and facilities. Rye Pier offers slightly more protection from prevailing southerly winds than some other sites. The sandy bottom and shallow depths (4-6 metres) make it an excellent spot for both scuba divers and snorkellers.
Other Sites
Aggregations have been observed at Sorrento, St Leonards on the Bellarine Peninsula, and Queenscliff. The crabs do not always choose the same site each year — the location shifts, and the reasons for site selection remain scientifically unknown. This unpredictability is part of what makes the event so compelling: only when the full moon approaches will divers know whether to head to Rye, Blairgowrie, St Leonards, or Queenscliff.
What You'll See Underwater
Nothing quite prepares you for the scale of a spider crab aggregation. The seabed, which on a normal dive would be a relatively featureless sandy bottom with scattered marine life, is completely covered by a moving, climbing, interlocking mass of crabs. They carpet the sand as far as visibility allows. They stack on top of each other on pier pylons, climbing over sponges and each other until the pylons appear to be made entirely of crabs. In some years, the piles extend almost to the surface in the shallows.
From above — looking down from the pier or while snorkelling — the aggregation appears as a vast dark mass on the seafloor, sometimes described as "big as a cricket field." Below the surface, you are surrounded. Crabs move slowly but deliberately, their long legs picking careful paths over and around each other. The collective rustling of hundreds of thousands of legs on sand and pylons creates a faint, eerie sound that experienced divers describe as one of the most unusual things they have heard underwater.
If your timing is right, you may witness the moulting process itself — a crab arching its back, splitting its old shell along a seam at the rear, and slowly extracting itself leg by leg from its former exoskeleton. The freshly moulted crab is pale and soft, noticeably different from the hard, encrusted shells around it. Discarded moult shells litter the seabed like ghost crabs, and it can take a moment to distinguish the empty shells from living animals.
Predators are drawn to the aggregation. Smooth stingrays cruise the perimeter, picking off vulnerable individuals. Seals may appear, creating dramatic interactions. Above the surface, seabirds gather on the pier and surrounding rocks, waiting for crabs in the shallows.
You Don't Need Scuba Gear
One of the most remarkable things about the spider crab aggregation is how accessible it is. Because it occurs in only 3 to 5 metres of water — often shallower — you do not need to be a certified diver to witness it.
- Snorkelling: A mask, snorkel, and fins are all you need. The crabs are clearly visible from the surface, and freedivers can descend for a closer look. Several operators offer guided snorkel experiences during the season.
- From the pier: In many years, the aggregation is visible as a large dark mass from the pier deck. Bring a pair of polarised sunglasses to cut surface glare and you can watch the spectacle without getting wet.
- Swimming goggles: Some visitors have reported seeing the crabs with nothing more than swimming goggles in clear, calm conditions.
Scuba diving does give you the advantage of extended bottom time and the ability to get right among the crabs at their level, but this is genuinely one of Melbourne's great marine spectacles for anyone — diver or not.
Planning Your Visit
Gear
This is a winter dive in water temperatures around 11-13°C. A 7mm wetsuit with hood and gloves is the minimum; many local divers wear drysuits for comfort during what can be long, slow dives spent observing behaviour rather than covering distance. For detailed advice on choosing the right exposure suit and accessories for Melbourne's cold water, see our cold water diving gear guide. A torch is useful for illuminating the deeper crab piles under the pier and revealing colour in the dim winter light.
Timing Your Dive
Start monitoring the Spider Crabs Melbourne Facebook group from mid-May. When sightings begin, plan to visit within the first week if possible — the peak at any given site can be brief. Weekday dives are significantly less crowded than weekends; once the aggregation makes the news (which it invariably does), popular pier sites become very busy with divers, snorkellers, photographers, and curious onlookers.
Conditions
Check wind and swell conditions before heading out. Calm days provide the best visibility and most comfortable diving. Rye Pier is more sheltered from southerly winds. Use the Shore Dives scoring tool to compare conditions at Rye, Blairgowrie, and other bay sites on the day.
Etiquette
The aggregation attracts crowds. Be considerate of other divers and snorkellers, especially at entry and exit points. Avoid standing on the crabs in the shallows — they extend right to the waterline in some areas. Maintain good buoyancy to avoid crushing crabs or stirring up silt that ruins visibility for everyone.
Photography Tips
The spider crab aggregation is one of the most photographed marine events in Australia, and for good reason — but it presents unique challenges.
Go wide. A fisheye or wide-angle lens is essential to capture the scale. The crabs stretch in every direction, and a macro or standard lens cannot convey the spectacle. A fisheye in the 10-17mm range (full-frame equivalent) is ideal.
Strobe positioning matters. Thousands of moving crabs stir up silt and sand, creating significant backscatter. Push your strobes up above the camera on extended arms — the "rabbit-ear" position — to top-light the scene. This produces even illumination with gradual fall-off into the background, rather than the harsh spotlight effect of side-mounted strobes.
Shoot from above. A higher angle looking straight down on the crab carpet conveys the density and scale most effectively. Lower angles turn the scene into an indistinguishable tangle of legs. For variety, look for crabs climbing pylons with colourful sponge gardens — the contrast of red sponges, blue water, and massed crabs makes for striking images.
Seek individual stories. Beyond the wide establishing shots, look for a crab in the process of moulting, a stingray hunting at the edge of the aggregation, or a lone crab climbing a pylon above the mass. These individual moments give your images narrative alongside the spectacle.
Plan your shots before descending. Water temperature is around 12°C, which limits comfortable bottom time even in a drysuit. Know what you want to shoot before you get in.
Conservation
Despite being one of the world's most spectacular marine wildlife events, the spider crab aggregation faces conservation challenges. Giant spider crabs can currently be legally harvested in Victoria with a combined bag limit of 15 per person per day, no size limit, and no closed season specific to the aggregation.
During the aggregation, when hundreds of thousands of crabs are concentrated in a small, shallow area and many are in their most vulnerable soft-shell state, harvesting pressure intensifies. Crab pots, hoop nets, and hand collection in and around pier sites create conflicts with divers and snorkellers, and fishing gear can damage the marine growth on pier pylons that supports other species including seahorses.
The Victorian National Parks Association has led a prominent campaign calling for a seasonal no-take period during the moulting aggregation. The campaign has broad community and business support. As of early 2026, a formal seasonal closure has not been implemented, though the issue continues to receive regulatory attention.
As divers, we can contribute by respecting the crabs — not handling or standing on them — and by reporting sightings to citizen science programs like the Spider Crab Watch initiative run by Deakin University through iNaturalist. The more data scientists collect about aggregation timing, location, and size, the stronger the evidence base for future conservation measures.
A World-Class Event in Melbourne's Backyard
The spider crab aggregation is one of those rare wildlife events that combines genuine scientific wonder with easy accessibility. You do not need a boat, expensive equipment, or advanced diving qualifications. You need a wetsuit, a willingness to dive in cold water, and the flexibility to go when the crabs arrive.
For Melbourne divers, it is the highlight of the winter diving calendar — proof that the colder months bring their own rewards. Start watching the Spider Crabs Melbourne Facebook group from mid-May, keep your gear ready, and when the first reports come in, do not hesitate. The crabs wait for no one.
Check current conditions at spider crab sites using the Shore Dives scoring tool, or explore Boat Dives for charter trips that may visit aggregation sites.