Marine life
Giant Spider Crab
Leptomithrax gaimardii
Tens of thousands of crabs piling into the shallows of Port Phillip Bay to moult, in one of Victoria's strangest dives.
Each year, giant spider crabs gather in Port Phillip Bay in enormous numbers — piles of them, sometimes several deep across the sand — to moult their shells in relative safety.
The aggregation is one of Victoria's most talked-about dives and one of the strangest sights in Australian diving: a moving carpet of crabs under a suburban pier in a few metres of water.
Safety in numbers is the logic. A crab that has just moulted is soft and defenceless for days, and being one of fifty thousand is better odds than being alone.
| Scientific name | Leptomithrax gaimardii |
|---|---|
| Typical size | Around 15 cm carapace, longer leg span |
| Peak months | May, June |
| Documented sites | 3 |
| States | Victoria |
- spindly legs with a compact, often weed-encrusted carapace
- usually reddish-brown, frequently overgrown with algae
- found in dense piles rather than singly during the aggregation
- Aggregates to moult, gaining protection through sheer numbers.
- Discarded shells litter the sand after a moult event.
- Attracts predators — rays and occasionally seals work the edges of the aggregation.
In-water etiquette
How divers behave around this species affects both the animal and everyone else's encounter.
- Do not pick up or move crabs, particularly soft freshly moulted ones.
- Keep fins clear; the aggregation is on sand and silts easily.
- Watch where you kneel.
Where to see them
Sites where this species is documented as a reliable encounter.
Often seen on the same dives
Frequently asked questions
When do the spider crabs aggregate in Port Phillip Bay?
Usually late autumn to early winter, most often May to July. The exact timing varies year to year and is not perfectly predictable.
Where is the best place to see them?
Blairgowrie and Rye piers are the best-known sites, both shallow shore dives.
Last researched: September 2026. Dive businesses change frequently — confirm current services, prices and requirements directly with the operator before booking.