The diving community in Melbourne, Australia, was shocked last month to find out the annual spider crab migration in Port Phillip Bay had started early. One diver contacted us to share her experience diving with the spider crabs last month and told us that typically these crabs migrate annually from April to June when the water cools.

As far as why the crabs migrate to the shallow bay, it’s still a bit of a mystery but it seems diver and photographer Chiharu Shimowada has a few ideas. Here’s more information about this incredible encounter in her words:

The most common theory I hear is that the spider crabs come into the shallows to molt and mate. The spider crabs need to molt to grow and get bigger. During the molting process, their shells are soft and take time to harden which makes the molted spider crabs extremely vulnerable to other predators (seals, rays, birds, etc). So they come together as a big group to increase their chances of survival.

I’ve been diving for more than 10 years around Australia and the world, yet this extraordinary event is one of my best diving experiences. A few years ago I witnessed this amazing spider crab migration at Rye Pier in Mornington Peninsula during the month of April. It took my breath away (you are not supposed to stop breathing while scuba diving, but it really did!), I could not believe the amount of crabs marching around — it was like layers of moving crab carpets!

I saw a post on a diving community Facebook page that mentioned the spider crabs were at the Blairgowrie Pier. At first I was not so sure, because it is so early to see the crabs this time of the year. Anyway, we decided to check it out. We went for a dive on Feb. 25 and it was excellent conditions for diving. There was a big group of spider crabs gathering around the Blairgowrie Pier. We certainly enjoyed diving with the spider crabs — watching them marching, hunting an octopus and climbing over each other, and got to witness their fascinating behaviors for quite some time.

We didn’t see many empty shells, so I assume they haven’t started molting yet. One of the dive shop owners told us that they might start molting when the water gets cool around April.

Their annual appearance in the bay is rather irregular; sometimes they stay for a long time or disappear and come back to the bay again at a later date. If you’re interested in seeing the crabs for yourself, I’d recommend keeping an eye on updates from local Melbourne area dive shops.

The best part of this experience is that you don’t even have to be a scuba diver to enjoy it. The crabs stay in very shallow waters (10 to 20 feet depths), so anyone can go and see this amazing event. All you need is a pair of swimming goggles to witness it!

Here are a few tips from Shimowada for anyone who plans to visit the area:

• Check for the wind and water condition
• Look out for boat traffic
• Do not disturb the crabs or any other marine lives by touching them and catching them

Video below was taken by Shimowada and shows the spider crabs feeding on an octopus.