Spider Crabs Melbourne

Sheree Marris freediving with the Giant Spider Crabs
Sheree Marris freediving with the Giant Spider Crabs. Photo courtesy of Sheree Marris.

Every year around May through to July one of the world's most interesting congregations of marine life happens as Giant Spider Crabs come together in the shallow waters off of the piers and beaches of Port Phillip. The Giant Spider Crabs (Leptomithrax gaimardii) are often sighted as they gather en masse on the sandy bottom of Port Phillip Bay, near Blairgowrie Pier and Rye Pier on the Mornington Peninsula.


Video by The Nature of Science.

When the Spider Crabs aggregate and march, this hungry army is easy to spot. They scavenge whatever food they can find, including the wildlife on the shells of one another. The spectacle of hundreds of large orange crabs against the bare, sandy sea floor is an amazing sight.

While scuba divers can stay longer on the bottom and get closer to the Spider Crabs, you don't have to be a scuba diver to see them. You can be a freediver or snorkeller using a mask, snorkel and fins, though as the water is somewhat cooler than you'd be used to in summer, you might want to have a wetsuit on as well. You can even see them from the surface by grabbing a pair of swimming goggles. When the Spider Crabs carpet the seabed around and under the local piers, they are so readily accessible that even those walking on the pier or beach nearby can appreciate this special event.

If you don't have snorkelling or scuba gear, please visit our dive shop in Rye to buy the equipment you need. If you're a certified scuba diver you can hire scuba gear from us.

For the latest information on Spider Crab sightings, please follow the Facebook group Spider Crabs Melbourne.

Spider Crabs Gathering to Moult

It's still a bit of a mystery as to what the aggregations are all about. We know that during the major gathering in May through to July they moult, but there may be other things they get up to as well.

All hard shelled marine crustaceans like the Giant Spider Crabs have to moult periodically. They outgrow their protective suit of armour, and then go through a process of shedding it so that they can grow bigger before their new skin hardens. The process of moulting can take up to an hour and all the crabs in an aggregation moult almost simultaneously.


Sped up version. Real event took 16 minutes. Video by Brett Illingworth.

Once they're out of their old hard shell the Giant Spider Crabs are vulnerable to lots of different predators like seals and rays, dolphins and birds. While their new shell hardens, most species of crabs hide in a rocky crevice or similar suitable sheltering place.

Our Giant Spider Crabs obviously choose a different 'safety in numbers' approach. Their gatherings often contain thousands of individual Spider Crabs, which in some places are heaped metres in height and almost out of the water in the shallows. They cover vast areas of the seabed in a mind blowing Spider Crab carpet.


Spider Crab mound. Video by PT Hirschfield (Pink Tank Scuba).

The gatherings can be thought of as an enormous schoolies party as most of the congregating Spider Crabs are undergoing their puberty moult that brings them into adulthood and sexual maturity. The females of many crab species are only sexually receptive when soft shelled just after mating. Thus it's tempting to think that a lot of mating activity goes on in the aggregations. However, this may not be the case with our Giant Spider Crabs. It's rare to observe them mating after moulting, thus they probably mate at other times once they've moved back into deeper waters.

When the Giant Spider Crabs Gather

The moulting process is probably determined by some biological cues, plus some environmental cues as well. What they are is yet to be determined.

What we do know is that the mass gatherings start to happen around May. At some point thereafter something triggers the moulting process. Typically it's all over by early July and the Spider Crabs move off into deeper waters.

Spider Crab Activities to Look For

While the gatherings of tens of thousands of Giant Spider Crabs is a spectacular thing to witness, there are plenty of more specific things to look out for.

The Moulting Process — Taking the time underwater to stop and watch a Spider Crab go through the full moulting process is amazing.


Timelapse Footage of Moulting Giant Spider Crab. Video by PT Hirschfield (Pink Tank Scuba)

Feeding Frenzy — When there are tens of thousands of Spider Crabs gathered en-masse in a location, it is natural that their predators will show up. Seals and rays, dolphins and birds are commonly seen feeding on the Giant Spider Crabs. This sudden influx of tender crab meat is probably an important part of the Port Phillip food chain.


Stingrays feeding on Spider Crabs in the shallows adjacent to Rye Pier. Video by Brett Illingworth.

Spider Crab Redecoration — Spider Crabs attach camouflaging material to their carapace to avoid being easily seen by predators. Plus algae, sponges and sea squirts set up shop on their shells helping to provide excellent camouflage. Many divers have been shocked to see what they thought was a stationery piece of seabed rise up and wander off! After moulting, all the camouflaging has to be redone on the new armour so there is plenty of this activity on show.

Essential Info When Wanting to See the Spider Crabs:

  1. PLEASE READ and FOLLOW EVERYTHING: This stuff is here for your safety and the safety of others. RE-READ it before you go looking so it's fresh in your mind.
  2. AT YOUR OWN RISK: NO animal sighting is worth dying for! We do NOT endorse or recommend ANYONE entering the water to look for or see Spider Crabs. We do not take any responsibility for anyone who chooses to do so. If you ever enter the ocean for any reason, that is entirely your own personal choice and at your own personal risk. You are entirely responsible for your own safety in the water. You must ONLY enter the water based on your level of training and experience (e.g. swimming, snorkelling, free diving, scuba diving). Often the crabs can be seen in the shallows, from the shore and from jetties.
  3. SAFETY FIRST: If you choose to enter the water there are MANY hazards and some of these may result in injury or death if not navigated properly. These include but are not limited to:
    NATURAL CONDITIONS (e.g. wind, rain, tides, visibility, currents, depth, water quality) all play a role and pose potential hazards. Sometimes the water is too polluted to swim safely, and you can check on that here: Yarra and Bay Beach Reports.
    BOAT TRAFFIC and jet skis on the surface can be deadly to careless swimmers, snorkelers and divers. Swimming, diving or snorkelling directly UNDER piers (but NOT in restricted areas) will keep you safer from boat traffic.
    DIVERS: Use a dive flag for your own safety. Stick to authorised dive zones and don't 'pop up' to figure out where you or your buddies are. If you can't navigate yourself safely underwater, don't get in the water and create hazards and life threatening situations for others and yourself. Fishing hooks and squid jigs can also cause injuries to those in the water.
    GEOGRAPHY: Familiarise yourself with the specific hazards, no-go zones (e.g. marina off-limit zones) etc of any specific locations where you are entering the water as these change from one site to the next.
    OTHER MARINE LIFE (e.g. sharks, blue ringed octopuses and stingrays) may also be present and may pose threat and harm if they are feeding or perceive themselves to be threatened.
  4. SHOW RESPECT: Spider Crabs are considered inedible and are not to be touched or manhandled. If you are privileged enough to see them, hands off and do not harass them in any way. That is, NO picking them up. NO catching them. Love, Respect, Protect, Kapish?
  5. CARE FOR THE UNDERWATER ENVIRONMENT: Don't be careless at the beach and in the underwater environment. You are entering someone else's natural habitat. Do not disturb or damage it in any way.
  6. ON THE MARCH: Spider Crabs are typically 'on the march' in January through March and can be seen briefly in large clusters that appear and disappear quite quickly (i.e. they can totally 'disappear' in a matter of minutes from where they were last seen). This can result in many 'false alarms' of 'They're here!' Keep an eye out for announcements on the Spider Crabs Melbourne Facebook page that they have settled to increase your chances of being able to see them.
  7. ONCE THEY SETTLE: Once the Spider Crabs have 'settled' for their mass moulting aggregation, they may be easier to access if they are in a safe and convenient location. Often they will be present for 1 to 2 weeks during which they can be most easily found. However, the crabs may settle in a place that is either unsafe, not accessible, or in a no-go zone, in which case you will not be able to access them. Keep an eye on the Spider Crabs Melbourne Facebook page for updates. You may witness behaviours such as crabs moulting and predators such as stingrays, octopuses and cannibal crabs feeding on them.
  8. SHARE WHAT YOU'VE SEEN: The Spider Crabs Melbourne Facebook page is designed to help gather observations about the movements and behaviours of the Spider Crabs. Please upload specifically WHERE and WHEN you've seen any groups of more than 30 crabs, along with any photos or videos showing numbers and behaviours that you've observed.

Common Sense and Safety First please, Spidey Fans.

Giant Spider Crab Field Guide

Name:Giant Spider Crab, Leptomithrax gaimardii (Milne Edwards, 1834)

Other Names: — Great Spider Crab

Brief description: — Carapace round, seaweed attached to body.

Description: — Carapace rounded and red-brown. Seaweed sometimes attached to body. Up to 16 cm wide (carapace).

Biology: — Giant Spider Crabs form large groups when breeding, often more than one hundred crabs to a group.

Habitat: — Seaweed, reef and sand areas, to depth of 820 m. Reefs. Soft substrates

Native status: — Native to Australia

Max size: — 16 cm

Diet: — Carnivore

Colours: — red brown

Distribution: — Southern Australia

Habitat types: — Marine

Depth: — Shallow (1-30 m), Deep (>30 m)

Water column: — On or near the sea floor

Harmful: — Not harmful but a nip from large claws could be painful

Acknowledgments

Many thanks to PT Hirschfield (Pink Tank Scuba), Sheree Marris, Brett Illingworth and others for the information you see on this page.

Taylor, J. & Poore, G.C.B., 2011, Giant Spider Crab, Leptomithrax gaimardii, in Taxonomic Toolkit for marine life of Port Phillip Bay, Museum Victoria, accessed 17 Feb 2017, http://portphillipmarinelife.net.au:8098/species/3976

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