Here’s a crab bag of interesting facts about spider crabs.  We’ve compiled answers to questions you always wanted to know and more.

Thanks to Dr Gary Poore (Museums Victoria) and our Marine Campaigner Shannon Hurley learn how and why moulting plays a crucial role in crab reproduction, along with other information that is just plain fascinating.

Crabs are amazing. Crabby encounters etch themselves into our memory – especially the annual migration of thousands of spider crabs in Port Phillip Bay.

You can take action to give our spider crabs safe harbour in moulting season.  We can protect our crabs and make sure this natural spectacle will be enjoyed for generations to come. 

Where do the Spider crabs that come to moult in Port Phillip Bay in winter migrate from? How do they know to go to shallow water to moult and why do they do this?

Much of a spider crabs life remains a mystery! We know there are spider crabs throughout Port Phillip Bay, and along other parts of Victoria’s coast, but we don’t yet know where exactly they come from.

It is believed they march into the shallows for safety in numbers while they undertake their moult, which is a very vulnerable part of a crabs life.

Crowding in large numbers means they are less likely to be picked off by predators such as rays (but crowding in the shallows also means like are more likely to be taken for human consumption).

Science tells us that some spider crabs also breed, between their moults, when soft shelled, (and this is the only time they can do this), but there are unknowns about what is the case for the spider crabs in Port Phillip Bay.

What does the tagging by the Victorian Fisheries Authority entail?

The Victorian Fisheries Authority (VFA) has tagged 15 spider crabs with satellite tags which are designed to be released from the spider crabs, once a week until all 15 have been released. This can track the location of spider crabs movements, and communicate information about the water conditions.

Further science is needed to ascertain estimates of population numbers of the crabs and to understand more about their life cycle.

What do we need to know to list a crab species as threatened?

Information such as estimates of population numbers, the number of mature individuals, the distribution or range of where the crabs are found, how much the population has declined by and over how many years, or if the population is fragmented.

The Flora and Fauna Guarantee Act 1988 (FFG Act) is the key piece of Victorian legislation for the conservation of threatened species and communities, and for the management of potentially threatening processes.

The State Government is currently going through the process of ensuring that the process and criteria for listing threatened species is consistent with federal legislation and other states.

More information:

Who is responsible for protecting the spider crabs? What are the fishing regulations?

There are a number of key players involved with responsibilities for protecting the spider crabs.

The marine environment including marine life, is managed under the guidance of the Marine and Coastal Act 2018, and subsequent Marine and Coastal Policy 2020. The Environment Minister ultimately has carriage of this act, along with the Department of Environment, Land, Water and Planning. All government departments and land managers with responsibilities for managing the marine and coastal environment need to adhere to this legislation.

The Victorian Fisheries Authority (VFA) is responsible for managing fishing, and in this case, the harvesting of spider crabs. Current fishing regulations allow for up to 30 crabs to be harvested per person, per day, however there is a proposal to reduce this bag limit to 15. The VFA along with the Minister responsible for Fishing and Boating, Melissa Horne has ultimate responsibility for managing harvesting and associated behaviour from the act of fishing. Other agencies however have a stake in the game with marine litter management sitting with the Environment Protection Authority (EPA), and management of the piers and jetties sits with Parks Victoria.

Addressing the issues experienced the past two seasons – environmental destruction, litter and pollution, and public safety concerns multiple agencies with responsibilities for public health and safety, and the protection of marine life sits across multiple agencies. This includes the Environment Protection Authority (EPA) responsible for marine litter, and Parks Victoria who are in charge of managing activities on many of the piers and jetties.

In regards to implementing a no-take season to protect the spider crabs, this decision ultimately sits with the VFA and the Minister for Fishing and Boating. But other key players mentioned above have an important role to play.

It is important to recognise that the crabs have value in their own right, and not just for the harvesting of them.

Could you please explain how moulting is crucial to successful mating of crabs? Is this the same for all crabs? Can you please recommend journal articles?

The sexes are separate in crabs. Males can be recognised by having a narrow  triangular ‘tail’ or abdomen and females having a wide rounded abdomen.  Males crabs produce sperm that is ejected from a pair of pores in the first segment of the last (fifth) pair of legs. Female produce eggs that are laid from a pair of pores on the first segment of the third pair of legs.

Spider crabs do not moult after becoming sexually mature so may mate when ‘hard shelled’, but it is uncertain if this is the case in the spider crabs in Port Phillip Bay. Some other species of spider crabs mate only between moults when ‘soft shelled’. Males guard or stand over females before mating, in some species for several days.

Various strategies are employed by crabs to transfer sperm from the male to the eggs in the female but all involve mating between two individuals ‘face to face’. In spider crabs a small mass of sperm is produced from each pore on the last male leg, enters a channel on the first limb of  the abdomen, pushed along this channel by the second abdominal limb, and put inside the female pore. All this happens on the individuals’ left and right sides.

Fertilisation of the eggs occurs as the eggs leave the female’s pores. The fertilised eggs are then attached by glue to hairs on the female’s abdominal limbs. Spider crabs carry many hundreds of developing embryos and are said to be ‘in berry’.

The embryos are carried by the female for several days, then hatch as larvae called a ‘zoea’ which looks not a bit like a crab. The zoea moults every few days while swimming in the plankton. Its final moult is to a ‘megalopa’, a stage that looks more like a crab and is able to settle on the seafloor. The larvae of the spider crab in Port Phillip Bay has never been studied and the megalopa never seen so little is known about how long this takes or how big the settling crab is.

This review is simple but may be hard to find. Others in the scientific literature are extremely detailed. There are several articles on the web with some detail.

  • Hartnoll RG (1969) Mating in the Brachyura. Crustaceana  16, 161-181.
What species of crabs should we keep an eye out for and what are there distinguishing features?

The Australian Government through its National Introduced Marine Pest Information System lists three species of crabs known to be invasive but not yet recorded from Australia.

Refer you to details at these sites.

How does ocean warming threaten our southern crabs?

We don’t know. Warmer temperatures along the southeastern Australian coast have been documented. Warmer water allows species confined to NSW to extend their range south to Tasmania. A notable example is the expanding impact of grazing sea-urchins now feeding on kelp forests in Tasmania.

Leptomithrax gaimardii favours cool water in bays and on the shelf but its upper temperature tolerance is not known. It migrates to shallow water during its breeding season only in winter. It is plausible that a persistent higher summer temperature in Port Phillip Bay would be deleterious to populations of spider crabs.

Are any crabs poisonous?

Poisonous yes, venomous no.

Some crabs are poisonous to eat and many fatalities have been reported from the tropical Pacific. With few exceptions, poisonous crabs are brightly coloured members of the family Xanthidae confined to shallow water and associated with coral reefs. They are distributed throughout the Indo-west Pacific from the Red Sea to Tahiti, including tropical Australia. The toxins involved are related to paralytic shellfish toxins. Eating poisonous crabs causes numbness, nausea and vomiting and death after few hours.

Xanthid crabs are found in Victoria but are small and relatively uncommon. I would not eat one.

Crabs can use their strong claws to nip unwary fingers. While this can be painful crabs do not sting like wasps or bees.

Some people have an allergic reaction, such as severe itching, when handling uncooked prawns. Similar allergic reactions to crabs may be possible.

  • Llewellyn LE, Davie PJF (1987) Crabs and other crustaceans. In Toxic plants and animals: a guide for Australia. (Eds Covacivech J, Davie P, Pearn J) pp. 126-135. Queensland Museum, Brisbane.
Are there crab infectious diseases we need to be alert for?

Probably not.