The greatest aggregations of megafaunaThe largest animals that represent the top of the trophic pyramid. These are the final building blocks in ecosystem structures for maximum entropy production. Megafauna can be measured at any spatial scale. The largest animal that ever lived on Earth is the Blue Whale. In a grassland, spiders could be considered megafauna The term is generally reserved for animals larger More on Earth will always defy human explanation. One case example are the spider crabs of Port Phillip Bay. Made famous by the BBC Blue Planet II series they gather every year in mid-winter. For a few weeks they carpet the sea floor clambering over each other to form pyramids. But because we share ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More with them, spider crab conservationWhy is animal conservation important? Animal conservation is important, because animals are the only mechanism to create biodiversity, which is the mechanism that creates a habitable planet for humans. Without animals, the energy from today’s plants (algae, trees, flowers etc) will eventually reach the atmosphere and ocean, much of it as carbon. The quantity of this plant-based waste is so More needs to form part of our culture. This means involving the whole community in their protection.
Why do spider crabs visit Port Phillip Bay?
The BBC say they might number in the hundreds of thousands. But no-one really knows this for sure. We do know it is likely that they gather together in order to moult.
Spider crabs are crustaceans, which means they have their skeleton on the outside. To grow they have to climb out of the old hard outer shell, expand, and grow a new one. This makes them soft and vulnerable for a few days afterwards.
After moulting, individual crabs can hunker down and shield themselves beneath hard-shelled others. Half-tonne smooth rays are among their greatest predator – another megafauna species that we know hardly anything about.
How long do spider crabs live for?
Science hasn’t yet found a reliable way to tell the age of crabs. Nonetheless, it’s thought that similarly-sized crustaceans like lobsters, might live for 50-100 years.
In 2008 a lobster called ‘George’ was caught for a seafood restaurant and was estimated to be 140 years old. Newspapers reported how a wildlife activist had bought the old lobster to release back into the wild. But this animal’s precise age is still impossible to verify.
A review in 2017 found that current aging techniques for crustaceans remain questionable. While we can count growth rings in eye stalks or in small bones in the stomach called ossicles this isn’t enough. Growth rings are laid down after each moult but we don’t know how often that happens for given species. Most likely it also varies depending on climate and individual circumstance. It’s likely we may never know exactly how old Port Phillip Bay spider crabs are.
It’s highly likely, however, that spider crabs live for a long time. How long depends on body size as in every moult the animal gets bigger. Moulting a bigger body takes up more and more energy. This eventually becomes too much so as this National Geographic article puts it “life just goes on until an inevitable end”.
Older also means wiser spider crabs
Once we realise spider crabs could live for many decades the mystery starts to get compelling. To know their importance, we have to imagine things, like: where do they come from? And how do they know where to go? There is a wisdom, just like ours, that enables them to live and be part of something bigger than themselves. A connection to the same ecosystems we rely on for our existence.
The older animals get, the wiser they become. Their intelligence isn’t individual, it’s collective. A s a group they perform miraculous feats of behaviour. As an animal their role in nature is every bit as important to us as it is to their kind.
How do spider crabs know where to go?
I recently wrote a blog describing some of the mysteries of migration. Monarch Butterflies, for instance, don’t instinctively know how to migrate. Their movements are as integrated with ecosystems as a drop of river water is, running through a gorge. This relationship between animals and the physical environment can be hundreds of thousands of years old.
Complex long-lived animals like spider crabs have developed their own culture. Genetics is only a starting framework. A spider crab’s genes might entice it to walk in a general direction but joining other crabs who ‘know where to go’, increases the chance of everyone’s survival. It’s this population-level intelligence that swings the balance between a successful (likeliest) species and one that goes extinct. It’s also the key to healthy ecosystems, as that is what animals build.
If we remove older and wiser animals from the environment, we remove the cultural knowledge, that connects them to nature. It’s this learned intelligence animals have that goes hand in hand with an ecosystem’s long-term stability. All over the world there are examples of animals that come and go, aggregate and disperse. This constant and cyclical ebb and flow, boom and bust, is akin to the Earth breathing in and out. But instead of air, animals collectively breath nutrientsEnergy and nutrients are the same thing. Plants capture energy from the Sun and store it in chemicals, via the process of photosynthesis. The excess greenery and waste that plants create, contain chemicals that animals can eat, in order to build their own bodies and reproduce. When a chemical is used this way, we call it a nutrient. As we More into systems, that provide our water, food and fair climate for living.
We need to respect spider crab aggregations. If we interrupt these we jeopardise processes that are bigger and more important than anything science alone can interpret or describe. We jeopardise our own future.
Where do the spider crabs come from?
We don’t know. One of the objectives of research by Deakin University students is to try to work out whether the crabs stay in the bay or disperse beyond it. For instance, they are placing acoustic tags on crabs that emit a regular ‘ping’. The bay’s entrance has a curtain of listening stations placed to hear if any cross the line.
This is quite possible as there are similar aggregations of spider crabs in northern Tasmania, just the other side of Bass Strait. Bass Strait and Port Phillip Bay are similar depths and covered in patches of reef and sponge-beds. During the last ice-ageA period of reduction in Earth’s temperature of between about 4–7 degrees that resulted in the rapid expansion of ice sheets and glaciation of much of the Earth’s surface. More these areas formed a land-bridge to Tasmania and the Yarra River cut a deep gorge through what is now Port Phillip Heads. My friend Matt Edmunds has recorded them in patches of deep reef in Port Phillip Heads, which is one of the deepest areas of the region, about 75m.
Today these ancient floodplains would be perfect habitat for single crabs to roam and graze. It’s distinctly possible some, if not all, of the population that enters Port Phillip Bay spread out all over the seafloor beyond our coast. These remarkable animals are transferring nutrients between widespread deep-reef systems offshore, to the shallow reefs of Port Phillip Bay.
The magnitude of spider crab impact
The impact of potentially hundreds of thousands of large crabs is immense. Here we have an ancient, long-lived animal, that congregates in huge numbers. They then re-disperse into surrounding areas. Individually they will be having an impact by foraging over their own area of seabed territory. Together, they will be impacting whole ocean systems. Migratory animals are enormously important for ecosystems and planetary processes. Each moult they will also deposit thousands of moulted skins. This concentrates a vast amount of nutrients into coastal systems where juvenile fish breed. All of this adds up to an undeniably huge effect.
Some scientists will tell you that knowing the age of spider crabs helps us create models for fisheries. But if you’re staring in the eye of an animal that’s older than you, it begs another question. Can our fisheries exist without them? Should we even be catching them at all? These are key questions for spider crab conservation. This is also the wonder of nature that we need to inspire people to protect them.
Science has a role but will never have all the answers
The most important question about spider crabs is: do they matter? Research can never answer that. The most compelling reason to protect spider crabs, therefore, begins with us respecting their age, wisdom, intelligence and culture like our own. To conserve wildlife we need to create a change in human values and that is on an emotional level.
Just scrape the surface of that understanding and a world emerges that’s not hard for us to appreciate. Animals exhibit similar traits to our own surviving Indigenous cultures and ancestry. The way animals behave connects directly with the stability of ecosystems of which we are part.
So, what do we do to protect them? This is where science has to step aside and let communities take the lead first. It means engaging everyone.
Protection begins by engaging communities
Unfortunately the occurrence of spider crabs in the bay attracted unwanted attention during COVID. The aggregations around Rye and Blairgowrie lured people and trucks intent on slaughtering the animals on mass. Locals were naturally appalled and fisheries were forced to impose limits.
This controversy has, sadly, led to an ongoing and rather strict moratorium on sharing information about where the crabs have congregated this year. People who wanted to see this natural marvel – the very same who would want to protect it – have been disappointed. Unfortunately this leads to ill-feeling and a disempowerment of the very people we need to create a change in human values.
Spider crab conservation will only come from building support from within communities. While it may seem like a good idea to restrict public access, this never works. We know this because the only successful conservation in the world comes from allowing communities to lead.
Ironically, the best examples of these are in some of the poorest areas of the world. It has taken many years to stop western scientists believing that protection requires wholesale exclusion of people. When this ‘ideal’ is imposed on less privileged local communities overseas it is described as a form of neo-colonialism. The same colonial attitude lives on today in our own back yard. Yet, when it come to effective conservation, there is no difference between a coastal town in Melbourne and a small village in Madagascar.
Conflicts of interest in conservation
The slaughter of crabs initially led local councils to vote for a ban on crab fishing. This is the very type of community-led outcome that could result in stable conservation outcomes. But for reasons explained above, centralisation of conservation control through universities and government bodies is par-for-the-course in many western countries. Community disempowerment soon takes over. That never ends well.
Very soon divers and snorkelers interested in seeing this extraordinary natural crab spectacle became excluded from knowing where they were even turning up. This way, it was suggested, any risk of fishing could best be eliminated. In a further twist, those same concerned people were encouraged to share sightings (but only with researchers) and this became ‘citizen science’.
What the public hasn’t been told, however, is that the citizen science is being fed into a larger project part-funded by fisheries. Hence, the research isn’t on behalf of communities since most local people are excluded from knowing where the crabs are located. Data will ultimately be used in a stock assessment for fisheries.
The Victorian Fisheries Authority report from April 2021 seeks to understand more about the species, so they can make ‘objective’ decisions and identifies that ‘Spider crabs are a highly prized seafood and commercially harvested around the world’. Then why exclude information sharing with other non-fishing users? Because then, the weight of economic data on the crabs, will end up being biased towards fisheries (which have a huge influence locally). Then it becomes only fishing groups who are the main beneficiaries … and they were vocal opponents to any controls on crab fishing in the first place.
We need to be doing more to engage anyone who cares
A diverse and committed group of divers and snorkelers, including many local families and children, have been excluded from enjoying an important natural history event. This includes 7,000+ members of the local snorkelling group. All of these compassionate advocates for conservation are prevented from promoting a deeper connection to crabs among themselves, their family and friends – from appreciating their own natural heritage and wildlife. Yet they remain key to their conservation.
Unfolding mysteries of the world, meanwhile, are too much for science to comprehend fully. The more we learn it seems the more we don’t know. The most important stories of all remain untold because science isn’t capable of doing that alone. And that’s the point. It’s the mystery of spider crab conservation and its significance on the lives and economy of all people that matters. Not the interest in science or fisheries.
If we’re going to preserve wildlife we have to be prepared to think a different way about animals and our connection to them. The mystery (what we don’t know) is more important than what we do. We desperately need to engage people in this wonder – not through TV series but in real life.
We haven’t many more years left to achieve that.
Change the way we work and scientists have a far greater role to play. One where the community asks for advice on how to behave to conserve spider crabs. This is a more constructive and rewarding job for any scientist. It’s one that works together with those who care the most about our ocean creatures.
Images of the spider crab aggregation
There is much we don’t know about wildlife, even in our own backyards, and this centuries-old cultural connection with healthy ecosystems and our own lives is a beautiful story but one I wasn’t allowed to share with people this year. Connecting with nature can only be achieved first hand.
This goes down as one of my top wildlife experiences but I’ve been instructed not to tell anyone where it is. I wish it could have been possible for many others to see it.
