Water

Science Short: The Black Swans of Albert Park

Wild Melbourne’s Science Short series is a collection of short video documentaries showcasing the wonderful research being done in Victoria. Each episode features a different scientist or research group, opening up the world of environmental research for the public to see. 

In this Science Short, Wild Melbourne venture to Albert Park Lake near the Melbourne CBD to follow the University of Melbourne Black Swan Research Group. Headed by the University of Melbourne’s Head of BioSciences, Raoul Moulder, the BSRG has been researching the black swan population at Albert Park for over a decade.

Raoul, along with Dr John Lesku from La Trobe University, is supervising PhD student Anne Aulsebrook as she investigates the effect that artificial lighting in urban areas is having on the biology of the swans. Artificial lights, such as street lamps, headlights and signs, are known to change how animals behave and respond to more natural cues.

Spider Crabs: No Rest for the Wicked

This is a guest post by Elodie Camprasse, a PhD student from Deakin University, Melbourne. 

Migration - when people hear this term, they usually picture herds of mammals (including people) or flocks of birds en route to places where they can find better conditions. However, did you know that Melbourne has a migration of its own in its underwater backyard? Giant spider crabs (Leptomithrax gaimardii) indeed put on a show every Winter in Port Phillip Bay. If you let me, I will take you on one of the most amazing dives where I was witness to this amazing event. Don’t worry though - you won’t have to get wet!

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  Giant spider crab ( Leptomithrax gaimardii) ; Photo: Elodie Camprasse

Giant spider crab (Leptomithrax gaimardii);
Photo: Elodie Camprasse

It is a chilly and dark Friday night and I am meeting with a bunch of very enthusiastic divers. We have all been looking forward to experiencing the annual spider crab migration. It is a first for me and even facing the cold, 12°C water of Blairgowrie could not take the excitement away. A nice, easy beach entry allows us to progressively get used to the water temperature before starting our actual dive under the pier. The first things we bump into are a tiny seahorse and a few stingarees, which I take as being good signs of the fascinating dive that is to come! As we swim towards the end of the pier, we cross paths with squids swimming in mid-water, porcupinefishes, big leatherjackets, colourful sea slugs and our torches reveal the vibrant colours of sponges. I am in awe already when we reach, at last, the spider crab aggregation. Here they are, hundreds of them, hanging out in about four to five metres of water. Most of them sport broken limbs and dull, old shells that they will need to get rid of within the next few weeks in order to grow bigger, this being the purpose of their migration. Crabs like to blend in and they tend to pick up bits and pieces from the sea floor and stick them on their shells in order to increase their camouflage. A few of them even ‘wear’ sponges on their heads and backs, ironically making them stand out even more as I am trying to spot the odd ones out to create better shots. As I focus on taking photos, a curious octopus cruises by to check me out.

Marching spider crabs in Blairgowrie. Photo: Elodie Camprasse

Marching spider crabs in Blairgowrie. Photo: Elodie Camprasse

This is only one of the few dives I was lucky enough to take part in during this yearly phenomenon. The other dives allowed me to witness the crabs’ movements and behaviours further, at various times of day and in different light conditions. Along with that, I was able to discover bizarre-looking critters, including tassled anglerfishes and stargazers, to name a few. I was amazed to observe the big piles that spider crabs often form at this season, sometimes reaching more than a metre in height, as they seek safety in numbers.

Spider crabs forming aggregations. Photo: Elodie Camprasse

Spider crabs forming aggregations. Photo: Elodie Camprasse

Towards the end of the migration, the last crabs switch from the sandy sea floor to the pylons and sponge gardens on the walls of the pier. This provides the now more isolated crabs with better protection. When they moult, the crabs release a specific scent that predators such as rays, seals and seabirds are able to pick up and follow to gorge themselves on freshly moulted and hence very soft individuals. They indeed need to wait a few days for their new, brighter-coloured shells to harden in order to become less vulnerable and leave the shallows to resume their solitary life in different parts of the bay. Watching the smooth stingrays circling in between the pylons and waiting for clumsy, freshly molted crabs to fall within their reach is quite a show! So too is observing the crabs extracting themselves from their old shells: a process that seems exhausting to them and can take up to approximately half an hour; it almost felt like I was watching a creature being born, right in front of my eyes. They usually free-fall in the water once they have managed to extract themselves from their old shells, rolling around the sea floor and looking stunned for a few minutes. Only when they are quick enough to recover their senses and climb back to the pylons are they able to escape the hungry rays. Nevertheless, more discrete critters join the frenzy. Small shrimps and seastars hang around the crabs to scrape the last bits of meat off the old shells. By this time, the bottom is strewn with discarded shells, adding to the already apocalyptic atmosphere.

Spider crab removing itself from its old shell. Photo: Elodie Camprasse

Spider crab removing itself from its old shell. Photo: Elodie Camprasse

There is still a lot of mystery revolving around the annual migration, which seems to happen at different sites in different years. Aggregations have indeed been observed at Rye, Sorrento and St Leonards in the past. Water temperatures or moon cycles might play a role in triggering the phenomenon, although this is only an educated guess. People used to believe that mating occurred after the crabs molted; this, however, is not supported by observations. There is not a lot of information about where these usually solitary animals spend the rest of the year and there have been aggregations in the forms of pyramids sighted at other times of year, whose purpose is unknown. Clearly, we still have a lot to learn about the migration and we will surely gain more information as enthusiastic divers and snorkelers continue to get in the water and share their sightings and behavioural observations on social media! So let us hope that next year, once again, there will be no rest for the wicked. 

For more, follow Elodie on Twitter!

An Ode to Wetlands

Water gives life, and where we find water we are often presented with a bouquet of life’s finest. You can find it blossoming from wetlands, both permanent and ephemeral, that are scattered from east to west. Splattered rather, as if a godly artist flicked her freshly-dipped paint brush across the canvas that is our state. In winter, rains fall and snows melt. The water of lakes and marshes rises and with it comes the growth of aquatic vegetation and an abundance of those tiny creatures that nibble at their fronds and stems. The tiny plant nibblers are nibbled themselves by other invertebrates that dart through the water column or haunt the sediments. Dragonfly larvae capture and devour infant mosquitoes in jaws reminiscent of a Ridley Scott movie, and caddisflies spin silky driftnets to snare floating morsels.

Such things are occurring all the time, out of view, out of mind, acted out in a realm almost removed from our own. We are the giants, and these tiny creatures are indifferent to our immense existence: an existence that shapes their own with every day that passes. We destroy their worlds; we fashion them to our liking, “stream-lining” streams and changing natural flows. The flows of ecosystems. The flow of life. It is easy to change things that you are indifferent to when you are human; when you are big and brainy. Yet, one might argue that we have not the excuses of these tiny creatures in our shared indifference. They lack the wherewithal to know us, but we possess the means of knowing them. And if ever we feel disconnected we need only observe our realm – our bigger realm – to know our world is one.

The salty marshes of Westernport lack snaky silhouettes. The Black Swans have gone to the newly birthed wetlands to feed and to breed. So too do many duck species, now flocking to freshwater for mating; dabblers and divers bringing splashes of colour to shimmering lakes of blue and grey. Wading birds sift through silt and sand, while Herons and Egrets eye the shallows for invertebrate-fat fish. Life gravitates to these places. It abounds at every level. For a time - brief to some, eternal to others - wetlands shine like stars against a backdrop of farmland and suburbia. Then, when the waters run dry, life disperses outwards across our land like a star turned supernova, scattering its atoms across the universe.

Few people possess the capacity to understand the eloquence of the physical and chemical processes that make up a star. Those that do are forced to observe from a distance, through machine and glass. We can all sit quietly, intimately, surrounded by the happenings of a wetland, seeing things with our own two eyes. Hearing the quarrels of ducks, smelling rich odours of endless origin, and feeling the one world of which we are apart all around and within us. Do not take such places for granted. They underpin all that we rely on, and will continue to do so as our city expands and our climate shifts. Find them and know them. See their changes and their constants. And know that when you are there, you are seated at both the origin and destination of life.