port phillip bay

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!

Sea Search Expedition: Mud Islands

Photo Credit: Phillip Wierzbowski

Photo Credit: Phillip Wierzbowski

Do you love beach combing, puddling around in rock pools and snorkelling in seagrass beds looking for sea creatures? Ever wondered if there was a way for you to get more involved in caring for your local marine environment?

The Parks Victoria Sea Search program offers communities the opportunity to help contribute to our knowledge of intertidal marine ecosystems and the spread of marine pests. This is a great way to learn more about your local marine national park, while collecting scientific data that helps to inform its conservation!

Wild Melbourne recently set out with a Sea Search expedition to Mud Islands at the mouth of Port Phillip Bay to get a feel for marine survey methods. With 40 odd keen marine beans, we headed out from Queenscliff on the 62ft research vessel - the Pelican.


Mud Islands, Port Phillip Heads Marine National Park

mud island high tide heading home.JPG

Despite the drab name, Mud Islands, Port Phillip Heads Marine National Park, is an area rich in biodiversity, deserving of its RAMSAR “wetlands of international importance” status.

Three silt islands, ever-changing in size and shape, surround a protected lagoon, to which wading birds retreat at high tide. Saltbush and succulents top what I hesitate to call dunes; the highest point a mere 4 metres above sea level. Sand flats dominate the intertidal zone and support vast numbers of wading birds, some of which migrate here in their thousands from as far away as Alaska and Siberia.

Probing the fine sands at low tide with specialised bills, plovers, curlews, knots and godwits hunt for worms and molluscs hidden beneath. The islands also support great nesting colonies of pelicans, terns, ibis, silver gulls, storm petrels, cormorants and royal spoonbills.

Sharp Tailed Sandpiper

Sharp Tailed Sandpiper

Come the breeding season this place will be a cacophony of nesting birds, but in late summer the islands are much quieter, the landscape strewn with their bones and feathers.

Dip below the surface and seagrasses carpet the flats, teeming with life as they protect juvenile fish, squid, crustaceans and sea stars, as well as animals specialised for life within and upon the blades.

Fiddler rays nestle between the leaves, well camouflaged with their brindled skin and “I’m-not-here” attitude. These lawns provide a vital nursery for many species, including the commercially valuable King George whiting.



Sea Search Survey

We settle on the sand, geared up in our wetsuits and underwater cameras, to hear the history of the islands and learn about the different ecosystems we will be surveying. After a verbal introduction to our survey methods, we set off in groups to conduct seagrass surveys and targeted patrols for invasive species, all under the watchful eye of Parks Victoria rangers.

Unfortunately, we ran out of time to conduct a proper survey of the feathered community. Still, we found a moment to train our binoculars upon some of the waders hiding in the lagoon. 

Mud Island's Breath-taking Lagoon. 

Mud Island's Breath-taking Lagoon. 


An ecosystem under threat!

Our time on Mud Islands ends with a discussion of the threats to the islands and Port Phillip Bay at large. Sitting close to the mouth of the bay, Mud Islands collect rubbish, seeds, and excess nutrients from urban streets and farm runoff.

Miscellaneous plastic and metal pieces litter the beach, while stubborn weeds including boxthorn and kikuyu grass resist the equally stubborn weeding efforts of local care groups.

Excess nutrients have caused masses of algal growth to smother the vital sea-grasses, reducing water flow and preventing light from reaching the plants. Without sunlight, the grasses are unable to photosynthesize, and start to die.

All are a poignant reminder that we each have an impact on the health of our bay communities. Around three and a half million people live around Port Phillip Bay, and everything that lands on our streets or goes down the drain ends up here.

We return to the Pelican, but don’t head home just yet. No trip through these parts is complete without a visit to the Australian fur seals and Australasian gannets at Chinaman’s Hat and Pope’s Eye. We get up close and personal while learning about these charismatic bay characters, and then it is time to draw our adventure to a close.

As the sun breaks through the clouds, the sails are raised and we turn for home, our party blissfully soaking up the sun on the Pelican’s deck.


Would you like to attend events like this, or become more involved in learning about and protecting your local marine habitat?

To get involved with Parks Victoria’s Sea Search program, head to http://parkweb.vic.gov.au/get-involved/volunteer/sea-search and send them an email at seasearch@parks.vic.gov.au .

Both Sea Search and Parks Victoria can also be found on Facebook.

You can find out more about the SV Pelican at http://www.svpelican.com.au

The Importance of Upwelling

 

Upwelling: many of you may have heard of it, and many of you may have no idea what it is. Regardless of what you may know, upwelling is extremely important in creating and sustaining life within all oceans. It is so important that, if missing, we might not be around much longer either.

Upwelling is the process where cool water is brought up from the depths of the ocean, full of delicious nutrients ready for consumption by tiny phytoplankton. This cool water replaces the warm, barren and ‘stale’ water that sits at the surface layer, giving the phytoplankton the food they need to produce the energy that is then bestowed upon the rest of the marine food web. Phytoplankton sit in the water and suck up all the nutrients surrounding them, which they then use to power growth.

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-->    A nudibranch off Blairgowrie pier: one of the
many creatures within Port Phillip Bay that benefits from upwelling in Bass
Strait.

A nudibranch off Blairgowrie pier: one of the many creatures within Port Phillip Bay that benefits from upwelling in Bass Strait.

Phytoplankton fill the niche space (a ‘sciencey’ word for a position in the food web) of a plant, as they are primary producers. Primary producers use sunlight to create energy using photosynthesis, as well as utilising the nutrients brought up from the depths of the ocean to move this energy up the food chain, allowing life to bloom. Phytoplankton therefore sequester the nutrients and convert sunlight into energy, and are in turn eaten by zooplankton, which are then eaten by larger and larger organisms, eventually reaching the apex predators, such as killer whales, dolphins and, my favorite, SHARKS! This process of nutrient acquirement makes up something you might have heard of before: the food chain!

If that description confuses you a little or doesn’t seem clear, we can use the analogy of a plant. Imagine any plant that has roots in the soil: the roots sit in the soil so that the plant can access the nutrients found there. Each root pulls in nutrients essential for life processes, such as phosphorus, nitrogen and potassium. These nutrients are then used to power the plant. Primary consumers then eat these plants, moving the nutrients up the food chain. Predators then chase these consumers and subsequently eat them, thus moving the nutrients up to the apex of the food chain.

In a nutshell, if you can’t get your nutrients, you can’t exist. For example, carbon is used in the construction of a plant by forming the backbone of most essential biomolecules, such as cellulose and starches. As many of you may already know from basic chemistry, carbon is pretty much the most essential element on earth, as it allows life to exist!

Now back to upwelling and why it is so important. Upwelling doesn’t occur all the time, as in the terrestrial environment there are seasons when not much food is available, and other times when there is. For Victoria in particular, the greatest period of upwelling is springtime. That’s right - NOW!

As a result of this huge upwelling, we see large marine animals visiting our coasts. Most would be aware that we have recently had humpback whales passing through our waters. You might be surprised to know that we also have the largest animal in the world moving into our waters during this period: the blue whale! It is amazing to think that microscopic phytoplankton can bring the biggest animal in the world to Australia’s own shores. Such an event truly draws attention to how influential these little critters are in the oceans of the world. It is even more amazing that something so small and (almost) invisible can be so integral to keeping the oceans afloat.

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-->      Australasian Gannets hanging out at Pope’s Eye. These are among the many seabirds that benefit from
the upwelling in Bass Strait.


Australasian Gannets hanging out at Pope’s Eye. These are among the many seabirds that benefit from the upwelling in Bass Strait.

 

So even though all the nature we can see with our eyes is both beautiful and vital when it comes to the survival of life, it’s the things that we maybe can’t see directly that are even more important. If you wish to witness the changes that upwelling can bring to the marine environment, jump in the water somewhere during the middle of winter, and you’ll find that it’s pretty quiet. Then go and have another look around early summer, sometime near Christmas, and experience the change in activity - you will be amazed!

Research: What impact does capture have on shark populations?

A Port Jackson Shark. Photo: The Age Newspaper

A Port Jackson Shark. Photo: The Age Newspaper

Elasmobranchs (sharks & rays) are amongst the most vulnerable animals to fishing pressure. Using species found in Port Phillip Bay, one of our members, Leonardo Guida, and his partner, Derek Dapp, explain how their research will improve both our understanding of elasmobranch biology and their conservation.

Check out Leo's amazing work in the video below thanks to Monash University's Faculty of Biological Science!