December 7, 2018

Simon Grove on Investigator
Simon Grove once dreamed of exploring remote coral islands, but now he's just as happy peering down a microscope at rafts of undescribed micro-molluscs from an altogether colder, darker place. Image: Fraser Johnston/CSIRO

seamounts blog logoDay 15: Simon Grove, Senior Curator, Invertebrate Zoology, Tasmanian Museum and Art Gallery

I’ve had a thing about molluscs since childhood. Many of our family holidays were built around good shell-hunting beaches. I even had my own seashell museum in my bedroom, foreshadowing by several decades my current job as a museum curator. Growing up on a diet of Willard Price adventure stories, I dreamed of exploring remote coral islands in the South Seas, though I would never have dared to imagine that my dreams might one day come true. Yet, in joining this voyage they have pretty much done so – except that the Seas are rather more South than I had imagined, and the coral islands are hidden from view beneath several hundred metres of cold water. Still, the molluscs are all the more intriguing for inhabiting such a cold and dark realm and for having kept their secrets from humanity right up to the present day. We really are discovering creatures that no-one even knew existed! That’s a real privilege.

Simon Grove on deck with a pile of shell grit
Simon fossicks for treasure in 'deep-sea biogenic grit' delivered on deck by the beam trawl. Image: Bethany Green/CSIRO

So, let me get down to the nitty-gritty. I use that expression deliberately, because it turns out that 'deep-sea biogenic grit' – the bits of broken coral and bryozoan that accumulate at the bottom of sea-mounts – is, when trawled from the depths, brought on-board the Investigator and diligently picked over and sorted through, a great source of sea-snails, and quite a few clams and other animals that together we call molluscs. I had expected to find a few species. What I hadn’t expected was to turn up one hundred and twenty-five species in a single trawl!

At this point I should explain that, for the most part, we’re not talking about tritons, whelks and conches here. True, there are a few large shells, but the diversity is in what we call the micro-molluscs, with shells barely a few millimetres long. If this comes as a surprise, bear in mind that nature is remarkably like one of those Mandebrot fractals (google it), or, I suppose, like Google Earth, where the more you zoom in, and the deeper you probe and explore, the more whole new worlds open up before your eyes. The shells of micro-molluscs are just as intricate as those of their larger cousins; it’s just that they’re small enough to slip through your fingers, and through most trawl-nets. How lucky, then, that one particular trawl was so gummed up with biogenic grit that even the fine material was brought to the surface.

Extracting micro-molluscs from the grit required hours of sorting petri-dishes full of the stuff under the microscope. Segregating each unto their own kind, another few hours.  And I’ve only just begun to try and identify them – a process that will continue once the shells are safely back at the Museum. At the moment it’s looking as though about two-thirds are undescribed – that is, they have no scientific names. Yes, I did say two-thirds. How special is that?

That said, I haven’t exactly been inundated with shells. That’s because about half of all these species are so far represented by merely one or two individual shells. They conform to a common pattern in nature, one I call ‘the commonness of rarity’: most species are rarely seen, rarely sampled, and rarely collected. Every time we sample, we get a few of them, but we can’t predict in advance which few we’ll get. That means that when we look at any natural community, we’re generally only seeing the much fewer species that are either so common or so showy that they more or less present themselves to us without us having to look too hard. Bear that in mind next time you’re out for a walk in the bush or at the beach.

As I was settling down to write this blog, the ship rolled from a monstrous westerly swell just as she was being turned around to line up for a new operation. For a moment the view out through the porthole was of an undersea world rather than the sky. Working at sea certainly presents its challenges! On the plus side, the very fact that I blithely carried on typing suggests that I’ve come a long way since I first boarded Investigator, when I could hardly move without being overcome with waves of nausea. This is not the balmy South Seas and coral atolls of my childhood imagination. It’s far more interesting than that.

montage of  shells
A montage of some of the larger deep-sea molluscs previously collected from the Tasmanian seamounts. Image: Simon Grove, TMAG
A spiky red crab
A picturesque crab from today's beam trawl. Image: Fraser Johnston/CSIRO

Notes on today's activities from Marine Biodiversity Hub Director, Nic Bax . . .

Light to moderate winds and gentle swells continued, with many birds and two species of whales visiting the ship – minke whale on one side and sperm whales (in the distance) on the other. First order of the day was completing the seamount recovery transects and that should have been that, however on checking some of the imagery on earlier transects closer to Pedra two transects were identified where the towed camera was more than two metres away from the seafloor and image quality was compromised, so we will have to repeat those on the way back.

Nevertheless after Main Matt was completed, we turned our attention to the Deepwater Coral Hunt transects, completing six of those and an additional ad hoc tow over an interesting feature at the end of an existing transect line. The highlight was a small unfished hill southeast of Main Matt. When we flew into the summit we saw at least four rays swimming away (which is double the number that we have seen in all previous surveys to the seamounts), and beneath them at 1300 metres a dense cover of Solenosmilia coral community that extended down to 1400 m, after which it rapidly became patchy, although small tufts were spotted as deep as 1500 m. This seems to be a quite common pattern in this area.

To stay an early onset of mutinous mutterings among the many taxonomic experts on board, who have not been able to apply their expertise while we focussed on towed camera transects, we ran a beam trawl over deep seafloor at 1300 m resulting in an undamaged trawl (the first so far) and a colourful mixed catch including 123 four-spined rattails, seven orange roughy (from two-years-old to mature, so about or more than 30-years old), at least 45 mollusc species, scaleworms, urchins, seastars, glass sponge, hard coral, soft coral, two nudibranchs, and some picturesque crabs. An interesting feature was the hundreds of ophiuroids or brittlestars entwined in the hard coral matrix and even in the many gastropods that also seemed to provide a base for the stony coral. This gives an idea of how interconnected and rich this deepwater coral matrix can be.

We’ll be staying in this area tomorrow to pick up some of the exploratory transects in shallow water. While we are starting to get some confidence about the lower limit of the Solenosmilia community in this area, we are really quite uncertain about its shallower limit. We haven’t seen it on some of the shallower fished seamounts where we had expected to see it, even if only in crevices and on unfishable faces, so are keen to understand more about its shallower limits.


Voyage date: 
Friday, December 7, 2018