We speak with Kerry Walton about the weird world of marine mollosucs
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Kerry Walton
Malacologist and invertebrate curator at Te Papa museum, New Zealand
I'm joined by Kerry Walton, malacologist and invertebrate curator here at the Museum of New Zealand. thanks for coming on and having a chat!
Thanks for having me!
So we were chatting a little bit yesterday about why it is so important to have a a grasp of the molluscan diversity. you were giving me a few examples of The Wider things you can tell just from having a decent understanding of the mollusc component to a habitat.
Yeah it's not unique to molluscs, but molluscs are probably the best example where you can use very high level of knowledge of one group to extrapolate and answer a bigger picture questions about ecosystem form and function, and diversity. Molluscs have a couple of advantages; for one, they're one of the largest animal groups there is, second only to the arthropods which are the insects and crustaceans. We've got between 200-300,000 living mollusc species and the majority of those (roughly 90%) produce a shell; which has a relatively high preservation potential. What that means is you can get and presence-absence data for molluscs which you couldn't for the majority of soft-bodied organisms. Remarkably of the roughly four and a half thousand living species we have in New Zealand, the majority have never been seen or collected alive. We assume they’re extent on the basis of known rates of shell degradation. But we collect their shells for depth assemblages by dredging at the base of seamounts or canyons, for example. Or for terrestrial species, by sieving leaf-litter and getting the small arboreal snails out of it that way.
So it's a combination of this really high diversity and the parts of them that we can identify to quite high level (like species level), preserve really well?
Yep, so a high diversity and high preservation potential. The fact that they produce a shell is very advantageous and there are some groups which are very much the exception, but most small species can be identifiable (with a degree of confidence by an expert) to species level from the shell alone. You don't need to dissect the anatomy, you don't need to sequence them.
You were telling me about how well they preserve, that they sort of do a self-buffering thing once they've decomposed to a certain level. You were talking about finding what would be a fossil anywhere, any other time, and it sort of looks like it died yesterday. They just preserve incredibly well.
There was a study published a few years ago that was carbon dating, what looked like, fresh shells on a beach and while the majority were fresh (in the sense of weeks to months old), some of them were thousands of years old, and the difference was barely perceptible with the naked eye. So we often make this distinction between ‘recent’ and ‘fossil’ when people will fight you over those terms, but fossilisation is the process of mineral replacement. Shells are minerals and the rate of mineral replacement in which minerals actually get replaced is contextual on the depositional setting. What the sediment contains, different pH values and so on. So what we have if you've got a carbonate-rich deposit which might be a bryozoan bed or a shell bed, where the majority of the sand or mud has a high carbonate content, that self buffers against acidic effect of the sea water. It means those shells can remain looking extremely fresh for perhaps ten thousand years or more. Conversely, if you have sub-optimal conditions e.g. a mud that has a low carbonate content, you can have a living mollusc that hasn't even bothered to die yet, that's already got a partially fossilised shell.
It's lived long enough to start fossilsing?
Effectively, yes. Or at least a highly-eroded shell is more common for deeper water taxa.
That's cool. And they can also pass through predator digestive tracts as well? Even fish poop can still have some recognisable material in?
Yeah going through fish guts is one of the easiest ways to collect deep sea molluscs, especially when they're from a hard bottom habitat that would be difficult to sample otherwise. A lot of the early deep sea mollusc collecting (in particular, by show collectors) is done by going through fish guts. And they're in a remarkable state of preservation: often the shells will pass through completely intact. For some species, it's hypothesized that the animal will pass through living, where they can seal the shell off completely with their opercula. And that's one potential means of dispersal for some of these species. Generally, the deeper you go, the fish will be more inclined to eat whatever they encounter so most deep sea fish that are frequently in contact with the bottom will nibble away at a mollusc where visible.
you were saying how this is really handy for habitats that are really difficult to sample, like Rocky habitats. the presence of these shells rolling downhill and in the guts of fish that are living in the area, indicates that this habitat is present even if we we can't sample it directly.
Absolutely and the problem with relying on a collection built from fish gut material is it's often poorly localized. You might have a region if you're lucky, but that's about it. But one of the big advantages of so many molluscs passing through fish, is that it means at the bottom of sea mounts that are quite difficult to sample, you risk losing a lot of gear. And you risk damaging a lot of probably sensitive coral habitats. So you don't want to go in recklessly to those. By sampling around the base of sea mounts or in the bottom of submarine canyons, you get all of these shells that have accumulated over centuries or millennia, potentially, depending on the sediment type, from higher up on those canyon walls. And by looking at those shells, we can identify certain characteristics. As much as a third of molluscs are obligate. Either parasites, or in a conventional relationship with certain corals, sponges, sea cucumbers. So we can, by having a good understanding of the ecology of some of these species and by having good natural history collections, more confidently say: ‘this genus feeds only on that genus’ or ‘lives between this depth range and that depth range’. We can start to reconstruct the unsampled habitats above by looking at this depth assemblage at the bottom of the seamount or canyon.
I feel like the wider mollusks (beyond the cephalopods) seem to get missed out. So I wanted to give them a chance to take the spotlight. There were a few you mentioned that were totally new to me. Some of the real obligate ones on whale carcasses and large carcasses that fall to the seabed. I had no idea these even existed!
The core of a whale fall community often comes down to (after you've had the initial successive cycles of the detrivores feeding on the flesh itself) bacterial action producing hydrogen sulphide, as the bones decay. And as the bones of cetaceans are much more oil-rich than the bones of equivalently sized fish, they produce a lot more hydrogen sulphide. So you end up with these quite unique communities that have some resemblance (certainly ecologically) with hydrothermal vent and methane seep communities. At family and sometimes genus level, you get a lot of the same species. There are very few species that are actually in common between those different habitat types.
Molluscs found at hydrothermal vents. Image credit - Craig Smith
So, we've got a considerable diversity of small mussel species that are unique to whalefall environments here in the South Pacific, and they are found throughout the world as well. These mussels are closely related to the hydrothermal vent giant mussels. They've got bacteria that they farm in their gills which metabolise sugars from the hydrogen sulphide, and the mussels absorb that sugar as their main form of nutrition. But we have a whole bunch of more easily overlooked invertebrates. Molluscs in particular, there are dozens of limpet species that are unique to organic falls and bones; squid beaks that are rotting etc.
and these are these are Specialists to each one of these?
Yep! One that we (my colleague Bruce Marshall and I) named a couple of years ago here at Te Papa was one we only discovered relatively recently that is unique to decaying baleen. And as far as I'm aware, that's the only species that has that unique habitat. It's a bit of a strange shape too, limpets haven't got a huge degree of flexibility in what shape they can be. Perhaps it's a cone, but it’s a cone that sits on top of a cylinder. One of the neat things about that, is if you look at the gap between the different scoots of the baleen, these limpets precisely compartmentalize that gap. The cylinder height is roughly half of the width of the gap, so you can have limpets on either side of the scoot and their little cones can interlock in the middle. So they're remarkably efficient at occupying almost all of the exposed surface of this decaying baleen.
So baleen is in the filter-feeding whales, the huge hair-like filters that they have instead of teeth.
The ‘hair-like’ is a key point too because chemically it’s hair-like: it's keratin. It's a remarkably inert long protein chain substance. The fact that anything derives nutrition from it at all was quite a surprise, let alone dedicate your life to it. I wouldn't want to guess the exact mechanisms of how it does so.
With the bone eating worms, there was a theory that they had evolved… and actually we've seen the scars… that they'd evolved in giant marine reptiles. And then, when the whales came along, they made that jump. But keratin in the deep sea seems very specific. So, would these be a more recent development? And where would they have come from? What were they eating before there were whales?
I love that question. And I've got no idea! There would have been keratin on ancient turtle carapaces, so there would have been some analogous structures that they might have transitioned from. But what that might be, I couldn't tell you. For most of these species, they're small mussels, they're small limpets. We have extensive fossil records of small mussels and limpets, but confidently equating this to one common ancestor or direct descendant of that one is bold.
But one of the things I do love about these groups, you get different species at hydrothermal vents, at methane seeps, at some of these organic falls. But there's one… I think it's in Japan, that was found in a polluted estuary near a wig factory. There is a thick layer of human hair in the estuary and this has created an anaerobic, anoxic chemo-symbiotic environment. So digging through the sediments and some of these polluted estuaries, you see these clams that are as happy as Larry.
“There is a thick layer of human hair in the estuary and this has created an anaerobic environment ... you see these clams that are as happy as Larry.”
That is so weird! Living on human wig hair is particularly weird. and so there's fundamental similarities when all these chemosynthetic habitats so we see not the same species but similar genera and families between the hydrothermal vents and the whale Falls or other other reducing environments. are there any unique adaptations to the vents in particular?
A lot of my work on vents has been trying to identify these often very limited or eroded samples. It's a toxic environment, it's a deep sea environment and it's a harder-to-sample environment. So we've got one or two shells collected by an ROV or a submersible and they all look the same. The main thing you need to overcome is the ethereal nature of venting systems. They will go through periods of activity and inactivity, and then the nature of the venting might change as well e.g. the temperature or the mineral content. So if you're a sessile benthic invertebrate, you can't move, you get what you're given. So one of the key adaptations is the long-lived larvae to allow, through these venting systems, persistence of taxa. Where at any individual site, might be a little bit vulnerable to some of the fluid dynamics of the vent itself. One of the more popularised groups are the ‘chainmail snail’ or ‘scaly foot gastropod’. They grow to about three or four centimeters. With the vents you get an extreme amount of heavy metals and various other things that are often toxic to life coming out of the water. When you're looking at a blacksmoker or a white smoker those are precipitates that are in liquid state when under pressure in the crust itself. But as they emerge, that pressure and temperature is released, they condense and so we get these chimneys forming of heavy metals. Some of the lighter elements generally form the clouds of the black and white smokers, which differ in what those elements actually happen to be. So much as the mangrove tree has to evolve a way to remove salt when it's in a high salt environment, that's excessively salty for healthy life. In some of these vent environments, the taxa have to evolve a manner of getting rid of some of the heavy metals that they ingest incidentally as part of their diet. The chainmail snail does this by secreting literal metal scales that line its foot. And it resembles dragon skin. I think they're remarkable.
Chain mail snail . Image courtesy of Dr Chong Chen
they're very goth. and there's color variations as well, there's like Reds and blacks and they're very cool looking!
Yeah, there's a pale one and… they would be a unremarkable brown but they often have other precipitates forming on them. So a lot of the colours we see of some of these deep sea molluscs, particularly in these unique environments, are a result of subsequent deposition of precipitates.
Could they be the starting nucleus of a lot of Manganese nodules could there be a tiny little shell in the middle of a lot of those?
I suspect so, but I don't know if I want to go public saying that. I think that's exactly what's going on there.
because there's been like Megalodon teeth and things like that. if they're precipitating manganese in life, then surely after a few hundred years…
I also wanted to touch upon the pelagics, but there is the teropods and a few other pelagic molluscs as well isn't there?
Yes, separate to the cephalopods, we've got a couple of different gastropod groups that are pelagic. We've got the Violet snails (Janthina) which are these extremely vivid purple or violet gastropods. They can grow up to about three centimeters and they float on the surface by forming bubble rafts. They've actually got counter shading, where the largest species Janthina janthina has a pale spire (which is the part that points down). So when viewed from the bottom, it resembles the sunlight, and is not that visible to a would-be predator. It actually looks like a sunbeam. So, these guys float on the surface and they hunt Portuguese Man of War ‘jellyfish’. So there's some unique species like that. We've got pelagic nudibranchs. We've got a couple of other very obscure pelagic groups. Perhaps the most ecologically important would be the pteropods or ‘sea butterflies’ which are abundant and largely unknown to most people because hey seldom wash up. Not all of them produce shells but most of them do and they'll be in the order of a couple of millimeters, up to the largest would be just shy of two centimeters. In its shell, the animal itself might be twice that size. Just with many of the other zooplankton, they'll float up and down according to what time of day it is and they form a substantial proportion of the diet for a very large amount of pelagic fish species.
getting onto the really deep stuff, we're getting mollusks right to the bottom of the deep trenches. I was always surprised at how their proteinaceous shell is like a soft membrane rather than a classic gastropod snail shell.
What's going on down there, is that: probably in large a part of the average mineral composition of the sediment in which these animals are living in. So most shells are comprised of two main layers, there's a carbonate layer which is the inside, or what you might think of as the shell itself. And then you've got what is your usually a thinner outer layer which is often translucent or transparent; often green or brown . This is chitinous; a similar structure to hair or fingernails, and that often brings the colour in some of the bivalves. Often it's the outer layer that gives a lot of the color and the shell itself is pale. According to needs or selective pressures, the molluscs can only work with what it has. If you haven't got much carbonate in the sediment, you can't produce a thick carbonate shell.
And then you've got to counterbalance that with the need for a thick shell according to rates of predation. Or who the predators actually are. Many predators can drill holes quite comfortably through shells almost regardless of their thickness. Some of the main predators in the deep sea would be octopuses, murex snails and moon snails. All three of those can drill holes through shells.
Is there a structural element to the shell or without predation pressure, can they be quite soft?
Yes a lot of shelled-species from shelled groups have lost their shell altogether. So, we've got some internal parasites that live inside sea cucumbers which are of groups where every other member of those groups have a fairly normal, perfectly thick shell. But where they live inside a sea cucumber, they're not worried about predation and those shells have evolved away.
So the shell is not a strict requirement?
For many taxa or environments it is. But again, that comes down to the selective pressures and what minerals are available in the first place for evolution to work with.
But it's not it's not providing any bodily structure? It’s about pushing through sediment or defending from predation?
It is in some species, but shells are not inherently necessary for molluscs for structure. So, for bivalves that have shells, if you took away the shell it's going to flop around and die - it's necessary. But you've got some groups where the animal fully envelopes the shell, and the shell has reduced. Maybe not disappeared entirely, but it no longer serves the same function that it used to have.
We talk about slugs and snails but a surprising number of slugs including the common Garden slug found throughout the world actually has a shell. It's got a tiny white shell; where the slug itself might be two-three centimeters and the shell might be three or four millimeters long.
I did not know that
Most common slugs will often have a little shell inside them. You could argue whether that's actually serving a structural function at all, or whether it's just left over but not such a burden that evolution has sought to get rid of it within just a few generations.
One thing we like to do is try and push against the annoying deep sea tropes. But I reckon I can throw it even broader. Is there anything you'd like to set the record straight on?
I've got a few…
One that that has come up in in regular conversation is that people keep flipping snails in Photoshop because the whirl looks better going the other way. Or it fits nicer into their composition.
It is remarkable how often you're looking at a photo of a snail and the shell coils in the wrong direction. And it makes you realise with birds or other animals or plants that don't have that same chirality, how often are these photos artistically or inadvertently flipped by the copy editor whoever it is.
So, the vast majority of coiled gastropod shells, coil in a certain direction. And there are very rare exceptions where through a genetic mishap, they might coil in the wrong direction, but it's an absolute minority. A couple of percent at most of species naturally coil in the sinisterly direction. There are a few species (and the examples that come to mind are terrestrial) where you've got both occurring in relatively high frequency; both sinisterly and dextrally coiled snails, within a conspecific population. There's an example I think from Europe where a snail is often eaten by a snake but the snakes would have an asymmetrical jaw where they could more easily feed on snails that coil in a certain direction. We create selection for snails that coil in less abundant direction, so you could call it ‘diversifying selection’ or ‘stabilizing selection’.
Have the snakes caught up yet?
There's probably one that got teased by his mates as a youngster, but now he gets more snails and he's bigger than the others.
Yeah it's just a constant arms race. but yeah, I've been walking around with you and seeing it like… ‘that snail’s on backwards, that snail's been flipped’. it's everywhere! and when you know to look for it…
I'm glad you brought that one up.
Thanks so much for having a chat!
It's a pleasure, thank you for having me!