Ocean acidification is complicated, case in point

I once wrote about the complicated way in which ocean acidification is mostly really bad for marine creatures with calcium carbonate shells/skeletons. Well, today, while reading a book I thought had nothing to do with ocean acidification, I came across a report of one such creature for whom the change is apparently for the better. (I’d expected to find all kinds of interesting information in Embryos in Deep Time, but this was a surprise…)

Dupont et al. (2010) studied common sun stars (above; Bernard Picton, habitas.org.uk), following the larvae right up to metamorphosis under current CO2 and pH values of their home seas, and also under a near-future predicted scenario with higher CO2 concentration and lower sea pH. Surprisingly, the larvae in the “future” tanks survived just as well, grew better, and showed no obvious defects in development or calcification compared to the control group.

The authors speculate that this might be related to the reproductive strategy of these animals. While the larvae of many echinoderms have very little yolk in their eggs and have to feed the moment they look vaguely like an animal, sun star larvae are provided with a lot of yolk that can sustain them until they’re ready to metamorphose. So they don’t have to face the burdens of hunting for food; all their energy can go towards growing, which might make them more resilient to harmful environmental effects.

I’m not sure I buy such a simplistic explanation – first, other echinoderms with a similar developmental strategy suffer quite badly in similar conditions; and second, they only examined one species during the early stage of its life cycle. In fact, the authors point out these exact same caveats. (Plus the creatures not only resisted acidification, they thrived.)

Whatever the mechanism, though, Dupont et al.‘s data show that there is at least one animal for which ocean acidification may be a boon. Considering that this guy happens to be a top predator in its ecosystem, that could have major consequences for said ecosystem.

Also, they are incredibly pretty. Echinoderms rock.

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Reference

Dupont S et al. (2010) Near future ocean acidification increases growth rate of the lecithotrophic larvae and juveniles of the sea star Crossaster papposus. Journal of Experimental Zoology 314B:382–389

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I couldn’t resist

Damn, I said I wasn’t going to talk about the Moroccan helicoplacoid-on-stalk, but it’s just so. Bloody. Amazing.

Here it is in its full glory, from the supplementary figures of Smith and Zamora (2013). Left is a cast of a young specimen, right is the authors’ reconstruction of the adult creature:

helicocystis_casthelicocystis_recon

So… the thing is a transitional form all right. It’s got a little stalk and cup like eocrinoids, built with a rather irregular arrangement of mineralised plates. On top of that it has a spiral body like helicoplacoids. It has ambulacra, the “rays” with porous plates where the tube feet that characterise living echinoderms can come out. This photo of the underside of a starfish is a pretty nice illustration of ambulacra (the white regions with little holes) and tube feet:

Even more interestingly, the new beastie (christened Helicocystis moroccoensis by the authors) seems to have five of them, like modern echinoderms (and a lot of extinct types, including eocrinoids). Helicoplacoids do have ambulacra, but only three or a single Y-shaped one, depending on interpetation.

Again unlike (one interpretation of) helicoplacoids but like modern echinoderms, the mouth of Helicocystis is right at the stalkless end. It’s also surrounded by an arrangement of skeletal plates that resembles more “conventional” echinoderms and has no equivalent in helicoplacoids proper. It’s about as neat a transitional form as you could hope for.

The question is which way the transition goes. It could be that the familiar five-rayed echinoderms are derived from a helicoplacoid-like ancestor, going through something like this guy. Or it could be that helicoplacoids are actually weird even for echinoderms, and their ancestors were more conventional stalked, five-armed beasties that lost their proper echinoderm shapes via something like Helicocystis.

Smith and Zamora actually did a phylogenetic analysis, but it’s not that helpful IMO. The tree in the paper is very pretty, and it says Helicocystis is the next branch after helicoplacoids on the path leading to “proper” echinoderms. The tree in the supplementary figures actually has measures of statistical support on it – which pretty confidently put Helicoplacus, Helicocystis, and a bunch of less weird echinoderms, together.

However, the relationships within that group are, shall we say, a little bit fluid. Granted, I come from a more sequency background and don’t often have to deal with morphology-based trees or parsimony as the method of analysis – but I’d definitely view a 56% bootstrap support with a big dose of scepticism, and this is the number they got for the hypothesis that Helicocystis is more closely related to “proper” echinoderms than to Helicoplacus. The other measure they display doesn’t make me any more confident about the relationship.

(I find it kind of amazing they got any resolution at all in that tree – with only 17 characters, some of which aren’t applicable to all species, and only nine species to begin with… yeah. The whole phylogenetic analysis is far from ideal even if it’s the best they could think of.)

So, based on that tree, the phylogenetic hypothesis they present is, at this point, just a plausible hypothesis. That doesn’t lessen the value of Helicocystis, though. The creature is still a damn neat transitional form – we just can’t be terribly sure which way the transition went.

There’s some interesting speculation in the paper about developmental evolution (yay!). Smith and Zamora point out that the spirally bit in Helicocystis looks like a complete helicoplacoid; the stalk and cup are kind of tacked onto that. The tissues of most modern echinoderm adults come from two different places: regular old tissues of the larva, and a special set of cells set aside for adult-making purposes*. So Smith and Zamora hypothesise that the two-part body of Helicocystis marks the point where this dual origin appeared. (Or, if they’re wrong about the phylogeny, the point where proto-helicoplacoids lost it?)

There’s also another interesting bit of evo-devo speculation (mixed with a bit of “eco”) about the stalk. Full-grown Helicocystis have pretty small stalks compared both to their own young and more typical stalked echinoderms. The authors wonder if this is because stalks for attachment originally functioned to help young echinoderms settle in a comfortable place, and only later became important for adults. I’m not sure how much sense that actually makes, and of course we only have a single species of Helicocystis to go by, but hey, ideas are fun.

Helicocystis has a random weird quirk as well, in that its spirals curl the opposite way to every proper helicoplacoid. That sort of variation happens even within species (e.g. in snail shells), but isn’t it a weird coincidence that such a unique creature should also twist the wrong way?

One thing is for sure: this beast is made of pure, distilled awesome. I think we should make a new Archaeopteryx out of it. Invertebrates need their evolutionary icons, too!

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*And that’s a nice reminder for me, because I thought they basically threw away the larva. Apparently I need a refresher on echinoderm development. Or just a reminder that not all echinoderms are sea urchins. The funny thing is a couple of years ago I actually specifically read and puzzled over literature discussing what comes from where in various echinderms…

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Reference:

Smith AB, Zamora S (2013) Cambrian spiral-plated echinoderms from Gondwana reveal the earliest pentaradial body plan. Proceedings of the Royal Society B advance online publication 26/06/2013, doi:10.1098/rspb.2013.1197

Echinoderm bonanza

Smith et al. (2013) has been sitting on my desktop waiting to be read for the last month or so. Man, am I glad that I finally opened the thing. I’m quite fond of echinoderms, and this paper is full of them. Of course. It’s about echinoderms. Specifically, it’s about the diverse menagerie of them that existed, it seems, a little bit earlier than thought.

The brief little paper introduces new echinoderm finds from two Mid-Cambrian formations in Morocco, which at the time was part of the great continent of Gondwana. As far as I’m concerned, it was worth reading just for this lineup of Cambrian echinoderms. I mean, echinoderms are so amazingly weird in such a variety of ways. They’re a delight.

Smith_etal2013-fig3_decorated

(The drawings themselves are from Fig. 3. of the paper; I rearranged them to fit into my post width, and the boxes are my additions. Dark box = new groups/species from Morocco, light grey box = known groups/species whose first appearance was pushed back in time by the Moroccan finds.)

Although none of the creatures above belong to the living classes of echinoderms, they display a wide range of body plans. You could say their body plans are more diverse* than those of living echinoderms. (And if you said that, the ghost of Stephen Jay Gould would nod approvingly.) For example, modern echinoderms tend to have either (usually five-part) radial symmetry (any old starfish) or bilateral symmetry that clearly comes from radial symmetry (heart urchins).

In these Early- to Mid-Cambrian varieties, you can see some five-rayed creatures, some that are more or less bilateral without any obvious connection to the prototypical five-point star, animals that are just kind of asymmetric, and those strange spindle-shaped helicoplacoids that look like someone took an animal with radial symmetry and wrung it out. And then there are all the various arrangements of arms and stalks and armour plates that I tend to gloss over when reading about the beasts. (Yeah. I have no attention span.)

The Morroccan finds have some very interesting highlights. The second creature in the lineup above is one of them. Its top half looks like a helicoplacoid such as Helicoplacus itself (first drawing). It’s got that characteristic spiral arrangement of plates and a mouth at the top end. However, unlike previously known helicoplacoids, it sits on a stalk that resembles the radially-symmetric eocrinoids (like the creature on its right). It’s a transitional form all right, though we’ll have to wait for future publications and perhaps future discoveries to see which way evolution actually went. It’ll already help palaeontologists make sense of helicoplacoids themselves, though, which I gather is a big thing in itself. The authors promise to publish a proper description of the creature, which is really exciting.

The other exciting thing about the Moroccan echinoderms is their age. As I already hinted at with my grey boxes, the new fossils push back the known time range of many echinoderm body plans by millions of years. This means that the wide variety of body plans we saw above was already present as little as 10-15 million years after the first appearance of scattered bits of echinoderm skeleton in the fossil record.

Smith et al. argue that this is a fairly solid conclusion based on the mineralogy of echinoderm skeletons. Organisms with calcium carbonate hard parts have a tendency to adopt the “easiest” mineralogy at the time they first evolve skeletons. Seawater composition changes over geological time; most importantly, the ratio of calcium to magnesium fluctuates. Calcium carbonate can adopt several different crystal forms, and the Ca/Mg ratio influences which of them are easier to make. So when there’s a lot of Mg in the sea, aragonite is the “natural” choice, whereas low Mg levels favour calcite.

The first appearance of echinoderms around 525 million years ago coincides with a shift in ocean chemistry from “aragonite seas” to “calcite seas”. Echinoderms and a bunch of other groups that first show up around that time have skeletons that are calcite in their structure but incorporate a lot of Mg. Since the ocean before was favourable to aragonite, it’s unlikely that echinoderm skeletons appeared much earlier than this date. In other words, echinoderm evolution during this geologically short period was truly worthy of the name “Cambrian explosion”.

That is, of course, if the appearance of echinoderm skeletons precedes the appearance of echinoderm body plans. The oldest of our Cambrian treasure troves of soft-bodied fossils, such as the rocks that yielded the Chengjiang biota of China, are roughly the same age as the first echinoderm skeletons. However, they don’t contain undisputed echinoderms as far as I can tell (Clausen et al., 2010). Proposed “echinoderms” from before the Cambrian are even less accepted. Of course, the unique structure of echinoderm skeletons is easy to recognise, but how do you identify an echinoderm ancestor without such a skeleton? (Is all that bodyplan diversity even possible without hard skeletal support?)

Caveats aside, this Moroccan stuff is awesome. And also, if my caveat proves overly cautious, echinoderms did some serious evolving in their first few million years on earth. A supersonic ride with Macroevolution Airlines?

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*OK, if I want to be absolutely pedantic, and I do, then body plans are disparate rather than diverse. “Disparity” in palaeontological/evo-devo parlance refers to how different two or more creatures are. Diversity means how many different creatures there are. Maybe I should do a post on that, actually.

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References:

Clausen S et al. (2010) The absence of echinoderms from the Lower Cambrian Chengjiang fauna of China: Palaeoecological and palaeogeographical implications. Palaeogeography, Palaeoclimatology, Palaeoecology 294:133-141

Smith AB et al. (2013) The oldest echinoderm faunas from Gondwana show that echinoderm body plan diversification was rapid. Nature Communications 4:1385

Before they became weird

Echinoderms are weird. They are supposed to be bilaterian animals, but they have abandoned bilateral (mirror image) symmetry for looking like fleshy stars, spiny boobs, strange flowers or funky sausages. When they first appear in the fossil record during the Cambrian period*, they show up with an even weirder menagerie of body plans ranging from almost bilateral through asymmetric to all sorts of variations and twists on the standard five-rayed body plan that we know and love. (Below: a selection of weird and wonderful Cambrian echinoderms from Zamora et al. [2012])

(We only know that some of these creatures were echinoderms or very close relatives thereof because they have skeletons with a unique spongy microstructure (stereom) only seen in echinoderms.)

I don’t know nearly enough about echinoderms to properly discuss the latest addition to the march of the weirdos, but damn me if I don’t at least give a proper fangirlish SQUEEE! to a new Cambrian echinoderm – with bilateral symmetry! Zamora et al. (2012) actually describe two fossil finds, but one of them is new specimens of a previously known animal. However, the other is brand new, and what a pretty thing, too! Behold Ctenoimbricata spinosa, straight out of science fiction – or a nightmare :-P! (OK, don’t start having Ctenoimbricata nightmares just yet. The whole animal was less than an inch long.)

The creature was reconstructed from fossils found in Middle Cambrian (about 510 million years old) rocks in northern Spain. The shape of its body and the arrangement of its many armour plates most closely resemble an obscure group of ancient echinoderms called ctenocystoids (represented by fossil A in the first picture). Typical ctenocystoids have slight asymmetries manifested as different arrangements of armour plates on their left and right sides. However, some are well-behaved bilaterians. That’s the other point of the paper: new fossils belonging to a previously known ctenocystoid demonstrate its symmetry. The authors think that the similarly symmetrical Ctenoimbricata was an even more primitive relative of ctenocystoids. In their view, echinoderms started out with mirror image symmetry, then became asymmetric, and only then did they evolve the radial symmetry starfish exemplify.

Ctenoimbricata, according to Zamora et al., is the most primitive echinoderm ever found. The fact that it doesn’t have a stalk or arms suggests that it wasn’t a filter feeder. Instead, it probably operated flat on the seafloor, gulping sediment and sifting out the edible bits. Since ctenocystoids are also stalk- and armless, this might mean that the last common ancestor of all echinoderms lived in a similar way, which has apparently been a matter of some debate. Yay!

Incidentally, I had no idea that Europe had such awesome Cambrian fossils. I thought the best sites were all at a minimum of a half-day plane ride away. So: double squee for our tiny spiny sandmower!

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*People have argued that a Precambrian fossil called Arkarua may be an echinoderm ancestor, but I wouldn’t bet on that. Just about the only thing those tiny imprints can be shown to share with echinoderms is the five-part symmetry, and it’s not like unusual body symmetries were… unusual for Precambrian animals.

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Reference:

Zamora S et al. (2012) Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution. PLoS ONE 7: e38296