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Two days ago, Ferrebeekeeper wrote about Earth’s magnetic field, an underappreciated invisible force-field which keeps the planet habitable by preventing solar wind from blowing away our atmosphere and oceans (we need those!). Long ago, Venus and Mars seemingly had liquid oceans and nice atmospheres, but something went wrong (?) with their magnetic fields a billion or so years ago, and just look at them now (tuts censoriously). But maybe we shouldn’t be so quick to judge our neighbors…Five hundred and sixty-five million years ago, the Earth underwent a magnetic crisis too.
Geologists have been studying fragments of plagioclase and clinopyroxene from the ancient continental shield of Canada to learn about the state of the planet’s magnetic fields in the ancient past. As they form, these crystals trap tiny magnetized iron fragments in place like the needles of little compasses. Scientists can thus study the deep history of the magnetosphere. As they studied magnetic crystals that were formed 565 million years ago, they found some troubling things: half a billion years ago, the Earth’s magnetic field was over 10 times weaker than what it is today. Additionally the poles were rapidly fluctuating between north and south at an unexpected rate.
A closer reading of all of this suggests that 550 million years ago the Earth’s magnetic field nearly collapsed! (for a look at what that means, just walk around Mars). Life was saved because the solid nickel iron core of Earth nucleated from the molten core at that time. Instead of a field collapse, our magnetic field became much stronger as the spinning solid inner core and the convection cycles of the molten outer core worked together to form a super geodynamo. Coincidentally, 541 million years ago is familiar to paleontologists as the inception of the Cambrian explosion, when multitudinous animal life forms appeared on Earth. It is such an important point that it divides the Phanerozoic (filled with mushrooms, megafauna, liverworts, and Roman centurions) from the Proterozoic (billions of years of bacterial soup). Just a coincidence?
A large single tunicate (blue) with a colony of smaller tunicates (saffron)
It’s time to talk tunicates! Many people blithely dismiss tunicates as primitive sack-like marine invertebrates which derive sustenance from filter-feeding. Although that is technically true, it is a very reductive and dismissive way to think about this ancient, ancient subphylum of animals. Tunicates are chordates…barely, but they are also classified as invertebrates. Because they mostly consist of delicate tissue sacks filled with fluids, the fossil record of tunicates is understandably exiguous, but it is believed they existed in Ediacaran times (circa 550 million years ago) and were part of the mysterious soft Ediacaran biota which blossomed into the Cambrian era’s suffusion of life forms. Tunicates probably closely resemble the basal organisms from which Pikaia and all other vertebrates (lynxes, caecilians, hummingbirds, triggerfish, humans, ichthyosaurs, turkeys, moeritheriums, and suchlike animals) sprang. Of course tunicates also resemble hydrozoans, mollusks, worms, and even arthropods—so they may be very basal indeed!
A free-swimming larval tunicate (microphoto by Wim van Egmond of Rotterdam)
All–or very nearly all–tunicates are hermaphrodites with both male and female reproductive organs (a single ovary and a lone testis). Not only do tunicates keep their romantic options open, they also metamorphose into different forms throughout their lives. The majority of tunicates have a free-swimming larval stage when they are motile (and have a little sliver of nerve chord). As they reach sexual maturity, their nerve chords disintegrate and they settle down to become sessile—attaching to a permanent base. Some tunicates live their entire lives as solitary individuals whereas others form colonies (like corals or siphonophores).
Not only do they have multiple genders they have multiple methods of reproduction
Colonial tunicates integrate at different levels depending on the species. In some, the zooids (the individual living organisms) merely live next to each other like coral or Brooklynites, whereas other tunicate colonies grow entwined and share common organs and anatomical structures. There are many different tunicates going by many different lifestyles and they have all sorts of crazy names. Wikipedia poetically avers that “…various species are commonly known as sea squirts, sea pork, sea livers, or sea tulips.”
Komodo National Park sea squirt (Polycarpa aurata) by nick Hobgood
As you could guess from these names, tunicates have an otherworldly beauty. Here is one which looks like a diseased zombie heart! Others look like transparent alien shrimp, fluorescent pens, or strangely hieroglyphed eyes. There are bioluminescent tunicates of the deep ocean, and pelagic tunicates that form long chains (with a single digestive tract running through the individual zooids). They live in coastal waters, pelagic waters, and in the depths.
Colonial tunicate with multiple openings in each zooid
Most of these zoology articles end with a sad coda about how the subject organism is threatened in the modern world–no so for tunicates! As humans overfish the oceans and drive countless teleosts to the edge of extinction, so-called primitive species like jellyfish and tunicates are flourishing! Acidification, climate change, and pollution seem to be resetting the great worldsea back to Neoproterzoic times. Additionally tunicates easily travel the world in ballast water and numerous species are becoming invasive pests (like the evocatively named carpet tunicate). In this troubled era, there is raw power in being a primitive protean organism with only a wisp of a nervous system (as we should have known just by looking at successful late-night comedians). Get used to the tunicate–not just an incredibly distant ancestor, but the once and future (and always) avatar of animal life in the oceans.
Chain of fluorescent tunicates. (photo by Francis Abbott/Nature Picture Library)
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