Riley Thompson describes the unique way that siphonophores grow. Rather than have one body with many specialized parts, they have many bodies that are each specialized for particular tasks. For more information, see our medium post.This episode of CreatureCast was created by Riley Thompson, based on a script that we wrote together. The animation is based in part on illustrations by Freya Goetz. More animations can be found at creaturecast.org, a project supported in part by the National Science Foundation grant DEB-1256695. Music by Coda.
Here is a little plant that starts it’s life high up in the tree tops, where it can find more light than the dark understory of the rainforest. As it grows though, soon getting enough water becomes limiting factor, and the plant will drop a shoot to the ground.
Matt Ogburn, a graduate student in Erika Edwards’ lab at Brown University, describes this little plant, the strangler fig, and explains how it eventually grows to take over the whole host tree and strangle it to death.
Artwork and editing by Sophia Tintori. Original score by Amil Byleckie. Thanks to Jo Dery for use of her studio. Video released under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 license.
We found more jellyfish being born in our lab this week!
Rebecca Helm, a Dunn lab graduate student, left a couple of bowls of salt water and hydroids out on the table overnight, instead of the refrigerator where they usually live at around 50 or 60 degrees fahrenheit. The next day she came in and found them doing this:
This particular animal is called Podocoryna carnea. Like most jellies and close relatives of jellies, it has a pretty elaborate life cycle. This one involves a free swimming jellyfish, and a larva that swims around then lands on the back of a hermit crab’s shell. Then the larva metamorphoses into a polyp, which buds more polyps, growing into a whole colony on the crab’s back. The colony is made up of lots of polyps that are all connected and share fluid through a web of tubes that circulate partially digested food. Some members of this colony will eventually bud new swimming jellyfish.
The video at the top is of one of the colonies we have growing in our lab. These polyps were given to us by friends, but they can also be collected from hermit crabs at the beach, then grafted onto slides. They seem to grow well on slides, and slides are much easier to take care of then crabs.
Some of the polyps in the video have pink balls growing around the top. These are the buds that will mature to become free-swimming jellyfish. If you look closely, you can see jellies of all stages of maturity growing, including some that are ready to break free. After they swim off they will continue growing. We’ll try to follow up on how that goes.
Video by Sophia Tintori, life cycle drawing by Perrin Ireland, both released under a Creative Commons Attribution-Noncommercial-Share Alike license. Thanks to Diane Bridge and Neil Blackstone for the Podocoryna colonies. Check out this earlier post of the other polyps we saw budding jellyfish in our lab.
The sense that the self exists somewhere close to the brain or heart is an intuitive one for humans. It also seems to apply to most of the animals we regularly encounter, even when they can regrow parts of their body. When a crayfish gets into a tight spot and loses one of its claws, the part of the crayfish with the head will regrow the lost claw, but the claw won’t regrow a body and head.
For many animals, though, there is no such essential center of the organism. When a flatworm gets its tail cut off, both the tail and head will fill in the missing parts and make two whole flatworms that are clones of each other. Its body is arranged such that there isn’t a single part of the animal that can be identified as the core.
Here is a bit of footage taken by Stephanie Spielman, an undergraduate in Casey Dunn and Gary Wessel’s seminar on the evolution of multicellularity at Brown University. The clip features the flatworm Dugesia tigrina swimming around the Dunn lab. It is released under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 license. The crawfish video is from Day at the River (1928), a video from DeVry School Films, Inc., which is under public domain.
Early last year, at the Australian Antarctic Division (AAD), I saw an unusual sight: the birth of a live Antarctic krill, Euphausia superba.
The newborn appeared on a video screen that projected the view of a camera poised over a petri dish. A tremulous form emerged from its egg with its legs beating furiously!
This event began a continuing conversation with krill research leader, So Kawaguchi.
Back in my Sydney studio, I worked with So’s words and images. He explained (by email) how krill grow, and sent me diagrams by John Kirkwood to work with. I also found data sets online of how krill appendages move (Uwe Kils). Piano music was improvised by an 11 year old friend, Sophie Green.
This is the first of some animations that I am making to more fully describe this elusive and most important creature.
Krill are central to the marine life food web. Their health is endangered as a result of oceans becoming more acidic (as carbon increasingly enters the atmosphere and then dissolves into the water).
A new research project at the AAD is to record changes in normal krill development in increasingly acid water. Next month (June 2010) I return to the AAD krill nursery to find out more about this research.
I will also record So Kawaguchi describe what he has identified as a circling krill mating dance. What a fine gesture of continuity!
This video is released by Lisa Roberts under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 license. More animations can be found at AntarcticAnimation.com.
In 2002, while out roaming the depths in Monterey Bay Canyon with the remote operated vehicle (ROV) Tiburon, MBARI scientist Robert Vrijenhoek stumbled upon a whale carcass on the ocean floor, and noticed that it had its own little ecosystem. When a whale has died, its skeleton drops to the ocean floor, creating a habitat island in the depths. Creatures apparently gather from far and wide to use the whale carcass’ nutrients and living space.
Scientists have categorized four stages of whale carcass ecosystems- first the “mobile scavengers” show up, such as sharks, crabs, hagfish. These guys pick away at what luscious meat remains. Snails, slugs and worms show up next to make use of the nutrient-rich poo (in science speak, “organically rich sediment”) the larger scavengers have left behind. The third stage is comprised of animals that rely on hydrogen sulfide gas emitted from the decomposing bones and organic sediments. These animals, like vesicomyid clams, depend on symbiotic bacteria that live inside their cells to make energy for the animal from sulfur based compounds. Free-living bacteria that also live off sulfur form in mats that coat the bones. The final stage of a whale bone’s community succession is the reef stage, when most of the nutrients the whale bone can provide have been exhausted, and the minerals remaining in the bone provide a surface for suspension and filter feeders, who rely on the ocean currents to bring food their way.
When Vrijenhoek and his colleagues were at depth checking out whalebone world, they noticed little red worms that they were unable to identify all over the remaining whalebones. They collected a sample and send the worms to worm expert Greg Rouse, who informed them they had discovered a new species. Related to tube worms that live at the mouths of hydrothermal vents, Osedax grow at their longest to be about the length of your index finger, and as thick as a pencil. Penetrating deep into the marrow cavities of the whalebones are their elaborate root systems. These roots house bacteria that help the worms extract and digest nutrients from the bone, as they lack stomachs and digestive tubes.
Perhaps most bizarre and enticing about the Osedax worm is that all the worms the scientists first discovered appeared to be reproductive females, with no males in sight. Eventually they found the tiny males living in tubes along the female’s trunk. An Osedax female essentially has a harem of up to fourteen males that do nothing else but provide sperm for the eggs she produces. Osedax males feed for their entire lives on yolk provisioned by the egg from which they hatched, like forty year olds living at home on Mom’s meatloaf. The males look strikingly similar to Osedax larva, suggesting that they are larva in arrested development that began producing sperm.
Most of the eggs exiting the female are already fertilized. But how do those little guys lying along her trunk scoot their sperm up to catch the eggs as they’re on the way out? And how, then, do larvae being flung into the dark beyond know whether to become male or female? It could be possible that sex determination depends on whether a larva lands on bone or lands on another female. Perhaps similar to hydrothermal vent worms, a juvenile becomes a male if it lands on a female and she releases a chemical, enticing it into her little harem, to do her reproductive bidding.
Mushrooms may look mundane, but they’ve got a lot going on underneath the surface. In animals, each cell in a body contains one nucleus, and each nucleus has 2 copies of the genome, one from the mother, and one from the father, which fused at fertilization. Unlike in animals, where the nuclei of the egg and sperm quickly join after the cells combine, the nuclei in mushroom cells stay separate. The reason for the difference boils down to the particular way fungi have sex.
Frisky fungi creep through the soil with long filaments. These moldy structures occupy the spaces between dirt, and allow the organisms to digest organic matter. They’re also great for mating. Fungi spend much of their lives with only a single nucleus. Except, that is, when two filaments cross paths.
When two lonely filaments find each other, the cells at the tip of the filaments fuse, and form new structures that have two nuclei per cell. This cell with two nuclei takes on a life of it’s own and divides many times to form a mushroom. Each mushroom cell contains a copy of each of the parent nucleus. The nuclei only fuse in the mushroom gills (pictured), just prior to the formation of mushrooms spores, which are then carried away by the breeze, off to seed the next generation of fungi.
Photographs of the basidiomycete Agaricus bisporus by Rebecca Helm.
Last month we posted a video of a siphonophore (one of the Dunn lab’s favorite animals) swimming freely in the ocean. In this next installment of CreatureCast, Casey Dunn describes how siphonophores help us question what we think of as an individual.
There are different ways to think of individuality. Individuality can refer to function- whether an organism operates and interacts with the world as a unit. A fish is a functional individual, but so is an ant colony. Individuality can refer to evolutionary descent. In this respect our liver is not an individual, there was no ancestral free-living livers out there that our liver is descended from. But our mitochondria are individuals in this sense. They evolved from free-living bacteria that became incorporated into other cells. Individuality can also refer to the process of evolution. In this sense an individual is any entity that has the properties necessary for evolution by natural selection- it reproduces and has variable heritable properties that influence the chances of survival. This could be a free living cell, a cell in a body, an entire multicellular organism, and even groups of organisms in some cases.
All of these definitions of individuality are in alignment in most of the organisms we are familiar with. A bird, a rose bush, and a fly are all individuals as functional entities, according to their ancestry, and as units of selection. This makes it easy to get lulled into thinking of individuality as a monolithic property.
A siphonophore colony is a functional individual. But a siphonophore colony is made up of many parts that are each equivalent to free living organisms such as sea anemones and “true” jellyfish. So by the evolutionary descent definition it is a collection of individuals. The colony as a whole is acted upon by natural selection, making it an individual in the sense of the process of evolution. But it is entirely unclear whether natural selection can act on the parts within the colony, as it does on our own cells when we get cancer, since we don’t know about the heritability between the parts of the colony.
Siphonophores, by forcing us to disentangle what we mean when we call something an individual, help us understand the evolutionary origins of individuality. These different aspects of individuality don’t necessarily evolve at the same time, and one or more of them can even be lost. Organisms like siphonophores provide glimpses of these different combinations of individuality.
The song New Homes is by Lucky Dragons, the siphonophore video is from Dr. Steve Haddock at MBARI, the podcast was produced by Sophia Tintori, and the video is published under a Creative Commons Attribution Non-Commercial Share Alike 3.0 license.
Look what we caught happening in our refrigerator.
While doing a fridge clean-out in the Dunn Lab, graduate student Rebecca Helm took a look at a forgotten bowl of Chrysaora colorata polyps from our friends Chad Widmer and Wyatt Patry at the Monterey Bay Aquarium. These creatures were left over from an RNA extraction we had done earlier for the Cnidarian Tree of Life Project, and were hidden in the back of the fridge, despite the labs strict ‘no pets’ rule.
Upon inspection, Rebecca noticed that the polyps were strobilating! This is a spectacular type of asexual reproduction, which is explained in more depth in Perrin Ireland’s post on the scyphozoan life cycle.
In this video, a polyp has pinched off into a stack of plate-like discs, called ephyrae. When they pop off of the end of the polyp, they each become a free swimming individual, and a direct clone of the parent polyp. Each ephyra will mature into adult bell-shaped jellyfish. Even before they break away from the poly, they are strongly pulsating as they flex their newly developed swimming muscles before birth.
Video by R. Helm and S. Siebert.
I recently attended a meeting between the Dunn and Wessel labs about the evolution of the mouth and anus. A new paper by Mark Martindale and Andreas Hejnol offers a hypothesis on the origin of these very important holes. Many animals, including jellyfish and their relatives (e.g. sea anemones and Hydra), have a single opening to their gut, so they eat food and release waste from the same opening. Most bilaterian animals (e.g. humans, fish, snails, and so on), which diverged from jellyfish a long time ago in the course of evolution, have two openings. These two holes create a through gut: a tube that takes in food at one end (the mouth) and releases waste at the other end (the anus). This raises a couple straightforward questions. Some animals have one hole, others have two—how did this happen? Does the single hole in one-holed animals correspond to the mouth or anus of animals with two holes?
There are a few hypotheses. Most invoke a hypothetical ancestor called the “Gastrea” which was postulated to have a single opening to the gut at the tail end of the animal and a sensory organ on the head end. This hypothesis relies largely on observations of jellyfish embryos. A single hole forms in the embryo, which then grows into a swimming larva. The “head” and “tail” ends were assumed to correspond to the swimming direction of the larva. There is a sensory organ is on the leading end, which was interpreted as the “head”, and a single orifice on the the trailing end, which was interpreted as the “tail”. This single hole was ascribed to be the anus since it was on the trailing end. This hypothesis therefore implies that the one hole in one-holed animals corresponds to the anus in two holed animals.
Molecular analysis, however, suggests otherwise. All animals start out in development with one hole, the blastopore. If there are two holes, the second hole forms later. The blastopore can arise at the top or the bottom of the embryo. In the jellyfish and their relatives the blastopore forms at the top of the embryo and becomes the dual-functioning hole of the adult. Blastopore formation is started by a protein called disheveled, which gets stuck at the top of the egg and then activates a specific set of genes. In the same location of jellyfish embryos, however, there are genes strikingly similar to the mouth genes of bilaterians. In the sea urchin, a bilaterian, these same mouth genes are also on the top of the embryo. However, disheveled has moved to the bottom. The blastopore forms at this new site of disheveled accumulation, rather than at the mouth. The mouth genes that remain on top still direct the formation of the mouth there. Martindale and Hejnol posit that moving disheveled from the top to the bottom of the embryo in some animals moved the location of blastopore, but that the mouth stayed put. In some bilaterians, like urchins and humans, the blastopore then became the anus. In this scenario all mouths are homologous to each other, whether the animal has one or two holes. The site of gastrulation, however, can move from the mouth to the anus and therefore can’t be used to identify which hole is which as it had been in the Gastrea hypothesis. It also indicates that jellyfish larva swim backwards, with their mouth on the trailing end.
By changing the location where a genetic program is activated, a huge and incredibly important change in body plan occurred. The same sets of genes appear in many different contexts within and across species. But the relationships between those genes are often consistent, as a sort of molecular module. In this case there are two distinct modules for mouth and blastopore, and they can be decoupled. Learning that genes evolve in modules was somewhat of an epiphany for me as a new student. Even if one does not care about the evolution of the mouth and anus, this story demonstrate the power of comparative evolutionary biology. Using model organisms (like fruit flies, mice, yeast, etc) to understand human biology is commonplace, but the study of evolution across a broader diversity of species can give us far more detail about what specific changes occurred to create the differences we see.
Photos by Casey Dunn. The sea anemone Nematostella vectensis, a cnidarian that has a single hole for eating, excreting, and shedding eggs and sperm, is on the left. This opening is at the top of the photo, between the tentacles. The annelid worm Buskiella vitjasi, whose through-gut can be seen through its transparent body, is on the right. Like other bilaterians, one end of its gut terminates in a mouth (at the top of the photo) and the other at the anus (at the bottom).