Bone Boring Worms

posted by Perrin Ireland / on May 25th, 2010 / in Annelids, Development, lifecycles

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.

A Tale of Two Nuclei

posted by Rebecca Helm / on May 16th, 2010 / in Development, Fungi, lifecycles

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.

CreatureCast – Picky Females

posted by Sophia Tintori / on January 14th, 2010 / in lifecycles, Podcast

A couple of weeks ago the Dunn lab went out after work, and we got to talking. There’s this thing that usually happens whenever we get together after a day in the lab or field– being a group where everyone focuses in one way or another on the diversity and evolution of reproduction and development, we start to tell stories about how animals reproduce. Someone mentions some surprising tidbit of reproductive biology they recently heard, and that sets it off. Then someone else remembers a weirder story, and tells it. This spurs someone else’s memory, and so on, and then I start feeling overwhelmed.

Well, this time we got caught up on the issue of female choosiness. It takes more energy and resources to make an egg packed with resources, or to raise offspring, or to carry a baby inside the womb, than it does to make sperm. This often leads females to be more selective about their mates than males are. We started talking about ways in which female choosiness manifests itself; sometimes through behavior, sometime through anatomy, and sometimes at the level of the cell. And then sometimes it is all for naught.

In this episode of CreatureCast Rebecca Helm, a graduate student in the Dunn Lab, recounts a few short stories about the many levels of reproductive selection.

Editing and animation by Sophia Tintori. We Want To Be Old by Bird Names. Photos of bowers by Mila Zinkova and Peter Halasz. Duck story from the research of Richard Prum and Patricia Brennan. Video of the inside of a comb jelly egg by Christian Sardet, Danielle Carré and Christian Rouviere, from the BioMarCell group. This video is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.

Stack of plates in action

posted by Sophia Tintori / on December 17th, 2009 / in Development, Jellies, lab, lifecycles

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.

Making babies like a stack of plates

posted by Perrin Ireland / on December 10th, 2009 / in Jellies, lifecycles

aurelia500

Our own lifecycles are pretty simple. Making babies requires sex. Sex creates offspring with new unique combinations of genes. Many organisms are also capable of asexual reproduction, which doesn’t involve sex (as the name implies) and involves only one parent. In most types of asexual reproduction, genes aren’t reshuffled and the offspring are genetic clones of their parent.

Unlike ourselves, many species have lifecycles that combine both  sexual and asexual reproduction. Take the moon jelly, for example. Moon jellies, also known as Aurelia aurita, are perhaps the quintessential jellyfish, with a typical umbrella-like medusa that travels on ocean currents. There is more to their lifecyle, though, than this swimming organism. The swimming medusa does use sex to make babies—but the babies don’t grow directly into swimming medusae. Medusae release their eggs and sperm into the water and these combine to form a zygote (the fertilized egg). The zygote then develops into a planula larva. The planula eventually sinks to the ocean floor and develops into a polyp, an organism that looks nothing like a medusa. Polyps are attached to the ocean floor, usually on a rock or other hard surface, and stay in one place their whole life. They have a mouth surounded by tentacles, just like the more familiar polyps of sea anemones and Hydra. These polyps, however, are incapable of having sex—they cannot make eggs and sperm. Instead, they reproduce asexually. They can asexually produce other polyps, but they can also asexually produce miniature medusae called ephyra. These are pinched off from the polyp’s mouth as if they were a stack of plates, with the most mature medusa on top. The ephyra then swim away, grow into mature medusae, and complete the lifecycle.

Thou shalt covet thy neighbor’s cnidocytes

posted by Christopher Laumer / on November 30th, 2009 / in lifecycles

Microstomum lineare 20x 3

Hydra viridis

Microstotum caudatum

A small clarification, dear reader: in a recent post about the fantastic stinging cells of the Cnidaria (jellyfish and their relatives), it was stated that only cnidarians possess these cnidocytes. It is surely true that only cnidarians can make these barb cells. However, the animal kingdom has found these diverse structures useful enough that thievery of a sort has evolved, in lineages as distinct as comb jellies and sea slugs.

Consider the case of Microstomum lineare, a common resident of organic slimes in slow segments of flowing waters worldwide. These tiny flatworms spend most of the year eating detritus and dividing asexually into new clones. When in need of defense, however, the worms seek out and consume bits of the freshwater cnidarian Hydra, a favorite study organism of biologists.  The parts of Hydra that are consumed are digested by enzymes in the gut which leave intact only the stinging part of the cnidocyte. Cells of the gut then enclose these stinging cysts, pass them off to cells of the connective tissue, and ultimately, to the skin, where they are used as a means of defense and prey capture, much as the Hydras themselves use them. Remarkably, Microstomum has found a way to prevent these otherwise hypersensitive cysts from firing until the very end stage of this process of manipulation. The cysts persist in the skin until used, and can be passed onto clonal offspring, grand-offspring, and beyond. Even clonal lines that have not been exposed to Hydra for tens of generations will exhibit this behavior, but a Microstomum with a full stock of cnidocytes will ignore Hydra completely.

Photographs of Microstomum lineare (top: whole animal, dorsal view; bottom: head, ventral view, showing stolen cnidocytes), and the tentacles of Hydra viridis, a favorite source of cnidocytes, were taken by Christopher Laumer.

Star colonies of sea squirts

posted by Perrin Ireland / on November 17th, 2009 / in Chordates, lifecycles

Botryllus_s

Botryllus schlosseri is a colonial tunicate (or sea squirt), so named because it lives in colonies that are communally covered by a leathery tunic. Its larvae bear a striking resemblance to vertebrates, and are even called tadpoles. The resemblance is not superficial or coincidental, tunicates and their kin are the closest living relatives of vertebrates. Each tadpole attaches itself to a rock, pier, or other hard surface in the sea, and metamorphoses into a sack-like adult that will spend the rest of its life stuck in that one spot.

Tadpoles are produced sexually–they arise from an egg that is fertilized by a sperm. Like many other animals, though, Botryllus also reproduces asexually by budding off clones of itself. Each adult (also called a zooid) produces a bud, and this bud in turn begins producing another bud even before its own heart begins to beat. These clones remain attached to each other in a star shaped group with common central opening, called a siphon, and continue to share resources through their connected circulatory systems. Once the colony is large and robust enough, usually with 5 to 10 members, each adult forms a pair of ovaries and testes, and the next generation of tadpole larvae can be produced.

This is the first in a series of illustrated lifecycles I’ll be posting to CreatureCast.