Making babies like a stack of plates

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

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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.

CreatureCast – Comb Jelly Movement

posted by Sophia Tintori / on December 6th, 2009 / in Comb Jellies, locomotion, Podcast (Student Contribution)

This installment of CreatureCast is the first of several contributions that were done as final projects by undergraduate students in Casey Dunn‘s Bio0410 Invertebrate Zoology class at Brown University. In episode 3 sophomore Lee Stevens discusses how comb jellies move the same way that many single-celled organisms do, which is remarkable given how much bigger comb jellies are.

This podcast is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Narrated and animated by Lee Stevens, with music by Tracky Birthday (this song, and also this one).

Pelagic plastic

posted by Erwin Keustermans / on November 30th, 2009 / in Science & Art

We have managed to create a huge memorial to human waste at a location that is remote from everyday human activity. In 2008 the predicted existence of a floating mass of pelagic plastic, a giant Garbage Patch, was confirmed in the stable waters of the North Atlantic gyre where plastic debris is accumulating over an area estimated to be twice the size of Texas.

Anna Hepler is a sculptor based in Portland, Maine. The subject of Hepler’s work is often the way a multitude of interlocked entities form a shape or a flock, spreading through space. On learning about the Garbage Patch, she incorporated it in a project for one of her first large scale installations. In January 2009 she spent a week together with eight assistants sowing together discarded plastic from a Portland recycling center. Once stitched together, the plastic nets formed a giant boat hull hanging from the walls and ceiling of the Center of Maine Contemporary Art in Rockport.

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An extended article about the project can be found here. Also, Hepler will be recreating this piece in the Portland Museum of Art in 2010 under the title ‘The Great Haul’.

Thou shalt covet thy neighbor’s cnidocytes

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

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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.

The art of knotting

posted by Stefan Siebert / on November 27th, 2009 / in Chordates

Hagfish have a skull, but no spine. They diverged from vertebrates prior to the origin of many other structures that are widespread within the group, including jaws. Hagfish are extremely important for understanding the origins of these key structures, but they are also famous for an unusual behavior—tying themselves in knots.

Hagfish have an eel-like body. They lead a bottom dwelling life and have a great sense of smell, but lack well developed eyes. When stressed, hagfish release a secretion that contains special filaments from glands along the body. When it contacts water, this secretion forms a massive slime and makes the hagfish an unpleasant bite for potential predators. To sneak out from this slimy shelter, the Atlantic hagfish, Myxine glutinosa, makes knots, which wring off the layer of slime. The knot, traveling along the body column, can provide a surface for the hagfish to push off. This enables the animal to pull its body out of the hole it makes in it’s prey’s flesh, or escape the grasp of a scientist.

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Pictured above are the heads of two Atlantic hagfish, whose bodies are burried in soft sediments. They were caught in the Gullmarfjord on the Swedish Westcoast.

Photo and video by Stefan Siebert. Video edited by Sophia Tintori. “I’m learning a song for Christmas” from Jack Pleasants.

Star colonies of sea squirts

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

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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.

Postdoc position in the Dunn Lab

posted by Casey Dunn / on November 6th, 2009 / in lab

Leucospermum

December 8th, 2009: The post-doc position has been filled. Thank you for your inquiries.

A joint postdoc position is open in the labs of Casey Dunn and Alexis Stamatakis as part of the iPlant Collaborative. The focus will be on phylogenomics, specifically data-set assembly and analyses. The tools will be developed as part of a large-scale effort to figure out how plants are related to each other, but will of course be relevant to the study of any group of organisms. The postdoc will be expected to spend time in both labs (in Providence and Munich). Please contact Casey or Alexis if you are interested.

Photo of Leucospermum flower by Casey Dunn.

Mating when you are stuck to a rock

posted by Stefan Siebert / on November 6th, 2009 / in Arthropods

If you are stuck to a rock it is tricky to get close enough to a partner to mate. One solution to this problem would be to release eggs or sperm into the open water, which is what many animals in this situation do. Acorn barnacles (Semibalanus balanoides), however, found a different solution. They have evolved the longest penis relative to their body size of any animal. In this video the penises of several barnacles are probing the neighborhood for mates. The penis is re-grown each mating season.

Acorn barnacles are hermaphrodites, each one has both male and female organs.  The specimens in the photos below have been removed from the rock and turned upside down. Two yellow egg clusters can be easily identified (top and middle images). The testis occur as white regions along the body of the animal which is here removed from its calcareous shelter (middle image). The penis is centered in between the feeding structures (bottom image).

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During the mating process, the penis of one barnacle is inserted into the body cavity of another and the sperm are released. Each barnacle can be fertilized by multiple partners, which means that a mothers offspring often have several different fathers. The developing embryos remain within the mother for several weeks before the larvae are eventually released.

Genetic paternity tests on the offspring performed by David Rand indicate that the closest neighbors are often not mates, which suggests that mating could be a selective process influenced by more than just the availability of partners. The barnacle penis shows adaptive plasticity—  J. Matthew Hoch has shown that barnacles growing where waves are stronger have thicker penises. The barnacles shown here were collected by Brown graduate student Patrick Flight who, together with David Rand, uses them as models to study genome evolution. Photos and video by Stefan Siebert.

Deadly bands

posted by Stefan Siebert / on October 28th, 2009 / in Jellies, Siphonophores

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Cnidaria is a group of animals that includes – among other things – jellyfish, corals and sea anemones. They take their name from the greek word for nettles (knide) because of to the sting and rash that a close encounter with them can cause. They elicit this response with a particular type of stinging cell that only they possess, the cnidocyte, which is arguably the most complex cell possessed by any animal. When triggered, a cnidocyte releases a hollow harpoon that penetrates prey organisms – or a swimmer’s skin- and injects toxins. These harpoons are microscopic, and there are many types of cnidocytes each with a different type of harpoon. Some create a painless sticky sensation, others are so powerful that a sting from just one cell can cause considerable burning.

The siphonophores, a group of colonial cnidarians, have multiple polyps and medusae that are specialized for tasks such as locomotion, feeding or reproduction. The picture on the left shows a feeding polyp (the prominent white structure in the center) of the siphonophore Nanomia bijuga. This feeding polyp is attached to the stem of the colony, which stretches across the top of this photo. Each feeding polyp has a single tentacle, and this tentacle has side branches with dense batteries of cnidocytes. Most of the cnidocytes are densely packed into a fascinating complex structure – the cnidoband. These are the orange spirals in the photos. The cnidoband ends in a filament (lower part of the picture) which contains sticky cnidocytes. The terminal filament makes first contact to the prey and sticks to it, which then tugs the cnidoband as the prey struggles. The cnidoband then stretches out and its cnidocytes fire as a unit, deploying their deadly power. These Nanomia bijuga were collected using the ROV Ventana with the friendly support of MBARI. Photos by Stefan Siebert.

Mitochondrial lenses

posted by Christopher Laumer / on October 21st, 2009 / in convergent evolution, optics

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When making observations of the tiny flatworms I study, I seldom paused to consider that they might also be looking back at me. However, when sampling in estuaries near Woods Hole, MA, I recently encountered several animals whose visual system proved impossible to ignore. On each eye, normally an inconspicuous black spot, I found a tiny spherical lens perfectly situated above their light-sensitive cells! These animals turned out to belong to a group of minuscule predatory flatworms that have an organ on their head (the acorn-looking thing) which they can shoot out rapidly to subdue their Lilliputian prey. The animal you see above is probably Toia ycia, which gets to be about half a millimeter long as an adult. We’re not sure if Toia can see in the same sense that your dog or goldfish can – I’d doubt it personally. But it’s likely that their eyes are more powerful than those of most other flatworms, which at best distinguish light from dark and can approximate the direction of the light source.

I’m apparently not the only person that’s found himself interested in these eyes—zoologists have been using electron microscopes to peer deep inside their cells for years. These investigations revealed quite a surprise. Most of us think of mitochondria as the “powerhouse” of the cell, as we learned in high school; some may remember learning about their origins as symbiotic bacteria. In these flatworms, however, these enslaved microbes serve another purpose: by accumulating refractive proteins, packing together, and becoming enlarged, these mitochondria have become lenses that focus ambient light onto the light-sensitive cells.

Toia and its ilk aren’t the only flatworms that do this: mitochondrial lenses seem to be a feature of quite a few distantly related flatworms. Some of these worms are free living, such as the beach predator Ptychopera westbladi, or the photosynthetic Dalyellia viridis. Others are more or less parasitic, as for example, Urastoma cyprinae, a pest of the commercial oyster, or the fish-gill parasite Entobdella soleae, a distant relative of the too-familiar tapeworm and liver fluke.

What are we to think of this spotty distribution of mitochondrial eyes in the flatworm tree of life? Maybe the most recent common ancestor of these organisms had an eye with a mitochondrial lens, and this feature was lost or unrecognisably modified in most descendant lineages. Another possibility, however, is that this represents a convergence – unrelated lineages of flatworms may have all found a way to build lenses with mitochondria, just as dolphins, sharks, and ichthyosaurs all independently became streamlined for drag reduction in the water. Without knowing exactly how these flatworm eyes develop, and in particular the cellular signals these animals use to guide their mitochondria to differentiate into lenses, it is difficult to distinguish between one or multiple origins of the mitochondrial lens.

Other organisms, though, have clearly discovered their own ways of making lenses with endosymbionts. Some dinoflagellates, single-celled photosynthesizers of the shallow ocean, have found a way to make lenses of their photosynthetic plastids, endosymbionts that were engulfed independently of mitochondria. Yet another clear de novo reinvention of the lens has occurred in the acoel Proporus venosus, a type of animal that was once considered a flatworm, but which has recently been shown to be as distantly related to to Toia it is to you or I. Below is a video I made of a Proporus venosus individual I captured in Sardinia.