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.

Marnix Everaert

posted by Erwin Keustermans / on October 15th, 2009 / in Science & Art
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Many people are familiar with the dazzling plates of Haeckel’s “Kunstformen der Natur” (http://en.wikipedia.org/wiki/Kunstformen_der_Natur). Haeckel set a standard for further similar undertakings, and at the same time stood firmly in a long tradition of documenting the abundance of strange creatures in the natural world. From a spectator’s point of view, it was and still is not easy to know or to see which creatures are real and which are imaginary, for the layman to decide which details are observed and which are made up from stereotypes, preconceptions or simply for reasons of symmetry or convenience.

Marnix Everaert’s (http://www.marnix-everaert.be) is a Belgian artist, a European expert on non-toxic printing techniques. His drawings remind the viewer of Haeckels pages.  Of course this is not the same encyclopaedic undertaking. There are differences of composition, for instance Everaert’s creatures are sometimes drawn on a common backdrop in a way that suggests that they share an imaginary space, while Haeckel’s items are often laid out on an empty page. Obviously Everaert is a contemporary artist and his style is looser than the standards that were set for 19th century illustrations, scientific or otherwise. Also, there seems to be more attention to structure than to detail, as if Everaert is taking elements from a repertory of geometric shapes that together constitute a generic type of creature. But for the viewer the question can be raised again: without more investigation or sound prior knowledge it is not possible to know what is real and what is imagined.

CreatureCast – Multicellularity

posted by Casey Dunn / on October 14th, 2009 / in Podcast

In Episode 2 of CreatureCast, Sophia Tintori and Cassandra Extavour talk about the evolution and development of multicellular organisms, and in particular the specialization of reproductive cells. Audio production and animations are by Sophia, who normally studies siphonophores in the Dunn lab. Music by Cryptacize.

With Episode 2, we are also happy to announce that you can now subscribe to the CreatureCast video podcast through Brown University at  iTunes U.

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This video is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.

A tale of two holes

posted by S. Zachary Swartz / on October 9th, 2009 / in Development

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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).