vroom vroom veligers

If I were to choose a favorite larval type, it would certainly be the veliger. While photos of these baby mollusks are lovely, their true charm can only be fully appreciated when you see them in motion.

Mollusks pass through an earlier larval stage, the trochophore, which is a larval type also seen in annelids. Although trochophores are cute in their own right, the lively action of the cilia on the margin of the velum and their tiny shells make veligers a special treat to run across in a plankton sample. Even nudibranchs, which lack a shell as adults, pass through a veliger stage.

Veligers have been famously celebrated in verse by Walter Garstang whose “Ballad of the Veliger” not only described the larvae, but also proposed an adaptive explanation for torsion. During development, molluscan larvae undergo a 180-degree twist of the body, which brings the gills and mantle cavity from where they start at the back of the body into position directly over the head. Perhaps, Garstang suggested in his poem, the mantle cavity provides a space for the larval head to retract into when threatened by enemies, which would make post-torsional larvae less vulnerable to predation. Almost six decades later, Pennington and Chia performed a series of feeding experiments and found no evidence to support this hypothesis, as a variety of predators, including hydromedusae, ctenophores, and crab larvae happily gobbled up abalone larvae whether they had undergone torsion yet or not. A complete understanding of the evolution of the gastropod body plan is likely to be rather more complicated than can be captured in verse.

Otter Cliffs field trip

The class went on a field trip yesterday to my favorite intertidal site on MDI, Otter Cliffs. Everyone saw various species of hydroids and bryozoans in the wild, as well as sponge. The stinkiness of the sponge provided a memorable demonstration of the secondary compounds used by these critters to protect themselves from being eaten. The pungent odor of rotten garlic mingled with gunpowder seems a convincing predator deterrent. We did happily sample the red alga, dulse, for an interesting intertidal snack.

Communicating Science via Film

I'm currently reading Randy Olson's Don't Be Such a Scientist, hoping to broaden my perspective on effective science communication. The book's subtitle, "talking substance in an age of style," captures what is challenging and aggravating to scientists who encounter resistance to evidence-based, fact-filled explanations about topics such as evolution and climate change.
Dr. Olson, a PhD from Harvard, with a background in evolution and ecology, also has an MFA in fimmaking from USC. He left the narrow arena of peer-reviewed journals and significantly expanded the audience he reaches through his website and films. Next week, he'll be at the University of Maine at Orono for three days, where panel discussions will follow screenings of several of his films, including Flock of Dodos: The Evolution-Intelligent Design Circus, Salt of the Earth: A Journey into the Heart of Maine Lobster Fishermen, and the improbably titled, Sizzle: A Global Warming Comedy. I'm heading up Thursday for Sizzle, leaving the COA campus at 2:30. Students, let me know if you'd like a ride.
He's come a long way as a filmmaker, but Randy Olson's early short, Barnacles Tell No Lies, is still an excellent illustration of the contrast between the accurate, thorough, and fact-filled, yet ultimately unilluminating approach of a college lecture (however unfairly stereotyped) and the power of humor and compelling visuals that film is so good at.

sponges: simply splendid

Sponges are typically described as the simplest of animals. In some ways, that’s a fair label. Their bodies are composed of only a handful of cell types responsible for all the functions in a living sponge: covering the outside, pumping water through the inside, digesting food, making the skeleton (yes, sponges have skeletons), and making gametes. These processes are all conducted at the cellular level, with no higher organization imposed. In contrast, there are hundreds of unique types of cells in humans, which are arranged into tissues, organs, and systems.
But sponges should not be underestimated; they have some special attributes.
Archaeocytes, a cell type that is dispersed throughout the body of the sponge, are totipotent, meaning they retain the ability to give rise to any type of cell in the sponge. Totipotency is the quality that makes human embryonic stem cells so attractive as a research tool and as a potential therapy for patients with certain types of cell damage due to disease or injury. Actually, embryonic stem cells cannot make the extra-embryonic membranes or placenta, so they are not, strictly speaking, totipotent, and are more precisely defined as pleuripotent. Virtually all animals begin life as a single, totipotent cell, the fertilized egg or zygote. (There are some exceptions to this assertion; can you think of an example?) During development, cell division and differentiation result in an increase in the number of cells and cell types, with various cell lines specialized for different tasks within the organism. Sponges are likely to be the oldest animals. Thus, the retention of a large population of totipotent cells could be an ancestral trait within our kingdom. Amazing regenerative ability and powers of asexual reproduction, which depend on totipotent or pleuripotent stem cells, are widespread among invertebrates. It’s our bad luck as mammals that our cells that are capable of differentiating into all cells in the body are so elusive, found only very early in development, when the embryo is but a tiny ball of cells.

Phylogenies

I'm teaching Invertebrate Zoology again this fall, and I intend to blog regularly throughout the term. For starters when I teach this class, before delving into the morphology, physiology, behavior, and ecology of the various groups, I like to think about the larger context and how invertebrates fit into the big picture of life on Earth. One way to visualize this big picture is in a phylogentic tree, which depicts patterns of evolutionary relationships among the groups represented by branches on the tree. This one includes everything from bacteria to people, although it gives short shrift to some important groups, like the Protists, and inflates others. It's odd that the mere 4500 mammalian species take up such a significant proportion of this figure, which represents tens of millions of species, at least. There are fuller treatments of mammalian phylogenic patterns available. Ignoring the bacteria for the moment, this tree of eukaryotes gives a rather different representation of protist diversity relative to the metazoans, which occupy only a modest corner in the lower right of the figure. The textbook for this class presents multiple versions of metazoan phylogeny, and we'll be discussing evolutionary relationships among animal phyla throughout the term.
I love phylogenetic trees and the evolutionary context they offer for considering life's diversity, although there are clearly others who are more committed to this view of the world than I am. This image is from Carl Zimmer's Science Tattoo Emporium, and if you're really excited about scientists acknowledging their passions in ink on their skins, you'll be happy to know there is a book coming out soon.

Bill and TED's malarial adventure

It's rather old news by now, but I wanted to bring your attention to the events at last week's TED conference. Bill Gates gave a talk that centered largely on education and socioeconomic patterns in the United States. He said plenty about teaching excellence and the impact of talented teachers on their students. He drew a number of conclusions, some I agree with, and some I would argue about. It's a rich subject, with a great deal of controversy, and high very high stakes. However, the headline-stealing event happened early in the talk when he was discussing malaria. As he opened a jar of mosquitoes in the auditorium, he said "there's no reason only poor people should have the experience." In the very next moment he assured the audience that those mosquitoes were not infected with malaria, and no one seemed panicked about contracting a dreaded disease. The only discomfort in the room seemed to stem from the knowledge that the attendees of the conference were part of a privileged society that spent more money on research and medicine for baldness than for malaria. It often seems to me that people who are out "raising awareness" might spend their time more productively "raising money." But Bill Gates has done that and through the Bill and Melinda Gates foundation continues to support work on a range of neglected diseases, including malaria. So I applaud the mosquito stunt. I wish there were an analogous act that could drive home that shameful fact mentioned by Gates in his talk that a poor person in the U.S. is more likely to go to prison than to earn a college degree.

snail tentacles

As I mentioned in a recent post, ants that are infected with the trematode, Dicrocoelium, are compelled by the presence of the parasite in their brains to march to the top of a blade of grass, bite down, and hang on. This behavior would seem to make it more likely that the fluke will be eaten by a grazing mammal like a cow or sheep, which needs to happen if the parasite is to complete its life cycle. It looks like a fascinating adaptation on the part of the parasite to modify the behavior of its host allowing the parasite to be transmitted more readily to the next host. Ideally, there would be data to document that the parasitized ants are eaten at higher rates than are uninfected ants. But this is not the ideal system in which to gather that kind of data. In fact, even though the manipulation of host behavior by parasites is not considered to be a controversial phenomenon, rigorous studies that are well supported by behavioral data are not as common as you might think. One of the best is Janice Moore's work on birds, pill bugs, and their acanthocephalan parasites, a system I also mentioned earlier. Moore discusses several other examples in her book, Parasites and the Behavior of Animals.
One of the earliest suggestions that parasites might modify the behavior or appearance of their host came from Wesenberg-Lund in 1931. He was studying another trematode, Leucochloridium. In typical trematode fashion, a snail serves as the initial host and a vertebrate, in this case a bird, serves as a final host. There is no arthropod (no insect or crustacean) anywhere in the life cycle, which is a challenge for the parasite, as the bird prefers insect prey and doesn't typically eat snails. To get itself from the snail into the bird, so the story goes, the fluke moves into the snail's tentacles, making them look like colorful, wiggly, and oh-so-tasty caterpillars, which the bird happily gobbles up, thus infecting itself. Alas, experimental confirmation of this tale is lacking. There haven't even been any reports of natural occurrence of predation by birds of these parasitized snails in the wild. But the behavior is pretty cool anyway, and you can be the judge regarding its adaptive significance and consider what sort of evidence would convince you that the parasite is modifying the snail host so that transmission to the bird is improved. As my father, my initial teacher in skeptical thinking, would say, "interesting if true."