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."

California dreaming

I was supposed to fly out to visit family in California for the holidays, but the nor-easter that blew through Sunday has adjusted our schedule rather dramatically. Our flight's been postponed 5 days. On the bright side, I love the white Christmas in Maine. It's also Nina's birthday, and Isaac Newton's, too, so there's much to celebrate. Unfortunately, my husband traveled a few days earlier to spend extra time with his mom and to work on a paper with a colleague in San Diego, so he's already been out there for some time. I keep reminding myself that we spent the first year of our marriage on separate coasts, while I was finishing grad school and he was starting up at COA. This helps put one Christmas 22 years later into perspective.

ant brains

Winter break and holiday plans aside, I'm already thinking about the parasites tutorial I'll be teaching next term. Carl Zimmer's book, Parasite Rex, makes the case very persuasively: parasites are so much more than grotesque little footnotes in life's pageant. They are major players in the ecology and evolution of many species. Of course, in addition to their ecological and evolutionary heft, many of them exhibit some really weird and creepy lifestyles. One of the most disturbing tricks that some parasites have developed is that of mind control. An infected host's behavior is altered by the parasite in a way that increases the ability of the parasite to complete its lifecycle. In many cases, parasites gain access to the final host, where reproduction occurs, by first infecting an intermediate host that is later eaten by the final host. Any behavior by the intermediate host that makes it more likely to be eaten benefits the parasite (but not the intermediate host, of course). Some parasites are able to manipulate theirs hosts, actually causing them to engage in reckless behavior. Pill bugs harboring acanthocephalan parasites may spend more time in dryer places and even prefer walking around on whiter surfaces.  This may not sound like especially exciting risky behavior, but compared to the safety of  the dark, moist leaf litter, where healthy pill bugs prefer to be, the infected pill bugs are more easily seen by birds, which gobble up the pill bugs and serve as the final host of their parasites.
Mind control of ants by liver fluke parasites has been colorfully presented in this video.

Endangered Marine Invertebrate

A physicist friend recently sent me a link to 20 strange and exotic endangered species, saying that the list was "sufficiently yucky and biological" to appeal to me, and it got me thinking about endangered marine invertebrates. The white abalone is the only marine invertebrate that's been listed as endangered under the Endangered Species Act. "Only one? Those marine inverts must be in pretty good shape," you might be tempted to surmise. But you're more sophisticated than that, so you probably wouldn't. There is a long list of assaults on marine invertebrate species: habitat loss, overharvesting, invasive species, disease outbreaks, pollution, global climate change, and more. Most marine invertebrates just haven't been studied in sufficient depth to be listed as endangered, and there are only a handful that are considered to be "threatened" or "species of concern," which are lesser categories than "endangered." To read more about threatened and endangered marine species, you can find plenty here.
White abalone have suffered dramatically from overharvesting. Here's the somewhat oversimplified ecological tale: Usually as population size goes down, individuals enjoy enhanced rates of growth and reproduction, because competition between individuals decreases as populations shrink. But there's a limit. When populations get extremely small (and what's extreme will vary with the ecology of each species), individuals may suffer reduced rates of growth and reproduction. If you do better with some neighbors around, the benefits of reduced competition that come with a shrinking population ultimately lead to other challenges, the most obvious being finding a mate. The general phenomenon of individuals doing worse as population size gets even smaller is called the Allee effect. This is what happened to white abalones. Mating in abalones is not a particularly touchy-feely process. Males and females shed gametes directly into the ocean where fertilization and subsequent embryonic and larval development occur. Fertilization cannot happen if spawning partners are too far away from each other; even a few meters can result dilution of gametes that is severe enough to eliminate the possibility of fertilization. Moving adult abalone closer to each other in the field, as well as spawning them in the lab to produce offspring are management measures that are being taken to rescue this species from the threat of extinction. There's plenty to read about regarding sustainable seafood (white abalone clearly not included) at this month's Carnival of the Blue.