Monday, October 4, 2010

polychaete setae

A conspicuous characteristic of most polychaetes are the many setae found on each segment. Here’s a photo of a fireworm, which I promised in class. Their setae are extended further from the body when the worm is disturbed so that an agitated worm is covered along each side with bright white tufts of setae. These setae contain a toxin, and just a slight brush up against them causes them to pierce the skin, break off, and cause a most unpleasant burning sensation. Look, but don’t touch.

Setae are definitely worth a close up look if you get a chance to see them under a compound microscope. The setae are formed during development and throughout the life of the adult worm as they break off during use. As larval polychaetes grow, the setae can be easily seen on each new segment.

Saturday, October 2, 2010

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.

Thursday, September 23, 2010

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.

Saturday, September 18, 2010

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.

Sunday, September 12, 2010

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.

Thursday, September 2, 2010


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.