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.