Hooked setae: tests of the anchor hypothesis

We tested the hypothesis that hooked setae function as anchors in three species of tubiculous polychaetes (Eudistylia vancouveri, Schizobranchia insignis, and Oweniafusiformis). All maintained position within their tubes when exposed to high pressures (up to 100-200 kPa) applied from the posterior direction (where it would tend to cause the tips of hooks to embed in the tube wall). When pressure was applied in the opposite direction, where hooks would not tend to embed in the tube wall, the worms were expelled from their tubes at lower pressures (30-100 kPa). The ability of these worms to maintain their position within their tubes was independent of body size. On the basis of these findings we made three predictions. First, worms that use their hooked setae as anchors should have those hooks located on the body in greatest number and size on the segments associated with greatest worm diameter. Second, as worms increase in size, setal armory should increase in a predictable way. The force that can be applied to extract worms from their tubes by suction feeding fish or wave action would increase as the area subject to suction increases (proportional to the cross sectional area of the tube). Therefore, we predict that setal armory should also increase as a squared function. Third, hooks or uncini should show patterns of wear or loss and/or the worms' bodies should show scars or wounds where the setae are most used (e.g., where worm diameter is at its maximum). All of these predictions were supported by the data and indicate that hooked setae function as anchors for tubiculous polychaetes. This is important for understanding the biology of these animals and has implications for using hooked setae as characters in phylogenetic analyses.