Discovery of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran Period.

The fossil record of the terminal Ediacaran Period is typified by the iconic index fossil Cloudina and its relatives. These tube-dwellers are presumed to be primitive metazoans, but resolving their phylogenetic identity has remained a point of contention. The root of the problem is a lack of diagnostic features; that is, phylogenetic interpretations have largely centered on the only available source of information-their external tubes. Here, using tomographic analyses of fossils from the Wood Canyon Formation (Nevada, USA), we report evidence of recognizable soft tissues within their external tubes. Although alternative interpretations are plausible, these internal cylindrical structures may be most appropriately interpreted as digestive tracts, which would be, to date, the earliest-known occurrence of such features in the fossil record. If this interpretation is correct, their nature as one-way through-guts not only provides evidence for establishing these fossils as definitive bilaterians but also has implications for the long-debated phylogenetic position of the broader cloudinomorphs.

Textures and traction: how tube-dwelling polychaetes get a leg up.

By controlling the traction between its body and the tube wall, a tube-dwelling polychaete can move efficiently from one end of its tube to the other, brace its body during normal functions (e.g., ventilation and feeding), and anchor within its tube avoiding removal by predators. To examine the potential physical interaction between worms and the tubes they live in, scanning electron microscopy was used to reveal and quantify the morphology of worm bodies and the tubes they produce for species representing 13 families of tube-dwelling polychaetes. In the tubes of most species there were macroscopic or nearly macroscopic (∼10 µm-1 mm) bumps or ridges that protruded slightly into the lumen of the tube; these could provide purchase as a worm moves or anchors. At this scale (∼10 µm-1 mm), the surfaces of the chaetal heads that interact with the tube wall were typically small enough to fit within spaces between these bumps (created by the inward projection of exogenous materials incorporated into the tube wall) or ridges (made by secretions on the interior surface of the tube). At a finer scale (0.01-10 µm), there was a second overlap in size, usually between the dentition on the surfaces of chaetae that interact with the tube walls and the texture provided by the secreted strands or microscopic inclusions of the inner linings. These linings had a surprising diversity of micro-textures. The most common micro-texture was a "fabric" of secreted threads, but there were also orderly micro-ridges, wrinkles, and rugose surfaces provided by microorganisms incorporated into the inner tube lining. Understanding the fine structures of tubes in conjunction with the morphologies of the worms that build them gives insight into how tubes are constructed and how worms live within them.

Resilin in dragonfly and damselfly wings and its implications for wing flexibility.

Although there is mounting evidence that passive mechanical dynamics of insect wings play an integral role in insect flight, our understanding of the structural details underlying insect wing flexibility remains incomplete. Here, we use comparative morphological and mechanical techniques to illuminate the function and diversity of two mechanisms within Odonata wings presumed to affect dynamic wing deformations: flexible resilin vein-joints and cuticular spikes. Mechanical tests show that joints with more resilin have lower rotational stiffness and deform more in response to a load applied to an intact wing. Morphological studies of 12 species of Odonata reveal that resilin joints and cuticular spikes are widespread taxonomically, yet both traits display a striking degree of morphological and functional diversity that follows taxonomically distinct patterns. Interestingly, damselfly wings (suborder Zygoptera) are mainly characterized by vein-joints that are double-sided (containing resilin both dorsally and ventrally), whereas dragonfly wings (suborder Epiprocta) are largely characterized by single-sided vein-joints (containing resilin either ventrally or dorsally, but not both). The functional significance and diversity of resilin joints and cuticular spikes could yield insight into the evolutionary relationship between form and function of wings, as well as revealing basic principles of insect wing mechanical design.

Polychaete chaetae: Function, fossils, and phylogeny.

In this article we review the phylogenetic distribution of major chaetal types within the Polychaeta, discuss what has been demonstrated about chaetal function in modern worms, and examine what is known about the evolution of chaete through the fossil record. We conclude with specific cautions about how chaetae are treated in phylogenetic analyses and make suggestions about how they could be used to provide a stronger phylogenetic signal.

Stream insects as passive suspension feeders: effects of velocity and food concentration on feeding performance.

Benthic suspension feeders are important components of aquatic ecosystems, often dominating the use of space and influencing patterns of material cycling between the water column and benthos. Biomechanical theory predicts that feeding by these consumers is governed by the flux (i.e., product of food concentration and velocity) of particulate material to their feeding appendages. We performed a laboratory flume experiment to test how feeding by larval black flies (Simulium vittatum Zett.) responds to independent manipulations of flow and food concentration. We quantified larval body posture, flick rate of the labral fans, and ingestion rate as a function of two concentrations of a baker's yeast/chalk suspension (0.96 and 4.44 mg l-1) and five water velocities (20, 30, 45, 60, and 90 cm s-1). Using analysis of covariance, we found that both flick rate and ingestion rate increased in a decelerating manner with increasing velocity, while fan height decreased linearly with increasing velocity. In contrast, food concentration had no effect on any aspect of feeding behavior. Thus, although both velocity and food concentration contribute to particle flux, our results indicate that the two were not substitutable under the range of conditions tested here.

Predator-prey interactions in a benthic stream community: a field test of flow-mediated refuges.

Ecological theory suggests that the impact of predation can be strongly modified by the existence of regions of the environment in which prey are less accessible to predators, which underscores the need for empirical studies examining the factors influencing the availability and importance of such prey refuges. Our study tested whether benthic microhabitats with high flows provide suspension-feeding larval black flies (Simulium␣vittatum) with a spatial refuge in which the negative impact of predatory flatworms (Dugesia dorotocephala) is reduced. We conducted a short-term field experiment in Chester Creek (southeastern Pennsylvania, United States) to examine how the number of black fly larvae inhabiting tile substrates responded to manipulated variations in flatworm abundance and current speed. The abundance of flatworms declined with increasing current speed, thereby creating the potential for sites with high flows to provide larvae with a refuge from these predators. Multiple regression analysis revealed that the final abundance of larvae exhibited a significant negative relationship to flatworm abundance and a significant positive relationship to current speed. After adjusting for variations in elapsed time and initial larval abundance, flow and predators explained 38% of the variation in the rate of change in larval abundance. The positive correlation between larval abundance and flow had two components: a positive, direct effect of flow on larvae, which arises because these food-limited consumers prefer to reside within sites with faster flows where they can feed at higher rates; and a negative effect of flow on predators, and of predators on larvae, which combine to yield a positive indirect effect of flow on larvae. This indirect effect demonstrates the existence of flow-mediated refuges (i.e., microhabitats in which the impact of predation is reduced due to high flows), although the effect accounts for a small proportion of total variation in larval abundance. A consideration of biomechanical relationships suggests that microhabitats with high flows are likely to create prey refuges in a wide range of freshwater and marine benthic environments. In particular, predators will often experience greater dislodgement forces than prey because of their larger size and because they project farther above the bed where current speeds are faster. Moreover, the ability to resist a given dislodgement force may be greater for many prey, especially those that are sessile or semi- sessile.

HOLDING ON BY THEIR HOOKS: ANCHORS FOR WORMS.

We examined the hooked setae of a dominant group of tube-dwellers, the polychaete annelids, and found a pattern of setation that is predictable by tube type, exclusive of worm taxon or orientation; we also demonstrated the mechanical significance of these hooked setae. When tube-dwelling worms belonging to different lineages are pressurized, they resist differentially as a function of the direction in which hooks face. The results are consistent with the hypothesis that hooks are used primarily to resist removal of worms from their tubes, are polyphyletic in origin, and are active agents of resistance.