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ABSTRACT Hydrostatic skeletons, such as an elephant trunk or a squid tentacle, permit the transmission of mechanical work through a soft body. Despite the ubiquity of these structures among animals, we generally do not understand how differences in their morphology affect their ability to transmit muscular work. Therefore, the present study used mathematical modeling, morphometrics, and kinematics to understand the transmission of force and displacement in the tube feet of the juvenile six-rayed star (Leptasterias sp.). An inverse-dynamic analysis revealed that the forces generated by the feet during crawling primarily serve to overcome the submerged weight of the body. These forces were disproportionately generated by the feet at more proximal positions along each ray, which were used more frequently for crawling. Owing to a combination of mechanical advantage and muscle mass, these proximal feet exhibited a greater capacity for force generation than the distal feet. However, the higher displacement advantage of the more elongated distal feet offer a superior ability to extend the feet into the environment. Therefore, the morphology of tube feet demonstrates a gradient in gearing along each ray that compliments their role in behavior. 

Abstract Larvae of many phyllodocidan annelids are planktotrophic, but the feeding mechanisms of larvae in this diverse clade are poorly known. Many larvae belonging to one large clade of phyllodocidans, Aphroditiformia, bear a bundle of long cilia attached to the left side of the prototroch, the oral brush, which they use in feeding. In 1936, D.P. Wilson observed that trochophore larvae ofNephtys hombergi, a member of the phyllodocidan family Nephtyidae, bore a strikingly similar bundle of long cilia on the left side of the body. Since Wilson's observation, numerous descriptions of nephtyid larvae have been published, but none remark on the presence of an oral brush. Here I show that metatrochophore I and II larvae ofMicronephtys cornutabear an oral brush, but that it is lost in the transition to the nectochaete stage, during which the larval mouth and foregut are also being remodeled to function in benthic feeding by juveniles. That an oral brush is clearly present in at least some larval stages of two genera suggests that oral brushes may be widespread in the feeding larvae of nephtyids, but have simply been overlooked for more than 80 years. Additional work is needed to make inferences on the evolutionary history of the oral brush in phyllodocidan annelids, and to distinguish among several hypotheses on the function of this peculiar group of cilia in larval feeding. 

Abstract The nonfeeding planktonic larvae of marine invertebrates typically lack larval feeding structures. One puzzling exception to this generalization is the annelid clade Sabellidae, in which nonfeeding larvae possess ciliary bands (specifically, food groove and metatroch) that, to the best of our knowledge, have no function other than in feeding. Nishi and Yamasu (1992b,Bulletin of the College of Sciences, University of the Ryukyus,54, 107–121) published a scanning electron micrograph showing that nonfeeding larvae of the serpulid annelidSalmacina dysterialso possess food groove and metatrochal cilia. Here I demonstrate that nonfeeding larvae ofSalmacina tribranchiataalso bear ciliary bands identifiable as food groove and metatroch by position. High‐speed video of ciliary beat patterns shows that, together with the prototrochal cilia, these bands function in an opposed band system. The presence of feeding structures in nonfeeding annelid larvae is thus more widely distributed than previously recognized. The presence of feeding structures may make evolutionary transitions to planktotrophy more likely, and may underlie an inferred origin of larval feeding in the common ancestor of one of the two major clades of serpulid annelids, Serpulinae. 

Abstract The serpulid annelidFicopomatus enigmaticusis a widely distributed invader of shallow‐water, brackish habitats in subtropical and temperate regions, where it has numerous damaging ecological and economic effects. Its distributional pattern suggests that temperature and salinity play important roles in limiting its distribution, but because other factors often covary with these, drawing strong conclusions from these patterns is difficult. In an effort to more clearly identify the effects of these factors, we examined tolerance to acute thermal (16–28°C) and salinity (0–35 psu) stress by larvae (5‐day exposure, unfed) and adults (14‐day exposure, unfed) ofF. enigmaticusin the laboratory experiments. Larvae showed higher mortality at the highest temperature tested 28°C; adult survival was unaffected by temperature. Neither larvae nor adults survived exposure to pure freshwater (0 psu), but survived well at salinities ranging 3.5–35 psu. In addition, high salinity did not slow tube growth in adults. These results suggest that salinity stress, in particular, does not directly limit the distribution ofF. enigmaticusto low‐salinity habitats. Experimental work on the distribution ofF. enigmaticusis uncommon in the literature, but is likely needed to identify the abiotic or biotic factors that limit the distribution of this frequently invasive species.