Effects of body size and shape on locomotion in the bat star (Patinia miniata).
In general, larger-bodied animals travel at higher speeds than smaller-bodied relatives across many phyla and modes of locomotion (Taylor et al., 1970; Schmidt-Nielsen, 1975). As limb length increases in large running birds, larger individuals run faster (Schmidt-Nielsen, 1975; Gatesy and Biewener, 1991): they take fewer, longer strides and cover ground more efficiently (Gatesy and Biewener, 1991). Larger scyphozoan medusae such as Aurelia aurita achieve greater absolute swimming speeds, and they do so via reduced pulse frequencies (McHenry and Jed, 2003). Both the common earthworm Lumbricus terrestris (Quillin, 1999) and the abalone Haliotis kamtschatkana (Donovan and Carefoot, 1997) show the same pattern: longer animals take larger strides to cover more ground at faster absolute velocities than smaller individuals.
Relative locomotion speeds (e.g., length-or mass-specific speed) may nonetheless decrease even when absolute speed increases with increasing body size. Such differences may reveal constraints or benefits experienced at larger sizes. In two genera of rotifers (Brachionus and Asplachna), body lengths moved per second decreases with increasing body length (Epp and Lewis, 1984). Despite an increased cost to locomotion at these larger body sizes, absolute velocity increased with body length (Epp and Lewis, 1984). Among terrestrial mammals that span a size range of 5 orders of magnitude, mass-specific velocity decreased as body mass and absolute velocity increased (Schmidt-Nielsen, 1975; Diaz-Iriarte, 2002). In steady-swimming sockeye salmon (Oncorhynchus nerka) size-specific swimming speed declines with increasing body size (Brett, 1965, as cited in Stemberger and Gilbert, 1987; Yanase and Arimoto, 2007). At maximum speed, larger abalone move fewer shell lengths per minute than smaller ones even though they crawl absolutely faster (Donovan and Carefoot, 1997).
Among echinoderms, however, the relation between body size and crawling speed is less clear. The Mediterranean sea urchin Paracentrotus lividus behaves like most other animals: larger individuals move at greater absolute crawling speeds (Domenici et al., 2003). The common sea star Archaster typicus also exhibits this pattern (Mueller et aL, 2011). However, crawling speeds of the sea stars Acanthaster planci, Linckia laevigata, and Protoreaster nodosus do not appear to vary significantly with body size (Mueller et al., 2011).
Echinoderms utilize a unique hydraulic system for feeding and locomotion: the water vascular system (Lawrence, 1987). Sea urchins and sea stars crawl while balanced on many flexible tubular podia that swing back and forth in a manner visually analogous to the limb movement of running mammals. However, the length and form of tube feet varies greatly among echinoderm taxa (Lawrence, 1987; Ferguson, 1995; Santos et al., 2005a). Urchins may also employ spines for greater support during locomotion (Domenici et al., 2003). The uniqueness, complexity, and diversity of the locomotory podia and water-vascular systems in echinoderms leave open many questions about even the most basic biomechanics of locomotion, including, do larger individuals generally move faster?
We therefore tested the effects of body size and arm number on maximum crawling speed in the bat star Patiria miniata (formerly Asterina miniata, Brandt, 1835), a common intertidal sea star (Asteroidea) found from Baja to Southern Alaska (Lamb and Hanby, 2008). Adults of this species typically possess five arms (pentaradial condition) (Hotchkiss, 2000). However, arm number varies naturally from four to nine (Hotchkiss, 2000). P. miniata is therefore an ideal species with which to study how locomotion speed varies with body size and number of arms. Because P. miniata exhibits a stereotyped escape response when exposed to effluent from the predatory sea star Solaster dawsoni, we could stimulate maximum velocity experimentally by triggering this fleeing behavior under laboratory conditions (Gaymer et al., 2004). We also tested for biased orientation of movement using an arm preference test (Polls and Gonor, 1975; Hotchkiss, 2000). Finally, we examined body form and posture of crawling bat stars to better understand the use of tube feet and the water vascular system during locomotion.
Materials and Methods
Specimens and testing protocol
Sixty specimens of Patiria miniata were hand-collected in October 2011 from Grappler Inlet near Bamfield, British Columbia, Canada (approx. 48[degrees]50'N, 125[degrees]07'W). Arm lengths ranged from 40 to 120 mm. Two medium-sized individuals of Soluster dawsoni (diameter of 20 cm from arm tip to arm tip) were obtained by scuba diving from the Bamfield Inlet narrows (approx. 48[degrees]50'N, 125[degrees]08'W). Prior to testing, animals were housed for one week in the flow-through seawater system at the Bamfield Marine Sciences Center. Groups of four P. miniata specimens were held in 20-1 tanks and fed crushed Mytilus edulis every 2 days. S. dawsoni specimens were kept in individual 9-1 tanks with a separate seawater supply to prevent pre-exposure of P. miniata to predator effluent (Gaymer et al., 2004) and were fed one Evasterias sp. (approx. diameter 10 cm) per week.
Prior to testing, individual specimens of P. miniata were fasted for 2 days to prevent any bias to crawling direction or speed as a result of the cardiac stomach being distended. To generate predator effluent, the running seawater supply was removed for 1 h from the 9-1 S. dawsoni tanks. Equal volumes of water from the two S. dawsoni tanks were combined to create the predator stimulus. To initiate the escape response in P. miniata, 10 ml of predator effluent was released from a syringe at a rate of 1 ml/s at about 3 cm above the aboral surface. All tests were conducted in a fixed-volume sea table with stationary seawater and no external seawater supply. Following each trial, the table was drained and the crawling surface scrubbed and rinsed with fresh seawater to remove residual S. dawsoni scent.
To characterize size and shape of P. miniata, high-resolution images were taken of the oral and aboral sides in air. Unedited images were analyzed using ImageJ image analysis software (Rasband, W. S., ImageJ, U. S. National Institutes of Health, Bethesda, MD, http://rsb.info.nih.gov/ij/, 1997-2010). Length and area measurements were recorded in addition to body mass because they to permit more thorough scaling analyses (Schmidt-Nielsen, 1975) of variables that might be functionally significant (e.g., ambulacral groove area versus total oral area). Arm length (AL) was measured from the center of the oral disk to the tip of each arm, and web length (WL) was meaured to the mid-point between arms (Fig. 1A). Surface area (SA) was measured as the total area of the oral surface (Fig. 1A). The ambulacral groove area (GA) was calculated as the total area on the oral surface containing tube feet (Fig. 1A). Wet mass (W) was measured to the nearest 0.1 g on a digital balance after blotting the oral side with a paper towel. Scaling coefficients were computed for pairs of variables (AL, SL, GA, and W) using reduced-major-axis slopes (RMA) of [log.sub.10] transformed data. RMA slopes provide more statistically accurate descriptions of the scaling relationship between two variables than least-squares linear regression when both variables have similar (or unknown) uncertainties (Smith, 2009). One-sample Student's t tests were used to test for departures from isometry. Null slopes for isometry were 2 for SA or GA versus AL, 1 for GA versus SA, and 0.67 for GA versus W.
Arm preferences during righting and bilateral-like movement have been observed in other sea star species (Polls and Gonor, 1975; Ji et al., 2012), so before performing velocity tests, we did a pilot study to assess possible directional biases in P. miniata crawling behaviors. Specimens were oriented randomly into four starting quadrants (defined by a standard x-y grid) relative to the madreporite. In each trial, 10 ml of S. dawsoni scent was applied to the center of the aboral disk (labeled a in Fig. 1B), and the P. miniata escape response was filmed. Preferred crawling direction was scored as the final location of the animal relative to the initial orientation. In the case of a crawling-direction change, the final position was considered the actively selected direction. An arbitrary arm-numbering system was used to characterize final location relative to the origin of each trial (Fig. 1 B). In this pilot test for biased crawling behavior, individual bat stars were exposed to predator scent only once.
An escape-response assay was also used to quantify the maximum crawling speeds of P. miniata. Initial orientation was standardized relative to the madreporite to minimize potential directionality bias (Polls and Gonor, 1975). To induce the bat stars to crawl in a "forward" direction, 10 ml of S. dawsoni scent was applied between two arms (labeled b in Fig. 1 B), 180[degrees] from an artificially chosen leading arm. Trials were run for about 5 min because this seemed sufficient for animals to reach maximum crawling speeds (see Fig. 4 below). As in the arm-preference testing, individuals were tested only once.
Table 1 Scaling analysis in Patiria miniata Dimensions Arm Expected RMA Slope Intercept * Number Slope ([+ or -] SE) ([+ or -] SE) SA vs. AL 5 2 1.64 -0.02 [+ or -] 0.07 [+ or -] 0.12 6 2 2.39 -1.08 [+ or -] 0.34 [+ or -] 0.66 GA vs. AL 5 2 2.08 -1.30 [+ or -] 0.13 [+ or -] 0.26 6 2 3.18 -3.42 [+ or -] 0.32 [+ or -] 0.63 GA vs. SA 5 1 1.27 -1.20 [+ or -] 0.08 [+ or -] 0.24 6 1 1.33 -1.31 [+ or -] 0.18 [+ or -] 0.57 GA vs. W 5 2/3 0.95 0.90 [+ or -] 0.12 [+ or -] 0.24 6 2/3 1.18 0.69 [+ or -] 0.31 [+ or -] 0.67 Dimensions [R.sup.2] n One-tailed t Scaling * test Mode (P value) SA vs. AL 0.94 39 <0.0001 (-) Allometric 0.82 11 0.28 Isometric GA vs. AL 0.84 39 0.54 Isometric 0.91 11 0.0042 (+) Allometric GA vs. SA 0.86 39 <0.0001 (+) Allometric 0.83 11 0.10 Isometric GA vs. W 0.47 38 <0.000l (+) Allometric 0.46 10 0.13 Isometric * SA oral surface area ([mm.sup.2]); AL. arm legth (mm); GA, ambulacral groove area ([mm.sup.2]); W, wet mass(g).
Video data for each trial were collected at 30 frames/s. These data were extracted into a series of single-second frames for image analysis using ImageJ. In each image, a repeatably identifiable point on the crawling bat star was marked, and the movement of this mark over time intervals of 10 s was used to measure displacement. Instantaneous velocity was calculated as displacement traveled in each 10-s time segment. These instantaneous crawling velocities were divided by each size variable to generate size-specific velocities and enable comparisons between five-and six-armed P. miniata. Size-specific crawling velocity curves were then linearized via [log.sub.10] transformation prior to statistical analysis.
Linear regressions and one-sample t tests were performed using the Java applet StatCrunch (2011). Chi-square tests compared the arm preference within and between five-and six-armed individuals of P. miniata with expected values based on the arbitrary arm classification system (Fig. 1B). For five-armed bat stars, arm class 1 and 2 were considered twice as likely to occur as arm class 3. In comparison, the three arm classes for six-armed bat stars were given equal weights. Analysis of covariance (ANCOVA) was used to compare slopes and intercepts between five-and six-armed stars. Analyses were repeated with the single highest and lowest values included or excluded to test how sensitive relations were to exclusion of extremal points.
Scaling relations differed between five-and six-armed individuals of Patiria miniata (Table 1). Total oral surface area (SA) was negatively allometric with arm length (AL) in five-armed bat stars but isometric in six-armed stars Conversely, ambulacral groove area (GA) varied isometrically with arm length in five-armed stars but exhibited positive allometry in six-armed stars. Therefore, six-armed individuals of P. miniata had proportionally more oral area and ambulacral groove area relative to arm length than five-armed individuals at larger body sizes (Fig. 2A, B; ANCOVA P = 0.0001, 0.008).
In contrast, slopes and intercepts of ambulacral groove area relative (GA) to oral surface area (SA) or body mass (W) did not differ significantly between five-and six-armed bat stars (Fig. 2C, D; ANCOVA P > 0.99). Ambulacral groove area was positively allometric with both oral surface area and wet mass in five-armed P. miniata but isometric in six-armed individuals (Table 1).
Crawling orientation and velocity
Preferred leading arm did not differ significantly from random (Fig. 3) in either five-armed (black bars, n = 42, [x.sup.2] P = 0.08) or six-armed (gray bars, n = 9, (2) P = 0.45) bat stars, nor did it differ significantly between five-and six-armed individuals ([x.sup.2] P = 0.25).
Instantaneous crawling velocities exhibited an acceleration phase followed by an oscillating plateau phase where average velocity no longer increased (Fig. 4). Both absolute and size-specific maximum crawling velocities decreased with increasing body size, independent of arm number (Fig. 5A-D). Mass-specific velocity decreased with increasing mass (Fig. 5B), as did length-specific velocity (Fig. 5C) and area-specific velocity (Fig. 5D). These relations did not differ significantly between five-and six-armed individuals (P > 0.1 for all slopes and intercepts, Fig. 5A-D).
During the oscillating plateau period, crawling individuals of P. miniata exhibited a regular vertical undulation when viewed from the side (Fig. 6A, B). In one five-armed individual, the period of this oscillation was about 8 s (Fig. 6C).
Locomotion in the bat star Patina miniata appears to differ from that of nearly all other animals (Schmidt-Neilsen, 1975), including some other sea stars (Mueller et al., 2011): maximum absolute crawling speed actually declined with increasing body size (Fig. 5A). These declines were, as expected, even more dramatic for mass-, length-and area-specific rates of locomotion (Fig. 5B-D) and contrast sharply with prior work in other adult echinoderms. In both the Mediterranean sea urchin Paracentrotus lividus and the common sea star Archaster typicus, absolute crawling speed increases with body size (Domenici et al., 2003; Mueller et at, 2011). Our repeatable but unexpected results for P. miniata raise many important questions about the mechanics of asteroid locomotion.
Somewhat surprisingly, maximum crawling speeds did not differ between five-and six-armed individuals of P. miniata despite some differences in scaling relationships. Addition of a sixth arm in P. miniata could potentially enhance locomotion through a proportional increase in ambulacral area or number of tube feet, or it could impair locomotion by increasing the complexity of coordination (Hotchkiss, 2000). However, even though total oral surface area and ambulacral groove area were proportionally larger in larger six-armed individuals (Fig. 2A, C), the presence of this sixth arm did not appear to enhance or hinder locomotion compared to five-armed bat stars of similar body size (Fig. 5A-D).
Differences in form and behavior may explain the differences observed between the size-dependence of locomotion in the bat star P. miniata and the sea urchin Paracentrotus lividus (Domenici et al., 2003). Urchins have longer and more flexible tube feet than sea stars, and some have the ability to crawl on their spines, which means tube feet are not the sole locomotory or support structures (Domenici et al., 2003). In contrast, on its oral side P. miniata has discrete bands of tube feet organized into grooves spanning the length of each arm. This may constrain the movement of larger individuals because tube feet in narrow grooves may be less efficient at accelerating a larger body mass. However, ambulacral groove area scaled isometrically with body mass in P. miniata (Fig. 2C), so groove area alone is not sufficient to explain the observed decline in absolute and size-specific crawling speed.
Table 2 Maximum crawling speeds in echinoderms Order (a) Species Crawling Radius Speed Modes (mm) (mm/s) (b) (c) ASTEROTDEA Forcipulatida Asterias forbesi normal/pursuit 60 1.3 [+ or -] [+ or -] 20 0.1/2.0 [+ or -] 0.5 Asterias rubens normal/escape < 50 mm 0.8 [+ or -] 0.2/3.3 [+ or -] 0.5 Asterias vulgaris pursuit 105 1.0 [+ or -] [+ or -] 35 0.5 Leptasterias normal/pursuit 60 0.5 poiaris [+ or -] [+ or -] 20 0.3/0.6 [+ or -] 0.3 Marthasterias normal 138 0.5 glacialis [+ or -] [+ or -] 13 * 0.2 Pycnopodia escape 260 20.4 helianthoides [+ or -] 5 ** Paxillosida Astropecten normal 100 2.5 aranciacus [+ or -] [+ or -] 50 0.8 Luidia ciliaris pursuit 235 50 Spinulosida Acanthaster planci normal 115 8.3 [+ or -] [+ or -] 75 2.5 Crossaster normal/escape 76 0.3 papposus [+ or -] 0.1/1.0 [+ or -] 0.2 Valvatida Archaster typicus normal 30 12.7 [+ or -] [+ or -] 20 5.4 Linckia laevigata normal 95 1.8 [+ or -] [+ or -] 55 0.5 Odontaster validus escape 40 0.9 [+ or -] 0.3 Oreaster normal/escape 14 3.3/5 reticulatus [+ or -] 6 Patiriella exigua normal/escape 10 0.6 [+ or -] [+ or -] 4 *** 0.1/1.5 [+ or -] 0.1 Patiria miniata escape 130 1.8 [+ or -] [+ or -] 80 0.6 Protoreaster normal 80 4.6 nodosus [+ or -] [+ or -] 60 1.2 ECHINOIDEA Mellita lata normal 25 0.3 [+ or -] [+ or -] 15 0.1 Clypeasteroida Paracentrotus normal 18 0.9 lividus [+ or -] [+ or -] 12.5 0.3 Echinoida Strongylocentrotus normal 25 0.03 droebachiensis [+ or -] [+ or -] 5 0.02 Order (a) Species Size Arm Podia Latitude Effect Number/Test Type (g) (d) Shape (e) (f) ASTEROTDEA Forcipulatida Asterias forbesi ? 5 FT TEM Asterias rubens ? 4-8 FT TEM (5 common) Asterias vulgaris ? 5 FT TEM Leptasterias ? 6 FT TEM poiaris Marthasterias ? 4-8 FT TEM glacialis (5 common) Pycnopodia ? many FT TEM helianthoides Paxillosida Astropecten ? 5 PT ST aranciacus Luidia ciliaris ? 7 PT TEM Spinulosida Acanthaster planci 0 many FT TR Crossaster ? many FT TEM papposus Valvatida Archaster typicus + 5 FT TR Linckia laevigata 0 5 FT TR Odontaster validus ? 5 FT POL Oreaster 0 5 FT TR reticulatus Patiriella exigua ? 5 FT ST Patiria miniata - 4-9 FT TEM (5 common) Protoreaster 0 5 FT TR nodosus ECHINOIDEA Mellita lata ? irregular - TR Clypeasteroida Paracentrotus + regular - ST lividus Echinoida Strongylocentrotus ? regular - TEM droebachiensis Order (a) Species Source ASTEROTDEA Forcipulatida Asterias forbesi Moore and Lepper (1997) Asterias rubens Mayo and Mackie (1976) Asterias vulgaris Drolet and Himmelman (2004) Leptasterias Thompson poiaris et al. (2005) Marthasterias Savy glacialis (1987) Pycnopodia Kjerschow- helianthoides Agersborg (1922) Paxillosida Astropecten Ferlin (1973) aranciacus Luidia ciliaris Brun (1972) Spinulosida Acanthaster planci Mueller et al. (2011) Crossaster Mayo and papposus Mackie (1976) Valvatida Archaster typicus Mueller et al. (2011) Linckia laevigata Mueller et at. (2011) Odontaster validus McClintock et al. (2008) Oreaster Scheibling reticulatus (1981) Patiriella exigua Stevenson (1992) Patiria miniata Current Study Protoreaster Mueller nodosus et al. (2011) ECHINOIDEA Mellita lata Kenk (1944) Clypeasteroida Paracentrotus Domenici lividus et al. (2003) Echinoida Strongylocentrotus Lauzon-Guay droebachiensis et al (2006) (a.) Species classified based on Vickery and McClintock (2000) [Asteroidea], and Kroh and Smith (2010) [Echinoidea]. (b.) Source: Radius (adult arm length from center of disc to arm tip) as reported in the original source paper, except for * from Gianguzza et al. (2009); ** from Brewer and Konar (2005); *** from Arrontes and Underwood (1991). (c.) Mean [+ or -] range to maximum or minimum value in the study. (d.) Dependence of crawling speed on size: (?) unknown, (0) none, (-) negative, (+) positive. (e.) Arm number for Asteroidea test shape for Echinoidea. (f.) Podia type for Asteroidea only: pointed-tip (PT), flat-tip (FT) from Vickery and McClintock (2000). (g.) TR = tropical (<20[degrees]), ST = subtropical (20[degrees] - 40[degrees]), TEM = temperate (40[degrees] - 60[degrees]), POL = polar (>60[degrees]).
Scaling of tube-feet numbers might influence the size-dependence of crawling speed in P. miniata and possibly other sea stars. If tube-foot number (and therefore total area of podial support) varies isometrically with arm length, then the mass supported per tube foot and per unit podial cross-sectional area will be higher in larger sea stars, which might impair locomotion at larger body sizes. Fewer tube feet per unit mass might also yield slower acceleration in larger individuals if the proportion of tube feet involved in locomotion at any one time does not change with size. The common ochre star Pisaster ochraceus appears to have proportionally more tube feet at larger body masses than expected from isometry (K. Hayne, University of Alberta, 2011, pers. comm.). This would reduce the amount of mass supported per tube foot and should yield relatively higher potential rates of acceleration in larger sea stars. Unfortunately, how tube-foot numbers scale in P. miniata remains unknown. The multi-armed Pycnopodia helianthoides offers a potential clue: this species appears to have proportionally more tube feet on the oral surface than P. miniata has, and it is one of the fastest moving sea stars in the northeastern Pacific, with a crawling velocity of 20 mm/s (Kjerschow-Agersborg, 1922; Van Veldhuizen and Oakes, 1981). Flow tube-foot size and number vary with body size in sea stars remains largely unexplored.
Substratum type might interact with the crawling body undulations of P. miniata in a way that also affects the size-dependence of absolute and relative crawling speeds. Crawling tests conducted on smooth plastic or glass surfaces (Domenici et al., 2003; this study) may not yield an accurate assessment of P. miniata maximum locomotion speeds, because this species normally lives on muddy or sandy bottoms (Rumrill, 1989). Echinoderms differ in their ability to attach to rough versus smooth surfaces: rough surfaces provide greater area for attachment and a stronger adhesion (Santos et al., 2005b). Changes to the amount of "stickiness" the tube feet experience while crawling on a substrate could potentially affect crawling speeds, but the magnitude of this influence seems to be species-specific (Mueller et al., 2011). The depth that moving tube feet might sink into a substrate during crawling could also have an effect, similar to how particle spacing influences the digging speed of clams (Nel et al., 2001). Therefore, additional velocity trials on sand, mud, and cobble might provide a more balanced view of P. miniata's crawling ability.
The relatively higher velocities exhibited by smaller individuals might possibly have resulted from enhanced sensitivity to predator stimuli. Smaller specimens of P. miniata show a stronger escape response (escaped more frequently and farther away) to Solaster dawsoni than do larger individuals (Van Veldhuizen and Oakes, 1981). Until they reach adulthood, juveniles in many species experience a greater threat of predation due to smaller size. If bat stars are more sensitive to S. dawsoni scent at a young age, it might explain the stronger escape responses observed by Van Veldhuizen and Oakes (1981) and the greater crawling speeds in this study. However, S. dawsoni individuals will capture and consume any size of sea star they encounter--even those twice their size (Van Veldhuizen and Oakes, 1981; pers. obs.), so smaller sea stars are not necessarily more vulnerable. Our results suggest that small specimens of P. miniata exhibit a more effective escape response purely for biomechanical reasons. Larger individuals of this species may simply be unable to move as quickly and get as far away from the predator.
Finally, the unusual vertical body undulation exhibited by crawling P. miniata (Fig. 6A, B) might help explain why maximum absolute crawling speeds were lower in larger individuals. This vertical undulation reveals that the arm margins intermittently drag on the substratum. If the number of tube feet scales isometrically with increasing body size, the greater mass that would have to be supported per tube foot might cause the arm margins to be in contact with the substratum for a greater proportion of the stride cycle, and the increased friction would tend to reduce crawling speed. The cost to lift the entire body mass up and forward during each stride could also be greater for larger animals due to increases in friction and the greater proportion of body mass relative to ambulacral groove area.
Despite its unusual morphology and crawling behaviour, maximum crawling speeds of P. miniata are on par with those of most other temperate sea stars (40-60[degrees] latitude) of similar body size (Table 2). Differences arise, however, when cold-water species are compared to those found in tropical (<20[degrees] latitude) and subtropical (20-40[degrees] latitude) waters. In general, sea stars native to warmer waters seem able to crawl faster than their cold-water counterparts (Fig. 7). The seven-armed Luidia ciliaris and multi-armed Pycnopodia helianthoides are notable exceptions. The voracious cold-water predator L. ciliaris is recognized as the fastest known sea star (Brun, 1972). The proportionally longer, pointed tube feet of this species may contribute to its impressive crawling speeds (Brun, 1972; Vickery and McClintock, 2000), but this cannot explain why P. helian-thoides is so fast, as it has flat-tipped podia. Retractor muscle and ampullae strength may play a role in the remarkable crawling speeds of L. ciliaris and P. helian-thoides. Enhanced movement of the tube feet could be achieved by increasing the cross-section of the retractor muscles (increased force of contraction), increasing the muscular control of the ampulla (to generate more hydrostatic pressure) (McCurley and Kier, 1995), or both. Enhanced tube-foot musculature has been observed in L. ciliaris but awaits confirmation in P. helianthoides (McCurley and Kier, 1995).
Clearly, many factors affect crawling speeds in sea stars. Water temperature, arm number, tube-foot number, and tube-foot form all need to be considered in future studies of sea star locomotion. Also, the use of standardized escape-response speeds, as done in the present study, may allow for more robust tests of differences among species.
Received 3 January 2012; accepted 5 May 2012.
* To whom correspondence should be addressed. E-mail: email@example.com
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EMALINE M. MONTGOMERY *, AND A. RICHARD PALMER
Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9; and Bamfield Marine Sciences Centre, Bamfield, British Columbia, Canada VOR 1BO
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|Author:||Montgomery, Emaline M.; Palmer, Richard A.|
|Publication:||The Biological Bulletin|
|Date:||Jun 1, 2012|
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