Ezequiel Meretta, P., & Rezende Ventura, C.R. (2021). Locomotion and righting behavior of sea stars: a study case on the bat star stellifera (). Revista de Biología Tropical, 69(S1), 501-513. DOI 10.15517/rbt.v69iSuppl.1.46392

DOI 10.15517/rbt.v69iSuppl.1.46392

Locomotion and righting behavior of sea stars: a study case on the bat star (Asterinidae)

Pablo E. Meretta1* Carlos Renato Rezende Ventura2

1. Estación Costera J.J. Nágera, Instituto de Investigaciones Marinas y Costeras, Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina-Universidad Nacional de Mar del Plata, Mar del Plata, Buenos Aires, Argentina; [email protected] (*Correspondence). 2. Departamento de Invertebrados, Laboratório de Echinodermata, Museu Nacional/Universidade Federal do Rio de Janeiro, Quinta Da Boa Vista S/Nº São Cristóvão, Rio de Janeiro, RJ20940-040, Brasil; [email protected]

Received 13-VIII-2020. Corrected 25-IX-2020. Accepted 29-X-2020.

ABSTRACT Introduction: The locomotion behavior of an organism involves the integration of aspects like body symmetry, sensory and locomotor systems. Furthermore, various ecological factors seem to be related to locomotion char- acteristics, such as foraging strategy, migration trends, response to predators and competitors, and environmental stress. Objective: To analyze locomotion and the influence of body symmetry in the crawling and righting movements of the sea star Asterina stellifera. Methods: We carried out laboratory experiments in aquariums in the presence/absence of water current and on a horizontal and vertical surface. Results: The speed is similar to speed in other species of similar size. Both the speed and linearity of displacement were independent of indi- vidual body size. A water current leads to faster crawling and straight paths, but there is no rheotaxis: streams do not affect locomotion. Speed and linearity of displacement were independent of individual body size. The displacement pattern described here may be an adaptation of organisms that present dense populations in com- munities with high prey abundance, as is the case of A. stellifera. Conclusions: Like other asteroids, this species did not show an Anterior/Posterior plane of symmetry during locomotion, or righting movement: it does not tend to bilaterality.

Key words: crawling; orientation; bilaterality; Asteroidea; rheotaxis; gravitaxis.

Crawling behavior comprises various spe- The reaction of sea stars to a water current cific aspects of , involving integration is variable: some species show a positive of body symmetry, sensory and locomotory sys- rheotaxis (Castilla 1972; Sloan, 1979, 1980; tems. Other physical and chemical cues influ- Sloan & Northway, 1982; Rochette, Hamel, ence the propensity to orient towards or away & Himmelman, 1994; Dale, 1997; Swenson from a stimulus, e.g. taxis mediated locomotion & McClintock, 1998), whereas others do not due to water flow, the presence of attractive or seem to respond to that stimulus (Dale, 1999). repulsive odorants (Dusenbery, 1992; Wyeth, The importance of water flow and chemical Woodward, & Dennis-Willows, 2006). stimulus as orientation cues is remarkable Locomotion traits of asteroids have called in many sea star species, but with contra- the attention of researchers for a long time. dicting results in differentiating the effect of

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) 501 each one separately (Sloan & Campbell, 1982; affect the crawling behavior of sea stars? How McClintock & Lawrence, 1981, 1984; Val- does a vertical surface affect its locomotion? entincic, 1983, 1985; Moore & Lepper, 1997). Do they behave bilaterally? Consequently, it is not clear whether asteroids We performed an experimental approach have a uniform or aleatory crawling behavior. under laboratory conditions to investigate Furthermore, the effect of gravity on locomo- physical and biological factors’ influence on tion is not well known. It is interesting to study the locomotion ability and movements’ orienta- this locomotion characteristic in species found tion to address these questions. We aimed to (1) in environments with high bottom heterogene- describe the effect of the individual’s body size ity, e.g. rocky boulders. (radius and body weight), water current (rheo- Several studies have also investigated taxis), and inclination plane (gravitaxis) on the movements oriented by one body axis in aster- crawling traits of sea stars and, (2) whether oids, i.e. do they move to the right or the left their plane of symmetry defines locomotion or backward/forward? These studies reached and up-righting movements. contradictory results: some species show a bilateral plane of symmetry (Cole, 1913; Polls MATERIALS AND METHODS & Gonor, 1975; Ji, Wu, Zhao, Wang, & Lv, 2012), but not others (Preyer, 1887; Ohshima, Collection and maintenance of aster- 1940; Montgomery & Palmer, 2012). oids: Samples of specimens occurred using Different ecological aspects of asteroids SCUBA from the port of Mar del Plata, Argen- (i.e. foraging behavior, migration pattern, tina (38°02’ S & 57° 31’30’’ W, 6-8 m depth). response to predators and competitors, and We immediately took the sea stars to the labo- environmental stresses) appear to be related ratory after collections. to their crawling actions (Feder & Chris- Two-week acclimatization of the individu- tensen, 1966; Gaymer & Himmelman, 2008). als took place in 300 l tanks with an open water Understanding the locomotion pattern (crawl- flow system that pumps seawater directly from ing speed, movement direction) would help the ocean during all experimental time. The characterize an organism’s different behaviors. aquarium seawater temperature used varied The sea star Asterina stellifera is an between 18-22 ºC. A timer kept the photope- omnivorous top predator that actively searches riod at 12 h light:12 h dark. In all tanks, the for its prey. This species has a wide variety individuals stayed on the observed field mean of prey items, from algae to benthic inverte- density (≈ 12 ind/m2) (Meretta et al., 2016). brates. Asterina stellifera can modify species Asteroids received a continuous food supply abundance in the subtidal community of Mar similar to those found in the natural environ- del Plata rocky bottoms. Large fragmented ment (Farias, Meretta, & Cledón, 2012). All boulders of big rocks and vertical walls of experiments occurred with one individual at a orthoquartzite blocks characterize the sampling time. Furthermore, each individual was used area (Genzano, Giberto, & Bremec, 2011). only once and in a single trial and then returned Therefore, studying this species’ locomotion to its environment. After each test, the aquari- pattern is a first step in understanding its um was emptied, scrubbed, rinsed, and cleaned foraging strategy (Meretta, Rubilar, Cledón, with fresh seawater, and refilled to avoid bias & Ventura, 2014; Meretta, Farias, Cledón, & from residual chemical cues. Ventura, 2016). Some basic questions related to sea stars Experimental design: We conducted locomotion arise. How do sea stars behave experiments to describe sea stars’ locomo- during locomotion? Do sea stars follow a usual tion behavior and the existence of a plane of locomotion pattern? Do bigger sea stars move symmetry during locomotion and up-righting faster than smaller ones? How does water flow movements. To characterize the locomotion

502 Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) behavior, we investigate a rheotaxis response Gravitaxis response: A fixed-volume of and the occurrence of a gravitaxis behavior. seawater with no external seawater supply and no flow characterizes the second type of exper- Rheotaxis response: We carried out experi- iment (still water treatment). The substrate was ments on the crawling behavior on a horizontal a glass of 80 x 80 cm marked with a 2 x 2 cm surface under two water-flow regimes: still grid, and the water column was about 90 cm. water (N = 120) and one-direction flow (N During vertical locomotion trials (N = 120), the = 120). A pump provided a continuous flow substrate was first in a horizontal plane. Imme- of filtered seawater across the experimental diately, we carefully moved the substrate in a aquarium through a PVC pipe to simulate and vertical plane, and the was allowed to standardize a one-direction water current. Sea- crawl freely. During trials, each sea star placed water entered the aquarium through the inflow alone inside the aquaria, started moving freely pipe on one wall and drained out back to the from the Ip, referred to by the madreporite sea through openings in an outflow pipe on the carefully placed at the origin (Fig. 1). opposite wall (Fig. 1). The current was nearly linear across the tank during trials at a velocity Crawling experimental measurements: of 1.5 cm/s–1 (observed with methylene blue For both experiments mentioned above, we dye). The current flow used was equivalent measured the sea stars’ wet weight (± 0.1 g) to a moderate current speed in the habitat and radius (± 0.1 mm; from the center of the of this species. disc to arm tip). Monitoring of each sea star Experiments occurred on an aquarium of trajectory displacement referred to the mad- 80 x 80 x 20 cm with a 2 x 2 cm grid marked reporite as a fixed point over time. Records off (Fig. 1), with the origin (0, 0) at the center of the sea star path occurred, dividing sec- to monitor the animals’ movements. Each sea tions per minute of locomotion. We called T star started moving from the center of the grid (total distance traveled) the sum of all parts (0, 0: initial position, Ip; Fig. 1), referred to by covered by an individual during one observa- its madreporite position. tion (Fig. 1). We recorded the instantaneous

Fig. 1. Diagram of the experimental aquarium (80 x 80 x 20 cm) with a bottom grid of 2 x 2 cm squares, showing the tank water flow and the initial (Ip) and final position (Fp) of the sea star. Grey arrows indicate water direction. Representation of one sea star trajectory, depicting total distance traveled (T) and linear displacement (Ld). The sea star orientation angle (Oa) represents the angle between Ld and water direction.

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) 503 crawling velocity over one-minute intervals. all angular measurements on a 360° grid and According to Montgomery and Palmer (2012) calculated clockwise. and Montgomery (2014), we calculated the All experiments finished when the sea average crawling speed (ACS) from the maxi- star reached one of the aquarium walls or mum instantaneous velocities (plateau) at the after 15 minutes of inactivity. The observation top of the speed-time curve. of the madreporite position occurred visually During each trial, we recorded the number every minute. of times a sea star changes its crawling direc- tion (DC). The linear displacement between the Bilateral response: We carried out experi- points that represent the initial and final posi- ments in aquariums (40 x 40 x 30 cm) with still tion (Ip and Fp, respectively) was called Ld. water to analyze the existence of a bilateral Trajectory straightness (linearity, L) is a dimen- plane of symmetry affecting locomotion and sionless value, and it means the ratio of Ld to T, up-righting movements in A. stellifera. For the first experiment, 120 sea stars were randomly L = Ld/T (Fig. 1). The L range is 0 to 1, where oriented relative to the madreporite and placed 0 is maximum sinuosity, and 1 is maximum in the aquaria center. Individuals were left to straightness. Classification of linearity levels crawl freely. We recorded the preferred crawl- was as follows: highly directional (higher than ing direction as the individual’s final arm posi- 0.7), partially directional (from 0.5 to 0.7), and tion relative to the initial orientation. For the non-directional (lower than 0.5) (Ferlin, 1973; second experiment, 100 individuals were turned Scheibling, 1981). upside down and allowed to turn back freely to Since moving in a straight line towards analyze righting movements. We record the any wall may also produce linearity of 1, we arms used during up-righting turning. recorded the orientation angles to distinguish We used three different arm-numbering between movements towards each lateral bor- systems to test the existence of a plane of der. We considered the sea star orientation as symmetry during both experiments mentioned the angle (Oa) between the straight line at the above (Fig. 2). To this end, the number given origin (0, 0) and the line representing the trajec- to each arm was as follows: (1) sea stars’ arms tory traveled (Fp coordinates, Fig. 1). For hori- were individually numbered clockwise from zontal crawling trials, an angle of zero pointed the madreporite, (2) the equivalent position directly to the current water source. We plotted of each arm across the madreporite plane of

Fig. 2. Aboral view of the three arm-numbering systems used to describe the sea star’s bilateral behavior. Arms numbered clockwise (1 to 5) from the madreporite. A. Arm numbering was assigned (numbers in bold) based on equivalent position across the madreporite plane of symmetry (according to Montgomery & Palmer, 2012). B. Arm numbering according to equal reference across the plane of symmetry located from arm 2 to interradius 4-5 (according to Ji et al., 2012).

504 Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) symmetry defined the arm numbering (accord- We used the open-access Software R (R ing to Montgomery & Palmer, 2012) (Fig. 2A), Core Team, 2019) for all statistical analyses and (3) the relative position of each ray across as following: the emmeans package for pair- the plane of symmetry located from arm 2 to wise comparisons (Russell, 2019), the circular interradius 4-5 identified this arm numbering package for the Rayleigh’s z-test and Watson- (according to Ji et al., 2012) (Fig. 2B). During Wiliams’ U2-test (Agostinelli & Lund, 2017), both trials, sea stars’ arms were identified (arm and the script developed by Hurd (2001) for numbering system) and followed. the G-test.

Data analysis: We used the Gaussian RESULTS linear regressions, a Generalized Linear Model (GLM) to analyze the sea star crawling behav- Sea star crawling behavior: The instan- ior in terms of average crawling speed (ACS), taneous crawling speeds presented an accelera- linearity (L), and the number of direction tion phase followed by an oscillating plateau. changes (DC) over a horizontal surface with The analyses indicated that ACS changes and without water current, and on a verti- according to water flow conditions and loco- cal surface for sea stars going downwards or motion plane (Table 1). In this sense, sea stars upwards. We calculated a Poisson GLM regres- crawled faster when subject to water current sion because DC is a count data (Zuur, Ieno, than still water or a vertical surface; and speed Walter, Saveliev, & Smith, 2009). In the same was slower going upwards than downwards way, we also analyzed the relation of ACS, (Table 1, Table 2, Fig. 3). Moreover, there were L, and DC with sea star size (radius and wet no differences in the number of sea stars that weight). We used the post-hoc Least Squares crawl downwards or upwards (51 downwards, Means pairwise comparisons to identify sig- 69 upwards; G-value = 2.417, P = 0.120). nificant differences across groups. Similarly, displacement linearity (L) over We used Rayleigh’s z-test for angular dis- the horizontal surface presented significantly persion to test whether the orientation angles higher values in the presence of a water cur- had a uniform distribution on the circle (an rent. On the other hand, displacement linearity expected result when there is no influence was similar in the absence of water current and on movement direction). The same test also when crawling on a vertical surface (Table 1, verified a significant directional trend in the Table 2). However, there were no significant sea stars’ movement due to the current water differences between going upwards or down- effect. Moreover, we used the Watson-Wiliams’ wards (Table 1, Table 2, Fig. 3). Consequently, U2-test for the mean angle formed by pairwise L’s values suggest that sea stars’ crawling samples to analyze directionality differences behavior in the water current regime tended to between water flow conditions (Zar, 1999). be directional (> 0.7). In the absence of water Also, we performed the log-likelihood ratio flow and faced to the vertical surface, their test for goodness of fit (G-test; Zar, 1999) to behavior ranged from partially directional (> compare the number of individuals that crawl 0.5) to non-directional (< 0.5) (Fig. 3). upwards against downwards during vertical The number of direction changes (DC) surface trials. during sea star locomotion differed between We carried out a log-likelihood ratio test experimental treatments. In the presence of the for goodness of fit (G-test; Zar, 1999) to assess current water regime, most individuals present- the existence of a bilateral plane of symmetry ed no differences in crawling direction. How- affecting locomotion and up-righting move- ever, in the still water, some sea stars had the ments. Then, we compare the individuals’ arm most remarkable direction changes during their preference in up-righting and crawling accord- locomotion (4 and 5 turns) (Table 1, Table 2, ing to the three arm-numbering systems. Fig. 4). On the other hand, when crawling over

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) 505 TABLE 1 Comparison of locomotion features related to sea star size, under two water regimes and surface inclination: using summary statistics (A) and Generalized Linear Model analyses (B) (N = 120/trial)

A) Summary ACS L DC statistics Wc 4.55 ± 1.06 cm/min 0.83 ± 0.11 1.18 ± 1.22 Sw 1.87 ± 0.80 cm/min 0.60 ± 0.20 1.64 ± 1.58 Vs 1.82 ± 0.99 cm/min 0.62 ± 0.20 1.08 ± 1.14 Up 1.26 ± 0.54 cm/min 0.63 ± 0.21 1.67 ± 1.15 Down 2.59 ± 0.98 cm/min 0.61 ± 0.19 0.29 ± 0.46 B) Dependent Chi-square Chi-square Chi-square d.f. P d.f. P d.f. P variables value value value T 2 581.29 < 0.001 2 182.61 < 0.001 2 118.69 < 0.001 (Wc – Sw – Vs) R 1 1.78 0.182 1 2.25 0.134 1 1.98 0.159 W 1 1.20 0.273 1 1.40 0.237 1 1.85 0.174 ACS – – – – – – 1 106.84 < 0.001 L – – – – – – 1 16.23 < 0.001 T 1 75.04 < 0.001 1 0.29 0.589 1 45.85 < 0.001 (Up-Down) R 1 1.13 0.287 1 0.001 0.975 1 0.73 0.393 W 1 1.42 0.234 1 0.39 0.534 1 0.11 0.739 ACS – – – – – – 1 1.92 0.166 L – – – – – – 1 0.36 0.546

ACS = average crawling speed; L = linearity; DC = direction changes; T = treatment; Wc = water current; Sw = still water; Vs = vertical surface; R = sea star radius; W = sea star weight.

TABLE 2 Post-hoc Least Squares Means pairwise comparisons of locomotion features between water current and surface inclination trials

Response ACS L DC variable Dif ± S.E. t-value P-value Dif ± S.E. t-value P-value Dif ± S.E. z-value P-value comparisons Sw – Wc -2.675 ± 0.120 -22.21 < 0.001 -0.227 ± 0.021 -10.98 < 0.001 -1.808 ± 0.204 -8.85 < 0.001 Sw – Vs 0.053 ± 0.116 0.46 -0.017 ± 0.026 -0.66 0.783 0.404 ± 0.117 3.47 0.002 Wc – Vs 2.727 ± 0.132 20.60 < 0.001 0.210 ± 0.021 10.06 < 0.001 2.212 ± 0.216 10.22 < 0.001 Down – Up 1.313 ± 0.152 8.66 < 0.001 -0.020 ± 0.037 -0.54 0.590 -2.029 ± 0.345 -5.88 < 0.001

ACS = average crawling speed; L = linearity; DC = direction changes; Wc = water current; Sw = still water; Vs = vertical surface; Dif = estimated difference between treatments. a vertical surface, sea stars showed the slight- On the other hand, the Rayleigh z-test est direction changes when moving downwards showed that the orientation angle distribution (Table 1, Table 2, Fig. 4). was not significantly different from unifor- Our results showed that there was no effect mity in all experimental crawling conditions of body size (radius and wet weight) on the (Fig. 5). Moreover, the orientation angles were locomotion traits (ACS, L, and DC) of A. stel- not significantly different between treatments lifera (Table 1). (Watson-Wiliams’s U2-test: water current-still

506 Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) Fig. 3. Average crawling speed (cm/min) and linearity regarding two water-flow regimes (still water, pink; water current, green; N = 120 sea stars/trial) and the plane of substrata (90° vertical plane, blue; N = 120 sea stars) going downwards (red) and upwards (aquamarine). water, U2 = 0.056, P > 0.1). Therefore, sea stars differ from random in any three-arm classes did not exhibit a preferential angle of movement. analyzed (Table 3). During righting, sea stars first extended Sea star bilateral-like behavior: During their arms upwards, attached two adjacent arms to the aquarium floor for support, and made the crawling behavior trials, all sea stars moved up-righting movement using the opposite arm. with two leading arms forward, two on the side, During the position-recovering, arm preference and one backward. Sea stars used different pair did not differ significantly from random in combinations of leading arms while crawling. any of the three-arm classes analyzed (Table Arm preference during locomotion did not 3). Therefore, the up-righting and crawling

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) 507 Fig. 4. The locomotion direction changes regarding the two water-flow regimes studied (still water and water current; N = 120 sea stars/treatment) and vertical surface trials (going downwards and upwards; N = 120 sea stars).

TABLE 3 Bilateral-like movement trials: Arm preference during locomotion (N = 120) and righting (N = 100) trials

Arm numbering G-test Arm classes 1 2 3 4 5 G P Crawling Trials arms individually numbered 0.17 0.22 0.24 0.22 0.15 2.983 0.561 madreporite axis 0.45 0.39 0.16 – – 1.485 0.476 arm 2 to inter-radius 4-5 axis 0.27 0.39 0.34 – – 3.180 0.204 Righting Trials arms individually numbered 0.23 0.20 0.24 0.15 0.18 2.757 0.599 madreporite axis 0.41 0.35 0.24 – – 1.429 0.489 arm 2 to inter-radius 4-5 axis 0.20 0.47 0.33 – – 2.463 0.292

508 Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) 2014). In conclusion, a species-specific vari- ability in the relationship between body size and movement speed in sea stars may occur. In accordance, our results show no association between crawling speed and radius nor wet weight for adult A. stellifera, regardless of the presence or absence of water current and plane of locomotion. There is a relationship between the ambulacral groove area (area on the oral Fig. 5. Orientation in the presence and absence of water surface containing tube feet) and the sea star’s current. Dots represent the position of a sea star at the end of the trial. The current water direction is at 0°. Arrows radius. (Mueller et al., 2011; Montgomery represent the mean angular vector. The P-values represent & Palmer, 2012; Montgomery, 2014). These the Rayleigh z-test for angular dispersion (expected points positive direct relationship between the radius uniformly distributed on the circle). θ, mean orientation and the ambulacral groove area might be more angle; r, mean vector length; N = 120 sea stars/treatment. significant in bigger sea stars than in small or medium ones (Montgomery & Palmer, 2012) such as A. stellifera. Thus, the absence of a experiments show no symmetric behavioral relationship between A. stellifera’s speed and plane of locomotion in A. stellifera. size needs further investigation. A possible explanation may arise from the relationship DISCUSSION between its ambulacral groove area and radius. On the other hand, it is noteworthy that Sea star crawling behavior: Describ- few studies have analyzed the crawling lin- ing sea star crawling movements can help to earity variation. The available data regarding elucidate different ecological aspects related the foraging behavior of sea stars describe to individual foraging behavior, migration, and changes in that parameter concerning prey relationships among individuals within and items (Moore & Lepper, 1997; Drolet & Him- among species. Since these behaviors involve melman, 2004; Thompson, Drolet, & Him- a complex integration of body symmetry, sen- melman, 2005; Barahona & Navarrete, 2010; sory system, and other body characteristics, Montgomery & Palmer, 2012). Although lin- it is not striking that contradicting crawling earity is a descriptive dimensionless parameter, actions could be described between and within it provides information about crawling behav- classes. iors concerning trajectory shape or directional Crawling speed values recorded for A. efficiency (Ricci, Barbanera, & Erra, 1998). stellifera are similar to those of other sea star In this study, individuals traveled a relatively species with comparable body sizes (40-70 straight trajectory (L > 0.7) on a horizontal sur- mm) living in temperate waters (Montgomery face and under the presence of a water current. & Palmer, 2012). We observed no L values differences between In general, animals present a positive individuals crawling on a flat surface with still relationship between body size and displace- water and a vertical surface, neither between L ment speed. Mainly, echinoderms show dif- values and the sea star size. Thus, by crawling ferent patterns in this relationship. However, in a linear trajectory, A. stellifera would not considering that the efficiency of displace- be walking the same path twice, optimizing ment depends on both the number of tube feet energy, e.g. avoiding sampling twice an area and body weight, it is expected that big sea during foraging. stars move faster than the small ones (Ferlin, For many marine organisms, water cur- 1973; Mueller, Bos, Graf, & Gumanao, 2011; rents trigger individuals’ movement (rheotaxis) Montgomery & Palmer, 2012; Montgomery, or help to detect prey cues (chemotaxis). In the

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) 509 latter case, it increases the chances of detecting time than they do to move downwards or on a chemical stimuli (Valentincic, 1983; Rochette horizontal bottom. Therefore, the gravitational et al., 1994; Mueller et al., 2011). Some stud- force helps sea stars to move downwards faster. ies reported evidence of chemoreception and positive rheotaxis for some sea stars species Sea star bilateral behavior: During (McClintock & Lawrence, 1981; Moore & crawling behavior trials, all A. stellifera indi- Lepper, 1997), but not in others (Sloan & viduals moved with two leading arms forward, Campbell, 1982; Mueller et al., 2011). Asteri- two on the side, and one backward, as reported na stellifera showed a random orientation for other sea star species (Reese, 1966; Ji et behavior both in the presence and absence of al., 2012). Similarly, during righting, sea stars water current regarding the crawling orienta- first extended their arms upwards, attached tion angle and linearity on a horizontal surface. two adjacent arms to the aquarium floor for Moreover, the water current caused faster and support, and made the up-righting movement straighter individual displacement, but not an with the opposite arm, as reported for other oriented movement with or against the water sea star species (Polls & Gonor, 1975). There- current. In other words, A. stellifera did not fore, the results obtained in this study are show rheotaxis. As stated before, natural selec- comparable to those. tion can optimize locomotion concerning prey Many studies analyze the bilateral behav- distribution and abundance over evolutionary ior of echinoderms, with contradictory results. time (Pyke, Pulliam, & Charnov, 1977). In this Most of them have focused on some regular sense, a randomly oriented but linear move- echinoids and asteroids. Sea urchins appear to ment may favor animals that belong to highly have no Anterior/Posterior axis of symmetry dense populations in communities with high when crawling in an open space (Parker, 1936; prey density (McClintock & Lawrence, 1981; Grabowsky, 1994; Yoshimura & Motokawa, Mueller et al., 2011). Asterina stellifera seems 2008, 2010; Yoshimura, Iketani, & Motokawa, to be well-adapted to moving randomly in a 2012). Similarly, Preyer (1887, as cited in dense community. This species’ population Cole, 1913) conducted some experiments with reaches densities of up to 72 ind/m–2 and lives ophiuroids (Ophiomyxa sp. and Ophioderma in a subtidal environment with abundant prey sp.) and found no preference for any particular in the South Atlantic (Farias et al., 2012; Mer- ray during locomotion. etta et al., 2014). Future studies on the crawling Regarding asteroids, some studies reported behavior of sea stars in the presence of prey the existence of a behavioral A/P axis or a will help to understand the foraging strategy tendency to have one during locomotion and of this species. righting in several species (Cole, 1913; Kjer- Regarding the inclination of the surface of schow-Agersborg, 1922; O’Donoghue, 1926; locomotion (i.e. bottom structure), our study Polls & Gonor, 1975; Ji et al., 2012; Ardor- showed that it affects this sea star’s crawling Bellucci & Smith, 2019). However, other sea behavior. The crawling speed of A. stellifera stars did not exhibit an A/P plane of sym- during trials was higher when the individuals metry during crawling nor righting (Preyer, moved downwards on a vertical plane than 1887, as cited in Cole, 1913; Ohshima, 1940; going upwards, probably due to the extra Montgomery & Palmer, 2012). Furthermore, mechanical work associated with the gravi- it is still unclear why some species use one tational force. However, there is no relation- combination of arms more frequently than the ship between sea star size (R and wet weight) others as leading arms (Polls & Gonor, 1975; and its vertical locomotion speed. According Ji et al., 2012; Ardor-Bellucci & Smith, 2019). to Domenici, González-Calderón, and Ferrari And also, why other species do not follow any (2003), sea stars adhere to the substrate to keep pattern (Ohshima, 1940; Polls & Gonor, 1975; themselves in a vertical plane for much more Montgomery & Palmer, 2012; this study). In

510 Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021) this sense, the potential existence of a tendency period, PEM was supported by a Ph.D. Fellow- toward bilateral behavior may be a species- ship funded by CONICET - Consejo Nacio- specific characteristic rather than a generalized nal de Investigaciones Científicas y Técnicas. feature of all echinoderms. CRRV is supported by Research Fellowships Asterina stellifera is another example of of the Conselho Nacional de Desenvolvim- a species that did not exhibit an A/P plane of ento Científico e Tecnológico (CNPq) and the symmetry during crawling nor righting. As is Fundação Carlos Chagas de Amparo à Pesquisa evidenced in the literature, there is a consider- do Estado do Rio de Janeiro (FAPERJ). able time lag in investigations on this subject. The methodology associated with experimen- RESUMEN tation and data analysis has changed. Thus, these results contribute as an example to the Comportamiento de locomoción y enderezamiento discussion regarding a functional bilateral axis en estrellas de mar: un caso de estudio sobre in adult asteroids and update the subject. la estrella Asterina stellifera (Asterinidae) In all asteroids, the water vascular system Introducción: El comportamiento de locomoción de and the nervous system assume a pentaradial un organismo implica la integración de aspectos como la symmetry. Radial distribution of sense organs simetría corporal, los sistemas sensorial y locomotor. Ade- allows sea stars to perceive their surrounding más, varios factores ecológicos parecen estar relacionados environment continuously from all directions con las características de la locomoción, como la estrategia and move towards or away from a stimulus de alimentación, las tendencias migratorias, la respuesta a los depredadores y competidores y el estrés ambiental. (prey or enemy). Understanding the mecha- Objetivo: Analizar el patrón general de locomoción y nisms by which sea stars detect and respond to la influencia de la simetría corporal en la locomoción y different stimuli will help elucidate ecological enderezamiento de la estrella de mar Asterina stellifera. aspects such as prey detection and selection, Métodos: Realizamos experimentos de laboratorio en avoid predators, and respond to changes in acuarios en presencia / ausencia de corriente de agua y en superficie horizontal y vertical. Resultados: La velocidad environmental conditions. es similar a la velocidad en otras especies de tamaño In conclusion, our results show that (1) A. similar. Tanto la velocidad como la linealidad del despla- stellifera presents no rheotaxis, but gravitaxis, zamiento fueron independientes del tamaño corporal indi- (2) locomotion speed and linearity are indepen- vidual. Una corriente de agua conduce a una velocidad de dent of body size, and (3) this species would desplazamiento mayor y a trayectorias más rectas, pero no hay reotaxis: una corriente de agua no afecta el patrón de not tend to bilateral behavior. locomoción. La velocidad y la linealidad del desplazamien- to fueron independientes del tamaño corporal individual. Ethical statement: authors declare that El patrón de desplazamiento aquí descrito puede ser una they all agree with this publication and made adaptación de organismos que presentan densas poblacio- significant contributions; that there is no con- nes en comunidades con alta abundancia de presas, como es el caso de A. stellifera. Conclusiones: Al igual que otros flict of interest of any kind; and that we fol- asteroides, esta especie no mostró un plano de simetría lowed all pertinent ethical and legal procedures Anterior / Posterior durante la locomoción o el movimiento and requirements. All financial sources are de enderezamiento: no tiende a la bilateralidad. fully and clearly stated in the acknowledge- ments section. A signed document has been Palabras clave: locomoción; orientación; bilateralidad; Asteroidea; reotaxis; gravitaxis filed in the journal archives.

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