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Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 501-513, March 2021 (Published Mar. 10, 2021)

Locomotion and righting behavior of sea stars:

a study case on the bat star Asterina stellifera (Asterinidae)

Pablo E. Meretta

Carlos Renato Rezende Ventura

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;

pabloemeretta@gmail.com (*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; ventura@acd.ufrj.br

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.

Ezequiel Meretta, P., & Rezende Ventura, C.R. (2021).

Locomotion and righting behavior of sea stars:

a study case on the bat star Asterina stellifera

(Asterinidae). Revista de Biología Tropical, 69(S1),

501-513. DOI 10.15517/rbt.v69iSuppl.1.46392

Crawling behavior comprises various spe-

cific aspects of animals, involving integration

of body symmetry, sensory and locomotory sys-

tems. Other physical and chemical cues influ-

ence the propensity to orient towards or away

from a stimulus, e.g. taxis mediated locomotion

due to water flow, the presence of attractive or

repulsive odorants (Dusenbery, 1992; Wyeth,

Woodward, & Dennis-Willows, 2006).

Locomotion traits of asteroids have called

the attention of researchers for a long time.

The reaction of sea stars to a water current

is variable: some species show a positive

rheotaxis (Castilla 1972; Sloan, 1979, 1980;

Sloan & Northway, 1982; Rochette, Hamel,

& Himmelman, 1994; Dale, 1997; Swenson

& McClintock, 1998), whereas others do not

seem to respond to that stimulus (Dale, 1999).

The importance of water flow and chemical

stimulus as orientation cues is remarkable

in many sea star species, but with contra-

dicting results in differentiating the effect of

DOI 10.15517/rbt.v69iSuppl.1.46392

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each one separately (Sloan & Campbell, 1982;

McClintock & Lawrence, 1981, 1984; Val-

entincic, 1983, 1985; Moore & Lepper, 1997).

Consequently, it is not clear whether asteroids

have a uniform or aleatory crawling behavior.

Furthermore, the effect of gravity on locomo-

tion is not well known. It is interesting to study

this locomotion characteristic in species found

in environments with high bottom heterogene-

ity, e.g. rocky boulders.

Several studies have also investigated

movements oriented by one body axis in aster-

oids, i.e. do they move to the right or the left

or backward/forward? These studies reached

contradictory results: some species show a

bilateral plane of symmetry (Cole, 1913; Polls

& Gonor, 1975; Ji, Wu, Zhao, Wang, & Lv,

2012), but not others (Preyer, 1887; Ohshima,

1940; Montgomery & Palmer, 2012).

Different ecological aspects of asteroids

(i.e. foraging behavior, migration pattern,

response to predators and competitors, and

environmental stresses) appear to be related

to their crawling actions (Feder & Chris-

tensen, 1966; Gaymer & Himmelman, 2008).

Understanding the locomotion pattern (crawl-

ing speed, movement direction) would help

characterize an organism’s different behaviors.

The sea star Asterina stellifera is an

omnivorous top predator that actively searches

for its prey. This species has a wide variety

of prey items, from algae to benthic inverte-

brates. Asterina stellifera can modify species

abundance in the subtidal community of Mar

del Plata rocky bottoms. Large fragmented

boulders of big rocks and vertical walls of

orthoquartzite blocks characterize the sampling

area (Genzano, Giberto, & Bremec, 2011).

Therefore, studying this species’ locomotion

pattern is a first step in understanding its

foraging strategy (Meretta, Rubilar, Cledón,

& Ventura, 2014; Meretta, Farias, Cledón, &

Ventura, 2016).

Some basic questions related to sea stars

locomotion arise. How do sea stars behave

during locomotion? Do sea stars follow a usual

locomotion pattern? Do bigger sea stars move

faster than smaller ones? How does water flow

affect the crawling behavior of sea stars? How

does a vertical surface affect its locomotion?

Do they behave bilaterally?

We performed an experimental approach

under laboratory conditions to investigate

physical and biological factors’ influence on

the locomotion ability and movements’ orienta-

tion to address these questions. We aimed to (1)

describe the effect of the individual’s body size

(radius and body weight), water current (rheo-

taxis), and inclination plane (gravitaxis) on the

crawling traits of sea stars and, (2) whether

their plane of symmetry defines locomotion

and up-righting movements.

MATERIALS AND METHODS

Collection and maintenance of aster-

oids: Samples of specimens occurred using

SCUBA from the port of Mar del Plata, Argen-

tina (38°02’ S & 57° 31’30’’ W, 6-8 m depth).

We immediately took the sea stars to the labo-

ratory after collections.

Two-week acclimatization of the individu-

als took place in 300 l tanks with an open water

flow system that pumps seawater directly from

the ocean during all experimental time. The

aquarium seawater temperature used varied

between 18-22 ºC. A timer kept the photope-

riod at 12 h light:12 h dark. In all tanks, the

individuals stayed on the observed field mean

density (≈ 12 ind/m

) (Meretta et al., 2016).

Asteroids received a continuous food supply

similar to those found in the natural environ-

ment (Farias, Meretta, & Cledón, 2012). All

experiments occurred with one individual at a

time. Furthermore, each individual was used

only once and in a single trial and then returned

to its environment. After each test, the aquari-

um was emptied, scrubbed, rinsed, and cleaned

with fresh seawater, and refilled to avoid bias

from residual chemical cues.

Experimental design: We conducted

experiments to describe sea stars’ locomo-

tion behavior and the existence of a plane of

symmetry during locomotion and up-righting

movements. To characterize the locomotion

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behavior, we investigate a rheotaxis response

and the occurrence of a gravitaxis behavior.

Rheotaxis response: We carried out experi-

ments on the crawling behavior on a horizontal

surface under two water-flow regimes: still

water (N = 120) and one-direction flow (N

= 120). A pump provided a continuous flow

of filtered seawater across the experimental

aquarium through a PVC pipe to simulate and

standardize a one-direction water current. Sea-

water entered the aquarium through the inflow

pipe on one wall and drained out back to the

sea through openings in an outflow pipe on the

opposite wall (Fig. 1). The current was nearly

linear across the tank during trials at a velocity

of 1.5 cm/s

–1

(observed with methylene blue

dye). The current flow used was equivalent

to a moderate current speed in the habitat

of this species.

Experiments occurred on an aquarium of

80 x 80 x 20 cm with a 2 x 2 cm grid marked

off (Fig. 1), with the origin (0, 0) at the center

to monitor the animals’ movements. Each sea

star started moving from the center of the grid

(0, 0: initial position, Ip; Fig. 1), referred to by

its madreporite position.

Gravitaxis response: A fixed-volume of

seawater with no external seawater supply and

no flow characterizes the second type of exper-

iment (still water treatment). The substrate was

a glass of 80 x 80 cm marked with a 2 x 2 cm

grid, and the water column was about 90 cm.

During vertical locomotion trials (N = 120), the

substrate was first in a horizontal plane. Imme-

diately, we carefully moved the substrate in a

vertical plane, and the animal was allowed to

crawl freely. During trials, each sea star placed

alone inside the aquaria, started moving freely

from the Ip, referred to by the madreporite

carefully placed at the origin (Fig. 1).

Crawling experimental measurements:

For both experiments mentioned above, we

measured the sea stars’ wet weight (± 0.1 g)

and radius (± 0.1 mm; from the center of the

disc to arm tip). Monitoring of each sea star

trajectory displacement referred to the mad-

reporite as a fixed point over time. Records

of the sea star path occurred, dividing sec-

tions per minute of locomotion. We called T

(total distance traveled) the sum of all parts

covered by an individual during one observa-

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.

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crawling velocity over one-minute intervals.

According to Montgomery and Palmer (2012)

and Montgomery (2014), we calculated the

average crawling speed (ACS) from the maxi-

mum instantaneous velocities (plateau) at the

top of the speed-time curve.

During each trial, we recorded the number

of times a sea star changes its crawling direc-

tion (DC). The linear displacement between the

points that represent the initial and final posi-

tion (Ip and Fp, respectively) was called Ld.

Trajectory straightness (linearity, L) is a dimen-

sionless value, and it means the ratio of Ld to T,

L = Ld/T (Fig. 1). The L range is 0 to 1, where

0 is maximum sinuosity, and 1 is maximum

straightness. Classification of linearity levels

was as follows: highly directional (higher than

0.7), partially directional (from 0.5 to 0.7), and

non-directional (lower than 0.5) (Ferlin, 1973;

Scheibling, 1981).

Since moving in a straight line towards

any wall may also produce linearity of 1, we

recorded the orientation angles to distinguish

between movements towards each lateral bor-

der. We considered the sea star orientation as

the angle (Oa) between the straight line at the

origin (0, 0) and the line representing the trajec-

tory traveled (Fp coordinates, Fig. 1). For hori-

zontal crawling trials, an angle of zero pointed

directly to the current water source. We plotted

all angular measurements on a 360° grid and

calculated clockwise.

All experiments finished when the sea

star reached one of the aquarium walls or

after 15 minutes of inactivity. The observation

of the madreporite position occurred visually

every minute.

Bilateral response: We carried out experi-

ments in aquariums (40 x 40 x 30 cm) with still

water to analyze the existence of a bilateral

plane of symmetry affecting locomotion and

up-righting movements in A. stellifera. For the

first experiment, 120 sea stars were randomly

oriented relative to the madreporite and placed

in the aquaria center. Individuals were left to

crawl freely. We recorded the preferred crawl-

ing direction as the individual’s final arm posi-

tion relative to the initial orientation. For the

second experiment, 100 individuals were turned

upside down and allowed to turn back freely to

analyze righting movements. We record the

arms used during up-righting turning.

We used three different arm-numbering

systems to test the existence of a plane of

symmetry during both experiments mentioned

above (Fig. 2). To this end, the number given

to each arm was as follows: (1) sea stars’ arms

were individually numbered clockwise from

the madreporite, (2) the equivalent position

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).

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symmetry defined the arm numbering (accord-

ing to Montgomery & Palmer, 2012) (Fig. 2A),

and (3) the relative position of each ray across

the plane of symmetry located from arm 2 to

interradius 4-5 identified this arm numbering

(according to Ji et al., 2012) (Fig. 2B). During

both trials, sea stars’ arms were identified (arm

numbering system) and followed.

Data analysis: We used the Gaussian

linear regressions, a Generalized Linear Model

(GLM) to analyze the sea star crawling behav-

ior in terms of average crawling speed (ACS),

linearity (L), and the number of direction

changes (DC) over a horizontal surface with

and without water current, and on a verti-

cal surface for sea stars going downwards or

upwards. We calculated a Poisson GLM regres-

sion because DC is a count data (Zuur, Ieno,

Walter, Saveliev, & Smith, 2009). In the same

way, we also analyzed the relation of ACS,

L, and DC with sea star size (radius and wet

weight). We used the post-hoc Least Squares

Means pairwise comparisons to identify sig-

nificant differences across groups.

We used Rayleigh’s z-test for angular dis-

persion to test whether the orientation angles

had a uniform distribution on the circle (an

expected result when there is no influence

on movement direction). The same test also

verified a significant directional trend in the

sea stars’ movement due to the current water

effect. Moreover, we used the Watson-Wiliams’

-test for the mean angle formed by pairwise

samples to analyze directionality differences

between water flow conditions (Zar, 1999).

Also, we performed the log-likelihood ratio

test for goodness of fit (G-test; Zar, 1999) to

compare the number of individuals that crawl

upwards against downwards during vertical

surface trials.

We carried out a log-likelihood ratio test

for goodness of fit (G-test; Zar, 1999) to assess

the existence of a bilateral plane of symmetry

affecting locomotion and up-righting move-

ments. Then, we compare the individuals’ arm

preference in up-righting and crawling accord-

ing to the three arm-numbering systems.

We used the open-access Software R (R

Core Team, 2019) for all statistical analyses

as following: the emmeans package for pair-

wise comparisons (Russell, 2019), the circular

package for the Rayleigh’s z-test and Watson-

Wiliams’ U

-test (Agostinelli & Lund, 2017),

and the script developed by Hurd (2001) for

the G-test.

RESULTS

Sea star crawling behavior: The instan-

taneous crawling speeds presented an accelera-

tion phase followed by an oscillating plateau.

The analyses indicated that ACS changes

according to water flow conditions and loco-

motion plane (Table 1). In this sense, sea stars

crawled faster when subject to water current

than still water or a vertical surface; and speed

was slower going upwards than downwards

(Table 1, Table 2, Fig. 3). Moreover, there were

no differences in the number of sea stars that

crawl downwards or upwards (51 downwards,

69 upwards; G-value = 2.417, P = 0.120).

Similarly, displacement linearity (L) over

the horizontal surface presented significantly

higher values in the presence of a water cur-

rent. On the other hand, displacement linearity

was similar in the absence of water current and

when crawling on a vertical surface (Table 1,

Table 2). However, there were no significant

differences between going upwards or down-

wards (Table 1, Table 2, Fig. 3). Consequently,

L’s values suggest that sea stars’ crawling

behavior in the water current regime tended to

be directional (> 0.7). In the absence of water

flow and faced to the vertical surface, their

behavior ranged from partially directional (>

0.5) to non-directional (< 0.5) (Fig. 3).

The number of direction changes (DC)

during sea star locomotion differed between

experimental treatments. In the presence of the

current water regime, most individuals present-

ed no differences in crawling direction. How-

ever, in the still water, some sea stars had the

most remarkable direction changes during their

locomotion (4 and 5 turns) (Table 1, Table 2,

Fig. 4). On the other hand, when crawling over

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a vertical surface, sea stars showed the slight-

est direction changes when moving downwards

(Table 1, Table 2, Fig. 4).

Our results showed that there was no effect

of body size (radius and wet weight) on the

locomotion traits (ACS, L, and DC) of A. stel-

lifera (Table 1).

On the other hand, the Rayleigh z-test

showed that the orientation angle distribution

was not significantly different from unifor-

mity in all experimental crawling conditions

(Fig. 5). Moreover, the orientation angles were

not significantly different between treatments

(Watson-Wiliams’s U

-test: water current-still

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

statistics

ACS L DC

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

variables

d.f.

Chi-square

value

P d.f.

Chi-square

value

P d.f.

Chi-square

value

(Wc – Sw – Vs)

2 581.29 < 0.001 2 182.61 < 0.001 2 118.69 < 0.001

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

(Up-Down)

1 75.04 < 0.001 1 0.29 0.589 1 45.85 < 0.001

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

variable

comparisons

ACS L DC

Dif ± S.E. t-value P-value Dif ± S.E. t-value P-value Dif ± S.E. z-value P-value

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.

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water, U

= 0.056, P > 0.1). Therefore, sea stars

did not exhibit a preferential angle of movement.

Sea star bilateral-like behavior: During

crawling behavior trials, all sea stars moved

with two leading arms forward, two on the side,

and one backward. Sea stars used different pair

combinations of leading arms while crawling.

Arm preference during locomotion did not

differ from random in any three-arm classes

analyzed (Table 3).

During righting, sea stars first extended

their arms upwards, attached two adjacent arms

to the aquarium floor for support, and made the

up-righting movement using the opposite arm.

During the position-recovering, arm preference

did not differ significantly from random in

any of the three-arm classes analyzed (Table

3). Therefore, the up-righting and crawling

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).

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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 classes

Arm numbering G-test

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

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experiments show no symmetric behavioral

plane of locomotion in A. stellifera.

DISCUSSION

Sea star crawling behavior: Describ-

ing sea star crawling movements can help to

elucidate different ecological aspects related

to individual foraging behavior, migration, and

relationships among individuals within and

among species. Since these behaviors involve

a complex integration of body symmetry, sen-

sory system, and other body characteristics,

it is not striking that contradicting crawling

actions could be described between and within

echinoderms classes.

Crawling speed values recorded for A.

stellifera are similar to those of other sea star

species with comparable body sizes (40-70

mm) living in temperate waters (Montgomery

& Palmer, 2012).

In general, animals present a positive

relationship between body size and displace-

ment speed. Mainly, echinoderms show dif-

ferent patterns in this relationship. However,

considering that the efficiency of displace-

ment depends on both the number of tube feet

and body weight, it is expected that big sea

stars move faster than the small ones (Ferlin,

1973; Mueller, Bos, Graf, & Gumanao, 2011;

Montgomery & Palmer, 2012; Montgomery,

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

surface containing tube feet) and the sea star’s

radius. (Mueller et al., 2011; Montgomery

& Palmer, 2012; Montgomery, 2014). These

positive direct relationship between the radius

and the ambulacral groove area might be more

significant in bigger sea stars than in small or

medium ones (Montgomery & Palmer, 2012)

such as A. stellifera. Thus, the absence of a

relationship between A. stellifera’s speed and

size needs further investigation. A possible

explanation may arise from the relationship

between its ambulacral groove area and radius.

On the other hand, it is noteworthy that

few studies have analyzed the crawling lin-

earity variation. The available data regarding

the foraging behavior of sea stars describe

changes in that parameter concerning prey

items (Moore & Lepper, 1997; Drolet & Him-

melman, 2004; Thompson, Drolet, & Him-

melman, 2005; Barahona & Navarrete, 2010;

Montgomery & Palmer, 2012). Although lin-

earity is a descriptive dimensionless parameter,

it provides information about crawling behav-

iors concerning trajectory shape or directional

efficiency (Ricci, Barbanera, & Erra, 1998).

In this study, individuals traveled a relatively

straight trajectory (L > 0.7) on a horizontal sur-

face and under the presence of a water current.

We observed no L values differences between

individuals crawling on a flat surface with still

water and a vertical surface, neither between L

values and the sea star size. Thus, by crawling

in a linear trajectory, A. stellifera would not

be walking the same path twice, optimizing

energy, e.g. avoiding sampling twice an area

during foraging.

For many marine organisms, water cur-

rents trigger individuals’ movement (rheotaxis)

or help to detect prey cues (chemotaxis). In the

Fig. 5. Orientation in the presence and absence of water

current. Dots represent the position of a sea star at the end

of the trial. The current water direction is at 0°. Arrows

represent the mean angular vector. The P-values represent

the Rayleigh z-test for angular dispersion (expected points

uniformly distributed on the circle). θ, mean orientation

angle; r, mean vector length; N = 120 sea stars/treatment.

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latter case, it increases the chances of detecting

chemical stimuli (Valentincic, 1983; Rochette

et al., 1994; Mueller et al., 2011). Some stud-

ies reported evidence of chemoreception and

positive rheotaxis for some sea stars species

(McClintock & Lawrence, 1981; Moore &

Lepper, 1997), but not in others (Sloan &

Campbell, 1982; Mueller et al., 2011). Asteri-

na stellifera showed a random orientation

behavior both in the presence and absence of

water current regarding the crawling orienta-

tion angle and linearity on a horizontal surface.

Moreover, the water current caused faster and

straighter individual displacement, but not an

oriented movement with or against the water

current. In other words, A. stellifera did not

show rheotaxis. As stated before, natural selec-

tion can optimize locomotion concerning prey

distribution and abundance over evolutionary

time (Pyke, Pulliam, & Charnov, 1977). In this

sense, a randomly oriented but linear move-

ment may favor animals that belong to highly

dense populations in communities with high

prey density (McClintock & Lawrence, 1981;

Mueller et al., 2011). Asterina stellifera seems

to be well-adapted to moving randomly in a

dense community. This species’ population

reaches densities of up to 72 ind/m

–2

and lives

in a subtidal environment with abundant prey

in the South Atlantic (Farias et al., 2012; Mer-

etta et al., 2014). Future studies on the crawling

behavior of sea stars in the presence of prey

will help to understand the foraging strategy

of this species.

Regarding the inclination of the surface of

locomotion (i.e. bottom structure), our study

showed that it affects this sea star’s crawling

behavior. The crawling speed of A. stellifera

during trials was higher when the individuals

moved downwards on a vertical plane than

going upwards, probably due to the extra

mechanical work associated with the gravi-

tational force. However, there is no relation-

ship between sea star size (R and wet weight)

and its vertical locomotion speed. According

to Domenici, González-Calderón, and Ferrari

(2003), sea stars adhere to the substrate to keep

themselves in a vertical plane for much more

time than they do to move downwards or on a

horizontal bottom. Therefore, the gravitational

force helps sea stars to move downwards faster.

Sea star bilateral behavior: During

crawling behavior trials, all A. stellifera indi-

viduals moved with two leading arms forward,

two on the side, and one backward, as reported

for other sea star species (Reese, 1966; Ji et

al., 2012). Similarly, during righting, sea stars

first extended their arms upwards, attached

two adjacent arms to the aquarium floor for

support, and made the up-righting movement

with the opposite arm, as reported for other

sea star species (Polls & Gonor, 1975). There-

fore, the results obtained in this study are

comparable to those.

Many studies analyze the bilateral behav-

ior of echinoderms, with contradictory results.

Most of them have focused on some regular

echinoids and asteroids. Sea urchins appear to

have no Anterior/Posterior axis of symmetry

when crawling in an open space (Parker, 1936;

Grabowsky, 1994; Yoshimura & Motokawa,

2008, 2010; Yoshimura, Iketani, & Motokawa,

2012). Similarly, Preyer (1887, as cited in

Cole, 1913) conducted some experiments with

ophiuroids (Ophiomyxa sp. and Ophioderma

sp.) and found no preference for any particular

ray during locomotion.

Regarding asteroids, some studies reported

the existence of a behavioral A/P axis or a

tendency to have one during locomotion and

righting in several species (Cole, 1913; Kjer-

schow-Agersborg, 1922; O’Donoghue, 1926;

Polls & Gonor, 1975; Ji et al., 2012; Ardor-

Bellucci & Smith, 2019). However, other sea

stars did not exhibit an A/P plane of sym-

metry during crawling nor righting (Preyer,

1887, as cited in Cole, 1913; Ohshima, 1940;

Montgomery & Palmer, 2012). Furthermore,

it is still unclear why some species use one

combination of arms more frequently than the

others as leading arms (Polls & Gonor, 1975;

Ji et al., 2012; Ardor-Bellucci & Smith, 2019).

And also, why other species do not follow any

pattern (Ohshima, 1940; Polls & Gonor, 1975;

Montgomery & Palmer, 2012; this study). In

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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

toward bilateral behavior may be a species-

specific characteristic rather than a generalized

feature of all echinoderms.

Asterina stellifera is another example of

a species that did not exhibit an A/P plane of

symmetry during crawling nor righting. As is

evidenced in the literature, there is a consider-

able time lag in investigations on this subject.

The methodology associated with experimen-

tation and data analysis has changed. Thus,

these results contribute as an example to the

discussion regarding a functional bilateral axis

in adult asteroids and update the subject.

In all asteroids, the water vascular system

and the nervous system assume a pentaradial

symmetry. Radial distribution of sense organs

allows sea stars to perceive their surrounding

environment continuously from all directions

and move towards or away from a stimulus

(prey or enemy). Understanding the mecha-

nisms by which sea stars detect and respond to

different stimuli will help elucidate ecological

aspects such as prey detection and selection,

avoid predators, and respond to changes in

environmental conditions.

In conclusion, our results show that (1) A.

stellifera presents no rheotaxis, but gravitaxis,

(2) locomotion speed and linearity are indepen-

dent of body size, and (3) this species would

not tend to bilateral behavior.

Ethical statement: authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are

fully and clearly stated in the acknowledge-

ments section. A signed document has been

filed in the journal archives.

ACKNOWLEDGMENTS

We are grateful to Mar del Plata Aquarium

for allowing us to use their facilities for aquari-

ums experimentation with their flow through

seawater system. During the experimentation

period, PEM was supported by a Ph.D. Fellow-

ship funded by CONICET - Consejo Nacio-

nal de Investigaciones Científicas y Técnicas.

CRRV is supported by Research Fellowships

of the Conselho Nacional de Desenvolvim-

ento Científico e Tecnológico (CNPq) and the

Fundação Carlos Chagas de Amparo à Pesquisa

do Estado do Rio de Janeiro (FAPERJ).

RESUMEN

Comportamiento de locomoción y enderezamiento

en estrellas de mar: un caso de estudio sobre

la estrella Asterina stellifera (Asterinidae)

Introducción: El comportamiento de locomoción de

un organismo implica la integración de aspectos como la

simetría corporal, los sistemas sensorial y locomotor. Ade-

más, varios factores ecológicos parecen estar relacionados

con las características de la locomoción, como la estrategia

de alimentación, las tendencias migratorias, la respuesta

a los depredadores y competidores y el estrés ambiental.

Objetivo: Analizar el patrón general de locomoción y

la influencia de la simetría corporal en la locomoción y

enderezamiento de la estrella de mar Asterina stellifera.

Métodos: Realizamos experimentos de laboratorio en

acuarios en presencia / ausencia de corriente de agua y en

superficie horizontal y vertical. Resultados: La velocidad

es similar a la velocidad en otras especies de tamaño

similar. Tanto la velocidad como la linealidad del despla-

zamiento fueron independientes del tamaño corporal indi-

vidual. Una corriente de agua conduce a una velocidad de

desplazamiento mayor y a trayectorias más rectas, pero no

hay reotaxis: una corriente de agua no afecta el patrón de

locomoción. La velocidad y la linealidad del desplazamien-

to fueron independientes del tamaño corporal individual.

El patrón de desplazamiento aquí descrito puede ser una

adaptación de organismos que presentan densas poblacio-

nes en comunidades con alta abundancia de presas, como

es el caso de A. stellifera. Conclusiones: Al igual que otros

asteroides, esta especie no mostró un plano de simetría

Anterior / Posterior durante la locomoción o el movimiento

de enderezamiento: no tiende a la bilateralidad.

Palabras clave: locomoción; orientación; bilateralidad;

Asteroidea; reotaxis; gravitaxis

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