Locomotion and Righting Behavior of Sea Stars: a Study Case on the Bat Star Asterina Stellifera (Asterinidae)
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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 DOI 10.15517/rbt.v69iSuppl.1.46392 Locomotion and righting behavior of sea stars: a study case on the bat star Asterina stellifera (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 animals, 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 animal 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.