Date Published: September 6, 2017
Publisher: Public Library of Science
Author(s): Morgan S. Pratchett, Zara-Louise Cowan, Lauren E. Nadler, Ciemon F. Caballes, Andrew S. Hoey, Vanessa Messmer, Cameron S. Fletcher, David A. Westcott, Scott D. Ling, Heather M. Patterson.
The movement capacity of the crown-of-thorns starfishes (Acanthaster spp.) is a primary determinant of both their distribution and impact on coral assemblages. We quantified individual movement rates for the Pacific crown-of-thorns starfish (Acanthaster solaris) ranging in size from 75–480 mm total diameter, across three different substrates (sand, flat consolidated pavement, and coral rubble) on the northern Great Barrier Reef. The mean (±SE) rate of movement for smaller (<150 mm total diameter) A. solaris was 23.99 ± 1.02 cm/ min and 33.41 ± 1.49 cm/ min for individuals >350 mm total diameter. Mean (±SE) rates of movement varied with substrate type, being much higher on sand (36.53 ± 1.31 cm/ min) compared to consolidated pavement (28.04 ± 1.15 cm/ min) and slowest across coral rubble (17.25 ± 0.63 cm/ min). If average rates of movement measured here can be sustained, in combination with strong directionality, displacement distances of adult A. solaris could range from 250–520 m/ day, depending on the prevailing substrate. Sustained movement of A. solaris is, however, likely to be highly constrained by habitat heterogeneity, energetic constraints, resource availability, and diurnal patterns of activity, thereby limiting their capacity to move between reefs or habitats.
The rate at which animals move has a major bearing on their ecology, influencing their exposure to different environments, foraging patterns, and biological interactions [1,2]. Short-term maximum rates of movement influence prey capture and/or predator avoidance, whereas sustained rates of movement affect the animal’s capacity to move between different environments and/or habitat patches . Ultimately, this locomotor capacity influences the fate of individuals, the structure of populations, and the evolution of species . Studies of animal movement (of both rates and patterns of movement) are important to understand temporal dynamics in the distribution of individuals and populations [3,4], and their corresponding interactions and impacts across environmental gradients and habitats.
Tube feet of crown-of-thorns starfish varied noticeably in size (length and diameter) both within and among individuals. The tube feet were shortest near the mouth (14.43 ± 01.09SE mm) and increased in length toward the middle of the arms (17.44 ± 1.360SE mm). Tube feet at the distal portion of the arm were equivalent in length (16.07 ± 1.220SE mm) though noticeably thinner, to tube feet in the central portion of the arm (Fig 1). There were also marked differences in the length of tube feet among individuals ranging from a mean of 1.61 ± 0.20SE mm for a starfish that was 28 mm total diameter, up to 25.44 ± 0.55SE mm for a starfish that was 415 mm total diameter. The number of pairs of tube feet recorded across the 42 A. solaris ranged from a mean of 23.67 ± 0.33SE per arm for a starfish that was just 27 mm total diameter, up to 66.33 ± 0.33SE per arm for a starfish that was 360 mm total diameter. Both the length (GLM: F1,41 = 22743, p < 0.05) and number of tube feet (GLM: F1,41 = 10148, p < 0.05) were positively related to the total diameter of individual starfish (Fig 2). Moreover, the size and abundance of tube feet (independently or in combination) did not account for individual variation in the mean velocity of crown-of-thorns starfish (based on AIC comparisons of alternative linear models) any better than did total diameter. As a consequence, size-based differences in mean and maximum velocity were analysed using total starfish diameter. Crown-of-thorns starfish (A. solaris) placed in the centre of an open and well-lit tank almost invariably and immediately moved toward the outer perimeter of the tank, consistent with an inherent avoidance of open habitats [17,18]. The main exception to this were smaller starfish that were able to seek refuge beneath larger pieces of coral rubble, and did not, therefore, progress to the outer perimeter of the tank. Furthermore, most starfish headed directly towards the black plastic sheet (see also ), highlighting the importance of vision in orientation by crown-of-thorns starfish and their inherent preference for habitat structure [18,19]. Beer et al.  showed that A solaris exhibit functional differentiation of arms during movement, whereby one or more leading arms are bent upwards, while trailing arms are flattened, to maximise the visual field in the direction of movement. In addition to this conspicuous locomotor posture, we observed that some starfish (mainly those with damaged arms) tended to prioritise certain arms, consistently moving in the direction that the dominant and intact arm(s) were facing. Similarly, Rosenberg  also suggested that Acanthaster spp. may have a physiological anterior-posterior axis as demonstrated by the use of lead arms during non-random behavioural orientation. This tendency occurred even if it resulted in them circling around to head towards the black plastic sheet. Accordingly, A. solaris with either damaged arms or strong polarity in the dominance of certain arms may not have detected the black plastic refuge, potentially explaining why some starfish did not move towards the black plastic sheet. Source: http://doi.org/10.1371/journal.pone.0180805