Homing behaviour by destructive crown-of-thorns starfish is triggered by local availability of coral prey

Abstract

Corallivorous crown-of-thorns starfishes (Acanthaster spp.) can decimate coral assemblages on Indo-Pacific coral reefs during population outbreaks. While initial drivers of population irruptions leading to outbreaks remain largely unknown, subsequent dispersal of outbreaks appears coincident with depletion of coral prey. Here, we used in situ time-lapse photography to characterize movement of the Pacific crown-of-thorns starfish (Acanthaster cf. solaris) in the northern and southern Great Barrier Reef in 2015, during the fourth recorded population outbreak of the starfish, but prior to widespread coral bleaching. Daily tracking of 58 individuals over a total of 1117 h revealed all starfish to move a minimum of 0.52 m, with around half of all tracked starfish showing negligible daily displacement (less than 1 m day^-1), ranging up to a maximum of 19 m day^-1. Movement was primarily nocturnal and daily displacement varied spatially with variation in local availability of Acropora spp., which is the preferred coral prey. Two distinct behavioural modes emerged: (i) homing movement, whereby tracked paths (as tested against a random-walk-model) involved short displacement distances following distinct 'outward' movement to Acropora prey (typically displaying 'feeding scars') and 'homebound' movement to nearby shelter; versus (ii) roaming movement, whereby individuals showed directional movement beyond initial tracking positions without return. Logistic modelling revealed more than half of all tracked starfish demonstrated homing when local abundance (percentage cover) of preferred Acropora coral prey was greater than 33%. Our results reveal facultative homing by Acanthaster with the prey-dependent behavioural switch to roaming forays providing a mechanism explaining localized aggregations and diffusion of these population irruptions as prey is locally depleted.

Keywords: behaviour; coral reefs; movement; predation; random-walk-model; time-lapse photography.

Figure 1.

(a) Map of study locations and sites at Lizard Island and the Swains System on the Great Barrier Reef, Australia; locations were separated by 1100 km, while sites within each location were separated by 2 km and 10 km at Lizard Island and Swains Reef, respectively. (b) Photographs of experimental set-up at the Swains Reefs location: (i) example of time-lapse camera standing approximately 1.5 m high and set squarely above an individual Acanthaster cf. solaris tagged with small pieces of pink flagging tape; (ii) close-up view of pink flagging tape tags, which spanned approx. 350 mm, that were placed gently over dorsal spines of the starfish enabling individuals to be identified and displacement measured the following day. (Online version in colour.)

Figure 2.

(a) Frequency distribution of daily displacement of 58 individually tracked Acanthaster individuals across northern and southern Great Barrier Reef, May/June 2015; the hatched bar for the 0–1 m bin indicates those individuals largely remaining within the camera field-of-view for approximately 20 h of time-lapse monitoring at 20-min intervals. (b) Average speed of Acanthaster across the diel cycle (midnight to midnight) derived from time-lapse photography of tracked individuals for starfish trackable for at least 1 h within time-lapse tracking field-of-view (n = 48). Lightly shaded regions indicate crepuscular periods (dawn and dusk) and dark shading indicates night-time during May/June. Data are averages (±s.e.) of all individual speed estimates occurring within bins of 0.01-day fractions (i.e. every 14.4 min from 00.01 to 24.00 h); the number of individuals in each day fraction bin (i.e. number of observations per bin) is shown as a grey trace on lower panel.

Figure 3.

(a) Boxplot of Ivlev's electivity index for different coral genera averaged across replicate time-lapse camera fields-of-view centred on Acanthaster individuals; y-axis indicates range of values revealing preference to avoidance using this index, non-overlapping bars on x-axis indicate significant groupings at alpha less than 0.001 based on Tukeys HSD. (b) Acanthaster displacement versus live cover of Acropora species; locations are shown as different symbols, see legend.

Figure 4.

Probability of Acanthaster emigrating from field-of-view relative to (a) cover of preferred Acropora prey, and (b) relative to time spent feeding. Inset values represent the cover of live Acropora coral prey representing a ‘lethal dose' of emigration for the population at LD75, LD50, LD25, respectively. The grey band gives the standard error for predictions about the fitted curve. (Online version in colour.)

Figure 5.

(a) Examination of Acanthaster movement relative to predictions of a random walk model; mean net squared-displacement is calculated over a maximum of five paths from predicted (solid line) and observed (closed circles) movement paths; dashed lines are 95% confidence limits for the predicted net squared-displacement based on a random walk model. Numbers in parentheses above the closed circles indicate the number of individuals observed, with most individuals within each step falling below the predicted random-walk line, i.e. with 94%, 100%, 88%, 100% and 100% of individuals falling below the predicted line for respective steps 1 to 5; indicating highly localized movement. (b) Probability of homing behaviour versus per cent cover of live Acropora sp.; homing was determined by visually inspecting time-lapse movies to determine if starfish returned to the same shelter within the field-of-view, plot as per figure 4 but with inclusion of LD95. (Online version in colour.)