Coral eating crown-of-thorns seastars (Acanthaster
) are abundant inhabitants of many coral reefs. Their specialized diet and frequent occurrence in high-density populations have caused extensive damage to coral reefs in the Indo-Pacific region for several decades [1
]. During population outbreaks thousands of adult individuals may appear on a reef and move across it in feeding-bands [2
]. Over time, local coral resources get depleted and the moving seastars reach the end of a reef or face a channel. In these situations A.
may leave its preferred reef substrate and move on the bare sandy bottom in the search of reef areas with sufficient coral cover [3
]. Although the spatial scales and individual numbers of migrations are not yet known, migrating adult A.
may subsequently contribute to the spread of outbreak populations or cause other reefs to be infected [7
]. Therefore, it is of considerable interest what triggers the onset of their movement and what cues might be used by A.
to direct their movement and orientate themselves between reefs, on sandy substrate bare of suitable food.
Several variables have been proposed to influence the movements of A.
: density of corals, exposure to wave action, temperature, time of day, light, and type of substratum, but also age or size, condition, or nutritional state [9
]. Most of these variables, however, only influence the speed of seastar movement or cause the seastar to actively avoid unpleasant areas. The direction of movement, however, may be guided by the presence of visual cues [12
], chemical cues (from food [15
], predators [16
], or conspecifics [17
]), or water currents [18
]. The visual sense of A.
was shown to be involved in navigation on sand towards reef structures from a distance of several meters [13
]. To successfully navigate between reefs, however, the reef structures have to be in sight, limiting the use of visual cues for long distance orientation on large, bare, sandy areas [14
]. It is commonly assumed that A.
navigate using their well-developed chemical sense [19
]. However, chemoreception in seastars has only proven its functionality over short distances [20
]. If chemoreception is used from a distance, then chemical cues need to be carried towards the seastar by water currents. The seastars may then follow a gradient of prey odors towards their source [21
]. For A.
navigating towards its coral food on the reef using chemoreception may be difficult, as the cues from corals are omnipresent and the flow patterns may be intricate and turbulent [14
]. However, localization of prey using chemical cues is most effective in subtidal environments and where currents flow in one direction [20
], and this may be the case in inter-reef areas. Seastars are also known to combine chemotaxis with their ability to perceive the mechanical stimulation by currents (rheotaxis). In the presence of chemical cues from food they may show positive rheotaxis (moving against the current) [22
] or cross-current movement [18
]. Stronger currents may thereby enhance the capability of seastars to locate their prey [18
]. In this context, the responsiveness to chemical cues from food is frequently increased by starvation [25
] and also results in a more consistent movement behavior [18
]. This may help A.
to overcome even larger distances between reefs where no food is available.
We aimed to investigate the potential of adult A. cf. solaris to navigate between reefs. In particular, we tried to identify cues, which may be used for orientation and factors, which trigger the onset of movement. We therefore analyzed the movement patterns of starved, fed and blinded A. cf. solaris in the field on sandy substrates under different strengths of water currents. Depending on the senses (vision, chemoreception, or mechanoreception) A. cf. solaris might use for orientation, we expected to see differences in movement patterns between treatments, such as an orientation of the seastars towards the reef or along water currents. In particular, we expected starved seastars to show the most directional movement and an orientation towards the reef (food source), while fed seastars may be less directed in their movement, but still being oriented towards the reef (seeking food and shelter). Blinded seastars should either not differ in their movement patterns from starved or fed, if the chemical sense is primarily used for orientation between reefs, or should show different movement patterns, if the visual sense is used for orientation.
A trigger for A.
to start its movements between reefs may be its state of nourishment. Our experiment showed that starved individuals left a safe shelter almost immediately after they were placed there, indicating that a poor nutritional state induces migration towards a new food patch. In contrast, fed individuals stayed there, sometimes for days. Blinded individuals were also in a good nutritional state, but started as early as starved individuals, a behavior that contradicts the hypothesis that nourishment induces movement. However, blinded individuals were obviously not behaving like starved or fed individuals. Some of them ended up very close to the shore (in Temae) and when they were recaptured they were often moving on sand, while others had found a small reef block to hide or feed. This supports the finding that vision is playing an important role in the orientation of A.
]. At the same time, the lack of visual information may have caused their early departure, as they may not have recognized being in a safe location, in the shade of a shelter.
The analysis of the movement patterns after the seastars had left the shelter indicate that there is a relationship between the strength of the water current and the direction of A.
movements on sand. Surprisingly, in strong current A.
followed its direction and showed negative rheotaxis independent of their nutritional state or their ability to see, even after several days of observation. This behavior is uncommon as often seastars show positive rheotaxis [22
], even in the absence of chemical cues in the water [24
]. Castilla and Crisp [34
] found a reversal of the normally observed positive rheotaxis in Asterias rubens
in the laboratory to be caused by the following factors: a sudden increase of the sea water temperature, a reduction of the sea water salinity below 25‰ S, a drop in the oxygen tension below 4.18 mL O2
/L, a pH of less than 6.9, and long periods of captivity under starvation. Although water parameters were not recorded in the study area, such dramatic changes are highly unlikely to have occurred during the experiments, especially given that no storms, heavy rains or swells took place. In addition, strong surge, which is suggested to alter A.
], can be excluded as an influencing factor, as experiments were only performed during very calm seas. Seastars may show negative rheotaxis in the presence of a cue that indicates potential predators or harmful conditions [34
]. However, this is also highly unlikely to have caused the observed movement patterns as such reactions have only been shown as a response to an immediate predator cue present in the water [17
] and, therefore, the immediate presence of a predator, which was not observed during the time of the experiment.
A factor that indeed might have caused the negative rheotaxis observed in the present study was starvation. However, blinded individuals also showed negative rheotaxis in strong currents, which is not directly attributable to their nutritional state. Additionally, instead of showing negative rheotaxis, other seastars even enhance their positive responsiveness to cues when starved [18
]. It might be assumed that the sand did not provide enough gripping surface to the tube feet and the seastars might, therefore, have been passively transported by the current rather than actively moving with it [3
]. However, the observations made during the experiments show that the seastars moved actively using their tube feet and could still change their direction of movement to the side and that several individuals could also move against the current. One remaining explanation for the observed downstream movement is that the seastars actively move with the current if it is too strong to save energy. The fact that A.
showed movement in random directions and no general downstream movement in weaker currents support this hypothesis. In even weaker currents (mean velocity: 0.054 m·s−1
) and on shorter timescales, also, no influence of currents on movement direction in A.
was observed [32
]. The only other field study that investigated movement patterns of tagged A.
was conducted on a solid reef structure, not like the present one on sandy substrate. Movement patterns here were random, as in the present study during weak currents, but no clear measurements of current strengths were made stating only that currents moved strongly in ‘both directions parallel to the shore’ [5
]. Such inconsistent movement directions as a response to a constant cue were also often found in chemoreception-studies of other seastars (reviewed by Sloan and Campbell [23
]). Still, in the present study, random movement directions were consistently shown over all treatments during weak currents. This suggests that either no cue for orientation could be detected by A.
or the cues were detected but the seastars did not react to them.
The probability that chemical cues from coral food were present in the water was very high, as in both experimental sites the water carried towards the seastars first had to pass the reef crest. Although, the water was not tested for chemical traces of corals, the question is why A.
was not moving towards the reef in the vicinity at all, considering that especially starved individuals should try to reach feeding grounds. This challenges the ability of A.
to successfully navigate between reefs and maybe even the functionality of chemoreception for long distance navigation. At the same time, the fact that blinded seastars followed the same inconsistent movement in weak currents as fed and starved ones implies that visual cues, which have been proven to be important in short-distance navigation [14
], were also not responsible for the observed movement patterns at long distances. The observation that some of the individuals in Maharepa had started moving into the deep water of the channel south to the release point, and that others in Temae had moved into shallower water in the direction of the beach, does rather imply that movement on larger scales between reefs may be random. Additionally, directed navigation or orientation seems to be limited to the very presence of a distinct chemical or visual cue only a few meters away. Still, in both experiments, movement of single individuals was generally highly directional, which may prevent re-encountering already traversed areas [33
]. Such a high directionality of movement has already been shown for A.
, although in shorter time scales and on artificial substrate [32
]. For movements between reefs this may be beneficial, as the distance covered is maximized, however, it still needs to move in the right direction to find a reef. A directed movement towards the reef was only shown in one individual from each treatment in Maharepa, which underpins that A.
is not efficient in finding reefs from a distance.