Sun J et al. (2022), Macroalgae and interspecific alarm cues regulat...

ECB-ART-50440

Sci Rep 2022 Mar 10;121:3971. doi: 10.1038/s41598-022-07889-8.

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Macroalgae and interspecific alarm cues regulate behavioral interactions between sea urchins and sea cucumbers.

Sun J , Yu Y , Zhao Z , Tian R , Li X , Chang Y , Zhao C .

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Sea urchins and sea cucumbers are mutually beneficial organisms in kelp ecosystem. As herbivores, sea urchins process kelp through feeding and egestion, providing inaccessible food for benthic consumers such as sea cucumbers. Sea urchins in turn profit from the sediment cleaned by sea cucumbers. However, behavioral interactions between them remain poorly understood, which greatly hampers our understanding on the relationship between ecologically important benthic species in marine ecosystems and the regulating mechanism. The present study investigated behavioral interactions between sea urchins Strongylocentrotus intermedius and sea cucumbers Apostichopus japonicus in laboratory conditions. We revealed that the presence of sea urchins caused significant higher speed movement of A. japonicus. Interestingly, the negative effects of S. intermedius on A. japonicus were significantly reduced in the shared macroalgal area. For the first time, we found the interspecific responses to alarm cues between sea cucumbers and sea urchins. Conspecific responses were significantly larger than the interspecific responses in both sea urchins and sea cucumbers. This indicates that interspecific response to alarm cues is an efficient approach to anti-predation and coexistence in mutually beneficial organisms. The present study shed light on the interspecific relationships and coexistence between sea urchins and sea cucumbers in kelp ecosystem.

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	Figure 1. Schematic diagrams for the experiments. (a) Strongylocentrotus intermedius and (b) Apostichopus japonicus. In experiment 1, either 20 sea urchins or 20 sea cucumbers in different tanks were recorded as the control group (c). Twenty sea urchins and 20 sea cucumbers in a tank were recorded as group E1 (d). In experiment 2, 20 sea urchins and 20 sea cucumbers in a tank with macroalgae Ulva lactuca were recorded as group E2 (e). Two injured S. intermedius and two injured A. japonicus were used as the source of alarm cues (f). In experiment 3, the behavioral responses to conspecific alarm cues of sea urchins and sea cucumbers were recorded as group E3 (g). The behavioral response to interspecific alarm cues of sea urchins and sea cucumbers were recorded as group E4 (h).
	Figure 2. Behavioral interactions exist between sea urchins and sea cucumbers. (a) Average centrifugal distance and (b) movement speed (mean ± SEM) of sea urchins in the control group and group E1. (c) Average centrifugal distance and (d) movement speed (mean ± SEM) of sea cucumbers in control group and group E1. (e) The initial positions (small hollow point) and terminal positions (large hollow point) of sea cucumbers (light red) and sea urchins (light blue). Tracking of the center position (line through solid points) of the sea cucumber group (bright red) and the sea urchin group (bright blue) in all the three trials.
	Figure 3. Macroalgae regulate the interactions between sea urchins and sea cucumbers. (a) Average centrifugal distance and (b) movement speed (mean ± SEM) of sea urchins in groups E1 and E2. (c) Average centrifugal distance and (d) movement speed (mean ± SEM) of sea cucumbers in groups E1 and E2. (e) The position of sea urchins (blue points) and sea cucumbers (red points) in groups E1 and E2.
	Figure 4. Sea urchins and sea cucumbers respond to the conspecific and interspecific alarm cues. (a) Average centrifugal distance and (b) movement speed (mean ± SEM) of sea urchins in the control group and group E3 and E4. (c) Average centrifugal distance and (d) movement speed (mean ± SEM) of sea cucumbers in the control group and groups E3 and E4.

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