Yu Y et al. (2022), High fitness areas drive the aggregation of the...

ECB-ART-50526

PeerJ 2022 Jan 01;10:e12820. doi: 10.7717/peerj.12820.

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High fitness areas drive the aggregation of the sea urchin Mesocentrotus nudus.

Yu Y , Sun J , Chang Y , Zhao C .

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Sea urchin aggregation is a common phenomenon in coastlines. However, it remains controversial whether sea urchins form resource aggregations or behavioral aggregations in a non-spawning season. To clarify, we studied the aggregative responses to food and predators in the sea urchin Mesocentrotus nudus when high fitness areas (HFAs) were scarce versus sufficient. By taking the occupied area of each sea urchin (test diameter + spines = 4.5 cm) as a square (4.5 cm × 4.5 cm), we set scarce HFAs for the sea urchins in Experiment 1 (the squares of HFAs: the area occupied by experimental sea urchins = 1:1) and sufficient HFAs for the sea urchins in Experiment 2 (the squares of HFAs: the area occupied by experimental sea urchins = 2:1). If M. nudus form resource aggregations, they would aggregate passively under the scarce HFAs conditions, but not in the sufficient HFAs conditions. Conversely, if M. nudus form behavioral aggregation, aggregation would occur in both scarce and sufficient HFAs. The present results showed that in the scarce HFAs, M. nudus in the food and predator groups were significantly closer to the food and further from predators, and had significantly more aggregated numbers in HFAs than those in the control group. Sea urchins did not aggregate in response to food or predators under the sufficient HFAs, although significantly more sea urchins of the experimental group was found in HFAs than that of the control group. Sea urchins (at least M. nudus) form resource aggregations that are driven by the scarce HFAs. This provides valuable information into the mechanisms of the aggregation of sea urchins.

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References [+] :

Harari, Mechanism of aggregation behavior inMaladera matrida Argaman (Coleoptera: Scarabaeidae). 1994, Pubmed

	Figure 1. Diagram of the experimental set-ups.The blue squares were the higher fitness areas (HFAs) and the green contained kelp. The upper part is the experimental set-up of the food and predator groups (A, C, E, G), the lower part is the experimental set-up for the control groups (B, D, F, H). The nets excluded sea urchins from the white areas in D and H.
	Figure 2. The number of aggregated sea urchins, the number of squares between sea urchins and nets, the number of sea urchins in higher fitness areas (HFAs) in the food (A, B, C) and the predator (D, E, F) tests in Experiment 1 (N = 8, mean ± SD).Significant differences are marked as , , ** for P < 0.05, P < 0.01, P < 0.001, respectively.
	Figure 3. The number of aggregated sea urchins, the number of squares between sea urchins and nets, the number of sea urchins in higher fitness areas (HFAs) in the food (A, B, C) and the predator (D, E, F) tests in Experiment 2 (N = 8, mean ± SD).Significant differences are marked as , * for P < 0.05, P < 0.01, respectively.