Effects of bait and stocking density on the behavior of Tripneustes gratilla

Sea urchins are keystone herbivores in coastal ecosystems, yet their high-density aggregations can trigger overgrazing and habitat degradation. The formation of grazing fronts is hypothesized to result from interactions between conspecific density and resource availability, but the behavioral mechanisms modulating individual movement—the fundamental unit of aggregation—remain poorly understood for many ecologically important species. This study aimed to dissect the independent and synergistic effects of population density and bait presence on the movement behavior of the collector sea urchin (Tripneustes gratilla). We tested whether both factors suppress movement intensity, whether their combined effect is synergistic, and whether non-contact signals, rather than physical interference, mediate this regulation. A two-factor laboratory experiment was conducted. Sea urchins test diameter: (9.36 ± 0.73) cm were exposed to three density levels (low: 0.5 ind./m²; medium: 4 ind./m²; high: 8 ind./m²) with or without a centralized bait patch in a 2 m² arena. Movement was recorded via overhead time-lapse photography (5 s intervals, 15 min trials). Trajectories were analyzed using ImageJ to quantify movement speed, net displacement, displacement distance away from the center, linearity index, and mean resultant length R. Two-way ANOVA, Tukey’s HSD, and mixed-effects models were used for statistical analysis. Both increasing density and bait presence significantly suppressed movement intensity. Speed and net displacement declined markedly from low to medium density speed: (1.47 ± 0.49) mm/s to (0.71 ± 0.20) mm/s, with no further reduction at high density (0.67 ± 0.24) mm/s, indicating a saturation effect. Bait presence reduced speed across all densities e.g., low density: (1.47 ± 0.49) mm/s vs. (0.60 ± 0.26) mm/s; medium density: (0.71 ± 0.20) mm/s vs. (0.34 ± 0.12) mm/s. The strongest suppression occurred under combined high density and bait presence, where movement nearly ceased speed: (0.20 ± 0.06) mm/s; net displacement: (10.41 ± 4.92) cm. A significant density × bait interaction (F ₂,₃₀ = 7.999, P = 0.002) confirmed synergistic effects on speed. Neither factor altered movement directionality (linearity index, mean result length R). Brief physical contact caused instantaneous speed reductions, but long-term suppression was uncorrelated with contact frequency or duration, strongly implicating non-contact chemical cues as the primary regulatory mechanism. The movement behavior of T. gratilla is synergistically modulated by population density and bait cues. This response manifests as density-dependent and food-induced suppression of movement intensity, without changes in path straightness, and is governed predominantly by non-contact signaling. This study provides mechanistic evidence linking individual behavioral plasticity to population-level grazing fronts. The finding that sea urchins drastically reduce mobility and concentrate activity near food under crowded conditions directly links individual decisions to ecosystem-level impacts. These insights inform predictive modeling and support behavior-based strategies for managing sea urchin populations to mitigate overgrazing and aid in the restoration of vulnerable macroalgal and seagrass ecosystems.