Loggerhead Turtles (<i>Caretta caretta</i>) Use Vision to Forage on Gelatinous Prey in Mid-Water

Identifying characteristics of foraging activity is fundamental to understanding an animals’ lifestyle and foraging ecology. Despite its importance, monitoring the foraging activities of marine animals is difficult because direct observation is rarely possible. In this study, we use an animal-borne imaging system and three-dimensional data logger simultaneously to observe the foraging behaviour of large juvenile and adult sized loggerhead turtles (Caretta caretta) in their natural environment. Video recordings showed that the turtles foraged on gelatinous prey while swimming in mid-water (i.e., defined as epipelagic water column deeper than 1 m in this study). By linking video and 3D data, we found that mid-water foraging events share the common feature of a marked deceleration phase associated with the capture and handling of the sluggish prey. Analysis of high-resolution 3D movements during mid-water foraging events, including presumptive events extracted from 3D data using deceleration in swim speed as a proxy for foraging (detection rate = 0.67), showed that turtles swam straight toward prey in 171 events (i.e., turning point absent) but made a single turn toward the prey an average of 5.7±6.0 m before reaching the prey in 229 events (i.e., turning point present). Foraging events with a turning point tended to occur during the daytime, suggesting that turtles primarily used visual cues to locate prey. In addition, an incident of a turtle encountering a plastic bag while swimming in mid-water was recorded. The fact that the turtle’s movements while approaching the plastic bag were analogous to those of a true foraging event, having a turning point and deceleration phase, also support the use of vision in mid-water foraging. Our study shows that integrated video and high-resolution 3D data analysis provides unique opportunities to understand foraging behaviours in the context of the sensory ecology involved in prey location.明确觅食活动的特征，是解析动物生活模式与觅食生态学（foraging ecology）的核心前提。尽管该研究方向意义重大，但海洋动物的觅食活动监测却颇具挑战，因为直接观测几乎难以实现。本研究同时搭载动物搭载式成像系统（animal-borne imaging system）与三维数据记录仪（three-dimensional data logger），在自然环境中观测大型幼体及成体蠵龟（Caretta caretta）的觅食行为。录像记录显示，蠵龟在中层水域（mid-water，本研究定义为深度超过1米的上层海水柱（epipelagic water column））游动时，会以胶质猎物为食。通过关联录像与三维数据，研究团队发现中层水域觅食事件均具备一个显著的减速阶段，该阶段与捕捉、处理行动迟缓的猎物密切相关。对中层水域觅食事件的高分辨率三维运动轨迹进行分析后发现：在171起事件中，蠵龟会径直朝向猎物游动（无转向点（turning point））；在229起事件中，蠵龟会在抵达猎物前平均5.7±6.0米处单次转向朝向猎物（存在转向点）。上述分析包含以游泳速度减速作为觅食代理指标、从三维数据中提取的推定觅食事件（检测准确率为0.67）。存在转向点的觅食事件多发生于日间，这表明蠵龟主要依靠视觉线索（visual cues）定位猎物。此外，研究还记录到一起蠵龟在中层水域游动时偶遇塑料袋的事件。该蠵龟接近塑料袋时的运动模式与真实觅食事件高度相似，均存在转向点与减速阶段，这进一步佐证了蠵龟在中层水域觅食时依赖视觉。本研究表明，将录像与高分辨率三维数据分析相结合，为从猎物定位相关的感官生态学（sensory ecology）视角解析觅食行为提供了独特的研究路径。