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

Abstract

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.

Movement patterns of marine turtles in the Mediterranean Sea: a review

“…This pattern too is in general accordance with recent findings obtained elsewhere showing how loggerhead spatial behaviour is actually characterised by a considerable complexity (Hatase et al 2002;Hawkes et al 2006Hawkes et al , 2007Rees et al 2010). This variability is likely linked to the loggerhead's plastic and opportunistic foraging habits (McClellan et al 2010;Reich et al 2010;Narazaki et al 2013), which allow them to exploit both neritic and oceanic foraging grounds.…”

“…Loggerhead turtles are carnivorous: for the first period of life they mainly feed upon pelagic prey at the sea surface, then they increasingly shift towards benthic prey (Bjorndal 1997), although they also feed through the whole water column (e.g. Narazaki et al 2013). The duration of the first strictly epipelagic period varies according to population and oceanographic features, being most extended in those turtles that disperse over wide oceanic regions (Musick & Limpus 1997).…”

Visual and behavioral evidence indicates active hunting by sperm whales

“…Davis et al 1999, Ponganis et al 2000, Watanabe et al 2004, Watanuki et al 2008, Kokubun et al 2013, Naito et al 2013, Narazaki et al 2013, Watanabe & Takahashi 2013, Nakamura & Sato 2014, Heaslip et al 2014, our systems did not allow us to observe direct feeding events of sperm whales, owing to 3 possible factors.…”

Application of a computer vision technique to animal-borne video data: extraction of head movement to understand sea turtles’ visual assessment of surroundings

“…This assumption is supported by the fact that green turtles have relatively clear horizontal vision among sea turtle species [21]. From the perspective of behavioral studies, vision is presumably of great importance for sea turtles when feeding, avoiding predators, and finding mates [8,22,23], although they also use hearing and chemoreception [24].…”