Materials & Methods While viewing a fixed-station underwater video from a coral reef in Moorea, French Polynesia we opportunistically observed a hawksbill sea turtle use its limbs while foraging, prompting discussion with experts in the field, and a broader survey for the occurrence and context of this behavior. We documented marine turtle foraging behavior from underwater surveys, web image and video searches (e.g., Google, YouTube, Vimeo, Flickr, Shutterstock), and the published literature. For this study, limb-use for feeding was defined as the intentional use of flippers, paws, tails, or feet to directly aid in the capture, processing, or transport of the animal’s food while in the marine environment. If we did not find limb-use feeding described in the published literature (e.g., searching Google Scholar and Web of Science), we conducted broader internet searches for video and images using the species’ common name or group (“green turtle” or “sea turtle”) combined with feeding terms (e.g., “feeding”, “foraging”, “eating”). Once an initial record was found, we conducted more in-depth searches for that species or group to determine the ecological context of the behaviors. Feeding strategies were grouped into broad behavioral categories and feeding stages based on previous study definitions (Hocking et al., 2017b), Table 1. Behavior Feeding stagea Definition Digging Capture Using one or both flippers or paws to remove benthic sediment in order to access benthic food. Striking Capture Using one or both flippers, or tail, to forcibly hit prey, usually to stun. Tossing Capture Using flipper or tail to project prey into the air, usually used to stun prey. Kerplunking Capture Slapping water surface with tail to cause a startle response in prey to aid in capture. Leveraging Processing Placing one or both flippers against benthic substrate to create tension while pulling food from substrate with mouth. Swiping Processing Moving one flipper against food to create tension while tearing food into smaller pieces with mouth. Holding Processing Using both flippers to keep food in place, either by squeezing flippers or gripping with claws while pulling food apart with mouth. Pounding Processing Using both flippers or paws to hold food while rapidly hitting against another object. Corralling Transport Using one or both flippers to guide loose food in a directed manner toward mouth. Lobtailing Transport Slapping water surface with tail during bubble-net feeding to corral prey together. DOI: 10.7717/peerj.4565/table-1 We defined marine tetrapods similar to previous studies (Kelley & Motani, 2015; Kelley & Pyenson, 2015). We excluded the polar bear (Ursus maritimus) as there is significant genetic admixture with a fully terrestrial species (Miller et al., 2012), and we excluded marine snakes as they lack external limbs. We initially included sea birds, yet limited observations to feeding occurring entirely in the marine environment. Foot-paddling, for example, is observed in a number of Laridae gulls, yet it occurs in terrestrial or mudflat habitats (Tinbergen, 1962) and so was not included. Due the difficulty of observing wild foraging behaviors for many marine tetrapods, the absence of documented limb-use while feeding here does not indicate the behavior does not or cannot occur. In light of this, our intent was to be descriptive, not exhaustive, in comparing the occurrence of these behaviors. Due to the relative rarity of this behavior, we grouped marine tetrapods into taxonomic families for comparisons. We broadly compared evolutionary, morphological, ecological, and behavioral factors to qualitatively determine if the presence of limb-use behaviors followed patterns across marine tetrapod families. Evolutionary relationships and divergence times are from Timetree.org (Hedges, Dudley & Kumar, 2006).

Conclusions The use of limbs to directly aid in foraging, while still relatively rare, is a strategy used by a variety of marine tetrapods. Despite being the oldest extant line of marine tetrapods, this is the first time such a wide range of limb-use has been described in marine turtles. We argue that these limb-use behaviors across marine tetrapods are limited by limb mobility and that the frequent use of forelimbs for other behaviors may promote the development of these feeding strategies. These observations provide additional insight into the diversity and possible evolution of limb-use behaviors.