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Seafinding By Hatchling Sea Turtles: Role of Brightness, Silhouette and Beach Slope as Orientation Cues
Abstract and Figures
Upon emerging from underground nests, sea turtle hatchlings immediately crawl toward the ocean. The primary cues used in orientation are visual but the nature of the visual cues was a matter of speculation. Hatchlings might also respond to secondary cues, such as beach slope. Experiments were carried out in an arena where specific visual and slope cues, simulating those present at nest sites, could be precisely controlled and manipulated. Subjects were green turtle (Chelonia mydas L.) and loggerhead (Caretta caretta L.) hatchlings. Both species oriented toward the more intensely illuminated sections of the arena. They also oriented away from dark silhouettes which simulated an elevated horizon, typical of the view toward land. Turtles responded primarily to stimuli (both silhouettes and photic differences) at or near eye level. When presented simultaneously with a silhouette and a photic gradient located in different directions, hatchlings oriented away from the silhouette and ignored photic stimuli. Under infrared light, both species oriented down slopes. However in the presence of nocturnal levels of visible light loggerheads ignored slope cues and responses of green turtles to slope were weakened. The data suggest that loggerhead and green turtle hatchlings usually find the sea by orienting away from elevated silhouettes. This is a prominent and reliable cue for species which typically nest on continental beaches.
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... Light pollution on marine turtle nesting beaches can disrupt natural cues used by hatchling turtles to orient to the ocean for their offshore migration [15]. Hatchlings typically move away from the tall, dark silhouette of dunes and vegetation and move toward the lighter horizon of the ocean [16,17]. However, artificial lights visible from a nesting beach can produce a light-trapping effect and disorient hatchlings, causing them to travel landward or in a circuitous manner; disorientation forces hatchlings to expend energy that is crucial for their offshore migration and increases their risk of predation and dehydration [18,19]. ...
... We intersected results from each viewshed model with turtle nest polygons, created a data matrix of the intersected results, and binarily classified nests as being exposed (1 = yes or 0 = no) to each building's viewshed. We randomly selected buildings at each of five levels (5,10,20,30,40,50) within the matrix to account for observed lighting levels (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) and for the possibility of higher levels . Using R programming language [41], we employed the dplyr package [42] to randomly sample buildings and the Matrix package [43] to summarize the number of nests visible from the randomly selected buildings. ...
Light pollution caused by poorly directed artificial lighting has increased globally in recent years. Artificial lights visible along marine turtle nesting beaches can disrupt natural brightness cues used by hatchling turtles to orient correctly to the ocean for their offshore migrations. Natural barriers, such as tall dunes and dense vegetation, that block coastal and inland lights from the beach may reduce this disruption. However, coastal areas are often managed toward human values, including the trimming of vegetation to improve ocean views. We used viewshed models to determine how reducing the dune vegetation height (specifically that of seagrape, Cocoloba uvifera) might increase the amount of artificial light from upland buildings that reaches a marine turtle nesting beach in Southeast Florida. We incorporated three data sets (LiDAR data, turtle nest locations, and field surveys of artificial lights) into a geographic information system to create viewsheds of lighting from buildings across 21 vegetation profiles. In 2018, when most seagrape patches had been trimmed to <1.1 m tall, female loggerhead turtles nested in areas with potential for high light exposure based on a cumulative viewshed model. Viewshed models using random (iterative simulations) and nonrandom selections of buildings revealed that untrimmed seagrape heights (mean = 3.1 m) and especially taller vegetation profiles effectively reduced potential lighting exposure from three building heights (upper story, midstory, and ground level). Even the tallest modeled vegetation, however, would fail to block lights from the upper stories of some tall buildings. Results from this study can support management decisions regarding the trimming of beach dune vegetation, any associated changes in the visibility of artificial lighting from the nesting areas, and modifications to existing lighting needed to mitigate light exposure.
... After emerging from their nests, hatchlings use visual cues to crawl directly toward the ocean (Irwin and Lohmann, 2003;Mrosovsky and Shettleworth, 1968;Salmon et al., 1992). Doing so makes it likely that they will encounter accumulations of algae on the lower beach. ...
... The algal wrack itself may not only act as a physical barrier, but also may elicit a crawl in the opposite direction, as does a landward silhouette. Hatchling turtles use visual cues to locate the sea by orienting away from elevated, dark silhouettes landward, and toward more open, brighter horizons seaward (Salmon et al., 1992). The movement of hatchlings parallel to the mat enabled at least a few turtles to find breaks in the mat barrier, exposing them to a brighter horizon that directed their crawl seaward. ...
Schiariti, J.P. and Salmon, M., 0000. Impact of sargassum accumulations on loggerhead (Caretta caretta) hatchling recruitment in SE Florida, United States. Journal of Coastal Research, 00(0), 000-000. Coconut Creek (Florida), ISSN 0749-0208. Hatchling loggerhead turtles emerge from subsurface nests on oceanic beaches at night, then crawl toward and enter the sea. Recently, increases in a floating algae (Sargassum) have been reported in the mid-Atlantic and Caribbean, resulting in large accumulation on Florida's beaches. The purpose of this study was to determine if, during the 2020 nesting season, these accumulations acted as a barrier that prevented the hatchlings from crawling to the sea. Seasonal changes in Sargassum density were recorded to determine when, and under what circumstances, hatchlings could cross the accumulated wrack. There was a significant overlap between when Sargassum accumulations peaked and when the turtles emerged, with the result that during the 2020 nesting season the number of hatchlings that entered the ocean was reduced by an estimated 22%. These results suggest that algal accumulations represent a significant threat that could potentially impede the recovery of loggerhead populations that currently are either threatened or endangered, worldwide.
... This miniature 'beach' was set to reduce the stress inherent to manipulating hatchlings and served for acclimation, whereby each test subject could move freely from the terrestrial to the aquatic environment. This beach gently sloped toward the flowing water, with this characteristic known to provide hatchlings with a means of orientating toward the sea (Lohmann and Lohmann, 1996;Salmon et al., 1992). Upstream (i.e. ...
Following their emergence on land, sea turtle hatchlings need to travel through the open ocean. Whether hatchlings can detect ecologically and functionally relevant chemical cues released in the marine habitat is unknown. We collected seawater at 6 and 27 km off the Brazilian coast, i.e. within and beyond the continental shelf. In a two-choice flume, we exposed post-emergent (<24 h old) loggerhead (Caretta caretta) turtles to these seawaters. Based on their life history, we posited that if hatchlings could distinguish between the seawater from these regions, they should prefer the oceanic seawater and/or avoid the coastal seawater. Hatchlings were tested singly and could access any parts of the flume. We recorded the seawater plume first visited and the time spent in each plume. Of all the first choices and time spent in a plume, nearly 70% involved the oceanic seawater. The ability of hatchlings to distinguish between seawaters could provide goal-recognition information.
... Rather, we assume that the forest itself, which determines the elevation of the horizon, served as a cue. Such a mechanism (but with an 'opposite' reaction) is known for hatchling turtles that recognize the direction to the sea based on both illumination levels and a mainly low horizon, as opposed to a high horizon on the side of land vegetation (Godfrey, 1995;Salmon et al., 1992). Similar mechanisms of orientation towards the forest can apparently be used by metamorphosed juvenile ambystomatid salamanders and American toads migrating from their pool (Rothermel, 2004;Rothermel and Semlitsch, 2002). ...
The orientation of naive animals during their first migration is extensively studied in birds and sea turtles, whereas the data for other groups such as amphibians are still scarce. To date, it is unknown whether young-of-the-year anurans perform a random or directional search for the hibernation sites, and what cues (global or local) do they use. We conducted a series of field experiments to study the orientation behavior of juvenile common frogs during their first wintering migration. We captured 1614 froglets from two subpopulations with different directions of migration and assessed their orientation in large circular outdoor arenas (20 m in diameter) on the opposite sides of the river.
Before the migration, froglets used local cues and moved back towards the forest (summer habitat). At the start of migration, the froglets do not move randomly: they navigate towards the river using local cues; later, however, before approaching the hibernation site, they memorize the compass direction of migration and follow it using global cues. Orientation along a memorized compass heading begins to dominate in the hierarchy of orientation mechanisms, and this predominance is maintained even after reaching the hibernation site. Unlike in birds, no innate direction of migration was found.
... Hatchlings generally emerge from nests at night and orient themselves from the nest site to the ocean, ideally as fast as possible . After emerging they generally show a preference of moving towards horizons which are low and bright, and moving away from horizons, which are dark and elevated (e.g., Limpus & Kamrowski, 2013;Lucas et al., 1992;Salmon et al., 1995) and using these cues they can navigate across the beach to the water. Exposure to coastal light pollution disrupts the natural orientation cues and leads to the disorientation and misorientation of hatchlings because lights obscure the natural horizons (Witherington & Bjorndal, 1991a, 1991b. ...
The globally widespread adoption of Artificial Light at Night (ALAN) began in the mid‐20th century. Yet, it is only in the last decade that a renewed research focus has emerged into its impacts on ecological and biological processes in the marine environment that are guided by natural intensities, moon phase, natural light and dark cycles and daily light spectra alterations. The field has diversified rapidly from one restricted to impacts on a handful of vertebrates, to one in which impacts have been quantified across a broad array of marine and coastal habitats and species. Here we review the current understanding of ALAN impacts in diverse marine ecosystems. The review presents the current state of knowledge across key marine and coastal ecosystems (sandy and rocky shores, coral reefs and pelagic) and taxa (birds and sea turtles), introducing how ALAN can mask seabirds and sea turtles navigation, cause changes in animals predation patterns and failure of coral spawning synchronization, as well as inhibition of zooplankton Diel Vertical Migration. Mitigation measures are recommended, however, while strategies for mitigation were easily identified, barriers to implementation are poorly understood. Finally, we point out knowledge gaps that if addressed would aid in the prediction and mitigation of ALAN impacts in the marine realm.
... Turtle hatchlings run toward the ocean because there is more light coming from the ocean and the light from the ocean can strike the hatchlings at a lower angle to the earth (Limpus, 1971;Mrosovsky, 1978;Salmon & Lohmann, 1989;Lucas et al., 1992). ...
The “cashew conundrum” is a seminal event in the history of economics. Professor Richard Thaler observed that his guests were happier not having the option to consume pre-dinner cashews. The fact that people can be happier with fewer options directly contradicts core assumptions in neoclassical economics, and is labeled an “anomaly” by behavioral economics. Far from being surprising, the cashew phenomenon is predicted by biological methods for understanding behavior. The cashew conundrum is not an anomaly, but rather an ordinary.
... That light is provided by celestial sources (stars and the moon) and in most instances it is the seaward view, as the dune and its vegetation in a landward direction absorb light while light is reflected from the water surface in the seaward direction (Lohmann et al., 1997). That difference in radiance between opposing horizons enables hatchlings to locate the ocean even when the uneven surface of the beach precludes a direct view of the sea, at least for a small hatchling (Limpus, 1971;Mrosovsky & Shettleworth, 1968;Salmon et al., 1992). ...
After completing embryonic development, marine turtle hatchlings emerge from their subsurface nest, generally at night, and crawl to the ocean ('sea finding'). That response depends upon the ability of the turtles to discriminate between the brighter seaward versus a dimmer landward horizon, followed by a positive phototaxis. While the crawls of most marine turtle hatchlings are well oriented and straight, those of leatherback hatchlings are sometimes interrupted by bouts of circling. We conducted experiments comparing the orientation and crawling behaviour of leatherbacks to those of loggerhead hatchlings to determine why those differences occur. The two species did not differ in the light spectra attracting the hatchlings, but leatherback thresholds for detection and for intensity discrimination were significantly higher than those of loggerheads. At the nesting beach, loggerheads under full (bright) or new (darker) moon conditions crawled straight to the ocean; circling rarely occurred. Leatherback crawls under a full moon were indistinguishable from those of loggerheads, but during new moon trials, when horizon brightness differences approached leatherback intensity discrimination thresholds, circling increased significantly. We conclude that circling is probably used by leatherbacks to reinforce orientation decisions when horizon cues become more difficult to discern. Circling could be costly as it lengthens the crawl and increases exposure to terrestrial predators. We hypothesize that those costs persist because other visual adaptations affecting sensitivity enhance the ability of leatherbacks to detect prey, mates or favourable habitats in an open ocean environment.
... It seems that hatchlings here favour dispersing in a westward direction, so that their tendency to move towards the different light treatments was not due to light attraction but simply a result of their directional preference. This hierarchy of cues has been reported elsewhere (Able 1991;Salmon et al. 1992;Salmon and Wyneken 1994) and it is possible that the influence of different cues may vary as hatchlings traverse nearshore waters on the way to the relative safety of deep water (Lohmann and Lohmann 1996;Okuyama et al. 2009). However, even if hatchlings were not attracted to, but simply came across the light (as that was a general direction travelled under ambient conditions) it appears that they might have been disoriented by the offshore light source as hatchlings took on average ~20% longer to transit the array when light was present (~13 min under ambient v. 16 min across most light treatments). ...
It is well known that light pollution disrupts the early dispersal of marine turtles. But now, light emitting diodes (LEDs) are replacing traditional lights, however, we know little about how they influence hatchling dispersal. Here, we used acoustic telemetry to assess the early in-water dispersal and predation rates of hatchlings in response to different intensities of LEDs ranging from 10 to 120 W. We found no effect of LEDs on hatchling bearing when lights were in the direction they dispersed under ambient conditions. When LEDs were not in their usual direction of travel, variability in mean bearing increased, and a change in bearing occurred with the highest light intensity. We found weak evidence that predation was also higher at this light intensity compared to ambient, and also in two of the lower light intensities (10 and 30 W), but only on one experimental night. We were unable to find a relationship between hatchling speed and time spent in the tracking area with light intensity. However, reduced sample sizes (due to predation) might have affected our ability to detect effects. Although more effort is required to increase the confidence in our findings, LEDs disrupted hatchling dispersal and are therefore likely to negatively affect their survival.
... Hatchling sea turtles emerge from their nests at night and immediately crawl toward the ocean, aided primarily by visual cues (Daniel and Smith 1947;Mrosovsky 1968;Witherington, Bjorndal, and McCabe 1990). Their sea-finding ability is driven by phototaxis (Celano et al. 2018;Witherington and Bjorndal 1991a), whereby hatchlings crawl toward bright, open horizons and away from tall, dark silhouettes (Ehrenfeld and Carr 1967;Mrosovsky and Shettleworth 1968;Salmon et al. 1992). Loggerhead sea turtles (Caretta caretta) exposed to artificial light exhibit spectrally biased phototactic responses, generally orienting toward shorter wavelengths and away from longer wavelengths, e.g., >560 nm (Mrosovsky and Carr 1967;Mrosovsky and Shettleworth 1968;Witherington and Bjorndal 1991a). ...
Coastal roadways with tall, full-spectrum streetlights along sea turtle nesting beaches present a challenge for managers seeking to balance protection of sea turtles with public safety. Many communities extinguish these lights during nesting season to avoid impacting nesting and hatchling sea turtles. Long-wavelength light emitting diodes (LEDs) offer an alternative for managers in these communities, but additional information on sea turtle response to these lights is warranted prior to installation. We conducted arena assays on Florida’s west coast to evaluate hatchling orientation when exposed to a shielded, long-wavelength (624 nm) prototype lamp compared to an adjacent beach with the streetlights turned off. We compared orientation in test and control arenas simultaneously over two consecutive nights, recording crawl direction and timing for individual hatchlings. Hatchlings in test and control arenas oriented correctly toward the ocean in all trials, with no differences in hatchling dispersion or circling. Thus, the fully shielded, long-wavelength LED streetlight fixture tested provides an appropriate option to minimize impacts to sea turtles along coastal roadways throughout the Unites States and elsewhere. As such, this alternative solution to extinguishing necessary streetlights can aid coastal managers in concurrently protecting nesting habitat and providing light for public safety.
... High temperatures negatively affect the digging. The hatchlings reach the surface at night and crawl towards the brightest horizon (moon, stars, and the reflections in the ocean) (Lucas 1992). Once they enter the water, they enter a period of continuous swimming for up to 1 week . ...
EAZA Best Practise Guidelines for Sea Turtles
The photic orientation of hatchling sea turtles during water finding behaviour was tested in a new type of experimental light field. The azimuthal vector diagram of this light field is sensitive to the detector's angle of acceptance. When this angle is small, for instance 1 degree, the diagram takes the form of a snail-shell. Moreover the major vector and the minor vector do not point in opposite directions as they do in the usual experimental light fields as produced by a lamp. The mean directions of hatchlings of Chelonia mydas and of Lepidochelys olivacea almost coincided with the major vector as measured with a detector with a horizontal angle of acceptance of 180°. The conclusions are that the animals moved in the “brightest direction” of the angular light distribution and not in the direction opposite to the “darkest direction”, and that they obtained information about the brightest direction by processing of photic stimuli impinging through an input cone with an impressively large horizontal angle of acceptance. Suggestions are given with respect to relevant taxis concepts.
A description is given of apparatus for studying orientation in turtles. The device consists essentially of a vertical shaft to which the experimental animal is fastened by its carapace, allowing only rotation of the body around the dorso‐ventral axis. The advantages are: 1. It permits locomotory movements (walking and swimming) to continue fairly undisturbed.2. It allows turtles swimming on the spot to behave quite naturally for days on end with respect to locomotory patterns and reactivity (light field, coast silhouette, food).3. It makes automatic recording of orientation direction and control of the stimulus situation easy to carry out.4. It can be operated in both laboratory and field conditions. For the recording and instant processing of directional data in the field a simple mechanical “computer” requiring no power supply was designed and connected to the vertical shaft.The merits of the apparatus are illustrated by the results of an experiment with hatchling green turtles (Chelonia mydas) under natural stimulus conditions on a nesting beach in Surinam, and by the results of a laboratory experiment with an (artificial) coast silhouette.
1. On a breeding ground for sea turtles in Surinam the horizontal vector diagram of the radiation field was measured. The mean orientation direction of sea‐finding hatchling green turtles (Chelonia mydas) was not related to the largest horizontal vector (brightest direction).2. Hoods were designed to hold attachments which would interfere with vision.3. The seaward orientation was disrupted in turtles with both eyes blindfolded for at least 24 hours. These animals showed a tendency to move down the slope of the beach.4. Turtles with one eye blindfolded for more than 2 hours oriented seawards.5. Sea‐finding orientation in the green turtle cannot be explained solely in terms of some photic (e.g. tropotactic) mechanism which permits progress in the brightest direction. In all probability the animal also orients visually with the help of a “multiple input unit system”; (Schöne, 1975).
The optic mechanisms used by hatchlings of the sea turtle, Chelonia mydas, in sea‐finding orientation, were studied in the laboratory. Optic stimuli were presented to these turtles attached to an apparatus for recording the orientation of animals walking on the spot.To find out whether turtles orient visually with the help of configurational cues such as the coastal silhouette, or photically with respect to a given vector of the angular radiance distribution (major vector ideally in seaward direction) the following simplified stimulus conditions which were considered relevant to the beach situation were devised: 1. anisotropic radiance field with egg‐shaped horizontal vector diagram;2. isotropic radiance field (as measured by a device with a sufficiently large angle of acceptance) inclusive of a configuration representing a coastal silhouette;3. an anisotropic radiance field combined with a configuration;4. a large configuration (uniform horizon) together with a number of smaller configurations (interrupted horizon).The orientation of the turtles confronted with each of the above conditions in the same order was: 1. in the direction of the major vector (the brightest direction);2. away from the configuration;3. away from the configuration;4. away from the smaller configurations.The conclusion is that Chelonia mydas hatchlings primarily orient visually but a photic system may take over under conditions that still have to be specified.
Field experiments with hatchling green sea turtles (Chelonia mydas) show that a positive phototropotaxis will account for many aspects of their ability to find the sea after emerging from the nest. The tropotactic reaction is discussed with respect to retino-tectal projections in reptiles and it is suggested that visual stimulation of one eye may initiate both ipsilateral and contralateral turning tendencies, depending on which part of the retina is stimulated. The influence of the sun's position and the effects of varying the spectral composition of light stimuli are outlined. Some complexities in experiments on the responses to light of frogs (Rana temporaria) and freshwater turtles (Pseudemys scripta elegans) are reported, and it is concluded that for correlating behaviour with physiological mechanisms a more dependable behaviour such as sea-finding in marine turtles offers some advantages.
The seaward orientation behavior of hatchling loggerhead turtles Caretta caretta when exposed to five different artificial light sources (high-pressure) and low-pressure sodium vapor, and yellow, red, and white incandescent lamps) was examined. Each light source affected hatchling sea-finding performance either in direction of orientation or width of dispersion. Hatchlings were attracted to light sources emitting short-wavelength visible light and long-wavelength sources that excluded intermediate wavelengths. A negative response was observed toward sources emitting predominately yellow light. For this reason, low-pressure sodium vapor (LPS) luminaires, which emit only yellow light, are expected to affect loggerhead hatchling sea-finding minimally, if positioned behind the primary dune. LPS luminaires positioned between emerging hatchlings and the ocean, however, will disrupt hatchling orientation.