"Red Light, Green Light: How Lighting Can Improve Hatchling Survival"
Sea turtle hatchlings find the ocean after emerging from their nest by moving towards lights near the horizon, and light color has been shown to elicit different responses, with red light attracting turtles less than blue light. However, these studies have only been conducted on a select group of sea turtle species (primarily greens and loggerheads). Hawksbill sea turtles (Eretmochelys imbricata) have not been tested for any such preferences and, considering that hawksbills nest under vegetation where it is presumably darker, one would expect their preferences and sensitivity to light to be different from other species. This has ramifications for implementing turtle-safe lighting, since what is safe lighting for greens and loggerheads may not be safe lighting for hawksbills. My project has two aims: 1) to determine the threshold of detection (light intensity) that elicits a phototactic response from hawksbill hatchlings across the visual spectrum, and 2) to determine if hawksbills show the same preference for shorter wavelengths (blue) of light compared to longer wavelengths (red) of light.
The procedure for both of these aims is similar: a hatchling hawksbill is placed into a Y-maze, and after a period of acclimation, allowed to orient within the maze. For the first aim, the hatchlings are presented with a light at the end of one arm and darkness at the other end. If the hatchling can detect the light it should move towards it. We lower the intensity in subsequent trials in a step-wise manner until the hatchlings are no longer moving towards that light. The lowest value that a hatchling moves toward is its detection threshold for that color of light. We then repeat this process for multiple colors across the spectrum. For the second aim, we present the hatchlings with two different colors of light at these threshold values, to determine a preference based on wavelength. We will also present hatchlings with red-shifted light at double the threshold value to see if relative intensity is the driving factor in orientation, rather than color.
The greatest benefit of this research is that it can be used to inform sea turtle-safe lighting practices for hawksbill nesting beaches.
Sea turtle hatchlings become disoriented by human lighting. Hatchling turtles find the ocean by looking for and moving towards the light of the moon and stars reflecting off the water (Daniel & Smith 1947), but lighting from homes and cities contributes to misorienting turtles, as artificial lights outshine the moon and stars (Salmon 2003). An inexpensive approach that can increase the survivorship of hatchlings is to adopt “turtle-safe” lighting (Witherington & Martin 2000). However, I hypothesize that what “turtle-safe light” is may depend on the species.
Previous research has found that green and loggerhead turtles orient towards blue light over longer wavelengths like red (Mrosovsky & Carr 1967). It has also been shown that these two species are able to detect light as low as 106 photon flux (Celano et al. 2018), the equivalent of starlight. Yet, research on visual acuity is lacking in other sea turtle species. Hawksbills (Eretmochelys imbricata) have different nesting patterns and prefer to nest in dense vegetation (Horrocks & Scott 1991; Kamel & Mrosovsky 2005), suggesting the need for more powerful visual acuity in hawksbill hatchlings than other species.
As part of my dissertation on vision and visual anatomy of hawksbill turtles, I have been studying the color preferences and acuity of hawksbill turtle vision. By utilizing new technology and greater precision for wavelength selection than in previous studies, I aim to determine the visual sensitivity of hawksbill turtle hatchlings. I hypothesize that, because of their unique nesting strategy, hawksbill hatchlings will be sensitive to light at lower thresholds than loggerhead and green sea turtles, but that they will still be attracted to shorter wavelength light (such as blue) over longer wavelength light (such as red). My recent preliminary data from Jamaica show that hawksbill hatchlings are more sensitive to orange light than green turtles, and more sensitive to green light than both greens and loggerheads. Thus, they are attracted to these colors at lower brightness. Understanding how hatchlings respond to different light colors and intensities is providing information for developing conservation strategies that mitigate the impacts of coastal development and anthropogenic light on hatchling survival.
The goal of my research is to determine the threshold of detection for a wide range of colors (wavelengths) and then use that information to test for color preference. I plan to work with local communities to implement better beachfront lighting practices across Jamaica to decrease hawksbill hatchling misorientation.
Study Site and Animal Care – I will experimentally test the behavioral responses of hawksbill hatchlings to light within a makeshift laboratory in Jamaica. With the help of local community members, I locate hawksbill nests during nightly beach patrols. Nests are marked with flags and monitored over the course of development. During nightly observations (1900-2300 hr), I collect newly-emerged hatchlings in an opaque cooler filled with moist sand for transport to the nearby makeshift laboratory. Hatchlings are maintained in darkness under ambient climatic conditions until a subset of hatchlings is tested. Thirty hatchlings are randomly selected and tested once before being placed in a separate opaque cooler. All hatchlings are released by 0300 hrs the same night and at the original collection beach.
Experimental Apparatus – Hatchlings are tested in a Y-maze with regulated light source at the end of each arm (Figure 1). Total length of the 10.2 cm diameter PVC Y-maze is 1 m, with each arm being 0.5 m in length. A GoPro Hero4 camera suspended above the chamber captures video of hatchling movements. For trials to be visible from above during recording, the 2 upper half of the PVC pipe lengthwise is removed. The bottom of the testing chamber is filled with 2 cm of natural, dry sand collected from the same nesting beach where turtles hatched, creating a flat, natural surface for hatchlings to crawl. A lightemitting diode (LED) light source is placed at the end of both arms. The light sources consist of six colored surfacemounted-device (SMD) LEDs in parallel on a circuit board. The LED color can be changed by switching circuit boards, and light intensity can be varied using a series of neutral density filters. In the detection threshold experiment, I provide a ‘no stimulus’ control. In the color choice experiment, there is a light source at the end of both arms of the maze. The entire apparatus is situated on level ground to ensure gravitropism is not a confounding variable. We have this apparatus housed in a dark room (ambient light reading of 0 photon flux) to minimize outside lighting effects, and on a rotatable table base to control for electromagnetic biases by randomizing the orientation of the apparatus.
Procedure – All hatchlings are tested for one treatment each night. Colored light at 385, 415, 470, 535, 555, 601, and 660 nm wavelengths are generated for the experiment. Hatchlings were tested using the up-down staircase statistical method, using 1.0 log steps down and 0.3 and 0.7 log steps up to determine detection threshold. A one-tailed binomial test is used to determine if a significant number of hatchlings are attracted to a specific intensity of light. If a nonsignificant number of turtles orient toward the light source, the turtles are orienting randomly and the detection threshold for that wavelength is determined. As another form of handedness control, hatchlings are presented with the choice of two identical white light sources, generated at both ends of the maze. In my comparison study, hatchlings orient based on two light sources, instead of a single light source and darkness. Hatchlings are presented with pairwise comparisons of light at the same wavelengths at their threshold values, as determined by the first study. The light hatchlings orient towards is considered the one turtles are most sensitive to.