In a previous study it was shown that loggerhead ( Caretta caretta ) eggs exposed during embryonic development to a distorted magnetic field produced hatchlings incapable of using magnetic information for orientation or navigation. My study was done to determine if hatchlings could recover from those effects when magnetic distortions were confined to shorter time periods (the first, second, or last third of embryonic development). Under those conditions, I found that orientation still failed and conclude that even a relatively short exposure to field distortions might potentially incapacitate hatchlings that use magnetic cues during offshore migration.

Many marine organisms (invertebrates, fishes) produce large numbers of small offspring (larvae) that differ in appearance and behavior from adults. That outcome occurs because larvae as they grow occupy one or more ontogenetic niches that select for phenotypes that differ from those that promote adult survival. Our purpose in this study was to determine whether similar associations also occur in marine turtles. To find out, we examined relationships between juvenile appearance (body shape) and the ontogenetic niches occupied by 3 species of marine turtles: leatherbacks ( Dermochelys coriacea ) and 2 cheloniids (green turtles, Chelonia mydas ; loggerheads, Caretta caretta ). Our measurements indicate that juvenile body shape differs the least from adult body shape in leatherbacks and the most from adults in the 2 cheloniid species. We present evidence from other studies showing that juvenile leatherbacks occupy ontogenetic niches that resemble adult niches, whereas during their ontogeny, both juvenile green turtles and loggerheads make radical niche shifts as they transition between oceanic and neritic habitats. These results are thus consistent with the hypothesis that many morphological and behavioral characteristics expressed by juvenile marine turtles, like those of other larval forms, are best understood as evolved responses shaped by the characteristics of their ontogenetic niches.

Marine turtles in the open ocean often forage in areas of enhanced productivity, but how they locate those sites and what they do once there is poorly understood. One possibility is that odors from prey might be informative. We explored those possibilities in laboratory experiments using juvenile marine turtles. We analyzed responses to an airborne by-product of predation (dimethyl sulfide or DMS) and to airborne and waterborne odors made from extracts of fresh prey (squid, jellyfish, penaeid shrimp, and algae [ Sargassum spp.]). Observations were made in a seawater-filled arena containing an enclosed air space. Airborne odors were delivered across the water surface and waterborne odors were introduced as extracts into the water. Tethered loggerheads ( Caretta caretta ) and leatherbacks ( Dermochelys coriacea ) failed to swim “upwind” in the presence of any airborne odor, suggesting that these stimuli, alone, do not evoke orientation from a distance. Free-swimming loggerheads and green turtles ( Chelonia mydas ), presented with either an airborne or waterborne squid odor, directed bites at the tank walls, at one another, and (most frequently) at a stationary visual stimulus (a small plastic sphere) anchored under water. DMS did not evoke a comparable response. We conclude that close to productive hot spots, both air- and waterborne prey odors elicit a similar visual search for underwater prey targets. A review of the literature suggests that at a distance, juvenile turtles might use magnetic maps to locate areas that include sites of greater oceanic productivity.

Hatchling marine turtles show a nocturnal rhythm of emergence from the nest. Here, we highlight aspects of the rhythm that are incompletely understood, with special reference to differences in the rhythm's expression among populations as well as its cessation at dawn. We postulate that the rhythm ceases because the hatchlings, as they dig toward the surface, acquire a time sense that suppresses activity if they approach the beach surface too late in the day.

Juvenile green turtles occupy home ranges on shallow reefs that parallel the southeastern coast of Florida. When disturbed, the turtles often flee eastward toward deeper water. We captured turtles at night and recorded their orientation during the next 2 days while they swam tethered inside a large circular outdoor pool and were exposed to the sun. The turtles from 2 reef sites were significantly oriented eastward. After 7–10 days of exposure to a laboratory photocycle advanced by 7 hours, turtles tested in the same pool were significantly oriented westward, the predicted direction if they used the sun for orientation. Westward orientation was unaffected by placing either disc magnets or brass discs of identical mass above the turtles' heads. These results are consistent with the hypothesis that orientation under clear skies is preferentially based upon solar cues. However, control turtles exposed for 7–10 days to a photocycle advanced by 1 hour were also expected to swim generally eastward, but, for unknown reasons, failed to show significant orientation. Our results therefore support the hypothesis that the sun is used as a compass, but additional experiments will be required to provide definitive evidence.

The flatback turtle is the only species of marine turtle that lacks an oceanic phase of development in its early life history. Instead, the turtles grow to maturity in shallow turbid shelf waters of tropical to subtropical Australia. We studied the development of diving behavior in neonate flatbacks to determine whether diving under those ecological conditions resulted in differences from leatherbacks ( Dermochelys coriacea ) and green turtles ( Chelonia mydas ) at the same age when diving in clear, deep oceanic waters. Data were obtained from flatbacks that varied in both age (1–7 weeks) and mass (38–100 g). Each turtle towed a miniature time–depth tag during a single 30-minute trial in shallow (≤ 12 m) turbid shelf waters near Townsville, Queensland, Australia. In total, 192 dives were recorded from 22 turtles from 4 nests. Most dives were short (< 100 seconds) and shallow (< 4 m), but even young turtles could dive to the bottom. The most common flatback dives had V- or W-profiles, whereas, in leatherbacks, most dives were V-profiles, and, in green turtles, the dives were either V- or U-profiles. Routine flatback dives were accomplished by swimming slowly (like leatherbacks), but, when sufficiently motivated, flatbacks could swim faster (> 1 m/s) than green turtles. They could also make repeated deep dives after surfacing only briefly to replenish their oxygen supply. Changes in performance (longer, shallower dives) were correlated with increases in mass but not age. We hypothesize that, as neonates, flatback dives enable the turtles to 1) search efficiently for prey throughout the water column under conditions of limited visibility, 2) minimize surface time so that even in murky water the turtles can return to previously attractive locations, and 3) swim rapidly to evade their predators.

ABSTRACT On many nesting beaches, hatchling marine turtles are exposed to poled street lighting that disrupts their ability to crawl to the sea. Experiments were done to determine how hatchlings responded to street lighting transmitted through 2 filters that excluded the most disruptive wavelengths (those < 530 nm; those < 570 nm). Filtered lighting, however, also attracted the turtles though not as strongly as an unfiltered (high-pressure sodium vapor) lighting. Filtering is therefore of limited utility for light management, especially since other alternatives (such as lowering, shielding, or turning off unnecessary lighting; use of dimmer lights embedded in roadways) are more effective.