The Development of Early Diving Behavior by Juvenile Flatback Sea Turtles (Natator Depressus)
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Chelonian Conservation and Biology, 2010, 9(1): 8–17 g 2010 Chelonian Research Foundation The Development of Early Diving Behavior by Juvenile Flatback Sea Turtles (Natator depressus) 1 2 1 MICHAEL SALMON ,MARK HAMANN , AND JEANETTE WYNEKEN 1Department of Biological Sciences, Florida Atlantic University, Boca Raton, Florida 33426 USA [[email protected]; [email protected]]; 2School of Earth and Environmental Sciences, James Cook University, Townsville 4811, Queensland, Australia [[email protected]] ABSTRACT. – 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. KEY WORDS. – Reptilia; Testudines; Cheloniidae; Natator depressus; dive profiles; behavioral development; swimming; hatchling Marine turtles are diving specialists that as larger phase of their development. This deficiency occurs juveniles and adults can submerge to depths and for because small turtles disperse over vast and largely durations that rival many marine mammals. Those diving unknown oceanic areas and, therefore, are unavailable for capabilities are the result of physiological (Kooyman study (Musick and Limpus 1997). Western Atlantic 1989; Lutcavage and Lutz 1997), morphological (Wyne- loggerheads (Caretta caretta) are the exception because ken 1997), and behavioral (Hays 2008) adaptations that for the first few weeks after they enter the sea, they can be together enable rapid gas exchange during typically brief found at ‘‘downwelling’’ sites adjacent to coastlines (Carr surface breathing episodes, enhanced mechanisms for 1987). Weeks to months later, they are transported oxygen storage and release, a streamlined body shape, and eastward by oceanic surface currents to ‘‘nursery’’ sites efficient propulsion (predominantly lift-based, ‘‘aquatic near the Azores, Canary Islands, and Madiera (Bolten flight’’; Wyneken 1997; Renous et al. 2008) during the 2003). But as small turtles, even surface-dwelling dive. Previous studies focused particularly on the dive loggerheads are difficult to locate and challenging to profiles of adult turtles, especially during long-distance observe while they reside in weed lines because they are migration between foraging sites and internesting behav- camouflaged and rarely move. This behavior probably ior near nesting beaches (Hays et al. 2006; James et al. promotes their survival as ‘‘float and wait’’ predators 2006; Rice and Balazs 2008) and by juvenile turtles (Witherington 2002) and also reduces their vulnerability foraging within circumscribed home ranges (van Dam and to predators. At this stage of development, diving in open Diez 1998; Seminoff et al. 2002; Makowski et al. 2006; water occurs infrequently. Seminoff and Jones 2006), where activity is typically These gaps in our knowledge are unfortunate because concentrated around important core areas (the best the survival strategies and associated adaptations of feeding and resting sites). young turtles in open water habitats are certainly In contrast to the relative wealth of data available for important and probably differ from those shown by larger large juvenile and adult marine turtles, little is known juveniles and adults in coastal waters that are less about diving frequencies, functions of dives, or types of vulnerable to a smaller subset of predators. The behavior dive profiles shown by young juveniles during the oceanic of young turtles is also of interest from a developmental SALMON ET AL. — Flatback Turtle Diving Ontogeny 9 perspective. With an increase in size, young turtles are (streamlined body shape, large flippers) and vigorous likely to become more competent as swimmers and activity shown by flatbacks after entering the sea (Salmon divers, which enables them to change how (and where) et al. 2009), coupled with a high probability of they search for food, what kind of prey they select, and encountering predators (Walker 1991a) should select for how they defend themselves against predators. Unfortu- a turtle capable of (3) rapid and powerful swimming nately, we know virtually nothing about any of these movements. A similar prediction was made earlier by aspects of their behavior. Walker and Parmenter (1990), who speculated that How can this situation be remedied? One possibility neonate flatbacks would be more powerful swimmers is to carry out ‘‘staged’’ experiments, that is, to introduce than the neonates of other marine turtle species. young turtles of particular size or age classes into an open-water environment and observe what they do. METHODS Studies of this kind must be done with care because there is always the possibility that environmental conditions Turtles. — We reared 33 hatchlings (6–11 per nest will be inappropriate and result in behavioral artifacts. from 4 nests) obtained from a rookery in Mackay, However, small turtles seem amazingly impervious even Queensland, Australia (lat 21u089S, long 149u119E). to highly simplified and artificial laboratory conditions. In Turtles were captured during January and February, either fact, they behave in ways that seem consistent with what as they emerged naturally or from the sand column above little we know about their ecology in the open ocean (e.g., the hatched eggs during the late afternoon just before their orientation relative to locations in gyre currents, Lohmann evening emergence. Hatchlings were immediately placed et al. 1997; migratory activity, Wyneken and Salmon into covered buckets that contained a shallow layer of 1992; Witherington 1995). moist sand and were transported within a few hours to In one such study, Salmon et al. (2004) compared the screened outdoor pools located in the Marine and development of diving and feeding behavior between Aquaculture Research Facilities Unit on the campus of young leatherbacks and green turtles. The turtles were James Cook University, Townsville, Queenland, Australia reared in the laboratory and, at 2-week intervals, several (lat 19u159S, long 146u459E). Each turtle was isolated in a were released offshore in deep water for a single, brief rectangular plastic container (34 cm 3 24 cm 3 18 cm trial before they were released. Observers recorded deep) that was continuously supplied with filtered behavior associated with each dive while dive profile (recirculating) seawater. The turtles began feeding within data were stored on miniature time–depth recorders a day after capture. They were provided with an in-house (TDR) that the turtles towed. These studies showed that manufactured diet composed of ground fish, gelatin, the 2 species differed in the type and frequency of dive human infant formula, freshwater turtle food pellets, and profiles, fed on different prey, and as they developed, reptile vitamins. The food was cut into small cubes that older leatherbacks made deeper dives whereas older green were dropped into the plastic containers; turtles dove to turtles made longer dives. These changes were apparent the bottom to consume the food (10%–15% of their body even over a relatively short (8–10 week) period of growth. weight daily). At capture and every 7–10 days thereafter, Here, we use similar methods to describe the diving we measured growth (straight-line carapace length [SCL]) behavior of young flatback turtles (Natator depressus) with vernier calipers (nearest 0.1 mm) and mass with an during the first 7 weeks of development. This species is of electronic scale (nearest 0.1 g). particular interest because, unlike other marine turtles, the Diving Trials. — Diving trials generally were carried hatchlings do not disperse into open oceanic waters; out during the morning and early afternoon to avoid late instead, they remain within the relatively shallow afternoon thunderstorms. A subset (n 5 22) of the 33 Australian continental shelf waters (Walker and Parmen- turtles we reared was used in these trials. Turtles were