Comparison of the Genetics and Nesting Ecology of Two Green Turtle Rookeries I.-J
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Journal of Zoology Journal of Zoology. Print ISSN 0952-8369 Comparison of the genetics and nesting ecology of two green turtle rookeries I.-J. Cheng1, P. H. Dutton2, C.-L. Chen1, H.-C. Chen1, Y-H. Chen1 & J.-W. Shea1 1 Institute of Marine Biology, National Taiwan Ocean University, Keelung, Taiwan, ROC 2 NOAA-Fisheries, Southwest Fisheries Science Center, La Jolla Shores Drive, La Jolla, CA, USA Keywords Abstract Chelonia mydas; Taiwan; nesting ecology; Wan-an Island; Lanyu Island; genetic We characterize the behavioral, phenotypic and genotypic characteristics of the structure. two main green turtle Chelonia mydas nesting populations remaining in Taiwan and examine how the differing ecology of the nesting environments on each island Correspondence may have influenced these life-history traits. Wan-an Island in the south-west I-Jiunn Cheng, Institute of Marine Biology, sector of the Taiwan Straits was found to be hotter and drier than Lanyu Island National Taiwan Ocean University, Keelung, located south-east of Taiwan in the Pacific Ocean. The frequency of nesting Taiwan 202-24, ROC. attempts and the internesting intervals were both significantly greater (mean Email: [email protected] nesting attempts: 15.4 vs. 15.2, mean internesting interval: 13.7 vs. 10.6 days) for the Wan-an nesters. Nests were deeper (69.6 vs. 69.0 cm) while the incubation Editor: Prof. Tim Halliday duration was shorter on this island (52.1 vs. 54.7 days). Green turtles were larger on Lanyu, but deposited smaller eggs. Nests on Lanyu had higher hatching Received 20 January 2008; revised 21 July success, hatchling emergence success and clutch survival rate (hatching success; 2008; accepted 22 July 2008 72.2 vs. 80.76%, hatchling emergence success; 47 vs. 64.1%, clutch survival rate; 67.7 vs. 70.3%). However, hatchlings on Wan-an were larger (HC; 47.6 vs. 46.6 doi:10.1111/j.1469-7998.2008.00501.x cm. HW; 23.9 vs. 21.7 cm). Analysis of mitochondrial DNA sequences obtained from a total of 53 nesters indicated that both rookeries are genetically distinct (FST =0.673, Po0.00001), with a single haplotype characterizing the Lanyu rookery (n=13). This lack of gene flow between the two rookeries is evidence for localized natal homing and is consistent with the morphological and behavioral differences that we detected between the two rookeries. These findings are surprising given the close geographic proximity of the two rookeries. Introduction tence of distinguishable stocks for management purposes (Norman, Moritz & Limpus, 1994; Encalada et al., 1996; Wan-an Island of Penghu Archipelago and Lanyu Island of Dethmers et al., 2006; Dutton et al., 2008). These studies Taitung County are the two main nesting sites of green have shown population structuring consistent with theories turtles Chelonia mydas, Linnaeus, 1758 remaining in Taiwan of broad natal homing to regional rookeries. A general (Fig. 1) (Cheng, 1998). Wan-an is located south-west from limitation to these studies is that they rarely compare Taiwan, on the continental shelf in the Taiwan Straits. rookeries that are closely spaced (o400 km), so the extent Lanyu, on the other hand, is south-east of Taiwan in the of connectivity on a local scale remains unknown. High Pacific Ocean. Satellite telemetry studies have shown that fidelity to particular nesting areas has recently been demon- the foraging sites for the green turtles nesting on Wan-an are strated elsewhere in green turtles (Lee, 2007) and this species distributed mainly on the continental shelves east of the certainly seems to maintain higher nesting area fidelity than Mainland China (Cheng, 2000), while foraging sites for the some other sea turtle species. The degree of demographic green turtles nesting on Lanyu may distributed mainly in connectivity between the two nesting populations in Taiwan Penghu waters south of Wan-an (I.-J. Cheng, unpubl. data). and with other populations in the Pacific is unknown. However, the sizes of these nesting populations are un- known, and it is unclear if the two rookeries represent distinct breeding populations. Purpose of the study Studies of maternally inherited mitochondrial DNA Sea turtles are known to have phenotypic and behavioral (mtDNA) in green turtles have been useful in understanding plasticity in response to varying environmental conditions the population structure and reproductive behavior of these (Hays et al., 2002; Glen et al., 2003; James et al., 2006) found highly migratory marine animals (Bowen et al., 1992; on nesting beaches. Local adaptations of nesting behavior FitzSimmons et al., 1997) and in demonstrating the exis- are associated with temperature, precipitation, sand Journal of Zoology 276 (2008) 375–384 c 2008 The Authors. Journal compilation c 2008 The Zoological Society of London 375 Comparison of life-history traits between two green turtle rookeries I.-J. Cheng et al. 118° 40′ 119° 20′ 120° 00′ 120° 40′ 121° 20′ 122° 00′ 122° 40′ Materials and methods Description of the study sites 25° 20′ Wan-an Island 24° 40′ Wan-an (231220N, 1191300E) is c. 7.4 km2 in size and located in the southern Penghu Archipelago, about 18 miles from the main Penghu Island. The average annual air tempera- 24° 00′ ture is 22 1C, and the average total annual precipitation is 1117 mm, occurring mostly in the monsoon and typhoon Taiwan seasons. ° ′ 23 20 There are 11 relatively flat sand beaches, all composed of Wan-an Island quartz propyrite, coral and shell debris. They range from 67 to 800 m in length, 20 to 100 m in width and are separated by 22° 40′ rocky outcrops. Turtles are known to nest on nine of the Lanyu Island beaches, all of which are located on the south and west sides of the island. The total length of the nesting beach is about 22° 00′ 4 km (Chen & Cheng, 1995; Cheng, 2006). km 0 71 21° 20′ Lanyu Island Lanyu (221000–080N, 1211500–600E) is in the Pacific Ocean Figure 1 Map of Taiwan with the location of two main sea turtle 2 nesting sites: Lanyu and Wan-an Islands showed on either side south-west of Taitung, Taiwan, measuring about 46 km . 1 of Taiwan. The annual average air temperature is 22.4 C and annual total precipitation exceeds 3077 mm (Natural Conservation Society ROC, 1988). The sand on the beaches is composed of quartz propyrite, inter-layered with muddy sediment. characteristics, vegetation coverage, beach width and slope Although there are six sandy beaches on Lanyu, prelimin- (Mortimer, 1990; Rivalan et al., 2005; Chen et al., 2007). ary surveys show nesting activity on only three beaches: The characteristics of the nesting environment in turn Ba-dai, big Ba-dai and Don-chin beaches. The areas of these influences the incubation success, sex ratio, hatchling mor- three beaches were about 5522, 17 000 and 15 766 m2 (Kuo, phology, and ultimately recruitment to the population 1999). (Hays et al., 2001; Glen et al., 2003). Glen et al. (2003) found phenotypic differences between green turtle hatchl- Environmental data ings from Cyprus and Ascension Island attributed in part to environmental factors such as temperature. Wan-an Sand grain characteristics and Lanyu are relatively close (c. 300 km); however, their respective climates are different, with over twice as much Sand samples were taken near nesting sites and stored dry in precipitation on average on Lanyu (Natural Conservation double-sealed plastic bags. The graphic mean (Mz; in mm) Society ROC, 1988). We characterize and compare the and inclusive graphic standard deviation (s1) of the beach nesting environments on both islands, and investigate sand were determined according to Folk (1974). whether there are differences in nesting behavior, morphol- ogy and hatching success. Secondly, we use mtDNA MzðfÞ¼ðf16 þ f50 þ f84Þ=3 sequencing to determine whether the two nesting popula- tions are demographically distinct. Anthropogentic im- z1 ¼ðf95 ðf5Þ=6:6 þðf84 ðf16Þ=4 pacts associated with urban development and tourism have where f16, f50, f84, f95 and f5 denote the proportion by been a concern for some time on Wan-an, but have also weight of total sand at 16, 50, 84, 95 and 16% of total phi(f) increased dramatically in recent years on Lanyu. The nest- value. Sand samples were collected from 1997 to 1998 and ing beaches on Wan-an were declared protected areas in from 2005 to 2006. 1995 (Chen & Cheng, 1995; Cheng, 2006). Study of green turtle nesting ecology on Lanyu was initiated in 1997 Climate data (Cheng, 1998). Understanding of the extent of demographic connectivity between the green turtle nesting populations on The meteorological data collected for the study include daily both islands, and the extent of adaptation to any local air temperature and precipitation on both Wan-an and differences in the nesting environment will be vital to Lanyu from 1997 until 2006. The data were purchased from developing management strategies appropriate for each the Central Weather Bureau of the ROC (Central Weather island. Bureau, 1997–2006). 376 Journal of Zoology 276 (2008) 375–384 c 2008 The Authors. Journal compilation c 2008 The Zoological Society of London I.-J. Cheng et al. Comparison of life-history traits between two green turtle rookeries Field surveys deepest position in the nest. On Lanyu, the clutch size was determined by counting the eggs in each nest. While on On both islands, field surveys were conducted between late Wan-an, the clutch size was determined by excavating the April and early October. Although Wan-an has been mon- nest after hatchling emergence, and counting the total itored since 1992, only data collected from 1997 to 2006 were numbers of hatched egg shells and unhatched eggs. Clutch used for comparative purposes, since the study on Lanyu frequency was determined as the number of nests deposited only began in 1997.