Comparative Study of Eggshell Morphology in Wild and Captive Olive Ridley Turtles Lepidochelys olivacea at Phuket, Thailand

 Sansareeya Wangkulangkul, Kumthorn Thirakhupt, Supot Chantrapornsyl

 Abstract

The objective of this study was to compare morphological and ultra- structural aspects of eggshells from wild and captive Olive ridley turtles. It was found that eggs from wild turtles were significantly lower in total weight, diameter, weight of eggshell, wet weight of yolk and calorific content of yolk than captive eggs. Studies of thickness of calcareous and fibrous layers of eggshells by stereo zoom microscopy indicated no significant differences between eggs laid by wild and captive turtles. Ultrastructure of the calcified layer of eggshell as revealed by scanning electron microscopy (SEM) showed irregularities in the eggshell structure of eggs laid by captive turtles.

Introduction

Thailand is one of the few countries world-wide with more than 25 species of turtles. A total of 28 species representing 26 genera in 6 families occur in Thailand (Sematong & Thirakhupt 1994). There are five species of sea turtle recorded in Thai waters including the Green turtle Chelonia mydas, the Loggerhead turtle Caretta caretta, the Hawksbill turtle Eretmochelys imbricata, the Olive ridley turtle Lepidochelys olivacea and the Leatherback turtle Dermochelys coriacea (Phasuk 1992). The Green.turtle and the Hawksbill turtle are common in the Gulf of Thailand while the Olive ridley turtle and the Leatherback turtle nest along the Andaman coast. Reports of nesting of the Loggerhead turtle in Thailand remain unconfirmed (Monanunsap & Charuchinda 1994).

Due to over-harvesting of eggs and other forms of exploitation along the Andaman coast of southern Thailand, decreased numbers of sea turtle nestings were reported. Observations at various nesting sites revealed an 82 % decline during 1979-1990 and an 87 % decline in 1993 at Ko Prathong; a 10 % decline during 1978-1982 and a 68 % decline in 1993 at Thaimuang beach; and an 88 % decline during 1978-1993 along the beaches of Phuket province (Chantrapornsyl 1992, 1996). Various efforts have been underway to increase sea turtles populations through a conservation breeding project at the Phuket Marine Biological Center (PMBC; Chantrapornsyl 1992). Eggs have been collected from various nesting sites along the west coast of Thailand and transferred for incubation at the center. Biological and behavioral data on sea turtles, in particular the Olive ridley, have been collected from a captive population for use as a basis for maintenance and breeding programs (Chantrapornsyl & Bhatiyasevi 1994). There has never been a comparative study on reproductive biology between wild and captive populations in Thailand.

Materials and Methods

Searches were made for Olive ridley nests along the Andaman coast (Fig. 1) from October 1997 to February 1998, and eggs were transferred to the PMBC. Eggs from a captive turtle were collected from artificial ponds at the PMBC during the same period. Clutch sizes were recorded and eggs were relocated to a laboratory within 3 hours after deposition. Fifty to one hundred eggs were stacked in styrofoam boxes (30 x 50 x 30 cm) covered with sand in order to prevent dehydration of the uppermost eggs. They were maintained at 3 1-34⁰C, and after 10 days of incubation, undeveloped eggs were separated.

Size and weight of each egg was measured upon arrival at the laboratory. Undeveloped eggshells after 10 days were weighed and preserved in 70 % ethanol. Thickness of eggshells was measured under stereo zoom microscope (x 4) with a stage and ocular micrometer. Yolk of the undeveloped eggs were measured for wet weight and oven-dried at 60^°C for 36 hours. Calorific content was then measured by combustion bomb calorimetry using an Automatic Bomb Calorimeter (Ac - 350, Leco, USA). Undeveloped eggshells were dehydrated, coated with gold and examined under a scanning electron microscope (SEM; JSM 35 CF).

Mean results were compared by analysis of variance (ANOVA), and Duncan's multiple range test was used to classify homogeneous subsets of the means. Probability of P≤0.05 was considered to be significantly different.

Fig 1: Western coast along the Andaman sea of Southern Thailand

Results

Only 1 clutch with 106 eggs from the wild population and 3 clutches of 118,24, and 127 eggs from captive populations were available, from which mean ± SE of egg weight, egg size, eggshell weight, thickness of calcareous and fibrous layers, yolk weight, and calorific content of egg were calculated (Table I).

Table I: Mean ±  SE of egg weight, egg size, eggshell weight, thickness of calcareous and fibrous layer, yolk weight, and calorific content of eggs from wild and captive turtles. Significant differences are indicated by differences in superscript.

Morphology of fresh eggs: The weight of eggs from the wild nest (22.0-26.0 g) was significantly lower than eggs from captive turtles (25.0-31.0, 26.0-34.0, 25.0-39.0 g). Regarding general shape, wild eggs were more circular while captive eggs were more oblong. The diameter of wild-laid eggs (32.5-37.5 mm) was significantly shorter than eggs from the captive nests (31.5-39, 35.0-39.3, 35.0-42.0 mm).

Morphology of the eggshells: There was only 1 undeveloped egg from captive nest No. 2, and it was excluded from further comparisons. There was a significant difference in eggshell weight between wild eggs (1.3-1.6 g) and captive eggs (Nest No. 3:1.3-3.7 g), but there was no significant difference in thickness of the calcareous layer (Wild: 0.05-0.13, Captive: 0.08-0.15, 0.03-0.10 mm) and fibrous membrane layer (Wild: 0.08-0.15, Captive; 0.10-0.20, 0.10-0.15 mm).

Yolk characteristics: Yolk weight was significantly different between wild (8.64-15.42 g) and captive eggs (10.09-15.16, 3.92-20.33 g). The calorific content of the yolk was also significantly different between wild and captive population.

Ultrastructure of eggshells: Shell units in the eggshell from the wild turtles were found to have a slightly domed shape which was different from eggs from the captive population (Fig. 2). The side view of the eggshells can be divided into a calcareous layer and a fibrous layer. The calcareous layer consist of crystal originated from the fibrous layer with a fan-like shape. It was found that eggs from captive turtles had fan-like units with some irregular structures at the top (Fig. 3). Bases of shell units were not different between wild and captive populations (Fig. 4).

Discussion

Eggs from wild populations were significantly smaller than eggs from captive populations in egg size, egg weight, eggshell weight, yolk weight and calorific content. However, there was no significant difference in thickness of the catcareous and fibrous layers of the eggshell. The size of fresh eggs from captive turtles is similar to egg dimensions of the first clutch of a captive Olive ridley turtle in 1992 (x = 38.5 mm, range = 37.4-40.5, n =122) reported by Chantrapornsyl & Bhatiyasevi (1994). The size and weight of fresh eggs from the wild population were lower than previous reports by Chantrapornsyl & Bhatiyasevi (1994; size: x = 33 mm, range = 30.3-37.0; weight: x = 39.5g, range = 37.0-39.5, n = 117). However, eggs from captive population also showed significant differences within the group, especially between clutch No.1 and clutches 2 and 3. The differences among the eggshell morphologies might be due to individual variation in health, fitness, diet, and competition within groups. Most captive eggs were oblong shape which was similar to findings by Sahoo et al. (1996), who reported that abnormal eggs of the Olive ridley turtle were oblong and smaller or larger than normal eggs.

The finding that calorific content of yolk in the captive population was significantly higher than that in the wild population may indicate that turtles in captivity could consume more food, but of less variety, than turtles in the wild. However, the numbers of clutches of both wild and captive animals were too low to draw conclusions with confidence.

The result that ultrastructure of eggshell consisted of 2 layers including calcareous and fibrous layers duplicates previous studies on other reptilian egg- shells (Packard & Hirsh 1986). The calcareous layer of eggshells from wild and captive animals share common characteristics, consisting of calcium carbonate crystals in aragonite form attach to a fibrous layer (Solomon & Baird 1976, Baird & Solomon 1979). It was found that Olive ridley's eggshell consisted of only aragonite form. This is unlike other turtles' eggshell that consists of calcite, vertaite and aragonite, in particular the leatherback, which has all three forms together (Sahoo et al. 1996)

The surface of eggshells from the wild population had regular nodular shell units on the upper surface, in agreement with reports of other sea turtle eggshells in Thailand by Khonsue (1993). For captive populations, it was found that outer surface had irregular aragonite axials as seen on side view of shell unit. Bases of shell units were found to have similar origin crystals, at which deposition started and calcite modification occurred.

Shells of wild populations showed regular arrangements with pores between the shell units, so that the embryo could exchange gas with the environment, while the captive eggs had irregular axials. It would be interesting to learn whether the irregularity of aragonite in eggshells from captive turtles affect the survivorship of the embryo and hatching success, and it is suggested that this be further studied.

a (x 800)

 
b (x 800)

Fig. 2: Top view of shell unit of eggshell from a) wild and b) captive turtles.

 
a (x 1000)

 
b (x 1,500)

 Fig. 3: Side view of shell unit of eggshell from a) wild and b) captive turtles.

 a (x 800)

 b (x 500)

Fig. 4: Base of shell unit of eggshell from a) wild and b) captive turtles.

Acknowledgments

We are grateful to Mr.Peter Paul van Dijk and Mr. Noppadon Kitana for their constructive discussions on sea turtle biology as well as polishing the writing. We are indebted to the TRF/BIOTEC Special Program for Biodiversity Research and Training for a supported research grant [BRT540014].

References

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