Temperature-dependent Sex Determination of a Sea Turde, Caretta ...

148 Shoji Tokunaga, Yuji Iwakiri and Yoshito Nakajima
1991). For sea turdes, high temperatures result in females and low temperatures
result in males. The temperature which gives 50% of each sex is called the critical
temperature (Pieau, 1976), the pivotal temperature (Mrosovsky and Yntema,
1980), the threshold temperature (Bull, 1980), or SDT50 (Limpus et al, 1985). In
this paper, the critical temperature was used to show the temperature giving 50% of
each sex, because this was the term first used to represent that temperature.
There have been some studies on TSD of Caretta caretta in the U.S.A. (Yntema
and Mrosovsky, 1980; Yntema and Mrosovsky, 1982; Mrosovsky, 1988) and
Australia (Limpus etal, 1985), but only a few reports are available on TSD inJapanese
populations of C. caretta and these were written in Japanese and published in a local
journal. Tokunaga (1991) first reported sex determination ofJapanese sea turtles
under constant temperatures. Asa preliminary report ofthis study, he reported that
nine hatched loggerheads from those incubated at 31°C (no correction for evapora
tive cooling) were females. Kamezaki and Kuroyanagi (1991) incubated one
clutch ofeggs from Atsumi, Aichi,Japan and reported that the sex ratio (number of
hatched turtles, hatching rate) of hatched turdes was 1.0 (34, 0.87) at 26°C, 1.0 (35,
0.90) at 28°C and 0.0 (37,0.95) at 30°C. He concluded that the critical temperature
of the clutch of C. caretta exists between 28 and 30°C.
This paper deals with TSD of C. caretta from Miyazaki, Japan. The critical
temperature of the loggerhead from Miyazaki and other localities in Japan, as well,
has not been identified so far. This study also intended to examine sex determina
tion underfluctuating temperature. The possible influence ofTSD on the hatchling
sex ratio is discussed.
Materials and methods
Eggs were collected from Miyazaki,Japan. The geography and the turtle nest
ing habitat of the beach have been described by Iwamoto etal (1985).
Experiments were conducted at the Laboratory of Ecology, Faculty of Science,
Kyushu University, in 1989. Three clutches were used for the experiments. Each
clutch was removed from its nest by YN after the turtle completed laying and was
packed in a Styrofoam box for transport. The eggs were sent to the laboratory by
commercial truck delivery with no attempt to control the temperature of the eggs.
The number of eggs, date of oviposition and the date of starting incubation ofeach
clutch were 127,June 30 and July 4 for the clutch A; 135,July 2 and July 7 for the
clutch B; 138,July 2 andJuly 7 for the clutch C.
Each clutch was divided into three to five groups (see Table 1). Each group of
eggswasincubated in a plasticcontainer (approx. 17 x 31 cm on the bottom) in two
layers and the bottom of the container was coveredwithwet vermiculite. A layerof
eggs, usually 28 eggs, was set on the vermiculite and covered with a mesh to keep air
spacebetween the eggs. Wetvermiculite was put on the mesh,and the secondlayer