Complete formation of the fertilization envelope developed during

Complete formation
of the fertilization envelope developed during 20 minutes, 18 minutes, and 14
minutes after insemination at low-temperature ranges, ambient temperature
ranges and high-temperature ranges, respectively (Fig. 9C). After 57 minutes,
the zygotes formed the first cleavage (Fig. 9D) in all the three treatments.
Following the development of the first cleavage, the second cleavage (Fig. 9E)
formed after 1hr 39 min, 1hr 36min, and 1hr 35min at treatment 1, treatment 2
and treatment 3, respectively. Several mitotic divisions followed including the
development of third cleavage (Fig. 9F), fourth cleavage (Fig. 10A) and fifth
cleavage (Fig. 10B). Early blastula began to develop at 8hr 40min in
low-temperature ranges, 8hr 40min in ambient temperature ranges, and 8hr 36min
in high-temperature ranges after insemination (Fig. 10C). During the continuous
development of the embryo, the gastrula stage was attained at 15hr 32min in
low-temperature ranges, 15hr 18min in ambient temperature ranges, and 15hr 17min
in high-temperature ranges (Fig. 10D).

    Prism (Fig. 10E), the earliest stage of the
larval development, developed at 23hr 48min, 23hr 36min and 23hr 33min at low-
temperature ranges, ambient temperature ranges, and high- temperature ranges,
respectively. After the sperm was mixed with the egg, the two-armed larvae
(Fig. 10F) formed at 36hr 24min, 36hr 12min, and 36hr 10min in low-temperature
ranges, ambient temperature ranges, and high-temperature ranges, respectively,
emerging as planktonic and lecithotrophic larvae. This larval stage is noted by
two small post oral (PO) arms and conical in shape.

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At almost every
stage, the zygotes exposed to the higher temperature has faster development
time compared to the other treatments. This is similar to the study of Brennand
et al (2010) where +3?C warming of the seawater resulted in faster growth and
increased size. On the other hand, the zygotes’ development in the lower
temperature is slightly slower. This is because cold temperature lowers the
activity of the enzymes inside the cell required to dissolve egg membranes
which trigger slower mitotic cell division (Rahman et al., 2009). And studies
suggested that temperature, among the abiotic factors, greatly affects the
growth and development of eggs, zygotes, and cleavages (Sewell & Young,
1999). The development of T. gratilla affected by different temperature levels
is almost similar with the developmental pattern of the other echinoids, like
the Salmacis sphaeroides (Rahman et al., 2012), Diadema setosum (Sarifudin et
al., 2016) and Lytechinus variegatus (Mc Edward, 1984). However, several
abnormalities in shape and development were being recorded by Sarifudin et al
(2016) with the embryonic development in blastula stage and consequently died
at temperatures lower than 19?C and higher than 33?C in D. setosum. Also, in
the study of Rahman et al. (2009) on the embryonic development of Echinometra
mathaei, as mentioned by Sarifudin et al. (2016), embryos and larvae developed
with abnormalities in temperatures below 16?C and higher than 34?C.

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