The exposed to low temperature (19-23?C) has higher

The
larval survival in the three treatments dropped drastically after 15 minutes of
exposure to different temperature ranges. Treatment 1 (low-temperature range)
dropped to 32%, Treatment 2 (ambient temperature range) downed to 21% and
Treatment 3 (high-temperature range) declined to 17%.  After 30 minutes of exposure, individuals in
the ambient temperature range has increased survival rate from 21% to 24% while
the other two treatments continuously dropped its survival rate.

A
35% increase in the survival rate of the two-armed larvae in ambient
temperature range after one (1) hour of exposure was noted. This may be due to
the reduced activity of the larvae at the start of the experiment and was
counted for mortality or may be due to the possible error in counting. With the
suitable temperature induced in Treatment 2, the live two-armed larvae are
significantly higher in number than Treatment 1 and Treatment 3, respectively.

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The
decrease in the survival rate may also be attributed, aside from temperature,
to the competition of the larvae with space, considering that the total number
of larvae per 500 mL was 32,500. The stocking density of the two-armed larvae in
their natural environment which is one (1) per mL (Mc Edward & Miner,
2007). Competition to space may be considered as a factor in their survival.

 Twenty-four (24) hours after exposure to
different temperatures, the two-armed larvae exposed at high temperatures has
significantly shown a rapid decrease in survival rate. Figure 11 shows lower
survival rate of the larvae in high temperature range, while higher in ambient
and low temperature ranges. Larvae exposed to 31?C or higher for 24-hours have
proven that regardless of the time interval, the higher temperature can cause a
significant effect on the survival rate of the two-armed larvae while those
exposed to low temperature (19-23?C) has higher survival rate over time,
compared to raised temperatures. The survival rate was highest at Treatment 2
(ambient temperature), as compared to the other treatments.

The
study conducted by Rahman et al (2009) concluded that thermal tolerance of
echinoids is stage dependent, having the prism and two-armed larvae stages
survived at temperatures between 30°C and 33°C, while the four-armed larvae
survived at temperatures between 30°C and 36°C for 2 hours. Longer exposure at
the high temperature significantly affects the survival rate of the two-armed
larvae and the threshold for a deleterious warming of the ocean can vary among
the developmental stages within and among the species (Byrne, 2010). And such
thermal threshold can vary among closely related species from similar
environments (Byrne et al, 2009).

Collin
and Chan (2016) recorded 0% survival rate at 32.3?C after 24 hours of exposure
for the early larval stage of Lytechinus variegatus. They concluded that early
larval stages including the two-armed larvae are more susceptible to thermal
stress comparing to the later stages of the life history of the echinoid being
studied.  Sewell and Young (1999) came up
with almost similar results having four-armed larvae of the Carribean sea
urchin, Echinometra lucunter to have survived up to 39?C compared to the earlier
larval stages which only survived up to 34?C after two hours of exposure to
high-temperature ranges. Their studies suggested that early life stages can
tolerate high temperature for a short period of time, which is very unlikely to
observe in their natural habitat wherein the larvae are planktonic and driven
by seawater currents into the shallow water tide pools and lagoons in the
intertidal zones during low tide. It is necessary for the larvae to survive
such high temperature around the habitat for a long time.

In
addition, increase temperature can significantly affect the larval survival
(Collin and Chan, 2016) because it heightens up the metabolism of the larvae
and results in failure of cellular processes when the thermal stability and
function of proteins are affected (Byrne, 2010).  The optimum preferable temperature for the
early life stages of T. gratilla is from 19-29?C (Rahman et al., 2009).

These
results suggest that tropical T. gratilla two-armed larvae are susceptible to
the predicted increase in seawater temperature and regional differences in the
extent of ocean warming highlighting the need of regional approach of
assessment of ecosystem change and risk to species.

     Temperature is considered to be the
primary environmental factor controlling the physiology, phenology, planktonic
larval duration and distribution of marine invertebrates (Byrne et al., 2010;
O’Connor et al., 2007; Sewell and Young, 1999). The response of T. gratilla
larvae to increased temperature reflected the typical pattern seen in echinoids
and other invertebrates with a balance between facilitation at certain levels
of warming (Byrne et al., 2010; Rahman et al., 2009). Temperature is well known
to control the pace of development in marine larvae (Byrne et al., 2010).      

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