Using thermal tolerance to predict changes in geographical
distribution in the sea star, Coscinasterias tenuispina Matthew
Okoneski, UNCW Honors Student Supervisor: Joseph R. Pawlik, Ph.D.
Department of Biology and Marine Biology, UNCW Abstract This study
was undertaken to determine how the geographic distribution of a
local sea star, Coscinasterias tenuispina, which is known to have a
very strict thermal tolerance, will be affected by potential rises
in water temperatures off the coast of North Carolina. The thermal
stress of C. tenuispina was tested using the righting time
response, which is the amount of time it takes for the animal to
return to a natural position after being inverted. Six individuals
of C. tenuispina were subjected to elevated water temperatures of
26, 27, 28, 29, 30, 31, 32, and 33C maintained by a waterbath
system for a 24 hour period before the thermal stress of the animal
was measured. There was a significant (determined by a One-way
ANOVA test and a Tukeys HSD test) difference between the average
righting time response with respect to increasing temperature,
indicating that elevated seawater temperatures place a considerable
amount of abiotic stress on the organism. The lethal temperature
was determined to be 33C. If the potential rises in water
temperature produce high levels of thermal stress, the distribution
of C. tenuispina will be affected. Materials and Methods
Determining Thermal Stress Levels and Thermal Tolerance The
righting time response is a measure, in seconds, of how long it
takes a sea star to return to the normal position (oral surface in
contact with the substrate) after it has been inverted (placed with
oral surface facing the water column). The righting time response
was measured with a stopwatch. According to Ubaldo et al., (2007)
longer righting times are indicative of higher levels of thermal
stress in sea stars. High temperatures affect the sea stars ability
to coordinate the complex movement needed in righting response
behavior. The righting time response was measured at 26, 27, 28,
29, 30, 31, and 32 C, which were temperatures determined to be
above the annual average surface temperatures where individuals of
C. tenuispina were collected (see Figure 1). Two trials at each
temperature were conducted for each individual (n=6). Figure 2
represents a flow-chart diagram for experimental protocol. Figure
2. A schematic representation of experimental protocol Manipulating
and Maintaining Temperature One-gallon jars were filled with
natural sea water of salinity 35 ppt, then placed in a water bath
system (Figure 3). The temperature of the water bath was adjusted
and maintained by two submersible aquarium heaters. Water
circulation was needed to avoid thermal stratification of the water
bath and to distribute heat evenly among the jars, and was
generated by a power head. Individuals of C. tenuispina were then
distributed into the jars, with one star occupying a single jar. In
order to ensure sufficient levels of oxygen were present in the
jars, air was supplied by a pump to each jar. The temperature
within the jars was recorded from a mercury thermometer and was
digitally monitored by HOBO Water Temp Pro V2 data loggers. Data
Analysis The righting time response in relation to an increase in
temperature was examined after finding the average for each
temperature class. A one-way ANOVA test was used to assess the
statistical significance of the variations in average righting time
response as a function of temperature increases. Tukeys HSD was
conducted to determine between which temperatures significant
differences in the average righting time were present. Figure 1.
Global annual mean surface water temperature in 2009 (C) Note:
White square indicates area of interest Figure 3. Water-bath
apparatus for manipulating and maintaining temperature Results and
Conclusions The average righting time response of C. tenuispina
increased as the water temperature increased, indicating an
increasing level of thermal stress on the sea stars (refer to
Figure 4). A one-way ANOVA was conducted to assess if this trend
was significant. (F calc = 5.701, F table(0.05, 6, 40) = 2.302).
Significant differences (determined by Tukeys HSD test) were found
between temperatures 32 C and 31C, and 30C and 29C. ( 8 7 = 6 5 = 4
= 3 = 2 = 1 ) Note: The average righting time of temperatures 32,
31, 30, 29, and 28 C were significantly different from the control
(21C). Water temperature of 33C was lethal to C. tenuispina. Based
on the negative effects that higher than normal water temperatures
have on feeding, righting, and larval development, a massive
mortality event of a large portion of the Bermuda and Brazilian
populations of C. tenuispina could be expected if annual surface
sea seawater temperatures continue to rise. Figure 4. The
relationship between average righting time of C. tenuispina in
seconds and temperature in 0 C (N=6) Note: (*) indicates that at
31C one sea star could not right itself. () indicates significant
difference from control. Figure 5. The speculative changes in
geographic distribution of C. tenuispina. Note: Green areas
indicate predicted concentrated areas of intertidal or subtidal
populations. Red areas indicate predicted areas of very scarce,
subtidal, populations (Geophylogeny from Waters and Roy, 2003).
Place sea stars in water bath Acclimation period: 24 hours Test
righting time of sea stars Return sea stars to control environment
Research Questions Are populations of C. tenuispina in tropical
waters above 25C (see Figure 1) experiencing thermal stress? If
individuals of C. tenuispina have a strict thermal tolerance, what
water temperature is lethal to the sea star? How will possible
increases in water temperature affect the geographic distribution
of C. tenuispina ?