Abstract

1. Using thermal tolerance to predict changes in geographical distribution in the sea star, Coscinasteriastenuispina Matthew Okoneski, UNCW Honors Student Supervisor: Joseph R. Pawlik, Ph.D. Department of Biology and Marine Biology, UNCW • Research Questions • Are populations of C. tenuispina in tropical waters above 25⁰C (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 ? 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 33⁰C maintained by a water‐bath 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 Tukey’s 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 33⁰C. 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 star’s 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 ⁰C, 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. tenuispinawere 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. Tukey’s HSD was conducted to determine between which temperatures significant differences in the average righting time were present. Place sea stars in water bath Acclimation period: 24 hours Figure 1. Global annual mean surface water temperature in 2009 (⁰C) Note: White square indicates area of interest Test righting time of sea stars Return sea stars to control environment Figure 3. Water-bath apparatus for manipulating and maintaining temperature 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). † 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. (Fcalc = 5.701, Ftable(0.05, 6, 40) = 2.302). Significant differences (determined by Tukey’s HSD test) were found between temperatures 32 ⁰C and 31⁰C, and 30⁰C and 29⁰C. (μ8≠μ7=μ6≠μ5=μ4=μ3=μ2=μ1) Note: The average righting time of temperatures 32, 31, 30, 29, and 28⁰C were significantly different from the control (21⁰C). Water temperature of 33⁰C 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 0C (N=6) Note: (*) indicates that at 31⁰C one sea star could not right itself. (†) indicates significant difference from control.