The rocky intertidal habitat is one of the most extreme examples of spatial and temporal heterogeneity in temperature (Dahlhoff et al. 2001, Hernandez et al. 2002). In the tropics, temperature is an important ecological driver of biological and ecological processes (Tewksbury et al. 2008a, Deutsch et al. 2015). Since tropical intertidal animals are already thought to be close to their thermal limits (Dahlhoff et al. 2001, Somero 2002, Davenport and Davenport 2005), tropical and high-shore species are expected to be particularly at risk to impacts from ocean warming (Tewksbury et al. 2008, Lima et al. 2009, Hofmann and Todgham 2010). The susceptibility of an organism to elevated temperatures can largely depend on the spatial and temporal distribution of thermal stress (Helmuth and Hofmann 2001, Harley 2008). For intertidal organisms, the availability and access to areas that can provide refuge from thermal stress may be key to their survival as the frequency and intensity of thermal stress events increases (Helmuth et al. 2006).
In the Hawaiian intertidal zone, one of the most abundant gastropods is the pipipi snail (Nerita picea). Pipipi typically occupies the area of the intertidal between the algal line and the uppermost wave-splash zone, and is an ecologically important grazer and culturally-significant species that has traditionally been utilized by native Hawaiians as a food source (Titcomb et al. 1978). Using the pipipi as our species of focus, we evaluated the distribution and utility of microhabitats as thermal refuges and aim to better understand how thermal microhabitat heterogeneity can contribute to the ecological resilience of the intertidal zone to climate change.
DETERMINING UPPER THERMAL LIMITS
To test the upper thermal limits of pipipi, we measured the snail's maximum heartbeat, Arrhenius Break Temperature (ABT), and the flatline temperature. These measurements correspond to critical points along the thermal tolerance curve (pictured above).
Using the data from each snail's heartbeat trace (in video above), we can measure the maximum heartbeat, ABT, and flatline temperature.
COMPARING UPPER THERMAL TOLERANCE TO FIELD RECORDED TEMPERATURES
The average temperature at which the pipipi's maximum heartbeat occurs lies below the highest recorded temperature at Makapu'u (the location where the snails were collected) and the average Arrhenius breakpoint temperature lies just above the maximum recorded temperature. This implies that the pipipi are already living at or near to their upper thermal limit.
COMPARING UPPER THERMAL LIMITS BETWEEN SNAILS FROM DIFFERENT MICROHABITATS
Initially, we hypothesized that snails found on exposed bare rock would have a higher upper thermal tolerance than those snails found in shaded or protected microhabitats during low tide. However, we did not find any difference in upper thermal tolerance limits between snails collected from different microhabitats.
STUDY SITES1. Koʻolina 2. Chun’s Reef 3. Turtle Bay 4. Makapuʻu 5. Kaloko
1. Quantify the thermal landscape of Hawaii's rocky intertidal zone.
2. Determine the type, distribution, and abundance of thermal refugia available to pipipi
3. Determine whether the snails are using microhabitats and if usage is related primarily to temperature.
4. Determine how close pipipi are to their upper thermal limits and if these limits are different between snails collected from different microhabitats.
To quantify the thermal landscape, we took thermal pictures (above) of each quadrat along the five vertical transects using an IR camera. From these pictures we were able to obtain max, min, and average temperature at each site. We were also able to quantiatively define microhabitats that might serve as thermal refuge by their difference in temperature from the exposed rock.
Based on their difference in temperature compared to the exposed rock, we found four microhabitats that could serve as thermal refuge- the micropool, vertical surface, crevice, and dry hole.
We wanted to know if the shaded/protected microhabitats were cooler than the bare exposed rock and we found that at at most sites, the protected microhabitats were significantly cooler than bare rock. However, this signifiance does depend on site.
HAWAII'S INTERTIDAL SHOWS A HIGH DEGREE OF THERMAL HETEROGENEITY
- Most microhabitats were significantly cooler than exposed rock but this depended on location
- There are thermal refugia available to pipipi and they appear to be taking advantage of this during low tide
PIPIPI MAY ALREADY BE LIVING AT OR NEAR THEIR UPPER THERMAL LIMITS
- Avg temperatures of exposed rock very closely matched the upper physiological limits of pipipi
- Pipipi from different microhabitats do not have a different upper thermal limit, meaning those snails found on bare exposed rock are living very close to their upper thermal limits
Overall, pipipi may already be living near the edge of their physiological capacity but microhabitats can provide thermal refugia. Thus, the role of microhabitats as thermal refuges may become even more important as our climate comtinues to warm.
Since we found that the microhabitats could provide thermal refuge, we wanted to know if the pipipi were actually using these microhabitats. We found that at every site, the snails are occupying these microhabitats during low tide.
IR IMAGERY AS A RAPID ASSESSMENT TOOL
- The portable infrared camera is able to take real-time high resolution images of a habitat from macro to landscape scale.
- The use of the IR images helped us discover Hawaii's rocky intertidal zone is a mosaic of temperatures with microhabitats that may help small mobile invertebrates survive warming. Similar types of questions and methodology can be used in many different scenarios, from climate change predictions to endangered species ecology.
NON-INVASIVE HEARTRATE MEASUREMENTS
- IR sensors provide a non-invasive method for measuring an animal's heartrate.
- Measuring an animal's heartrate has been a critical methodology in thermal conservation biology since it is an effective index for whole organism physiology. Until recently, the only techniques available for monitoring heartrates in many animals were invasive (e.g. implanting electrodes into the pericardial cavity). Now, we are able to perform non-invasive heartrate analysis on a wider range of animals such as vulnerable and protected species.
- With this new technique, we can improve our ability to predict the responses of species of a changing environment such as increases in temperature or even levels of pollution.
BASELINE TEMPERATURE MEASUREMENTS FOR HAWAII'S ROCKY INTERTIDAL
- These data provide baseline temperature measurements that can easily be added onto more comprehensive shoreline measurements done by other organizations or communities that are interested in flora and fauna changes over time.