Background

Rising Temperatures Threaten Crab Populations and the Future of Marshlands

Climate change, or the long–term shifts in weather patterns resulting from rising levels of carbon dioxide in the atmosphere, is an existential threat to humanity (United Nations, n.d.). Not only has this phenomenon impacted weather patterns, it has also increased the frequency of extreme weather events, resulting in over 1,452.84 square kilometers of salt marshland being lost across the world between 2000 and 2019 (Campbell et al., 2022). Marshes, defined as regularly flooded wetlands containing saturated soil that supports soft–stemmed vegetation, serve as carbon sinks, which store more carbon than they emit (Marshland, n.d.; Salimi et al., 2021). 

Marshlands, similar to rain forests, are packed with vegetation that take in carbon through photosynthesis and release on average about half the carbon dioxide they store through respiration (Huntingford et al., 2017). Additionally, marshlands store a greater amount of carbon because they have both a high level of photosynthetic vegetation and carbon rich soil (Marshland, n.d.; Salimi et al., 2021). Marshland soil is primarily composed of decomposed matter; therefore, all carbon held in previously living organisms is contained in the soil (Marshland, n.d.; Salimi et al., 2021). In fact, marshland habitats account for approximately 20–30 percent of the total amount of the Earth’s landmasses that can store carbon, which makes them a critical environment to preserve in the battle against climate change (Salimi et al., 2021). As temperatures rise and carbon sinks continue to be destroyed, carbon is released into the atmosphere at a higher rate than it can be stored on land (Salimi et al., 2021). Consequently, the increasing atmospheric carbon dioxide supports the rise of marshland temperatures and creates even more far reaching challenges for the species who rely on these habitats (Salimi et al., 2021). For example, it is estimated that 450 metric tons of carbon dioxide are released annually as a result of climate change’s destruction of marshlands (Pendleton et al., 2012). To illustrate this, that is equivalent to the volume of carbon dioxide released by 97 million cars in one year (Pendleton et al., 2012). Therefore, the destruction of marshlands results in a self–perpetuating cycle that poses a dangerous threat to the future of the earth.

An additional threat to marshland ecosystems is the rising prevalence of algae blooms. In a recent study, Dai et al. found that algal blooms increased in frequency by 59 percent across 54 large marine ecosystems between 2003 and 2020 due to climate change (2023). These areas of algae overgrowth are known as “dead zones” because of their lack of oxygen and high levels of harmful toxins produced by the blooms (United States Environmental Protection Agency [US EPA], n.d. -b). This is because photosynthetic algae populations absorb a great deal of oxygen, which blocks sunlight from underwater species and creates an obstacle to their survival (Center for Disease Control, n.d.). Dead zones cause mass die offs of aquatic species, forcing them to migrate to new environments, or otherwise attempt to survive in a habitat devoid of oxygen. It was reported that a dead zone was recently found off of the coast of Tampa, Florida killing upwards of 1,700 tons of aquatic life living in the marshland zone (University of South Florida Ocean Circulation Lab, n.d.). Thus, the challenge of algae overgrowth poses a major threat to the structure and endurance of food webs in marshland ecosystems, particularly as climate change continues to exacerbate the proliferation of these blooms (Griffith & Gobler, 2020). Therefore, if the trend in rising temperatures continues, a potential surge of algae dead zones puts certain aquatic species at risk of extreme population decline. The resolution to this problem is an unexpected one: the fiddler crab.

In marshland zones, fiddler crabs are a keystone species due to their vital role in the health and endurance of marshland ecosystems (Rosenberg, 2020). Fiddler crabs perform this duty in a wide array of marshlands throughout West Africa, the Western Atlantic, the Eastern Pacific, the Indo-Pacific, and the Algarve region of Portugal (Vianna et al., 2020). Within these environments, fiddler crabs play a key role as detritivores, or organisms who rely on organic waste products, dead plants and animals, and small eukaryotic organisms, such as algae (collectively known as detritus) (Britannica, 2020). By consuming detritus, fiddler crabs manage populations of microalgae, or unicellular microorganisms that create algal biomass through photosynthesis (Ruane et al., 2010). In marshlands, fiddler crabs account for the consumption of up to 70 percent of microalgal biomass, which is the total mass of the organism in the habitat (Johnson et al., 2020). 

As previously stated, large, uncontrolled populations of microalgae form blooms that release toxins into marine environments, which cause mass die–offs of aquatic species to occur (US EPA, n.d. -b). Thus, the control fiddler crabs assert over microalgal populations plays a major role in protecting aquatic marshland species. In fact, Johnson et al. found that during flooding tides, microalgae populations are capable of restoring 100 percent of their original biomass within a day of being consumed by fiddler crabs, demonstrating the necessity of fiddler crab populations to to prevent microalgae overgrowth (2020). Without fiddler crabs, microalgae blooms could continue to grow unregulated, posing an immediate threat to the environment. Rising temperatures not only threaten to create excessive algae blooms in marshlands, but they also threaten to drive marshland species, such as fiddler crabs, away from their homes and reduce their populations. 

In 2014, North American fiddler crabs were found 80 km north of their usual habitat — the bounds of which were determined ten years prior (Johnson, 2014). Researchers observed that, as average surface temperatures rose in marshlands, fiddler crabs began to migrate north to seek cooler climates (Johnson, 2014). In this way, migration serves as a mechanism of thermoregulation, or a behavior through which an organism regulates its internal body temperature (Osilla et al., 2022). This behavior is critical to this study, because an organism's thermoregulatory behaviors dictate how they respond to rising temperatures, and understanding this relationship can help researchers predict the impact of climate change on species migration in the years to come. It is projected that a 3°C increase will take place by the year 2100 (Vianna et al., 2020); however, the extent of temperature’s impact on the distance traveled by fiddler crabs is unknown. Therefore, to address this knowledge gap, a study could be conducted evaluating fiddler crab migratory distance from its regular habitat when it experiences a 3°C increase from its standard temperature range. To investigate this correlation in a controlled research lab, crabs will be placed on one side of an enclosed temperature controlled room where the temperature is 3C above the mean habitat temperature of their regular environment, and the temperature on the opposite side of the room would be set to their mean habitat temperature. To track their movement, a camera would be used to calculate the total distance traveled over the course of 30 days. The data gathered from this study could help researchers to determine the relationship between fiddler crab migration and the continued increases in the average temperature of marshland zones. This information is crucial because the migration of fiddler crabs leads other marshland species, especially their predators, to migrate along with them. 

The recent migration of fiddler crabs has disrupted the diets of the marshland birds who feed on them forcing them to move northwards. Fiddler crabs are a key part of the diets of species like herons and egrets, and their recent migratory patterns have led these predators to move north in search of new sources of food (Zeil et al., 2006). Between 1966 and 2013, 305 species of North American birds were observed to have migrated north of their regular habitats by a minimum of 40 miles (approximately 64 km) (US EPA, n.d. -a). This resulted in changes in the predator–prey relationship existing within marshland ecosystems. Rising temperatures have not only affected predators, but also the prey themselves. Fiddler crabs have been left more susceptible to predation as physiologic processes begin to slow from heat stress.

Fiddler crabs over time have evolved to inhabit various visual and physiological adaptations necessary in avoiding predators. Crabs, like many other crustaceans, possess a unique visual capability known as polarized vision which enables them to detect light waves that oscillate in a specific direction (How et al., 2015). While polarized vision offers fiddler crabs several advantages for detecting aquatic predators, the primary advantage polarized vision offers for detecting bird predators is enhanced contrast (Bagheri et al., 2022). In watery habitats such as marshlands, sunlight scatters, becoming polarized, and this polarized light creates patterns on the water's surface, on the bodies of other aquatic creatures, and in the sky. These patterns enable crabs to see their environment with greater contrast, making it easier for them to detect approaching predators and enabling them to more quickly initiate their escape responses (Bagheri et al., 2022). 

For crabs attempting to outrun their predators and reach their burrows for protection, speed becomes the next crucial part of their survival. In Allent and Levinton's research, they evaluated the impact of the heavy major claw on male Uca pugilator speed (2007). Across all four groups of crabs tested — with major claw intact, without major claw, with major claw intact and added weight, and without major claw but with added weight —, maximum sprint speed was approximately 0.4 m/sec for all conditions demonstrating that sprint speed is independent of both claw size and mass (Bengt J. Allent & Levinton, 2007). Hence, the fiddler crabs' ability to run at similar maximum speeds regardless of claw size has proven to be a beneficial adaptation in escaping the threat of predators. Given that male fiddler crabs have evolved over time to develop larger major claws to attract more mates for reproduction, this speed adaptation maximizes both survival and reproductive fitness. Fiddler crabs with larger major claws and therefore larger mass have evolved physiologically to overcome the decrease in speed with added weight that affected other species (Bengt J. Allent & Levinton, 2007). Fiddler crabs have evolved over time to optimize the balance between speed and claw size. However, with rapid temperature increases inevitable with climate change, fiddler crabs may struggle to adapt as the rate of temperature increases outpaces the evolutionary rate they can adapt physiologically (Bengt J. Allent & Levinton, 2007). 

If crabs have uniquely adapted to overcome physical limitations affecting speed, what then if anything does decrease fiddler crabs' speed and therefore chances of survival under attack? Given that fiddler crabs are ectotherms — an organism that relies on external processes to control its body temperature —, their survival is threatened by even the smallest of temperature increases (Hews et al., 2021). They are also, therefore, prone to heat stress. With the known rising temperatures in marshland environments, it is especially important to understand how climate change will affect the physiological processes of these temperature delicate species. 

Temperature Effects on Fiddler Crab Physiology and Survival 

 Given that even a small margin of change in environmental temperatures can have drastic implications on crab behavior, it is vital that a standard air temperature range for crab habitats is determined now so researchers can find solutions to combat temperature increases in marshland zones globally. Wilkens and Fingerman evaluated crab behavior at increasing air temperatures and they were able to correlate a decline in fiddler crab survival rates with above average air temperatures (1965). In their experiment, they selected 100 North American fiddler crabs and evaluated them in groups of 10 in a controlled lab setting. There, they exposed the crabs to nine temperature conditions ranging in three degree increments from 21°C to 45°C. Through this method, they determined that the standard air temperature range for fiddler crabs was 25°C–30°C after exposing crabs to a spread of air temperatures and determining their rate of survival (Wilkens & Fingerman, 1965). Specifically, Wilkens and Fingerman demonstrated that as the air temperature rose by 3°C, from 21°C to 39°C, the crab’s rate of survival decreased by 3 percent between each condition. However, at a temperature of 42°C, the crab’s rate of survival sharply declined to 10 percent, representing a 72 percent change in their survival rate (Wilkens & Fingerman, 1965). Therefore, it can be deduced that as average temperatures continue to rise across the globe, fiddler crab populations will dwindle. 

Weinstien and Full conducted an experiment evaluating the effects of ambient temperatures on changes in body temperature in Ocypode quadrata (1994). They also observed how these changes affected locomotor performance: the performance of muscle functions (Peyré-Tartaruga & Coertjens, 2018). While various methods were used to study aerobic capacity, metabolic cost, and endurance of sustained, terrestrial locomotion, each method included 3 experimental groups of crabs with body temperatures of 15°C, 24°C, and 30°C exercising on a treadmill (Weinstein & Full, 1994). For performance measurements taken at rest, crabs were exposed to ambient temperatures of 15°C, 24°C, 30°C, or 35°C in a respirometer within an incubator, and body temperatures were recorded at 5–minute intervals for 60 minutes (Weinstein & Full, 1994). For performance measurements taken while exercising, crabs were exposed to an ambient temperature of 30°C or 35°C in a treadmill chamber, and body temperature was measured in 30 to 60 second intervals for the duration of exercise trials (Weinstein & Full, 1994). 

To better understand the methodology and accuracy of these measurements, it is important to examine how body temperature was measured. A hole was drilled into the carapace of each crab, and a thermocouple — a sensor that measures temperature that consists of two different types of metals joined together at one end — was placed into the drilled hole  (Thermocouple, n.d.). After body temperature was recorded, the crabs were returned to an environmental chamber set at 24°C (Weinstein & Full, 1994). Fiddler crabs were provided with access to 30 percent–50 percent seawater for at least 3 hours before any further experimentation (Weinstein & Full, 1994). At the beginning of the experiment, they measured body temperature during rest and body temperature during exercise. They found that when crabs were exposed to an ambient temperature of 15°C, body temperature decreased and stabilized near 15°C within 45–50 minutes, and body temperature at the beginning of exercise periods was approximately 6°C below the ambient temperature (Weinstein & Full, 1994). These results mean that crabs' internal body temperature is largely influenced by ambient temperatures which has been shown to drastically affect crab behavior and physiology.

Allen et al. observed the effects of internal body temperature on locomotor performance in male Uca pugilator, and they found that increasing internal body temperatures resulted in a decrease in speed (2012). In their study, 25 crabs over the course of eight trials were evaluated — 200 crabs in total — with body temperatures ranging from 20°C to 48°C. Internal body temperature was controlled using a water bath to ensure that the heating was similar to what the crabs experience in the field on warm sunny days (Allen et al., 2012). To evaluate crab speed, they placed crabs on a straight 1.80 m x 0.20 x 0.24 m raceway (Allen et al., 2012). At the beginning of each trial, crabs were released at one end of the raceway and physically chased to the other end (Allen et al., 2012). It is important to note that this form of predator stimulus is not reflective of a true predator–prey interaction in the marshland. Trials were recorded using a digital camera placed above the raceway, and from this, velocity was measured by the time it took the crabs to travel a predetermined distance (Allen et al., 2012). These stimuli were highly variable; however, the data from the experiment still demonstrates consistent correlation between body temperature and speed. Allen et al.found that the fiddler crabs' sprint speed decreased by roughly 20 percent when their internal temperature increased from 25°C–40°C. Additionally, sprint speed dropped to zero once a temperature of 43°C was reached (Allen et al., 2012). Similar to other species, certain physiologic functions at extreme temperatures become impossible for fiddler crabs as heat stress causes their bodies to shut down. Based on this data, it is apparent that as internal body temperatures increase, maximum speed decreases among male Uca pugilator populations. 

The studies conducted by Wilkens, Weinstein and Full, and Allen et al. provide a comprehensive understanding of how fiddler crabs' physiological processes are altered due to climate change, impacting marshland ecosystems (1965; 1994; 2007). These studies show a strong correlation between ambient temperature, internal body temperature of fiddler crabs, and their locomotor performance, highlighting the vulnerability of fiddler crabs to temperature fluctuations and providing further evidence for the susceptibility of ectotherms to changes in environmental temperatures. This decline in locomotor performance due to rising temperatures could lead to increased predation given that fiddler crabs are highly reliant on speed to escape predators and seek safety in their burrows. As climate change causes the temperatures in marshland environments to rise at rates faster than physiologic adaptations can occur, it is likely that we could see a dramatic decrease in the fiddler crab population, posing a direct threat to the balance of marshland ecosystems.

The potential ripple effects of decreased fiddler crab populations as a result of climate change are also profound. Altered migratory patterns and increased algae blooms could disrupt multiple interconnected marshland species within these habitats. The decrease in the fiddler crab population as a result of decreased survival under predation would mean fewer crabs regulating algae causing an increase in algae blooms. Additionally, as fiddler crab speed decreases, the volatility in migratory patterns of crabs could increase. This poses a problem for predators, such as marshland birds, which have timed migrations north for spring and summer. They may face a scarcity of crabs, which are being killed at a higher rate by predators earlier in the season. Consequently, climate change not only causes crabs to move north, but also leads their predators to follow them. This could potentially leave marshlands devoid of the key species that sustain them. Although some knowledge exists, further research is necessary to fully understand the implications of temperature fluctuations on marshland ecosystems and develop strategies to mitigate potential adverse effects.

Proposed Methods to Address Current Knowledge Gaps 

Currently, it is known that crabs use speed as a means of survival. As mentioned above, the effect of increasing body temperatures on speed has been studied in the male North American fiddler crab species Uca pugilator; however, little is known about the direct effects of ambient temperatures on speed, more specifically with females. By studying the impact of ambient temperature increases on fiddler crab speed, a more direct comparison could be made to the effects rising marshland temperatures would have on these crabs. Additionally, comparing the impact of rising temperatures on both male and female North American fiddler crabs would help scientists better understand temperature's effect on not only the species as a whole but also potential effects on reproduction. 

It is important to examine differences in speed between both sexes as significantly different speeds could mean increased predation for the slower sex. This in turn could create a population imbalance leaving fiddler crabs without partners to produce offspring. From previous research conducted, crabs have already been shown to exhibit sex–specific differences in other physiological processes. For instance, female Uca panacea crabs have been found to have a higher feeding rate than males by almost 15 percent (Tina et al., 2016). While on the other hand, on a per claw basis, males showed a significantly faster feeding rate than females by almost 60 percent (Tina et al., 2016). Researchers have also made similar observations among male U. panacea and U. bengali populations showing sexbased biological differences are present across various fiddler crab species (Tina et al., 2016). Thus, even though fiddler crabs may live in the same region, their sex-specific niches create a clear difference in their behaviors, which could mean that heat stress impacts male and female crab physiology differently. 

To address the aforementioned knowledge gap, I will conduct a study to investigate the escape speed of the North American sand fiddler crab, Uca pugilator, in response to above–average air temperatures consisting of both 27°C–29°C and 31°C–33°C with a 15–minute period of acclimation to temperature. Specifically, my study will evaluate the question, to what extent does warming have an impact on the escape speed of male and female crabs in marshland habitats? Based on the information previously presented regarding fiddler crab’s escape speed in response to a predator stimulus under increases in temperature, I hypothesize that the Uca pugilator’s escape speed will decrease as the temperature of the air in its environment increases due to their reliance on thermoregulatory behaviors as ectotherms. 

To evaluate this question, I plan to observe both male and female fiddler crabs under three temperatures, including a negative control temperature that has not been modified from the standard temperature range of the experimental setting. To generate positive control data points, crabs will be exposed to a high, but non–fatal temperature range of 31°C–33°C. This range is ideal because it is known to generate slower travel speeds among male North American Fiddler Crab, so I hypothesize that it will elicit a similar response among female and male North American fiddler crabs (Allen et al., 2012). et al., 2020). While this temperature range has been used in previous experiments evaluating various crab behaviors in response to temperature increase with male North American Fiddler crabs, I hypothesize that it will elicit a similar response in the female population's escape speed following a stimulus under a lower temperature and range. Then, the crabs will be exposed to an experimental temperature range of 27°C–29°C to investigate their escape speed following a stimulus under slightly lower temperatures. This second range was chosen because the impact of these temperatures on escape speed of Uca pugilator has not been studied. In addition, this range mimics the currently predicted increase in marshland temperatures in the northern hemisphere by the year 2100 (Vianna et al., 2020). Therefore, by gathering new data on the North American fiddler crab's speed and reaction time for a temperature range of 27°C–29°C, I will contribute to understanding the impact of warming on marshlands.

As previously mentioned, Allen et al. similarly investigated speed in North American fiddler crabs increased internal body temperatures on the speed of North American fiddler crabs (2012). In my proposed study, a similar setup will be utilized to ensure reliable results. Specifically, I intend to use similar terrarium environments and methods of gathering and evaluating data to create a controlled environment. A key difference is that my study will specifically focus on examining the effects of ambient temperatures on fiddler crab speed by utilizing heat lamps, rather than manipulating internal body temperatures as was done in Allen et al's. experiment (2012). This approach will provide valuable insights into how ambient  temperatures impact crab locomotion. 

Understanding the impact of above-average temperature ranges on the escape speed of North American fiddler crabs, Uca pugilator, is crucial given the limited knowledge on this subject. While my primary aim is to analyze the effects on North American fiddler crabs, findings from this study could also serve as a basis for comparing their responses to those of their South American counterparts. This comparative analysis could further reveal if there are any similarities or differences in how these distinct species respond to increased temperatures. The data gathered from this study, in combination with what is currently known about Northern American fiddler crab's physiologic responses to increased temperatures, can be used to determine realistic solutions in order to conserve this keystone species, and in effect, marshland habitats across both hemispheres. This is crucial because a greater breadth of species–specific data can help enable researchers to identify patterns of behavior exhibited by fiddler crabs in response to climate change from the North American fiddler crab. These observations can in turn be used to predict the consequences of rising temperatures to better protect marshland ecosystems. They can additionally be useful in protecting other keystone species whose physiological response is similarly impacted by the ecological stressors of global warming not only within marshlands, but also in other zones around the world. Climate change and warming in habitats is exacerbated by human activity, and it is important to acknowledge and address our contributions to rising temperatures across the globe. Collectively, this data can inform practices to limit human carbon emissions and protect keystone species to ultimately limit the detrimental impacts of warming worldwide.