Differential effects of water loss and temperature increase on the physiology of fiddler crabs from distinct habitats

Metabolic rates of different demographics in the sand fiddler crab Leptuca pugilator

Studies on animal energetics often focus on standardized metabolic rates to facilitate comparison across systems. Yet these standardized measurements often do not capture the realistic demographic and environmental variation that is common in natural settings. Rather, individuals included in these studies are often non-reproducing, uninjured, resting adults that have not recently eaten-far from a representative sample. We measured the respiratory rates of the sand fiddler crab Leptuca pugilator in air immediately after capture in the field, and compared rates between males, females of different reproductive states, and juveniles. As expected, we show that metabolic rates were influenced by body mass and activity level. We also show that being vitellogenic or gravid had only minor impacts on metabolic costs of females. Importantly, we demonstrate how considering demographics allows for the detection of phenomena that would otherwise go unnoticed. We found that field metabolic rates of L. pugilator in air are as much as an order of magnitude higher than previous standard metabolic rates measured on post-prandial, quiescent individuals. These higher rates may reflect a combination of high activity and active digestion, as fiddler crabs actively feed during low tide periods. Our results highlight the importance of considering differences in sex, life history stage, and reproductive state of organisms in fluctuating environments, such as intertidal habitats, when assessing energy expenditure.

Influence of environmental patterns on the population structure and secondary production of the fiddler crab Uca maracoani (Latreille) in the Amazon mangroves

Uca maracoani is a fiddler crab found in estuaries along the western Atlantic coast, with a notable preference for euhaline environments. This study aimed to analyze the population structure and dynamics of this species in an estuary on the North Coast of Brazil, specifically in an area of the upper estuary where seasonal rainfall fluctuations result in significant changes in salinity. Monthly crab samples were taken from December 2013 to November 2015, together with measurements of environmental variables, such as water and climate parameters. The population maintains a balanced sex ratio; however, males are generally larger, with lower mortality rates and longer lifespans than females. Reproduction is continuous but mainly takes place in the dry season when salinity levels are higher (above 12‰). Higher crab densities have been observed during the rainy season when, despite lower salinity levels (below 10‰), the conditions for survival (food availability and milder climate) seem to be more favorable. The estimated average annual biomass and production for the population were 2.62 g AFDM m-2 and 5.43 g AFDM m-2 year-1, respectively, characterized by a high turnover rate (P/B = 2.10 year-1). Our results suggest that U. maracoani has thriving populations in the Amazon coast's mangroves, benefiting from the vast muddy intertidal zone and the high organic content delivered by the estuaries.

Tolerance and behavioral responses of crabs in disturbed mangroves during a heatwave event

We assessed the tolerance, safety margins, and behavioral responses to extreme conditions of the mangrove fiddler crab Leptuca speciosa during a heatwave event (May of 2022), in the Yucatán Peninsula, Mexico. In the field, L. speciosa demonstrated aggregation behavior, congregating in areas that were above the water level to escape the extreme water conditions. In the laboratory, we determined that the upper critical thermal limits (UT99) ranged from 40.2 °C and 42 °C. For salinity, the lethal concentration was LC99 = 39 psu. Our study showed that L. speciosa, one of the most conspicuous and resilient inhabitants of mangroves, had no safety margin and low tolerance to the climatic conditions as measured in the heatwave, and displayed a protective behavior. Considering that the frequency and intensity of heatwaves have been predicted to increase in the next few years, the combined multiple stressors effect may increase the vulnerability of mangrove organisms.

Sharing is living: The role of habitat heterogeneity in the coexistence of closely related species

In biologically diverse ecosystems, an essential process to support competing species to coexist is ecological differentiation. Habitat heterogeneity is, hence, important in establishing species abundance and richness, favoring the coexistence of species due to habitat partition. In this context, shading and species thermal tolerance can be good factors to elucidate the role of habitat heterogeneity in the habitat partition among closely related species. Herein, we study shading effects in microhabitat selection, behavior, and physiological limitation on two species of fiddler crabs (Leptuca leptodactyla and Leptuca uruguayensis). Indeed, shading conditions influenced fiddler crabs species proportion over time, with L. leptodactyla more associated with nonshaded/warmer areas while the L. uruguayensis to shaded/cooler ones. They also adjusted their behavior differently from each other to deal with thermal stress. Finally, we have demonstrated that these effects are related to species' physiological limitations. We conclude that biologically diverse ecosystems, such as intertidal regions from estuaries (e.g., mudflats and mangroves), support the coexistence between closely related species by reducing competition due to habitat partition.

Can tolerances of multiple stressors and calculated safety margins in fiddler crabs predict responses to extreme environmental conditions resulting from climate change?

To comprehend mangrove crab responses to predicted global climate changes, we assessed submersion and desiccation survival durations and salinity tolerances and upper thermal limits in fiddler crabs from Isla del Carmen, Yucatán Peninsula. Based on their tolerances of extreme ambient conditions, we also calculated safety margins using abiotic monitoring data. The two most terrestrial species, Minuca rapax and Leptuca panacea, exhibited submersion tolerances of from 22 to 40 h, and desiccation tolerances of from 30 to 55 h; LC₅₀'s were ≈45‰S and UT₅₀'s were ≈40 °C. The two least terrestrial species, M. vocator and L. speciosa, were less tolerant of all experimental challenges, showing submersion and desiccation tolerances of <6 h, and LC₅₀'s of 36‰S and UT₅₀'s of 38 °C. While these fiddler crabs inhabit niches closer to their salinity and desiccation/submersion tolerances than to their temperature limits, all are clearly vulnerable to the multiple stressors that accompany anticipated global climate change.

Losers can win: Thermoregulatory advantages of regenerated claws of fiddler crab males for establishment in warmer microhabitats

Fiddler crab males present a hypertrophied claw, which is used for sexual and aggressive displays, fights with competitors, and has been proposed as an important thermoregulatory organ for heat control. Two claw morphologies can be observed within fiddler crab populations: brachychelous claws (unregenerated) and leptochelous claws (regenerated). The leptochelous morphotype presents less muscle mass and longer fingers, resulting in a less advantageous weapon in fights. Considering their slender shape, we hypothesized that the leptochelous morphotype would present lower thermal inertia and be more efficient at body heat dissipation. We evaluated the role of the fiddler crabs' claw shape as a heat sink and how this influences their distribution between unshaded and shaded microhabitats. We tested in the field whether the proportion of adult male Leptuca uruguayensis with leptochelous claws was higher in unshaded microhabitats than shaded ones. In the laboratory, we tested if heat transfer between the body and claw is higher in leptochelous males than in brachychelous males. Spontaneous waving behaviour and active time above the surface were compared between both morphotypes in the field during the hottest period of the day. Leptuca uruguayensis with regenerated claws comprised more than 60% of the sampled male population of unshaded microhabitats compared to 18% in shaded microhabitats. Leptochelous males showed a mean heat transfer between body to claw 35% higher than that observed for brachychelous males. During the hottest period of the day, brachychelous males waved approximately 28% more than leptochelous males. Moreover, brachychelous males spent less time under the sediment surface since the surface temperature increased, while activity of leptochelous males was not associated with a temperature increase. Therefore, regenerated claws may be advantageous for the establishment of L. uruguayensis males in warmer and unshaded microhabitats because they are more efficient for heat loss and allow crabs to cool down, spending less time waving. Our study shows the relevant context of winners and losers in the face of climate change and highlights the importance of morphological variations in thermoregulatory structures for the occupation of thermal niches.

Towards more integration of physiology, dispersal and land-use change to understand the responses of species to climate change

The accelerating biodiversity crisis, for which climate change has become an important driver, urges the scientific community for answers to the question of whether and how species are capable of responding successfully to rapidly changing climatic conditions. For a better understanding and more realistic predictions of species' and biodiversity responses, the consideration of extrinsic (i.e. environment-related) and intrinsic (i.e. organism-related) factors is important, among which four appear to be particularly crucial: climate change and land-use change, as extrinsic factors, as well as physiology and dispersal capacity, as intrinsic factors. Here, I argue that these four factors should be considered in an integrative way, but that the scientific community has not yet been very successful in doing so. A quantitative literature review revealed a generally low level of integration within global change biology, with a pronounced gap especially between the field of physiology and other (sub)disciplines. After a discussion of potential reasons for this unfortunate lack of integration, some of which may relate to key deficits e.g. in the reward and incentive systems of academia, I suggest a few ideas that might help to overcome some of the barriers between separated research communities. Furthermore, I list several examples for promising research along the integration frontier, after which I outline some research questions that could become relevant if one is to push the boundary of integration among disciplines, of data and methods, and across scales even further - for a better understanding and more reliable predictions of species and biodiversity in a world of global change.

Tropical saltmarshes are important to juvenile fiddler crabs but not as refuges from large predators or high temperatures

Tropical saltmarshes' role as a refuge for macrofauna is poorly known. We tested the hypotheses that: i) there is an increase in abundance of early life-stages of fiddler crabs associated with an increase in vegetation density (thermal and/or predation refuge) and ii) predation are lower within saltmarsh vegetation. The abundance of early life-stages increased with both vegetation density and temperature. Megalopa and juveniles might be adapted or benefit from high temperatures found on tropical saltmarshes and thus do not need protection from it. Predation was higher in saltmarshes than in nearby non-vegetated areas, thus tropical saltmarshes can not be considered a refuge from relatively large predators for juveniles of fiddler crabs. Although not offering refuge against temperature and predation, tropical saltmarshes are important habitats for the recruitment of early life-stages of fiddler crabs. Future studies should consider the evaluation of tropical saltmarsh hydrodynamics in the settlement and permanence of juveniles.

Temperature-related heart rate in water and air and a comparison to other temperature-related measures of performance in the fiddler crab Leptuca pugilator (Bosc 1802)

Performance in poikilotherms is known to be sensitive to temperature, often with a low-sloping increase with temperature to a peak, and a steep decline with increasing temperature past the peak. We complemented past measures of performance by measuring heartbeat rates of the fiddler crab Leptuca pugilator in water and in air as a function of a range of temperatures previously shown to affect other measures of performance. In water over a range of 20-50 °C, heartbeat increased steadily to a peak at 40 °C and then steeply declined to near zero at 50 °C. In air, heartbeat also increased, but to a peak at 35 °C and then with a gentler decline than was found in water. Part of this different response may be due to evaporative water loss, which reduced body temperature in air, and therefore thermal stress, relative to body temperature when crabs were immersed in water. Increased availability of oxygen from air, according to the oxygen and capacity-limited thermal tolerance hypothesis, likely increased aerobic scope past the thermal peak, relative to within water, where oxygen delivery at higher temperatures may have been curtailed. We compared the heart rate performance relations to two previous measures of performance - endurance on a treadmill and sprint speed, both done in air. The peak performance temperature increased in the order: treadmill endurance time, sprint speed, heart rate in air, and heart rate in water, which demonstrates that different performance measures give different perspectives on the relation of thermal tolerance and fitness to temperature. Endurance may therefore be the limiting upper thermal stress factor in male fiddler crabs, when on hot sand flats. Temperature preference, found to be for temperatures <30 °C in air, could be a bet-hedging evolutionary strategy to avoid aerobic scope affecting endurance.

Effects of temperature increase on the physiology and behavior of fiddler crabs

Intertidal organisms rely on physiological and behavioral adjustments to maintain homeostasis under warm exposure. We examined the effects of the temperature increase related to climate warming on the physiology and behavior of two fiddler crab species: Leptuca uruguayensis, which inhabits mostly vegetated areas, and Leptuca leptodactyla, that inhabits unvegetated areas. We hypothesized that L. uruguayensis would be more sensitive to warming than L. leptodactyla. Crabs were exposed to different temperatures: 27 °C (control), 31 and 33 °C (+4 and +6 °C, respectively) for 15 days to evaluate their physiological responses (oxygen consumption, Q₁₀, ammonia excretion and hepatosomatic index) and for 2 days to observe their behavioral responses (feeding rate and duration of burrow retreat). We also tested in situ the effect of air, surface, and body temperatures on the claw-waving display of both species. We found that species were affected differently by increasing temperature. Leptuca uruguayensis showed adaptation limit (Q₁₀ <1), increasing oxygen consumption. Also, in comparison with control, L. uruguayensis decreased the ammonia excretion at 31 °C, but not at 33 °C, indicating a compensatory mechanism to cope with thermal stress. In contrast, L. leptodactyla was able to adjust its metabolic rate to temperature rise (Q₁₀ ~3) and reduce ammonia excretion, suggesting changes in the energetic substrate and amino acid catabolism. Higher temperatures reduce the hepatosomatic index of both species, indicating increased use of energy reserve (although only the L. uruguayensis feeding rate was reduced). Furthermore, warmer temperatures increase the duration of burrow retreat in both species, potentially impacting social interactions, such as mating. Temperature increase did not affect the claw-waving display frequency, suggesting that other factors may affect this behavior, e.g., the presence of females and competitors. Specific behavioral (e.g., microhabitat selection) and morphological attributes (e.g., larger major claw) might benefit the thermoregulation of each crab species since no differences in body temperature were found between them in situ. Therefore, fiddler crabs that inhabit vegetated areas are more vulnerable to higher temperatures and may change its geographic range as a result of climate warming, while fiddler crabs that inhabit unvegetated areas are more tolerant to temperature rise and may have a competitive advantage under a temperature increase scenario.

Point-of-care testing for measuring haemolymph glucose in invertebrates is not a valid method

Blood glucose is widely used as a physiological parameter for vertebrates and invertebrates. However, its measurement in the field is often difficult due to the need for expensive and non-portable equipment. Point-of-care (POC) devices, originally intended for human use, are increasingly being used for measuring blood parameters of animals in the field. In this regard, POC glucose meters are becoming valuable tools for conservation physiologists, as glucose can be a useful indicator of stress response. In invertebrates, the use of POC glucose meters is still scarce, and no study yet has evaluated their usability in crustaceans and molluscs. We tested if a POC device can be used to measure haemolymph glucose in two widely used models, Leptuca thayeri and Perna perna, compared with a standard laboratory method. The device was unable to measure glucose in P. perna haemolymph due to equipment inaccuracy and low glucose concentration in this species (10.13 ± 6.25 mg/dL). Additionally, despite the device being capable of measuring glucose in L. thayeri haemolymph, Bland-Altman plots showed a strong bias and wide limits of agreement, and Lin's concordance correlation coefficient showed a weak concordance between methods. When simulating experimental conditions, POC results differed from those found using the standard method. We conclude that POC glucose meters are unsuitable for assessing glucose in mussels and should not be used in crabs as results are inaccurate.