https://orcid.org/0000-0002-8126-6323
Figures
Abstract
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.
Citation: Stancil C, Smith N, Fletcher LS, Anderson L III, Griffen BD (2024) Metabolic rates of different demographics in the sand fiddler crab Leptuca pugilator. PLoS ONE 19(10): e0308617. https://doi.org/10.1371/journal.pone.0308617
Editor: Khor Waiho, Universiti Malaysia Terengganu, MALAYSIA
Received: January 19, 2024; Accepted: July 27, 2024; Published: October 28, 2024
Copyright: © 2024 Stancil et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper and its Supporting information files.
Funding: This work was funded by a grant from the Roger and Victoria Sant Educational Endowment for a Sustainable Environment.
Competing interests: The authors have declared that no competing interests exist.
Introduction
The rate at which an organism uses energy provides insight into the connection between energy intake via consumption and the expenditure of energy to fuel essential biological processes. Because consumers influence energy flow by recirculating matter through ecosystems, understanding their metabolic rates is crucial to explaining ecosystem function [1]. An organism’s growth and reproduction are constrained by its energy budget; the amount of energy available for egg production depends on the amount consumed and the amount allocated to metabolic functions. Energy budgets and the study of energetics in general are valuable tools that have been used across a wide range of ecological settings and questions, to determine growth and reproductive costs [2], to understand breeding ecology and nesting [3], to explain maternal investment and sex allocation [4], and to predict physiological ecology and life histories [5]. The energetics of individual organisms are not constant, but vary with the dynamics of the ecosystems in which they live, as well as with their own individual state. The impacts of dynamic habitats are particularly apparent in ecosystems, such as marine intertidal habitats, that experience drastic daily shifts in physical conditions with the rising and falling of the tide.
Several general conceptual arguments suggest that an organism’s metabolic rate may differ in and out of water. For instance, during aerial exposure intertidal organisms experience broader temperature fluctuations, which greatly impacts metabolic rates of poikilotherms that predominate in intertidal habitats [6]. Additionally, intertidal organisms often display behaviors that correlate with tidal phase, such as increased foraging activity during high tide compared to low tide [7], that can strongly influence metabolic costs [8, 9]. These tidally-dependent feeding patterns not only influence metabolic costs by changing activity levels, but also alter temporal patterns in metabolic rates due to energetic costs of digestion (i.e., specific dynamic action) [10]. For intertidal species that are most active during low tide, assessing metabolic needs during low tide periods may provide a very different picture of energetics than in-water measurements that are more common in the literature.
Metabolic rate can also vary with several factors that are specific to the individual organism, such as body size [11, 12], activity level [13], injury [14], sex [15], or body condition [16]. An individual’s developmental stage and sex can also be expected to affect metabolic rate. In many species, juveniles allocate a large amount of their energy toward growth, to decrease the likelihood of being predated upon, to reach reproductive maturity more quickly, or to be able to better tolerate harsh environmental conditions [12, 17–19]. Once mature, their energy is divided between reproduction, continued growth (for some), maintenance, and other activities based on species-specific life history [20]. Differences in energy allocation with life history stage may also affect an organism’s contribution to overall ecosystem processes, further shaping their community. For instance, fish that grow to smaller body size experience higher individual turnover due to increased predation risk, and are thus responsible for a large amount of energy flux, relative to larger-bodied species, within coral reef communities [21].
Salt marsh systems are intertidal habitats that cover extensive coastal area throughout much of the southeastern part of the United States. The proportion of time that regions of the marsh are inundated vary widely with marsh elevation, ranging from <1% annually (in upper marsh regions) to >70% annually (in lower marsh regions) [22]. Previous work on salt marsh inhabitants suggests a significant difference between aerobic and aquatic energetics [23]. However, the energetic differences between demographic segments of populations has received little attention. Instead, energetics in marsh systems have often focused on populations through measuring energy use by ‘average’ individuals and then scaling this up based on the number of individuals in the population (e.g., [24, 25]). While it is important to look at overall energetics of a population, ignoring demographic differences between individuals can lead to an incomplete picture at the population level, and certainly fails to capture broadly representative energy dynamics at the individual level.
Fiddler crabs are intertidal decapods commonly found in salt marsh habitats around the world. The numerically abundant decapod in the salt marshes of southeastern North America is the sand fiddler crab, Leptuca pugilator (Bosc 1802; Crustacea, Ocypodidae; previously Uca pugilator). The geographic range of L. pugilator spans from Alabama to Florida in the Gulf of Mexico, and from Florida to Massachusetts on the Atlantic coast [26]. As with other species, demographic information such as sex and size have been consistently overlooked in examining the energetics of this species [23, 27–29]. Populations of this species are most abundant in upper marsh habitats where they spend the majority of their time exposed to aerial, low tide conditions. We used an in-situ closed system respirometry method [30] to measure aerial respiration rates of male and female L. pugilator of a range of sizes. Our measurements allowed us to examine how metabolic rate varied with temperature, body size, sex, reproductive state (vitellogenic and gravid), developmental stage, activity level, and limb loss and regrowth. We test the hypothesis that each of these factors influences the in-air energy expenditure of L. pugilator during low tide periods.
Methods
Preparation and collection
Between July 12 and July 15, 2021, we collected crabs each morning during low tides from salt marshes within Hobcaw Barony in Georgetown, South Carolina, USA (33°20’5.61"N 79°11’36.34"W). We collected 274 crabs by hand and from burrows using trowels (since gravid females do not occur on the marsh surface). Of the 274 crabs, 158 were female (57.7%), 89 were male (32.5%), and 27 were juveniles (9.9%). All crabs were immediately transported to a screened-in outdoor lab for the Baruch Institute for Marine and Coastal Sciences run by the University of South Carolina. Crabs were then maintained in containers with lids (i.e., in shade conditions) with approximately 1 cm of water to maintain moist conditions while they waited to be measured. Crabs were measured in blocks of 10 at a time, and processing all individuals took an average of 4.5 hours each day. Following methods of Fletcher et al. [31], each was placed in a separate metabolic chamber made from clear 150-mL polypropylene syringes (length 14.9 cm, diameter 4.4 cm), the tips of which had been plugged with silicone. Each chamber also had an 8-mm diameter hole drilled into the end that served as a port for extraction of air samples (see Fig 1).
A) Plunger, B) barrel with crab inside, C) port that was covered by septa during trial to create a sealed chamber and through which the gas sample was extracted at the end of a trial, D) tip of chamber sealed with silicone.
Depending on the size of the crab, we adjusted the chamber volume of the syringe, and thus the volume of air. The volume used was determined in preliminary trials and ranged from 30 ml for the smallest crabs to 120 ml for the largest crabs. This approach ensured that volumes were sufficiently small to ensure measurable changes during a trial, but sufficiently large to prevent hypoxic conditions due to oxygen depletion. Crabs were then left undisturbed for 5 minutes to adjust to the experimental chambers. While we made efforts not to disturb crabs during this adjustment period and subsequent measurement periods, handling has been shown in other crab species to elevate metabolic rates for several hours [32]. The extent of similar impacts in L. pugilator are not known. However, given that all crabs in our study were treated in an identical manner, these disturbance effects do not compromise our ability to make comparisons between crabs within our study. During this adjustment period, we measured the air temperature, relative humidity, and barometric pressure using a BTMeter (model 100-AAP).
Trial procedure
Trial procedures followed those described by Fletcher et al. [31]. A trial was initiated by placing a septum (Bridge Analyzers, Inc.), designed for use in headspace analysis, over the sampling port and recording the time the chamber was sealed. The septa prevented any gas exchange between the chamber and environment, and also provided a surface that could be pierced by the gas probe’s needle at the end of the trials.
Throughout the duration of a trial, we observed the activity of each crab using visual scans at one-minute intervals. A crab was considered active if it was moving at all within the experimental chamber, and all movements were considered to be equal. During each scan, we recorded whether or not a crab was active. At the end of the trial, we summed the number of active observations to create an activity score and then used the trial duration to calculate the proportion of time active.
We varied the duration of the trial for individual crabs (20–90 minutes, longer for smaller crabs) so that, in combination with chamber volume, changes in oxygen concentration were detectable, but sufficiently minimal to avoid hypoxic conditions. The use of different chamber volume and trial duration for each crab did not affect our estimate of oxygen consumption, since these were explicitly included in the calculation of metabolic rate for each crab, as described below. Oxygen levels in experimental chambers always remained well above levels that cause anoxic conditions for air-breathing crustaceans [33]. Specifically, the minimum final O2 level encountered was 18.95%, with mean final levels at 20.04% ± 0.30%. Thus, the decrease was small compared to saturated atmospheric levels of 20.94%. Similarly, CO2 levels in experimental chambers always remained well below levels that cause hypercapnia [34]. Specifically, the maximum final CO2 level encountered was 0.99%, with mean final levels at 0.41% ± 0.16% (i.e., atmospheric levels 0.04%).
Each crab was used only once in this study. At the completion of a trial, we noted the time (for trial duration) and measured the final oxygen content of the chamber by inserting a needle through the port and extracting a gas sample using a multi-gas oxygen probe (Forensics Detectors™, model # FD-600, 0.01% resolution) with built-in pump (0.5 L/min). Crabs were then placed into individually-numbered sample bags and immediately frozen. Crabs were shipped on dry ice to Brigham Young University in Provo, UT and stored at -80°C until dissection.
Dissection
We thawed the crabs to room temperature by submerging in room-temperature water. We then measured the carapace width, counted the number of missing limbs, and the number that were regenerating based on the presence of limb buds. Female crabs were inspected for evidence of eggs and were recorded as gravid or non-gravid. Female crabs were additionally dissected to determine whether they were vitellogenic. We did this by removing the ovary using dorsal dissection and examining it under a dissecting microscope to determine the presence of eggs. Crabs were determined to be “juveniles” if they were lacking external reproductive morphology and internal gonad tissue during dissection. To ensure sufficient sample size for analyses of juveniles, male and female juveniles were pooled together. We then dried crabs individually to constant weight at 60°C and the final dry weight was measured using a Mettler Toledo scale (DualRange model XS205).
Calculations
We calculated metabolic rate for each crab individually using Equation 4.4 from [30]:
where VolO2 is the volume of oxygen consumed by the crab, V is the volume of the test chamber, FiO2 is the initial fractional oxygen concentration, FeO2 is the final fractional oxygen concentration, VolH2O is the volume of water vapor in the chamber, and RQ is the respiratory quotient.
To find V, the volume of air in the syringe chamber, we subtracted the crab’s volume from the recorded volume of the chamber. We determined crab volume by dividing the dry weight by 0.26, which is the empirically-determined factor used for crustaceans to convert dry weight to wet weight [35]. The volume is the wet weight divided by the density. Because crab density was an unknown variable, we used 1.1 g/cm3 for our equations (crabs sink in water, which has a density of 1.0 g/cm3), following Fletcher et al. [31]. These methods introduce uncertainty to our estimates of chamber volume; however, the degree of error remained low due to the small ratio of crab size to chamber volume.
FiO2 and FeO2, the initial fractional concentration of oxygen (i.e., 0.2094, the atmospheric concentration) and the fractional concentration at the end of the trial, were both measured at the time of the trials using the multi-gas oxygen probe as described above. We included a small amount of ambient temperature water (~0.5 ml) in each chamber during each trial so that water vapor was saturated [30]. We then determined the water vapor pressure using the relative humidity at the start of each trial and the mean temperature during each trial (measured at 10-min intervals throughout) and employing the water vapor pressure calculator found at respirometry.org. We used this estimate of water vapor pressure to calculate the VolH20 as outlined in [30].
Finally, we set the respiratory quotient (RQ) to 0.85. Without knowing the exact ratio of protein, carbohydrates, and lipids in the diet, a value of 0.85 yields the lowest possible error from over- or underestimation of the RQ, with a maximum error of 3% [34]. However, our error was likely much lower than this because, as a deposit feeder, L. pugilator omnivorously consumes a combination of epipelic algae and meiofauna [35], and thus the assumed RQ of 0.85 is likely close to accurate.
After obtaining VolO2, we used Equation 2.1 from [30] to correct the metabolic rates to standard temperature and pressure using the average temperatures (T) and barometric pressures (BP) that we measured during each trial. Finally, we divided standardized VolO2 by the duration (minutes) of the trial for each crab and multiplied by 60 to convert oxygen consumption to a rate: volume oxygen consumed per hour.
Statistical methods
Using R v.4.1.2, we used linear models to analyze data from male, female, and juvenile crabs separately. The response variable in each was the corrected mL O2 consumed per hour. Our predictor variables in the full model were dry mass (grams), proportion of time active during the trial, number of limbs missing, number of limbs regenerating, average ambient temperature during a trial (°C), as well as whether crabs were gravid or vitellogenic (for females). We first fit a full model that included all these predictor variables. We did not include any interaction terms because of the large number of predictor variables and because we had no a priori expectations about which interactions might be biologically or ecologically important. We then used the step function in the base package of R to determine the best-fitting model based on AIC [36].
In addition, for each demographic separately, we also used linear regression to determine whether there was a correlation between the ambient temperature and the time that crabs were active. The results of these analyses were used to further understand the results of the main analyses described above.
Results
We measured oxygen consumption of 274 crabs total, including 158 adult females, 89 adult males, and 27 juveniles (sexes combined). Of these, 26 (16.5%) females were missing limbs (61 limbs missing total), with 21 of these missing limbs in the process of regenerating. Similarly, 7 (7.9%) males were missing limbs (36 limbs missing total), with 12 of these regenerating. Just 2 (7.4%) juveniles were missing limbs and 1 of these was regenerating. Most missing limbs were walking legs, with only 5 females missing a claw (3 regenerating) and 5 males missing a claw, 3 of which were major claws (4 regenerating). Of the females, 40 were gravid and 46 were vitellogenic. These were not mutually exclusive, as four females were both gravid and vitellogenic.
Overall, for males, females, and juveniles, oxygen consumption was influenced strongly by body size and was also influenced to varying degrees by activity level during the trials, by reproductive state, by injury, and by temperature during the trials. Below we describe these impacts for each of these groups individually.
The oxygen consumption of male crabs increased by 1.41 ± 0.07 ml O2 per hour for every 1-gram increase in dry crab body mass (t = 20.26, p < 0.0001, Fig 2). The oxygen consumption of male crabs increased by 0.55 ± 0.13 ml O2 per hour as the proportion of time active during a trial increased (t = 4.35, p < 0.0001, Fig 2). Male crab oxygen consumption also increased by 0.15 ± 0.07 ml O2 per hour for each additional limb that was regenerating (t = 2.09, p = 0.04, Fig 2). The oxygen consumption of male crabs decreased slightly by -0.08 ± 0.01 ml O2 per hour for each 1°C increase in temperature (t = -10.59, p < 0.0001, Fig 2). No other variables were included in the best-fitting model.
Symbols show number of limbs regenerating, where circle = no limbs regenerating; triangles = 1 limbs regenerating; plus = 2 limbs regenerating. Symbol size shows relative proportion of time active during the trial, where the smallest symbol is complete inactivity and the largest symbols are active throughout the trial. Symbol color shows the temperature during the trial, following the scale given on the top of the figure.
The oxygen consumption of female crabs increased by 2.23 ± 0.13 ml O2 per hour for every 1-gram increase in dry crab body mass (t = 16.77, p < 0.0001, Fig 3). Female crabs that were gravid had rates of oxygen consumption that were 0.15 ± 0.05 ml O2 per hour lower than females that were not gravid (t = -3.07, p = 0.003, Fig 3). Oxygen consumption also increased by 0.23 ± 0.10 ml O2 per hour as the proportion of time that crabs spent active increased (t = 2.36, P = 0.02, Fig 3), and decreased by 0.05 ± 0.01 ml O2 per hour for each 1°C increase in average temperature (t = -8.50, P < 0.0001). No other variables were included in the best-fitting model.
Circles show females that are not gravid, while triangles show females that are gravid. Symbol size and color are as described in the caption for Fig 2.
The oxygen consumption of juvenile crabs increased by 2.87 ± 0.33 ml O2 per hour for every 1-gram increase in dry crab body mass (t = 8.79, p < 0.0001, Fig 4). Oxygen consumption also increased by 0.31 ± 0.07 ml O2 per hour with an increase in the proportion of time that crabs were active during the trials (t = 4.19, p = <0.0003, Fig 4). No other variables were included in the best-fitting model.
Circles show crabs with no limbs missing, while triangles show crabs with a single limb missing. Symbol size and color are as described in the caption for Fig 2.
We found that differences in activity level were weakly correlated (R2 = 0.02) with ambient temperature for adult females (t = 2.06, P = 0.04), while there was no change in activity level of adult males (t = -0.38, P = 0.7) or juveniles (t = -0.25, P = 0.81) with changes in average temperature.
Discussion
As expected, the oxygen consumption rates for male, female, and juvenile L. pugilator were influenced by body mass and activity level. Though these differences should be expected, many studies of L. pugilator fail to include basic information on the size of study individuals. It follows that groups with various age, size, or sex ratios could expect to see differences in their overall oxygen consumption rates. We also found that oxygen consumption rates were influenced by the number of limbs missing (in juveniles only) and regenerating (in males only). Differences here were possibly due to vagaries driven by the relatively small numbers of individuals in each demographic that were missing or regenerating limbs. In female crabs, being gravid had a minor impact, while being vitellogenic had no impact. Counterintuitively, the oxygen consumption rate of adult males and females decreased with temperature, though this effect was small relative to other factors and was driven by a small number of data points where crabs were completely inactive (and therefore had low metabolism), but where ambient conditions were warm.
Our findings show that as the size of fiddler crabs increases, their oxygen consumption rates increase; however, the increase in oxygen consumption rates was much more dramatic with increases in body size for females and juveniles, compared to males. This suggests that males pay a lower cost for being large than that paid by females. The primary morphological difference between the sexes is the presence of a single enlarged claw in males. This strong sexual dimorphism in fiddler crabs has spurred previous studies to investigate the metabolic cost of the large male claw. Allen and Levinton [37] found that the presence of the claw increased metabolic costs for individuals by ~8% compared to declawed individuals. In contrast, previous work reported no significant differences in oxygen consumption between declawed and clawed male L. pugilator during rest or activity [38]. The actual metabolic cost of the single large claw in males is therefore not clear. Our findings suggest that the relatively low cost of greater body size in males compared to females may potentially compensate for any additional cost paid by the male associated with this enlarged claw.
In previous experiments [38], male L. pugilator had average resting in-air oxygen consumption rates of approximately 0.2–0.3 mL O2/g/hr [38]. A previous study examining the aerial oxygen consumption rate of L. pugilator found that oxygen consumption rates on land were 0.061 mL O2/g/hr [23]. Similar rates were found for male L. pugilator by Full and Herreid, who report a resting oxygen consumption rate of 0.049 ± 0.010 mL O2/g/hr [39]. However, oxygen consumption rates measured on all groups here were considerably higher than those reported in these previous studies. Several likely explanations explain this difference. First, these previous studies reported oxygen consumption per unit wet body mass, whereas we report rates per unit dry body mass. We did not measure wet body mass prior to drying animals and so cannot make direct comparisons. However, for decapods, dry body mass is on average 0.25 the wet body mass [35]. Thus, we would expect our rates to be 4-fold higher than these previously reported rates simply because of the use of dry vs. wet body masses.
Second, as mentioned above, oxygen consumption rates reported here were likely elevated due to stress from handling of experimental animals. Metabolism after handling can remain elevated in crabs for several hours [32], and thus surely had an impact on our study, though the magnitude of this impact is unknown. However, all individuals in our study were treated similarly, and these procedures should therefore not have influenced the relative oxygen consumption rates between demographic groups, or the impacts of reproduction and injury reported here. However, the absolute rates we report were almost certainly elevated by our handling procedures, likely explaining some of the difference between rates reported here and previously reported rates.
Third, our individuals were not in a resting state, but were active in the experimental chambers (65% of time on average, range 4–100%). Relatively few studies have measured metabolic scope in crabs, but a study on blue crabs (Callinectes sapidus) found that active oxygen consumption rates during vigorous exercise were approximately 5 times higher than resting rates [40]. A similar study on exercising ghost crabs (Ocypode quadrata) found oxygen consumption rates that were approximately 3.5 times higher than resting rates [39]. Assuming that the difference between active and resting rates are similar for L. pugilator, this likely explains a large amount of the high oxygen consumption that we measured. This is supported by the two previously mentioned studies of male L. pugilator that found oxygen consumption to increase up to 4.4 times [41] and 4–5 times [38] when crabs were exercised on a treadmill. Although our subjects were not being intentionally exercised, their increased activity, and therefore increased oxygen consumption, could perhaps be explained by the initial stress of collection and further handling.
Fourth, our crabs were captured in the field during the day when fiddler crabs are often actively feeding. Animals that have recently eaten have increased metabolism that represents the cost of digestion (i.e., specific dynamic action). Metabolism generally spikes for a few hours after eating, and then gradually declines as the remaining undigested food decreases. Studies in other crab species show that oxygen uptake spikes for about 5 hours after feeding, before it gradually declines again over the next 20 hours [10, 42], and that metabolism is about 3 times higher after food consumption compared to starved crabs [43]. Post hoc visual inspection of dissected dried crabs in our study revealed that 65% of them visibly had food in their cardiac stomach, which would have affected their oxygen consumption rates.
Fifth, the environmental temperature during our trials was highly variable, ranging from 16.9°C to 32.7°C. Increased temperatures have been shown to both increase and decrease the oxygen consumption rate of Leptuca species [29], and have also been linked to higher lactate dehydrogenase which may indicate increased energetic demand [44]. In our trials, temperature had a relatively modest effect by reducing oxygen consumption rates slightly in adult males and females. The reason for the decrease in metabolism with temperature is unclear, though similar results have been noted before in this species [29]. These authors hypothesized that metabolism may be depressed for individuals that are near their thermal maximum, which is 39.5°C [45]. However, this was unlikely the cause, as maximum temperatures during our metabolism trials was 32.7°C. It is also unclear how the temperatures we recorded relate to marsh surface temperatures experienced at our site by crabs during summer periods where crabs use behavioral thermoregulation via burrow use to modulate body temperatures [46].
Thus, several factors may be responsible for the relatively high absolute oxygen consumption rates we report here, and these rates may be more similar to ’field metabolic rates’ (i.e., metabolic rates expected of animals during normal day-to-day activity, digesting, etc.) as opposed to resting or standard metabolic rates when animals are inactive and not eating. Overall, while it is not clear how to interpret the absolute values we present, the relative values may be clearly compared across demographics given the similar methods used with all crabs in our study.
Our study fills several large knowledge gaps in our understanding of metabolism in L. pugilator. We looked at several nuanced situations that had not previously been examined in other studies, including maturity, reproductive state, missing limbs, regenerating limbs, and sex. According to our findings, being gravid did not increase the oxygen consumption rates of female crabs. Counterintuitively, our results showed a very slight decrease in the oxygen consumption rates of gravid females when compared to non-gravid individuals. This difference was statistically significant, but may have limited biological importance. These results suggest that for L. pugilator, carrying a clutch of eggs may not require large amounts of energy. However, it is possible that these low costs of egg carrying could reflect energy reallocation from other processes, such as growth and maintenance [47–49]. Gravid females were extracted from burrows where they generally remain while they are gravid [50], during which time they are presumably relatively inactive.
Interestingly, being vitellogenic did not appear to be metabolically expensive in L. pugilator. A possible explanation is that females are using lipids already stored in the hepatopancreas, rather than producing new lipids. Known patterns of lipid storage in other crab species are consistent with this explanation. Alava et al. [51] demonstrated in the mud crab Scylla serrata, that ovary lipid content peaked during late vitellogenesis and corresponded with a decline in hepatopancreas lipid content. It is possible that L. pugilator similarly mobilizes stored lipids from the hepatopancreas to produce their ova. In this case, the metabolic cost of vitellogenesis may have largely been paid previously during the process of lipid synthesis in the hepatopancreas. Further research is necessary to examine this hypothesis and to examine the cost of lipid mobilization in crabs.
Although previous work examines L. pugilator limb regrowth in different salinities and chemical stressors [52], and the effect of losing the major chela on heart rate [53], no study has previously looked at the metabolic cost of missing limbs, including walking legs. We found that adult male crabs experienced an increase in oxygen consumption rate with the number of limbs regenerating (juveniles had an increase in metabolism with the number of limbs missing, but this reflected just two individual animals and should not be overintepreted). This increased cost during limb regeneration may reflect the energy of protein synthesis for replacing lost limbs [54], and would likely be compounded with additional associated costs of limb loss, including increased energy requirements related to moving, feeding, mating, predator avoidance, etc. that were not measured in this study.
In summary, our results provide an important comparison between oxygen consumption rates of male, female, and juvenile L. pugilator under similar field conditions, and simultaneously examine other individual physiological states that may influence metabolism, including reproductive state and nonlethal injury. Our field methods can be easily adapted to measure low tide oxygen consumption rates in other marsh invertebrates in the field immediately following collection. Our results highlight the importance of considering individual variation when attempting to draw conclusions about energy expenditure of the larger population.
Supporting information
S1 Data. This file contains all data used for the analysis in the current paper.
https://doi.org/10.1371/journal.pone.0308617.s001
(CSV)
Acknowledgments
We thank the Baruch Institute for Marine and Coastal Sciences at the University of South Carolina for use of their field station. This work was funded by a grant from the Roger and Victoria Sant Educational Endowment for a Sustainable Environment.
References
- 1. Loreau M. Consumers as maximizers of matter and energy flow in ecosystems. The American Naturalist. 1995 Jan 1;145(1):22–42.
- 2. Pecquerie L, Petitgas P, Kooijman SA. Modeling fish growth and reproduction in the context of the Dynamic Energy Budget theory to predict environmental impact on anchovy spawning duration. Journal of Sea Research. 2009 Aug 1;62(2–3):93–105.
- 3. Ruxton GD, Houston DC. Modelling the energy budget of a colonial bird of prey, the Ruppell’s griffon vulture, and consequences for its breeding ecology. African Journal of Ecology. 2002 Sep;40(3):260–6.
- 4. Rutkowska J, Koskela E, Mappes T, Speakman JR. A trade-off between current and future sex allocation revealed by maternal energy budget in a small mammal. Proceedings of the Royal Society B: Biological Sciences. 2011 Oct 7;278(1720):2962–9. pmid:21325329
- 5. Desforges JP, Marques GM, Beumer LT, Chimienti M, Blake J, Rowell JE, et al. Quantification of the full lifecycle bioenergetics of a large mammal in the high Arctic. Ecological Modelling. 2019 Jun 1;401:27–39.
- 6. Barnes H, Barnes M. Seasonal changes in the acutely determined oxygen consumption and effect of temperature for three common cirripedes, Balanus balanoides (L.), B. balanus (L.) and Chthamalus stellatus (Poli). Journal of Experimental Marine Biology and Ecology. 1969 Dec 1;4(1):36–50.
- 7. Silva AC, Boaventura DM, Thompson RC, Hawkins SJ. Spatial and temporal patterns of subtidal and intertidal crabs excursions. Journal of Sea Research. 2014 Jan 1;85:343–8.
- 8. Brown FA Jr, Bennett MF, Marguerite Webb H. Persistent daily and tidal rhythms of O2‐consumption in fiddler crabs. Journal of Cellular and Comparative Physiology. 1954 Dec;44(3):477–505. pmid:13233297
- 9. Bas C, Lancia JP, Luppi T, Méndez‐Casariego A, Kittlein M, Spivak E. Influence of tidal regime, diurnal phase, habitat and season on feeding of an intertidal crab. Marine ecology. 2014 Sep;35(3):319–31.
- 10. McGaw IJ, Whiteley NM. Effects of acclimation and acute temperature change on specific dynamic action and gastric processing in the green shore crab, Carcinus maenas. Journal of Thermal Biology. 2012 Dec 1;37(8):570–8.
- 11. Robinson WR, Peters RH, Zimmermann J. The effects of body size and temperature on metabolic rate of organisms. Canadian Journal of Zoology. 1983 Feb 1;61(2):281–8.
- 12. Deegan LA. Changes in body composition and morphology of young‐of‐the‐year gulf menhaden, Brevoortia patronus Goode, in Fourleague Bay, Louisiana. Journal of fish biology. 1986 Oct;29(4):403–15.
- 13. Newell RC. Factors affecting the respiration of intertidal invertebrates. American zoologist. 1973 May 1;13(2):513–28.
- 14. Archie EA. Wound healing in the wild: stress, sociality and energetic costs affect wound healing in natural populations. Parasite Immunology 35: 374–385, 2013. pmid:23808526
- 15. Tomlinson S, Phillips RD. Differences in metabolic rate and evaporative water loss associated with sexual dimorphism in thynnine wasps. Journal of Insect Physiology 78: 62–68, 2015. pmid:25935839
- 16. Beauvieux A, Queiros Q, Metral L, Dutto G, Gasset E, Criscuolo F, et al. Energy allocation trade-offs between life-history traits in the Mediterranean sardine: an ecophysiological approach. Marine Ecology Progress Series 701: 99–118, 2022.
- 17. Sogard SM. Size-selective mortality in the juvenile stage of teleost fishes: a review. Bulletin of marine science. 1997 Jul 1;60(3):1129–57.
- 18. Biro PA, Post JR, Abrahams MV. Ontogeny of energy allocation reveals selective pressure promoting risk-taking behaviour in young fish cohorts. Proceedings of the Royal Society B: Biological Sciences. 2005 Jul 22;272(1571):1443–8. pmid:16011918
- 19. Stallings CD, Coleman FC, Koenig CC, Markiewicz DA. Energy allocation in juveniles of a warm-temperate reef fish. Environmental Biology of Fishes 88: 389–398, 2010.
- 20. Kuzawa CW. Developmental origins of life history: Growth, productivity, and reproduction. American Journal of Human Biology 19: 654–661, 2007. pmid:17639581
- 21. Depczynski M, Fulton CJ, Marnane MJ, Bellwood DR. Life history patterns shape energy allocation among fishes on coral reefs. Oecologia. 2007 Aug;153:111–20. pmid:17436023
- 22. Krask JL, Buck TL, Dunn RP, Smith EM. Increasing tidal inundation corresponds to rising porewater nutrient concentrations in a southeastern US salt marsh. Plos one. 2022 Nov 28;17(11):e0278215.
- 23. Teal JM. Respiration of crabs in Georgia salt marshes and its relation to their ecology. Physiological Zoology. 1959 Jan 1;32(1):1–4.
- 24. Teal JM. Energy flow in the salt marsh ecosystem of Georgia. Ecology. 1962 Oct 1;43(4):614–24.
- 25. Cammen LM, Seneca ED, Stroud LM. Energy flow through the fiddler crabs Uca pugnax and U. minax and the marsh periwinkle Littorina irrorata in a North Carolina salt marsh. American Midland Naturalist. 1980 Apr 1:238–50.
- 26. Barnwell FH, Thurman CL. Taxonomy and biogeography of the fiddler crabs (Ocypodidae: Genus Uca) of the Atlantic and Gulf coasts of eastern North America. Zoological Journal of the Linnean Society. 1984 May 1;81(1):23–87.
- 27. Vernberg FJ. Studies on the physiological variation between tropical and temperate zone fiddler crabs of the genus Uca. II. Oxygen consumption of whole organisms. The Biological Bulletin. 1959 Aug;117(1):163–84.
- 28. Levinton JS, Volkenborn N, Gurr S, Correal K, Villacres S, Seabra R, et al. 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). Journal of thermal biology. 2020 Feb 1;88:102502. pmid:32125988
- 29. da Silva Vianna B, Miyai CA, Augusto A, Costa TM. Effects of temperature increase on the physiology and behavior of fiddler crabs. Physiology & behavior. 2020 Mar 1;215:112765.
- 30.
Lighton J.R., 2018. Measuring metabolic rates: a manual for scientists. Oxford University Press.
- 31. Fletcher LS, Bolander M, Reese TC, Asay EG, Pinkston E, Griffen BD. Metabolic rates of the Asian shore crab Hemigrapsus sanguineus in air as a function of body size, location, and injury. Ecology and Evolution. 2022 Sep;12(9):e9297. pmid:36177136
- 32. Wilson CH, Nancollas SJ, Rivers ML, Spicer JI, McGaw IJ. Effects of handling during experimental procedures on stress indices in the green shore crab, Carcinus maenas (L). Marine and Freshwater Behaviour and Physiology. 2021 Mar 4;54(2):65–86.
- 33. Schmitz A, Harrison JF. Hypoxic tolerance in air-breathing invertebrates. Respiratory physiology & neurobiology. 2004 Aug 12;141(3):229–42. pmid:15288596
- 34. Burnett LE. The challenges of living in hypoxic and hypercapnic aquatic environments. American Zoologist. 1997 Dec 1;37(6):633–40.
- 35. Ricciardi A, Bourget E. Weight-to-weight conversion factors for marine benthic macroinvertebrates. Marine ecology progress series. 1998 Mar 12;163:245–51.
- 36.
Burnham KP, Anderson DR, 1998. Model Selection and Multimodel Inference: A Practical Information-Theoretic Approach, 2nd Ed. Springer, New York.
- 37. Allent BJ, Levinton JS. Costs of bearing a sexually selected ornamental weapon in a fiddler crab. Functional ecology. 2007 Feb 1:154–61. http://www.jstor.org/stable/4139397
- 38. Tullis A, Straube CH. The metabolic cost of carrying a sexually selected trait in the male fiddler crab Uca pugilator. Journal of Experimental Biology. 2017 Oct 15;220(20):3641–8. pmid:28794227
- 39. Full RJ, Herreid CF 2nd. Aerobic response to exercise of the fastest land crab. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. 1983 Apr 1;244(4):R530–6. pmid:6837768
- 40. Booth CE, McMahon BR. Aerobic capacity of the blue crab, Callinectes sapidus. Physiological zoology. 1992 Nov 1;65(6):1074–91.
- 41. Full RJ, Herreid CF. Fiddler crab exercise: the energetic cost of running sideways. Journal of Experimental Biology. 1984 Mar 1;109(1):141–61.
- 42. Robertson RF, Meagor J, Taylor EW. Specific dynamic action in the shore crab, Carcinus maenas (L.), in relation to acclimation temperature and to the onset of the emersion response. Physiological and Biochemical Zoology. 2002 Jul;75(4):350–9. pmid:12324891
- 43. Burggren WW, Moreira GS, Maria do Carmo FS. Specific dynamic action and the metabolism of the brachyuran land crabs Ocypode quadrata (Fabricius, 1787), Goniopsis cruentata (Latreille, 1803) and Cardisoma guanhumi Latreille, 1825. Journal of experimental marine biology and ecology. 1993 Jun 21;169(1):117–30.
- 44. Principe SC, Augusto A, Costa TM. Differential effects of water loss and temperature increase on the physiology of fiddler crabs from distinct habitats. Journal of Thermal Biology. 2018 Apr 1;73:14–23. pmid:29549987
- 45. Teal JM. Distribution of fiddler crabs in Georgia salt marshes. Ecology. 1958 Apr 1;39(2):186–93. https://www.jstor.org/stable/1931862
- 46. Hews S, Allen Z, Baxter A, Rich J, Sheikh Z, Taylor K, et al. Field-based body temperatures reveal behavioral thermoregulation strategies of the Atlantic marsh fiddler crab Minuca pugnax. Plos one. 2021 Jan 6;16(1):e0244458. pmid:33406524
- 47. Kozłowski J, Czarnołęski M, Dańko M. Can optimal resource allocation models explain why ectotherms grow larger in cold?. Integrative and Comparative Biology. 2004 Dec 1;44(6):480–93. pmid:21676734
- 48. Griffen BD, Alder J, Anderson L III, Asay EG, Blakeslee A, Bolander M, et al. Latitudinal and temporal variation in injury and its impacts in the invasive Asian shore crab Hemigrapsus sanguineus. Scientific Reports. 2022 Oct 3;12(1):16557. pmid:36192531
- 49. Griffen BD, Bolander M, Blakeslee A, Crane LC, Repetto MF, Tepolt CK, et al. Past energy allocation overwhelms current energy stresses in determining energy allocation trade‐offs. Ecology and Evolution. 2023 Aug;13(8):e10402. pmid:37560183
- 50.
Christy JH, 1980. The mating system of the sand fiddler crab, Uca pugilator. Cornell University.
- 51. Alava VR, Quinitio ET, De Pedro JB, Priolo FM, Orozco ZG, Wille M. Lipids and fatty acids in wild and pond‐reared mud crab Scylla serrata (Forsskål) during ovarian maturation and spawning. Aquaculture Research. 2007 Oct;38(14):1468–77.
- 52. Stueckle TA, Shock B, Foran CM. Multiple stressor effects of methoprene, permethrin, and salinity on limb regeneration and molting in the mud fiddler crab (Uca pugnax). Environmental Toxicology and Chemistry: An International Journal. 2009 Nov;28(11):2348–59.
- 53. Levinton J. Thermal stress: the role of body size and the giant major claw in survival and heat transfer of a fiddler crab (Leptuca pugilator). Journal of Experimental Marine Biology and Ecology. 2020 Sep 1;530:151428.
- 54. Peng ZL, Yin BX, Ren RM, Liao YL, Cai H, Wang H. Altered metabolic state impedes limb regeneration in salamanders. Zoological Research. 2021 Nov 11;42(6):772. pmid:34643071