Locomotion Energetics of the Ghost Crab: I. Metabolic Cost and Endurance

Arthropods have evolved an extraordinary array of solutions to the problems of locomotion on land (Herreid & Fourtner, 1981). They possess different numbers of walking legs (four in a hermit crab to over 180 in a millipede), a variety of leg lengths and positions, numerous stepping patterns, different musculoskeletal arrangements (from muscle levers to hydraulics in spiders), a range of body shapes (long in millipedes and round in some crabs), different styles of locomotion (forwards vs sideways travel) and different oxygen transport systems (from gills to tracheae). Nevertheless, each solution must allow for the repeated generation of the necessary propulsive forces. Although the various potentials and limitations of particular locomotor designs have often been hypothesized (Manton, 1977), actual determinations of function that allow comparison are lacking. The following two papers quantify steady-state locomotor performance, so that the effects of variation in design can be delineated. The arthropod selected for this study was the eight-legged sideways travelling ghost crab, Ocypode quadrata. The present paper examines metabolic energy input and endurance capacity as a function of speed, body mass and ontogeny. A second paper (Blickhan & Full, 1987) describes the mechanics of locomotion, including the locomotor style, force production, and the mechanical energy output used to move the centre of mass.

Two important variables used to compare locomotor performance are the metabolic energy cost of locomotion and endurance capacity. The metabolic energy required for terrestrial locomotion has been best studied in mammals and birds as a function of body mass and speed (Taylor, Heglund & Maloiy, 1982). Metabolic energy during steady-state, submaximal exercise is derived primarily from aerobic metabolism (Seeherman, Taylor, Maloiy & Armstrong, 1981). In most species, steady-state oxygen consumption increases linearly with an increase in speed and the minimum amount of metabolic energy necessary to move 1 g of animal 1 km (C_min) is approached at the higher submaximal speeds (Taylor, Schmidt-Nielsen & Raab, 1970). C_min is a regular function of body mass (Taylor et al. 1982). Small vertebrates, on a per gram basis, demand more metabolic energy to travel a given distance. Undoubtedly, a suite of size-dependent constraints, such as the cost of muscle force production and the storage of elastic strain energy, determine the scaling of metabolic cost. If size-dependent constraints on metabolic cost are conservative among animals then C_min should scale intra-and interspecifically with body mass in a manner relatively independent of locomotor design. Ghost crabs are appropriate for examining intraspecific scaling of energetic cost, because individuals highly adapted for locomotion vary by nearly 40-fold in body mass (Hafemann & Hubbard, 1969; Warner, 1977) and have considerable aerobic capacity, in contrast to other crustaceans (Full & Herreid, 1983). Previous research on interspecific comparisons of mammals, birds, reptiles, an amphibian and some arthropods suggests that C_min is relatively independent of locomotor design (Bennett, 1982; Full, 1986; Herreid, 1981 ; Taylor et al. 1982). Determining C_min for a ghost crab with its unusual style of locomotion can be used to examine this hypothesis for an arthropod design that is as large as that of some mammals, birds and reptiles.

Mammals and lizards appear to differ greatly in their maximal capacity to consume oxygen. In both groups, smaller animals attain maximal oxygen consumption at lower speeds than do larger animals (Taylor et al. 1980; Garland, 1982, 1983). However, the speed at for a lizard is only one-tenth of that predicted for a mammal of the same mass (Garland, 1983). Assuming the capacity to consume oxygen is a major variable determining endurance at speeds near ⁠, then small animals of both groups should have a low endurance capacity. Lizards should fatigue at much lower speeds than mammals (Bennett & Ruben, 1979). If the patterns observed in lizards represent ectotherms in general, then ghost crabs should also fatigue at relatively slow speeds. Moreover, chitinous gills and an open circulatory system in crabs could severely restrict oxygen uptake (Taylor, 1982). If oxygen consumption is severely limited by the crustacean design, then crabs should possess only a modest endurance capacity, even in comparison to other ectotherms, such as lizards.

Ocypode quadrata (Fabricius) were collected from the beaches surrounding the Duke University Marine Laboratory in Beaufort NC, USA. Plastic containers filled with 50 % sea water to a depth of 1 cm were used to house each crab. All animals were kept in an environmental chamber on an L: D 14h: 10 h photoperiod at 24°C and fed fresh fish 2–3 times a week. Only intermoult animals were used. Crabs were divided into three groups based on mass: small (2g), medium (27g) and large (71g).

Ghost crabs were exercised on a treadmill which was enclosed in a respirometer. Three treadmills were used to accommodate crabs of different size. The chamber volumes were 125, 1432 and 2355 ml for small, medium and large crabs, respectively. was determined by open-flow respirometry. Air flow rate was 50 ml min⁻¹ for the small crabs, 300 ml min⁻¹ for the medium and 400 ml min⁻¹ for the large animals. Air leaving a chamber was dried (Drierite) and CO₂ was removed (soda lime). O₂ concentration was monitored with an oxygen analyser (S3A, Applied Electrochemistry), which was interfaced with an integrating chart recorder (Linear Instruments Model 282). Mass-specific, instantaneous was determined as described previously (Herreid, Prawel & Full, 1981b). All values were corrected to STP.

Crabs were given a 30-min rest period on the treadmill before exercise. Animals were exercised for 20 min at a single speed after the rest period. A 30-min recovery period followed the exercise bout. was measured continuously during rest, exercise and recovery. Steady-state oxygen consumption was determined during the final 10 min of a 20-min exercise bout. The range of speeds selected, within a given mass range, was based on the animal’s endurance and walking ability. The fastest speed was chosen so that animals could sustain the rate for 20 min, but when exercised longer fatigued shortly thereafter. The slowest speed represented the rate at which an animal could walk and maintain a consistent pace without undergoing extraneous movements. In most cases, each crab was exercised at every speed. An individual was tested only once a day.

Maximal oxygen consumption was determined by a progressive maximal test (Full & Herreid, 1983). Following a 30-min rest period, a crab was exercised at an intermediate to high speed. Upon attaining a steady-state oxygen consumption for 3 min, the speed of the treadmill belt was increased by 0·03 km h⁻¹ until a new higher steady-state was attained. Three minutes after the new steady-state had been reached, speed was increased again. was defined as the attained when two consecutive incremental increases in speed resulted in no further increase in ⁠.

The contribution of anaerobic metabolism to sustained exercise was determined by measuring lactate content. Crabs were given a 30-min rest period on the treadmill before exercise at a single speed. After the rest period or 5, 10, 15 or 20 min of exercise, crabs were quickly removed from the treadmill and frozen in liquid nitrogen (i.e. in 2–3 s). Experiments were immediately terminated if the crabs struggled at any time. After freezing, the whole body of small animals was completely pulverized in a mortar pre-cooled with liquid nitrogen. Animals of intermediate mass (16-3 g) were divided into parts (i.e. body, chelae and walking leg pairs) after the initial freezing. Each part was then pulverized and analysed separately. Tissue powder was placed in a perchloric acid solution and centrifuged. The concentration of L-lactate was determined by a spectrophotometric procedure (Sigma diagnostic kit no. 826-UV). The glycine-hydrazine buffer recommended was modified by adding EDTA and HC1 to prevent drift in absorbance readings (Full & Herreid, 1984).

After a 10-min rest period on the treadmill, crabs were exercised to fatigue at a selected speed. Speeds ranged from 0·2 to 0· 9 km h⁻¹. Crabs maintained a consistent pace without prodding until just before fatigue. An animal was considered fatigued when it failed to maintain pace with the treadmill belt, dragged its abdomen or did not respond to three successive prodding attempts. The experiment was halted if endurance time exceeded 2h. Experiments during which crabs walked erratically or struggled were aborted.

The value during the final 15 min of the pre-exercise period was averaged to obtain the resting (Table 1). Mass-specific resting was a function of body mass:

where is in ml O₂g⁻¹ h⁻¹ and M_b represents body mass in g (Fig. 1).

increased rapidly to a steady-state at the onset of exercise (Fig. 2). The time to attain 50% of the (t_1/2on) was independent of speed (F_(2,32) = 1·47; P = 0·24), but was dependent on body mass (F_(2,32) = 9·04; P = 0·001). Crabs of intermediate mass (27 g) had a significantly lower t_1/2on (28·2s; N = 5) at high speed (87–92% of ⁠) compared with medium-sized (2g) (42·6s;N=5; t = 3·0; P = 0·02) and large (71g) crabs (55·8s; N = 4; t = 2·5; P = 0·02).

declined rapidly to resting rates after exercise. decreased to 10% of the resting at all speeds within 15 min. The largest crabs (71 g) would not become quiescent, therefore true recovery could not be determined.

was calculated by averaging for the last 10 min of exercise. increased linearly with speed (Fig. 3; Table 1). The slopes of the regression equations, which represent the minimum amount of metabolic energy necessary to move 1 g of crab 1 km (C_min), decreased significantly with an increase in body mass (analysis of covariance, P< 0·001). y-intercept values were elevated compared to mean resting rates for all groups; 71-g crabs showed the greatest elevation — fourfold.

The regression equations relating and to body mass were significantly different (t = 17·3 ; P < 0·003). Large crabs (71 g) had greater aerobic factorial scopes ⁠.

Whole body lactate (WBL) content in the 2-g crabs did not continue to increase after had been attained, even at speeds eliciting 92% of (Fig. 4A). Preexercise WBL was 240 ± 0·36pmolg⁻¹ (N=3). No increase in WBL over preexercise levels occurred at the slow (57% of ⁠; F_(2,6)⁼ 1·9; P = 0·22) and medium (72% of⁠; F_(2,6)= 3·7; P=0·09) exercise speeds. WBL content increased during the first 10 min of exercise at the fastest speed (92 % of ⁠), but decreased as exercise continued past 10 min.

WBL content of 16-g crabs depended on speed (F_(2,10)⁼ 40·6; P< 0·001) and exercise duration (F_(1,10)⁼ 14·0; P<0·005). WBL increased over resting levels at 0·4 and 0·6 km h⁻¹ (Fig. 4B). WBL content at 0·4 km h⁻¹ after 20 min of exercise was not significantly greater than levels measured after 10 min (t = 1·98; P= 0·073). Net WBL production did not occur during in 16-g crabs until exercise exceeded 68 % of (Fig. 4B). This is actually a conservative estimate, because these crabs were smaller than the intermediate group (27 g) used for measurements and were therefore exercising at speeds closer to their maximal aerobic speed (i.e. >68% of ⁠). WBL increased throughout the exercise period at 0·6kmh⁻¹. WBL was significantly greater during exercise at 0·6 km h⁻¹ than at 0·4 km h⁻¹ after both 10 (t = 3·58; P = 0·005) and 20min (t = 4·83; P = 0·001).

The metabolic energy cost of locomotion for the ghost crab was dependent on body mass and speed (Fig. 3). Smaller, younger crabs used higher rates of oxygen consumption on a per gram basis at any speed. Elevated costs were due to (1) higher resting metabolic rates, (2) elevated y-intercepts (i.e. the intercept of the vs speed function compared to ⁠) and (3) a steeper incremental increase in with an increase in speed (C_min, minimum cost of locomotion). Each component increased the total metabolic energy required to travel a given distance. Smaller crabs (2 g) had the highest mass-specific and, therefore, the highest total massspecific cost of locomotion at each speed (⁠⁠; Fig. 5). The largest crabs (71 g) with the lowest mass-specific had the lowest C_t values, except at the low speeds. For crabs of all masses, C_t decreased with speed, because and the y-intercept became a smaller percentage of the total metabolic cost at the higher speeds.

Endurance time (T_end) decreased as exercise speed increased (Fig. 6). Crabs of intermediate mass (27 g) showed the greatest endurance capacity over the range of speeds tested (T_end = 0·036v^−4·02 where T_end is in h and v is in km h⁻¹ ; r² = 0·81 ; analysis of covariance, P< 0·001), whereas the 2-g crabs had the poorest capacity (T_end = 0·0097v^−2·96; r² = 0·82). Large crabs (71g) showed less stamina (T_end⁼ 0·019V^−3·35; r²= 0·80) than 27-g animals.

The ghost crab’s rapid adjustment of O₂ uptake from the environment during submaximal exercise was comparable to other gas transport systems in meeting the O₂ demands of aerobic pathways in muscle, despite its reliance on gills and an open circulatory system. A value was observed at all speeds (Fig. 2). The time required to attain 50% of (t_1/2on = 30–60 s) was well within the range observed for mammals ventilating lungs (t_1/2on = 25–90s; Cerretelli et al. 1979; Marconi et al. 1982), insects using tracheae (t_1/2on = 30–180 s; Herreid & Full, 1984) and another species of ghost crab, O. gaudichaudii, depending on gills (t_1/2on = 50—150 s; Full & Herreid, 1983). The kinetics of O. quadrata remained rapid as speed was increased to workloads near ⁠. Furthermore, effective O₂ transport during submaximal exercise appears to be maintained as O. quadrata increases in body mass; t_1/2on values for crabs varying in mass from 2 to 71g were within the ranges for mammals and insects. It should be noted, however, that 71-g crabs did have significantly slower adjustments to than smaller crabs, especially at speeds approaching ⁠.

The slowest kinetics of ghost crabs were considerably faster than the sluggish responses seen in five other species of terrestrial and semiterrestrial crabs. In species of Cardisoma, Uca and Gecarcinus never attains a steady state during 10–20 min of treadmill exercise, even at slow speeds (Full & Herreid, 1984; Herreid, Lee & Shah, 1979; Herreid, O’Mahoney & Full, 1983; Wood & Randall, 1981a). In humans, prolonged delays in achieving (i.e. drift) have been correlated with increasing blood lactate levels and may result from hormonal and temperature changes (Davis, 1985). The actual mechanism behind the difference between ghost crabs and other crab species remains obscure. Nevertheless, the ghost crab’s capacity to elevate rapidly was associated with a lesser dependence on anaerobic sources.

Using net lactate production as an index, accelerated glycolysis in ghost crabs appears to play only a minor role in energy production during steady-state submaximal exercise, even at speeds at which 70—90% of was required (Fig. 4). These findings are consistent with previous studies on mammals and lizards (Seeherman et al. 1981; Seeherman, Dmi’el & Gleeson, 1983).

Rapid glycolysis can also contribute at the onset of submaximal exercise before O₂ uptake adjustments are completed. WBL content during submaximal exercise in ghost crabs increased during the first 10 min of exercise (Fig. 4); thereafter, WBL content declined or remained the same as a was established. Studies on humans (Cerretelli, Pendergast, Paganelli & Rennie, 1979), mammals (Seeherman et al. 1981), lizards (Gleeson & Bennett, 1982), anuran amphibians (Taigen & Beuchat, 1984) and lungless salamanders (Full, 1986) have also shown lactate accumulation at the beginning of submaximal exercise (i.e. >50% ⁠). Other crab species, such as U. pugilator, rely on rapid glycolysis early in exercise to a much greater extent. These crabs require near maximal rates of at all speeds and are unable to increase rapidly to a steady state during 10–20 min of exercise. The delay is similar to that measured in sedentary humans forced to exercise (Cerretelli et al. 1977, 1979). Just as in these untrained subjects, crabs also depend more on accelerated glycolysis to power exercise before is attained. In fiddler crabs, 40–70% of the ATP requirement for a 15-min exercise bout is supplied by anaerobic metabolism (Full & Herreid, 1984).

Based on the patterns of aerobic and anaerobic metabolism in ghost crabs, it appears that any limitation in the rate of O₂ uptake during submaximal exercise is not necessarily inherent in the design of the crab’s O₂ transport system. This conclusion is perhaps less surprising in the light of recent studies of lungless salamanders exercising on treadmills (Full, 1986). The increase in O₂ uptake for these diffusionlimited, lungless vertebrates is also relatively rapid (t_1/2on⁼ 100s), and accelerated glycolysis contributes little during until >85 % of is attained.

Extraordinarily different O₂ transport systems appear to have a similar capacity to increase O₂ delivery above resting rates. Ghost crabs depend on gills and an open circulatory system for O₂ transport during exercise. Yet is not different from that predicted for an ectotherm, such as a lizard, of the same body mass and temperature (Bennett, 1982). Ocypode quadrata had the capacity to elevate its 6·4-to 8-fold over resting rates (Table 1). The capacity to increase was somewhat less than that of the ghost crab, O.gaudichaudii (12-fold; Full & Herreid, 1983), but was well within the range (5-to 15-fold) reported for mammals (Taylor et al. 1980), reptiles (Bennett, 1982) and even a lungless salamander (Full, 1986).

It should be emphasized that the factorial increase in depends on the ratio of to Similar aerobic factorial scopes do not necessarily indicate a similar ⁠. Factorial scopes can be elevated if is low. For example, ghost crabs and lungless salamanders (Full, 1986) have a similar aerobic factorial scope, yet the of the salamander is only half that predicted for a ghost crab of a similar mass and body temperature. The high factorial scope is due to the fact that plethodontid salamanders have a very low (Feder, 1976). Similar factorial scopes can also result from proportional increases in both and ⁠. The aerobic factorial scope of the ghost crab was comparable to that measured in mammals, even though the absolute is 20-fold greater in the mammal.

Above ⁠, rapid glycolysis contributes throughout the exercise period in mammals (Seeherman et al. 1981), lizards (Seeherman et al. 1983) and lungless salamanders (Full, 1986). A similar dependence most probably existed for the 16-g O. quadrata at high speeds (0·6 km h⁻¹); WBL increased linearly during 20 min of exercise, but no direct measurements of were available for that body mass (Fig. 4B). Full & Prestwich (1986) have shown that both rapid glycolysis and depletion of high-energy phosphates contribute in ghost crabs at speeds demanding energy well above (i.e. 1·4–2·3 kmh⁻¹ for a 27-g crab). Other crustacean species also show increases in lactate levels during vigorous exercise, presumably at or above (Burke, 1979; McDonald, McMahon & Wood, 1979; Wood & Randall, 1981b). Unfortunately, activity levels have seldom been quantified. During controlled exercise in fiddler crabs, WBL content increases at a constant rate throughout the exercise period at even relatively slow speeds (0·06–0·46kmh⁻¹ ; Full & Herreid, 1984). In contrast to ghost crabs, other crustaceans have a relatively modest capacity to increase above their resting rates (i.e. 2-to 6-fold; McMahon, 1981). Consequently, they always appear to be exercising near ⁠, even at relatively low levels of activity.

Small animals, regardless of their locomotor design, appear to demand more energy than larger animals to transport 1 g of body mass over a distance. Evidence for this statement has come from interspecific comparisons of adult mammals, birds, reptiles and some arthropods (Herreid, 1981; Bennett, 1982; Taylor et al. 1982). The present study suggests a similar intraspecific trend.

Elevated costs in small ghost crabs resulted from (1) higher resting metabolic rates, (2) elevated y-intercepts (i.e. the intercept of the VS speed function compared to ⁠) and (3) a steeper incremental increase in with an increase in speed (C_min). Metabolic maintenance costs are often subtracted in order to compare animals of different body masses and temperatures. Certainly, the validity of these baseline subtractions is somewhat questionable (Stainsby, Gladden, Barclay & Wilson, 1980). For example, the resting metabolic component is often assumed to remain constant during exercise, even though this assumption is largely untested. Even with removed, the 2-g crabs still demanded more energy than the larger crabs to move 1 g of their mass 1km. This energy cost represents the net cost of transport (C_net), the amount of energy over rest required to move 1 g of animal 1 km. Surprisingly, the largest crabs (71 g) had a greater C_net than 27-g crabs at speeds up to 0·5 kmh⁻¹. Intraspecific scaling of C_net did not follow interspecific trends (i.e. a decrease with increased body mass). The largest crabs (71 g) actually required higher net mass-specific rates of oxygen consumption at low speeds compared to crabs less than half their mass (Fig. 3). This resulted in the relatively large y-intercept to ratio of 4·0 (Table 1). In birds and mammals, this ratio ranges from 0·4 to 2·3 (Fedak, Pinshow & Schmidt-Nielsen, 1974; Taylor et al. 1970). The reason for the relatively high energy costs at low speeds in large crabs (71 g) is unknown.

C_t and C_net both decreased at the high speed to a minimum (C_min) which equals the slope of the vs speed relationship (Fig. 5). Two points are apparent from the C_min determination for ghost crabs. (1) The relationship between C_min and body mass was similar to that found for interspecific comparisons of adult mammals, birds, reptiles and some other arthropods (Herreid, 1981; Bennett, 1982; Taylor et al. 1982). A smaller crab requires a greater minimum amount of energy to move 1 g of its body 1 km. As ghost crabs grow larger, the energy cost of moving 1 g of their body a given distance at speeds near declines. (2) C_min appears to be relatively independent of locomotor design, even when sideways travelling octapedal arthropods are considered. C_min for ghost crabs fell within the 95 % confidence interval of the relationship given for vertebrates (Fedak & Seeherman, 1979; Taylor et al. 1982). Additional support for this conclusion is found for six-legged insects (Herreid, 1981; Herreid, Full & Prawel, 1981a; Lighton, 1985), small Panamanian ghost crabs (Full & Herreid, 1983) and four-legged hermit crabs (Herreid & Full, 1985).

The present results are consistent with the hypothesis that the time course of force development and decay is the primary determinant of the metabolic cost of locomotion (Taylor, 1985), although this does not rule out the possibility that other size-dependent constraints are involved. Even though drastically different in design, crabs and vertebrates of a similar mass cycle their limbs, and therefore operate their muscles, at similar frequencies (Blickhan & Full, 1987).

Endurance capacity correlates with the speed at which is attained. However, endurance capacity may not be a simple function of body mass within a species as has been described for interspecific comparisons. Endurance capacity in ghost crabs decreased exponentially with an increase in speed, just as is found in humans (Wasserman, VanKessel & Burton, 1967), lizards (John-Alder & Bennett, 1981) and other crab species (Wood & Randall, 1981a; Full & Herreid, 1983, 1984).

Yet endurance capacity did not increase regularly with body mass (Fig. 6). Instead, endurance capacity was correlated with the speed at which was attained ⁠. In lizards (Bennett & John-Alder, 1984) endurance capacity also correlates with ⁠. At speeds approaching the ⁠, anaerobic metabolism supplements aerobic sources and endurance capacity declines. In mammals increases as a function of body mass (Taylor et al. 1980). Smaller mammals attain at lower speeds, primarily due to a higher mass-specific C_min. Lizards show a similar increase of with body mass, except that the absolute only one-tenth of that predicted for a mammal of the same mass (Garland, 1982, 1983).

The ontogenetic comparison in ghost crabs did not follow the same relationships observed interspecifically for lizards and mammals. Crabs of intermediate mass (27 g) had the highest and the best endurance capacity. The 2-g crabs had a high mass-specific C_min, as do other small animals, and therefore had a lower and the poorest endurance. The largest crabs (71 g) had a low mass-specific C_min, but used large amounts of energy even at slow speeds (i.e. an elevated y-intercept). As a result, 71-g crabs attained at lower speeds than did 27-g crabs. The largest crabs (71 g) were able to sustain speeds for only 1/2–1/3 as long as a crab half their mass (e.g. 0·4kmh⁻¹ for 30min).

Overall and ⁠, for ghost crabs were similar to values found for lizards (Garland, 1982, 1983), but were much lower than the values predicted for mammals. Ghost crabs (30–70g) use a walking gait as speed is increased up to 1·4 kmh⁻¹, where they switch to a slow run (Blickhan & Full, 1987). In marked contrast to mammals, ghost crabs attained at speeds below this transition from a walk to a slow run, at approximately the mid-speed used for a walk. For animals locomoting within a gait, such as walking in humans (Margaria, Cerretelli, Aghemo & Sassi, 1963) and horses (Hoyt & Taylor, 1981), the minimum energy cost to travel a given distance is found near the mid-speed for that gait. This speed is also the operating speed preferred by the animal. At higher or lower speeds within the gait, the metabolic cost per unit distance increases. A similar situation could exist in crabs. The speed at which was attained corresponds to the range of speeds over which a pendulum-like mechanism minimizes mechanical and probably metabolic energy (Blickhan & Full, 1987). At faster walking speeds, the energy-conserving mechanism becomes less effective in minimizing energy cost, and additional metabolic energy must be supplied by anaerobic sources because is exceeded. Ghost crabs can change gaits and run at speeds 20 times faster, but only for short periods and with the aid of anaerobic metabolism.

Many thanks to C. F. Herreid for all phases of this work and to William Kirby-Smith for the collection of animals.