OXYGEN CONSUMPTION BY THE HERMIT CRAB, CLIBANARlUS V/ITTATUS (BOSC, 1802) IN

DECLINING OXYGEN TENSIONS

ANI\‘ELIESE MARGARETE WERNICK and CARLOS HENRIQUE SILVA PENTEADO

Departamento de Zoologia, lnstituto de Biocidncias, Universidade Estadual Paulista “Jlilio de Mesquita Filho”. 13SOGRio Clara. Slo Paula, Brazil

(Receic~~d 6 July 1982)

Abstract---l. Oxygen consumption and its relationship to stepwise declining oxygen tension were exam- ined in the common striped hermit crab, Clibanarius cittutu,s.

2.,Weight_specific oxygen consumption varied with body weight (W), according to the equation log l’o, = 2.1639 + (-0.419log W).

3. Shell-less ipdividuals of l-2g wet wt, where found to be “oxygen conformers”. since oxygen consumption (Vo,) decreased with declining oxygen tensions. At ambient oxygen tensions below 35.4 mmHg, oxygen consumption remained constant, suggesting an increased ventilation.

4. C. cittatus was found to survive in oxygen-free seawater for 5.5 hr, and no significant differences were found in oxygen consumption rates, for shelled and shell-less crabs, measured in water and air.

5. The use of a K,/K, index of oxygen independence, showed that larger animals were better able to maintain oxygen-independence during hypoxia than smaller individuals.

6. C. rittutus displayed a pattern of no oxygen debt, once returned to normoxia.

INTRODUCTION

Most aquatic invertebrate groups exhibit a diversity of responses to oxygen tension. Some are regulators, other are conformers (Hill, 1976), although Mangum & Van Winkle (1973) concluded that these are merely two ends of a spectrum of responses for species. It has also been suggested that a species can be either a regulator or a conformer depending upon such factors as temperature, salinity, habitat, activity, size, molting cycle and acclimation period in the experimental chamber (see Herreid, 1980).

Crustacea are among the best studied invertebrates with respect to declining oxygen tensions and re- sponse to hypoxia. Some species conform to a hypoxic environment such as Orconectes immunis (Wiens & Armitage, 1961), Homarus vulgaris, which was previously described as a conformer (Thomas, 1954; Taylor, 1981) when allowed time to settle in a respirometer, maintained the normoxic oxygen uptake during moderate hypoxia (Butler et al., 1978). Other species such as Curcinus maenus (Taylor, 1976; Taylor et al., 1977), Austropotamobius pallipes (Wheatly & Taylor, 1981), Orconectes airi/is (McMa- hon et al., 1975) and Callinectes supidus (Batterton & Cameron, 1978) can regulate oxygen uptake down to low levels of poz. However, there is little information on responses to progressive hypoxia for anomurans. Sarojini & Nagabhushanam (1968) reported that the rate of oxygen consumption of the hermit crab Dio- genes bicristimanus was unaffected by changes in oxy- gen content of the water between 7.0 and 4.0 ml/l. but that the rate decreased considerably down to 2.5 ml/l. Davenport et al. (1980) showed that Pagurus bern- hardus is an oxygen conformer and that this species survived in oxygen-free seawater for 7 hr. while the

sublittoral anomurans Guiutheu strigosu (Bridges & Brand, 1980) showed an oxygen independence. influenced by size and activity.

The common striped intertidal hermit crab Cliha- nurius cittutus is frequently found exposed at low tide on mud flats and among rock shores and thus is likely to encounter periods of environmental hypoxia, es- pecially in the summer months. In our laboratory in- dividuals very often remain voluntarily exposed to air for long periods. When covered by water, C. cittatw maintains its level of activity, but remains quiescent and retreats into its shell, when exposed to air.

In the present investigation we studied the respirat- ory metabolism of the shell-less C. cittatus at stepwise declining oxygen tensions, to assess whether the crabs are regulators or conformers under oxygen-deficient conditions. We related these measurements to the size of the animals and to the repayment of oxygen debt after return to normal oxygen tension. The study in- cludes oxygen consumption of shelled C. rittuttrs measured in air and water.

MATERIAL AND METHODS

Hermit crabs, CIibunarius rittatu,s, were collected at S%o Sebastiio, Sio Paula, Brazil (23 49’ S and 45 24’ W) and transported to the laboratory of the Instituto de Biociin- cias at Rio Clara, Sio Paula, Brazil. The specimens were maintained in filtered and recirculated seawater with 35”,,,, salinity and 23 k 2°C temperature for at least 2 weeks prior to the experiment, They were fed fish slices and main- tained in good health for several months.

Hermit crabs (wet wt 0.735-5.738 g) were removed from their shells using a vice. Crabs obviously injured in this process were discarded. but the majority showed no appar- ent ill effects of removal from the shell. Only intermolt males and non-ovigerous females were used in subsequent

749

750 ANNELIESE MARGARETE WERNICK and CARLOS HENRIQUE SILVA PENTEADO _

200 $ ; N

.9 50- . .

c 0.1 1.0 2.0 3.0 4.0 6.0

Weight lg )

Fig. 1. Relationship between the respiratory rate and size in CIibanarius cittatus at normoxia, 25°C and salinity 35”,,,,

experiments. Shell-less crabs were maintained without feed- ing in individual plastic cups for at least 24 hr before they were used in any experiments.

Respiratory rates (r’oJ were obtained in a Warburg apparatus at an experimental temperature of 25°C. Filtered seawater (4 ml, 3Y’/, S) was introduced in flasks (volume about 60 ml) and 0.5 ml of 14% KOH was used as CO2 absorbant. In each series of experiments the flasks were thoroughly perfused with a mixture of N, and O2 of the desired percentage of oxygen (for not less than tOmin). Respiratory rates were measured under normoxic and hypoxic conditions for 1 hr, following a 30 min period of adjustment for each experimental no,. The perfusion mix- tures contained 21, 15, 10, $2.5 and 1% of oxygen. Accord- ing to a local mean barometric pressure of 708 mmHg, the oxygen tensions (p,J used were 148.68, 106.20, 70.80, 35.40, 17.70 and 7.08 mmHg, respectively.

The influence of declining oxygen tensions on respiration was determined according to the procedure of Tang (1933) recently reviewed by Bayne (1971) for three lamellibranchs. In this case, the same animal is used in successive hours of experiment in progressively declining oxygen tensions. Its respiratory rates (voJ are used in a po2/vo2 x po2 plot and a K, (n value)/l(, (b value) ratio is inferred from the regres- sion equation. The Ki/K, ratio is considered as an “oxy- gen dependence index”; when low, it indicates indepen- dence of ambient oxygen tension. Tang’s procedure allows also one to relate K,/K* ratios to size to evaluate the influence of weight (or age) on respiratory regulation.

After exposure to low oxygen tension, animals were returned to normoxic conditions and respiratory rates were measured to examine rates of oxygen debt repayment once the normoxic state was restored.

The data were expressed as ~1 OZ/g (wet wt)/hr.

RESULTS

Oxygen consumption and body weight The relationship between weight-specific oxygen

consumption and body weight at normoxia is presented as a log-log relationship in Fig. 1, where oxygen consumption increased significantly with de- creasing body weight. The slope (b - 1) and intercept (a) of the regression line describing this relationship were -0.419 and 2.1639, respectively (r = 0.66, P < 0.01, N = 20, 25°C).

Oxygen consumption in declining oxygen tensions When vo2 values of 8 Clibanarius uittatus weighing

l-2g were plotted against stepwise declining oxygen tension, they showed an oxygen dependence down to a po, of 35.4mmHg. Then oxygen consumption remained constant until very low po2 values of 7.08 mmHg (Figs 2 and 3). Otherwise, no critical oxygen tension (PC) was observed. When the animals returned to normoxic conditions, they repaid some oxygen debt, but the O2 consumed was less than expected and not significant (P > 0.05, between I’,, post- hypoxia and PO, pre-hypoxia).

Effect of size on oxygen consumption at declining oxy- gen tensions

Animals of increasing weight (0.982-5.738 g) were used in order to evaluate effects of size on the K,/K, ratios, i.e. in the “oxygen dependence index”, thence of regulation, according to Tang’s (1933) and Bayne’s (1971) procedures. Values for this index were calcu- lated for each experiment from regression equations fitted, by the method of least squares, to the curves of P~,I~~, against po2, which showed correlation coef- ficients varying from 0.80 to 0.99 being in all cases significant (P < 0.05). These values were plotted against the weight-specific oxygen consumption in Fig. 4 and the relationship may be expressed as

K,jK2 = 0.1604. v&‘s6

[correlation coefficient = 0.57, P < 0.05 for 14 degrees of freedom, and Syx (standard error of esti- mate) = 0.27111.

The results showed that for C. &tutus smaller indices correspond to bigger animals (with small VoZ) suggesting a higher oxygen dependence in small ani- mals in which the capacity of regulation was smaller than in large ones.

Oxygen debt and size The repayment of an oxygen debt in relation to size

was also investigated. When the ratio described by Bayne & Livingstone (1977), vo2 after-hypoxia/I’o/,2 before-hypoxia was compared as a measure of oxygen

1 17.7 35.4 70.8 c6.2 40.7

PO, (mmktg )

Fig. 2. Oxygen consumption (PoJ at different oxygen ten- sions in percentage of Vi+ before hypoxia for 8 crabs, C.

vittatus, weighmg between 1 and 2 g.

Oxygen consumption by the hermit crab

NORM - __~ ii Y POXIA -_c NORM

Tvne (hours I

751

Fig. 3. Hypoxia experiment on X crabs, C. cirfutus, weighing lL?g. The bars represents the po, of the ammal chamber for I hr each. Oxygen consumption during this period is also depicted by the filled

circles.

debt of individuals of different size. there was no cor- relation (r = 0.17, P > 0.05) between size and the magnitude of the apparent oxygen debt subsequently repaid (Fig. 5). Some animals showed a depressed oxygen consumption after return to normoxia, whereas others presented a “sub-normal” oxygen debt, which was size independent. The mean value of the ratios was 1.13 & 0.39 SD. Otherwise. the mean

100

?’ . , /

6 /

I

I I I”111 I1

IO 30 40 50 100 200 300

wo*(jJIoz~~vhT)

Fig. 4. The “oxygen-dependence index” (K,,!Kz) for C. rif- tutws plotted against the weight-specific oxygen

consumption.

values of J& during pre-hypoxia (116.19 + 39.00 ~1 0,/g per hr) and PO1 post-hypoxia (125.61 + 52.52 ~1 OJg per hr) showed no significant differences (P > 0.05), suggesting that C. uirtatus displayed a pattern of “no oxygen debt”. i.e. the oxygen consumption was rapidly restored to prehypoxic conditions once nor- moxia had returned.

20- .

.

.

. .

. ??

? ?? ?

? ?

? ?

? ? ? ? ? ? ? ? ? ? ?? ? ? ? ? ?

. . . .

-i

:: 0.1 0.7 1.0 2.0

Wetghf lg)

Fig. 5. The relation between “oxygen debt index” and size in C. rittatus at 25-C and salinity 35”,,,,. A ratio > 1.0 signi- fies the repayment of an oxygen debt; a ratio < I.0 signifies

depressed post-hypoxic I/&.

752 AYXNELIESE MARGARETE WERNICK and CARLOS HENRIQUE SILVA PENTFADO

Anaerobic surciual

Five shell-less specimens of C’libanarius (1.785-4.733 g wet weight) were placed in a one litre sealed container of oxygen-free seawater (produced by nitrogen bubbling). They were inspected for dead specimens at intervals of 30 min, 1. 1.5, 2, 2.5, 3, 4.5 and 5.5 hr. When exposed to anaerobic conditions C. aittatus first showed increased activity and then became extremely sluggish. It proved to be impossible to determine accurately mortality half-times for anoxia, since animals which seemed to be dead re- covered after returning to aerobic conditions. After 5.5 hr in oxygen-free seawater, only one individual (2.762 g wet wt) was found to be dead after a half hour of perfusion by pure oxygen.

Oxygen consumption in air and water

It is well known that Clibanarius rittatus is able to survive for prolonged periods in air (Young, 1978). Thus, we investigated the ability of this hermit crab to respire in water and air. For this, oxygen consump- tion rates of each of five shelled (1.7552.64 g wet wt) and five shell-less (1 X%3.84 g wet wt) specimens were first measured for 1 hr in water and then for 1 hr in air. The results obtained for shelled crabs were 84.32 _t 27.03 ~1 OJg per hr in water and 83.46 f 16.92 ~1 OJg per hr in air and 79.02 k 53.20 ~1 0,/g per hr and 70.42 f 20.15 ~1 0,/g per hr, for shell-less crabs, respectively. In both cases no significant differences in oxygen consumption were observed.

DISCUSSION

The dependence of respiration upon size is well documented within the Crustacea, where the weight- specific oxygen consumption rates generally decrease with size. The b - 1 value found for C. aittatus (-0.419) is well within the range reported for other decapods, although the b - 1 values show inter and intraspecific variation, For the same species, Wernick (1982) found for shelled crab, a b - 1 value of -0.28. Shumway (1978) has reported a h - 1 of -0.332 for adult hermit crab Pagurus bernhardus measured at 15°C and using dry weight, while Bridges & Brand (1980b) recorded a value of -0.247 for the same spe- cies, measured at 10°C and wet weight. Young (1963) reported a b - 1 of -0.390 for P. hir.sutiuscu/us, at 20°C while Scholander et a/. (1953) found values of about -0.15 for various crustacean species, including the tropical hermit crab Clibanarius antillensis.

Shell-less Clibanarius cittatus decreased their oxy- gen consumption when oxygen tension declined step- wise, down to a p,,, of 35.4 mmHg and therefore could be considered “oxygen conformers”. Davenport et al. (1980) allowing shelled Pagurus bernhardus to exhaust the oxygen supply within the respirometer, reported that these hermit crabs were “oxygen conformers”, too. However, in the present paper, oxygen consump- tion remained constant at po, values from 35.4 to 7.08 mmHg. Mangum & Van Winkle (1973) and Her- reid (1980) proposed that curvilinear responses are expected from animals with a changing conductance pattern. The high level of oxygen consumption at low oxygen tensions observed for C. oittatus may reflect the cost of increased ventilation needed to obtain suf-

ficient oxygen at reduced tensions, as suggested by Beamish (1964) and Hill (1976) for the goldfish, Caras- sius uuratus. In that study, there was no systematic change in oxygen consumption when oxygen tension was lowered from 160 to 90 mmHg, but then oxygen consumption rose, as the tension was dropped from 90 to 65 mmHg, due to an increasing ventilation and then finally declined rapidly, as the tension was low- ered. Hyperventilation was also described in 15 C acclimated crayfish Austropotamobius pallipes down to a PC of 40mmHg (Wheatly & Taylor, 1981) but then ceased at lower tensions. Taylor (1981) reported that Carcinus maenas had a PC for li,l of 40 mmHg at 10°C which increased to 60mmHg at 18 ‘C. Venti- lation was unaffected by progressive hypoxia down to 30 mmHg at 1O’C but increased significantly below 60 mmHg at 18°C. Due to the high energy cost. in- creased branchial pumping is probably only useful as a short-term response to hypoxic conditions, enabling the animal to combat short natural hypoxic periods or allowing time for other adaptative mechanisms to develop (McMahon & Wilkens. 1975). C. cittatus, exposed only for short periods to hypoxic conditions in their habitat, may show such types of responses. McMahon & Wilkens (1975) reported that in Homarus americanus experiments of short duration, characterized by short acclimation to the experimen- tal chamber followed by a fairly rapid decrease to low oxygen tensions, showed that oxygen consumption decreased progressively with decrease in ambient oxy- gen tension. But, oxygen consumption was main- tained over a wide range of external tensions when animals were acclimated to the experimental con- ditions over two or more hours. Mangum & Van Winkle (1973) concluded that conformity is to be expected of smaller or less complex invertebrate forms, although in their study of 31 marine inverte- brates, representing a number of phyla, there was no compelling correlation between habitat and oxygen response.

When exposed to anaerobic conditions C. Gttatus survived for 5.5 hr, but they are unable to perform any locomotory ability. which indicates that the anoxic circumstances are very unfavorable, as shown for other crabs, such as Carcinus maenas (Spaargaren, 1977). Similar results were reported for Pagurus herh- nardus (Davenport et u/., 1980) which can survive for 7 hr without an oxygen supply, although mortality is rapid thereafter.

C. Gttatus is able to survive for prolonged periods in air and in our laboratory we often observed that some animals migrated from seawater into air and remained voluntarily exposed for long periods. The results we obtained for I&, in animals with and with- out shells, exposed to air and water were not signifi- cantly different, and were similar to results reported by Taylor & Butler (1978) for 15C acclimated Car- cinus maenas exposed for 3 hr to water and air. This was supported by the results reported by Wallace (1972) and Newell et u/. (1972) for the same species, for these authors found that Vo, in air was approxi- mately 75:d of the measured rate from crabs sub- merged in seawater. However, the method used by these authors required shaking of the respirometers to equilibrate the gas and liquid phases. which leads to excited levels of activity in the crabs (Wallace, 1972).

Oxygen consumption by the hermit crab 753

Using Tang’s transformation to give some qua@- tative value to regulatory ability with changes in Vo, levels. Bayne (1971) and Taylor & Brand (1975) in marine bivalve molluscs, and Bridges & Brand (1980) for the anomurans Corystes cassivelaunus and Galathea strigosa, reported an increase with increas- ing size in the ability to maintain oxygen indepen- dence during hypoxia. Although Tang’s transform- ation is difficult to apply when oxygen-independence approaches a linear function, we observed similar results for C. vittatm

Herreid (1980) in his review, attempted to classify the various types of aerobic responses that are com- monly seen in organisms during recovery from hypoxia. once normoxic conditions are restored. We observed. that independent of size, specimens of C. ritttrtus showed a complex relationship, with values for the oxygen debt ranging from positive to appar- ently negative. and when a mean ratio was deter- mined, a pattern of “no oxygen debt” was found, i.e. Vol was rapidly restored to prehypoxic conditions once normoxia had returned. Herreid noted that poikilometabolic conformers would be expected to have this type of recovery pattern, for they do not generate significant anaerobic endproducts.

Further detailed studies must be undertaken in CIihanarius aittatus to identify the physiological and biochemical processes involved during hypoxia and on the return to aerobic conditions.

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