OXYGEN CONSUMPTION OF THE TERRESTRIAL MITE ALASKOZETES ANTARCTICUS (ACARI: CRYPTOSTIGMATA)

Transcription

1 J. exp. Biol. (1977), 8, \With figures Printed in Great Britain OXYGEN CONSUMPTION OF THE TERRESTRIAL MITE ALASKOZETES ANTARCTICUS (ACARI: CRYPTOSTIGMATA) BY WILLIAM BLOCK* Department of Zoology, School of Biological Sciences, University of Leicester, Leicester LEi jrh, England (Received 17 November 197) SUMMARY Analysis of 148 measurements of individual respiration rate showed that although respiration was linearly related to live weight on a double log scale, there were significant differences between rates at o, and + C. Proto- and deutonymphal metabolic rates were higher than other stages, especially at + C. Q values ranged from 2-07 to 3-83 over o to + C. Equations relating individual respiratory rate to live weight and temperature for A. antarcticus, and metabolic rate to temperature for species of Antarctic terrestrial invertebrates were developed. Comparison with temperate data indicated considerable cold adaptation in the Antarctic species with 3-5 times increased metabolism. It was calculated that % of the energy assimilated may be used in respiration by A. antarcticus. INTRODUCTION The maritime Antarctic affords a unique opportunity for the environmental physiologist to study the effects of low temperatures on terrestrial arthropods. The terrestrial arthropods there consist almost entirely of Acari and Collembola, and although respiration rates have been measured for the collembolan, Cryptopygus antarcticus Willem (Tilbrook & Block, 1972; Block & Tilbrook, 1975), there is little information on the physiology of the terrestrial mites. Accordingly, the present study of oxygen consumption of a common cryptostigmatid mite was undertaken at Signy Island, South Orkney Islands, during the southern summer in Alaskozetes antarcticus (Michael) of the family Podacaridae is endemic to the Antarctic and Sub-Antarctic regions, and is one of the most southern over-wintering land animals at almost 78 0 latitude (Wallwork, 197). There are two distinct subspecies: A. antarcticus antarcticus and A. antarcticus intermedius. This study was concentrated on the former subspecies, which is widely distributed throughout the Antarctic Peninsula area. The habitats of A. antarcticus are varied, from inter-tidal debris, penguin guano, seal wallows, and bird nests to the undersides of loose rocks, bones, crustose lichens and algae, especially Prasiola crispa (Lightf.) Menegh. Present address: Life Sciences Division, British Antarctic Survey, Madingley Road, Cambridge CB3 oet.

2 70 W. BLOCK (Gressitt, 197; Strong, 197; Tilbrook, 197; Wallwork, 1973). It occurs often in dense aggregations which may include all stages from egg to adult. Aggregations are present throughout the year, and development, moulting and maturation occur within them. A. antarcticus is a scavenger feeding on detritus, mostly of vertebrate origin. It is one of the largest Antarctic terrestrial arthropods, but the adult is only ca. 1 mm in body length, and all stages are slow moving. The objectives of the present study were to investigate the influence of body weight and temperature upon the oxygen consumption of a range of individual mites representing the post-embryonic life stages of A. antarcticus. In addition, data were collected on the variations in metabolism due to sex and breeding condition of the adult. This project was complementary to a long-term ecosystem research programme of two terrestrial sites at Signy Island, which is being undertaken by the British Antarctic Survey (Tilbrook, 1973). METHODS Great care was taken to ensure that collected animals were maintained and studied at temperatures representative of their summer habitat conditions (December-March). Samples of live mites for respirometry were collected daily by hand from habitats close to the B.A.S. Research Station on Signy Island. Field temperatures were measured at the time of collection by a Grant Model S thermistor thermometer. The experimental temperatures were o, +5 0 and + C, and as far as possible the measurements of oxygen uptake were made within 5 0 of the field temperature. Once collected, the mites were handled entirely in a cold room at the experimental temperature. The mites were sorted and identified to their various life stages under a x 1 stereomicroscope and then weighed individually on a Beckmann electromicrobalance (LM 500) sensitive to o-i fig. The micro-respirometer used was a Cartesian Diver (Linderstaam-Lang, 1943; Holter, 1943), and stoppered divers were utilized (Zeuthen, 194) with gas volumes in the range /tl. The respirometer had a capacity for 14 measurements at one time, and was also in the controlled temperature room. The respirometer water-bath temperature was controlled to ± o-oi C. Mites were placed singly in the divers, and at least 30 min was allowed for equilibration after loading into the respirometer. Respiratory measurements were made over 5- h, and the individual activity of the mite was recorded. Following this the animals were removed, killed in 75 % ethyl alcohol, mounted in 80 % lactic acid on slides and examined. Confirmation of life stage by the setation of the anal and genital areas in the adults, and the genital papillae in the juveniles (Wallwork, 192) was then made, along with observations on sex, egg number, and gut contents. Calculations of individual and weight-specific oxygen consumption rates of A. antarcticus were as described for C. antarcticus in Block & Tilbrook (1975), except that individual live-weight measurements, rather than estimated values, were used for the weightspecific rates. RESULTS Oxygen consumption and live weight The mean individual live weights of each life stage of A. antarcticus, which were calculated from the material used for the respirometric measurements, are given in

3 Oxygen consumption of Alaskozetes antarcticus 71 Table 1. Mean live weights (fig) of individuals of each life stage of Alaskozetes antarcticus used for respiratory measurements at Signy Island. The mean weight ± 8.E. and the number of individuals (n) weighed are also given. Significant differences between the mean weights of life stages are indicated below (NS: not significant) Life stage Larva Protonymph Deutonymph Tritonymph Adult male Adult female (gravid and non-gravid) Adult female (gravid) Adult female (non-gravid) Mean live weight ±s.e. (jig) 1329 ± ± I-IO I5-97 ± Significance: Larva - protonymph, NS; Protonymph - deutonymph, P<o-ooi; Deutonymph - tritonymph, P < o-ooi; Tritonymph - adult male, NS; Tritonymph - adult female (gravid and nongravid), P < o-oi; Adult male-adult female (gravid), P < o-oi; Adult male-adult female (nongravid), NS. Table 1. There was a steady live-weight increase from the larval to the deutonymphal stage, each being approximately double the weight of the previous stage. The largest weight increase (almost 3 x) occurred between the deuto- and the tritonymph. The adult female was significantly heavier (mean live weight of both gravid and non-gravid individuals) than the tritonymph (P < o-oi). Both gravid and non-gravid females separately were generally heavier than the males, and the mean weight of the combined gravid and non-gravid females was significantly different (P < o-oi) from the male. Within each life stage, there were no significant differences in live weight of material used at each temperature. Log oxygen consumption rate ( x io" 3 fi\ O g ind" 1 h -1 ) was plotted against log live weight (fig) for each animal measured at each of the three experimental temperatures (Fig. 1). Linear regressions were fitted to these data, and the resulting equations and correlation coefficients are shown in Table 2. There was a steady increase in respiration rate with increasing live weight at each temperature. The regression lines for and 4- C were almost parallel, indicating that the relationship of log oxygen consumption to log live weight was similar at these two temperatures. At o C, however, the regression line diverged from those of +5 0 and + C over the upper portion. To investigate this further, the homogeneity of the three regressions was examined using the method of Ostle (193). This tests whether the combined data can be represented by a single regression line. The overall regression equation for respiration rate (^o,) on h ve weight (W) for the three temperatures combined was log J^Oi = \og w W The analysis and variance ratio tests (Table 3) showed that the three lines were not homogeneous (P < o-ooi) and could not be represented by a single regression. However, the slopes of the lines were not significantly different, and the failure of the overall regression equation resulted from between-temperature differences. Further, the mean respiration rate at each temperature (P < o-oi) and the within and between temperature slopes (P < o-ooi) were significantly different. Therefore, the relationship of respiration to weight in A. antarcticus was similar at the n

4 W. BLOCK 50 Live weight Oig) Fig. i. Oxygen consumption as a function of live weight at o, +5 and + io C for Alatkoseta antarcttcus. Data are plotted on log 18 scales, and individual measurements with the fitted linear regression line are shown for each temperature., Larva; A> protonymph; D, deutonymph;, tritonymph; A, adult male; O. adult female. three temperatures, but significant differences in respiration rate were detected between temperatures. Examination of the individual weight-specific respiration rates compared to live weight shows that there was only a very slight negative correlation (Table 2, Fig. 2). The correlation coefficients for each temperature were not significantly different from zero. Therefore weight-specific respiration rate was not correlated with live weight at these temperatures in A. antarcttcus. These results contrast with those for the Antarctic collembolan, C. antarcticut, reported by Block & Tilbrook (1975). For this species, the +5 C regression line of individual respiration rate against live weight was significantly different from those of o and + C. It was concluded that the smaller, immature individuals had a

5 Oxygen consumption of Alaskozetes antarcticus 73 Table 2. Linear regression equations of oxygen consumption (log y) on live weight (log x: fig) for Alaskozetes antarcticus at o, +5 0 and + io C. The number of determinations (n) and the correlation coefficient (r) (P < o-ooi throughout) are also given Temperature ( C) n a b ± 83- r x io-* fa O, ind" 1 h" 1 o 40 o-8n +o-830±o-o ±o-osa a o-97±o-o ft\ O, g- 1 live wt h" 1 o 40 +a-i88 o-i9±o-o8 " ii o-oa9±o-osa a * Table 3. Summary of tests for homogeneity of linear regressions of log oxygen consumption (yixio" 3 fi\ O 2 ind" 1!!- 1, on log live weight (x: fig) for Alaskozetes antarcticus (D.F. = degrees of freedom; s.s. = sum of squares; M.S. = mean of squares) Source of variation Temperature ( C) o Sum of residuals: Within temperatures Increase in sum of residuals: Between temperatures Total D.F i 145 a 147 [Si + S t + S t Si = s t = 5,- S T = 8.8 I-97I Residuals D.F a 1 1. Test for homogeneity: VR = ((S,-S 1 )/4)/(S,/i4a) = 19-57; D.F. 4, 14a; P < o-ooi. a. Test for identity of slope within temperatures: VR = (5 I /a)/(5 1 /i + 2) ; DJ. 2, 142; NS. 3. Test for linearity of temperature means: VR = (S,/i)/(S 1 + S,)/i44 = 7-47I3; D-F. 1, 144! P < Test for identity of within and between temperature slopes: VR = (S t /i)/(s 1 +S,)/i44 = -9953; D.F. i, 144; P < o-ooi significantly higher metabolic rate at +5 C, which may be a seasonal adaptation to summer temperatures in the region of +3 0 to +5 C. A. antarcticus at o C exhibited a different metabolism-weight relationship from that at + 5 and + C, but this was not significant. The metabolism of A. antarcticus in respect to live weight was not affected by a rise in temperature to + C, which was above the summer norm. Considering the weight exponent b in the relationship log P Ot => a + b log W (where P 0 : respiration rate, and W: live weight) for A. antarcticus (Table 2,) b varied from to 0-97 with a mean of over the C temperature range. These values are generally higher than those found for terrestrial mites. Berthet (194) determined that b = 0-72 for 1 species of temperate oribatids, and gave a range of b values: (o C), (+ S c )» "459 (+ 1O C) and (+15 C) for the adult

6 74 W. BLOCK a Live weight (/ig) Fig. 3. Weight-specific oxygen consumption as a function of live weight at o, +s and +io C for Alaskozetet antarctiau. Log scales are used, and the linear regression line calculated on individual measurements is shown for each temperature. mite Steganacarus magnus Nicolet. The coefficients of the slope of the lines at + io C and +15 C did not differ significantly, and both these groups taken together gave a value of These results were partially explained by Berthet in that the correlation coefficient between log oxygen consumption and log live weight was highly significant at all temperatures except + 5 C, and also that the variability of oxygen uptake determinations was greatest at o C, presumably due to decreased sensitivity of the diver. For four species of phthiracarid mites, comprising 44 individuals (mostly adults), Wood & Lawton (1973) determined a weight exponent b of at + IO ^- In three oribatid species (all life stages) the value of b ranged from to For a further 12 species of Cryptostigmata covering 92 individual adults, b was at the same temperature. They considered that, after weight, activity was the most important factor influencing mite respiration. However, the results for A. antarcticus were obtained on resting animals and the effect of activity on metabolism can largely be excluded. Webb (1975) calculated a mean b value of O-8 for all life stages of S. magnus at +18 C. He concluded also that two regression lines adequately represented the relationship between respiratory rate and live weight: one for adults- (In P Oi = hi W-1-819) and another for juveniles (hi F o, = 0-51 In W ). Weight therefore, has a major influence upon metabolism of the micro-arthropods studied, but it is not a constant effect either within or between species at differing temperatures within their normal range. This may be due to the varying degree of chitinization of the exoskeleton between individuals and species. Amongst the mites and Collembola, A. antarcticus has the largest weight exponents so far determined. Oxygen consumption during development In order to examine differences in oxygen uptake between individual life stages of A. antarcticus more closely, the mean respiration rates for each stage at each temperature (Table 4) were calculated from the individual data in Fig. 1. Mean oxygen consump-

7 Oxygen consumption of Alaskozetes antarcticus 75 Table 4. Mean oxygen consumption rates of each life stage of Alaskozetes antarcticus at o, and + C. Mean data individual- 1 andg~ x with the S.E. of the mean and the number of determinations (n) are shown Oxygen consumption Life stage xio-vo, ind-'h" 1 o C (Mean! S.E.) ± n + 5 C (Mean ± S.E.) ± ± ±i ± ±3-877 n ia io C (Mean±8.B.) ± io-737±rii ±8-985 n Larva Protonymph Deutonymph Tritonymph Adult male Adult female Mi 0, g" 1 h~ l Larva Protonymph Deutonymph Tritonymph Adult male Adult female ± S± I3'39S ± ± ± ± ± i58-i34±»4-i ± i7i-522±23-ii ± ± ± O ±5:»-7»4 3O3-782± ± ± ± tion per individual increased steadily with mean live weight of the juvenile stages at each temperature (Fig. 3 a). The mean adult male respiration rate decreased slightly below the tritonymphal rate at o C but increased above it at + 5 C, and also at + C where it continued the juvenile trend. There was much more variability in the results for males at + C than at and o C. A similar situation was seen for the mean female respiratory rate over the three experimental temperatures, except that at + 5 C the female rate continued the juvenile trend and did not increase as in the male. Marked differences between the juvenile stages occurred in mean weight-specific respiration rates (Fig. 3), both within and between temperatures. At o C the mean protonymphal rate was depressed compared to both the larval and deutonymphal rates. Thereafter, there was a steady decrease in metabolic rate with increasing live weight, with the adult female having the lowest rate. At + 5 C there were no significant differences between the juvenile stages; but the adult male rate was distinctly increased compared to the female level. At + C metabolic rate increased greatly from the larval to deutonymphal stage, thereafter a steady decline ensued, with no significant differences between the tritonymph and adult male and female. The results stress the considerable differences in oxygen consumption between the various life stages and these are further complicated by differing metabolic responses to the experimental temperatures. There are few published data on respiration rates throughout development in terrestrial mites. In Nothrus sihestris Nicolet, the protonymphal stage had a high rate (213 1 fi\ 0 2 g" 1 h -1 ) compared with other juvenile stages and adults (range fil 0 2 g- 1 h- 1 ) at + C (Webb, 199). The deutonymphs of Nothrus pajustris C. L. Koch and Parachipteria toiumanni Hammer had significantly higher respiration rates than the protonymph at + C (Wood & Lawton, 1973). Again, the deutonymphal metabolic rate of S. magnus at +18 C was considerably higher than that of the ather nymphal stages, but so also was larval metabolism (Webb, 1975). It seems that Ihere are differences both between species and between temperatures in this respect

8 7 W. BLOCK 40 -L P D r O X a g D. E i i i i i i i t i i i i t 300 I.c 200 O 3 0 I i I I I I I I I Mean live weight (/ig) Fig. 3. Mean (±8.E.) oxygen consumption rate plotted against mean live weight for each life stage of Alcukozetes antarcticus at 0, +5 and + C. (a) Mean oxygen consumption per animal; (J) mean oxygen consumption per g live weight., Larva; At protonymph; D, deutonymph;, tritonymph; A, adult male; O» adult female. (*) 200 for the five mite species for which there are data. In A. antarcticus the proto- and deutonymphs show the greatest metabolic response at + C. The sex ratio of the weighed individuals (see Table 1) was 1 female: rzi male. There were no significant differences in oxygen uptake per individual and per g for male and female A. antarcticus at each of the three temperatures studied. Further comparisons of the mean respiration rates of male and gravid female did not reveal any

9 Oxygen consumption of Alaskozetes antarcticus 77 Table 5. Respiration rates individual- 1 and g- 1 of gravid and non-gravid female Alaskozetes antarcticus at the three experimental temperatures. The mean ±S.E. and (n) are given Adult females Tempera- t * \ Respiration rates ture ( C) Gravid Non-gravid xio"vl O, ind^h- 1 o (5) (3) (5) (3) (7) Not measured o (5) (2) ± (s) ± (3) (7) Not measured differences. This was surprising in view of the fact that the female was heavier (Table 1) and a large proportion (77 %) of the females were carrying eggs. The mean number of eggs was 4-3 per gravid female, and it is interesting to note that gravid females only occurred in the samples from 27 January onwards. These results are similar to C. antarcticus (Block & Tilbrook, 1975), in which the effect of sex on respiratory rate was not detected at the same temperatures. Table 5 shows a further breakdown of the respiration data for adult females into gravid and non-gravid components at each experimental temperature. No data were obtained for non-gravid female A. antarcticus at + C, and thus the comparison is restricted to the two lower temperatures. The gravid female rate was higher than that of the non-gravid female at + 5 C on an individual basis, but a multiple regression analysis of oxygen consumption of the adult female on live weight and number of eggs and prelarvae failed to confirm this difference. For mean weight-specific oxygen uptake the gravid rate was lower than the non-gravid female at both o and + 5 C. Thus there was a slight reduction in metabolic rate due to an increase in the female weight component (Table 1). In N. silvestris, Webb (199) found that gravid females showed a 25 % increase in weight-specific oxygen uptake compared to non-breeding adults (non-gravid females and males). In S. magnus the egg content of the female was more important than live weight or number of prelarvae in its effect upon respiratory rate at + C (Webb, 1975). Wood & Lawton (1973) studying a range of oribatid mites, recorded that in five out of seven species, the gravid female was heavier than the non-breeding adults (i.e. males and non-gravid females) and usually exhibited higher respiratory rates than these at + C. However, significant differences were only established between individual rates'for Ceratoppia bipilis (Hermann) and 5. magnus, and between weightspecific rates for Damaeus onustus C. L. Koch and 5. magnus. These trends are similar to those found in A. antarcticus. Oxygen consumption and temperature Fig. 4 (a) shows the mean individual oxygen consumption plotted against temperature for each life stage of A. antarcticus. For all stages there is a steady increase of respiration rate with temperature over the experimental range. There is a separation into two groups in this respect: one consisting of larva, protonymph and deutonymph, and the other of tritonymph, adult male and female. This may be a reflexion of an BXB 8

10 W. BLOCK Temperature ( C) Fig. 4. Mean oxygen consumption rates of each life stage of AUukozeta antarctiau as a function of temperature, (a) Mean oxygen consumption per animal; () mean oxygen consumption per g live weight., Larva; A, protonymph; D, deutonymph;, tritonymph; A, adult male; Oi adult female. increased individual metabolism from early in the tritonymphal instar, which occurred at all three temperatures, due to an increase in activity. The range of oxygen consumption values over all life stages is small at o C, greatly increased at + 5 C, and largest at + C. At and + C the order of increasing metabolism follows the developmental cycle, but at o C, the tritonymph has a higher level than the adult male and female. At + 5 C the oxygen consumption of the male exceeds that of the female, whereas they are similar at o and + C. A more uniform pattern for all stages emerges when the weight-specific respiration rate and temperature relationship

11 Oxygen consumption of Alaskozetes antarcticus 79 Table. Temperature coefficients (Q ) over three temperature ranges for each life stage o/alaskozetes antarcticus calculated from respiration rates per individual Life stage Larva Protonymph Deutonymph Tritonymph Adult male Adult female o-s C S»a a3 478 S- C a-44 o-io C a a is examined (Fig. 4). The larval stage showed the least increase of metabolism (g- 1 h- 1 ) from o to + C. Q values were calculated for each instar of A. antarcticus for the temperature ranges 0 to +5 C, to + C and o to + C, using the mean respiratory rate animal" 1 (Table ). Over the full o to C range, there was only a small variation in temperature response from a Q of 2-07 (deutonymph) to 3-83 (protonymph). If Q values for the component temperature ranges are examined, more striking differences are apparent. Between o and +5 C, Q varied greatly from i'98 (tritonymph) to 7-23 (adult male), and between +5 C and + C it varied only slightly from 1*78 (larva) to 3* (protonymph). The deuto- and tritonymph stages exhibited least change in Q w, whereas the adult male showed most change over the ranges examined. It is concluded that environmental temperature changes within the normal summer range for A. antarcticus at Signy Island elicit a complex series of metabolic response patterns, which may be associated with activity of the mites. Again, as for the collembolan, C. antarcticus (Block & Tilbrook, 1975), there are very marked changes in Q above and below + 5 C. Diurnal temperature fluctuations in summer in the habitats of A. antarcticus in the maritime Antarctic are probably large ( 5 0 to +1 C; Chambers, 19), but at Signy Island the mean summer temperature is in the range 0*5 to +9 C. The total exposure time of the individual to temperatures around + C in summer is probably small. This may account for the increased variability in respiration levels especially of the adults (Fig. 3 a) and juveniles (Fig. 3 b) at + C. The results suggest, therefore, that there may be metabolic adaptation to summer temperatures in A. antarcticus. Few calculations of Q have been made for Acari. For 1 species of oribatid mites, Berthet (194) found a Q range of 3-5-5*7 with a mean of 4-0 from o to +15 C. A Q of 2*5 was calculated for N. silvestris for the temperature range + io to +20 C (Webb, 199), and similarly a Q of 2-03 for S. magnus over the range +11 to + 25 C (Webb, 1975). These were all adults of temperate species, but they have some similarity with the Qio values for the various stages of A. antarcticus over o to + C (Table ). The larva, protonymph, adult male and female of A. antarcticus have higher temperature coefficients than Webb's species, but lower than Berthet's species. The Q values of these instars of A. antarcticus approach Berthet's mean for adult mites over a similar span of temperature. Compared with a Q^ range of i > 99-2 < 54 for C. antarcticus (Block & Tilbrook, 1975) over the same temperature interval, Q w values for A. antarcticus are generally higher. It is expected that cold-adapted arthropods will -a

12 80 W. BLOCK exhibit a greater response to changing temperature than temperate species, and this is confirmed by A. antarcticus. This is also true for male blowflies (Tribe & Bowler, 198) in which standard metabolism is temperature dependent over the range io -3O C, but not for many marine invertebrates (Newell & Pye, 1971). In Littorina littorea (L.) standard respiratory rate is almost independent of temperature, but the active rate is markedly temperature dependent and the point beyond which a decline occurs varies seasonally. The mechanisms of metabolism-temperature interaction are little understood at present, and current research on A. antarcticus is directed towards this end. An attempt was made to determine if the environmental temperature at the time of collection of the mites from the field influenced their respiration rate as measured in the divers. A multiple regression equation between respiration (log J^O : x - 3 fil O 2 ind" 1 h" 1 ), live weight (log W: fig) and field collection temperature (T: C) was calculated on the individual data at each experimental temperature. The experimental temperatures, mean field collection temperatures (FCT) and the regression equations were: (, () + 5 C(FCT: C);P Oi = PF T(P < 0< 05) : o C);P Ol = W (NS) By testing the regression sums of squares of FCT derived from a comparison of the multiple (above) and linear regressions (Table 2) against the residual sums of squares it was found that FCT only had a significant (P < 0-05) effect on respiration rate at + 5 C. This contrasts with C. antarcticus where a similar effect was detected at o C. DISCUSSION The results of the present study showed that the relationship between oxygen consumption rate and live weight remained constant in A. antarcticus over the temperature range o to + C. Significant differences in respiration level were detected between these temperatures. This is in contrast to a similar study of the collembolan C. antarcticus (Block & Tilbrook, 1975). Comparative data for other species suggest that weight is the major influence on metabolism in micro-arthropods, but this is not constant within or between species and it may vary with temperature. This is probably caused by the varying degree of sclerotization of the different life stages and species for which data exist. The influence of temperature on oxygen consumption of A. antarcticus was also important. In terms of individual respiration and temperature, there were two lifestage groups, one consisting of larva, proto- and deutonymph with low rates, and the other composed of tritonymph, adult males and females with high rates. The early instars (proto- and deutonymph) exhibited the highest weight-specific respiration rates, particularly at + C. Also, the various life stages showed markedly different temperature responses as indicated by Q. The effect of temperature changes, which are representative of those occurring in the natural environment, upon cold-adapted species such as A. antarcticus require further investigation. Comparative respiration data for temperate mites have been recorded by Berthet

13 0 Oxygen consumption of Alaskozetes antarcticus o "3.. o x 50 I 0-5 = (*) + C J I I I Mill ' I I i HIM I I I I III + C 0-5 I I I I I 1111 I I I I Illl I I I I I I Live weight Fig. 5. Comparison of oxygen consumption as a function of live weight at + C for Alaskozetes antarctiau and temperate mite species. Double log scales are used, (a) Individual data for adults of 39 species of Cryptostigmata:, Alaskozetes antarcticus (Present study);, is species (Berthet, 194); T, 1 species (Webb, 199);, ai species (Wood & Lawton, 1973); A, 1 species (Webb, 1975). Regression lines have been fitted for 39 species of Cryptostigmata (C C), 32 species of Mesostigmata (Webb, 1970; Wood & Lawton, 1973) (M M), and 5 species of Prostigmata (Wood & Lawton, 1973) (P P). () Data for all life stages of six species of Cryptostigmata:, Alaskozetes antarcticus (Present study);, Nothrus silvestris (Webb, 199); A, Damaeus onustus; U, Nothruspalustris; x, ParacUptena foiumamti (Wood & Lawton, 1973); Y, Steganacarus magnus (Webb, 1975). Regression line* have been fitted for A. antarcticus ( ) and five temperate species excluding A. antarcticus

14 82 W. BLOCK Table 7. Regression equations of log respiration rate (x io" 3 /A O t md~ x A" 1 ) on log live weight (jig)for six species of oribatid mites at + C. The regressions are calculated on the mean rates for post-embryonic life stages of each species, and the number of measurements (n) together with the correlation coefficient (r) are given (P < o-ooi throughout) Species Authority n a ±8.B. r Alaskozetet antarcticus Nothrtu tilvutru Damaeus ortustus Nothrtu pahatru Parachipteria tmumamti Steganacarus magmu Present study Webb, 199 Wood & Lawton, 1973 Wood & Lawton, 1973 Wood & Lawton, 1973 Webb, ± ± ± ± ± ± (194), Webb (199,1970,1975) and Wood & Lawton (1973). At o C, adults of four species of oribatids over a similar weight range (calculated from Berthet, 194) had much lower ( x lo^/il Oa ind" 1 h" 1 ) respiration rates than adult A. antarcticus (Table 4). Also, at +5 C, adult A. antarcticus showed much higher levels of metabolism, compared to adults of 1 oribatid species (Berthet, 194) but this difference was not so pronounced at + C. Fig. 5 (a) is a double log plot of all the available respiration data at + C against live weight of adult Cryptostigmata. The equation for the fitted regression line (C) of respiration rate (J^o, : x IO" 8 /*! 0 2 ind" 1 h" 1 ) on live weight {W: fig) excluding A. antarcticus is log V Qt = \og lf) W (n = 48, r = , S.E. b = ±o-o44)- Published data for respiration rates at + C of adults of 19 species of Mesostigmata (Webb, 1970; Wood & Lawton, 1973) and adults of five species of Prostigmata (Wood & Lawton, 1973) allow the following equations to be derived on the same basis: Mesostigmata (M): log^j^o, = log TT ( n = 22, r = +0891, s.e. b = ±0-097), Prostigmata (P): log P Oi = log I0 W-o-2i8 (n = 5, r = , S.E. b = ±0-127). It is concluded that on the basis of the available data for temperate mites (adults only), the Cryptostigmata have a lower weight exponent (b) compared to the other two groups (Fig. 5 a). Adults of A. antarcticus at + C have respiration rates comparable to Mesostigmata of similar weight. Respiratory data covering all the post-embryonic life stages of individual mites are very limited. Fig. 5 (b) compares results for six species of Cryptostigmata including A. antarcticus for which life-stage data are available at + C. Linear regressions have been fitted for each species, and the regression equations are given in Table 7. Over the upper part of its weight range, individuals of all life stages of A. antarcticus have much higher respiration levels than other species, which suggests some degree of metabolic adaptation in this species. Variation in the regression coefficient occurs

15 Oxygen consumption of Alaskozetes antarcticus 83 between species, and the b value for N. silvestris (0-930) approaches that of A. antarcticus ; the remaining species having lower weight exponents. A general relationship linking oxygen consumption and live weight has been derived for the six oribatid species in Fig. 5(): log P 0, = 0-14 log W (n = 38, r = , s.e. b = ±0-049). A similar relationship of log e J^Oi = 0-7 log e W 0-0 has been derived from data covering adults and nymphs of several oribatid mites (Chapman & Webb, 1977). By contrast, analysis of life-stage respiration and weight data for four species of Me8ostigmata (Wood & Lawton, 1973) gives the following relationship: log I^Oi = log 1Q W ( n = 1, r = +0-82, S.E. b = ±0-128), indicating that live weight has a greater influence on the measured respiration rate over the life cycle in these mites than in the Cryptostigmata. The present study reports a b value ranging from (o C) to ( + C) for A. antarcticus. This is generally higher than that of other Antarctic invertebrates which have been studied: (o C) to (+io C) for cultured C. antarcticus (Tilbrook & Block, 1972), (+ 5 c ) to + O<82 5 ( + IO c for field ) C. antarcticus (Block & Tilbrook, 1975) and ( + 5 and + C) for the tardigrade Macrobiotus furciger J. Murray (Jennings, 1975). Individual respiration is almost directly dependent upon live weight rather than surface area in A. antarcticus. Zeuthen (1947) concluded that animals < 1 g in weight do not obey the surface law of V r Ot = a W 0 "* 7, and this is confirmed by examination of the limited micro-arthropod data available. Live weight has been shown to be a major factor affecting individual respiration levels in such arthropods, but its effect varies within the species according to developmental stage and between species over their normal temperature range. Studies of respiration and growth rates together, over a range of field temperatures, in selected arthropods are required to clarify this. Considering the effect of temperature alone on the respiration of A. antarcticus, a regression analysis of the individual data for the three experimental temperatures allows the following equation to be derived: log J^Ot = (n = 148, : r = +0-37, S.E. b = ±o-on), where Po t respiration rate (x io" 3 fd O 2 ind^h" 1 ) and T: temperature ( C). A mean Q value may be calculated as the antilog of x coefficient b, which is 3-31 and this corresponds to the average value for the range of temperature coefficients given in Table. However, the weak regression and correlation coefficients confirm that temperature alone does not influence respiration significantly in this species. Berthet (194) transformed his individual weight data to a standard weight equal to the mean of all the mites used in his respiration experiments. For adult data from 1 species at +5 0, +io and +15 C he calculated the relationship, log I^Oj = ( n = 4-8)- The regression coefficient for temperature is not significantly different from that of A. antarcticus, but further comparison is precluded because of life-stage differences in the data. The effects of both live weight and temperature on individual respiration of A. antarcticus can be examined by means of a multiple regression, the equation log t r Ot = log ft+o-o42i9t' (n = 148) being calculated, where ^0, : respiration rate (x io" 3 fil O t ind" 1 h~ x ), W: live weight (fig) and T: temperature ( K). This was further developed in respect of the absolute temperature by analogy with Arrhenius' law to give log J^Ol = log W o-^ztyt (n = 148), where F 0 : respiration rate (xio~-*/il O a ind -1 h -1 ), W: live weight (jig) and

16 84 W. BLOCK T: ix io*/t absolute. This general relationship is representative of the active life stages of A. antarcticus over the temperature range o to + io C. This allows a comparison with Berthet's (194) generalization for adults of several species over the temperature interval to + i5 C,wherelog V o% = \og W o-$*lt. It can be seen that weight exerts a greater influence on respiration level in A. antarcticus than in the temperate species, and that temperature has a slightly reduced effect on respiration for the Antarctic mite. In order to compare the equations resulting from the present study with those from Berthet (194), substitution in the equations was made for an average mite (83-88 fig) calculated from the live weights of all the life stages of A. antarcticus together with adults of Berthet's 1 species. Two temperatures, and + C were used which represent the overlap of the two experimental temperature ranges. The calculated respiration rates were: A. antarcticus equation+ 5 C, t r Ot = 2-2 x io" 3 /A O 2 ind~ x h -1 1 species equation+ 5 C, X r Ol = 77-2 xio^/il O 2 ind" 1 h -1 A. antarcticus equation + C, V Ol = x IO" 3 /d O 2 ind" 1 h -1 1 species equation+ C, V r Ot = x io^/il O 2 ind" 1 h -1 The respiration values derived by the A. antarcticus equation are 3-45 times greater than those from Berthet's equation at + 5 C and 2-73 times greater at + C. This trend is continued over the complete weight range of all the material at each temperature. Although there are considerable differences, such as life stage and habitat, between the two sets of mites on which the equations are based, it is clear that the Antarctic species has a much higher level of oxygen consumption than temperate species over the range +5 0 to + C. It is concluded that for A. antarcticus there is translation of the metabolism-temperature curve but no rotation (Precht, 1958). It is suggested that this species occupies an intermediate position in respect of its metabolism being partially independent of environmental temperature fluctuations (Hazel & Prosser, 1974), a similar situation to the majority of invertebrate poikilotherms. The available data on metabolic rate (weight-specific oxygen consumption) and live weight, which have been reported for a range of Antarctic terrestrial invertebrates, are shown in Fig.. For A. antarcticus, only Marsh (1973) is comparable, and his average value lies within the range of results for the present study at + 5 C. The collembolan C. antarcticus is generally smaller in size, and its metabolic rate (jil O a g" 1 h -1 ) is higher than that of A. antarcticus. The other collembolan Isotoma klovstadi Carpenter is larger than C. antarcticus, but higher metabolic rates have been measured for this species even at 4 C (Strong, Dunkle & Dunn, 1970). Gamasellus racovitzai (Trouessart) (Goddard, 197a), a mesostigmatid mite, although of similar weight to A. antarcticus, has a higher metabolism probably due to its greater locomotory activity at low temperatures associated with its predatory role. The Antarctic Prostigmata are amongst the smallest arthropods to be measured with a Cartesian diver microrespirometer, and several of them are very active species. Hence their higher metabolic rates (Goddard, 197; Block, 197), compared with other mites and the Collembola. Considering all the data for Antarctic terrestrial invertebrates, a relationship of metabolic rate to temperature over the range 4 0 to +22 C has been derived as:

17 Oxygen consumption of Alaskozetes antarcticus Temperature ( C) Fig.. Metabolic rate as a function of temperature for ten species of Antarctic terrestrial invertebrates. A regression line has beenfittedjtothe combined data ( ). Data are taken from various sources: 0, Alaskozetes antarcticus (Present study);, A. antarcticus (Marsh, 1973); A Cryptopygus antarcticus (Block & Tilbrook, 1975),, C. antarcticus (Dunkle & Strong, 197a);, C. antarcticus (Marsh, 1973); Y, Isotoma klovttadi (Strong, Dunkle & Dunn, 1970), x, Gamasdlus racovitzai (Goddard, 197 a); +, five species of Prostigmata (Goddard, 197), and Oi Macrobiotus furciger (Jennings, 1975). l gio ^o, = T(n = 24, r = , s.e. b = ), where P Oi : metabolic rate (/il O a g" 1 h" 1 ) and T: temperature ( C). The overall Q calculated from the regression coefficient b is 3*04, which is slightly higher than the majority of values found for temperate micro-arthropods. Comparison of this relationship for Antarctic species with that derived from 9 species of temperate Acari over o to r + 25 C, where log P o, = (71=9, = + -44, s.e. b = ), indicates that there is no significant difference between the regression coefficients. Therefore it may be concluded that the metabolic response to temperature of the two groups of Antarctic and temperate terrestrial invertebrates is similar over their normal temperature ranges, but the Antarctic group has a generally elevated level of metabolism. This elevation is 3 to 5 times the metabolic rate of temperate Acari over the range o to + 20 C. It is possible, knowing the respiratory rate of an animal and the calories lost for each unit of oxygen consumed, to calculate the minimum energy required to support metabolism. Using anoxy-calorific equivalent of 4-74 calories ml- 1 of oxygen consumed (Petrusewicz & Macfadyen, 1970) for an animal feeding on a mixed diet and with an RQ of 0-82, and the respiratory rates of A. antarcticus determined at Signy Island (Table 4), it was calculated that from Qarva at o C) to (adult male at + C) /ical ind" 1 24 h" 1 were required for maintenance metabolism. Marsh (1973) using a mean respiration value of x io- 3 /*! O 2 ind^h" 1 at +5 C,

18 8 W. BLOCK an ingestion rate of fig dry weight of food 24 h" 1 and a calorific food equivalent fo 47 /teal fig- 1 determined that 78-2 % of the energy assimilated was utilized in respiration for an individual A. antarcticus of 81-0 fig weight. Marsh's mean respirationfigureat + 5 C falls between the tritonymphand adult values in the present study (Fig. 3 a), but his mean weight corresponds to midway between the deuto- and tritonymphal stages (Table 1). Notwithstanding these differences, a similar calculation of the proportion of energy used by A. antarcticus in maintenance metabolism, based on the Signy Island respiration data at +5 C and a mean weight of Si-Ofig, gave a value of 82-3 %. This is rather higher than Marsh's estimate, but together the two values suggest that the majority of energy assimilated by A. antarcticus is used in respiratory metabolism. It is concluded that this is probably a feature of microarthropod energetics in habitats with low environmental temperatures, leaving only a small proportion of energy for production of tissues and young. Hence, these animals have slow growth rates and generally long life cycles compared to temperate species (Block, 195). There has been little attention paid to metabolic compensation to temperature amongst terrestrial poikilotherms of high latitudes. In studies of arctic insects, Scholander et al. (1952) found scant evidence for a relative elevation in respiratory activity at low temperatures. This is in contrast to the results of the present study. Most coldadapted terrestrial arthropods are probably dependent on exposure to relatively high environmental temperatures, for albeit short periods, for their activity and development. At Signy Island, habitat temperatures in the summer for A. antarcticus are mostly in the range 5 0 to +9-5 C, when growth and reproduction are maximal. As temperatures fall below o C, a capacity to survive mechanical damage due to freezing rather than to compensate metabolically becomes more important. With the severe winter conditions ofits habitat (minimum temperature 2O to -3O C), A. antarcticus must be able to withstand freezing in most of its life stages. Investigations are presently in progress to determine if the capacity to withstand freezing is linked to a facility to supercool, and to elucidate the possible mechanisms involved. I thank the British Antarctic Survey for support throughout the Antarctic summer season, the Leverhulme Trust for a Research Fellowship, the Royal Society for a travel grant and Leicester University for leave of absence, without which this research would not have been possible. Finally, I appreciate the criticism of draft manuscripts by Drs P. J. Tilbrook, R. R. Harris and N. R. Webb. REFERENCES BERTHFT, P. (194). L'activiti des Oribatides (Acari: Oribatidae) d'une Chftnaie. Mint. Init. r. Set. nat. Belg. 15a, BLOCK, W. (195). The life histories of Platynothrus pdtifer (Koch 1839) and Damaeut clavipes (Hermann 1804) (Acarina: Cryptostigmata) in soils of Pennine moorland. Acarologia 7, BLOCK, W. (197). Oxygen uptake by Nanorchestes antarcticus (Acari). Oikos 37, BLOCK, W. & TILBROOK, P. J. (1975). Respiration studies on the Antarctic collembolan Cryptopygus antarcticus. Oikos a, CHAMBERS, M. J. G. (19). Investigation* of patterned ground at Signy Island, South Orkney Islands: II. Temperature regimes in the active layer. Bull. Br. Antarct. Surv. io, CHAPMAN, S. B. & WEBB, N. R. (1977). The productivity of a Calhma heathland in southern England In The Ecology of some British Moors and Montane Grasslands (ed. O. W. Heal and D. F. Perkins). Springer-Verlag (in the press).

19 Oxygen consumption of Alaskozetes antarcticus 87 DUNKLB, R. & STRONG, F. E. (1972). A digital electrolytic micro-rcspirometer. Ann. ent. Soc. Am. 5, GODDARD, D. G. (197a). The Signy Island Terrestrial Reference Sites: VI. Oxygen uptake of Gamatdhu racovitzai (Trouessart) (Acari: Mesostigmata). Bull. Br. Antarct. Sure. 45, 1-. GODDARD, D. G. (197). The Signy Island Terrestrial Reference Sites: X. Oxygen uptake of some Antarctic prostigmatid mites (Acari: Prostigmata). Bull. Br. Antarct. Surv. 45, GRESSITT. J. L. (197). Notes on arthropod populations in the Antarctic Peninsula- South Orkney Islands area. Antarct. Res. Series io, HAZEL, J. R. & PROSSER, C. L. (1974). Molecular mechanisms of temperature compensation in polilotherms. Physiol. Rev. 54, HOLTER, H. (1943). Technique of the Cartesian Diver. C. r. Trav. Lab. Carlsberg Ser. Chim. 24, JENNINGS, P. G. (1975). The Signy Island Terrestrial Reference Sites: V. Oxygen uptake of Macrobiotus furdger J. Murray (Tardigrade). Bull. Br. Antarct. Suro. 41 & 43, LLNDBRSTRBM-LANG, K. 11.(1943). On the theory of the Cartesian Diver micro-respirometer. C r. Trav. Lab. Carlsberg Ser. Chim. 24, MARSH, J. B. (1973). Radioisotopic determination of the ingestion rates of three species of Antarctic arthropods: Cryptopygus antarcticus Willem (Collembola: Isotomidae), Belgica antarctica Jacobs (Diptera: Chironomidae) and Alaskozetes antarcticus (Micheal) (Cryptostigmata: Podacaridae). M.Sc. thesis, University of California at Davis. NEWELL, R. C. & PYE, V. I. (1971). Quantitative aspects of the relation between metabolism and temperature in the winkle Littorina littorea (L.). Contp. Biochem. Physiol. 38B, OSTLE, B. R. (193). Statistics in Research. Iowa State University Press. PETRUSEWICZ, K. & MACFADYEN, A. (1970). Productivity of terrestrial animals, I.B.P. Handbook No. 13. Oxford: Blackwells. PRECHT, H. (1958). Concepts of the temperature adaptation of unchanging reaction systems of cold blooded animals. In Physiological Adaptation (ed. C. L. Prosser). Am. Physiol. Soc SCHOLANDER, P. F., FLAOO, W., WALTERS, V. & IRVINE, L. (1953). Climatic adaptation in arctic and tropical poikilotherms. Physiol. Zool. 2, STRONG, J. (197). Ecology of terrestrial arthropods at Palmer Station, Antarctic Peninsula. Antarct. Res. Series, STRONG, F. E., DUNKLE, R. L. & DUNN, R. L. (1970). Low-temperature physiology of Antarctic arthropods. Antarct. J. U.S. 5, 123. TILBROOK, P. J. (197). Arthropod ecology in the Maritime Antarctic Antarctic Res. Series, TILBROOK, P. J. (1973). The Signy Island Terrestrial Reference Sites: I. An introduction. Bull. Br. Antarct. Surv. 33 & 34, 5-7. TILBROOK, P. J. & BLOCK, W. (1972). Oxygen uptake in an Antarctic collembole Cryptopygus antarcticus. Oikos 33, TRIBE, M. A. & BOWLER, K. (198). Temperature dependence of 'standard metabolic rate' in a poikilotherm. Comp. Biochem. Physiol. 25, WAIXWORK, J. A. (192). A redescription of Notaspis antarctica Michael, 1903 (Acari: Oribatei). Padf. Insects 4, (4), WALLWORK, J. A. (197). Cryptostigmata (oribatid mites). Antarct. Res. Series, WALLWORK, J. A. (1973). Zoogeography of some terrestrial micro-arthropoda in Antarctica. Biol. Rev. 48, WEBB, N. R. (199). The respiratory metabolism of Nothrus silvestris Nicolet (Acari). Oikos 20, WEBB, N. R. (1970). Oxygen consumption and population metabolism of some mesostigmatid mites (Acari: Mesostigmata). Pedobiologia, WEBB, N. R. (1975). Respiratory metabolism of Steganacarus magnus (Acari). Oikos 2, 43-. WOOD, T. G. & LAWTON, J. H. (1973). Experimental studies on the respiratory rates of mites (Acari) from beech-woodland leaf litter. Oecologia 12, ZEUTHKN, E. (1947). Body size and metabolic rate in the animal kingdom. C. r. Trav. Lab. Carlsberg Ser. Chim. 2, ZBUTHEN, E. (194). Microgasometric Methods: Cartesian Divers (pp ). 2nd Int. Congr. Histo- & Cytochemistry (ed. T. H. Schiebler, A. G. E. Pearse and H. H. Wolff). New York: Wiley.

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