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The effect of water quality on the size and fecundity of Asellus aquaticus (Crustacea:Isopoda)
101
Aquufic Toxicokrgy, 1 (1981) 101-l I2 ElsevierINorth-Holland Biomedical Press
THE EFFECT OF WATER QUALITY ON THE SIZE AND FECUNDITY OF ASELLUS AQUATICUS tCRUSTACEA:ISOPODA)
M.R. TOLBA and D.M. HOLiilCH tleporlmPnt of Zod~g~ the ~~~i~ers~~y~ No~~~~~~~ NG7 .?ftf3. En&& (Received I5 December 1980; accepted 20 February 1981)
Samples of Asellus uquotincli (L.) were collected from a numb,rr of freshwater sites in centra! England exhibiting either clean, semi-polluted or polluted condition!,. The ;:lean sites contained naturally daaying organic matter but the polluted rites also contained sewage and toxicants. The asellids were found to vary in the maximum length attained, and in pereonal width and fresh hod! weight. in response to water quality. In asellids from polluted sites the body size (length x aterage pereonal width) and wet weight were significantly lower than those from a clean site. Asellids frc.: . semipolluted sites showed an intermediate condition. The number of eggs contained in the marsupia of ovigerous females increaseswitn body size. However, the actual number of eggs is dependent upon the water quality. there being more eggs in the marsuaia of all sizes of ovigerous females from clean sites than those from polluted sites. Asellids from semi-p&luted sites showed an intermediate condition. Ovigerous females from polluted sites, kept in clean water, produced Fg generations which, -hen grown in clean water, showed a significant increase in body size and weight. Ft generation males, derived from clean water females and grown up in clean or polluted * ater at 20 ‘C, showed a significantly Ia:ger increase in width for any given body length in clean water than they did in polluted water. Care must therefore be taken when comparing growth rates of animals from different geographisal
locations which exhibit different degrees of water quaiity. Key words: Aselfus; isopod: water quality; size; fecundity
INTRODUCTION
The biological impact which pollution has on the freshwater environment is usually measured in terms of changes in population density and community structure (e.g. Learneret al., 1971; Etryceei al., 1978). Increasedlevels of organic pollution for instance may lead to a decreasein species diversitybut to an increasein the number of individuals of a tolerant species. Asellus aquaticus is a case in point, oecurriIlg commoniy in relatively clean water, as part of a diverse commurlity (Adcock, 1979), but reaching much higher densities in organically polluted rivers, where species diversity is low (Hawkes and Davies, 1974; Bryce et al., 1978; Aston and Milner, 1980). Numerous experiments have been reported in the literature concerning the effects which various pollutants have on the survival of individual 0l66-445X/81/MxIo-oooU/$02.50
d Elsevier/North-Holland
Riomedical Press
species, However, few workers have attempted to evaluate the effect of total water quality on the growths of individuals within a ~pulatio~. Stebbing (1979) has attempts this with the marine hydroid, ~ff~~~~~~~~ff _&WOSZ.We found that coionies produced diff~ent growth patters and frequencies of gonozooids when subjected to natural water af different quality in the laboratory. Holdich and Tolba (1980) have shown that water quality and temperature have an effect on the rate of ~~~rnent of A, ~~~~~~~ eggs, with potlnted water speeding up de~~e~oprn~tof the eggs of &an water a&lids, thus enabling them to quickly get over a critical phase in she life&tory, when placed under stress. The present study was undertaken to assess the effect that water quality has on the size and fecua~ty of post-marsupial A. ff~~ufjc~. Aston and Milner (198O}found that the longest individuais of this crustacean were associated with locations on the River Trent (in the Midlands region of Eng~~d) which exhibits the heaviest organic ~l~ution loads. However, length is not always the best measure of ‘size”, e(en though it has been extensively used in studies of As&s growth (Steel, 1%1: Andersson, 1969; Prus, 1972, 1977a,b; Marcus et al., 1978; 0kland, 1978; Rossi and Vit~l~ano-Tadini~ i978; Adcock, 1979; Willoughby and Marcus, 197% Astou and Milner, t%O). When isopods moult they usually increase in width as we!1as length. and weight increases over the intermoult period. It will be shown in this paper that water quality affects the weight and body size (length x average pereonal width) of A. ~~~~t~~s - water of poor quality depressing these parameters. A nunther of workers listed above have reported a positive relationship between the length of ovigerous female A. ~47~utict~ and the number of eggs iuitially contained within the marsupium. However, individuals of the same length do not necessarily have the same sized marsupial as this i? aiso dependent upon their width. Therefore the relationship between body size and egg number was also examined at a number of locations exhibiting different degrees of water quality.
Samples of A. aquatics were collu:ted manually every month from IO March 1978 to 15 March 1979 f?om six main &es within the Trent River system. At least tOOindividuals were collected on every occasion, and usually many more. One site (Notti~ham Canal} was considereri to contain relatively clean water, it contains very few toxicants. aIthough it was eutrophic. It had a much higher Trent Biotic Index (Woodiwiss, I%4) than the other sites. The other five sites all showed varying degrees of pollution (e.g. sewage and heavy metals) mainly derived from industriitl and domestic sources. In addition, samples were collected during the summer of t979 from a number of other sites for comparative purposes, but details of the variables were not recorded (‘Fable I). All the sites were contained within the area bordered by latitude 52’1 YN and 53O3o’N,and longitude 2” IO’W and 0” iO’E, Each individual aseihd longi:r than 3 mm, below which it is difficult to separate
104
the sexes, was staged: male, non-ovigerous female or ovigerous female. Numbers of early stage eggs in the marsupia of the latter were counted. Measurements (Fig. 1) of body length and body width were made using a graticule under a dissecting microscope. Individuals were then blotted dry on filter paper and weighed. Ovigerous females, with eggs at stage A or B (Holdich and Tolba, 1981) were isolated from collections made at Nottingham Canal and at three polluted sites, i.e. River Erewash, and Water Orton, above and below Minworth Sewage Works, on the River Tame. The females were allowed to incubate their broods in polluted or clean water (Nottingham tap-water). When the juveniles were released 200 from each site were allowed to grow in the same type of water they had been incubated in. The experiments were continued for 90 days and the asellids were measured every 14 days. Food was provided in the form of decaying elm leaves and the temperature was maintained a; approximately 20 “C. The measurement of the variables listed in Table 1 were made with the equipment outlined in Holdioh and Tolba (198 I) every month from 10 March 1978 to 15 March 1979. Separate regression analyses were carried out for all measurements of the body length/width, body size/wet weight and body size/egg number relationships using
f-ig. I. Diagrams of female (A) and male (B) A. aquatinrs to show positions from which measurements of body length and pereon,al widths were taken.
105
the standaid equation: Y = A + BX, where Y is the width, wet weight or egg number, X the length or body size, A the calculated Y intercept and B the slope. Comparisons of the regression data were carried out for pairs of sites to show whether or not the differences were significant. RESULTS
The relationship between body length and pereonal width in males and females
It can be seen from Fig. 1 that the maximum pereonal width of males and of females lies at different levels, being a! pereonite 3 in females and pereonite 6 in males. The minimum width occurs across pereonite 1 in both sexes. A gross comparison of random collections from a clean and a polluted site clearly shows a difference in body size between the two sites (Fig. 2), those from the clean site being much larger. The maximum lengths of males and females collected from clean sites during the period March, 1978 to March, 1979 were 20 mm and 16 mm respectively, for polluted sites the lengths were 14 mm and 12 mm respectively. If body lengths are plotted against the mean widths of pereonites 1 and 6 for males, and pereonites 1 and 3 for females, there is an obvious increase in width with an increase in body length (Fig. 3). However, if the regression lines are compared for animak from clean and polluted w;:rer there is a significant difference (P s 0.001) for the male data, and also for ti!e female data, although the difference between males and females is not so clear-czt. Ttrerefore, both males and females of any length over 3 mm from clean sites are significantly wider than the corresponding
big. 1. Photographs ol’ random GUII~IC\ of :i. uqrturrtvr.~from il clean (A) and a polluted (BI site.
Fig. 3. Regression lines of mean pereonal width on body length for females (a, Nottingham Canal; c. Water Orton above) and maks (b, Nottingham (a) Y = 0.463 + 0.334X.
f -- 0.99;
r = 0.99; (d) Y = -0.476
(b)
+ 0.3!2X,
Canal; d. Water Orton above). Regression data:
Y = 0.911 + 0.222X.
r = 0.96;
(c)
Y = - 1.040 + 0.422X.
r = 0.99.
b
Bodysize
mm?
Fig. 4. Regression lines of egg number on boidy size for ovigerous females from (a) Nottingharl
Canal;
(b) Saltfleet Ha,ven; (c) Attenborough; (d) Wnter Orton above; (e) R. Erewash. Regression data: (a) Y = -11.340+7.200X, r = 0.w
r-0.99;
(d) Y = -32.040
(b)
Y=
-26.170+6.170X,
r=0.99;
+ 5.370X, I = 0.99; (e) Y = -23.870
(c)
Y=
-31.63Ot6.ooOX.
+ 4.540X, r = 0.99.
sexes from polluted water. As length is not therefore a good measure of the size of as asellid, the body size (length x average pereonal width) is used in future comparisons between sites.
The relationship between body size and the number of marsupial eggs
One might expect that long ovigerous females will have a larger marsupium than short individuals. Measuring length only, however, may hide the fact that females which are the same length may be of a different width; wider females may therefore have a Iarger marsupium and so be able to incubate more eggs. If egg number is plotted against the body size there is an increase for every size of female (Fig. 4). However, if the regression lines for sites having different degrees of water quality (Table I) are compared. then it can be seen that egg number varies considerably. All sizes of ovigerous females from the wliuted sites (Water Orton above and R. Erewash) have significantly lower egg number (P ~~0.001) than those found in Nottingham Canal. The water-filled gravel pits at the Aricnborough Nature Reserve have in recent years become semi-polluted due to the fact that water from the R. Erewash enters it now. The Great Eau River at Saltfleet Haven is miIdly polluted from domestic and agricultural sources. Both these sites show egg numbers which are higher than the polluted sites but lower than the clean site. The relatio qship between body size and wet weight
Even though animals may appear to have similar body sizes their weights ma) differ due to a number OFinternal factors, e.g. the reproductive state, the quality of
Fig. 5. Regression lines of wc~ weight on body size of males (a) and females tb) from Nottingham Canal and of mates (d) from Water Orton above. Also shown is 11~seffrvt of clean water on Water Orton above juveniles(c),
Regression data: (a) Y = - 3.715 + I .440X. r = 0.98; (b) t’ =
(c) Y = -2.220
t lAMOX. r = 0.98; (d) Y = 0.598 + 0.800X. r = 0.88.
3.520 + 1.060x’. f = 0.96~
Fig. 6. ~qp+on lines of wn:sweight on body size of mules from Wuter Orton hetow (e), R. Errwash (d) and Snardlow le). Also shown ia the el’tix~ on H. Er,uwash(a) and Water Orton below (h) juveniles elf
~r~}wjt~~(hem up is clcon water. Regression data: (a) Y = - 3.202 + 1.3XLY. r fx 0.96; th) 1’ z )- 2.220 c ).&@Rx, r ~7O.YL:. (c) Y c t.SYH t 0.970X. r r~ 0.95; fd) Y = 1,923 t O.MOX, I 1 0.9% (c) Y 7 3.248
I O.WX. r 2:O.YS.
Fig. 7. Reeressionslines of wet weight or: body sire ol’ males from Somcrsham pond (a).Attcnborough (b), lairham &rook (c), Wilford (d) and Kingsbury (c). Regression dulu: (al Y =: -3.7H4 + 1.540X, I.864 + c .i 0.9X; (b) Y = ‘2.23 c I.OlOX, r .z 0.94; (C) Y = .- 0.954 + 0,070x, r z 0.X; fd) Y =. O.YWIX.
r “z o.uz;
(C) Y .&
2.454
t 0.900X,
r = 0.99.
their foo.f. whether they have eaten recently or not, and disease. In addition, errternalI:Yctorssuch as water quality may also hsve an effect. When the wet weight of P, ~~~u~i~~~ is plotted against body size there is a linear relationship (Figs. S-1). A. uquuricus from Nottingham Canal show a positive increase in wet weight with every increase in body size - that for males being larger than that for females (Fig. 5). ??.owever,individuals from Water Orton above show a much smaller increase (Fig. 5). The difference between males for the two sites is signi~cant (P < 0.001). Individuals fram a water-filled quarry in Somersham (a clean site) were sir$lar to ,thoscfrom Nottilngham Canal (cf. Figs. 5 and 7). Individuals from polluted sites on the R, Tame (Kingsbury and Water Orton below}and the R. Trent (Shardlow and Wilford) were similar to those frum Water Orton above, whilst those from sites exhibiting semi-poollutedwater (Fairham Brook and Attenborough) were in between those from Water Orton and Nottingham Canal (cf. Figs. 5, 6, 7).
ln order to assess the effect of clean and polluted water on the growth of A. u9~Qt~e~~ a number of ovigerous females rvere transferred from Water Orton above and below, and from the R. Erewash, and kept in clean water throughout their brooding periods. When FI generations resulting from these female:; were grown up for 90 days in clean water they showed a significant increase in body size and body weight compared with individuals collected from the field (Figs. 5 and 6). This is particularly noticeable in the larger individuals from the R. Erewash (P s 0.01) (Fig. 6). To show that water quality affects growth, in terms of width, individuals derived from clean water females were grown up in clean water or poliuted water (R. Tame) at the same temperature (20 “C). When mean pereonal width is plotted against body length there is a significant difference (P s 0.05) in the growth of asellids from the two sites - width being depressed in those growing in polluted water (Fig. 8). DfSCUSSlON
Two implicati~l~s are apparent from the results obtained in this study. Firstly, as mentioned in the introduction, many workers have used body length to assess growth in A. aqtmicus, and they have made comparisons between their populations and those of other workers. It has been shown in the present paper that in a fairly small geographicA area of England the body size and weight of individuals of the same length can differ sig~ficantly. Therefore, future studies should include a
110
abjuration of o:her parameters besides length to obtain a true indication of ‘growth’. Second.ly , the quality of the water in which populations of A. uquuticus jive &x:ts their grctwth and the fecundity of the females. in this paper the effect of total water quality has been examined, the effects 0’ organic pollution and that of toxicants in the water have not been distinguished. The clean site which was exexsmhd mg&trly ~Nott;ngham Canal) contra large amounts of dads natural vegetation but the ievels of toxicants were very low. Species diversity was high at t.his site but the population densitiesof the isopod wetlSlover than in the ~llut~ sites (To&a, unpubl. obs.). lndivid~~s were large and ov@erous females produced large broads. The sites studied on the R. Tame, R. Erewash and R. Trent are ah known to be polluted by sewage and toxicants and to have a low species diversity (Severn ‘Gent 1. ater Authority, 1979), and independent observations have confirmed this (Table I). Biological oxygen demand, conductivity, and levels of phosphates, chloricbs and heavy metals such as lead, copper, zinc and ~dmium all tended to be higher than in Nottingham Canal. Oxygen levels were fairly high and pH levels normal at all sites and these did not appear to be a limiting factor in the ovulations studiecl. All she polluted sites contains high densiti~ of A. uq~ut~~~, possibly due to the large amounts of food and lack of competitors and predators. Howevfer, individuals at these sites tended to be smaller and females produced smaher broods. Temperature levels variqd between the sites over the study period. In the R. Tame, 6:. Trent and R. Erewash water temperatures were consistently higher than for the &an and semi-polluted sites due to the input of heated effluents from andustry, in particular electricity generation plants, Only Nottingham Canal froze to any depth in the winter months, The sites at Fairham Brooh, Saltfleet Haven and Attenb~~rough all had high species diversity in the summer months. but were subject to low levels of organic and chemical pollution from agricultural and domestic sources. interestingly, the results from these sites were intermediate between those of P~ottingham Canal and the polluted sites. The only detaitecicomparative study which has been made on A. uguutiws from a number of sites ii that of Aston and Milner (1980). They found an inverse relationship between the lengths of ovigerous females and the mean t~~ratu~ in the R. Trent - the smallest individuals occurring where temperatures wete high and organic pollution tow. Other stages of the life-history showed a similar trend, However, they also found that organic pollution in the same river fed to an increase in population density and to an increase in the length of males and non-ovi BUS females. Aston and Brown (1975) have found similar results for leeches the R. Tame. Althoug:? increased organic matter may provide more food for A. uquctticusin the R. Treitt the actual amount of growth attained by the isopods less than they ace crpable of - as shown by our transfer e~~~rnents. This s that other factors such as heavy metals may be depressing growth in polluted parts of ti.le R. Trent, In Nottingham Canal, where the comes from natura ly decaying vegetation and not from sewage, the isop&s are able to grow to a larger size.
According to Aston and Milnet (1980) the relatively high temperatures in certain stretches of the R. Trent may bring forward sexual maturity in A. uqwricus by diverting energy from growth to egg production, but may also have the effect of making animals smaller due to the facr that more energy has to be utilized for activity and maintenance at the higher temperatures. It seems likely that if more utilized for activity and maintenance and less for growth then less production - thus resulting in smaller broods, as From our results it could be concluded that the small size tions where temperatures are relatively high, may also be effect of non-thermal pollutants on growth. Thermal pollution tically with certain chemical pollutants thus having a greater occur in non-thermally polluted water. In our transfer eqximts jutik from polluted water were grown at 20 “C, which is somewhat n temperature for any of the sites studied (Table I). and yet m the absence of non-thermal pollutants. There was also a signirint difference betwee th in terms of width for clean site FI individaals at 20 ‘C. Holdich and Tolba (1980) found that grown in clean and polluled the developmental r&es, over a range of temperatures, were similar for A. uquaticus from clean and pol sites when they were grown in water from their home . (When clean site were transferred to polluted water they reacted by developing at a faster rate - presumably a response to environmental stress.) However, in the field. the post-marsupial stages of polluted site asellids may well develop faster in polluted water than in clean water, but the actual amount oi growth they achieve is significantly less. More work now needs to be carried out tc, assess the underlying physiological differences between A. uquuficus from clean and polluted sites, and to determine the effects of individual pollutams on growth.
Thanks are due to Zagazig University, Egypt and the Egyptian Government for financial support. and lo Professor P.N.R. Usherwood for the provision of laboratory facilities. REFEREM-ES
112
Aston, R.J. and A.G.P. Mibner. 1980. A comparison Of populations Of the isopod A~~I%u aqualkws above and txlow power stations in organically polluted reachesof the River Trem. Freshwat. Biol. 10. l-14. &ycq D., i.M. Caffoor, C.R. Dale and A.F. Jarrelt. 1978. Macroinvertebrates and the bioassay of water quality, a report based on a survey of the River Lee. Nelpress, London, 30 pp. aepafimmt of the Environment, 1975. River Pollution Survey of England and Wales - updated 1973. River Quality and Discharges of Sewage Effluents. H.M.S.O.. London, 6) pp. Hawkes, H.A. and L.J. Davies, 1974. Some effecls of organic enrichment on ‘rmthic invertebrate ccrmmunitiesin stream riffles. In: The scienMic management of animal ond plant communities for conservation. edited by E. Duffy and A.J. Watt. 1 Ith Symposium of the tlritish Ecological Society 1970. pp. 271-293. Holdich, D.M. and M.R. Tolba, 1981. The effect of temperature and water quality on the in vitro development of AX&U ixpu~icus (Crust.acea:lsopoda) eggs. Hydrobiologia, f&227-236. Learner, M.A., R. Williams, M. Harcup ant B.D. Hughes, 1971. A survey of the macro-ftuma of the River Cynon. a polluted triba!ta:y of the River Taff (South Wales). Freshwat. Biol.“:. 339-367. Marcus, J.H.. D. Sutcliffe and L.C. Willouphby, 1975. Feeding and growth of AS&AS uquafio1s (Isopoda) on food items from the litloral of Windermere, including green leaves of Elodtw cwn&eti. Freshwat. Biol. 8, 505-519. Okland, K.A.. 1918. Life hisiory and growth of Asellus uquaricus (L.) in relation to environment in a eutrophic lake in Norway. Hydrobiologia 59. 243-259. Prus, T., 1972. Energy requirement. and transformation efficiency during development of .&r//us aquaricus (Crusracea. Isopoda). Pol. Arch. Hydrobiol. 19, 97-l I?. Prus, T., 1977a. Experimental and field studies on ecological energetics of A&us uquaricus (lsopoda). 111. Population dynamics and the background of macrobenthos occurrence in the littoral zone of Powsinskie Lake. Ekol. Pol. 25, 59-74. Plus. T., 1977h. Experimental and field studies on ecological energetics of A&us aquaticus (Isopoda). IV. Energy budget of a population. in the littoral zone of Powsinskie Lake. Ekol. Pol. 25. 593-623. Rossi. L. and Ci. Vitagliano-Tadini, 1978. Role of adult faeces in the nutrition of larvae of Asrllur aquatims (Is~pda). Oikos 30, 109-I 13. 13evernTrem Water Authority. 1979. Water quality 1977/78. Birmingham. 203 pp. Qebbing. A.R.D..
1979. An experimental approach tu the determinants of biologisal water quality.
Philos. Tram.. R. Sot. London. Ser. B 286, 465-481. Slecl. E.A.. 1901. Some observationson the life history of Asellusuquarkw (L.) and .qsel/us m~r&vw.s Racovitza (Crustacea:Isopoda). J. ZooI., Lond. 137, 71-87. Willoughby. L.G. and J.H. Marcus, 1979. Feeding and growth of the isopod ~lsellusuqumirus 0.1 actinomyceetes.considered as model filamenlo:Js bacteria. Freshwat. Biol. 9. 441-449, Woodiwiss. F.S., 1964. The biologkd system of stream classification used by the Trent River Board. Chcm. Ind. 1I, 443-447.
Aquufic Toxicokrgy, 1 (1981) 101-l I2 ElsevierINorth-Holland Biomedical Press
THE EFFECT OF WATER QUALITY ON THE SIZE AND FECUNDITY OF ASELLUS AQUATICUS tCRUSTACEA:ISOPODA)
M.R. TOLBA and D.M. HOLiilCH tleporlmPnt of Zod~g~ the ~~~i~ers~~y~ No~~~~~~~ NG7 .?ftf3. En&& (Received I5 December 1980; accepted 20 February 1981)
Samples of Asellus uquotincli (L.) were collected from a numb,rr of freshwater sites in centra! England exhibiting either clean, semi-polluted or polluted condition!,. The ;:lean sites contained naturally daaying organic matter but the polluted rites also contained sewage and toxicants. The asellids were found to vary in the maximum length attained, and in pereonal width and fresh hod! weight. in response to water quality. In asellids from polluted sites the body size (length x aterage pereonal width) and wet weight were significantly lower than those from a clean site. Asellids frc.: . semipolluted sites showed an intermediate condition. The number of eggs contained in the marsupia of ovigerous females increaseswitn body size. However, the actual number of eggs is dependent upon the water quality. there being more eggs in the marsuaia of all sizes of ovigerous females from clean sites than those from polluted sites. Asellids from semi-p&luted sites showed an intermediate condition. Ovigerous females from polluted sites, kept in clean water, produced Fg generations which, -hen grown in clean water, showed a significant increase in body size and weight. Ft generation males, derived from clean water females and grown up in clean or polluted * ater at 20 ‘C, showed a significantly Ia:ger increase in width for any given body length in clean water than they did in polluted water. Care must therefore be taken when comparing growth rates of animals from different geographisal
locations which exhibit different degrees of water quaiity. Key words: Aselfus; isopod: water quality; size; fecundity
INTRODUCTION
The biological impact which pollution has on the freshwater environment is usually measured in terms of changes in population density and community structure (e.g. Learneret al., 1971; Etryceei al., 1978). Increasedlevels of organic pollution for instance may lead to a decreasein species diversitybut to an increasein the number of individuals of a tolerant species. Asellus aquaticus is a case in point, oecurriIlg commoniy in relatively clean water, as part of a diverse commurlity (Adcock, 1979), but reaching much higher densities in organically polluted rivers, where species diversity is low (Hawkes and Davies, 1974; Bryce et al., 1978; Aston and Milner, 1980). Numerous experiments have been reported in the literature concerning the effects which various pollutants have on the survival of individual 0l66-445X/81/MxIo-oooU/$02.50
d Elsevier/North-Holland
Riomedical Press
species, However, few workers have attempted to evaluate the effect of total water quality on the growths of individuals within a ~pulatio~. Stebbing (1979) has attempts this with the marine hydroid, ~ff~~~~~~~~ff _&WOSZ.We found that coionies produced diff~ent growth patters and frequencies of gonozooids when subjected to natural water af different quality in the laboratory. Holdich and Tolba (1980) have shown that water quality and temperature have an effect on the rate of ~~~rnent of A, ~~~~~~~ eggs, with potlnted water speeding up de~~e~oprn~tof the eggs of &an water a&lids, thus enabling them to quickly get over a critical phase in she life&tory, when placed under stress. The present study was undertaken to assess the effect that water quality has on the size and fecua~ty of post-marsupial A. ff~~ufjc~. Aston and Milner (198O}found that the longest individuais of this crustacean were associated with locations on the River Trent (in the Midlands region of Eng~~d) which exhibits the heaviest organic ~l~ution loads. However, length is not always the best measure of ‘size”, e(en though it has been extensively used in studies of As&s growth (Steel, 1%1: Andersson, 1969; Prus, 1972, 1977a,b; Marcus et al., 1978; 0kland, 1978; Rossi and Vit~l~ano-Tadini~ i978; Adcock, 1979; Willoughby and Marcus, 197% Astou and Milner, t%O). When isopods moult they usually increase in width as we!1as length. and weight increases over the intermoult period. It will be shown in this paper that water quality affects the weight and body size (length x average pereonal width) of A. ~~~~t~~s - water of poor quality depressing these parameters. A nunther of workers listed above have reported a positive relationship between the length of ovigerous female A. ~47~utict~ and the number of eggs iuitially contained within the marsupium. However, individuals of the same length do not necessarily have the same sized marsupial as this i? aiso dependent upon their width. Therefore the relationship between body size and egg number was also examined at a number of locations exhibiting different degrees of water quality.
Samples of A. aquatics were collu:ted manually every month from IO March 1978 to 15 March 1979 f?om six main &es within the Trent River system. At least tOOindividuals were collected on every occasion, and usually many more. One site (Notti~ham Canal} was considereri to contain relatively clean water, it contains very few toxicants. aIthough it was eutrophic. It had a much higher Trent Biotic Index (Woodiwiss, I%4) than the other sites. The other five sites all showed varying degrees of pollution (e.g. sewage and heavy metals) mainly derived from industriitl and domestic sources. In addition, samples were collected during the summer of t979 from a number of other sites for comparative purposes, but details of the variables were not recorded (‘Fable I). All the sites were contained within the area bordered by latitude 52’1 YN and 53O3o’N,and longitude 2” IO’W and 0” iO’E, Each individual aseihd longi:r than 3 mm, below which it is difficult to separate
104
the sexes, was staged: male, non-ovigerous female or ovigerous female. Numbers of early stage eggs in the marsupia of the latter were counted. Measurements (Fig. 1) of body length and body width were made using a graticule under a dissecting microscope. Individuals were then blotted dry on filter paper and weighed. Ovigerous females, with eggs at stage A or B (Holdich and Tolba, 1981) were isolated from collections made at Nottingham Canal and at three polluted sites, i.e. River Erewash, and Water Orton, above and below Minworth Sewage Works, on the River Tame. The females were allowed to incubate their broods in polluted or clean water (Nottingham tap-water). When the juveniles were released 200 from each site were allowed to grow in the same type of water they had been incubated in. The experiments were continued for 90 days and the asellids were measured every 14 days. Food was provided in the form of decaying elm leaves and the temperature was maintained a; approximately 20 “C. The measurement of the variables listed in Table 1 were made with the equipment outlined in Holdioh and Tolba (198 I) every month from 10 March 1978 to 15 March 1979. Separate regression analyses were carried out for all measurements of the body length/width, body size/wet weight and body size/egg number relationships using
f-ig. I. Diagrams of female (A) and male (B) A. aquatinrs to show positions from which measurements of body length and pereon,al widths were taken.
105
the standaid equation: Y = A + BX, where Y is the width, wet weight or egg number, X the length or body size, A the calculated Y intercept and B the slope. Comparisons of the regression data were carried out for pairs of sites to show whether or not the differences were significant. RESULTS
The relationship between body length and pereonal width in males and females
It can be seen from Fig. 1 that the maximum pereonal width of males and of females lies at different levels, being a! pereonite 3 in females and pereonite 6 in males. The minimum width occurs across pereonite 1 in both sexes. A gross comparison of random collections from a clean and a polluted site clearly shows a difference in body size between the two sites (Fig. 2), those from the clean site being much larger. The maximum lengths of males and females collected from clean sites during the period March, 1978 to March, 1979 were 20 mm and 16 mm respectively, for polluted sites the lengths were 14 mm and 12 mm respectively. If body lengths are plotted against the mean widths of pereonites 1 and 6 for males, and pereonites 1 and 3 for females, there is an obvious increase in width with an increase in body length (Fig. 3). However, if the regression lines are compared for animak from clean and polluted w;:rer there is a significant difference (P s 0.001) for the male data, and also for ti!e female data, although the difference between males and females is not so clear-czt. Ttrerefore, both males and females of any length over 3 mm from clean sites are significantly wider than the corresponding
big. 1. Photographs ol’ random GUII~IC\ of :i. uqrturrtvr.~from il clean (A) and a polluted (BI site.
Fig. 3. Regression lines of mean pereonal width on body length for females (a, Nottingham Canal; c. Water Orton above) and maks (b, Nottingham (a) Y = 0.463 + 0.334X.
f -- 0.99;
r = 0.99; (d) Y = -0.476
(b)
+ 0.3!2X,
Canal; d. Water Orton above). Regression data:
Y = 0.911 + 0.222X.
r = 0.96;
(c)
Y = - 1.040 + 0.422X.
r = 0.99.
b
Bodysize
mm?
Fig. 4. Regression lines of egg number on boidy size for ovigerous females from (a) Nottingharl
Canal;
(b) Saltfleet Ha,ven; (c) Attenborough; (d) Wnter Orton above; (e) R. Erewash. Regression data: (a) Y = -11.340+7.200X, r = 0.w
r-0.99;
(d) Y = -32.040
(b)
Y=
-26.170+6.170X,
r=0.99;
+ 5.370X, I = 0.99; (e) Y = -23.870
(c)
Y=
-31.63Ot6.ooOX.
+ 4.540X, r = 0.99.
sexes from polluted water. As length is not therefore a good measure of the size of as asellid, the body size (length x average pereonal width) is used in future comparisons between sites.
The relationship between body size and the number of marsupial eggs
One might expect that long ovigerous females will have a larger marsupium than short individuals. Measuring length only, however, may hide the fact that females which are the same length may be of a different width; wider females may therefore have a Iarger marsupium and so be able to incubate more eggs. If egg number is plotted against the body size there is an increase for every size of female (Fig. 4). However, if the regression lines for sites having different degrees of water quality (Table I) are compared. then it can be seen that egg number varies considerably. All sizes of ovigerous females from the wliuted sites (Water Orton above and R. Erewash) have significantly lower egg number (P ~~0.001) than those found in Nottingham Canal. The water-filled gravel pits at the Aricnborough Nature Reserve have in recent years become semi-polluted due to the fact that water from the R. Erewash enters it now. The Great Eau River at Saltfleet Haven is miIdly polluted from domestic and agricultural sources. Both these sites show egg numbers which are higher than the polluted sites but lower than the clean site. The relatio qship between body size and wet weight
Even though animals may appear to have similar body sizes their weights ma) differ due to a number OFinternal factors, e.g. the reproductive state, the quality of
Fig. 5. Regression lines of wc~ weight on body size of males (a) and females tb) from Nottingham Canal and of mates (d) from Water Orton above. Also shown is 11~seffrvt of clean water on Water Orton above juveniles(c),
Regression data: (a) Y = - 3.715 + I .440X. r = 0.98; (b) t’ =
(c) Y = -2.220
t lAMOX. r = 0.98; (d) Y = 0.598 + 0.800X. r = 0.88.
3.520 + 1.060x’. f = 0.96~
Fig. 6. ~qp+on lines of wn:sweight on body size of mules from Wuter Orton hetow (e), R. Errwash (d) and Snardlow le). Also shown ia the el’tix~ on H. Er,uwash(a) and Water Orton below (h) juveniles elf
~r~}wjt~~(hem up is clcon water. Regression data: (a) Y = - 3.202 + 1.3XLY. r fx 0.96; th) 1’ z )- 2.220 c ).&@Rx, r ~7O.YL:. (c) Y c t.SYH t 0.970X. r r~ 0.95; fd) Y = 1,923 t O.MOX, I 1 0.9% (c) Y 7 3.248
I O.WX. r 2:O.YS.
Fig. 7. Reeressionslines of wet weight or: body sire ol’ males from Somcrsham pond (a).Attcnborough (b), lairham &rook (c), Wilford (d) and Kingsbury (c). Regression dulu: (al Y =: -3.7H4 + 1.540X, I.864 + c .i 0.9X; (b) Y = ‘2.23 c I.OlOX, r .z 0.94; (C) Y = .- 0.954 + 0,070x, r z 0.X; fd) Y =. O.YWIX.
r “z o.uz;
(C) Y .&
2.454
t 0.900X,
r = 0.99.
their foo.f. whether they have eaten recently or not, and disease. In addition, errternalI:Yctorssuch as water quality may also hsve an effect. When the wet weight of P, ~~~u~i~~~ is plotted against body size there is a linear relationship (Figs. S-1). A. uquuricus from Nottingham Canal show a positive increase in wet weight with every increase in body size - that for males being larger than that for females (Fig. 5). ??.owever,individuals from Water Orton above show a much smaller increase (Fig. 5). The difference between males for the two sites is signi~cant (P < 0.001). Individuals fram a water-filled quarry in Somersham (a clean site) were sir$lar to ,thoscfrom Nottilngham Canal (cf. Figs. 5 and 7). Individuals from polluted sites on the R, Tame (Kingsbury and Water Orton below}and the R. Trent (Shardlow and Wilford) were similar to those frum Water Orton above, whilst those from sites exhibiting semi-poollutedwater (Fairham Brook and Attenborough) were in between those from Water Orton and Nottingham Canal (cf. Figs. 5, 6, 7).
ln order to assess the effect of clean and polluted water on the growth of A. u9~Qt~e~~ a number of ovigerous females rvere transferred from Water Orton above and below, and from the R. Erewash, and kept in clean water throughout their brooding periods. When FI generations resulting from these female:; were grown up for 90 days in clean water they showed a significant increase in body size and body weight compared with individuals collected from the field (Figs. 5 and 6). This is particularly noticeable in the larger individuals from the R. Erewash (P s 0.01) (Fig. 6). To show that water quality affects growth, in terms of width, individuals derived from clean water females were grown up in clean water or poliuted water (R. Tame) at the same temperature (20 “C). When mean pereonal width is plotted against body length there is a significant difference (P s 0.05) in the growth of asellids from the two sites - width being depressed in those growing in polluted water (Fig. 8). DfSCUSSlON
Two implicati~l~s are apparent from the results obtained in this study. Firstly, as mentioned in the introduction, many workers have used body length to assess growth in A. aqtmicus, and they have made comparisons between their populations and those of other workers. It has been shown in the present paper that in a fairly small geographicA area of England the body size and weight of individuals of the same length can differ sig~ficantly. Therefore, future studies should include a
110
abjuration of o:her parameters besides length to obtain a true indication of ‘growth’. Second.ly , the quality of the water in which populations of A. uquuticus jive &x:ts their grctwth and the fecundity of the females. in this paper the effect of total water quality has been examined, the effects 0’ organic pollution and that of toxicants in the water have not been distinguished. The clean site which was exexsmhd mg&trly ~Nott;ngham Canal) contra large amounts of dads natural vegetation but the ievels of toxicants were very low. Species diversity was high at t.his site but the population densitiesof the isopod wetlSlover than in the ~llut~ sites (To&a, unpubl. obs.). lndivid~~s were large and ov@erous females produced large broads. The sites studied on the R. Tame, R. Erewash and R. Trent are ah known to be polluted by sewage and toxicants and to have a low species diversity (Severn ‘Gent 1. ater Authority, 1979), and independent observations have confirmed this (Table I). Biological oxygen demand, conductivity, and levels of phosphates, chloricbs and heavy metals such as lead, copper, zinc and ~dmium all tended to be higher than in Nottingham Canal. Oxygen levels were fairly high and pH levels normal at all sites and these did not appear to be a limiting factor in the ovulations studiecl. All she polluted sites contains high densiti~ of A. uq~ut~~~, possibly due to the large amounts of food and lack of competitors and predators. Howevfer, individuals at these sites tended to be smaller and females produced smaher broods. Temperature levels variqd between the sites over the study period. In the R. Tame, 6:. Trent and R. Erewash water temperatures were consistently higher than for the &an and semi-polluted sites due to the input of heated effluents from andustry, in particular electricity generation plants, Only Nottingham Canal froze to any depth in the winter months, The sites at Fairham Brooh, Saltfleet Haven and Attenb~~rough all had high species diversity in the summer months. but were subject to low levels of organic and chemical pollution from agricultural and domestic sources. interestingly, the results from these sites were intermediate between those of P~ottingham Canal and the polluted sites. The only detaitecicomparative study which has been made on A. uguutiws from a number of sites ii that of Aston and Milner (1980). They found an inverse relationship between the lengths of ovigerous females and the mean t~~ratu~ in the R. Trent - the smallest individuals occurring where temperatures wete high and organic pollution tow. Other stages of the life-history showed a similar trend, However, they also found that organic pollution in the same river fed to an increase in population density and to an increase in the length of males and non-ovi BUS females. Aston and Brown (1975) have found similar results for leeches the R. Tame. Althoug:? increased organic matter may provide more food for A. uquctticusin the R. Treitt the actual amount of growth attained by the isopods less than they ace crpable of - as shown by our transfer e~~~rnents. This s that other factors such as heavy metals may be depressing growth in polluted parts of ti.le R. Trent, In Nottingham Canal, where the comes from natura ly decaying vegetation and not from sewage, the isop&s are able to grow to a larger size.
According to Aston and Milnet (1980) the relatively high temperatures in certain stretches of the R. Trent may bring forward sexual maturity in A. uqwricus by diverting energy from growth to egg production, but may also have the effect of making animals smaller due to the facr that more energy has to be utilized for activity and maintenance at the higher temperatures. It seems likely that if more utilized for activity and maintenance and less for growth then less production - thus resulting in smaller broods, as From our results it could be concluded that the small size tions where temperatures are relatively high, may also be effect of non-thermal pollutants on growth. Thermal pollution tically with certain chemical pollutants thus having a greater occur in non-thermally polluted water. In our transfer eqximts jutik from polluted water were grown at 20 “C, which is somewhat n temperature for any of the sites studied (Table I). and yet m the absence of non-thermal pollutants. There was also a signirint difference betwee th in terms of width for clean site FI individaals at 20 ‘C. Holdich and Tolba (1980) found that grown in clean and polluled the developmental r&es, over a range of temperatures, were similar for A. uquaticus from clean and pol sites when they were grown in water from their home . (When clean site were transferred to polluted water they reacted by developing at a faster rate - presumably a response to environmental stress.) However, in the field. the post-marsupial stages of polluted site asellids may well develop faster in polluted water than in clean water, but the actual amount oi growth they achieve is significantly less. More work now needs to be carried out tc, assess the underlying physiological differences between A. uquuficus from clean and polluted sites, and to determine the effects of individual pollutams on growth.
Thanks are due to Zagazig University, Egypt and the Egyptian Government for financial support. and lo Professor P.N.R. Usherwood for the provision of laboratory facilities. REFEREM-ES
112
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