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2 J. Fish Biol. (1986) 28, Biology of the acid-tolerant fish species Umbra pygmaea (De Kay, 1842) L. H. T. D e d er e n *!, R- S. E. W. L eu v en*, S. E. W endelaar Bo n g a I and F. G. F. O y en* * Laboratory o f Aquatic Ecology, f Department o f Animal Physiology, Catholic University, Toernooiveld, 6525 ED Nijmegen, The Netherlands ( Received 17 July 1985, Accepted20 September 1985) In The Netherlands, the mudminnow Umbra pygmaea mainly occurs in water bodies with a ph ranging from 3-5 to 4-0, an alkalinity less than 0-Imeq 1_I and a calcium content below 100 nmol 1_ 1. The abundance of mudminnows is inversely related to the ph and the presence of predatory fish species. Experimental studies show that U. pygmaea is extremely acid-tolerant. Exposure to media with ph values ranging from 4-0 to 7-0 did not cause mortality or significant changes in blood plasma osmolarity and haematocrit. At ph 3-0 no mortality was recorded although a strong decrease in the plasma osmolarity was observed. Exposure to ph 2 8 and calcium concentrations of 100 or 500 xmol l ' 1 for 10 days caused 70% and 40% mortality respectively. The growth, reproduction and feeding of mudminnows are described. Age-growth and length-age relationships were calculated. Gonadal development started between September and January. The highest Gonado Somatic Index was recorded in spring. Spawning occurred from April to May. Fecundity of a sample of females was assessed. Nematocera provided the main item of diet of U. pygmaea; the microfauna ingested was dominated by Cladocera. The adaptations of mudminnows to acid environments are discussed. I. IN T R O D U C T IO N T he E ast A m erican m udm innow, Umbra pygm aea (De K ay, 1842), is one o f the five now living representatives o f the U m bridae (W ilson & Veillieux, 1982). The world distrib u tio n o f the um brids is shown in Fig. 1. Umbra pygm aea is native in inland w aters in the eastern p a rt o f the U.S.A. (Schrenkeisen, 1938; W heeler, 1969). A b o u t 70 years ago this species was introduced into W estern Europe. Subsequently, U. pygm aea has been recorded from France, (Spillmann, 1959), The Federal G erm an R epublic (Ladiges & Voigt, 1965; M uus & D ahlstrom, 1968), T he N etherlands (K leijn, 1968; V ooren, 1972; Leuven et al., 1984«, b) and Belgium (Poll, 1949; B urney, 1984). In N o rth A m erica, U. pygm aea has been found in quiet, m ud-bottom ed, often heavily vegetated stream s, sloughs and ponds, particularly along the m argins (Lee, 1980). H ow ever, little is know n ab o u t the ecology and life history o f this fish species; In E urope, U. pygm aea has been observed in small rivers, lakes, pools, ditches and sluggish or stag n an t w aters in peat bog areas (V ooren, 1972). Recently it has been reported th a t the E ast A m erican m udm innow is extremely acid-tolerant and survives in w aters w here no native fish can live (Leuven et al., 1984a, b). The distribution o f U. pygm aea in T he N etherlands has been extended during recent decades. M any acid and acidifying soft-w ater systems have been invaded by this species /86/ $03.00/0 *986 The Fisheries Society of the British Isles
3 308 L. H. T. DEDEREN ETAL. I Fig. 1. The world distribution of the Umbridae (after Spillman, 1959; Ladiges & Voigt, 1965; Vooren, 1972 Scott & Crossman, 1973; Wilson & Veillieux, 1982, Leuven et al., 1984/)). In AmpnVn t u " CmS u p y%maea m ay PlaY a significant role as a to p predator, limi (K irtla n H 'U P ^ n e e m enon ^ as been described for the related species U. ofsoecial intp 6 M agnuson, 1983). Thus the biology o f m udm innow s is is S o w n a h o m l T C ^ Seem t0 be WeI1 ad apted to ac^ w ater. So far, little umbrids to liv e h P ysiological and ecological m echanism s w hich enable these um brids to live under acid conditions. a c i d - t o l e r a n c e w a s s,u d d T a l a b t a t o ^ l ; e r i m t, 0 n f m " d m i n n W S ' T h e FIELD STUDIES II. M E T H O D S (alkalinity < 2 meq 1'^ js o lf tw a t!!!? investigated in 96 isolated soft-water systems southern and eastern parts of THp m ^m or*5n<^ P ls and small lakes) are situated in the offish species, 54 w a t e ^ ^ K 18;? rder t0 8et a reasonably complete list gear, i.e., V-shaped dipnets fvkes oin ast t^r?e times with different types of fishing body was made and the presence i ^ and seine-nets. A survey of the entire water remaining 42 waters were sampled nni? &1VCaJ^unc^ance of U. pygmaea determined. The individuals were collected ' If U' m * m a wls present in a catch, 5-40 sex, length and total weight w ge determination. S S in 70% ethano1' At the laboratory the e opercular bone was removed and used for rekti " ShiP be,w'» «W U - described b, Ihe power function: W = al p (1) fitting. y eight) L body length, and the constants a and (3 were estimated by curve
4 ft* ACID-TOLERANCE OF MUDMINNOWS 309 / v A The age-length data for fish older than 50 days were fitted to the model (Brody, 1945): L = L x (1 e~ki'~'0>) (2) where L is body length, t age, L x the arithmetically determined ultimate length, and K and t0 are species specific constants. The parameters in equation (2) were estimated according to Von Bertalanffy (1934, 1938). In addition, the equation L = Pek! (3) where I is body length, t age, and k a species specific constant, was applied in order to describe the length-age relationship in younger fish (Brody, 1945). The time-derivative of ^ equation (2) G 1 = A :L x o ' ( 4 ) Where L is body length, t age, L r the arithmetically determined ultimate length, and K and t0 are species specific constants, gives a description of the growth-rate of the fishes. To fit the data to the models, the GLM and NLIN regression procedures of SAS (1982) were applied. For statistical comparison of the length-weight and length-age relationships for males and females the best fitting equations (1, 2) were transformed to linear models using logs. Statistical differences between the equations calculated were tested using F tests (Fisher, 1924). Prior to spawning, the ovaries of some specimens were preserved for counting the total number of eggs per fish. The length-fecundity and weight-fecundity relationships were described by the model Y= ax b (5) <rw (Babiker& Ibrahim, 1979). The oesophagus and stomach of the preserved mudminnows were dissected and the contents identified. From January until May 1984 approximately 20 individuals of U. pygmaea were ^ sampled fortnightly in the Groot Huisven. This moorland pool is situated East of Oisterwijk (5I 35'N 5 16'E) and has a surface area of 5 ha and a maximum depth of 1-5 m. The physico-chemical properties of the Groot Huisven are given in Table I. All A fishes captured were preserved in 70% ethanol The Gonado Somatic Index (GSI) of a fish was calculated as: GSI = (gonad weight) (body w eight-gonad weight)-1 x 100%. a f-. At all locations, ph measurements were carried out with a Metrohm E488 ph-meter and a model EA152 combined electrode. Alkalinity was measured by titration of 100 ml of water with 0-01 m HC1 down to ph 4-2 and the acidity by titration with 0-01 m NaOH up to ph 8-2. Electrical conductivity (EC) and temperature were measured with a model 33 YSI-SCI portable conductivity meter. The measured conductivity was corrected for temperature and ph according to Vangenechten et al. (1981). A Dentan model FN5 meter was used for the measurement of the turbidity. Water samples were passed through a Whatman GF/C filter, collected into two 100-ml iodinated polyethylene bottles and preserved by adding 0 5ml of a 200 mg 1 1 HgCl2 solution. In one of the two samples some grains of citric acid were added in order to prevent precipitation of metals. All samples were transported to the laboratory in a refrigerated container and stored at - 20 C until analysis. A i J LABORATORY STUDIES Fully controlled laboratory experiments were performed in order to study the acidtolerance of U. pygmaea. The experimental set-up has been described by Roelofs et al. (1984). All experiments were conducted in glass containers (25 x 25 x 30 cm) which were placed in a water bath. The temperature of the water bath was maintained at 20 C by means of a Neslab type Coolflow 75 cooling aggregate. The medium in each aquarium was continuously refreshed (1 litre h -1) from black polyethylene stock containers by means of a Multifix Constant pump unit. To avoid oxygen deficiency or high carbon dioxide pressures, an inundated air-tube was placed in each container. All experiments / S
5 310 L. H. T. DEDEREN ETAL. T a b le I. The ranges of some physico-chemical parameters o f 21 Dutch waters harbouring Umbra pygmaea and the average values of these parameters for the G root Huisven Parameter Range* Groot Huisvent ph Alkalinity Acidity E C ' V cr NO"2 n o ; nbj p o ;~ - so;- Na K + C a+ + Mg+ + Si Fe Mn A1 Cd Turbidity (meq 1_ *) (meq 1-1) (ns cm -1) (nmol 1 (nmol 1 (nmol 1 (nmol 1 ( n m o l 1 (^imol 1-1 (nmol 1_1) (nmol 1~*) ( n m o l 1 ~ x) (nmol 1_ *) (nmol 1 (nmol 1 (nmol 1 (nmol 1 (nmol 1 (ppm) Range in average for 21 sites (1983). t Average values 1984 (n = 19). T a b le II. The chemical composition of the water used in the experiments in nmol 1 1 C a or 500 so;~ 32-0 K cr 800 or li N a+ 532 F~ 0-08 M g Br" 0-94 Sr I" h 3b o h c o ~3 2-7 p trace elements were conducted in synthetic culture media (Table II), prepared by the addition of sea salt and CaCl2 to twice demineralized water. After acclimatization to the basic medium (ph = 5-7), replicates of 10 individuals were exposed to water of different ph and calcium content. In order to prevent shock effects the ph in the aquaria was changed gradually. All fish were incubated at the ultimate ph for a period of 10 days. During the experiments the animals were fed Tubifex. The ph of the media was adjusted daily by adding H 2S 0 4 or KOH. The mortality rate of the fishes was recorded and probability of survival predicted using PROBIT analysis (SAS, 1982). vi Îer cut,tlt}g the tails of anaesthetized fish, blood was collected from the caudal blood vessels in heparinized haematocrit capillaries. After centrifugation o f the blood in a
6 ACID-TOLERANCE OF MUDMINNOWS 311 T a b le III. The presence and relative abundance of Umbra pygmaea (%) in poorly buffered waters in The Netherlands with different ph (n, number of waters sampled; n.d., not determined) ph < 5 ph > 5 3-5< ph <8-1 (77 = 28) («= 26) («= 96) Presence Relative abundance* N.D. * Percentage U. pygmaea of the total number of fish caught. Heraeus-Christ haematocrit centrifuge (3 min) the haematocrit percentage was measured and the osmolarity of the plasma estimated with a Vogel osmometer. The blood parameters (P<0-05) were tested for statistical significance with the Kruskal & Wallis one-way analysis by ranks. CHEM ICAL ANALYSIS Magnesium, manganese and iron were estimated with a Beckman model 1272 Atomic Absorbtion Spectrophotometer, and aluminium and cadmium with an IL Video AA/AE Spectrophotometer. Sodium and potassium were determined flame-photometrically and sulphate gravimetrically using a Technicon I Autoanalyser. Orthophosphate (Henriksen, 1965), nitrate and nitrite (Kamphake et al., 1967), chloride (O Brien, 1962), ammonia (Grasshoff & Johannsen, 1977) and silicon (Campbell & Thomas, 1970) were analyzed colorimetrically with a Technicon II Autoanalyser. III. R ESU LTS CHARACTERIZATION OF THE HABITATS In 1983 U. pygm aea was observed in 21 (22% ) o f 96 soft w aters investigated (Table III). The ranges o f the physico-chem ical properties o f these habitats are show n in Table I. U. pygm aea has a rather wide range o f occurrence w ith respect to the ph of the w ater. T he h ab itats are characterized by low alkalinity and ionic content. In som e system s the acidity and (heavy) m etal concentrations are rather high. F igure 2 illustrates th at U. pygm aea m ainly occurs in waters with a ph between 3-5 an d 4-0, an alkalinity below 0-1 m eq 1 1 and a calcium concentration below 100 (imol P 1. These w aters are relatively small (< 1 0 ha) and shallow (average d ep th 1-5 m). T he vegetation is m ainly dom inated by Juncus bulbosus L., M olinea caerulea (L.) M oench and Sphagnum spp. The presence an d th e average relative abundance o f U. pygm aea in com parison with o ther fish species are given in T able III. The presence of U. pygm aea in soft w aters seems to be independent o f the ph. The percentage o f w ater bodies which h arb o u r m udm innow populations is m ore or less the same for systems o f different ph -classes. T he average relative abundance of U. pygm aea, however, is strongly related to the ph. In circum neutral and alkaline w aters (ph > 5 ) the m udm innow s only account fo r 1-6% o f the total num ber of fish in the catches. In these systems U. pygm aea can be accom panied by Abramis brama (L.), Blicca bjoerkna (L.), Cyprinus carpio L., E sox lucius L., Lepomis gibbosus L., Perea flu via tilis L., R utilus rutilus (L.), Scardinius erythrophthalm us (L.) an d Tinea tinea
7 312 L. H. T. DEDEREN ET AL. o ' JZL HA/d n C r\ ' ' I - ' ' ' 7 T Alkalinity (m eq I ' 1) ( c ) n Q-j Calcium content («mol I- ) Fig. 2. Frequency distributions of the (a) ph Umbra pygmaea. (b) alkalinity and (c) calcium content o f 21 waters harbouring extremdyacidwatersis p r e d a t o r y s p e S s t a x ^ a c i d w a t e s w h i c h t h e harbouring these scede? tin«1 * flu via tilis w ere ab sen t. In w aters F or five waters the minimum6 \ lv num ^ er f m udm innow s was alw ays low. (Table IV). The total binma S a ^ stock o f m udm innow s was estim ated 21-3 kg ha *. bl mass? er unit o f surface area varied betw een 0-8 and FOOD The Fig- 4. M ore than 80% C? tents ^ m udm innow s is show n in ingested m acro-inv erteb rates w ere N em ato cera;
8 ACID-TOLERANCE OF MUDMINNOWS 313 Fig. 3. The relationship between the number of mudminnows as a percentage of the total number (n 1019) caught and the ph of the waters sampled. (* Pereafluviatilis and/or Esox lucius present.) T a b le IV. The estimated minimum standing stock of some mudminnow populations TT* Alkalinity* Minimum standing stock Sites Coordinates ph* (m e q r l) (k g h a- 1} Klein Aderven 51 34' 5 13' Groot Huisven 51 35' 5 16' Peetersven 51 22' 5 28' Kiezelven 51 24' 5 36' G at van Klercks 51 23'5 55' Average values E phem eroptera and A carina each represented 5%. Only a small num ber of G astro p o d a, C orixidae and O d o n ata were recorded (total < 5 % ). The im portance o f the O ligochaeta could not be estim ated because only rem nants of chaetae were found. F igure 5 shows th at m ost o f the N em atocera ingested were C hironom idae (49-7% ) and O rthocladiinae (36-7%), while only low percentages o f T anypodinae (8-6 % ), C eratopogonidae (6-0%) and C haoboridae (1% ) were recorded. T he zo o plankton ingested consisted of about 90% C ladocera, 10/o C opepoda and few O stracoda and R hizopoda [Fig. 4(b)]. The com position of the diet and the relative im portance o f several food items varied w ith age and habitat. AGE AND GROWTH T he w eight-length relationship o f the m udm innow s in the G root Huisven is
9 314 L. H. T. DEDEREN ET AL ( b ) _ Cl R. _ S_ A G xm C CoH0 Rh Toxa Fig. 4. The relative number of several food items of Umbra pygmaea. (a) Macrofauna: Nematocera (N), Ephemeroptera (E), Acarina (A), Gastropoda (G), Corixidae (C), Odonata (O) and Remainder (R). (b) Microfauna: Cladocera (Cl), Copepoda (Co), Ostracocra (O) and Rhizopoda (Rh); the Cladocera are subdivided into Chydorus sphaericus (C), Acantholeberis curvirostris (A), Simocephalus vetulus (S) and the remainder species (R). CeratoDoao^dapd^ Cn thneimj-t;ocera taxa 'n tlle stomaclls of Umbra pygmaea., Chaoborus; ceratopogomdae, H, Orthocladimae;, Chironominae; H, Tanypodinae. gf n in Fig. 6. Fitting of 547 d ata pairs to a pow er function (eq u atio n 1) resulted ^ = L 3'55 males and fe m a le Because the w eight-length relationship for m ade in the data presented (Fig. g ^Ustically ( P > 0 ' 51) 110 sex discrim ination was was significsa n % S d iffe re n t'* ^ ^ re la tio n sh ip for U. pygm aea. T his relationship and the total J L u f?, he two sexes (^ < ). F o r m ales, females ion o f fish older th an 50 days the p aram eter estim ates of
10 ACID-TOLERANCE OF MUDMINNOWS 315 Length (cm) Fig. 6. The length-weight relationship of Umbra pvgmaea (n = 547). Age (days) 7. The age-length relationship o f Umbra pygmaea (n = 547). The curves drawn are calculated according to Von Bertalanfly (1934, 1938) and Brody (1945) , <?;
11 316 L. H. T. DEDEREN ET AL. T able V. Parameter estimatesfor the age-length model L = L X( 1 e~ K<'~'o>) [equation 2] for Umbra pygmaea n LM(cm) t0 (year) A-(year 1) r? <? Population ? 8 O I16 <D No. of individuals F ig. 8. The length-frequency distribution of male and female Umbra pygmaea («= 451). equation 2 are given in T able V. T he predicted u ltim ate length o f fem ales is approxim ately twice th at o f males. Also, t0 and K are different for m ales and females. In determ ining the length-age relationship o f fish younger th an 50 days, the sexes were n o t separated because sex determ ination o f young fish is difficult. Putting the 105 age-length d ata pairs into equation 3 resulted in: L = g1183' where L is in cm, t in years, and an d r = The grow th of U. pygmaea is also sex-dependent. U sing eq u a tio n 4, it was calculated that the grow th rate o f both sexes decreases w ith age. Initially m ales grow faster than females; the grow th rate for b o th sexes is sim ilar w hen they are a out 150 days old, after which females grow faster th an m ales o f the sam e age. ese differences result in a similar length for m ales an d fem ales a t an age o f approxim ately 660 days (Fig. 7). Fem ales older th an 660 days are usually longer than males. The predicted ultim ate lengths are an d 8-2 ± 1-4 cm for females an d males respectively (Table V). The length-frequency distribution o f the m udm innow s sam pled is presented in lg, ' * he longest female and m ale were 12-7 an d 8-6 cm respectively. The predicted ultim ate lengths given in Table V agree w ith these values. T he lengths m ost frequently m easured were 5-7 and 5-4 cm for m ales an d fem ales respectively.
12 ACID-TOLERANCE OF MUDMINNOWS 317 Fig. 9. The sex ratio of Umbra pygmaea in relation to age. F,g.,0. The average Gonado Somatic Index (GSI,; with Groot Huisven during 1983 and The highest GSI for females ,<*. REPRODUCTION,.. O f the (ƒ. pygm aea cau g h t 53% were females. H w e «r. * e sex r P on the age o f the fishes (Fig. 9): w ithin the group of 0 + fish 7 2 /. were males, at age 6 years the percentage o f m ales had decrease to o Qentem ber and F igure 10 shows th a t gonadal d e v d o p i ^ t s t a r t ^ M d January, the highest average G S I was reached m spring, occurred 10-5% an d % for m ales an d fem ales respectively, and th at spaw g
13 318 L. H. T. DEDEREN ET AL. Fig. 11. The fecundity of Umbra pygmaea in relation to the body weight (n - 50). Fig. 12. The fecundity of Umbra pygmaea in relation to the length (n - 50). from April to M ay. G onadal m aturation was observed in all fishes older than I year, and in 0 + fishes w ith a length exceeding 4-7 cm. T he rep ro d u ctio n o f m udm innow s seems n ot to be affected by low p H. T h e eggs and fry o U. pygmaea were observed in waters w ith a p H > The fecundity-weight and fecundity-length relationships are illu strated in Figs II & 12 respectively. The fecundity rises w ith increasing w eight an d length. The estimates o f the fecundity are given by the equations: jf= 2-15 W' 5' 11 («= 49; r = 0-88) F 2-61 L 2'58 («= 49; r = 0-88) where F is fecundity (i.e., num ber of eggs ju st before spaw ning), W w eight in g, and L length in cm.
14 ACID-TOLERANCE OF MUDMINNOWS 319 T a b le VI. LC(E) values expressed in ph with 95% confidential limits C.L., for Umbra pygmaea incubated in culture media with different calcium contents (E, percentage expected mortality) (Ca2 + ) = 100 nmol r 1 (Ca2+) = 500 nmol 1 1 ph 95% C.L. ph 95% C.-L. LC LC LC ACID-TOLERANCE AND W ATER CALCIUM LEVELS C om pared w ith the survival after 10 days in m edia with a calcium content o f 100 nm ol 1 1, the probability o f U. pygm aea survival was higher in m edia with a calcium content o f 500 nm ol I-1 (Fig. 13). Some lethal concentrations (LC) and their 95% confidence limits are presented in Table VI. The average osm olarity o f the blood plasm a o f fish incubated in m edia with different ph and calcium content varied betw een 182 and 322 m O sm ol I-1 (» = ) (Fig. 14a)_and it is evident th at the osm olarity o f the blood is approxim ately 300 m Osm ol 1 1 under norm al conditions, i.e., p H > 3. A fter exposure o f m udm innow s to m edia w ith a ph of 2'8 or 3, significantly low er osm olarity levels were recorded th an in fish exposed to w ater a t ph > 4 (ƒ><0-008). T he plasm a osm olarities o f fish exposed to m edia w ith different calcium contents and a ph > 4 were of the same m agnitude and varied betw een 261 and m O sm ol I " 1. A lthough in m edia at ph 3 the osm olarity was reduced by ab o u t 15% at both calcium concentrations used, no m ortality was observed during the experim ental period. However, a decrease of the osm olarities by approxim ately 25% resulted in a probability o f death of <0-40 and 0-70 in w ater at ph 2-8 and a calcium content o f 500 nm ol 1 1 and n m o l 1 ~ 1 r e s p e c t i v e l y ( F i g. 1 3 ). The h aem atocrit values of the blood are presented in [Fig. 14(b)]. The haem a- tocrit ranged from 42 to 72% (n = 110). The lowest values were recorded in fish exposed to w ater a t p H 5. T he levels m easured after exposure to ph 3 were significantly higher th an those at higher ph levels ( P < 0-045). IV. D ISC U SSIO N In T he N etherlands, U. pygm aea has been observed in waters with low alkalinity, ph and ionic content and relatively high (heavy) m etal concentrations and carbon dioxide pressure. In o u r experim ental m edia with calcium contents of 100 and 500 nm ol l -1, after 10 days 50% m ortality occurred at ph (ph LC50) 2-86 and 2-79, respectively. U n d er laboratory conditions the ph LC50 ([Ca ] = 140nm ol I 1) for U. limi was 3-05 (Rahel & M agnuson, 1983). Thus, U. pygm aea seems to be m ore acid-tolerant than U. limi. In naturally acid waters the abundance of m ost fish species is low (E IFA C, 1969; T onn & M agnuson, 1982, R ahel & M agnuson, 1983). Recently m any authors have described the deleterious im pacts o f acidifying precipitation on fish populations in poorly buffered waters. A lso cu ltu ra l acid ificatio n results in the loss o f fish species o r even entire fish
15 320 L. H. T. DEDEREN ET AL. Fig. 13. The probability of survival for 10 days of Umbra pygmaea in relation to the ph and calcium concentration of the experimental media , Ca2+=500 umol I-1 ; A, Ca2+ = 100 nmol I-1. assemblages (Schofield, 1976; W right & Snekvik, 1978; D rablos & T o llan, 1980 Harvey et al., 1981; O verrein et al., 1981; Johnson, 1982). A cute exposure to water with a ph between 3-5 and 5-0 is lethal for m ost fish species. T he E uropean Inland Fisheries Advisory Com m ission (1969) has concluded th a t it is unlikely for any fish species to survive for m ore th an a few hours in w aters w ith a p H ranging from 3-0 to 3-5. In the present paper, however, it h as been p o in ted o u t th a t U. pygm aea tolerates this p H range. R ahel & M ag n u so n (1983) also fo u n d the closely related central m udm innow ( U. limi) to be extrem ely acid-tolerant. The m echanism s enabling these fish species to live in acid w ater are n o t well know n. U ndoubtedly b o th u m b rid species h ave evolved special physiological and structural adaptations to acid water. Therefore, a com parison o f the biology of acid-sensitive species with th at of m udm innow s m ay elucidate som e o f these m echanism s. O sm oregulatory stress, im paired gas exchange, reproductive failure, and changes in the predator-prey systems are considered as th e m ain reasons fo r the decline or disappearance o f fish populations in acidifying system s (F rom m, 1980; Overrein et al., 1981; W ood & M cd onald, 1982; M cd onald, 1983«; H ow ells et a., 1983). Furtherm ore, it is evident th a t n o t only the ph b u t also the changes in ot er environm ental factors (alkalinity and ion co n ten t, heavy m e ta l co n cen trations and carbon dioxide pressure) m ay be involved in the m o rtality o f fish in 1983)WaterS ^E IF A C 1969; F ro m m 198 ; M cd onald, 1983a; H ow ells et a l, OSMOREGULATION aci undcr circum stances norm ally results in d eterio ratio n o f the ionic ase regulation, increased m ucus secretion, changes in gill stru ctu re and
16 A C ID -T O L E R A N C E OF M UDM INNOW S 321 F i g. 14. The osmolarity of the plasma (a) and the haematocrit of the blood (b) of Umbra pygmaea a 10-day incubation in water of various phs at two calcium concentrations (Mean + s.d., n 10) , Ca2 + = 500 jjmol 1 1; O O, Ca2 + = 100 nmol 1 '1. im peded oxygen u p tak e and tran sp o rt (From m, 1980; W ood & M cd onald, 1982, M cd onald, 1983a). O u r experim ental d ata show that when the ph of the m edia varies betw een 7 and 4, plasm a osm olarity of U. pygm aea is m aintained at a relatively co n stan t level. E xposure to p H 3 results in a m arked decrease o f the osm olarity but n o t in m ortality. W ater at ph 2-8 causes severe haem odilution and m ortality. M cd onald (1983a) surm ized th at not only the ph but also the calcium content of the w ater is o f considerable im portance. It is well know n th at calcium in the water im proves the survival o f freshw ater fish in acidified w ater (W right & Snekvik, 1978; Leivestad et al., 1980; M cd onald et al., 1980; Brown, 1981; M cd onald, 19836). T he results o f o u r experim ents support these findings also. Since w ater an d ion balance is u nder endocrine control, horm ones are undoubtly closely im plicated in the process o f ad aptatio n to acid w ater (W endelaar Bonga e t al., 1984a, b). Studies on the role o f the endocrine system in the adaptation m echanism o f U. pygm aea to acid environm ents are in progress.
17 322 L. H. T. DEDEREN ET AL. GAS EXCHANGE A cid-intolerant fish often respond to acid exposure by an increased release of mucus from m ucous cells in the gill area. A num ber o f au th o rs have suggested that a thick m ucous layer upon the gills w ould im pede oxygen tran sfer and contribute to tissue hypoxia (M cd onald, 1983a). F u rth er, an acid discharge in aquatic systems m ay liberate sufficient carbon dioxide from (bi)carbonate in the water and in the sediment to be either directly toxic or, in the p H -ran g e 5-6, even lethal for fish (EIFA C, 1969). Acid stress leads to blood acidosis, and thus will interfere w ith haem oglobin- 02 transport via the well-known B ohr & R o o t effect (cf. P acker, 1979). However, mucus accum ulation, and increased C 0 2 pressures an d blood acidosis, do not necessarily im pair gas exchange in m udm innow s. Investigations by G eyer & M ann (1939) and Safford Black (1945) have show n th a t the sw im bladder of m udm innow s acts as a supplem entary organ for respiration, enabling airbreathing. Safford Black (1945) pointed o ut th a t under respiratory stress U. limi draws extensively on the oxygen in its sw im bladder. C arb o n n ier (1874) (cf. Poll, 1949) had already noticed th at air-breathing o f U. pygm aea resulted in a decrease of opercular m ovem ent. It still rem ains to be established w hether a decrease in opercular m ovem ent reduces the effects o f acid m edia on o ther gill functions, e.g., ion regulation. Furtherm ore, m udm innow s are sluggish fish, an d their low activity will result in a low respiration rate. T he haem atocrit o f the blood o f U. pygmaea ranged from 42% to 72%. In com parison w ith o th er fish species, the upper limit is relatively high. F o r o ther air-breathing fishes Jo h an sen (1970) described a m axim um haem atocrit o f 47%. T he h aem ato crit o f the b lo o d o f U. pygmaea tended to increase with decreasing ph. The sam e p h en o m en o m has been observed for Salmo gairdneri R ichardson (W ood & M cd o n ald, 1982). W hether the increased haem atocrit results in a higher gas exchange capacity is unknow n. REPRODUCTIVE FAILURE M any fish species show reproductive failure in acid environm ents. The reproduction process m ay be negatively affected by reduced spaw ning (Beamish & Harvey, 1972), decreased functioning o f the gam etes (E IF A C, 1969), failure of fertilized eggs to develop or hatch (Peterson et al., 1980), and hatching of eform ed fry (R unn et al., 1977). In an acid m oorland pool (average ph : 4-9) we observed norm al developm ent of gonads and the presence o f fertilized eggs and ry o U. pygm aea; even at ph 3-5 successful reproduction was observed. PREDATOR-PREY RELATIONS Acidification of fresh w ater affects aquatic biota at all trophic levels (D rab l0 s & o an, 0, Overrein et al., 1981; Johnson, 1982). K ey organism s fo r tran sferring energy rom lower to higher trophic levels m ay be w iped o ut by acidification. An aci Wat rs^ s negatively affect the population density and grow th o f fish. vnriptvlnf f asz (1984), U. limi is a generalist feeder, capable o f using a are- tvz sites anc* f ocl items. W e have established th a t N em ato cera descrihph frs 11^P rtan^ 00cl items in the diet o f U. pygm aea. T he sam e has been and therp ef ^ 1968). M any N em atocera species are acid-tolerant no evi ence th at the biom ass o f N em atocera is reduced in acidified \
18 A C ID -T O LER A N C E OF M U DM INNOW S 323 waters (Leuven et al., 1986). Therefore, it is unlikely th at in acid w aters the abundance and productivity o f m udm innow s is lim ited by their food supply. R ahel (1984) reported th at U. lim i coexists with centrarchids, cyprinids and fish predatory species like Pereaflavescens M itchell: it appeared to coexist by selecting a different m icro h abitat from that o f its potential predators. C lady (1974) found only one U. lim i in the stom achs o f 400 sm allm outh bass, M icropterus dolomieu Lacepede, in a M ichigan bog lake, although the m udm innow s were ab undant in the shallow p arts o f the lake. C ontrastingly, our d ata indicate th at the abundance of U. pygm aea m ay be influenced by the presence o f the fish predatory species Perea fluviatilis and E so x lucius (Fig. 3). However, in some low alkalinity waters that did not h arb o u r fish predators the abundance o f m udm innow s was also low. In addition to predation, o ther factors, e.g. com plete freezing or dessication of the pool, m ay be im p o rtan t for the abundance o f U. pygmaea. A rnholt & A hl (1936) (cf. G eyer, 1940) noticed th at for a population o f JJ. kram eri W albaum the ratio between females and males was 1-5. F o r U. pygm aea the sex ratio was also in favour o f the females (IT) and highly dependent on the age of the fish. Vitali & Braghieri (1984) described the same phenom enon for Barbus barbus plebejus (Valenciennes) and Leuciscus cephalus cabeda (Risso). The populations of these fishes also show ed a preponderance of females, especially at old age. M udm innow s have so far been econom ically unim portant. M aw (1968) and Slavin et al. (1977) noted th at m udm innow s m ay be possible agents for m osquito control in sem i-perm anent pools. M uch attention has been given to the osteology and the phylogenetical relationships o f U m bridae (Cavender, 1969; Sytcevskaya, 1976; O brhelova, 1978; W ilson & Veillieux, 1982). K ligerm an et al. (1975, 1984) used U. pygm aea fo r m utagens tests. In the present study it has been shown th at m udm innow s are extraordinarily acid-tolerant, so they m ay be useful as an experim ental m odel for the study o f the physiological adaptation o f fish to acid water. A part of this study has been financed by the Ministry of Housing, Physical Planning and Environment, Directorate Air (Project LB 131). The authors are greatly indebted to Prof. D r C. den Hartog, Dr G. van der Velde and Dr J. A. A. R. Schuurkes for critically reading the manuscript; to J. G. M. Roelofs for stimulating discussions; to Dr J. F. M. Geelen for identification of microfauna; and to M. A. L. Brouwers and L. M. r. J. Meuffels for technical assistance during chemical analysis. Our thanks are extended to the Dutch Society for Promotion of Nature Reserves, the State Forest Service and the Brabantian Landscape Foundation for permission to visit their properties, and to e Graphics Department of the Faculty of Sciences for preparing the illustrations. References Arnholt, P. & Ahl, E. ( 1936). F rem dländische Süssw asserfische. Braunschweig. Barbiker, M. M. & Ibrahim, H. (1979). Studies on the biology of reproduction in the cichlid Tilapia nilotica (L.): gonadal maturation and fecundity. J. Fish Biol. 14, ,, Beamish, R. J. & Harvey, H. H. (1972). Acidification of the La Cloche mountain lakes, Ontario and resulting fish m ortalities./. Fish. Res. Bd Can. 29, Brody, S. (1945). Bioenergetics and growth. New York: Reinhold Publ. Corp pp. Brown, D. J. A. (1981). The effects of various cations on the survival of brown trout, Salmo trutta at low phs. J. Fish Biol. 18,
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