Pupfish, Cyprinodon Rubrofluviatilis

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Interspecific Interactions and Limiting Factors of Abundance and Distribution in the Red River Pupfish, Cyprinodon rubrofluviatilis

ANTHONY A. ECHELLE, ALICE F. ECHELLE and LOREN G. HILL University of Oklahoma Biological Station, Willis 73462

ABSTRACT: The Red River pupfish, Cyprinodon rubrofiuviatilis, was positively associated in occurrence with Fundulus zebrinus, Hybo- gnathus. placitus and Notropis bairdi and negatively associated with a large, predominantly fresh-water (salinity < 10 ppt) complex composed of Notropis lutrensis, Gambusia aflinis, Lepomis spp. and Pimephales spp. Although occurring in dilute waters, the pupfish was virtually restricted to saline conditions (salinity > 10 ppt) where few other species occurred. An apparent lack of intrinsic limiting factors was demonstrated. The pupfish seems adapted to a broad ecological niche, creating competition with an array of fresh-water forms that are better adapted to exist in richer faunas. A W to E filter-bridge based on decreasing numbers of saline streams seems to be the major factor limiting eastward distribution of the pupfish. Competition and direct behavioral interaction with the pupfish seemed to be major factors depressing F. zebrinus populations in saline waters. A variety of "broad adaptations" of the pupfish may help to confer a numbers advantage over F. zebrinus, magnifying the effects of behavioral interaction. These effects may also help make the pupfish successful at moderate salinities (10-20 ppt) where there are at least 12 potentially competing species. INTRODUCTION In this paper we examine possible limiting factors in the distribution and abundance of the Red River pupfish, Cyprinodon rubrofluviatilis (Family: Cyprinodontidae), and discuss behavioral and competitive relationships between the pupfish and other species in its geographical range. Particular emphasis is placed on interactigns with the plains killifish, Fundulus zebrinus, the sympatrically occurring species most closely associated, ecologically and phylogenetically, with the pupfish; the ecological roles of these two species are compared in an analysis of competitive exploitation and interference. Most species of Cyprinodon occur in restricted, often isolated, bodies of water with few other fishes (Miller, 1948, 1968; Liu, 1969), an observation that may be causally related to the absence or low abundance of predator and/or competing species. Indeed, pressures exerted by introduced exotic fishes are implicated in the extinction or near extinction of various cyprinodontid fishes (cf. Miller, 1961; Hubbs and Deacon, 1964; Deacon and Bunnell, 1970). To date, however, there are no publications providing detailed treatment of ecological relationships between cyprinodontids and other fishes. Evi- dence has been presented suggesting competitive feeding relationships between Cyprinodon variegatus and other cyprinodontoids and between two species of Fundulus (Harrington and Harrington, 1961, and Thomerson and Wooldridge, 1970, respectively) . Martin (1968) 109 110 THE AMERICAN MIDLAND NATURALIST 88(1) gave attention to some aspects of our study, and Liu (1969) sum- marized the literature on levels of interspecific cohabitation and sympatry for various species of Cyprinodon.

METHODS AND MATERIALS A systematic series of collections was made at 59 localities in western areas of the Red River drainage in Oklahoma and Texas during two seining trips, 8-14 June and 4-8 July 1969. These collections were scattered as randomly as possible within the geographical range of the pupfish. Data recorded at each collection site included general comments concerning turbidity, width and depth of the stream, vegetation, bottom type, speed of the current and total dissolved solids content ( = salinity). Salinity was estimated with an electrical dissolved solids meter calibrated with standard NaCl solutions. Seining was chine with a 10-ft "common sense" seine., An attempt was made to sample, with short seine hauls, all different habitats at each collection site. Each species was enumerated and recorded according to the seine haul(s) in which it was taken. For each species, the number of individuals collected at each locality was corrected to a standard of five seine hauls by dividing the number of seine hauls in which it was collected into five and multi- plying the result by the total number of individuals of the species taken at the site. This was an attempt to make the abundance of the various species comparable from one locality to the next--on the assumption that individual seine hauls containing a particular species can be equated with the "preferred" habitat of that species; by cor- recting for a standard number of seine hauls containing the species the preferred habitat is also standardized. In figuring product-moment correlation coefficients for proportionate data (e.g., Fig. 4), the an- gular transformations of the proportions were used as prescribed by Sokal and Rohlf (1969). Data from 67 collections in the University of Oklahoma Museum of Zoology were used in conjunction with our data in determining species associations (Tables 2 and 3). Only those collections from within the range of the pupfish were used. These collections were made by Dr. Hague Lindsey and Russell Bates in the summer of 1963 as part of the Oklahoma Biological Survey. These workers were very thorough, and we believe their collections were representative of the fish populations. Of the collections made in the present study, only those taken from free-flowing streams are considered in Figures 2-4. Behavioral observations were made on the Red River pupfish in almost every month of the year at several localities in the field from spring 1967 to January 1970. The details of this study were presented by Echelle (1970). During that study and since, we have made numerous collections and general observations on the fishes of western areas of the Red and Brazos rivers; these data have provided a broad base upon which some of the inferences of this study are made. 1972 ECHELLE, ECHELLE AND HILL: PUPFISH 111

GEOGRAPHICAL RANGE AND ABUNDANCE The Red River pupfish occurs in the Red and Brazos rivers of western Oklahoma and Texas (Fig. 1) . The western boundary of its geographical range corresponds with the High Plains escarpment. The pupfish occurs commonly in western areas of its range and sporadically in eastern areas. Its easternmost occurrence in the Red River drainage is a small unnamed creek (herein called Oscar Creek) and adjacent areas of the Red River proper near Oscar, Jefferson Co., Okla. Our easternmost record from the Brazos drainage is from the Brazos River near Seymour, Baylor Co., Texas. The pupfish occurs abundantly everywhere in the Salt Fork of the Brazos, but is rare in the Double Mountain Fork; in 10 collections at seven localities on the latter stream we collected a single specimen (18 mm, TL) approximately 60 stream miles from the Double Mountain-Salt Fork confluence. We know of no specimens from the Clear Fork of the Brazos or the Washita River of the Red River drainage.

ENVIRONMENTAL CHARACTERISTICS The larger streams in the geographical range of the pupfish have shallow, low-gradient watercourses with little emergent vegetation and usually wide, flat, sandy bottoms. Water levels fluctuate extensively becoming swollen and swift after periods of moderate to heavy rain-

I 00°

OKLAHOMA! TEXAS j

35Q—

RED RIVER

z BRAZOS 25 ml 5 RIVER

I CIO° Fig. 1.—Geographical range of the Red River pupfish based on collections made in the present study and catalogue records at the University of Oklahoma Museum of Zoology. 1 = Washita River; 2 = "Oscar Creek" locality; 3, 4 and 5 = Salt Fork, Double Mountain Fork and Clear Fork, respectively, of the Brazos River 112 THE AMERICAN MIDLAND NATURALIST 88(1) fall and often receding to intermittency in dry periods—especially near the upper terminae. These fluctuations in flow often cause abrupt local changes in turbidity, dissolved solids content, and the position and topography of the watercourse. Much of the western half of the area is underlain by beds of salt (primarily chlorides) with extensive areas of overlying gypsum. Ground water percolating through these areas and surface waters flowing over the beds of gypsum dissolve large quantities of minerals and many streams carry high concentrations of dissolved solids; oil field pollutants, low precipitation (ca. 50 cm/year) and high evaporation rates also contribute to this effect. The chemical characteristics of the major streams in the Red and Brazos river drainages are given by the U. S. Public Health Service (1964) and Irelan and Mendieta (1964), respectively. In these streams there is a W to E gradient of decreasing salinity due to increasing numbers of fresh-water tributaries. Salini- ties above 10 ppt are often obtained in western areas. Salinities of 1 ppt or less are predominant in eastern areas, especially in the smaller streams. However, there is considerable local variation with extremes of fresh and saline waters occurring in both general areas. Diel fluctuations in air temperature from 0 C or below at dawn to 20 C in the afternoon are common in the winter, and fluctuations from 20 C to 38 C or higher occur regularly in the summer. Minimum air temperatures are near —15 C; maxima are near 43 C. Temperature in shallow flowing water varies directly with air temperature and usually differs by less than 2 C. Shallow pools of relatively quiet water are found in most areas; often having thermal gradients as great as 3-4 C from the surface to depths of 30-50 cm, they afford some protection from extreme air temperatures. Usually, however, these refuges are small and separated by long stretches of shallow exposed water. Table 1 shows the most common species of fishes in the Red River drainage within the range of the pupfish; a similar fauna occurs in the Brazos drainage. In general the fauna is depauperate compared with areas in eastern Oklahoma and Texas where greater varieties occur (Hubbs, 1957, pers. observ.) . This is correlated with greater degrees of habitat instability and structural homogeneity in western areas. Within the geographical range of the pupfish, collections of less than five species were taken from waters with salinities of 10-20 ppt or more (Fig. 2) and in transient fresh-water streams that would be dry during periods of low rainfall. SPECIES ASSOCIATES Our collections and those of Lindsey and Bates were similar in regard to the predominantly collected species and the percentage occurrence of each species in collections containing pupfish (Table 1) ; the plains killifish, Fun dulus zebrinus kansae, was the most common associate of the pupfish in both collection series. (We follow the in- ference of Miller, 1955, and the conclusions of Drewry, 1967, in re- garding kansae a conspecific of zebrinus.) Contingency analysis of joint occurrence (Table 2) demonstrated three major complexes of 1972 ECHELLE, ECHELLE AND HILL: PUPFISH 113 positively associated fishes among the 12 most frequently collected species: (1) C. rubrofluviatilis-F. zebrinus; (2) Hybognathus placitus- Notropis bairdi-Notropis percobromus-Hybopsis aestivalis, and (3) Notropis lutrensis-Gambusia affinis-Lepomis cyanellus-Lepomis megalotis-Pimephales promelas-Pimephales vigilax. We refer to these as the rubro-zebrinus, placitus-bairdi and lutrensis-affinis associations, using the names of the two dominant species (in numbers) for the latter two. From the few collections that we have made in the Brazos River, it appears that the same associations exist with the substitution of N. buccula for N. bairdi, N. oxyrhynchus for N. percobromus, and F. z. zebrinus for F. z. kansae; the members of these pairs are very

TABLE 1.—Fishes taken within the geographical range of Cyprinodon rubrolluviatilis in 59 collections made in 1969 and 67 collections made by Lindsey and Bates in 1963. In the 1969 series the species are arranged in order of decreasing frequency in collections containing rubrofiuviatilis. N = total number of collections containing the species; % = per cent frequency of occurrence in collections containing rubrofiuviatilis; ppt = range of salinities at which the species was collected

ppt 1969 1963 Species % N % N Cyprinodon rubrofluviatilis 0.4-51.7 28 29 Fundulus zebrinus .3-43.2 90 44 93 44 Hybognathus placitus 1.2-21.7 52 26 66 41 Notropis bairdi .4-21.7 52 28 66 38 Notropis lutrensis .3-13.0 52 44 55 49 Gambusia affinis .3-19.7 45 33 45 41 Lepomis cyanellus .3-19.7 34 31 21 20 Hybopsis aestivalis 1.2-19.6 17 13 45 31 Pimephales promelas .3-13.0 14 10 38 30 Lepomis megalotis .4- 7.0 14 15 7 13 Notropis percobromus .7- 9.5 10 9 21 18 Notropis potteri 4.2-19.7 10 4 10 13 Pimephales vigilax .3- 2.8 10 11 21 17 Carpiodes car Pio .9- 5.8 10 8 10 19 Lepomis humilis 4.6-11.0 10 8 14 12 Ictalurus melas .4- 8.2 7 9 0 2 Micropterus salmoides .4- 8.2 7 6 3 2 Dorosoma cePedianum 1.4- 5.8 3 2 10 14 Notropis stramineus .5- 1.7 3 2 7 9 CyPrinus carpio .5-12.5 3 5 0 5 Lepomis microlophus 2.8 3 1 0 0 Lepisosteus osseus 4.5- 9.5 2 2 7 2 Notropis venustus 0 0 0 1 Notropis buchanani 0 0 0 2 Notemigonus chrysoleucas 2.4 0 1 0 0 Phenacobius mirabilis 2.1-10.0 0 3 10 9 Ictiobus sp. 6.5 0 1 0 1 Ictalurus punctatus .9- 9.5 0 3 10 19 Ictalurus natalis .4 0 1 3 2 Noturus nocturnus 0 0 0 2 Pomoxis annularis .2 0 1 0 2 Percina phoxocephala 0 0 0 1 APIodinotus grunniens 0 0 0 I 114 THE AMERICAN MIDLAND NATURALIST 88(1) similar and seem to represent ecological equivalents in their respective drainage systems. Much overlap existed among the three associated species groups; they often occurred together and each usually occurred with individuals of one or both of the other groups. Close association of the rubro- zebrinus combination and the placitus-bairdi complex is implicit in the tendency for species of both groups to show negative associations with species of the lutrensis-affinis complex (Table 2). Separate analyses of joint occurrence between each of the four species of the placitus- bairdi complex and C. rubrofluviatilis and F. zebrinus are shown in Table 3. Significant deviation from chance grouping is confirmed for all four analyses. The chi-square distributions for the H. placitus and N. bairdi tests suggest that both are closely associated in occurrence with the rubro-zebrinus association; however, compared with N. bairdi there was a greater tendency for H. placitus to occur in collections not

r1=23

1 0 n=6

6 NUMBER OF SPECIES 4 n=7

0-4 4-8 8-10 10-20 20+

SALINITY (ppt) Fig. 2.—Number of species collected vs. salinity at the collection locality. Black bar = one standard error above and below the mean; vertical line = range TABLE 2.—Results of 2 X 2 contingency tables comparing the presence or absence of the 12 most abundant species in the 126 collections represented in Table 1. Numbers in parentheses = number of collections containing the species; ns ----- non- significance, pos = significant positive relationship, neg = significant negative relationship, * = p <.05, ** = p <.01 C. rubrofiuviatilis (26) pos** F. zebrinus (32) ns ns H. placitus (17) Collections with 5 species or fewer ns ns pos** N. 'bairdi (13) ...- .... N. percobromus (1) n = 47

...... -- H. aestivalis (3) ECHELLE, ECHELLE neg** neg* ns N. lutrensis (20) ...... P. promelas (5) ...... P. vigilax (2) neg** ns pos** ... L. cyanellus (9) ------.... L. megalotis (3) neg** ns ns pos* ns .... G. affinis (16) C. rubrofiuviatilis (31) pos** F. zebrinus (56) ns ns H. placitus (51) Collections with 6 species or more

ns ns pos** N. bairdi (50) AND HILL: PUPFISH ns ns pos** pos* N. percobromus (24) n = 79 ns ns pos** pos** pos* H. aestivalis (38) ns ns ns ...... neg** N. lutrensis (73) ns ns ns neg*** ns ns pos* P. promelas (35) ns ns neg* neg ns ns .... ns P. vigilax (26) ns ns neg** ns neg** neg** ns ns ns L. cyanellus (42) ns ns ns ns ns neg* ns ns ns L. megalotis (24) ns ns neg** neg** ns neg** ns ns ns ns G. affinis (54) C. rubrofiuviatilis (57) pos** F. zebrinus (88) ns ns H. placitus (68) All collections ns ns pos** N. bairdi (63) ns ns pos** pos** N. percobromus (25) n = 126 ns ns pos** pos** pos** H. aestivalis (41) neg** ns ns ns pos** ns N. lutrensis (93) ns ns ns ns ns ns pos** P. promelas (40) neg* ns ns ns ns ns pos** pos** P. vigilax (28) neg** ns neg* ns ns ns pos** pose pos* L. cyanellus (51) neg** ns xis ns ns ns pos** ns pos** pos** L. megalotis (27) neg** ns ns n5 ns ns pos** pos** pos** pos** ns G. affinis (70)

TABLE 3.-Results of selected three-species contingency analyses of presence-absence data for the 126 collections represented in Table 1. rub = C. rubrofiuviatilis, zeb = F. zebrinus, pla = H. placitus,, bai = N. bairdi, aes = H. aestivalis, per = N. percobromus; numbers in parentheses = per cent frequency of occurrence in the collections; asterisks = largest chi-squares for positive deviations in each analysis Species present O"') rub (45.2) rub rub rub Total zeb (69.8) zeb zeb zeb chi-square pla (53.1) pla pla pla Observed 32 21 2 14 2 21 19 15 Expected 21.0 18.7 9.1 25.2 8.1 22.6 11.1 9.8 THE AMERICAN MIDLAND NATURALIST Deviation +11.0 +2.3 -7.1 -11.2 -6.1 -1.6 +7.9 +5.2 Chi-square 5.76* 0.28 5.54 4.98 4.59 0.10 5.62* 2.76 29.63 <.005 rub rub rub - rub - - zeb zeb - zeb - zeb - bai (49.2) - bai bai - bai - Observed 32 21 1 15 3 20 14 20 Expected 19.2 20.2 8.4 23.7 8.7 24.4 10.2 10.6 Deviation +12.8 +0.8 -7.4 -8.7 -5.7 -4.4 +3.8 +9.4 Chi-square 8.54* 0.003 6.52 3.19 3.74 0.79 1.41 8.34* 33.53 G.005 rub rub rub - rub - zeb zeb - zeb - zeb - - aes (34.9) - aes aes - aes - Observed 17 36 1 12 3 23 14 20 Expected 13.8 25.8 6.2 16.6 11.2 31.4 7.3 13.6 Deviation +3.2 +10.2 -5.2 -4.6 -8.2 -8.4 +6.7 +6.4 Chi-square 0.74 4.04* 4.36 1.27 6.00 2.24 6.15* 3.01 27.81 G.005 rub rub rub - rub - zeb zeb - zeb - zeb per (21.4) - per per - - per - Observed 9 44 0 8 4 27 10 24 Expected 8.6 31.2 3.7 10.3 13.5 37.8 4.4 16.4 Deviation +0.4 +12.8 -3.7 -2.3 -9.5 -10.8 +5.6 +7.6 Chi-square 0.02 5.25* 3.70 0.53 6.78 3.09 7.13* 3.52 30.02 G.005 1972 ECHELLE, ECHELLE AND HILL: PUPFISH 117 containing either of the cyprinodontids. Neither the pupfish nor F. zebrinus was closely associated with H. aestivalis or with N. percobromus. It is evident that the species groups were not discrete, well- segregated ecological units; nevertheless, there was nonrandom assort- ment which seemed related to habitat characteristics. The pupfish and F. zebrinus were ubiquitous at high ( > 10 ppt) salinities (Fig. 3). However, F. zebrinus, but not the pupfish, was also common in fresh waters, occurring in 83% of the collections from waters above 10 ppt and in 74% of those from more dilute waters, compared with 100% and 26%, respectively, for the pupfish. Despite these differences, the probability was near .90 that a collection containing the pupfish would also contain F. zebrinus (Table 1), accounting for the high positive correlation obtained for the occurrences of these two species. The wide occurrence of F. zebrinus in the collections (Fig. 3) accounts for the lack of significant negative or positive association between it and members of the other associations. The positive association between the rubro-zebrinus complex and H. p/acitus and N. bairdi was due to the frequent occurrence of the latter two at salinities of 10-20 ppt. The greatest frequency of contact between C. rubro-

-I • = F. zebrinus 464 t 300- all collections 50 r= -24, p> .10 collections with pupfish 0) 260- r=-.41, .1 0>p>.05 40 x;. = C. rubrofluviatIlls 7 220- r = +.72, p <.01 30 - 1 -c

20 -9.

I 0

1 00-

60120 1 0 20 . U41 30 40 COLLECTION NUMBER Fig. 3.—Number of C. rubrofluviatilis compared with the number of F. zebrinus in the collections. Each bar represents a single collection arranged according to increasing salinity at the collection site; the line graph shows the trend of increasing salinity. r =. product-moment correlation coefficient for abundance versus salinity, considering just those collections containing the species in question; p = probability that the observed relationship was a random occurrence 118 THE AMERICAN MIDLAND NATURALIST 88(1) fluviatilis and the placitus-bairdi association was in saline waters (10-20 ppt) of larger streams. Comparison of the presence or absence of the three associations against stream size showed a positive relationship between the occurrence of the placitus-bairdi combination and wide, as compared with narrow, stream beds (chi-square = 13.91, p < .01) , while the rubro-zebrinus and lutrensis-affinis combinations showed no significant relationship with stream size. The negative association between species of the placitus-bairdi and lutrensis-affinis complexes was due to virtual restriction of the former to larger streams and high frequency of occurrence at higher (10-20 ppt) as well as at lower salinities; the latter complex, though common in larger fresh-water streams, was ubiquitous in small fresh-water systems and infrequent at salinities of 10-20 ppt; differential occurrence with respect to salinity also accounts for the negative correlation between the occurrence of the rubro-zebrinus and lutrensis-affinis associations. Based on our collections, the upper salinity tolerances of species in the lutrensis-affinis complex seemed to be between 10 to 20 ppt (Table 1) ; this corresponded with the salinities at which species diversity declined in the collections (Fig. 2). The upper limits for H. placitus and N. bairdi are probably slightly above 20 ppt, but exact limits are not well demonstrated since there was a substantial gap, from 22.0 to 35.1 ppt, in the salinities at which collections were made. Excepting the occurrence of H. placitus and N. bairdi at 21.7 ppt and H. placitus at 21.2 ppt, C. rubrofluviatilis and F. zebrinus were the only species collected at salinities above 20 ppt. There seems to be little difference in the salinity tolerances of the latter two species. On 14 July 1971 both species were collected from an intermittent pool of the Elm Fork, 8 miles N Vinson, Harmon Co., Okla., where the chloride content was 110 ppt (measured by mercuric nitrate method, Hach Chemical Co.) and the total dissolved solids content was 140-150 ppt (estimated with a hydrometer calibrated with standard NaCI solutions), which approximates the maximum at which cyprinodontoid fishes have been found (Barlow, 1958) . That there may be some difference in salinity tolerance is suggested by the occurrence of approximately 1000 dead killifish (15-25 mm, TL) and only a single dead pupfish (25 mm, TL) on the bottom of the pool. Two miles downstream the salinity was 86 ppt, and large and small individuals of both species were common, although the ratio of pupfish to killifish was 100:1. Thirty-five miles farther downstream the salinity was 20 ppt and N. bairdi and H. placitus were abundantly present as were the two cyprinodontids; no other fishes were present.

ASPECTS OF INTERSPECIFIC INTERACTION N. lutrensis and G. affinis occurred in a large proportion of the collections containing pupfish (Table 1) ; however, at localities supporting the three, either the pupfish or the other two species were usually in low abundance, and usually other fishes were present. On the other hand, the pupfish sometimes occurred with F. zebrinus, H. placitus and 1972 ECHELLE, ECHELLE AND HILL: PUPFISH 119

N. bairdi at localities where these were the only species present and where all four were fairly abundant. The latter combination seemed to form a relatively consistent ecological unit (Table 3), and it is of particular interest to examine the ecological role of each species. This approach provides insight into the kinds of relationships that may exist between the pupfish and other fishes in more complex faunas. Habitat preference.—H. placitus and N. bairdi occurred primarily in shallow depressions in the current, along the edges of deeper pools, and in deeper areas of shallow backwaters. In waters of moderate to rapid flow the pupfish was rigidly bottom-oriented, seeming to require protection from the current provided by turbulence around vertical irregularities of the bottom; in quiet waters they commonly swam in midwater and near the surface, but they showed greater affinity for the bottom at all times. The pupfish occurred most abundantly on sandy shoals where the water formed a shallow layer of minimal current, in shallow backwater areas, and around the edges of deeper pools. F. zebrinus occurred more frequently at the surface and in gravelly riffles than the other species; otherwise it was similar to the pupfish in microdistribution. F. zebrinus is a stronger swimmer than the pupfish and, in flowing waters, swims more easily and more frequently near the surface; it is, however, more bottom-oriented than other members of the genus in Oklahoma. There was a great deal of overlap in the microdistributions of the four species, but a tendency toward habitat segregation was evident, particularly on warm days. Feeding.—The gut contents of C. rubrofluviatilis resembled those of F. zebrinus and were notably different from those of N. bairdi and H. placitus (Table 4). N. bairdi contained mainly animal items and very little bottom sediments; the small amounts of substrate material suggest that some animals may be obtained from the bottom. Bottom sediments were a large part of the gut contents of the pupfish and F. zebrinus, and represented the total gut content in H. placitus; diatoms and other algae, protozoans and organic debris are some of the possible nutriment sources in this material. The gut contents of 50 additional pupfish (Echelle, 1970) and 220 F. zebrinus (Echelle and Mosier, ms.) from other localities resemble those shown in Table 4 for these species; the major departure was a wider variety of animal items and greater amounts of vegetable material (primarily filamentous algae) in both species. There is some doubt whether much of the latter material can be digested (Martin, 1970) . Although not quanti- fied, the pupfish ingests more substrate material and unidentifiable debris than F. zebrinus, suggesting that the former feeds more indis- criminately. This is supported by observations of feeding behavior in the two. Both feed primarily by nipping and digging on the bottom and counts of as many as 60-70 such acts per min are common in the pupfish during peak activity, while counts of 30-40 are more common in F. zebrinus; the latter seems more deliberate and investigative in feeding than does the pupfish. The competitive feeding relationship between F. zebrinus and the 120 THE AMERICAN MIDLAND NATURALIST 88(1) pupfish is heightened by similarities in microdistribution and diurnal feeding chronology. In both species the rate of feeding varies directly with temperature, apparently as a result of changes in metabolic demands; as a result, on summer days, feeding activity gradually in- creases from early morning until midafternoon when a state of almost constant feeding activity is attained (Echelle, 1970; Echelle et al., 1971) . Little feeding occurs at night. In areas where pupfish were abundant, feeding F. zebrinus were frequently confronted with aggressive members of the feeding pupfish population and breeding males. As noted by Minckley and Arnold (1969) for C. bifasciatus, female and nonbreeding male Red River pupfish often defend small feeding territories, especially at high temperatures, from which they threaten and charge any approaching fish. These acts were directed even at fishes much larger than the pupfish and frequently disrupted feeding activity in individual F. zebrinus. As noted above, the pupfish is more active and less investigative when feeding than F. zebrinus; accordingly, it is less susceptible to feeding disruptions. These differences are evident both in the field and in aquaria. Spawning behavior.—The breeding characteristics of the pupfish and F. zebrinus are similar in a variety of ways and represent a major avenue of competition. Koster (1948) and Echelle et al. (1971) described various aspects of behavior in natural conditions for F. zebrinus; similar descriptions were presented by Echelle (1970) for the Red River pupfish. Except where otherwise noted, the following description is based on the latter two papers.

TABLE 4.—Contents of the anterior 1/3 of the digestive tracts of 10 specimens each of Cy prinodon rubrofiuviatilis and three other species from a single collection at 1830 hr, 14 July 1971, Elm Fork of Red River, 9 miles N, Mangum, Greer Co., Olda., where these were the only species present. % = estimated per cent of total gut content; tr = trace; for each animal item listed, numbers in parentheses = per cent frequency of occurrence, numbers not in parentheses = total number found; L .= larvae, P = pupae, A = adult CyPrinodon Fundulus Notropis Hybognathus rub rofluviatilis zebrinus b airdi Placitus (36-41)* (48-57) (40-50) (62-69) Bottom sediments 78% 76% tr 100% Vegetable matter 12% tr tr tr Animal items 10% 25% 100% Chironomidae L 61(40) 83(30) 2(10) P 1(10) 3(20) Ceratopogonidae L 2(20) 8(10) Collembola 1(10) Coleoptera (A) .... 2(-20) Lepidoptera (L) 1(10) Gastropoda 1(1-0) *Size range in mm, SL. 1972 ECHELLE, ECHELLE AND HILL: PUPFISH 121

The spawning season in Oklahoma is from February to November in the pupfish and March into October in F. zebrinus; the permissive ranges of temperatures for spawning are 13 to 34 C and approximately 20 to 34 C, respectively. Reproduction occurs daily, with local variation, except on excessively cool days. The males of both species defend large breeding territories where courtship and spawning occur. In cold weather territorial defense occurs only in the afternoon when water temperatures approach the permissive levels for spawning. In late spring and in summer pupfish males defend territories from daylight until dark even when afternoon temperatures exceed the permissive range for spawning. In contrast, F. zebrinus males defend territories only during the reproductive period of the day, which is relatively short on warm days. At all times of the year, pupfish males begin defending breeding territories earlier in the day than F. zebrinus. The territories tend to occur in clusters, usually in shallow, quiet water, but also in shallow areas directly in the current. Often, at the peak of spawning, in areas supporting dense populations of either species, practically every available area is occupied by breeding males. Perhaps as a reflection of lower aggression, the territorial organ- ization of F. zebrinus is more flexible than that of the pupfish, with more overlap between neighboring territories and higher tolerance of intruders. At the peak of spawning male pupfish defend rigid territories from which all intruders except receptive females are excluded. Courtship is strikingly similar, usually involving a single male and a female and culminating in a clasp on the bottom of the stream during which a single egg is expelled. The highly aggressive nature of the male pupfish frequently causes aborted courtships as he leaves the female to chase intruders; even intrusion into the breeding territory by nonbreecling conspecifics and members of other species usually leads to disruption of courtship activity. The male F. zebrinus is less susceptible to this type of disruption, for, although aggression during courtship is common between breeding males, they seldom leave the female to displace intruders from the territory. Rather, a small area around the female is defended against competing males. Because of the tendency for the breeding males of the pupfish and F. zebrinus to locate their territories in similar areas, numerous interspecific agonistic bouts were observed. Being more motile in courtship, F. zebrinus seems to require more space than the pupfish; this often led to encounters with several territorial pupfish males in succession when given F. zebrinus males were courting. On several occasions male pupfish physically disrupted the spawning act in F. zebrinus by ram- ming into pairs that were clasping on the bottom. In Oscar Creek on 20 July 1969 male pupfish interfered with almost every courtship at- tempted by a particular F. zebrinus male. The reciprocal activities were rarely observed, but during courtship male pupfish sometimes left the female to chase individual F. zebrinus that were entering the territory. Predation.—Inter- and intraspecific predation between the pupfish and F. zebrinus may be major sources of predation on these fishes in 122 THE AMERICAN MIDLAND NATURALIST 88(1) areas where they are the predominant species. Egg predation may be especially heavy as a result of the bottom-oriented feeding and spawning habits of the two. Because of greater size and mobility, F. zebrinus probably is the greater predator on young fishes, and, in fact, one 50 mm female had a 25 mm F. zebrinus in its intestine (Echelle and Mosier, ms.) ; the pupfish, because of its more active, in- discriminate feeding habits, probably is the greater threat to the eggs. Perhaps related to this is the apparent tendency for Cyprinodon variegatus to ingest more insect eggs than Fundulus spp. (Harrington and Harrington, 1961) . F. zebrinus males defend breeding territories only during the reproductive period of the day, thereby leaving the fertilized eggs unprotected for long periods. This may be somewhat compensated for by the tendency for the spawning act to bury the egg well beneath the surface of the substrate. However, pupfish are in- tensive bottom feeders and have behavioral patterns that function to excavate large amounts of substrate, apparently for the purpose of exposing buried food items (Minckley and Arnold, 1969) . Individual pupfish males tend to defend breeding territories at all hours during daylight, affording more protection for the fertilized eggs.

LIMITING FACTORS OF ABUNDANCE AND DISTRIBUTION Martin (1968) stated that "Cyprinodon variegatus, C. rubrofluviatilis, and 'C. bovinus-like' fishes . . . are not limited in their distribution by Ca2+, Na, K+, or Mg2+ or by any combination of these ions. . . ." He showed that the majority of localities at which Cyprin- odon did not occur had levels of all ions and ion ratios within the range at which pupfish were found. Neither do Red River pupfish seem limited by the total dissolved solids content of the water; we collected them at salinities ranging from 0.4 ppt to near 150 ppt. They were more frequent at higher salinities, but this may be more reflective of differential abundance than occurrence; considering only those collections containing pupfish, they were much more abundant, thus more easily collected, at the higher salinities (Fig. 3) . Pupfish occurrence, albeit sporadic, in very dilute waters as well as at high salinities suggests that low dissolved solids per se are no barrier to distribution. We have kept them in fresh-water (1-2 ppt) tanks where they produced offspring that grew in size until removed from the tanks 3 months after hatching. These findings are not surprising since other Cyprinodon are noted for euryhaline ability (cf., Barlow 1958; Kinne, 1960; Simpson and Gunter, 1956) . Kinne (1960) and Kinne and Kinne (1962) gave evidence which suggests that (1) Cyprinodon macularius, another pupfish occurring in dilute waters as well as at high salinities, is more efficient, meta- bolically, at salinities of 15 and 35 ppt than in "fresh water," and (2) optimum salinities are determined in early incubation and persist into adulthood. This suggests that habitat preference, with the pupfish gravitating toward high salinities, may partly account for the restriction of the pupfish to areas of western Oklahoma and Texas. If this effect 1972 ECHELLE, ECHELLE AND HILL: PUPFISH 123 were intense, further examination of possible limiting factors would be superfluous; however, the pupfish was occasionally collected in moderate abundance in dilute waters (e.g., collections 4 and 31, Fig. 3), suggesting that other factors must be operating. The Red River pupfish is one of the most euryplastic fishes, suggesting that physical or chemical characteristics of the environment would not be major limiting factors. Lowe Hinds and Halpern (1967) provided evidence that C. macularius is beha viorally, morphologically and physiologically adapted to tolerate conditions of low dissolved oxygen, and C. rubrofluviatilis seems to have similar adaptations. To our knowledge, the highest successful incubation temperature in fishes is 36.5 C in C. rubrofluviatilis (Echelle et al., 1972) and the widest known thermal range for spawning in fishes may be that noted above for the pupfish (13-34 C). Feeding occurs at temperatures as low as 4. C and as high as 40 C or above. Neither does the occurrence of the pupfish seem affected by environmental factors such as turbidity or structural features. Habitats supporting pupfish varied from turbid to very clear, from those with soft mud bottoms to others of sand and gravel, and from areas with dense growths of vegetation (e.g., filamentous algae and Potamogeton) to those with little or no such growths. The current varied from slight to moderately swift. Spawning occurred on sand, soft mud bottoms, the surface of stones and within mats of vegetation and plant debris. Despite euryhaline abilities and the apparent lack of other intrinsic limiting factors, C. rubrofluviatilis is effectively restricted to the more saline waters within its geographical range. The pupfish occurred in dilute waters primarily in western areas where saline waters, and therefore high pupfish concentrations, are most abundant. It is virtually absent from fresh-water situations in more eastern areas. The easternmost extent of the range in Oklahoma is Oscar Creek, a small saline (30 ppt is common) stream in Jefferson Co. that receives oil field brine from surrounding areas. The pupfish is always abundant in this stream, but to our knowledge has never been collected in the small fresh-water streams of Jefferson Co. or adjacent counties and it is rare in the Red River proper, which is relatively dilute, 1-2 ppt, in this area. Although capable of thriving in fresh waters the pupfish is primarily restricted to saline conditions, occurring only sporadically in dilute waters and then primarily in western parts of its geographical range where the chances are greater that individuals will wander into such areas from saline waters with dense populations of pupfish. The absence of the pupfish in the Washita River and Clear Fork of the Brazos, and its rarity in the Double Mountain Fork of the Brazos probably are accounted for on this basis. The headwaters of these streams are similar in physical aspect to streams where pupfish were abundant, but, unlike the latter, they do not carry heavy loads of dissolved solids. In the absence qf intrinsic limiting factors, it follows that causative factors of the virtual exclusion of C. rubrofluviatilis from fresh waters 124 THE AMERICAN MIDLAND NATURALIST 88(1) are to be found in extrinsic pressures, namely competition and/or predation. Comparison of the numbers of pupfish against the numbers of species in the collections demonstrated a significant negative correlation (r = —.66; 01 < p < .05) ; a similar correlation was obtained for proportionate abundance of pupfish (Fig. 4A). Figure 4A is more

C. rubrofluviatIlls F. zebrinus 100 r = -.79 all collections p <.01 r=-23, p>.10 80 sallpIty <10 ppt r = -.47, .05 >p>.01

60 so

40 A 20 •

• 8 12 4 8 12 NUMBER OF SPECIES C. rubrofluviatilis F. zebrinus PROPORTION r=+.88 r= -.30 p< .01 .10>p> .05 80-

60-

40.

20 -

Li! .111 1111.1 I INCREASING SALIN TY Fig. 4.—The proportions contributed by C. rubrofluviatilis and F. zebrinus to the total number of fishes in the collections represented in Figure 3, compared with the number of species collected and the relative salinity at the collection site. X = F. zebrinus present, closed circles = C. rubrofluviatilis present, X in a small circle = both species present, large circles = those collections taken at salinities of 10 ppt or greater. In C and D each line represents the proportion for a single collection with collections arranged according to increasing salinity. Notation for correlation coefficients same as in Figure 3 1972 ECHELLE, ECHELLE AND HILL: PUPFISH 125 suggestive of an acutely negative exponential than a straight line relationship between pupfish abundance and species density. It appears that, with increased species density, the pupfish population diminishes precipitously to a near minimal level at relatively low numbers (5-6) of cohabiting species. With increasing species density a trend of gradual extinction is obtained; this would occur concomitantly with decreasing salinity (Fig. 2). In west Texas and Oklahoma the pupfish probably exists in fresh waters only because of reinforcement from saline waters. Reinforcement would be progressively reduced with increasing distance from centers of abundance resulting in a W to E filter bridge decreasing the probability of the pupfish extending its range farther eastward. It is possible that, in dilute waters, the pupfish is under low level physiological stress which would assume importance when added to other pressures. The possible effects of physiological inefficiency seem secondary to interspecific pressures, for our observations suggest that in the absence of other fishes the pupfish would be successful in fresh waters. Using Hutchinson's (1957) terminology, the "fundamental niche" of the pupfish probably encompasses dilute waters as well as saline waters, but the "realized niche" is effectively restricted to the latter conditions. The species of the lutrensis-affinis complex (Table 2) are the most likely ones effecting exclusion of the pupfish from fresh water since these were the most ubiquitous fresh-water forms. Ecological in- compatibility between species of the lutrensis-affinis complex and the pupfish could account for the negative associations shown in Table 2; however, such analyses do not demonstrate interspecific phenomena (Hurlbert, 1969), for similar associations could result from habitat selection eliminating competition or other such factors as causative agents. DISCUSSION As might be expected for species adapted to exist in depauperate faunas, Cyprinodon species seem to occupy broad ecological niches. Kaill (1967) stated, "The behavior of Cyprinodon variegatus is flexible. It seems to be made up of components assembled in a non- stereotyped manner which are then modified to the demands of a particular habitat." In the same species, Harrington and Harrington (1%1) noted a shift from primarily carnivorous habits to essentially herbivorous habits, depending upon the availability of animal items. Examples of broad adaptation in C. rubrofluviatilis are: (1) it is omnivorous, (2) although occurring primarily near the bottom, it is frequently seen feeding at all levels in the water; (3) the reproductive season probably encompasses that of all other species in its geographical range; (4) spawning occurs under a wide range of temperature; and (5) breeding territories occur in a variety of situations. Such factors would create competition with a wide spectrum of species, an effect that might be especially critical in dilute waters where the pupfish may be at metabolic disadvantage. 126 THE AMERICAN MIDLAND NATURALIST 88(1)

Cross (1967) reported prolonged spawning seasons and/or flexibility in choice of spawning sites in the prairie streams of Kansas by a variety of fishes that are among the more common species in the geographical range of the pupfish; he postulated a need for flexibility in unstable environments. We surmise that fishes of prairie streams should show a variety of broad adaptations when compared with close relatives in more stable and complex environments. Broad adaptations should tend to confer numbers advantages in density- dependent competitive interactions. This would be important in homo- geneous environments where there would be extensive interspecific overlap in ecological requirements. It appears that, while a certain level of broad adaptation may confer adaptive advantage in fresh- water prairie streams, the pupfish is overextended in this respect, and is excluded from these waters by an array of competing forms which, although operating in rather expanded niches, are somewhat more narrowly adapted and thereby better adapted for richer faunas. Though this study falls short of actual proof, it seems likely that the "realized niche" of C. rubrofluviatilis is primarily restricted to saline waters through competitive exclusion by fresh-water faunas. The competitive relationship between the pupfish and other fishes is undoubtedly heightened by the unusually high degree of structural homogeneity in the streams of western Oklahoma and Texas. Typi- cally, these are shallow with flat sandy bottoms and little emergent vegetation, offering little opportunity for species to segregate according to differential adaptive peaks. It is difficult to assess the possible effects of predation in restricting the pupfish primarily to depauperate faunas, but it probably has some influence. Its small size (max. = 60 mm, TL; modal adult size, near 30 mm), although possibly affording metabolic advantage in harsh conditions (Lowe, Hinds and Halpern, 1967), probably makes the pupfish more susceptible to predation than if it were, say, sunfish size. In addition it is a slow swimmer compared with forms like F. zebrinus and Notropis, and it leads a fairly open, exposed life compared with others such as Gambusia affinis. In the geographical range of the pupfish, F. zebrinus seems more successful in fresh-water faunas than the pupfish and, in fact, showed a slight negative correlation between abundance and salinity (Figs. 3, 4D). The latter was unexpected since this species is frequently docu- mented as occurring predominantly in saline waters (Metcalf, 1959; Cross, 1967; Hubbs, 1957). Our observations of F. zebrinus in saline waters of the Arkansas River drainage of Oklahoma are in accord with the literature; we have seen dense populations in saline pools at the Great Salt Plains State Park (Alfalfa Co.), and, in July 1969, they were collected in abundance-321 individuals in five seine hauls— from Salt Creek (5 miles S, Okeene, BlaMe Co.) at a salinity of 16 ppt. Similar levels of abundance were not observed at comparable salinities in the geographical range of the pupfish. In our experience F. zebrinus seems to occur most abundantly in areas where there are few other 1972 ECHELLE, ECHELLE AND HILL: PUPFISH 127 fishes; this is especially true in south-central Oklahoma where the more complex eastern biota interdigitates with the western biota. Its feeding habits, reproductive biology and microdistribution are similar in many respects to the Red River pupfish, suggesting that the mechanisms limiting its distribution and abundance may be similar to those pro- posed for the pupfish. As in the pupfish, the proportionate abundance of F. zebrinus varied negatively with species density; however, this is shown only if those collections taken from saline waters are excluded from consideration (Fig. 4) . Despite low species densities (Fig. 2), the abundance of F. zebrinus was depressed at high salinities in the presence of C. rubrofiuviatilis. F. zebrinus seems to prefer shallow waters with sand bottoms (Cross, 1967; Koster, 1957), and, as these are typical features of the saline waters supporting pupfish, suboptimal physical habitats cannot explain the depressed abundance of the former in such waters; thus, negative effects of the cohabiting pupfish population probably are the primary influences. Figure 4B suggests that the effect exerted by the pupfish on the abundance of F. zebrinus in saline waters approximates that exerted by richer faunas in dilute waters. It is as if the more broadly adapted pupfish expands in numbers in response to the absence of other species, saturating the habitat and thereby reducing the resources that might otherwise be available for an expanding F. zebrinus population. The tendency for the pupfish to begin spawning earlier in the year and at a broader range of temperatures probably helps to confer a numbers advantage, magnifying the effects of such density-dependent factors as egg predation and competition for food and spawning sites. Con- ceivably, the tendency for pupfish males to begin defending breeding territories earlier each day might give them a type of "prior residence" advantage (see Braddock, 1949) over F. zebrinus males in competition for breeding space. Additional behavioral effects would be the greater levels of activity and aggressiveness exhibited by the pupfish. Factors such as differential growth rates, fecundity, longevity, etc., probably influence the relative success in areas of sympatry (Hubbs, 1971), but our data are too scanty in this respect for a comparison of the demo- graphic dynamics of the two species. Incubation time in the pupfish (Echelle et al., 1972), at comparable temperatures, is shorter by days than in F. zebrinus (Wilson and Hubbs, 1972), and the latter is about 5 mm longer at hatching. The smallest spawners that we observed were 15-20 mm, TL, in the pupfish and 25-30 mm, TL, in F. zebrinus. Although such characteristics may have some effect the behavioral differences alone probably suffice to maintain the abundance of the pupfish over F. zebrinus at high salinities. The characteristics that give the pupfish competitive advantage over F. zebrinus at high salinities may operate to make the pupfish successful at moderate salinities, 10 to 20 ppt, where at least 12 other species are potential cohabitants (Table 1), especially since the latter species might be at metabolic disadvantage. The high level of aggression exhibited by the pupfish may be ad- 128 THE AMERICAN MIDLAND NATURALIST 88(1) vantageous in simple faunas and maladaptive in rich faunas. At the level of individual fitness, aggressiveness would seem to confer greater selective advantage in systems where aggression would increase the reproductive output of the individual relative to those animals at which aggression is directed. This would be maximal in simple faunas where resources are in short supply. Cyprinodon species seem to be adapted for simple faunas where all or almost all cohabitants are direct competitors. As the pupfish occurs in high densities in such situations, aggressiveness would have obvious advantages. The highly aggressive nature of the pupfish would be a disadvantage in complex faunas where selective advantage would seem to favor segregation of species according to adaptive peaks, resulting in minimal interspecific interaction and maximum efficiency of energy utilization. Competitors would be less dense in such situations and indiscriminant aggression would be waste- ful of energy. The lower level of aggression in F. zebrinus may offer a partial explanation of its greater success, compared with the pupfish, in fresh waters where greater numbers of species occur. In summary, it appears that interspecific interaction is the primary factor influencing the observed pattern of distribution and abundance in C. rubrofluviatilis. This study does not resolve the problem of which species are responsible for excluding the pupfish from fresh waters, but the answer probably does not lie in a one-on-one competitive relationship where species A excludes species B. Rather, it appears that the combined effects of a variety of species are responsible. In the four-species system occurring in large streams at moderate salinities and consisting of Hybognathus placitus, Notropis bairdi, F. zebrinus and C. rubrofiuviatilis, the former two are somewhat removed from competition with the pupfish through habitat and food-niche partition- ing, and F. zebrinus emerges as the major competitor. Indeed, high levels of behavioral similarity, competition and aggressive interaction were observed between the pupfish and F. zebrinus. Although these effects seem responsible for the depression of the abundance of F. zebrinus at high salinities where the pupfish is the only cohabiting species, they probably do not account for the exclusion of the pupfish from fresh waters. High flexibility, particularly in food and breeding habits, and a high level of aggression would seem to make the pupfish compatible with, and/or an effective competitor against, any other single species in its geographical range. Assuming, however, that these attributes contribute to a general inefficiency in restricted niches, the pupfish would be excluded from waters supporting a variety of more specialized species.

Acknowledgment.—This paper is based on part of a Ph.D. dissertation submitted by A. A. Echelle to the University of Oklahoma graduate faculty. Financial assistance was provided by the University of Oklahoma Biological Station and a grant to A.A.E. from the Oklahoma College Teacher Develop- ment Fund which was provided by Cameron State College, Lawton, Okla. Drs. C. D. Riggs, C. C. Carpenter and H. P. Clemens offered helpful criticisms.

1972 ECHELLE, ECHELLE AND HILL : PUPFISH 129

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LOWE, C. H., D. S. HINDS AND E. A. HALPERN. 1967. Experimental cata- strophic selection and tolerances to low oxygen concentration in native Arizona freshwater fishes. Ecology, 48:1013-1017. MARTIN, F. D. 1968. Some factors influencing penetration into rivers by fishes of the genus Cyprinodon. Ph.D. Dissertation, Univ. Texas, Austin. 87 p. . 1970. Feeding habits of Cyprinodon variegatus (Cyprinodontidae) from the Texas coast. Southwest. Natur., 14:368-369. METCALF, A. L. 1959. Fishes of Chautauqua, Cowley and Elk counties, Kansas. Univ. Kans. Publ. Mus. Natur. Hist., 11:345-400. MILLER, R. R. 1948. The cyprinodont fishes of the Death Valley system of eastern California and southwestern Nevada. Misc. Publ. Mus. Zool. Univ. Mich. No. 68. 155 p. . 1955. An annotated list of the American cyprinodontid fishes of the genus Fundulus, with the description of Fundulus persimilis from Yuca- tan. Occ. Pap. Mus. Zool., Univ. Michigan, 568:1-27. . 1961. Man and the changing fish fauna of the American Southwest. Papers Mich. Acad. Sci., Arts, Letters, 46:365-404. . 1968. Two new fishes of the genus Cyprinodon from the Cuatro Cienegas basin, Coahuila, Mexico. Occas. Pap. Mus. Zool., Univ. Mich., 659:1-15. MINCKLEY, W. L. AND E. T. ARNOLD. 1969. "Pit digging" a behavioral feeding adaptation in pupfishes (Genus Cyprinodon). J. Ariz. Acad. Sci., 5:254-257. AND M. ITSKOWITZ. 1967. Ecology and effects of intestinal gas ac- cumulation in a pupfish (Genus CyPrinodan). Trans. Amer. Fish. Soc., 96:216-218. SIMPSON, D. G. AND G. GUNTER. 1956. Notes on the habitats, systematic characters and life histories of Texas salt water cyprinodontes. Tulane Stud. Zool., 4:115-134. SOKAL, R. R. AND F. J. ROHLF. 1969. Biometry. W. H. Freeman and Co., San Francisco. 776 p. THOMERSON, J. E. AND D. P. WOOLDRIDGE. 1970. Food habits of allotopic and syntopic populations of the topminnows Fundulus olivaceus and Fun- dulus notatus. Amer. Midi. Natur., 84:573-576. UNITED STATES PUBLIC HEALTH SERVICE. 1964. Arkansas-Red river basins water quality conservation: Report on a basic study of water quality, sources of natural and manmade salt pollution, and suggested corrective measures. U. S. Dep. Health, Education and Welfare, Washington, D. C. 71 p. WiLsoN, S. AND C. Hasps. 1972. Developmental rates and tolerances of the plains killifish, Fundulus kansae, and comparison with related fishes. Tex. J. Sci., in press.

SUBMITTED 12 AUGUST 1971 ACCEPTED 15 OCTOBER 1971