Description and Ecology of texana n. sp. (Crustacea: ) from the Edwards Plateau of

MICHAEL M. STEVENSON and ALEX E. PEDEN

•

Reprinted from THE AMERICAN MIDLAND NATURALIST

Vol. 89, No. 2, April, 1973, pp. 426-436 University of Notre Dame Press Notre Dame, Indiana Description and Ecology of Hyalella texana n. sp. (Crustacea: Amphipoda) from the Edwards Plateau of Texas

MICHAEL M. STEVENSON Department of Zoology and Oklahoma Biological Survey, University of Oklahoma, Norman 73069 and ALEX E. PEDEN British Columbia Provincial Museum, Victoria, British Columbia, Canada

ABSTRACT: Hyalella texana, a new of amphipod , is described from Clear Creek Spring, Menard Co., Texas, 16.7 km W of Menard. It is distinguished from H. azteca, which also occurs at this locality, by the presence of four large tergite spines, enlarged mouth parts, more antennal segments and narrower body and appendage seg- ments. Each species is found predominantly in different habitats of the spring area. INTRODUCTION In his study on the fresh-water amphipods of glaciated , Bousfield (1958) indicated that only one species of the Hyalella occurred in , the United States or Canada. This form, H. azteca Sauss., is usually distinguished from all other North American fresh-water genera by the absence of a secondary flagellum on the first antenna and the presence of dorsal tergite spines on the first two abdom- inal segments, although some populations are known to have individuals without such spines (Smith, 1875) . Bousfield reported that most popu- lations of Hyalella from Canada had individuals with a single spine on abdominal tergites 1 and 2 only, while specimens from Prince Edward Island had an additional spine on tergite 3. He also noted individuals with single spines on two, three or four body segments from Grand Etang, Quebec, and indicated those from the Queen Charlotte Islands approached an edentate condition. Similar variability has been re- ported in the United States, resulting in proliferation of several names for allopatric populations. Weckle (1907) synonymized all such North American forms with H. azteca and said that variation in spine number was taxonomically insignificant. has been reported from springs in Texas by Hubrecht (1943), Hubrecht and Mackin (1940) and Weckle (1907) , and we have collected the species from most drainages on the Edwards Plateau of this state. We found the typical form with two tergite spines to be widely distributed, although a few localities also had indi- viduals with three spines. The situation at Clear Creek Spring on Clear Creek Ranch, Menard Co., Texas, was most unusual in having, in addi- tion to a large population of typical Hyalella with single spines on the first two abdominal tergites, another population of larger and more laterally compressed Hyalella with four large tergite spines: a single spine each on abdominal segments 1-3 and on the last thoracic seg- 426 1973 STEVENSON AND PEDEN : HYALELLA TEXANA 427 ment. The four-spined form was most abundant near the head springs, and the two-spined form was more numerous downstream. Moreover, we found a zone of sympatry where there were no intermediate adults with three tergite spines or other intermediate characters. In addition, one of us (Peden) kept both species in aquaria for many months and, although each species reproduced readily during that time, no hybrid individuals were found among the progeny. Furthermore, each form was found in distinct habitats in the spring area and the species ful- filled the criteria for a biological species concept. Because the two-spined species is widely distributed and is most similar to published descriptions of H. azteca (E. L. Bousfield, pers. comm.), the other form which is apparently endemic to Clear Creek must be considered as an undescribed species. We, therefore, describe this form as Hyalella texana and provide a description of the ecology of both species occurring at Clear Creek.

Hyalella texana n. sp. Material examined.—Holotype and paratypes were collected on 20 July 1968, and 5 September 1971 at Clear Creek Spring, Menard Co., Texas. The holotype, a male 8 mm long, and an ovigerous female allotype are deposited at the National Museum of Canada. Two dozen paratypes are deposited there as well as at the U.S. National Museum and the British Columbia Provincial Museum. The description is based on 50 male and 50 female specimens. Diagnosis.—An orange-colored species of Hyalella with a darker orange middorsal stripe extending from the first thoracic segment to the last abdominal segment; first antenna less than half the length of the second, and no accessory flagellum; the second antenna over half the length of the body; last thoracic and first three abdominal segments each bearing a middorsal and posteriorly projecting spine; coxal plates short and narrow, anterior ones not overlapping mouth parts and the remaining ones projecting only slightly over more distal appendage segments; second distal segment of male's second gnathopod has four setae on the posterior border, but only two setae present on the same segment on the female's gnathopod. There are three large teeth on the inner plate of the maxillipeds and small apical spines on the inner anterior surface of this plate. In sample populations there are disproportionally more females. Description.—Body greatly compressed laterally, uniformly orange except for a conspicuous and darker middorsal orange stripe between the first thoracic body segment and the last abdominal tergite. Eyes small, nearly round, often with an indistinct border; distance between each eye greater than eye diameter. First antenna less than half the length of the second; first segment of peduncle longer than the second segment, and both segments longer than third; flagellum about one and one-half times as long as peduncle and composed of 10-20 sparsely setose segments. Second antenna over half the length of the body, peduncle length 428 THE AMERICAN MIDLAND NATURALIST 89(2)

greatly exceeding that of the first antenna; fifth segment is longer than the fourth, and each of the latter two longer than the second or third segment; flagellum longer than the first antenna and with 17-30 sparse- ly setose segments. Mouth parts enlarged. Maxillipeds are broad-based, overlapping the anterior mouth parts and extending beyond the level of the basal antenna segments. Inner plates of maxillipeds shorter than outer plates and narrowly overlapping; distal margin has three enlarged teeth with only a few setae on the inner margin but several stiff spines on the anterior distal surface and inner lateral margin. Outer plate of maxillipeds broad at the rounded distal margin and with many setae along the inner margin and anterior surface; second palp seg- ment with well-produced lobe possessing several fine setae on inner margin; fourth segment weak with a long, slender nail. Second maxilla has inner and outer plates roughly the same length and with many strong and elongate setae on the distal margin; anterior surface of the inner palp has two naked setae near upper distal margin. First maxilla has an inner plate with two stiff spines at the squared distal margin, an outer plate with usually 7-9 long serrate spines and a small palp that ends in a sharp point. Mandible with a strong and broad-based molar, a rather large protuberance on the ventral outer edge, weak setae on the inner edge, two elongate setae between the molar and a row of three equal-sized spines, and a cutting edge with 3-4 teeth (Fig. 1) .

mandible 1st maxilla 2nd maxilla maxillipeds

Fig. 1.—Comparison of mouth parts of Hyalella texana and H. azteca from Clear Creek Spring 1973 STEVENSON AND PEDEN : HYALELLA TEXANA 429

First gnathopod of the males elongate. Carpus broadened at the distal end with a distinct posterior lobe possessing many setae; propodus rather narrow, its palm transverse but slightly convex; dactyl strongly curved with scales on the outer distal margin and setae on the inner margin to nail; nail fits into a slight notch at the posterior end of the palm; a single tooth lateral to the notch. Fourth appendage segment with pectinate scales on posterior rounded margin; anterior and pos- terior margin of propodus with similar scales distally. First gnathopod of the female is similar to that of the male (Fig. 2) . Second gnathopod of male much larger than first. Carpus short, with narrow lobe curving over and extending halfway up the posterior border of propodus; many elongate setae at distal end. Propodus slightly longer than its greatest width, broadening distally with the anterior margin curved and unarmed; posterior margin straight with 2-3 setae toward the distal end; palm oblique, curving downward into a large notch at the posterior angle; both notch and palm provided with several elongate stiff spines and small setae. Dactyl strongly curved with tip fitting into the notch on palm; with a single seta on the middle to outer margin; usually five on the inner margin. Posterior border of the second segment with two setae; fourth segment with scales on the posterior border of apical protuberance (Fig. 2) . Second gnathopod of females slender. Carpus similar to that of first gnathopod of males and females. Propodus weak, elongate and slightly broadened at distal end; palm transverse but slightly arcuate

1st gnathopod 1st gnathopod 2nd gnathopod 2nd gnathopod

J/ . 0 / 0 I ,...... , M 0J ", ''t-5-i&w- NOR

0 ,...., flt,1 ...,., whiIitho„, r Fig. 2.—Comparison of first and second gnathopods between Hyalella texana and H. azteca from Clear Creek Spring 430 THE AMERICAN MIDLAND NATURALIST 89(2)

with many stiff spines and several hairs. Dactyl only slightly curved, not extending the length of palm, with a few scales on outer margin and a few short hairs on the inner margin. Second appendage segment with two large spines on the posterior margin, scales on anterior and posterior distal margin of propodus. Scales are especially dense on the rounded posterior border of the fourth segment (Fig. 2) . Peraeopods 1 and 2 small, with narrow segments and many long spines; terminal segments elongate and strongly curved. Peraeopods 3-5 large, with many fine spines. Second segment of peraeopods 3 and 4 slightly expanded and with slight serration and sparse armament around the posterior border. Second segment of peraeopod 5 large, strongly serrate and with many setae. Pleopods elongate, plumose, and with elongate hairs on the distal segments. Uropods spinose, with stouter armament in females and distal spines upturned in the males; inner ramus of the first uropod equal to peduncle length. Telson slightly angular, more broad than long and with two setae at the tip. First three coxal plates short and narrow, the fourth broader but short. Anterior plates not extending ventrally to cover the mouth parts, but narrowly overlapping posteriorly, last three thoracic plates small, bibbed and not greatly overlapping. Last thoracic and first three abdominal tergites greatly produced dorsally to form a large, posteriorly projecting spine on each body segment, fourth spine often projecting upward. Epimera naked (Fig. 3) . Lateral sternal gills absent on the second segment. Brood plates, flat, sharply pointed on second segment, squared at setosed distal margin of remaining segments. Body length frequently exceeds 8 mm.

last thoracic seg. 1st abdominal 2nd abdominal 3rd abdominal te lson 0 V N

0 0 SR IRA V , N 1 MI Fig. 3.—Comparison of last thoracic and first three abdominal body seg- ments and telson between Hyalella texana and H. azteca from Clear Creek Spring 1973 STEVENSON AND PEDEN : HYALELLA TEXANA 431

Differences between H. texana and H. azteca.—The body of H. texana is dorsally narrower and much more compressed anteriorly than that o f H. azteca from Clear Creek. All juveniles of H. texana above 2 mm in total length (measured from anterior margin of head to tip of telson when telson is held in horizontal plane) have two spines on the first and second abdominal tergites. But as H. texana grows, a third spine develops on the last thoracic segment and is found on indi- viduals of 3 mm total length. An additional fourth tergite spine develops on the third abdominal segment of individuals longer than 5 mm. In contrast, H. azteca of 2 or more mm in total length retain only two tergite spines on the first and second abdominal segments. These spines are very short compared to the longer, often overlapping spines of H. texana. In both species there is an increase in the number of antennal segments with increased body growth (Fig. 4), but the number of flagellar segments on the second antenna of H. texana is about twice as large as that of similar-sized H. azteca (Fig. 5) . As indicated by Fig. 6, relative sizes of the fourth coxal plate are sexually dimorphic and also diagnostic for each species since they are proportionally smaller in H. texana.

I 6-

1 4 b. 1.229 (±0974)

13=0.986 (+ .0932) 1 2 II= 1.052 (+ .0877)

b.0.822(+ .0782) I 0

8

6 First Antenna (no. of segments)

4 I I I I I I 2 3 4 5 6 7 8

Body Length (mm) Fig. 4.—Comparison of number of first antenna segments to body length for males and females between Hyalella texana ( ) and H. azteca ) from Clear Creek Spring (b = regression coefficient ± 95% confi- dence interval; n = 30 for each sample) 432 THE AMERICAN MIDLAND NATURALIST 89(2)

Color differences can also be used to identify specimens of each species from Clear Creek. In contrast to the uniform orange color and darker orange middorsal stripe of H. texana, H. azteca is usually greyish and not uniformly colored. Also, orange pigment usually occurs as vertical bands along the sides of the middle tergites in the latter species while orange spots typically occur at the junction of the tergites with the coxal and pleural plates. We have not found specimens of H. azteca with a middorsal stripe.

28 -

26 - el Ix 1.624 (±2129) 24-

,/ b=1.352 (+ .1070) 22 - , 1 /0

20- / / ^ I 8 - /

I 6-

0 I 4 - 052 (+ 0504) (+ .0989) 9

6 -

4 -

I 2 3 4 5 6 7 8

Body Length (mm) Fig. 5,—Comparison of number of second antenna segments to body length for males and females between Hyalella texana ( ) and H. azteca ) from Clear Creek Spring (b = regression coefficient ± 95% confi- dence interval; n = 30 for each sample)

1973 STEVENSON AND PEDEN : HYALELLA =CANA 433

ECOLOGY AND DISTRIBUTION Clear Creek is a permanently flowing stream issuing from several large springs at the base of a limestone cliff and from several smaller seeps downstream. A small dam, located about 75 m from the head springs, produces an impoundment of about 100 m x 75 m with a maximum depth of 2 m (Fig. 7) . A second dam, about 1 km farther downstream, forms another impoundment about 100 m wide that con- tains slow-moving stream water with dense stands of submergent vegetation. Several hundred meters below this second dam there is a third dam, below which the stream narrows and flows rapidly to- ward the San Saba (see Hubbs, 1971, for a detailed description of these impoundments) . These man-made dams have affected the original stream environment and altered the distribution of aquatic vegetation. Most of the upper impoundment is occupied by a large submergent stand of a finely leafed form of Ceratophyllum sp. that is rooted to the detritus and limestone rubble of the bottom. This species of plant produces great masses of fronds which occupy most of the surface layers of the pond. We also found a few patches of Ludwigia sp. inshore, several floating mats of filamentous and scattered stands of water lilies (Nuphar sp.) in this upper impoundment. Near

2.2

2.0 b= 1.142 (+ .0158)

1.8

b= 0.833 (+ .0251) 1.6 b= 1.021 (+.025I)

1.4 b= 0.745 (+ .0162)

1.2 Fourth Coxal Plate ( L+W in mm)

1.0 I 1 1 I I 1 1 I 1 2 3 4 5 6 7 8

Body Length (mm) Fig. 6.—Comparison of the size (length and width) of the coxal element of the fourth appendage for males and females between Hyalella texana ( and H. azteca ( ) from Clear Creek Spring (b = regression coefficient ± 95% confidence interval; n = 30 for each sample) 434 THE AMERICAN MIDLAND NATURALIST 89(2) the overflow area below the upper dam, the aquatic environment appears to be similar with mixed stands of Myriophyllum sp. and Ceratophyllum sp., but a moss (Fissidens) also covers the bottom at depths below a half meter. Farther downstream from the overflow area occur either mixed stands or concentrated patches of other plants such as Potomogeton spp., Nuphar spp. and the stiffer fronded Ceratophyllum demersum. Much of the shallow inshore waters below the upper dam has a muddy substratum with sparse vegetation. The spring waters of the upper impoundment have little diurnal or seasonal fluctuation of temperature (21 ± 1.5 C). Similar relatively constant temperatures occur near the seeps and middle of the stream below the upper dam. In contrast, the waters of the shallow inshore areas downstream are subject to much greater fluctuation with recordings of 7 C to 22.5 C having been made between November and March and 21 C to 30 C between April and October (see Hubbs, 1971) . Hyalella texana was numerous and the only species of amphipod present in the upper impoundment region of relatively constant tem- perature, where the fronds of Ceratophyllum sp. and floating algal mats occurred. It was also found in the gravel and roots of indigenous

Fig. 7.—Proportion of Hyalella texana and H. azteca at different collecting sites in the Clear Creek area. Blackened portion of circles indicates H. texana and open portion of circles indicates H. azteca (• = spring outflow) 1973 STEVENSON AND PEDEN : HYALELLA TEXANA 435 vegetation (e.g., Nasturtium, Ludwigia) in the upper impoundment and seeps farther downstream. Below the upper dam, H. texana was abundant in the fronds of the fine-leafed Ceratophyllum sp., very numerous in the mats of Fissidens immediately below the overflow area, but only sparsely distributed in fronds of C. demersum and Myriophyllum sp. Hyalella azteca was not found above the upper dam or in the main springs or seeps below, although it was found with H. texana in the mixed vegetation near the influence of seeps below the upper dam and was the only species of amphipod in the shallow areas of sparse vegetation and fluctuating temperature. In such areas H. azteca was usually found in detritus and at water depths of less than a half meter. Although H. texana was found no farther than a few hundred meters downstream from the spring sources, H. azteca was found in detritus and shore-line habitats to approximately 5 km downstream. Our estimates of population densities for H. texana ranged from 1300 per m3 in the fine-leafed Ceratophyllum sp. to 9555 per m3 in the few algal mats above the dam and to 33,000 per m3 in the Fissidens mats immediately below the dam. At localities where only H. azteca occurred, the numbers of amphipods were generally under 1900 per m3 using standardized dip nets or Ekman dredge samples. DISCUSSION On the Edwards Plateau of Texas we have not found any other populations of amphipods possessing four tergite spines. However, a few populations which possess individuals with three spines and whose relationships to H. azteca and H. texana are unclear were found in other spring areas of the plateau. Because Clear Creek has other endemic organisms such as the mosquito fish Gambusia heterochir, excellent opportunity exists for coevolutionary studies on the several organisms that have apparently evolved here. For instance, recent studies by Hubbs (1971) indicate that the geographically restricted G. heterochir, which occurs mostly above the upper dam, feeds very substantially on H. texana, whereas the widely distributed G. affinis feeds only occasionally on this species. Because G. heterochir has a more powerful jaw structure, sturdier teeth and more blunt gill rakers, it would appear to be better equipped than G. affinis for eating amphipods. On the other hand, H. texana has the larger tergite spines and this, with possible avoidance be- haviors, might make it more difficult to be eaten. We believe future studies could demonstrate these morphological attributes to be the results of coevolutionary interactions between predator and prey species. Certainly from an evolutionary and ecological viewpoint, the unique biota of Clear Creek deserves further study. Acknowledgments.—We wish to thank Dr. Clark Hubbs for obtaining per- mission for our access to the Clear Creek area and Dr. E. L. Bousfield for his advice and encouragement in this study. Additional aid in making collections was given by Nancy Stevenson and Harry G. Chichester. Figures are by Mrs. Ginna Davidson. 436 THE AMERICAN MIDLAND NATURALIST 89(2)

REFERENCES BOUSFIELD, E. L. 1958. Fresh-water amphipod of glaciated North America. Can. Field-Natur., 72:55-113. Huass, C. 1971. Competition and isolation mechanisms in the Gambusia affinis X G. heterochir hybrid swarm. Texas Mem. Mus. Bull., 19:1-47. HUBRECHT, L. 1943. Studies on the Nearctic fresh-water Amphipoda. III. Notes on the fresh-water Amphipoda of eastern United States, with descriptions of ten new species. Amer. Midi. Natur., 29: 683-712. AND J. G. MACKIN. 1940. Description of nine new species of fresh-water amphipod crustaceans and new localities for other species. Ibid., 23: 187-218. SMITH, S. I. 1875. Report on the amphipod crustaceans of Colorado. Annu. Rep. U.S. Geol. Surv. Territories for 1873, 1875:608-611. WECKLE, A. 1907. The fresh-water Amphipoda of North America. Proc. U.S. Nat. Mus., 32:25-58.

SUBMITTED 18 MAY 1972 ACCEPTED 21 JULY 1972