The Hydrobiid Snails (Gastropoda: Rissoacea) of the Cuatro Ctenegas Basin: Systematic Relationships and Ecology of a Unique Fauna

THE HYDROBIID SNAILS (GASTROPODA: RISSOACEA) OF THE CUATRO CTENEGAS BASIN: SYSTEMATIC RELATIONSHIPS AND ECOLOGY OF A UNIQUE FAUNA

ROBERT HERSHLER Edwards Aquifer Research and Data Center Southwest Texas State University San Marcos. Texas 78666-4615

ABSTRACT

Results of the study of the morphology, systematics, and ecology of the Cuatro Cidnegas hydrobiids are given. Contrary to previous thought, no subfamilies of hydrobiids are endemic to the basin: all taxa studied belong to either the Nympho- philinae or Littoridininae, subfamilies widespread tlroughout North America. Of the nine genera (five endemic) and 13 species (nine endemic) found while sampling a large portion of the basin drainage, one genus and three species are new (but will be described elsewhere), and two species are new to the basin. The six nominal species of Mexipyrgus are reduced to one, M. churinceanus. The diverse fauna is paxtitioned among three habitat types: species with large, thickened shells inhabit large springs and their outflows; minute, blind, unpigmented species are restricted to smaller groundwater outlets; and a third set of species inhabits smaller stxeams of the basin, Within large springs, micro-habitat partitioning occurs as seParate species predominate either in soft sediment, on aquatic vegetation, or on travertine. At least one species reproduces year-round in the thermal waters of large springs. Phenetic and phyletic analyses show that three of five endemic genera closely resemble non-endemics found in the basin. The close similarity of endemics to non-endemics, lower level of endemism than once thought, lack of marked differentiation within the basin, and close proximity of the basin drainage to outside waters suggest a local and recent origin for the endemic taxa.

RESUMEN

Se reportan resultados del estudio de la morfologfa, sistem6tica, y ecoloSa de los hydrobioides de Cuatro Cidnegas. Contra lo que se habia pensado, ninquna subfamilia de hydrobioids es enddmica ala cu€nca: todos los taxa investigados pertenecen a Nymphophilinae 6 Littoridininae , subfamilias extendidas por todo Norte America. De los 9 g/eneros (5 end6- micos) y 13 especies (9 endCmicas) encontrados en muestras tomadas sobre una porci6n grande de la cuenca, 1 g6nero y 3 especies son nuevas, y 2 especies son nuevas para la cuenca. Las 6 especies nominales de Mexipyrgus estan reducidas a l, M. churinceanus. La fauna diversa estd repartido entre 3 tipos de Mbitats: las especies con conchas grandes y gmesas habitan manantiales grandes y sus desagu.es; las especies diminutas, ciegas, y sin pigmentaci6n estan restringidas a salidas pequefras de aguas sub- terr-aneas; y el tercer grupo de especies habita arroyos pequefros. Dentro de los manantiales grandes se ve la repartici6n de micro-Mbitat donde especies distintas predominan en sedimento suelto, en vegetaci6n acu6tica 6 en travertino. Por lo menos una especie se reproduce durante todo el afio en las aguas termales de los manantiales grandes. Et an-alisis fenotfpico y fildtico muestra que 3 de los 5 gCneros endCmicos son muy parecidos a los no end-emicos encontrados en la cuenca. La semejanza casi completa de los enddmicos a los no end6micos, el nivel mas bajo de endemismo que se habia pensado, la falta de diferenciaci6n dentro de la cuenca y la proximidad cercana del drenaje de la cuenca a aguas afuera indican un origen local y reciente de los taxa end6micos.

INTRODUCTION.-Of the various groups of organisms North and Central America. Apart from their high ende- found in Cuatro Cidnegas, the hydrobiid snails have been mism, the hydrobiid snails of Cuatro Ci-enegas are of inter- credited with the highest level of endemism. Taylor (1966a) est for the following reasons: 1) the shells of many of thc considered three subfamilies, five genera, and 12 species of species are relatively large, sculptured, or color-banded these tiny aquatic snails as endemic to the basin. Local (Fig. 1), whereas most hydrobiids have small, smooth endemism of freshwater snail faunas, involving a few shells without color; 2) the high local diversity of the species, is common in the arid American Southwest (Taylor hydrobiid fauna, with nine genera and 12 species found 1966b, Russell 1971, Pratt 1977), but the high level of within a 30 x 40 km valley, is unparalleled in North rnd endemism of the Cuatro Ci6negas snail fauna is unique in Central America and compares favorably with that of the

Henhler, R. f9E4. The Hydrobiid Snails (Gaotropoda: Ricsoacea) of the Cuatro Ci€negas Basin: Syrtematic Relrrtionrhipr and Ecology of a Unique Fauna. Journal of the Arizona-Nevada Acadcmy of Scicnce 19:61-76. 62 JOURNAL OF THE ARTZONA-NEVADA ACADEMY OF SCTENCE vol,. t9

Figurc 1. Photographs of shells of hydrobiid opecies found in the Cuatro Ci6negas Basin. The shells are not printed to the aamc enlargement. A) Nympbopbilus nincklqti (shell length, 8.0 mm); B)Nympbopbilusn. sp. (4.25 mm);C) "Stiobia"n, sp. (f .f 5 mm); D) Mexipyrgus cburinceanas (3.76 rnm); E) Durangonella coahuilae (3.60 mm); F) Mexitbauma quadripaludium (7.83 mm); G) Cocbliopina riograndensis (shell width, 3.00 mm); lJ'y C. milleri (shell width, 3.26 mm); ll Paludiscala caratltba (shell length, 2.40 mm);J) Coabailix n. sp. (shell width, 1.20 mm); K) C. bubbsi (shell width,0.87 mm); L) Orygo- ceras (?l sp. (rhell width, 2.26 mm). ISST,JE I, 1984 THE HYDROBtrD SNAILS OF TIIE CUATRO CIENECAS BASIN 63

large hydrobiid faunas of ancient lakes (Table 1); and 3) on samples from only twelve localities (Taylor 1966a): six differentiated populations of snails distributed among the allopatric species of. Mexipyrgus were described from a total insular springs of the valley offer an opportunity to study of seven localities. evolution in a "natural laboratory" (Taylor and Minckley A detailed presentation of the morphology of the r966). Cuatro Cidnegas hydrobiids, together with a systematic Early classifications of hydrobioids (i.e., those rissoa- revision of the fauna (including descriptions of new taxa), cean snails that rescmble Hydrobia in morphology of either is presented elsewhere (Hershler f983). The purposes of shell, penis, operculum, or radula) (Berry 1943; Morrison this paper are: 1) to summarize changes in the classification 1949), including that originally offered for the Cuatro of the snails and to discuss new taxa (and those new to the CiCnegas fauna (Taylor 1966a), were based on these charac- basin); 2) to present sampling data and to discuss patterns ters and a few other external morphological features (see of distribution of the hydrobiid species arnong the aquatic Table 2). It now is known that such characters have con- habitats of the basin and other aspects of their ecology; and verged in rissoacean taxa that are not closely rclated on the 3) to discuss the origin, evolution, and endemism of the basis of overall soft-part anatomy (Davis 1979). Such con- hydrobiid fauna. verg€nces zrxe ramp:urt among hydrobiid snails and study METHODS.-Snails were collected from 103 localities in of entire soft-part anatomy is necessary both to recognize the basin. One hundred localities were concentrated in an these convergences and to clarify systematic relationships arc surrounding the northern tip of the Sierra de San (Davis 1979). Endemic freshwater snail faunas frequently Marcos (fig. 2) and encompassed parts of four of the five- include taxa with thickened, sculptured, or otherwise- seven drainages of the basin (Minckley 1969; LaBounty peculiar strells (Annandale 1919, Vermeij and Covich 1978) 1974). Soft sediments, largely consisting of snail copropel, that represent adaptations to local ecological conditions. were sampled using a fine hand sieve. Bunches of vegeta- Classifications weighted in favor of such shell features tion (Clara, Utricularia, Nymphaea) were collected at ran- may inflate the uniqueness of such taxa. Some of the dom, washed, and the dislodged snails then picked from the endemic snails of Lake Tanganyika, for instance, have residue under a dissecting microscope. Snails were picked thickened, sculptured, "marine-like" shells and once were from individual travertine pieces and blocks using fine considered to be oceanic relicts and classified as members forceps. To collect the tiny snails that frequent ground- of modern or Jurassic marine families (see Pilsbry and water outlets, ordinary domestic (cotton) mops were placed Bequaert 1927; Yonge f938). Later anatomical studies into small springheads to serve as colonization substrates showed that they were modified members of widespread (method suggested by W. L. Minckley). Mops were periodi- freshwater cerithiacean families (Leloup 1953): the peculiar cally removed, washed, and the snails picked from the resi- shells masked a more conventional soft-part anatomy. due under a dissecting microscope, Dissection methods and The original description of the Cuatro CiCnegas hydrobiids by Taylor (1966a) stands as a fine contribution for Table 2. Diagnostic features listed by Taylor (1966a) for that time period and stimulated my own research. Yet in hydrobiid subfamilies found in Cuatro Gi'enegas. light of the above, the classification offered for the snails is suspect because it was based largely on shell and external Subfamily Diagnostic features features. Additionally, within the data set used, the numerous subfamilies described do not have distinctive sets of Cochliopinae Shell trochoid to planispiral. features (see Thompson 1968; Davis 1979). The descrip- Littoridininae elongate tions of species are suspect also because they were based Shell to turriform; penis with bulbous or elongate lobes. generic Table l. C,omparison of and cpecific diversities Nymphophilinae* Shell large, trochoid; opercu- among hydrobiid faunag of ancient lakes (from data com- lum multispiral; penis with pilcd by Bose 1978) and the Cuatro Ci6negas Basin (Hersh- elongate filament and blood lcr 1983). sinus; vas deferens in elevated ridge alongmantle cavity floor. Number Number Location of genera of species Mexithaumatinae* Shell with bristly periostra- cum; anterior end of pallial Lake Baikal, Soviet Union 5 40 oviduct reflected; mantle edge papillate; pig- Lake Tanganyika, East Africa 3 3 tentacles with ment strip along length. Lake Ohrid, Yugoslavia 5 39 Lake Titicaca, Peru-Bolivia '' 2L Paludiscalinae* Shell with lamelliform costae. Lake Inle, Burma t c *Considered endemic to the Cuatro Cfenegas Basin by Cuatro Ci6negas, Mexico 9 t2 Taylor (1966a). a* JOURNAL OF TIIE ARIZONA.NEVADA ACADEMY OF SCIENCE vol,.l9

100

5o zg q8 71 51 75, >77 62 /1 70 57, ;JJi I

9lt "V g2---/ ,, 4'-95

SIERRA DE SAN MARCOS to ----J N

o t 2 9lm

Ffrrc 2. f|1p of tbc portion of thc buil ttet wu intenrivcly nnplcd (from llcrrblcr f9t3). llnnbcrr (f-f00) rcicr to ol}ctlol bcdlda. DuLGd [ncr rcfcr to higrtioD curb. Arrowr bdicrtc urtcn tlrt coltbuc to flow towrtd tb crt. - ISSUE I,1984 THE IIYDROBIID SNAILS OF THE CUATRO CIENEGAS BASIN 65

other techniques of morphologic Btudy are described else- this duct: the pallial oviduct has no ciliated sperrn groove where (Hershler 1983). in the ventral channel. This penis of littoridinines lacks RESULTS.-Changes in classification. Recent anatomical glandular ridges, but may have other specialized gland studies have demonstrated that hydrobioid snails represent a types (Figs. 4C, D). Littoridinines also differ from the polyphyletic assemblage that includes taxa belonging to the Nymphophilinae-Hydrobiinae in that their cephalic ten- Hy&obiidae and Pomatiopsidae, families characterized by tacles often have complex, hypertrophied, surficial ciliary divergent anatomical groundplans (Davis 1979). Among tracts (Ci, Fig. 4A), while the tentacles of the latter lack the Hydrobiidae, two basic anatomical groups are seen these complex tracts. This subfamilial classification of the (Davis et al. 1982, Hershler f 983). One group includes the Hydrobiidae, while admittedly based on few characters, Hydrobiinae and Nymphophilinae (sensa Thompson 1979, does place emphasis on a major difference in overall ana- Davis et d. 1982), in which spenn are received into tfie tomical groundplan (i.e., ventral channel verzus spermathe- paltial svidqct of the female (Apo + Ppo, Fig. 3A) and cal duct). This classification is deemed tentative, not only travel along an internd ciliated channel (Vc) of the capsule because at least 90% of the North and Centrd American gtand (Apo) towards the sperm-receiving bursa copulatrix hydrobiid species are unknown in terms of anatomy, but (Bu) and geminal receptacle (Sr). Nymphophilines are also because recent work (Ponder 1983) suggests that characterized by a peniswithglandular ridges (Glr, Fig.48), character-states of the female reproductive system (such elevated regions with paired rows of small glands discharg- as the a.bove) may also have arisen iteratively in the evolu- ing through a centf,al slit; whereas species in the Hydro- tion ofvarious rigsoacean groups. biinae have a penis lacking specialized glands. In thc other Thompson (1979) demonstrated that Nymphophilus, group, the Uttoridininae (senw Davis et al. 1982), sperm endemic to Cuatro Ci6negas and previously considered the enter a spermathecal duct (Sd, Fig. 38) of the female, sole genus in this subfamily (Taylor 1966a), was similar in which is at least partly scparate from the pallial oviduct anatomy to hydrobiid snails from other parts of North (wholly scparate in some taxa, Fig. 3D), and travel along America and added several gcriera to this subfamily. My

Figurc 3. Schcmetic dnwinp of thc fcude rcproductivc morphologieo of lt) Hydrobia, Bl Spuntkkb, C,) Coabdlir, sil D) Llcrhbaurc (frou llcnhlcr f 9t3). Apo = ceprulc gland; Bp = brood pouch; Bu = buna copuletrir; Ov = oviduct; Ptlo = albumen glend; Sd B rpcrmrthccsl duct; Sdu * rpcrm duct; Sr = ccmind receptecle; eld Vc = ventrel chenncl. 66 JOURNAL OF THE ARIZONA.NEVADA ACADEMY OF SCIENCE vot,. 19

studies indicate that dl Guabo Ci6negas hydrobiids studied features. I gathered morphological data on specimens from can be placed in the Nymphophilinae or the Uttoridininae 33 populations and found that whilc patterns of geographi- (also widespread with numerous taxa): there are no sub- cal variation were evident, the purported differences in families endemic to the basin. My classification of the shell features - arnong nominal species often were blurred Cuatro C,fenegas hydrobiids is contrasted with the original in given populations, or even intergrade, and distinct species classification in Table 3. The purported distinctive fcatures thus could not be recognized (Herstrler f 983). of the three nominal subfamilies erectcd by Taylor (1966a), New taxa. A list of the hydrobiid specier found in the which were congidered endemic to the basin (Tablc 1), are basin is given in Table 4. The new taxa will be described nnconvincing. For example, the penis of. Nymphophilus elsewhere (Hershler 1983). Coahuilix n. sp. (Iig. lJ) is has glandular ridges as do thosc of all nymphophilines, and much larger than C. hubbsi, and differs in other shell all hydrobiids have the vas deferens in a ridge (of varying features as well when sympatric. Nytnphophilur n. sp. height) on the floor of the mantle cavity (Thompson 1968, (fig. 18) is much smaller in sizc than lV. minchleyi and 1979). The reflected pallial oviduct it Mexithauma is lacks the pronounced basal kcel of the shell seen in the found in many hydrobiide and is associated with a vivipar- latter. These two species are allopatric to one another: ous reproductive habit (Hershler f 983). Pigment streaks on Nymphophilus n. sp. is known only from the southeast tentacles also axe common arnong hydrobiid snails (Fretter lobe of the basin, whereas N. minckleyd is restricted to and Graham 1962, Hershler and Davis 1980). Lamelliform the northeast and western lobes. "Stiobia" n. ry. (Fig. costae on the shell, seen in Paludiscala, also are found in lC) represents a new genus of Nymphophilinac (alro unrelated taxa (e.g., Lanzah). found elsewhere in M6xico), similar to Stiobia (Thompson The six nominal species of Mexipyrgus axe reduced to and McCaleb 1978). one, M. churinceanus Taylor (fig. lD). Thc nomial species Two species new to the basin werr discovered during were described on the basis of minor differences in ghell my collecting. Cochliopina riograndeasr's (Fig. 1G), pre-

D D Ffrue 4. Head, pcnis, and pcnial glandr of hy&obioid snails. A) Dorsal aspect of the head, rhowing the hypertrophied citiary tuftr (Ci) on thc tcntaclc; B) Dorral arpcct of the penic, obowing glandula,r ridgcs (Gh); G; D) Penial gland types (apocrinc and mammiform glando, rerpectivcly); Ci = cilliary tufts, Glr = glandular ridges, pl6 = penal lobe, Vd = vas deferens.' rssuE l,1984 THE HYDROBIID SNAILS OF THE CUATRO CIENEGAS EASIN 67

viously known from the nearby Rfo Salado de Nadadores Table 4. Hydrobiid species found in the Cuatro Ci'enegas and other Rfo Grande tributaries (Taylor 1966a), was Basin. collected in Laguna Santa Tecla in tle southeastern lobe of tlre basin. Orygoceras(?) sp. (Fig. 1L), previously *Coahuilix hubbsi Tavlor 1966a known from a single spring in western Texas (Taylor *Coahuilix n. sp. 1974), was discovered living in a single small spring in the * P aludis cala c ararnb a Taylor 1 9 66a basin. A frestr strell of this species has been found since at a second locality in western Texas (Davis 1983). * M e xit h auma q u adrip alu diun Taylor 1 9 66a ECOLOGY.-As mentioned above, the hydrobiid fauna Cochiliopina riograndensis Pilsbry and Ferriss 1906 of Cuatro Ci6negas also is noteworthy for its high diversity. xCo milleri Taylor 1966a How are the 12 species distributed among the aquatic chiliopina habitats of the basin? Most aquatic habitats in the basin * Durangonella co ahuilae Taylor 1966a are spring-fed, and vary greatly in basic type (i.e., spring *M e xipyrgu s churinc e anus Taylor 19 66a pool, stream, matsh, lake), size, and temperature (Minckley 1969). For the purposes of describing the distribution of * Ny mp ho philus min ckley i Taylor 1 966a the various snail species, it is convenient to divide the *Nymphophilzs n. sp. springs into two classes: small springs, with spring pool "Stiobia" n, sp. areas of < 9 mz. whose outflows run as shallow streams for only tens of meters before terminating in a marsh or dis- Orygoceras(?) sp, appearing underground; and larger springs (Minckley 1969, fiir. f+, 15), with spring pool areas in excess of 900 m2, *Species endemic to the basin. whose outflows are large streams. Discussion is based on copropel or intensive collecting at Localities 1-100 (Fig. 2): species Utricularia); soft sediment composed of snail not found in this arca (Cochliopina riograndensis and' an algal-detritus mixture, often rnixed with small traver- Nyrnphophilus n. sp.) are discussed only briefly. tine pieces and shell debris; and large-sized travertine Large springe and their outflows. The large springs pieces (Fig. 5). Mexipyrgus churinceanus, Mexithauma com- have considerable micro-habitat diversity, including aquatic quadripaludiurn, and Nymphophilus minckleyi are vegetation of several types (sedges, Nymphaea, Chara, monly found in the large springs; while Durangonella coahuilae and Cochliopina milleri are much less common. Table 3. Subfamilial pliacement of the Cuatro Ci6negas This may be explained partly by the presence of mollus- hydrobiid genera (based on the results of this study) con- civorous cichlid fishes in the large springs (Deacon and trasted with that of Taylor (1966a). Minckley 1974, Fig. 7): the former three snails have large and thickened shells, possibly affording resistance to the Taylor (1966a) This study snail-crushing fish, whereas the latter two species are fragile-shelled. Family Hyrobiidae Family Hydrobiidae In all springs sampled, M. churinceanus was restricted Subfamily Cochiliopinae Subfamily Littoridininae to soft sediments (in which it burrows), sometimes achiev- Coahuilixl Coahuilixl ing extremely high densities ranging to 40,000/m2 (datz Cochliopina Palud,iscalal Subf amily Littoridininae Mexithaumal Mexipyrgusl CochlioPina Durangonella Durangonella Subfamily Nymphophilinae2 MexipyrgusL Nymphophitusl Subfamily Nymphophilinae Subfamily Mexithaumatinae2 Nymphophitus\ MexithaurnaL "Stiobia"S Subfamily Paludiscalinae2 Sobf"-ily Unknown Paludiscalal Orygoceras(?)4

lc.rro, endemic to the Cuatro Cidnegas Basin. 2s.,bf"*ily considered endemic to the Cuatro CiCnegas Basin by Taylor (1966a). Figure 5. Photograph showing substratc diversity in Laguna 3N"* g.rror. Mojarral East. Note the Nympbaea (water lily), soft copro- 4Systematic pel sediment, and travertine blocks. Vertical distance ir status uncertain. 4 m; water depth is 1 m. 68 JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCTENCE vol,.19

from box core sampling). Nymphophilus minckleyi and on dense growths of Chara in the deeper portions of the Mexithauma quadripaludiun usually arc found on hard spring, substrates (plant or travertine). The massive foot and The other two species occasionally found in large trochoid shell of these snails are presumably adaptations springs, Durangonella coahuilae and Cochliopina milleri, to a clinging mode of bf.e. Nymphophilus minckleyi may usually are restricted to soft sediment and aquatic vege- also require a hard substrate on which to deposit its egg tation, respectively. Cochliopina riograndensis eo,;rd, Nyrn- capsules (M. quadripaludiurn is viviparous). When N. phophilus n.sp. are on aquatic vegetation in large springs minckleyi and M. quadripaludiunz coexist in a spring that in the Southeastern lobe of the basin. has aquatic vegetation and travertine in abundance, each Prior to rnan-made alterations, most of the large predominates on the substrate to which it is best adapted springs gave rise to large streams that terminated in shallow (the former on vegetation, the latter on travertine). Data playa lakes (Minckley 1969). In general, abundances of on substrate utilization by the three species in Laguna all of tlre above species, except Durangonella coahuilae, Mojarral ^East, a large limnocrene (pool area of about decrease downstream from a spring source. Several environ- 9,000 mz), are given in Table 5. Only M. churinceanzs was mental changes occur downstream tlrat may account for common in soft sediments; N. rninckleyi wag common and these decreases. Aquatic vegetation may disappear or M, quadripaludium rare on aquatic vegetation; and M. become patchy and soft sediments frequently become quadripaludium was three timcs as common as N. minck- covered with an algal layer. Water temperature fluctua- leyi on under surfaces of travertine blocks (the three sub- tions may become marked. At Locality 30 (fig. 2), for strates are shown in Fig. 5). Thus, in a large spring with instance, water temperatures at the springhead varied from micro-habitat diversity, each of the three species predomi- 31.0-34.5oC, over a four-day period during the summer nates in a separate micro-habitat. In springs with reduced (maxi-mini thermometer readings), while 77 m down- micro-habitat diversity, either N. minckleyi or M. quadri- stream where water disappeared into a swallet, the tempera- paludium is rare or absent, and the species present often tures varied from 20.6-37.8oC during tlre same period. "switches" to the micro-habitat usually occupied by the Salts become more concentrated downstream as well other, suggesting at least a partly competitive basis for (Minckley and Cole 1968). In the Churince system (Local- patterns of substrate use. In Laguna Churince, for instance, ities 1-9, Fig. 2), M. churinceanus, M. quadripaludium, and N. minckleyi was extremely rare, travertine was largely N. minckleyd have not been found below the large pools absent, and M, quadripaludiurn was extremely abundant (Locality6), which differ greatly from the springhead in terms of substrate, water chemistry (Minckley and Cole Table 5. Relative abundances (%) of. snail species within 1968), and water temperature vari;ations. Durangonella micro-habitats in Laguna Mojarral East (sampling during coahuilae, on the other hand, can tolerate these environ- 4l2O-4l3Ol8Ol. mental changes and is abundant not only in these pools, but also in the playa lake, Laguna Grande (Locality 9). Micro-habitat Srrall springs and their outflows. Small springs also generally have warm water (29-33oC). Riparian vegetation Soft typically covers the springheads and sometimes part of the Species sedi Nymph- CharaS Traver- streams as well. Micro-habitat diversity is reduced relative mentl oro2 tine4 to tlre large springs. Chara mats and soft sediments composed of mixtures of silt, organic mud and travertine axe Mexiphyrgus the only abundant substrates. Of the five species found in churinceanus 0.99 0.03 large springs, only D, coahuilae also is common in the smaller springs. One group of blind, unpigmented species Mexithauma (Paludiscala caramba, Coahuilix hubbsi, Coahilix n. sp., quadripaludium 0.01 0.06 0.11 0.76 and Orygocerasl?l sp.) is restricted largely to heads of these small springs (and possibly subterranqan waters), Nymphophilus while a second group of species (D. coahuilae, "Stiobia" minckleyi 0.91 0.89 0.24 n. sp.) abounds not only in springheads, but also in the streams. Species. of the second group have dark body Total number pigment that may serve as a "sunscreen" (shells are trans- of snails parent, as are those of the first group), protecting the vital collected 3468 1219 25r 498 organs of the snails from the intense insolation of open stream reaches. lF.o* 56-10 minute collecting efforts. Data from the abundances of snail species in mop samples from 38 springheads are given in Table 6. At a 2Fro* 10 samples of plant bunches. given mop were 3Fro- springhead, several samples taken at vary- 5 samples of plant bunches. ing intervals during 1980 and 1981. Snails were picked 4Fro- 110 pieces averaging 12.5 x 30.0 cm. from each sample until a total of 100 individuals was found. Relative abundances varied littlc among samples rssuE l.1984 THE HYDROBIID SNAILS OF THE CUATRO CIENEGAS BASIN

Table 6. Abundances of snail species in mop samples from 38 small springheads (localities shown in Fig. 2). L = locality, N = number of eamples, Pal = Paladiscalo carambo, C.b. = Coabuilix bubbsi, C.n.sp. = Coahuilix n. sp., "S " = "Stiobia" n. sp., Dar = Durangonelh coabuilae, Nym = Nympbophilus minckleyi, Mex = Mexipyrgus cburinceanus.

C.h. C.n.sp. "S" Dur Ny* Mex Total

, 2L 10 11 L2 9_ I 7b lb L4 1- 2 I 13 16 15 2- t7 298 315 16 998 85 I 63 459 706 t7 5 118 1 2 23 t44 l8 46 1 I 16 25 23 295 3 98 24 4 43L 6 23 460 25 214 I 15 26 140 6 46 27 I 110 ll0 28 4 237 , 4L 48 329 31 316 2 1 98 2 119 3i 15 c 33 2 200 6 206 34 2 2LL 1 I 213 35 2 203 , 6 2ll 38a* 413 35 1 214 35 298 b* 2t I 6 8 40 1- I t 52 L2 131 10 L43 56 l1 I 18 20 DI 338 290 328 58 219 29 3 t37 188 59 229 29 60 2 186 I 4 4 195 61 I 100 , 44 147 62 2 199 135 334 63 8 444 3 447 64 4 450 40 3 1 494 65 6 633 9 3 6 t04 IJD 66 25L 8 59 67*'k l0 982 119 t5 100 1216 68 2lL 8 19 69 2 142 2 L44 72 24 4 , 11 2- t0l 27 128

Total ro7 5090 350 43 12r5 t354 18 80 71

*"a" and "b" from small springs joining the larger cool spring at Locality 38. **Orygoceras(?) sp. also found in subsequent samples from this spring. 70 JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE vol-. 19

Table 7. Analysis of abundance data from the 23 springs listed in Table 5 from which samples totalled more than 100 individuals. Species abbreviations as in Table 6.

Species

Pal c.h. C.n.sp. "s" Dur

Number of springs found in (of the 23) 2r 11 t2 16 t4

Number of springs in which species comprised:

a) more than 0.25 of the sample t4 6 4

b) more than 0.75 of the sample T2 3 2

of a given springhead, and data were pooled for each the snails disappeared only when the pool temporarily spring. Paludiscala caramba was by far the most common dried during drought. Durangonella coahuilae does not, species in springheads (Table 7). Of the other blind species, however, extend into the saline waters of the basin, both Coahuilix hubbsi and. Coahuilix n. sp. were less com- In summary (Table 8), hydrobiid species of the basin mon, and only a few specimens of Orygoceras(?) sp. were are divided into three groups with differing habitat usages: found in unsorted samples from a single spring. Proof is those dwelling in large springs and their outflows, blind lacking that the blind, unpigmented species inhabit sub- species restricted to groundwater outlets, and two species terranean waters as well; the mops may be colonized not common in the outflows of small springs (one of which only by snails washed from underground, but also by sur- also occupies marshes and playa lakes). Predominant face-dwelling snails living in the springhead (such as Duran- species in large springs usually occupy different micro- gonella coahuilae and n. sp.). To distinguish "Stiobia" Table 8. Summary of the deployment of hydrobioid between these two sets of snails, a fine mesh net could be species among the various habitats in the basin. "++" - capped over the groundwater outlet, filtering snails from commonr tt+t'= rare. the watering stream, but not allowing surface-dwellers to crawl in. This method has been successfully used to sample Habitat faunas of artesian wells (Holsinger and Longley 1980). Large Small springs While blind species were never found in thc open springs Marshes, reaches of stream, they occasionally extended below spring- and pools, heads when channels were covered by riparian vegetation Head Outflow Species outflows lakes (mimicking a subterranean environment). At Locality 63, a small thermal (33-35oC) spring issued into a pool (Brown Mexipyrgus 1974, Fig. 5), then flowed 170 m as a shallow stream churinceanus before entering a marsh. Paludiscala caramba was abundant on plant and travertine surfaces for the upper 83 m of Mexithauma stream, which had complete vegetative cover. Below 83 m, quadripaludium vegetative covered ended and no P. cararnba were found despite intensive collecting that yielded quantities of Nymphophilus Durangonella coahuilae artd "Stiobia" n. sp. At Locality rninckleyi 67, nine mop samples taken 3 m below the springhead, Cochliopina where riparian vegetation completely covered the stream, milleri yielded 67 P. caramba, 520 C. hubbsi, L0 Coahuilix n. sp., as well as quantities of the pigmented species. Durangonella The two pigmented species, D. coahuilae and "Stiobia" coahuilae ++ ++ n, sp., are abundant'in both soft sediments and,Charamats ++ of shallow streams. No clear-cut differences were seen in "9tiobia" n. sp. ++ their micro-habitat usage. Paludiscala speciee the basin, Other aquatic habitats. Of all of only cafamba Durangonella coahuilae is common in playa lakes, pools and marshes, habitats with sluggish water circulation and pro- Coahuilix nounced water temperature and chemistry fluctuations. For hubbsi + example, only D. coahuilae was collected from a series of + groundwater-fed pools that flucuated in size as the water Coahuilix n, sp, table rose and fell (Localities 7, 8). During 1981 water Orygoceras(?) sp. temperatures varied from 9.5-35,6oC in one of these pools; rssuE 1, 1984 THE HYDROBIID SNAILS OF THE CUATRO CIENEGAS BASIN

habitats. As the fauna is large, partitioning of habitats Reproductive ecology. Near-constancy of water tem- occurs as expected, with no more than three or four species peratures in large spring pools of the basin is well docu- common at any locality. The diversity of habitats occupied mented (Minckley 1969, Arnold 1972) and is reflected in by hydrobiids may be due partly to absence of other the apparent year-round reproduction of Nyrnphophilus groups of gill-breating snails (i.e., viviparids, pleurocarids). minckleyi in Laguna Mojarral East (Fig. 6). A large sample Adaptations of the species to their respective habitats and of Nymphaea (water lily) was collected from the south-west micro-habitats merit more studv. corner of this spring at 11 monthly intemals during 1980-

L2 2.1 3.6 4.8 6.O 7.2 8.4 4.8 6.0 7.2 1.2 2l 7.2 8.4

bq cI o u3 o t!L Shell Lensft (m-)

1981. The date of eollection and number of snails are given for each histogram. JOURNAL OF THE ARIZONA.NEVADA ACADEMY OF SCIENCE vol,. 19

1981 and washed to r€move snails. which were then mea- (exclusive of Orygoceras[?] ) all belong to either the Nym- sured. Water temperatures throughout the year ranged phophilinae or Littoridininae (Table 3). Of the endemic from 26 to 32oC. For all 1l samples, juveniles (shell genera, Nymphophilus belongs to the Nymphophilinae, length < 1.25 mm) comprised at least 48% of the total whereas all other ar€ Littoridininae. Nymphophilus and collection (Fig. 6), with a March peak of about 80%. "Stiobia," the other nymphophiline genus in the basin, Individuals with mature gonads (seen in dissection), and egg differ in at least 10 morphological features (Table 9), but capsules (on Nymphaea leaves) were common in all samples. many of those differences are probably correlates of their Note the paucity of intermediate-sized (2.0-6.0 mm) snails great size disparity (Nymphophilus = 3.5-8.4 mm shell in all samples (Fig. 6), which may be due to intensive length; "Stiobia" = 1.0-2.0 mm), and they may be closely predation by cichlids in this spring. Tiny juveniles perhaps related as tley share several unusual features, notably the are overlooked by the fish whereas large adults may have small number of glandular ridges on the penis. thick enough shells to resist crushing forces of the molari- A comparison was made among the six littoridinine form pharyngeal teeth of the cichlids. While comparable genera of the basin involving 36 characters: six (17%) from data are not available for other spe cies of the basin, embry- the operculum and shell; seven (19%) from non-reproduc- onic young were found in the brood sacs of all viviparous tive aspects of anatomy; eis.}rt (22%l from the male repro- species ( Durango nella c o ahuilae, M e xipyrgus c hurinc eanus, ductive anatomy; and 15 (42%) fuom the female reproduc- M e x i t h au m a q u ad rip alu d iu rn, C o c h lio p in a m ill e ri ) through- tive anatomy (see Hershler 1983 for a list of characters). out the year. Percentage difference among these taxa was determined by DISCUSSION.-Relationships among taxa. To confidently counting differences between pairs and dividing by total assess origins and relationships of a locally endemic fauna, a numbers of characters shared by the two taxa. A pheno- thorough knowledge is required of the systematics and dis- gram, based on simple averaging of differences between tribution of that group over a much larger area. Such data taxa, is given in Fig. 7A. Two pairs of very similar genera are available, for example, for freshwater fishes of Mdxico (<29% difference) are each comprised of an endemic and and adjacent regions, and thus the origins and affinities of non-endemic genus (Mexipyrgus, Durangonella; Mexi- various endemic fishes of Cuatro Cidnegas appear fairly thauma, Cochliopina). The other pair of genera (linking at clear (Miller 1978, Minckley 1978). Stirdy of the hydro- 49% difference) are both endemic to the basin. bioids of Mdxico and adjacent regions lags far behind that A hypothesis of phyletic relationships uunong Hydro- of fishes; while numerous species have been described, bla (Hydrobiinae), the six Cuatro Ci€negas littoridinines, neither anatomical information nor detailed distributional and Spurwinkia, the only other North American littori- data are available for any species. Thus, while relation- dinine known in terms of complete soft-part anatomy ships among various members of the Cuatro Cidnegas fauna (Davis et al. 1982), is shown in Fig. 78. Clades were can be clarified, their relationships to members of other defined on the basis of shared, unreversed, and presumably hydrobioid faunas remain unclear. derived character states (numbers !.-10, Fig. 8) from aspects As mentioned above, the Cuatro Ci6negas hydrobiids of soft-part anatomy (see Hershler 1983 for list). "A"

Table 9. Ten morphological differences between "Stiobia" anld Nympbophilus.

"Stiobia" Nymphophilus

Operculum with 3.5 whorls. 5.5-6.0 whorls.

Osphradium short. Osphradium elongate.

Central tooth of radula with pair of basal cusps. 3 pairs.

Male gonad overlaps stomach. Male gonad posterior to stomach.

Male gonad a single-lobed mass. Male gonad bushJike.

Penis witl single glandular ridge. 1-3 glandular ridges.

Penial lobe slender, with single fold. Penial lobe stout, with many folds.

Bolster and ventral channel poorly developed. Bolster and ventral channel well developed.

Bursa small (0.2f of pallial oviduct length), dorsal Bursa large (0.32 of pallial oviduct length), posterior to to pallial oviduct, with a short duct. pallial oviduct, with a long duct.

10. Opening of common genital aperture at end of Opening of common genital apertute lateral to pallial oviduct. pallial oviduct. ISSUE I,1984 THE HYDROBIID SNAILS OF THE CUATRO CIENECAS BASIN

Mexithauma

CoEh'tio'Fi-nE Durangone lla

Mexipyrgu

Paludiscala

Coahuilix

Hydrobia Spurwinkia Pal. Goah. Mexip. Mexit.

1-3

Figure 7. Phenogram (A) and cladogram (B) showing relationships among the littoridinine genera of Cuatro Ci6negas (from Herehler 19Sg). For the cladogram, Pal. = Paladiscala, Qoah. = Coabuilix, Mexip. = Mexipyrgus'Durangonella group, and Mcrit. = Mexitbauma-Cocbliopina group. Paludiscala and Coabuilix share character-states 5-7 (hence the circle enclosing thcn). 74 JOURNAL OF THE ARTZONA-NEVADA ACADEMY OF SCTENCE vol,. 19

represents a hypothetical, ancestral hydrobiid. The strong The basin drainage, while currently isolated, lies in concordance between the grouping of Cuatro Ci6negas lit- close proximity to the Rfo Grande (= Rfo Brauo del Norte) toridinine genera in the phenogram and cladogram sugg€sts drainage to the east and has likely had repeated connections that the groups constitute true clades. Distinguishing with these waters in the recent past (Minckley 1969, 1978). features of these three groups are given in Table 10. Note Taxa assignable to the Nymphophilinae and Littoridininae also that of the six genera, two (Coahuilix and, Paludiscala) on the basis of penis type (i,e., Fontelicella and Pyrgo- are egg-layers while the other four brood their young. phorus, respectively) are found in nearby waters of nor- Origin and endemism of the fauna. The idea that most thern M€xico and southwest Texas. of the endemic snails are ancient (tens of millions of years) The lack of marked differentiation of snail taxa within relicts unrelated to other extant taxa of the region is the basin may also suggest a young fauna; only three of the prevalent in the literature (Taylor 1966a; Minckley 1969; nine genera have a second species in the basin and in only Parodiz 1969), but was based on classifications of the snails one case (Coahuilix spp.) are congeners sympatric. It that inflated their seeming uniqueness. Unfortunately no should be noted, however, that even over a long period of fossil record, documenting timing of colonization(s) of and time speciation within the basin might be infrequent as diversification within the basin, is known. However, my the basin is small and its drainages have continually shifted results suggest a more local and recent origin for the ende- through time (reducing the probability of continuous mic snails. isolation of given springs). I have shown that endemism is on a smaller scale than The now-recognized smaller scale of endemism, simi- previously believed; no subfamilies are endemic to the larity of endemics to non-endemics within the basin, basin. Of the five genera considered endemic to the basin, proximity of the basin to outside drainage, and low level I suspect that two, Paludiscala artd, Coahuilix, are in fact of intra-basin differentiation point to a r€cent (late Ceno- not so restricted, but have not been recorded elsewhere zoic?) and local origin for the endemic snail fauna. Note because the techniques necessary to collect these minute tfiat the three genera that are definitely endemic to the snails from groundwater outlets generally are not used dur- basin (Nymphophilus, Mexipyrgus, and Mexithaurna) arc ing fieldwork. Snails with shells resembling those of the all inhabitants of large springs. Rapid iz sdtz evolution of above are known from the Edwards Aquifer of Texas these genera (from the non-endemics they resemble) could (Pilsbry and Ferriss 1906, Fullington 1978). Note that have occured as a result of intense cichlid predation within two of the three blind, unpigmented crustacean genera the large springs selecting for large, thickened shells, with originally described from and considered endemic to soft parts changing less so. Rapid evolution of freshwater groundwater outlets in tlle Cuafto Ci6negas Basin, Mexi- snail genera has been documented in several cases (Davis ueckelia and, Mexistenasellus, were discovered later in cave 1979, 1981; Stanley 1979). waters in southern M6xico (Magniez 1972, Argano 1973, ACKNOWLEDGMENTS.-My co-advisors, Drs. Steven M. Holsinger 1973) and elsewhere (Cole, present symposium). Stanley and George M. Davis, provided constant encour- Table 10. Distinguishing features of the three groups of Cuatro Ci6negas littorid.inines.

Group Features

Pa lud i s c a la- C o a hui lix Animd blind, unpigmented; oviduct coils, gonopericardial duct, seminal receptacle absent (Fig. 3C); sperm duct (Sdu) connects bursa (Bu) and oviduct (Ov) at point where the latter enters tlre albumen gland (Ppo, Fig. 3C); females oviparous; caecum of stomach greatly reduced; penis with apocrine gland (rig. ac).

M e xit hau m a-C o c h lio oina Shell with strong spiral sculpture (Figs. lF-H); tentacles with hypertrophied ciliary tracts (Fig. 4A): pallial oviduct with slight posterior bend (Fig. 3D); seminal receptacle (Sr) opens directly into oviduct (Ov, Fig. 3D); anterior end of brood pouch (Bp) strongly reflected and very muscularized (Fig. 3D); penis without specialized glands.

M e x ip y r gu s -D ur an g o n e lla Shell elongate-conic to turriform (Figs. lD, E)jpallialoviduct with posterior coil in several complex loops; albumen gland reduced to mere glandular smear; seminal receptacle (Sr) connects with oviduct (Ov) via a short sperm duct (Sdu, Fig. 3B); anterior end of brood pouch (Bp) weakly reflected and slightly muscularized (Fig. 3C); penis with ciliated tip. ISSUE I,1984 THE HYDROBIID SNAILS OF THE CUATRO CIENEGAS BASIN l5

agement during the tenure of this project. Dr. Davis sug- DEACON, J. E. and W. L. MINCKLEY. L974. Desert gested the project to me, shared his vast knowledge of fishes. Pp. 385-488. In.' Desert Biology, Vol' II hydrobioid snails with me, and provided the space and (G. W. Brown,Jr., Ed.), Academic Press, NY. facilities at the Academy of Natural Science of Philadel- FRETTER, V. and A. GRAHAM. 1962. British Prosobranch phia necessary for completion of the project. Drs. W. L. Molluscs. Ray Soc,, London, 755 pp. Minckley, G. Longley, I. Kornfield, and Mr. J. Landye FULLINGTON, R. W. 1978. The Recent and Fossil fresh- discussed aspects of the project with me. water gastropod fauna of Texas' Unpubl. Ph.D. Dis- I thank the Mexican govemment for providing the sert., N. Tex. State Univ., Denton. necessary permits for collecting freshwater molluscs in HERSHLER, R. 1983. The systematics and evolution of their country. Fieldwork in Cuatro Cidnegas was aided the hydrobioid snails (Gastropoda: Rissoacea) of the by Dr. C. Dunn, Mr. D. Bereza and numerous local towns- Cuatro Cienegas Basin, Coahuila, Mexico. Malacologia. people. In Press. Financial support for the project came from Sigma , and G. M. DAVIS. 1980' The morphology Xi. the National Shell Club, the Theodore Roosevelt of Hydrobia truncata (Gastropoda: Hydrobiidae): Memorial Fund, and the Geological Society of America. relevance to systematics of Hydrobia. Biol. Bull. A Jessup Scholarship partly supported my stay at t}te 1 58:195-2 19. Academy of Natural Science of Philadelphia. HOLSINGER, J. R. 1973. Two new species of the subter- Research for this paper was done in partial fulfill- ranean amphipod genrs Mexiweckelia (Gammaridae) ment for a Doctor of Philosophy degree at Johns Hopkins from Mexico and Texas, with notes on the origin and University, Baltimore, Maryland. I thank Drs. A. Metcalf' distribution of the genus. Pp. 1-12. 1n.' Studies on W. Ponder, and two anonymous reviewers for their com- the cavernicole fauna of Mexico and adjacent regions. ments on the manuscriPt. Assoc. Mexican Cave Stud. Bull.5. and G. LONGLEY. 1980. The subter' CITED LITERATURE ranean amphipod crustacean fauna of an artesian well ANNANDALE, N. 1919. The gastropod fauna of old in Texas. Smithsonian Contrib. Zool' No. 308' 62 pp. lake beds in upper Burma. Rec. Geol. Soc. India 50: LABOUNTY, J. F. L974. Materials for the revision of 209-240. cichlids from Northern Mexico and Southern Texas, ARGANO, R. 1973. Mexistenasellus magniezi n. sp.' a USA (Perciformes: Cichlidae). Unpubl. Ph.D. Dis- blind aquatic isopod from Veracruz, Mexico. Acca- serat., Ariz. State Univ., Tempe. demia Nazionale dei Lincei 171:97-103. LELOUP, E. 1953. Gasteropodes. Exploration Hydro- ARNOLD, E. A. 1972. Behavioral ecology of two pup- biologique du Lac Tanganyika (1946-1947). 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PILSBRY, H. A. andJ. BEQUAERT. 1927. The aquatic 1966b. Summary of North American mollusks of the Belgian Congo, with a geographical Blancan non-marine mollusks. Malacologia, 4 tl-17 l. and ecological account of Congo malacology. Bull. 1974. The tertiary gastropod Orygoceras Amer. Mus. Nat. Hist.53:69-202. found living. Archiv fur Molluskenkunde, 107:93-96. and J. H. FERRISS. 1906. Mollusca of the , and W. L. MINCKLEY. 1966. New world Southwestem states Il. Pp. 123-L75. .fn.. Proc. Acad. for biologists. Pac. Disc., 19: 18-22. Nat. Sci. of Phila. THOMPSON, F. G. 1968. The aquatic snails of the family PONDER, W. F. 1983. Review of the genera of the Hydrobiidae of peninsular Florida. Univ. Fla. Press, Barleeidae (Mollusca: Gastropoda: Rissoacea). Rec. Gainesville. 268 pp. Aust. Mus. In Press. 1979. The systematic relationships of the PRATT, W. L. 1977. Hydrobiid snails of the Moapa warm hydrobiid snail genus Nyrnphophilus Taylor 1966 and spring complex, Nevada. West. Soc. Malacol., Ann. the status of the subfamily Nymphophilinae. Malacol. Rep. 1977 10:7. Rev., 12:41-50. RUSSELL, R. H. 1971. Molluscaof Fish Springs, Juab -. , and J. E. McCALEB. 1978. A new fresh- County, Utah: rediscovery of. Stagnicola pilsbryi water snail from'a spring in Eastem Alabama. Amer. (Hemphill, 1890). Gr. Bas. Nat., 3l:223-236. Midl. Nat., 100:350-35 7. STANLEY, S. M. 1979. Macroevolution:' Pattern and VERMEIJ, G.J. and A. P. COVICH. 1978. Coevolution of Processes, W. fI. Freeman and Co., San Francisco. freshwater gastropods and their predators. Amer. Nat., 132 pp. 1 I 2: 83 3-843. TAYLOR, D. W. 1966a. A remarkable snail fauna from YONGE, C. M. f 938. The prosobranchs of Lake Tangan- Coahuila, Mexico. Veliger, 9 :152-228. yika. Nat., L42:464-466.