MALACOLOGIA, 1981, 21(1-2): 209-262 DIFFERENT MODES OF EVOLUTION AND ADAPTIVE RADIATION IN THE POMATIOPSIDAE (PROSOBRANCHIA: MESOGASTROPODA) George M. Davis ^ Academy of Natural Sciences, Nineteenth and the Parkway, Philadelphia, PA 19103, U.S.A. ABSTRACT Two subfamilies of the Pomatiopsidae are shown to have different tempos and modes of evolution. Data for the Triculinae are not new but represent a synthesis of several data sets (Davis, 1979, 1980; Davis & Greer, 1980). Data for the Pomatiopsinae with emphasis on the Tomichia radiation of South Afhca are new. The distribution of modern pomatiopsid taxa is vicariant, a relict distribution with a secondary elaboration in Southeast Asia and the Far East extending to North America. There are eight pomatiopsine genera, one each in South Africa, South America, and Australia; one genus is found in an arc from western China to the Philippines and Sulawesi with taxa reaching Japan; two are endemic in Japan; one is found in fHanchuria, Japan, and western U.S.A.; one is endemic in the U.S.A. There are 16 triculine genera, all but one of which are located entirely in Southeast Asia or western China. Tricula extends in an arc from India through China to the Philippines and in an arc through Burma to Malaysia. The Triculinae have undergone an extraordinary endemic radiation in the Mekong River, yielding three tribes, 1 1 genera and over 90 species in a period of about 12 million years. This burst of cladogenesis was apparently driven by extrinsic processes correlated with the massive tectonics caused by the Himalayan orogeny that led to the formation of the major river systems of Southeast Asia, and western China. The morphological changes in the entirely aquatic group of snails that marked the entrance into various new adaptive zones involved a series of innova- tions in the female reproductive system, the male reproductive system posterior to the penis, and the central tooth of the radula. Bursts of speciation following each morphological innovation or series of correlated innovations yielded clusters of species that are considered discrete genera. The genera are separated by distinct gaps defined by morphological distances that are meas- ures of morphological changes indicative of entrances into new adaptive zones. Pomatiopsine taxa are aquatic, amphibious, or terrestrial. Modes of evolution in the Pomatiopsinae of the southern continents are in marked contrast to those in the Triculinae. In South Africa there are, at most, eight species of Tomichia with an evolutionary history of at least 80 million years. In Australia there are, at most, nine species of Coxiella. Tomictiia and Coxiella are very similar anatomically. No burst of cladogenesis or considerable speciation is seen. Species of Tomictiia do not differ very much in anatomy. The apparent low rate of speciation and lack of cladogenesis correlate with the lack of tectonic upheaval and gradual climatic changes since proto-Tomichia and proto-Cox/e//a were separated by the breakup of Gondwanaland. The limited Tomictiia radiation is apparently in response to increasing aridity spreading from west to east in South Africa since the breakup of Gondwanaland. Speciation has not involved morpho- logical modification but rather, adaptation to different ecological settings: freshwater streams, freshwater lakes, amphibious ecotones, temporary brackish water pools. Preadapted morpho- logical features for an amphibious existence were probably the large, powerful foot and the elongate spermathecal duct. The tempo of the Mekong River triculine evolution is rapid (R = about 0.40 contrasted with a slower rate (R = about 0.139) for the Tomichia radiation. The mode of triculine evolution is rapid, episodic speciation involving considerable morphological innovation and cladogenesis, all associated with extreme tectonism. The mode of Tomichia evolution involves a physiological radiation with low morphological diversity associated with gradual climatic change and general absence of tectonism. INTRODUCTION In considering tempos I am concerned with rates of cladogenesis, the number and extent Modes and tempos of evolution above the of adaptive radiations in phyletically allied species level are highly relevant topics for clades (per unit time), and the rate of extinc- contemporary students of biological evolution, tion of species and lineages. By extent of ""Supported by U.S. National Institutes of Health grant No. AM 1373. (209) 210 DAVIS adaptive radiation, I mean the number of spe- cies of a single radiation and the different niche dimensions these species occupy. In considering modes of evolution, I am concerned with how organisms respond to the selective pressures of different types of changing environments, and with how organ- isms respond to different rates of environ- mental cfiange. The presumption Is made that speciation and evolution above the species level will not occur in environmental stasis. The purpose of this paper is to demonstrate two vastly different modes and tempos of evo- lution in the rissoacean family Pomatiopsidae. One mode involves a radiation of consider- able morphological uniformity but physiologi- cal divergence in a setting of gradual environ- mental change. The other mode involves a radiation exhibiting numerous morphological innovations associated with rapid tectonic environmental changes. The most important comparisons made here involve the extraor- dinary triculine radiation in the Mekong River and the more modest Tomichia radiation in South Africa. Data pertinent for discussing the triculine radiation have been published (Davis, 1979, 1980; Davis & Greer, 1980). Data for the Tomichia radiation are new. Two different clades are involved, because the Mekong River radiation belongs to the Triculinae and Tomicliia is a member of the Pomatiopsinae. Together these two subfami- lies comprise the Pomatiopsidae as recently defined (Davis, 1979). The family Pomatiopsidae The origin and evolution of the family have been discussed with emphasis on the adap- tive radiation of the Triculinae in the Mekong River (Davis, 1979). The evolutionary topol- ogy of the family is shown in Fig. 1 based on the hypothesis that the Pomatiopsidae evolved and diverged into two Gondawanian subfamilies prior to the breakup of Pangaea. Published zoogeographical, morphological, and paleontological data (Davis, 1979) are consistent with the following concepts: 1 ) the distribution of modern pomatiopsid taxa is vicariant. There is a relictual distribution in the southern continents with a secondary elabo- ration in the Far East extending to North America (Table 1). 2) Triculinae and Pomatiopsinae were introduced into the Asian mainland via the Indian Plate. 3) The patterns of disthbution of Pomatiopsidae throughout Asia and North America and the direction of evolution of derived morphological character states indicate a direction of evolution from Gondawanaland to Asia (Davis, 1979). The subfamily Triculinae The subtending of the Asian continent by India initiated the Himalayan orogeny begin- ning in the Oligocène some 38 million years ago (Molnar & Tapponier, 1975). The orogeny began at the western end of the mountain chain and spread eastward as the Indian Plate rotated, bringing the northeast corner into contact with the Asian mainland in the Miocene. As the Tibetan region was lifted from the sea, drainage patterns were initiated that became the major rivers of Southeast Asia and much of China. These are the Irrawaddy, Salween, Mekong, and Yangtze rivers. Estuahne and finally fluviatile deposits were laid down in northern Burma at the end of the Miocene; in the Pliocene the sediments of the Irrawaddy River became entirely fresh- water (Pascoe, 1950). It is apparent that proto-Triculinae were in- troduced from the Indian Plate into the newly forming drainages of the Asian mainland (Davis, 1979, 1980; Davis & Greer, 1980). All Triculinae thus far studied are entirely fresh- water in streams, lakes, and rivers. They ex- tend in three arcs. One arc extends from north- western India through China to the Philippines. The second arc extends from India through northern Burma and western Yunnan, China and throughout the Mekong River drainage but ending in northern Cambodia. The third arc extends through northern Burma, northwest- ern Thailand into Malaysia. Tricula, the genus with the most general- ized morphology and least derived character states (Davis & Greer, 1980) is found along each of these arcs. Taxa with the most de- rived character states are found endemic in the Mekong and Yangtze River drainages and in lakes in Yunnan, China between the rivers (Davis, 1980; Davis & Greer, 1980). These derived taxa are IHalewisia and Pachydrobia of the Triculini and all members of the Lacunopsini and Jullieniini. As shown in Table 1 , of 1 6 genera and 1 20 species of Triculinae, 10 genera and 92 species (76.7%) are en- demic to the Mekong River drainage. POMATIOPSID EVOLUTION 211 PRESENT MIOCENE 120 CRETACEOUS 140 FIG. 1 . Phyletic topology of the Pomatiopsidae with time given in millions of years (on a log scale) from the Jurassic to the present. Branching points; 1. Triculine and pomatiopsine lineages established in Gond- wanaland pnor to the breakup of the southern continent. 2. Divergence to form the Jullieniini (left grouping) in the Miocene. 3. Radiation of specialized Lacunopsis (Lacunopsini), which diverges from the Triculini. Lacunopsis, on shell characters, resembles marine and freshwater Neritidae. Some species converge on Anculosa (Pleuroceridae), Littorina (Littorinidae), or Calyptraea (Calyptraeidae). 4. Seven genera evolved in the Miocene, probably much at the same time. Pachydrobiella (PA) converges on Pachydrobia (PAC) of the Triculini in shell shape and structure. 5. Anatomical and shell data clearly indicate that Hydrorissoia (HY) and Jullienia (JU) diverged from a common ancestor. 6. A late Miocene radiation took place in Japan, giving rise to the endemic genera Blanfordia (B) and Fukuia (F), and Cecina (С). Cecina spread to western North America, while Pomatiopsis (P) occurs only in the U.S.A. 7. Blanfordia and Fukuia have either diverged from a common ancestor or are the same genus. Data thus far available support the former interpretation. A. Aquidauania, South America. B. Blanfordia, Japan. С Cecina, Japan, Manchuria, U.S.A. CO. Coxiella, Australia. F. Fukuia, Japan. H. Halewisia, Mekong River. HU. Hubendickia, Mekong River. HY. Hydroris- soia, Mekong River. JU. Jullienia, Mekong River. KA. Karelainia, Mekong River. L. Lacunopsis, Mekong River. O. Oncomelania, China, Japan, Philippines, Sulawesi. P. Pomatiopsis, U.S.A. PA. Pactiydrobiella, Mekong River. РАС. Pachydrobia, Mekong River, PAR. Paraprosothenia, China. Mekong River (Thailand, Lao). S. Saduniella, Mekong River. T. Tomicfiia, South Africa. TR. Tricula, India, Burma, China, Philippines, Mekong River (from Davis, 1979). 212 DAVIS >« со о. со р) S'a ф 5 О) CD N ТЗ I- + ф со с 3 о ф со с ел Ol О Е о CL ел ■^ О ^ со ■■ _1 Я -ТЭ ■D О 5 -О ■? Е 5(3 со ел о со li coo ш w со' с со ^ га га га -а "5 С га ел < c\j т- c\j см ел ^ О •^ ■? Е ^(3 ю 3 ел ral3 ^? Е-? (3^ ел 3 га •• _1 га '■о тз о -? Е га се ел О га _1 га _| га 1- - - ф > СП с о ф со .~~. со т- О) •>- .-^^-^ о -^ со со + ез) 05 ел > га -О •■5 2 о й Ф еч 5 ï Ф CJ ел Ll оа ф ел ел О О со I -С о со О ехз Ф Ф « О тз с га ел ■> га со со со со со ф >. га ._ ел С Ф ёя О _ ел ^ S- о о га с _| э is ел Ф 5 t о с га" га о *^ ф Е < ел с га га га 'с га Е ел га х: Q. С га i га 3 о со со ■D С С 52 га га -^ CL Q.^ га га 13 га га ел < с га а. га ел ел ф га га. У - %^< о..ь 3 tí 3 га -"^ со с: о т О Э со 1- см •>- о ^%^ •^ см см ■^ г^ со ^ со СП со ^ ел« !2 i - > га ел га5 Е Û < га ■ "О .га < < ^^< СО ^ СО 3 S О СЗ--2 Ф "Ч: QQO О) т}- 05 00 сз) '^ со аз ^'т- "^ в ^ со oö Q ni л io ■ n 1 ^ ra< = CD о Э OU. .ra 5 CD Ф E о Ü с: О см со ..- со ю '- со с . о с ь^ il CD .о о о 00 со ■о с га ш с .2 Ф э to сз> 00 га > га ш со о со со •о СО 00 ф о ел об Ф ел ел О О .га с ф "3 О) CJ) ел ■> га Û CD .С CD 5 СО 00 см '- СП О) Ш Ф ф ф .ел со •§.2 ¿о «5 га J о. СП 5 ф га тз с га с с < о 05 ТЗ С га ±: CÛ со о га со ^ о Q. г CD О. ф с TD га со ф и Е ф тз с ф см Э. ел Ф о Ф О. ел О СМ га ф с ф О) со О) ф и ф а. ел СП см га Ф С Ф О) со < t- О (Л 0} (J ф CL (Л i3 со (Я ео .ео й с: .с POMATIOPSID EVOLUTION 213 The tribes and genera of the Triculinae are separated by discrete qualitative morphologi- cal gaps (Davis, 1979, 1980; Davis & Greer, 1980). Sonne 28 characters are of use in rec- ognizing these taxa because the taxa have shared derived states of these characters and/or uniqueness of certain derived states (Table 2). Of these characters, 14 are frorл the female reproductive system (50%), seven are from the male reproductive system (25%) (only one is from the penis), four are shell characters (14%), two are radular characters (7%), and one is osphradial (4%). The Triculinae provide an excellent oppor- tunity for studying how higher taxa evolve. The monophyletic assemblage (Davis, 1979) is large enough to explore how species of vari- ous adaptive zones have radiated, and to un- TABLE 2. A list of 28 characters that are used to recognize tribes and genera of the Triculinae. References to illustrations or discussions of character-states are given; these are one or more of Davis, 1979, 1980 (= 1980a below); Davis & Greer, 1980 (= 1980b below); Davis et al., 1976. Shell 1. shape 2. sculpture 3. size 4. thickness Central tooth 5. anterior cusp morphology 6. size of blade supports Osphradium 7. length Female reproductive system 8. gonad morphology 9. coiling of the oviduct posterior to the bursa copulathx 10. position of the opening of the seminal receptacle 11. length of seminal receptacle 12. oviduct configuration at the bursa copulatric region 13. length of the bursa copulathx relative to length of palliai oviduct 14. length of duct of the bursa copulathx 15. position of the palliai oviduct relative to the columellar muscle. 16. Coiling of the spermathecal duct 17. encapsulation of the spermathecal duct 18. vestibule of the spermathecal duct 19. extension of the spermathecal duct into the mantle cavity (= sperm uptake organ) 20. position of opening of the spermathecal duct into the bursa copulatrix complex of organs 21 . method by which sperm enter female repro- ductive system at the postenor end of the mantle cavity Male reproductive system 22. gonad morphology 23. position of coiling of the seminal vesicle 24. relative length of the vas deferens (Vd^) be- tween the gonad and seminal vesicle 25. coiling of the vas deferens posterior to the penis 26. position where vas deferens leaves the prostate 27. penis has stylet or papilla 28. status of vas efferens 1979, figs. 28-30; 1980a, fig. 7 1979, figs. 28-30; Table 12; 1980a, fig. 7 1979, figs. 28-30; Table 1 1 ; 1980a, fig. 7; Table 6 1979, figs. 28-30; 1980a, fig. 7 1979, fig. 4; 1980a, fig. 6 1979, fig. 4; 1980a, fig. 6 1976, fig. 7 1979, figs. 11-15; 1980a, fig. 11 1980a, figs. 4, 8, 13 1979, figs. 3, 11-18; 1980b, fig. 10 1979, fig. 12 1979, fig. 3 1979, figs. 12, 13 1979, figs. 11-16; 1980a, fig. 13 1979: 107 1979, fig. 12 1980b, fig. 7 1980b, fig. 7 1979, fig. 14C; 1980b, fig. 10 1979, fig. 3; 1980a, figs. 8, 13; 1980b, fig. 10 1979, fig. 3; 1980a, figs. 5, 8; 1980b, fig. 10 1979, fig. 19; 1980a, fig. 11; 1980b, fig. 9 1979, figs. 11-15; 1980a. fig. 12 1980a, fig. 12 1979, fig. 12A 1979, figs. 14, 15 1976, fig. 10; 1979, fig. 10; 1980b, fig. 9 1979, figs. 11-15; 1980a, fig. 11; 1980b, fig. 9 214 DAVIS derstand the directions of morphological change that permitted the crossing of thresh- olds of various adaptive zones to new adap- tive zones. In the Triculinae, as in other higher taxa, we see four aspects of adaptive radiation: first order adaptive radiations, null radiation, sec- ond order adaptive radiations, and macro- adaptive radiation. The term adaptive radiation was first used by Osborn (1918) and fully exploited by Simp- son (1949) who stated: "Adaptive radiation is, descriptively, this often extreme diversification of a group [e.g. mammalian or reptilian radia- tion] as it evolves in all the different directions permitted by its own potentialities and by the environments it encounters." Stanley (1979) stated: "Adaptive radiation is the rapid pro- liferation of new taxa from a single ancestral group." These authors are discussing what I call here macro-adaptive radiation, a higher taxon or a higher taxon clade that is, in fact, recognized as such because of its component clades. The Triculinae are a macro-adaptive radiation. A first order radiation is equated with a genus, which is a composite of at least two, but usually more than two species. The en- trance into a new adaptive zone made possi- ble by a new morphological or physiological innovation is associated with the rapid prolif- eration of new species that fill various niche dimensions. A null radiation is a monotypic genus, a taxon recognized by the discrete morphological gap from other genera to which it is phyletically allied. Such a genus may be the basis for a first order radiation of the fu- ture, or represent a dead-end due to the very nature of the morphological innovation(s) that distinguishes it. Planispiral Saduniella of the Triculinae is such a genus. A second order radiation involves two or more phyletically allied first order radiations and can be equated to named taxa between generic and high taxon clades under discussion. Within the Triculinae, the tribes Triculini and Julli- eniini are second order radiations. Detailed discussions of the evolution of de- rived character-states and taxa with those states have been given (Davis, 1979, 1980; Davis & Greer, 1980). In review, the most pro- found changes involved the reproductive sys- tems as the progenitors of the modern Tricu- linae adapted to the evolving Mekong and Yangtze River systems. Changes were es- sentially in two directions involving two clades, the Lacunopsini and Jullieniini. These changes show divergence from Tricula, which -*- OVATE -;.^ CALYPTRAEA RADIANS FIG. 2. Shells of representative species of the Lacunopsini showing diversity in shell shape and showing a closer relationship of the Lacunopsini to the Triculini than to the Jullieniini (also see Fig. 1). The marine mesogastropod Calyptraea radians is illustrated to show how similar the species is to L fischerpietti. These two species are highly convergent in shape, growth patterns, and sculpture (from Davis, 1979). POMATIOPSID EVOLUTION 215 has the most generalized character-states. Many of these derived innovations are corre- lated with swift-water habitats as has been shown statistically (Davis, 1979). There is a lack of species with generalized character- states adapted to swift-water habitats. The Lacunopsini (Fig. 2) most likely evolved from an ancestor that also gave rise to Tricula bollingi (Davis & Greer, 1980). A single first order radiation is involved, all in the Mekong River. The niche dimensions filled are swift- water habitats on rocks where species differ- ences are seen in shell shape and sculpture, and positional relationships in the water col- umn involving rock slope, depth, rock surface, degree of current. Shell shapes are astonish- ing for freshwater hydrobioids as shapes con- verge on those of marine Neritidae, Littohni- dae, and Fossaridae. The most remarkable changes in the reproductive system are the loss of the seminal receptacle as seen in Tricula and the development of several ac- cessory seminal receptacles, and the degree to which the pericardium is modified and used to accommodate sperm during reproduction. All species are similar in that the central tooth is a derived type (Fig. 5) modified for scraping food from rock. The Jullieniini (Fig. 4) comprise one of the most spectacular second order molluscan radiations ever seen in freshwater. This radia- tion in the Mekong River has five first order radiations and two null radiations. We know too little about the Chinese genera Litho- P SPINOSÄ H EXPANSA HALEWISIA P BAVAYI PACHYDROBIA LACUNOPSINI JULLIENIINI TRICULINI FIG. 3. Shells of representative species of the three genera of the Triculini. The implication of this tree-like configuration is that Pachydrobia has more derived character states than does Tricula, reflected in certain shell features, e.g. ribs, tjosses (odd lump[s] on the shell), and solitary spines. Also implied is the basal status of Tricula relative to the divergent tribes Lacunopsini and Jullieniini, which have more derived char- acter states (also see Fig. 1). Note also the increase in size (only L. aperta is drawn at a larger scale, as indicated by the 5 mm scale bar) in P. variabilis, P. fischeriana, etc., compared with Halewisia and Tricula (from Davis, 1979). 216 DAVIS glyphopsis, Delavaya, Fenouilia, and Para- pyrgula (Table 1 ) to say anything about them. Incremental derived changes in the female reproductive system are in the direction of in- creasing volume and complexity of the repro- ductive organs (Davis, 1979, 1980). The generalized hydrobioid oviduct is thrown into a 360° complex with the seminal receptacle and spermathecal duct (Fig. 6). This 360° loop is small in diameter in the least derived genus (Karelainia) and increases markedly in di- ameter in the more derived genera. The gonad is the generalized pomatiopsid type in Karelainia and is considerably modified in morphology in the more derived genera. Elongation of the seminal receptacle is seen in only a few species of Hubendickia while extreme elongation is seen in more derived genera such as Paraprososthenia, Jullienia, Hydrorissoia, and Pachydrobiella. Extreme elongation and recurving or coiling of various sections of the vas deferens are seen in the more derived genera and especially pro- nounced in the most derived genera, Jullienia and Hydrorissoia. Increasing complexity in the reproductive system is associated with exploitation of dif- fering (even if slight) reproductive strategies. Increasing bulk and complexity of the repro- ductive system are associated with the Mekong River thculine fauna (Davis, 1979). These species are colonizers and opportunis- tic species in a river that goes through an an- nual cycle of rampaging floods during the monsoon season (June through November) to relative quiet and shallow flow during the dry season (December through May). The floods bring high density-independent mor- tality because of the distribution of habitats and the sweeping away of snails from low- water depositional areas. There are high re- productive rates in the single short low-water breeding season available to these annual species. The relative volume of reproductive organs discussed above coupled with the tremendous biomass of young produced (see Davis, 1979) attest to comparatively great amount of energy put toward reproduction (contrast Pomatiopsinae, Davis, 1979: 69). Growth and reproductive activities of Mekong River species are remarkably in phase with the annual hver cycles. Different groups of species mature, reproduce, and die at different times once the dry season begins and water levels begin to drop. All Triculinae are semelparous as far as is known. Once Pachydrobia reproduces, the reproductive system slowly disintegrates. This is first seen in the male where the penis begins to disinte- grate; it is later seen in the female where the ovary and palliai oviduct disintegrate. The snails live on for a month or more after the onset of this disintegration process. Once Tricula aperta has laid its eggs, it dies and there is a period of about one month when no adults are seen and no hatched young can be found. Additionally, there is a temporal division of river habitat as regards maturation and repro- duction. A given habitat may have one group of species at one period of low water that re- produce and die, to be replaced by different species that hatch, grow to maturity, etc. (Davis, 1979). The temporal division keeps pace with the annual cycle of habitat emer- gence. As water levels begin to decrease in October, habitats begin to emerge and form. First island masses and the larger waterfalls appear, followed by smaller islands, embay- ments between islands, lakes and pools on islands, smaller rapids, sandbars, and finally shallow quiet areas allowing for considerable mud deposition. From mid-October or No- vember through June most habitats are free FIG. 4. Shells of representative species of the seven genera of the Juilieniini grouped to reflect relationships and a radiation of shell types within each genus. The trend from bottom to top is one of generalized to specialized both in shell features and anatomy. Spiral and nodulate sculpture is derived. Jullienia is most specialized in terms of sculptural patterns, large size, and odd shapes (e.g. flattening of the base of the shell in some species) as well as anatomy. In Hubendickia, the shells, depending on the species, are smooth or ribbed. Nodes are seen on the adapical ends of the ribs in two species. In Paraprososthenia, shells range from smoothly ribbed, with solid spiral cords, or with spiral rows of nodes. P. hanseni has moфhs ranging from smooth, one spiral row of nodes to several spiral rows of nodes on the body whorl. Hydrorissoia and Jullienia are, on the basis of anatomy, phenetically very similar. Together with Paraprososthenia they form the Jullienia complex. Karelainia parallels Paraprososthenia in shape and sculpture but diverges consider- ably in anatomy. Note that K. davisi has several morphs. Fossarus foveatus is shown as an example of convergence between unrelated taxa. F. foveatus is similar to species of Jullienia in shell shape and sculpture. F. foveatus is in the marine family Fossaridae. All shells are drawn to the same scale except the six Jullienia with the 5 mm scale bar (from Davis, 1979). POMATIOPSID EVOLUTION 217 TRICULINI 218 DAVIS from flooding and destruction caused by tfie monsoons. Because of the floods, the con- figurations of sandbars, islands, and rapids change yearly. A population that flourished in a muddy depositional area one year may be buried under stones and cobbles the next year. Species with the most derived reproduc- tive systems appear to grow and mature rap- idly and to reproduce during lowest water. Taxa with the most generalized systems re- produce during higher water periods before and after the four-month lowest-water months (Davis, 1979). The foregoing discussion has involved 75% of the derived characters. Different feeding habits involve yet another niche dimension especially exploited in the second order Jullieniini radiation. This is reflected by the morphology of the central tooth of the radula (Fig. 5). The generalized central tooth seen in the Triculini and all Pomatiopsinae is found only in a few species of Hubendickia of the Jullieniini. Species of all other genera have derived types of teeth. Finally, shell char- acters reflect adaptations to different micro- habitats and perhaps to living in sympatry with different species (Figs. 2-4). Only two or three species of Hubendickia have the smooth, ovate-conic, small shell that is the generalized hydrobioid type (Davis, 1980). Modification of shell characters from generalized to most derived follows a parallel course in each of the two second order and Lacunopsini first order adaptive radiations of the Triculinae. There is a net increase in size, and there appears to be a progression from smooth to ribbed, nodu- late ribs, reticulate sculpture, spiral noded cords, and finally odd spines and nodes. There is another progression from ovate- conic to diverse symmetric shapes including planispiral, and finally to asymmetry. In the Jullieniini the trends in increasing complexity of the reproductive systems generally parallel the three trends in shell characters and the trends in central tooth morphology. It is in Hubendickia that we have an indica- tion that certain sculptural character-states are related to species living in sympatry. We see a possible case of character displace- ment. At Khemarat, Thailand five species of Hubendickia live sympatrically. It is common to find four species in great numbers (hun- dreds) in a handful of algae. Each of these species has a distinctive shell sculpture in- volving ribs. One of these species was called H. spiralis Brandt because of pronounced spiral micro-sculpture. These species crawl over each other continuously. It seems prob- able, although it is untested, that sculpture serves for species recognition for mating pur- poses. It was determined on the basis of over- all morphological similarity that H. siamensis spiralis was a synonym of H. sulcata (Bavay) of the lower Mekong River (near Cambodia) as was also H. siamensis Brandt of the Mun River that flows into the Mekong River at the isles of Ban Dan (Davis, 1979). No other spe- cies of Hubendickia lives in the Mun River where one finds the population of H. sulcata referred to by Brandt as H. siamensis. Snails of this population entirely lack spiral micro- sculpture. Over 100 miles south of Khemarat at Khong Island there are more than 50 spe- cies of Triculinae but few species of Huben- dickia. Populations of Hubendickia are rarely sympatric in the sense that they are found FIG. 5. Central teeth of representative species of Triculinae and Pomatiopsinae compared with the central tooth of Hydrobia totteni. A, B. Stylized drawings showing structures of the central tooth. Note that the blade (Bl, blackened layer) is a layer fused on the dorsal aspect of the blade support (Bsu). The lateral view of the tooth is shown in В and ЕЕ. C-E. Hydrorissoia hospitalis (Triculinae: Jullieniini). F, G. ¡Hubendickia cylind- rica (Triculinae: Jullieniini). H. Saduniella planispira (Triculinae: Jullieniini). I. Paraprososthenia levayi (Triculinae: Jullieniini). J-L. Jullienia harmandi (Triculinae: Jullieniini). M. Pachydrobia variabilis (Triculinae: Thculini). N. Hubendickia coronata (Triculinae: Jullieniini). 0-Q. H. gochenouri (Triculinae: Jullieniini). R.H. polita (Triculinae: Jullieniini). S. H. pellucida (Triculinae: Jullieniini). T. Oncomelania tiupensis (Pomatiop- sidae: Pomatiopsinae). U. Hydrobia totteni (Hydrobiidae: Hydrobiinae). V-Y. Hydrorissoia elegans (Triculinae: Jullieniini). Z. Lacunopsis cónica (Triculinae: Lacunopsini). AA-CC. Halewisia expansa male (Triculinae: Triculini). DD. Karelainia davisi (Triculinae: Jullieniini). ЕЕ. Tricula aperta (Thculinae: Triculini). FF. Jullienia acuta (Triculinae: Jullieniini). GG. Pachydrobiella brevis (Triculinae: Jullieniini). HH. Halewisia expansa female (Triculinae: Triculini) All teeth without /мт bars are drawn to the same scale as ЕЕ. Z was drawn at Уз the magnification of ЕЕ. Note the multiserrated blade of J-L and GG and the pauciserrated blade of AA (from Davis, 1979). Acu, anterior cusp; Be, basal cusp; Bl, blade; Bsu, blade support; Edg, edge of the blade support; Fa, face of the tooth; L, length of tooth; La, lateral angle; L of Acu, length of antehor cusp (to the Edg); Led, lateral edge of tooth face. POMATIOPSID EVOLUTION 219 Acu 220 DAVIS living intermixed on the same substrate in the same area. Spiral microsculpture is weakly developed in a few populations of H. sulcata, found on only some individuals of other popu- lations, and is entirely lacking from individuals of yet other populations. It is evident that in the absence of high incidence of congeneric sympatry, spiral microsculpture breaks down. Many shell shapes are clearly interpretable when one observes how the species live. The shells of one species converge on the shells of phyletically totally unrelated groups be- cause the animals of these different groups position themselves on various substrates in the same way. The resemblance of various Lacunopsis species to marine Littorina has been discussed in detail elsewhere (Davis, 1979, 1980). Tricula of the Triculini radiation (Fig. 3) has the most generalized morphology and is rep- resented in the Mekong River by only one species, T. aperta (Temcharoen). The one successful Triculini radiation in the Mekong River involves Pachydrobia. Again, this spe- cies-rich radiation involves innovations in the female reproductive system and establish- ment in a range of habitat types as reflected in a range of shell morphologies that fit the trends discussed above. It is evident that entrance into an adaptive zone, which permitted a new first order radia- tion of Triculinae, enabled some species of that radiation to overlap many niche dimen- sions of species of other first order radiations. A single scoop of a hand sieve (500 ml capac- ity) through a muddy substrate often yields several thousand snails of eight to ten species of three to six genera (Davis, 1979). Numer- ous species in sympatry on a rock or patch of mud or small area of sandy-mud is the rule, not the exception. The snails do not seem to be resource limited unless it is for space for egg deposition. A number of species do occupy unique space. An example is Lacunopsis fischer- pietti Brandt, the largest triculine in the Me- kong River (shell diameter of 15 to 18 mm), which closely resembles the marine species Calyptraea radians. L fischerpietti lives one or two per boulder on the vertical faces of huge boulders, facing the swiftest current. Other examples are: Lacunopsis liarmandi, which lives at the interface of swiftly flowing water and air. Jullienia costata lives crowded by the thousands, packed shell to shell, on vertical cliff walls in rushing waterfalls, splashed continuously by the spray. Some populations of Hubendicl