Brotia species are remarkably plastic not only in their phenotypic appearance. Similar taxa have in the following been considered conspecific, which has bloated the synonymies (e.g., Rensch, 1934; Benthem Jutting, 1956; Brandt, 1974). However, in many cases it remained unclear (and unattended) to which extent shell parameters really varied within single species. Most recent data suggests that intraspecific variability of morphological characters including the shell frequently was overemphasized, which has lead to erroneous taxonomic conclusions. This has been exemplified also for other pachychilids, such as Jagora by Kohler & Glaubrecht (2003). Consequently, one of the main results of the current study is the conclusion that in Brotia, 20th century authors have frequently gone too far in synonymizing taxa for exhibiting a similar shell. Instead, a quite contrasting picture is revealed herein showing that Brotia species in general are much more restricted be means of their morphological variability as well as their distributional range than assumed before.

Shell Shape: Most pachychilid species have highly turreted shells with about up to 12 whorls. This feature is found in all major clades as a predominant character. Few species have conical or even globular shells, such as B. armata, B. paludiformis, or B. pagodula. These species live attached to stones and boulders in swiftly flowing streams while other species are found buried in or crawling on substrata of all kinds.
It has been shown by Urabe (1998) for Semisulcospira reiniana that individuals inhabiting riverine habitats have a more conical shell than specimens from stagnant waters as a phenotypic response to environmental pressures. Although this observation refers to phenotypic responses only, a conical shell can be considered as adaptation to strong water currents repeatedly obtained by Asian pachychilids.

Size and Thickness: In general, shell size and thickness may be controlled by the availability of nutrients (Fromming, 1956), but also by the harshness of physical environmental factors (Vermeij, 1972), parasitism (Wright, 1966), or predation (Zipser & Vermeij, 1978; Reimchen, 1982; Reid, 1986). Nevertheless, there is substantial evidence that shell growth rate and adult size are also under genetic control (Vermeij, 1980). In Brotia, variability in shell size among conspecific specimens of same age is considered lower than formerly supposed. Only in few cases, shells may vary for about the twofold between populations from different environments: Specimens of B. torquata from Lake Manindjau are considerably smaller than those from adjacent rivers. In other cases, however, inhabitants of lakes are larger than riverine forms (e.g., B. sumatrensis from Lake Toba). A possible explanation could include the limitation of certain nutrients due to interspecific competition in one case and the presence of predators, such as shell crushing crabs as discussed for Tylomelania in Sulawesi (Rintelen et al., 2004) or simply the fact that large shells are prone to dislodgement in rivers but not in lakes in the other case.

Sculpture: Freshwater gastropods in general are notorious for their plasticity in form and sculpture (e.g., Davis, 1971; Fretter & Graham, 1984; Urabe, 2000). Similarly, among Brotia shell sculptures vary considerably and are used as a conspicuous feature to distinguish among species. Shells may be completely smooth or sculptured by strong axial ribs, spiral cords, spiny nodules, and/or spines. The degree of intraspecific variability, however, seems to differ greatly. In general, variation of the shell morphology, and thus also sculpture, has been considered to hav a genetic basis and a strong sculpture shall be adaptive against predators or physical environmental factors (e.g., West & Cohen, 1996). It has been shown that sculp¬tured shells are more tolerant of a crushing load than are smooth shells with the same shell mass (Urabe, 2000). Some studies have further demonstrated that shell morphology shows a great deal of phenotypic plasticity controlled by physical or biological factors (e.g., De Wolf et al., 1997), such as the substratum (Urabe, 2000). While phenotypic plasticity within single species has not been addressed in this study, it can be confirmed that shell form and sculpture are correlated to the substratum: species with smooth shells were always found on sandy or pebble substrata, whereas species with armed shells live on gravel, stony bottoms or sit on boulders (Glaubrecht & Kohler, 2004, for Brotia species of the Kaek River). It is assumed that a sculpture not only prevents the animals from being preyed upon, which seems to be a rather imaginary threat when sittinq directlv in the water current, but
from the influence of physical forces. A well-developed sculpture, however, is unfavourable when crawling in the sand as it would increase the friction with the substratum. Accordingly, different shell sculptures may have evolved as result of ecological and morphological diversification, in some cases induced by competitive interaction between the different species.

Colour. In Brotia shell colour is uniform, from yellowish brown to olive brown, dark brown or almost black and overall not very helpful for species recognition. In some species, dark spiral bands may be present; axial flames that can be observed in other pachychilids, such as Pachychilus, Adamietta, and Paracrostoma, are generally lacking.