RICHLING, I. (2004): Classification of the Helicinidae: Review of Morphological Characteristics Based on a Revision of the Costa Rican Species and Application to the Arrangement of the Central American Mainland Taxa (Mollusca: Gastropoda: Neritopsina). - Malacologia, 45 (2): 195-440.
The present study combines a taxonomical revision of the poorly known Costa Rican Helicinidae, with a detailed investigation of certain morphological structures with respect to their relevance for systematics, culminating in a discussion of the arrangement of the Central American mainland species.
The revision of the Costa Rican species is based on the examination of nearly all type material, coupled with extensive field work and investigations of the collections of the Instituto Nacional de Biodiversidad de Costa Rica and the Florida Museum of Natural History, Gainesville, along with perusal of additional historical material. With minor exceptions, all these species were investigated with respect to the features of shell, operculum, surface sculpture of embryonic shell and teleoconch, internal shell structures, radula, and female reproductive system. In addition, analyses of morphometry and sexual dimorphism were carried out. Faced with a limited amount of material, it became necessary to develop a new preparation method to separate the soft body from the shell without damaging either.
For the higher classification and comparative analysis of the different morphological characteristics, similar examinations emphasizing formerly poorly studied or neglected characteristics, such as embryonic shell and female reproductive system, were carried out for 17 additional species representing the most important related Central American supraspecific taxa using their type species when available. For taxa with inaccessible material, data from the available literature were critically incorporated.
For Costa Rica, 15 species were recognized, among them seven new species, partially published in Richling (2001) - Helicina echandiensis, H. talamancensis, H. monteverdensis, H. chiquitica, H. escondida, Alcadia (Microalcadia) hojarasca, and A. (M.) boeckeleri - and two new subspecies - H. punctisulcata cuericiensis, and H. beatrix riopejensis. Other previously subspecifically separated taxa (H. funcki costaricensis Wagner, 1905; H. tenuis pittieri Wagner, 1910) were shown to fall within the range of intraspecific variability. Records of the Guatemalan and Mexican species Helicina oweniana L. Pfeiffer, 1849, and subspecies, H. amoena L. Pfeiffer, 1849, as well as those of H. fragilis Morelet, 1851, were proven to be based on faulty identifications and were therefore excluded from the Costa Rican fauna. This fact, together with the recognition of the several new species, shows that the faunal composition of Costa Rica is much more distinct from that of the northern areas than previously assumed. The transitional zone of Nicaragua, however, still remains widely uninvestigated. Only Helicina tenuis L. Pfeiffer, 1849, being ecologically very tolerant, Lucidella lirata (L. Pfeiffer, 1847), and Pyrgodomus microdinus (Morelet, 1851) are wide-spread, extending from Mexico to Costa Rica, perhaps even farther south. The distribution of the typical Costa Rican species follows the topographical subdivision created by the Central Cordilleras, along with its corresponding effects on the climate.
Contrary to former assumptions, certain features of the female reproductive system proved very useful for the classification of the Helicinidae. For the first time, monaulic conditions have been recognized for Helicina and Eutrochatella, necessitating the correction of previous descriptions in this respect. Furthermore, the monaulic or diaulic state is characteristic of the genera and is paralleled by consistent changes in the embryonic shell structure. Because primitive members of the Helicinidae possess a diaulic system, the monaulic condition is regarded as the derived state. The Central American genera Helicina, Alcadia, Eutrochatella, Lucidella and Schasicheila were properly distinguished and described by this, as well as by other differences in the female reproductive system. The anatomies of the type species of Helicina and Alcadia were examined for the first time, and earlier descriptions of Eutrochatella and Lucidella were corrected in major points. On the basis of this new evidence, the assignment of traditional subgeneric units of Helicina and Alcadia, previously based mainly on vague radula and shell characteristics, was especially reassessed. The subgenera Sericea and Analcadia were transferred to Helicina, as well as the mainland land species summarized under the preoccupied taxon "Gemma". Tristramia, Oxyrhombus, Pseudoligyra, Oligyra, Succincta, "Cinctella" (also preoccupied) and Punctisulcata were confirmed in their association with Helicina. Due to its monaulic condition, the former genus Ceochasma is reduced to a subgenus of Helicina. In addition, exemplary non-type Antillean species were studied, including Helicina jamaicensis Sowerby, 1841, which had to be shifted to Alcadia s.l., and Alcadia (Analcadia) platychila (von Muehlfeldt, 1816), which is now assigned to Helicina s.s. On one hand, the new arrangement excludes Alcadia as previously known from the Central American mainland, but, on the other hand, examination of the newly discovered Costa Rican species Helicina hojarasca and H. boeckeleri required the establishment of a new subgenus of Alcadia, Microalcadia n. subgen. on the mainland, based mainly on the features of the female reproductive system and embryonic shell structure. The occurrence of Alcadia with only a few diminutive species on the mainland of Central America corresponds to the distribution of the genera Eutrochatella/Pyrgodomus and Lucidella.
The Central American mainland species of Helicina seem to show a closer relationship among each other than to the northern South American subgenera Analcadia and Sericea. The Brazilian taxon Angulata, previously a subgroup of Helicina, deviates remarkably in embryonic shell structure and shows differences in anatomy that still require final confirmation, and it thus deserves recognition as a separate genus.
Contrary to the well-supported differentiation at the generic level, the attempt to characterize subgroups of the Central American mainland species of Helicina has been only partially successful. Certain similarities in teleoconch surface structure, relative development of the accessory structures of the female reproductive system, and the degree of sexual dimorphism become obvious and are discussed to some extent, but intermediate characteristics complicate a satisfactory solution. Besides Ceochasma, three Central American mainland subgenera are recognized: Oligyra, Tristramia most closely resembling Helicina s. s., and "Gemma". The latter preoccupied name is tentatively retained, because the proposal of a new name seems inappropriate at this stage.
Investigation of the morphological features other than the embryonic shell sculpture and the female reproductive system revealed the following additional results, mainly based on the Costa Rican species of Helicina:
Characteristics of teleoconch, operculum, and radula, previously regarded as substantial for classification, were repeatedly demonstrated to be subject to convergent development, thus limiting their value for systematics. Different examples are given, such as the T-shaped lateral of the radula or periostracal hairs, and further evidence is provided by the necessary re-arrangements outlined above. Nevertheless, these features play a supplementary or supporting role.
The mantle pigmentation of arboreal Helicinidae is closely related to the transparency of the shell and functionally replaces shell color in thin shells. The physiological possibility of an obviously adaptable mantle pigmentation could provide the opportunity for survival with thin, transparent shells as adaptation to the limited availability of calcium carbonate. Whereas varying and patterned mantle color are characteristic for arboreal thin-shelled species, the color of the head and foot is seldom species specific.
Size differences of the embryonic shell have not previously been studied for Helicinidae. Embryonic shell size is shown to increase with the shell size within a group of related species and also altitude within different populations of a species. Furthermore, it may show a certain species specificity. Preliminary data on Lucidella and Eutrochatella/Pyrgodomus suggest a consistently smaller embryonic shell size than in Helicina or Alcadia.
Internal shell structures - axial cleft and muscle attachments - seem characteristic for certain systematic units, for example, Lucidella and Schasicheila. The length of the axial cleft is confirmed to be constant within a species, but, contrary to former assumption, it is not related to the whorl count.
The data on sexual dimorphism given in this study represent the most comprehensive approach to date to analyze this phenomenon for Helicinidae. The sexual dimorphism may manifest itself in differences in volume, a male's size being only about 62-70 % of that of the female's, but formerly assumed deviations in shape could not be proved to be of significance for species of Helicina. A certain value for the degree of differences in uncovering systematic affinities is indicated.
© Ira Richling, Germany