María R. Alonso

Testing limiting similarity in Quaternary terrestrial gastropods

Abstract The hypothesis of limiting similarity, which postulates that morphologically and/or ecologically similar species will differ enough in shape, size, or other variables to minimize competition, has been controversial among ecologists and paleoecologists. Many studies have reported the occurrence of limiting similarity in modern environments or in time-averaged fossil deposits; however, empirical high-resolution time series demonstrating limiting similarity over longer time scales are lacking. We have integrated radiocarbon-calibrated amino acid dating techniques, stable isotope estimates, and morphometric data to test the hypothesis of limiting similarity in late Quaternary land snails from the Canary Islands over a period of 42,500 years. We tested for both ecological character displacement (two closely related species will differ in size in order to minimize competition in sympatry and these differences will be minimized in allopatry) and community-wide character displacement (overdispersion of body size among competitors in a guild). Multiple proxies of body size consistently show that two endemic congeneric pulmonate gastropod species (Theba geminata and T. arinagae) maintained a difference in size from ∼42,500 b.p. through the last occurrence of T. arinagae 14,900 b.p., with a concomitant trend of a decreasing body size. Theba geminata body size did not converge on that of T. arinagae and variation in T. geminata body size did not increase significantly following the extinction of T. arinagae; therefore, ecological character displacement and release did not occur. Community-wide character displacement was found in only one time bin over the last 42,500 years. These results suggest that limiting similarity is a transient ecological phenomenon rather than a long-term evolutionary process. This study not only demonstrates the problems inherent in biological “snapshot” studies and geological studies of time-averaged deposits to test limiting similarity adequately, but it also presents a more adequate research protocol to test the importance of interspecific competition in the history of life.

ECOLOGICAL FIDELITY OF PLEISTOCENE–HOLOCENE LAND SNAIL SHELL ASSEMBLAGES PRESERVED IN CARBONATE-RICH PALEOSOLS

Abstract Studies that assess the ecological fidelity—preservation of the original community—of terrestrial shell accumulations are uncommon but essential to infer accurate changes in past ecosystems. When live-dead comparisons are unavailable, the taxonomic agreement between differing taphofacies may be used to evaluate the fidelity of ancient shelly assemblages. This approach was used to approximate the fidelity of Quaternary land snails preserved in carbonate-rich paleosols from the northeastern islets of the Canary Archipelago. Such macroscopic alteration as fragmentation, corrosion, carbonate coating, and color loss affected shells, however, microscopic analyses concluded substantial diagenetic alterations unlikely. The shell abundance negatively correlated with fragmentation, suggesting that a higher proportion of shells may be a consequence of higher shell input rate and lower shell destruction rate rather than lower sedimentation rate, as predicted by taphonomic models. Strongly and weakly altered taphofacies significantly differed in species abundances. Substantial taphonomic bias was improbable, however, because both taphofacies contained taxa with comparable durability. Temporal fluctuations in taphonomy and ecology suggest variable environmental conditions operated through time. The overall decline in shell abundance from the last glacial to interglacial paleosols may be explained by a decline in humidity and reduced island surface area resulting in lowered snail proliferation, and in turn, a decreased net shell input rate. This study emphasizes that the original community is preserved within the studied terrestrial shell accumulations regardless of the degree of taphonomic alteration. Measures of past taxonomic richness and diversity, therefore, may be used as a reliable measure of the original snail community.

Revision of the species group Napaeus variatus (Gastropoda, Pulmonata, Buliminidae) from the Canary Islands, with description of five new species*

The Napaeus variatus (Webb & Berthelot, 1833) species group from the Canary Islands is reviewed from literature information, types, other museum specimens, and newly collected material. Anatomy data is newly provided for six species. Five new species are described: N. elegans and N. esbeltus from Tenerife, N. orientalis and N. beguirae from La Gomera, and N. exilis from Gran Canaria. Eight species lack a diverticulum and are referred to the subgenus Napaeus (Napaeus) Albers, 1850, whereas five species are provided with a diverticulum and arc referred to Napaeus (Napaeinus) Hesse, 1933. Four species cannot be assigned to a subgenus due to a lack of information. Keys of species from the different islands are provided.

On the relationships of the genus Napaeus (Gastropoda: Pulmonata: Enidae) with the descriptions of four new species from the Canary Islands

Species of the Napaeus subgenera show incongruences between genital anatomy and molecular phylogenetic data. In this study, four new Napaeus species from the Canary Islands are described. Shell drawings of the 56 known Napaeus species were obtained from shell photographs for comparison. Differences between some genital system character-states of the species described here and those of the original subgenera descriptions are evident. Consequently, in the absence of phylogenetic analyses, the new species are not assigned to the Napaeus subgenera. In addition, the possible relationships among Macaronesian enids are preliminarily evaluated. Within the Macaronesian region, enids are exclusively present in the Canaries and the Azores. The absence of enids from Madeira, combined with the differences in genital anatomy between the Canarian and Azorean enids and the relatively large distance between these archipelagos, may indicate different immigration pathways (Africa/Europe, respectively) and possibly different phylogenetic origins for the Canarian and Azorean enid populations.

The Genus Obelus Hartmann, 1842 (Gastropoda, Pulmonata, Helicoidea) and its Phylogenetic Relationships

Abstract The genus Obelus , so far known from shells only, is re-described on the basis of the anatomy of its genital system. Its range is here restricted to north-western Africa and the Canary Islands. Four species are traditionally listed for the Canary Islands: Obelus despreauxii , O. moderatus , O. mirandae and O. cyclodon . The presence of the last species on the Canary Islands is doubtful, whereas the fossil/subfossil presence of O. pumilio is documented. The species Helix morata also belongs to the genus, as well as a new species, O. discogranulatus sp. nov. The genital systems of all confirmed extant Canarian species are shown. Obelus has a peculiar vaginal stimulator appendix (“appendicula”) which is homologous to the penial appendix of the Orthurethra and to the stimulatory organ of other Stylommatophora, with the exception that the A 3 portion is missing. The genus is diagnosed by characters of the vaginal stimulator appendix, such as a curved, finger-like A 2 portion of the appendicula ending proximally in a blind, well-developed muscular sac, and by the slender tubular A 4 portion arising laterally from the muscular sac duct. We conclude that Obelus belongs to the Cochlicella group because it shares an appendicula with the other genera of the group. However, it differs in anatomical details of this structure. The Cochlicella group should be recognized as a separate family Cochlicellidae Schileyko, 1972, with close relationships to the Helicellidae and the Hygromiidae. A new diagnosis for Cochlicellidae is proposed and its phylogenetic relationships are discussed. This is: Notes on the Malacofauna of the Canary Islands, No. 46.

Two new Napaeus species from La Gomera and La Palma (Canary Islands) (Gastropoda: Pulmonata: Enidae)

Two new species of Napaeus are described, from La Gomera and La Palma (Canary Islands). Both species are small, that from La Gomera being the smallest known Napaeus, its shell surface area (plane view) is more than ten times smaller than that of N. bertheloti. The two new species actively disguise their shells with lichens and soil, presumably to reduce predation.