Influence of setting,sieve size,and sediment depth on multivariate and univariate assemblage attributes of coral reef-associated mollusc death assemblages from the Gulf of Aqaba

Measures of diversity and ecology of marine invertebrate assemblages depend on a variety of factors including environmental conditions and methodological decisions. In this study, the influence of such factors on multi- and univariate assemblage parameters of molluscan death assemblages from the Gulf of Aqaba (Red Sea, Jordan) was evaluated. Sediment samples were collected at two coral reef types, a patch reef at 13 m of water depth characterized by fine-grained sediments and a Millepora-fringing reef with coarse-grained sediments at 5 m of water depth. The upper and lower 10 cm of the sediment column were separately removed and sieved with mesh sizes of 1 and 2 mm. A large dataset of 6400 bivalve and gastropod shells was compiled to evaluate how setting, sediment depth, and sieve size influenced taxonomic composition and species richness, species-abundance patterns and the Shannon–Wiener index, the number of drilled shells per species and drilling frequency (DF) of the assemblage. Setting had the strongest impact on all aspects, followed by sieve size, but sediment depth was insignificant, probably due to complete homogenization of the sediments by reworking and bioturbation. Multivariate assemblage parameters distinguished much better between categories (setting, sieve size) than univariate measures. Sieve size-related disagreements recognized between the two higher taxa are mostly due to the underlying difference in body-size distribution of bivalve and gastropod assemblages. We conclude that species richness and other ecological characteristics of molluscan death assemblages in coral reef-associated sediments will most strongly reflect habitat complexity of the sites chosen, are significantly influenced by methodological decisions (i.e., sieve size), will only poorly preserve temporal patterns, and the results will differ between bivalves and gastropods.     

Diversity,palaeoecology and systematics of a marine fossil assemblage from the Late Triassic Cassian Formation at Settsass Scharte,N Italy

A marine fossil assemblage from the Late Triassic (Early Carnian) Cassian Formation is reported. It was retrieved by bulk sampling, including wet sieving and quantitative picking, and by quantitative surface collection. The collection consists of c. 460 specimens (foraminiferans not included) representing 54 species. In terms of abundance and species richness, it is strongly dominated by molluscs, especially gastropods. 97 % of the individuals are molluscs. The most abundant species are the gastropods Goniospira armata, Schartia carinata and Helenostylina convexa, followed by the scaphopod Plagioglypta undulata and the bivalve Palaeonucula strigilata. Disarticulated echinoderm ossicles (mostly echinoids, crinoids, few holothurians) comprise almost all of the rest of the assemblage. The studied assemblage shows moderate diversity, similar to those of previously reported assemblages or associations from basinal settings. However, it differs considerably in taxonomic composition from previously described associations of the Cassian Formation. The abundance of small gastropods is a result of their primary abundance in these ancient living communities and of the sampling method (sieving at 0.5 mm), because most of the previous associations were obtained by performing surface collections, in which small gastropods are easily overlooked. The fauna is interpreted as an autochthonous soft bottom assemblage dominated by species that lived in low epifaunal to shallow infaunal habitats. Detritivory, deposit feeding and microcarnivory represent the main feeding types. Most of the species were fully motile but slow, and either infaunal (scaphopods, nuculids, the gastropod Domerionina) or epifaunal (most other gastropods, echinoids). The present assemblage underlines the pronounced heterogeneity of the Cassian biota. The low grade of lithification and diagenetic alteration facilitates bulk sampling and the investigation of small species. This minimizes possible sampling and preservation biases, so the studied assemblage reflects the alpha diversity of this ancient living community to an unusually high degree. The following gastropod taxa are new: Helenostylina convexa n. sp., Schartia carinata n. gen. n. sp., Schartiinae n. subfam.; Cassianastraea Bandel non Volz is replaced with Bandelastraea nom. nov.     

The earliest known estuarine bivalve assemblage, Lower Ordovician of northwestern Argentina

The oldest known estuarine bivalve assemblage is documented from the Lower Ordovician (upper Arenig-lower Llanvirn) Alto del Cóndor Formation, which crops out in the Cordillera Oriental of northwestern Argentina. This unit displays most of the diagnostic sedimentary attributes of estuarine environments. Biotic components include low-diversity trace fossils and a peculiar bivalve fauna consisting of the new genera Konduria, Pseudoredonia, and Pucamya, and the new species Redonia condorensis. This constitutes the earliest known occurrence of bivalves in brackish waters, suggesting that the capability of this clade to colonize estuarine environments developed early in their radiation.     

Early Ordovician bivalves from Dali, West Yunnan, China

A bivalve fauna of Early Ordovician (late Arenig) age is described from the Hsiangyang Formation of the eastern part of West Yunnan, China. The fauna contains elements in common with Early Ordovician faunas of southern Gondwanan areas such as the Montagne Noire and Morocco and with those of Avalonia, together with several previously undescribed taxa. The following taxa are new: Biseriodonta simplex gen. et sp. nov.; Glyptarca sinensis sp. nov.; Trigonoglyptarca magna gen. et sp. nov.; Erhaiconcha xiangyangensis gen. et sp. nov.; Fasciculodonta impressa gen. et sp. nov.; Yunnanoredonia laevis gen. et sp. nov.; Daliella gen. nov.; Goniophorina ( Goniophorina ) contracta sp. nov.; Haidongoconcha radialis gen. et sp. nov.; Eopterinea aequiconcha gen. et sp. nov. The bivalve fauna includes the most diverse glyptarcid fauna and the earliest nuculanid and paracyclid known hitherto.   Eastern West Yunnan belonged to the Indochina terrane and it is concluded that the bivalve fauna represents a high-latitude assemblage and that the Indochina terrane should be considered a component part of the Peri-Gondwanan continent in the Ordovician; its bivalve faunas contrast with those of neighbouring terranes which have affinities with those of lower latitudes.     

New data on the terrestrial gastropods from the Oligocene-Miocene transition in the Valley of Lakes, Central Mongolia

The Oligocene-Miocene terrestrial gastropod fauna of the Valley of Lakes, Central Mongolia, is described for the first time based on SEM images. These allowed detailed anatomical analyses and resulted in better species delimitation and taxonomic rectifications. The assemblage comprises six pupilloid species of the genera Gastrocopta, Vallonia, and Pupoides. Vallonia stworzewiczae Neubauer, sp. nov. is introduced as a new species. Two samples from the Loh Formation exposed at the Hotuliin Teeg locality have yielded excellently preserved specimens. Based on a small mammal fauna, these samples are proposed to span the interval of the Oligocene-Miocene boundary. A comparison with ecological requirements of modern congeners suggests the presence of rather open but vegetated habitats in the Valley of Lakes during sedimentation of the Loh Formation.     

A novel trophic relationship between cassid gastropods and mysticete whale carcasses

Large cassid gastropods found in close association with Oligocene mysticete whale skeletons in a deep water setting indicate a novel trophic relationship. Bones of early mysticetes are preserved in close association with a recurring assemblage of bivalve taxa that are common components of fossil and recent hydrothermal vent and cold methane seep localities. Living members of these bivalve groups harbor sulfide-oxidizing autotrophic bacteria and comparisons with recent whale-falls indicate that they utilized bone-oil seepage. But unlike recent whale-falls, this fossil invertebrate assemblage is volumetrically dominated by large individuals of the cassid gastropod Liracassis apta that are preserved with their apertures adhering to the bone surfaces. Comparisons with recent cassoidean taxa and their ecological settings suggests that Liracassis apta belongs to the subfamily Oocorythinae, not Cassininae, and was an opportunistic scavenger in deep water, low oxygen environments. Oocorythine gastropods radiated rapidly on the northeastern Pacific margin during the Late Eocene and Oligocene to become the most commonly recorded gastropod in shelf and slope settings.     

Sarmatian (Late Middle Miocene) gastropod assemblages of the Central Paratethys

Summary A rather diverse gastropod fauna from Sarmatian deposits of the Austrian/Hungarian Eisenstadt-Sopron Basin was studied. The fauna derives from two layers of clay and silt within a siliciclastic section at St. Margarethen in Burgenland (Austria). These layers are interpreted as littoral mudflats which formed during the Sarmatian (Late Middle Miocene) along the western coast of the Central Paratethys. Strong shifts in the composition of the gastropod fauna, dominated by Potamididae (Cenogastropoda: Cerithioidea), within each layer indicate successions of limnic-fluvial to oligohaline, brackish-littoral, and marine-littoral environments. These shifts in facies are reflected by an alternational of thePotamides hartbergensis assemblage,Granulolabium bicinctum assemblage, and thePotamides disjunctus assemblage. The speciesJujubinus turriculus (Eichwald, 1850),Gibbula buchi (Dubois, 1831), andCylichnina elongata (Eichwald, 1830) are reported for the first time from the Sarmatian of the Paratethys.Mitrella agenta nov. sp. (Neogastropoda: Columbellidae) is introduced as a new species. These species might represent relics of the diverse Badenian fauna but could also prove a minor ingression of marine species from an adjacent bioprovince in the Late SarmatianMactra “Zone”.     

Predatory gastropod boreholes in an Eocene molluscan assemblage from Nigeria

Arua, Ingela & Hoque, Mominul. 1989 01 15: Predatory gastropod boreholes in an Eocene molluscan assemblage from Nigeria. Lethaia . Vol. 22, pp. 49–59. Oslo. ISSN 0021–1164.
Morphometric data of 1, 346 boreholes belonging to eleven gastropod species and nine bivalve species have been studied. On the basis of hole-wall geometry and concentricity of the inner and outer holes, six different borehole types are identified: (1) concave with concentric openings (A-type); (2) concave with aconcentric openings (B-type); (3) planar tapering with concentric openings (C-type); (4) planar tapering with aconcentric openings (D-typc); (5) vertical with concentric openings (E-typc); and (6) concave-planar with aconcentric openings (F-type). The preferred gastropod and bivalve prey species for each bole type are named. In identifying the borers of a particular hole type, the mode of life of the most preferred prey species is used as a diagnostic parameter. Preferred infaunal prey species are dominantly bored by naticids, preferred epifaunal prey is mainly bored by muricids. The presence of a boss at the base of an inoomplete hole and the presence of countersunk holes are the additional criteria for identifying naticid borings. The hole type analysis suggests that hde types A, B and D belong to muricids and C, E and F to naticids. All the naticid and muricid hole types studied have a preferred site for boring. They are located purposefully where they would intercept internal organs of the prey. It appears that in ornamented bivalve shells these predators would rather drill between ribs than on top of the ribs. □ Gastropoda, predation, boreholes, molluscan assemblage, Eocene. Ameki Formation, Nigeria. Ingela Arua, Department of Geology, University of Nigeria, Nsukka. Nigeria; Mominul Hoque, Geo-Mechanis Inc., Elizabeth, PA 15037, U. S. A.; 14th October, 1987 .     

Ecological aspects of a silicified bivalve fauna from the Silurian of Gotland

The silicified Wenlockian (Silurian) bivalve fauna from MÖllbos, Gotland, is part of a life assemblage. The vast number of shells show unusual phenomena, e.g. shell repair, pearl and tumour formation, etc. A number of shells contain epibionts and bored, round holes. Presumptive predators of the bivalve community are discussed. Size-frequency distribution of the two most abundant species possibly reflects age classes. The fauna, comprising eleven species, is dominated by deposit-feeders (90 %). They exhibit niche diversification, including at least three different feeding levels within the sediment.     

Late Triassic (Late Norian) gastropods from the Wallowa Terrane (Idaho,USA)

A diverse Late Triassic (Late Norian) gastropod fauna is described from the Mission Creek Limestone of the Wallowa terrane (Idaho, USA). Sample standardization by rarefaction analysis indicates that the fauna is even more diverse than the Late Triassic gastropod fauna from the Pucara Formation (Peru) which represents the most diverse gastropod fauna from South America. The gastropod fauna consists of 66 species; several genera are reported for the first time from North America. A high percentage of the species are highly ornamented and several have distinct siphonal canals. This suggests that the appearance of truly Mesozoic elements among the gastropods began before the Mesozoic Marine Revolution in other clades. The fauna is dominated by high-spired strongly ornamented procerithiids, a group more characteristic for the Jurassic. Comparison of the present fauna and the Iranian Nayband Formation gastropod fauna show that the procerithiids underwent a first global radiation in the Late Triassic. The high number of new species in this fauna suggests that sampling of Late Triassic gastropod faunas is still incomplete and hinders palaeobiogeographic considerations. Previous suggesions that gastropod faunas from the Wallowa and Wrangellia terranes resemble each other and are distinct from those of Alexander, Chulitna, and Farewell terranes are basically corroborated. The gastropod fauna of the Mission Creek Limestone differs considerably from that of the western and central Tethys but shares several taxa with the Late Triassic gastropod fauna of the Pucara Formation in Peru. Thus, the Hispanic corridor was probably not present in the Norian but opened only in the Early Jurassic. The subfamily Andangulariinae is introduced and placed in the Zygopleuridae. The generaSpiniomphalus, Nodoconus, Gudrunella, Blodgettella, Idahospira, andSiphonilda and the subgenusCryptaulax (Wallowax) are introduced. 27 species are erected. A lectotype is designated forCryptaulax rhabdocolpoides Haas, 1953.      

Mollusks of the Utlyuk Liman (Sea of Azov): A Review of Species Composition with Remarks on Distribution and Ecology

Summarized data on the fauna composition, distribution, and ecology of gastropod and bivalve mollusks of the Utlyuk Liman in the northwestern part of the Sea of Azov is presented. The total number of mollusk species identified was 63; 43 species belonged to the class Gastropoda, and 20 species, to the class Bivalvia. The distribution of mollusks in the liman has extremely irregular character, whereas the distribution of species along the marine shore of Biryuchii Ostrov spit is more homogeneous. Euryhaline Mediterranean species represent the core of liman malacofauna; some taxa of the Ponto-Caspian zoogeographical complex (Dreissena polymorpha and species of the genus Theodoxus) and invader species from distant sea basins (Mya arenaria and Anadara inaequivalvis) were also identified.     

Discovery of a Charophyte locality and Biostratigraphy of the continental Eocene deposits of Oued Méridja (southwestern Algeria)

The continental deposits of Oued Méridja area (west of Bechar, southwestern Algeria) have been assigned by previous authors to the early Eocene on the basis of lacustrine gastropod fauna (Pseudoceratodes). This study reports new paleontological and biostratigraphic data from the Oued Méridja section. Recent field investigations resulted in the discovery of an important charophyte assemblage, composed of five species belonging to four genera: Maedleriella cristellata, Maedleriella aff. cristellata, Harrisichara aff. leptocera, Peckichara disermas and? Gyrogona sp. This association allows to suggest a late Thanetian to early Ypresian age for this locality.     

Facies of two important Early Triassic gastropod lagerstätten: implications for diversity patterns in the aftermath of the end-Permian mass extinction

Two important lagerstätten of Early Triassic gastropods, the Sinbad Limestone (Utah, USA) and the Gastropod Oolite (North Italy) yield about 40% of all described Early Triassic species. This great contribution to the global diversity and the exceptional good preservation render high information content, which characterizes fossil lagerstätten. The Smithian Sinbad Limestone contains the most diverse Early Triassic gastropod fauna. At the type locality, it occurs in single, probably storm-induced shell bed within a series of high energy deposits underlain by intertidal microbial mats and subtidal oolite/peloid shoals. The main shell bed contains about 40 invertebrate taxa. Gastropods, scaphopods, and bivalves are most abundant and form an assemblage, which is dominated by small neritaemorphs, the opisthobranch Cylindrobullina convexa and the scaphopod Plagioglypta (annulated tubes). This assemblage lived on shallow, subtidal soft-bottoms based on sedimentological and ecological characteristics. The Dienerian (to Smithian?) Gastropod Oolite Member (North Italy) has extremely abundant, probably salinity-controlled gastropod faunas with low species richness. Almost monospecific assemblages of Pseudomurchisonia kokeni as well as assemblages with about four species are present in the Gastropod Oolite. Modern hydrobiid mudsnail faunas which are adapted to strongly fluctuating salinity in intertidal to shallow subtidal coastal areas form probably a suitable model for the Gastropod Oolite biota. Gastropods from the Werfen- and Moenkopi-Formation lagerstätten are well preserved compared to other Early Triassic deposits. The high contribution to the global diversity of just two sites suggests very incomplete sampling and preservational bias. However, the low richness of the major faunas reflects depauperate Early Triassic faunas and slow recovery from the Permian/Triassic crisis.     

Messinian Lago-Mare mollusc fauna from the Gorgona Island slope, Tyrrhenian Sea

Late Miocene Lago-Mare macrofossiliferous sediments were recovered in the northeastern Tyrrhenian Sea by dredging the continental slope off Gorgona Island, Tuscan Archipelago, at 300-470 m depth. The fossil assemblage consists of a rich lymnocardiid bivalve fauna dominated by Pontalmyra ex gr. P. incerta (Deshayes), associated with Dreissena ex gr. D. rostriformis (Deshayes), Pontalmyra cf. partschi (Mayer), “Limnocardium” sp., the gastropods Melanopsis narzolina (D’Archiac), Melanopsis sp. and cf. Saccoia sp. All bivalve taxa recognized at species level are of Paratethyan (Pontian) affinity and widespread in the Late Miocene of the Mediterranean Basin while M. narzolina has so far only recorded from the Mediterranean Basin. This finding represents the most diverse Lago-Mare macrofauna reported thus far from any submerged location in the Mediterranean Basin and documents that the post-evaporitic Cusercoli Formation contributes to the syn-rift neoautochthonous units of this sector of the Northern Tyrrhenian Sea.     

Taphonomy of the Upper Mesozoic tetrapod Teete locality (Yakutia)

The fauna of the Teete locality (Neocomian, Yakutia) includes bivalve and gastropod mollusks, palaeoniscid and chondrostean fishes, caudate and anuran amphibians, choristoderes, lizards, phytophagous and predatory dinosaurs, and theromorphs (cynodonts). The flora includes mosses, horsetails, lycopods, ferns, and conifers. The petrographic study showed the prevalence in the section of volcano-sedimentary rocks. The Teete paleobiota dwelled on the alluvial-lacustrine plain in conditions of warm humid climate influenced by intense volcanic activity.     

An unusually diverse mollusc fauna from the earliest Triassic of South China and its implications for benthic recovery after the end-Permian biotic crisis

A single carbonate coquinoid lens from the Griesbachian (Early Triassic) of Shanggan, South China, yielded 11 bivalve species described in this study in addition to four gastropod and one ammonoid species reported elsewhere. This makes the Shanggan fauna one of the richest mollusc faunas from the early post-extinction interval after the end-Permian mass extinction event. Four of the present genera are long-term survivors, five are holdovers that went extinct at the end of the Griesbachian or later in the Early Triassic, and seven first appear in the Griesbachian. Three new bivalve species are described: Myalinella newelli nov. sp., Scythentolium scutigerulus nov. sp., and Eumorphotis shajingengi nov. sp. The genus Astartella, previously assumed to have vanished at the end of the Permian, is reported for the first time from the Early Triassic, which also removes Astartidae from Early Triassic Lazarus taxa. The small growth size of the Astartella specimens supports an earlier hypothesis that many of the Early Triassic Lazarus taxa did not survive in unknown refuges but were simply overlooked due to the scarcity of easily observable large-sized specimens. Ecologically, a comparatively high proportion of infaunal bivalve species (4/11) is remarkable for the early post-extinction interval, supporting the impression of a relatively advanced recovery state. Moreover, abundance-data of the bivalve-gastropod community reveal a remarkably low dominance index (D = 0.17) that is suggestive for advanced recovery and stable environmental conditions. It is proposed that the Shanggan fauna represents a late Griesbachian benthic recovery event that coincided with the appearance of similarly diverse benthic faunas in Oman and Primorye. A high proportion of genera that have previously not been reported from the Early Triassic indicate that the prevalence of poor preservation conditions is a major obstacle in identifying early phases of recovery from the greatest crisis in the history of metazoan life. The early recovery of benthic faunas reported in this study questions previous claims of a prolonged lag phase as a consequence of the extraordinary extinction magnitude or the persistence of adverse environmental conditions.     

A Middle Miocene endemic freshwater mollusc assemblage from an intramontane Alpine lake (Aflenz Basin,Eastern Alps,Austria)

The mollusc fauna of the early Middle Miocene (Langhian) intramontane Alpine Lake Groisenbach is described for the first time. The shells derive from the Feistring Formation in the Aflenz Basin in Austria, which was covered by Lake Groisenbach. The assemblage is moderately diverse with 12 gastropod and 2 bivalve species, suggesting shallow lacustrine and fluvial settings. Among the gastropods, only Theodoxus crenulatus (Klein, 1853) is known from other Miocene localities, whilst all other species are documented so far only from Lake Groisenbach. None of the Early and Middle Miocene lake systems of the Alpine-Carpathian Foredeep and the Balkan Peninsula displays any faunistic resemblance with this new fauna. Even coeval lake faunas from the close-by Graz Basin have no species in common with Lake Groisenbach. This pattern points to a surprising endemicity and biogeographic fragmentation in the Central European freshwater systems during the Early and Middle Miocene. The uniqueness of the newly described fauna is also indicated by the completely erratic occurrence of the otherwise African-Mediterranean genus Bulinus, which is unknown from all other central European Miocene freshwater systems. Emmericia roetzeli Harzhauser and Neubauer nov. sp., Nematurella zuschini Neubauer and Harzhauser nov. sp., Romania fastigata Neubauer and Harzhauser nov. sp., Odontohydrobia groisenbachensis Neubauer and Harzhauser nov. sp., Odontohydrobia pompatica Neubauer and Harzhauser nov. sp., Odontohydrobia styriaca Harzhauser and Neubauer nov. sp., Planorbis austroalpinus Harzhauser and Neubauer nov. sp., Gyraulus sachsenhoferi Harzhauser and Neubauer nov. sp., Bulinus corici Harzhauser and Neubauer nov. sp., Ferrissia crenellata Harzhauser and Neubauer nov. sp. and Stagnicola reinholdkunzi Harzhauser and Neubauer nov. sp. are introduced as new species.     

Temporal dynamics of gastropod fauna on subtidal sandy sediments of the Ensenada de Baiona (NW Iberian Peninsula)

The temporal variation of the gastropod fauna inhabiting sandy sediments of the Ensenada de Baiona (Galicia, Spain) was studied at three subtidal sites from February 1996 to February 1997 by means of quantitative sampling. A total of 5,463 individuals representing 51 gastropod species and 22 families were found. The family Pyramidellidae was the most diverse in number of species (11 species), followed by Rissoidae and Trochidae (4 species each). The dogwhelk, Nassarius reticulatus, and the rissoid snail, Rissoa parva, were the numerically dominant species at the three studied sites; those and other abundant species showed their greatest densities by the end of summer and the beginning of autumn. In general, univariate measures of the assemblage (number of species, abundance, diversity and evenness) showed variations through time; greater values were recorded between summer and autumn depending on the site. Multivariate analyses done on abundance data showed certain seasonality in the evolution of the assemblage as expected for shallow subtidal sandy sediments at temperate latitudes; those seasonal changes were mostly related to variations in abundance of numerically dominant species. Although the measured sedimentary variables did not show significant correlations with faunal univariate parameters, sediment heterogeneity due to the presence of mats of Zostera marina L. and shells of dead bivalves might explain the differences in composition of the gastropod assemblage among sampling sites.     

Early Miocene reef- and mudflat-associated gastropods from Makran (SE-Iran)

A new gastropod fauna of Burdigalian (early Miocene) age is described from the Iranian part of Makran. The fauna comprises 19 species and represents three distinct assemblages from turbid water coral reef, shallow subtidal soft-bottom and mangrove-fringed mudflat environments in the northern Indian Ocean. Especially the reef-associated assemblage comprises largely new species. This is explained by the rare occurrence of reefs along the northern margin of the Miocene Indian Ocean and the low number of scientific studies dealing with the region. In terms of paleobiogeography, the fauna corresponds well to coeval faunas from the Pakistani Balochistan and Sindh provinces and the Indian Kathiawar, Kutch and Kerala provinces. During the early Miocene, these constituted a discrete biogeographic unit, the Western Indian Province, which documents the near complete biogeographic isolation from the Proto-Mediterranean Sea. Some mudflat taxa might represent examples of vicariance following the Tethys closure. The fauna also displays little connection with coeval faunas from Indonesia, documenting a strong provincialism within the Indo-West Pacific Region during early Miocene times. Neritopsis gedrosiana sp. nov., Calliostoma irerense sp. nov., Calliostoma mohtatae sp. nov. and Trivellona makranica sp. nov. are described as new species.     

A circa-decadal change in the gastropod fauna on a tidal flat in an island mangrove estuary

An investigation on gastropod fauna was carried out on a tidal flat in the Nagura Estuary on Ishigaki Island, the Ryukyu Islands in 1989 and 1998 using similar methods. 470-480 quadrats covering ca. 1900 m2 were surveyed during low tides from February to April in each year. Of the total 19 species recorded, the ranges of eight species had varied significantly between the two surveys, with six species expanding their range and two species contracting their range. Percentage in abundance of muddy-bottom species and tropical (<29 degrees N) species increased significantly between the two years. Topography of the flat also changed: the mouth of the river was narrowed and the elevated sections of the tidal flat expanded. During the period from 1984 to 1998, the farmland development around the study site caused influxes of soil into the estuary and the sea-water temperature was rising. These results suggest that the topographical changes due to soil influx and the rising temperature affected the gastropod assemblage at the study site, by increasing the abundance of muddy-bottom species and tropical species. The methodology used in this study, i.e. surface observation at low tides, includes more than 95% of the gastropod fauna, demonstrating the usefulness of surface counts for the study of soft-bottom fauna.