Neritid Snails Research Articles

A draft genome of the neritid snail Theodoxus fluviatilis.

The neritid snail Theodoxus fluviatilis is found across habitats differing in salinity, from shallow waters along the coast of the Baltic Sea to lakes throughout Europe. Living close to the water surface makes this species vulnerable to changes in salinity in their natural habitat, and the lack of a free-swimming larval stage limits this species' dispersal. Together, these factors have resulted in a patchy distribution of quite isolated populations differing in their salinity tolerances. In preparation for investigating the mechanisms underlying the physiological differences in osmoregulation between populations that cannot be explained solely by phenotypic plasticity, we present here an annotated draft genome assembly for T. fluviatilis, generated using PacBio long reads, Illumina short reads, and transcriptomic data. While the total assembly size (1045 kb) is similar to those of related species, it remains highly fragmented (N scaffolds = 35,695; N50 = 74 kb) though moderately high in complete gene content (BUSCO single copy complete: 74.3%, duplicate: 2.6%, fragmented: 10.6%, missing: 12.5% using metazoa n = 954). Nevertheless, we were able to generate gene annotations of 21,220 protein-coding genes (BUSCO single copy complete: 65.1%, duplicate: 16.7%, fragmented: 9.1%, missing: 9.1% using metazoa n = 954). Not only will this genome facilitate comparative evolutionary studies across Gastropoda, as this is the first genome assembly for the basal snail family Neritidae, it will also greatly facilitate the study of salinity tolerance in this species. Additionally, we discuss the challenges of working with a species where high molecular weight DNA isolation is very difficult.

Differences in the expression of soluble proteins in freshwater and brackish‐water ecotypes of the snail Theodoxus fluviatilis

The neritid snail Theodoxus fluviatilis has formed regional subgroups in northern Europe, where it appears in both freshwater (FW) and brackish water (BW) in coastal areas of the Baltic Sea. These ecotypes show clear differences in osmotolerance and in the modes of accumulating organic osmolytes under hyperosmotic stress. We reasoned that the expression patterns of soluble proteins in the two ecotypes may differ as well. BW snails have to deal with a higher salinity (up to 20‰) than FW snails (0.5‰) and also cope with frequent fluctuations in environmental salinity that occur after heavy rains or evaporation caused by extended periods of intense sunshine. Therefore, the protein expression patterns of specimens collected at five different FW and BW sites were analyzed using 2D SDS-PAGE, mass spectrometry, and sequence comparisons based on a transcriptome database for Theodoxus fluviatilis. We identified 89 differentially expressed proteins. The differences in the expression between FW and BW snails may be due to phenotypic plasticity, but may also be determined by local genetic adaptations. Among the differentially expressed proteins, 19 proteins seem to be of special interest as they may be involved in mediating the higher tolerance of BW animals towards environmental change compared with FW animals.

A neritid gastropod name “<i>Theodoxus dniestroviensis </i>Put’, 1972” is a junior subjective synonym of <i>Th. fluviatilis </i>(L., 1758): decision based on the topotypic specimens study

The nominal species name “ Theodoxus dniestroviensis ” was commonly accepted as introduced by Put’ in 1972 for neritid snails occurring in the upper part of the Dniester River basin. Currently an earlier publication [Put’, 1957] has been found in which this name is established in accordance with the ICZN guidelines, so the dating should be amended to Put’, 1957.The type series of Theodoxus dniestroviensis is apparently lost but the samples from the type locality were recently traced in the National Museum of Natural History of the National Academy of Sciences of Ukraine (Kiev). An illustrated comparison of topotypic specimens with other morphotypes of regional river nerites is provided. Given the variation in conchological characteristics observed among the specimens of Th. fluviatilis , we consider Th. dniestroviensis to range within its intraspecific variability. This makes Th. dniestroviensis a junior subjective synonym of Theodoxus fluviatilis (Linnaeus, 1758).

Expression levels and activities of energy-yielding ATPases in the oligohaline neritid snail Theodoxus fluviatilis under changing environmental salinities.

ABSTRACTThe aquatic gastropod Theodoxus fluviatilis occurs in Europe and adjacent areas of Asia. The snail species has formed two genetically closely related subgroups, the freshwater ecotype (FW) and the brackish water ecotype (BW). Other than individuals of the FW ecotype, those of the BW ecotype survive in salinities of up to 28‰. Coastal aquatic ecosystems may be affected by climate change due to salinization. Thus, we investigated how the two Theodoxus ecotypes adjust to changes in environmental salinity, focusing on the question whether Na+/K+-ATPase or V-ATPase are regulated on the transcriptional, the translational or at the activity level under changing external salinities. Animals were gradually adjusted to extreme salinities in containers under long-day conditions and constant temperature. Whole body RNA- or protein extracts were prepared. Semi-quantitative PCR- and western blot-analyses did not reveal major changes in transcript or protein abundances for the two transporters under low or high salinity conditions. No significant changes in ATPase activities in whole body extracts of animals adjusted to high or low salinity conditions were detected. We conclude that constitutive expression of ATPases is sufficient to support osmotic and ion regulation in this species under changing salinities given the high level of tolerance with respect to changes in body fluid volume.

The neritid snails of Brunei Darussalam: their geographical, ecological and conservation significance

Neritid snails are diverse and conspicuous in tropical coastal environments. They can serve as indicators of environmental change and can provide conservation information. In the present review of the neritid species of Brunei Darussalam, we report sixteen species, including seven new records from estuarine, mangrove and rocky-shore habitats. These records update distributions across the Central Indo-Pacific realm, specifically the Palawan/North Borneo ecoregion. Under-sampling, species misidentifications and locally-rare species undermine the accuracy of records in previous studies for the region. Three of the rocky-shore and two of the mangrove species collected here are represented by single specimens; these rocky-shore species are possible colonizers from neighbouring regions and the scarcity of the mangrove species likely refers to under-sampling. We present novel shell characteristics that readily distinguish between Neripteron violaceum and Nerip. cornucopia. We describe the ecology and habitat use of the Brunei species and discuss conservation issues.

The mitochondrial genome and phylogenetic position of a marine snail Nerita (Heminerita) japonica (Gastropoda: Neritimorpha: Neritidae)

The mitochondrial genome of the neritid snail Nerita (Heminerita) japonica (Mollusca: Neritimorpha) from Kumamoto, Japan was determined by whole-genome sequencing. This mitogenome is comprised of 13 protein-coding genes, 2 ribosomal RNA (12S and 16S) genes, and 22 transfer RNA genes, with the same gene order as in the other species of the family Neritidae. A likelihood-based phylogenetic reconstruction recovered the subgenus Heminerita (including N. japonica as its type and N. yoldii from China) as monophyletic and sister to a clade with four species of the subgenera Nerita and Theliostyla.

Contributions of biogeographical functions to species accumulation may change over time in refugial regions

AbstractAimElevated biodiversity is the result of the cradle, museum or sink functions. The contributions of these three functions to species accumulation and their changes through time remain unknown for glacial refugia. Additionally, our understanding of the role these functions played during pre‐glacial periods is limited. We test for changes in contributions of functions through time leading to the current diversity patterns using a model refugium and taxon.LocationAnatolia, Western Palaearctic.TaxonFreshwater neritid snails (genusTheodoxus).MethodsAssessments were made to define molecular operational taxonomic units (MOTUs) forTheodoxusand reaffirm the genus as a suitable model taxon with elevated interspecific diversity in noted glacial refugia. Thereafter, we constructed a time‐calibrated multilocus Bayesian phylogeny of mtDNAandnDNAby using both fossil data and published substitution rates. Ancestral area estimation was performed on the phylogeny to define the contribution of the functions through time.ResultsAccumulation ofTheodoxusdiversity in Anatolia over the Miocene–Pliocene transition is attributed to the museum function, but its contribution was small as only few divergence events occurred. The cradle function dominated during the Pliocene and Early Pleistocene, when most interspecific diversity built up and extant lineages in Anatolia were established. The sink function acted from the Middle Pleistocene to present‐day, but with only a small contribution to the total extant Anatolian interspecific diversity.Main conclusionOur results do not entirely mitigate the role glacial cycles played in species accumulation, but highlight Ice Ages may have been less effective in forcing temperate aquatic interspecific diversity into more opportune areas. The elevated diversity in refugia may rather be the result of earlier in situ diversification. Elevated interspecific diversity attributed to the legacy of glacially forced retreats may need to be re‐evaluated in cases where refugia have long and complex geological histories such as Anatolia. These results highlight the importance of considering species accumulation through a temporal perspective to adequately explain present‐day biodiversity patterns.

Operculum shape variation in Theodoxus Montfort, 1810 (Gastropoda: Neritidae).

The operculum morphology of neritid snails of genus Theodoxus Montfort, 1810 yields important species-specific taxonomic characters. This study is the first attempt to describe morphological differences in snails based on operculum shapes using a geometric morphometric approach. We examined the variability of opercular shapes between sexes and among populations and species based on 91 opercula of adult specimens belonging to six populations of three species of Theodoxus from the central Balkan and the southern border of the Pannonian plain. There are no sex-related differences in operculum shape and size in the studied species. The presence of shape variations influenced by size (allometry) was confirmed for T. danubialis (C. Pfeiffer, 1828) indicating that allometry could be a component of the morphological variation of this species. At the intraspecific level, phenotypic plasticity of operculum shape was found in T. danubialis and T. fluviatilis (Linnaeus, 1758). Of particular interest is the lack of morphological variability in the rare and endangered T. transversalis (C. Pfeiffer, 1828). At the species level a clear morphological distinction of T. fluviatilis from T. danubialis and T. transversalis was detected, confirming that the operculum with a rib could be used as a species-specific morphological character. The main morphological differences between species are an outwardly-stretched opercular/apophysal rib and a shortened tip of the opercular plate in T. fluviatilis, compared to the same structures in T. danubialis and T. transversalis.

The trade-off between digestibility and phenol content influences the food choice of the obligate seagrass-feeding neritid snail Smaragdia souverbiana

Grazing influences the structure and function of seagrass meadows. Like terrestrial plants, seagrasses have evolved nutritional, structural and chemical mechanisms to either tolerate or resist grazing. Phenols are common secondary metabolites that have multiple roles, including grazing deterrence, and their content in seagrass tissues can vary both among species and within a single species in response to biotic and abiotic processes. Such variability influences the grazing behaviour of species that consume seagrass, but few studies have addressed mesograzers that consume or damage seagrass, such as the nerite Smaragdia souverbiana. We assessed the effect of environmentally- mediated differences in phenol concentration on these seagrass mesograzers by testing: ( 1) if phenol content is different between sites and seagrass species ( Zostera muelleri vs Halophila ovalis) and ( 2) if S. souverbiana inflicts different amounts of damage on different seagrasses from different sites, according to their phenol content. Phenol content was consistently lower in Z. muelleri than H. ovalis, and concentrations were about 60% lower in both species at one of two sites in eastern Moreton Bay, Queensland. These differences corresponded to different foraging patterns of S. souverbiana, with the snails generally choosing the seagrass species with the lowest phenol content. These results suggest that S. souverbiana chooses among locally available seagrasses based on a trade- off between phenol content and digestibility ( related to cell size). This work demonstrates how subtle local processes can create complex changes in seagrass- grazer interactions with potential ecological consequences at local and regional scales.

Hatching plasticity in the tropical gastropod Nerita scabricosta

AbstractHatching plasticity has been documented in diverse terrestrial and freshwater taxa, but in few marine invertebrates. Anecdotal observations over the last 80 years have suggested that intertidal neritid snails may produce encapsulated embryos able to significantly delay hatching. The cause for delays and the cues that trigger hatching are unknown, but temperature, salinity, and wave action have been suggested to play a role. We followed individual egg capsules of Nerita scabricosta in 16 tide pools to document the variation in natural time to hatching and to determine if large delays in hatching occur in the field. Hatching occurred after about 30 d and varied significantly among tide pools in the field. Average time to hatching in each pool was not correlated with presence of potential predators, temperature, salinity, or pool size. We also compared hatching time between egg capsules in the field to those kept in the laboratory at a constant temperature in motionless water, and to those kept in the laboratory with sudden daily water motion and temperature changes. There was no significant difference in the hatching rate between the two laboratory treatments, but capsules took, on average, twice as long to hatch in the laboratory as in the field. Observations of developing embryos showed that embryos in the field develop slowly and continuously until hatching, but embryos in the laboratory reach the hatching stage during the first month of development and remain in stasis after that. Instances of hatching plasticity in benthic marine invertebrates, like the one in N. scabricosta, could greatly enhance our ability to investigate the costs and benefits of benthic versus planktonic development, a long‐standing area of interest for invertebrate larval biologists.

Drilling predation on neritid egg capsules by the muricid snail Reishia clavigera

The egg capsules of neritid snails (Gastropoda: Neritimorpha) are made of a tough inner organic layer and an outer calcareous layer for further reinforcement. Such hard capsules are less susceptible to mechanical damage compared with soft or thin capsules of other gastropods, and there have been only a few reports of predation on neritid eggs. Here, we describe drilling predation on the reinforced egg capsule of Nerita japonica by Reishia clavigera (Caenogastropoda: Muricidae) on a high intertidal rocky shore in central Japan. The muricid makes a small, elliptical drill hole in the capsule lid by the use of both an accessory boring organ (ABO) and a radula, as it does in mollusc and barnacle shells. The etching around the hole by the acid secretion from the ABO, which can be observed under a dissecting microscope and by scanning electron microscopy, should provide an effective way to identify muricid predation on neritid eggs, probably not only by R. clavigera but also by other species in the warm temperate to tropical regions all over the world. Reishia clavigera made drill holes in c. 14% of the capsules, making it possibly the main cause of mortality for neritid eggs at the study site. However, this mortality rate is lower than those previously reported for caenogastropod eggs with softer capsules. All penetrated capsules of N. japonica were white (rather than brownish) and newly laid, suggesting that their acid-resistant organic layer had yet to harden at the time of radular rasping. The thick organic layer and calcareous coating are an effective armour, not only against the usual type of grazing but also against more sophisticated predation involving the chemical and mechanical boring of Muricidae.

Coalescent and biophysical models of stepping‐stone gene flow in neritid snails

Marine species in the Indo-Pacific have ranges that can span thousands of kilometres, yet studies increasingly suggest that mean larval dispersal distances are less than historically assumed. Gene flow across these ranges must therefore rely to some extent on larval dispersal among intermediate 'stepping-stone' populations in combination with long-distance dispersal far beyond the mean of the dispersal kernel. We evaluate the strength of stepping-stone dynamics by employing a spatially explicit biophysical model of larval dispersal in the tropical Pacific to construct hypotheses for dispersal pathways. We evaluate these hypotheses with coalescent models of gene flow among high-island archipelagos in four neritid gastropod species. Two of the species live in the marine intertidal, while the other two are amphidromous, living in fresh water but retaining pelagic dispersal. Dispersal pathways predicted by the biophysical model were strongly favoured in 16 of 18 tests against alternate hypotheses. In regions where connectivity among high-island archipelagos was predicted as direct, there was no difference in gene flow between marine and amphidromous species. In regions where connectivity was predicted through stepping-stone atolls only accessible to marine species, gene flow estimates between high-island archipelagos were significantly higher in marine species. Moreover, one of the marine species showed a significant pattern of isolation by distance consistent with stepping-stone dynamics. While our results support stepping-stone dynamics in Indo-Pacific species, we also see evidence for nonequilibrium processes such as range expansions or rare long-distance dispersal events. This study couples population genetic and biophysical models to help to shed light on larval dispersal pathways.

Dispersal and upstream migration of an amphidromous neritid snail: implications for restoring migratory pathways in tropical streams

Summary1. The amphidromous life cycle of several species of neritid snails, shrimp and gobies throughout the tropics includes juveniles that migrate from the ocean to breed in fresh water. In many Hawaiian streams, the decline of Neritina granosa, an endemic gastropod, has been associated with habitat degradation and water withdrawal, which are common factors affecting tropical rivers around the world.2. We investigated the effects of water withdrawal and density on dispersal and upstream migration of N. granosa using three experimental treatments: (i) reduced flow (RF) owing to a stream diversion, (ii) natural flow (NF) and (iii) natural flow with artificially increased snail density. For each treatment, snails were differentially tagged and released in a stream without a natural, extant population of N. granosa.3. Capture rates ranged from 17 to 65% over a 63‐day period following release. Captures on 2–6 days after release measured initial dispersal and migration, whereas longer‐term migration rates were calculated from snails captured 16–63 days after release. Snails under NF displayed positive rheotactic behaviour, with only 3–12% demonstrating initial downstream movement. Under RF, 22–77% of snails moved downstream or showed no bias either way.4. Initial mean upstream migration rate (UMR) was 0.25, 0.66 and 1.16 m day−1 under RF, NF and natural flow with increased snail density, respectively. Longer‐term migration rates did not differ significantly between treatments, and the overall mean was 0.62 m day−1.5. Principal component analysis and generalised linear models were used to identify habitat characteristics important to UMR, with habitat and reach‐scale hydraulics as the most important factors. The relationship between discharge and UMR suggested it would take 11–35 years for snails to migrate past the most upstream water diversion. However, rates from published studies of neritid snail species migrating en masse or in long lines under natural situations suggested that N. granosa could migrate above stream diversions within 72 days–2.5 years (when in an aggregation) and 29 days–1.1 years (when following in long lines of individuals).6. An understanding of upstream neritid snail migration can be used for the management and conservation of this and other migratory species in tropical streams.

Predation on hardest molluscan eggs by confamilial snails (Neritidae) and its potential significance in egg-laying site selection

Neritid snails (Gastropoda: Neritimorpha) protect their eggs in a hard capsule, of tough conchiolin, reinforced by mineral particles derived from the faeces and stored in a special sac near the anus and oviduct opening. Predation on this arguably hardest of molluscan egg capsule is described and illustrated here; neritids of the freshwater to brackish-water genera Clithon and Vittina, generally classified as herbivores, feed facultatively on the eggs of various confamilial species after breaking the reinforced capsule lid by means of prolonged radular rasping. Intensive predation pressure by these common inhabitants in Indo-West Pacific coastal streams may have given rise to the remarkable egg-laying behaviour of Neritina on the shells of other living snails. Our laboratory examination showed that Neritina species deposited clusters of egg capsules more frequently on the living shell than on other substrates, and that the predation rate was significantly lower on this moving ‘nursery’. Predation rate was even lower on the small egg capsules of Clithon and Vittina themselves, which were deposited one by one in the depressions on the rough surfaces of stones.

Exploiting Macrofauna Diadromy for Assessing Anthropogenic Impact in American Samoa Streams1

Stream biomonitoring is increasingly used to identify and monitor changes in water quality, stream habitat, and even the surrounding watershed. An effective biomonitoring protocol must comprise attributes able to discriminate human-caused changes from natural variation. We attempted to identify such attributes for streams of American Samoa, which, in turn, might also have widespread applicability to other oceanic islands. Owing to the diadromous nature of the macrofauna, we assessed species richness, diversity, composition, dominance, and biomass of freshwater fishes, crustaceans, and mollusks in 50 m sections in midreaches of five streams with and five streams without anthropogenic influences at the estuarine reach. We electrofished for fishes and crustaceans, and we picked mollusks from stream substrates. We discovered that two species of neritid snails of the pan-Pacific genus Clithon were significantly more abundant in the midreach of streams undisturbed by human impacts at the estuarine reach, making them potentially useful bioindicators throughout the South Pacific.

Hierarchical contribution of river–ocean connectivity, water chemistry, hydraulics, and substrate to the distribution of diadromous snails in Puerto Rican streams

Diadromous faunas dominate most tropical coastal streams and rivers, but the factors controlling their distribution are not well understood. Our study documents abiotic variables controlling the distribution and abundance of the diadromous snail Neritina virginea (Gastropoda:Neritidae) in the Caribbean island of Puerto Rico. An intensive survey of N. virginea density and shell size, and channel substrate, velocity, and depth was conducted at microhabitat, habitat, and reach scales of a coastal plain reach of the Río Mameyes between August and December 2000. In addition, the inland extent of distribution (stream-network scale) and presence (regional scale) of N. virginea were surveyed in 32 coastal rivers around the island during summer 2001 and 2003. At the microhabitat scale, snail density and microhabitat electivity were greater in patches consisting of a mix of boulders and cobbles than in other types of substrate. At the habitat scale, snail density increased with depth. At the reach scale, snail density increased with fast and turbulent flows (riffle > pools > pond), whereas snail size showed the opposite pattern. At the regional scale, populations were present in 13 of 32 streams. Populations of N. virginea were not found in rivers that were disconnected from the ocean for most of the year because of channel dewatering, formation of sediment bars at their mouths, and low mean monthly discharge (Q = 0.69 m3/s). In contrast, rivers with N. virginea populations had a permanent (Q = 4.04 m3/s) or seasonal (Q = 2.88 m3/s) connection to the ocean over the year. At the regional scale, the inland distribution of populations was not correlated with stream gradient, but was negatively correlated with concentrations of SiO2, P, and acid neutralizing capacity of the water. Populations colonized montane reaches in only 5 rivers, all of which were forested and protected. Our study highlights the importance of taking a hierarchical approach in managing tropical coastal rivers, and the usefulness of neritid snails as biological indicators of the physical and chemical integrity of rivers.

Paraspermatogenesis of Cerithioidean snails: Retention of an acrosome and nuclear remnant

Paraspermatozoa of Cerithioidea are oligopyrenic with a central nuclear remnant surrounded by glycoprotein bodies and an anterior acrosome complex. Posteriorly, the parasperm have one long and several small flagella. Biosynthesis of proteins begins in the rough endoplasmic reticulum (RER), early in paraspermatogenesis, prior to the degradation of the nucleus. The breakdown of the nucleus has features characteristic of apoptosis but a nuclear remnant remains that is composed of intact DNA. The acrosome complex of parasperm is Golgi-derived, forming posteriorly and migrating anteriorly along the plasma membrane to the apex of the nuclear remnant as the paraspermatid matures. This mechanism of acrosome formation is similar to that in euspermiogenesis in neomenioid aplacophorans and neritid snails and is plesiomorphic to mollusks. However, eusperm acrosomes of Cerithioidea form differently. In the paraspermatid, small, dense granules secreted by the Golgi body fuse to form larger granular vesicles, many of which are released by exocytosis into the seminal fluid. These granular vesicles stain red with acridine orange at pH 7 under 489-nm light, indicating that they are lysosomes. The retention of a nuclear remnant and development of an acrosome complex in the parasperm of cerithioideans, as well as the secretion of lysosomes into the seminal fluid, suggests novel functions for these unusual germ cells.

Stream crossings and the conservation of diadromous invertebrates in South Pacific island streams

Abstract Most non‐insect invertebrates that occur in streams on tropical Pacific islands are diadromous; they live as juveniles and adults in streams but have larvae that migrate to the ocean to complete their development before returning to fresh water. The type of crossing used in the construction of roads to traverse small streams can impede upstream migration and, consequently, colonization of diadromous fauna above the stream crossing. A stream in the Opunohu Valley, Moorea, French Polynesia, had the same diadromous fauna of atyid shrimps, palaemonid prawns, and neritid snails occurring above and below an 8 m wide, 10 m long, bottomless culvert (i.e. an open‐bottom bridge) built to flow over natural substrates. However, no diadromous species were found upstream of two 1 m diameter, 8 m long concrete pipes used for a culverted stream crossing. The increased shear stress in the pipe and subsequent downstream erosion of the stream bed probably inhibited upstream migration. Modifications to culverted stream crossings, which are widely used on tropical islands, can reduce migration barriers and prevent upstream loss of diadromous stream fauna. Copyright © 2004 John Wiley & Sons, Ltd.

Intestinal Ultrastructure of Nerita picea (Mollusca: Gastropoda), an Intertidal Marine Snail of Hawaii

AbstractThe neritid snail Nerita picea is a marine prosobranch mollusc which resides high in the intertidal zone on the Hawaiian Islands. Since other studies have shown considerable variations in molluscan gut histology and the relatively few recent ultrastructural reports have revealed novel cellular structures in the molluscan gastrointestinal tract, this investigation was directed toward ultrastructural clarification of the neritid intestine.Seven principal cell types constituted the intestinal architecture, including absorptive cells, zymogen cells, neural and endocrine cells, myocytes, pigment and gland cells. The intestinal epithelium was composed mainly of tall ciliated (9 plus 2 complement of microtubules) columnar absorptive cells which also possessed microvilli, extensive deposits of non‐membrane‐bound lipid‐like droplets, and large reservoirs of glycogen‐like granules. Less frequent, columnar zymogen cells contained numerous large zymogen secretory granules and possessed microvilli but not cilia. Small endocrine‐like cells with secretory granules were observed basolaterally between some absorptive cells, resembling mammalian gut endocrine cells. Nerve fibers were prevalent in close association with the epithelial cells. A thin layer of non‐striated muscle was present, as well as a serosally located gland composed of storage cells with a granular matrix and large granules.

Life history features of some macroinvertebrates in a French Polynesian stream

The invertebrate fauna of the Opunohu River catchment in Moorea, French Polynesia, studied 1988–1989, was numerically dominated by four species of neritid snails, one species of thiarid snail, two species of atyid shrimp, two species of palaemonid shrimp, and ten species of coenagrionid, chironomid, and simuliid insect larvae. Numbers of egg capsules of neritid snails were higher in riffles (maximum x density > 10,000/m2 surface area) than pools, were highest in riffles in September and pools in August, and were lowest in both habitats in March and December. The snail Neritina canalis Lamarck and the shrimp Atyoida pilipes Newport, Caridina weberi DeMan, Macrobrachium lar Fabricius, and Macrobrachium australe Guerin‐Menévílle are diadromous: they enter freshwater as juveniles and migrate upstream; their larval offspring return to the ocean for growth and development. Ovigerous females of A. pilipes and M. lar were mainly found in austral fall; ovigerous females of C. weberi and M. australe were found in both austral fall and spring. Ovigerous females were the largest individuals of each shrimp species collected. Mean size of N. canalis, A. pilipes, C. weberi, and M. lar was generally larger in upstream than downstream reaches; no such difference was found for the non‐diadromous thiarid snail Melanoides tuberculata Müller. Coenagrionid, chironomid, and simuliid insect larvae have multiple cohort populations but at least one simuliid species occurs as a single cohort. Downstream habitat alteration may strongly influence the distribution of diadromous species.