Neritidae

Abstract: 1. The streams of French Polynesia contain several species of Neritidae and Thiaridae (Mollusca: Gastropoda). The neritids are dioecious and amphidromous with a freshwater adult stage and a poorly known, marine larval stage. The thiarids are parthenogenetic and viviparous, and rely on passive dispersal for colonisation of new habitats. 2. Populations of the neritid Clithon spinosus and the thiarids Melanoides tuberculata and Thiara granifera were analysed using mitochondrial DNA sequences from COI to compare the population structure of the snails at three different scales: between streams (N = 9), between islands (N = 4), and between age and distance of paired islands. 3. The amphidromous C. spinosus showed no evidence of genetic isolation at any of the scales tested (F st values ≤ 0.02). Parsimony analyses resulted in two haplotype clusters separated by a three-step segment, which were not linked to geographic isolation. The larval phase of C. spinosus is most likely a long-lived planktotroph and a very effective disperser. 4. Two haplotypes of M. tuberculata, separated by 16 base pairs, were found. Both haplotypes were found in snails on all islands, and individuals representing both were often collected in the same habitat. One haplotype of T. granifera was found. M. tuberculata has the characteristics of the 'general-purpose genotype' of clonal population structure and although it relies on passive dispersal, it has colonised nearly all freshwater habitats on the islands.

Abstract: The freshwater stream fauna of tropical oceanic islands is dominated by amphidromous species, whose larvae are transported to the ocean and develop in the plankton before recruiting back to freshwater habitat as juveniles. Because stream habitat is relatively scarce and unstable on oceanic islands, this life history would seem to favor either the retention of larvae to their natal streams, or the ability to delay metamorphosis until new habitat is encountered. To distinguish between these hypotheses, we used population genetic methods to estimate larval dispersal among five South Pacific archipelagos in two amphidromous species of Neritid gastropod (Neritina canalis and Neripteron dilatatus). Sequence data from mitochondrial cytochrome oxidase I (COI) revealed that neither species is genetically structured throughout the Western Pacific, suggesting that their larvae have a pelagic larval duration (PLD) of at least 8 weeks, longer than many marine species. In addition, the two species have recently colonized isolated Central Pacific archipelagos in three independent events. Since colonization, there has been little or no gene flow between the Western and Central Pacific archipelagos in N. canalis, and high levels of gene flow across the same region in N. dilatatus. Both species show departures from neutrality and recent dates for colonization of the Central Pacific archipelagos, which is consistent with frequent extinction and recolonization of stream populations in this area. Similar results from other amphidromous species suggest that unstable freshwater habitats promote long-distance dispersal capabilities.

Abstract: Massive upstream migrations of neritid snails (Neritidae: Gastropoda) occur in tropical and subtropical streams worldwide, but their seasonality and proximate causes are unknown. We monitored massive upstream migrations of Neritina virginea for 99 weeks, and conducted a detailed study of snail density, size, and hydraulic descriptors in lower Rio Mameyes, northeastern Puerto Rico. The study assessed the 1) timing and seasonality of upstream migration, 2) size composition of migratory aggregations, 3) patterns of habitat use, and 4) role of floods on upstream migration. Massive upstream migrations (500-3000 ind/m 2 ) were observed in 44 of 99 weeks of observation. While N. virginea aggregations occurred at random time intervals, they were clumped during rainy periods. Migratory aggregations consisted mostly of small individuals (5-7 mm). Greater mean density was consistently observed in a stable riffle than in an unstable run (115.7 and 17.8 ind/m 2 , respectively), but mean density increased and mean size reduced in both reaches during the first 7 upstream migratory events. N. virginea density and size dynamics differed between reaches as a function of habitat hydraulics. While juveniles used the stable riffle as a permanent habitat and preferred passageway, they also used an adjacent, unstable reach after storm events. Density variation was correlated with days postflood (>3.5 m 3 /s) in both reaches. Our observations indicated that massive upstream migrations of N. virginea juveniles occur at least once a month, presumably as habitat-dependent responses to floods.

Abstract: . The endemic Hawaiian gastropod Smaragdia bryanae is a specialized marine herbivore that uses the endemic seagrass Halophila hawaiiana as both food and habitat. These small neritids, their grazing scars, and their egg capsules are found year-round on seagrass leaves, where they feed on protoplast contents released as the sharp outer-lateral teeth of the snail's radula puncture leaf epidermal cells; the contents of these cells are likely swept into the mouth by the long, wispy cusps of the marginal teeth. Structural differences from the typical neritid radula include elongated outer-lateral teeth with two sharply pointed cusps, delicate marginal teeth reduced in both size and number, and a compressed central section. Snails grazed on leaves of H. hawaiiana steadily in laboratory culture, and grew and reproduced on this diet. In laboratory choice experiments, snails did not graze the thalli of any of six macroalgal species growing near seagrass where snails were collected, and strongly preferred occupying seagrass. Seagrass samples from five field sites on Oahu and one on Maui showed from 30% to 94% of leaves damaged, with 11% of the total leaf standing area grazed. Snails are smaller (mean length 2.74±0.32 mm, mean width 2.15±0.17 mm, n=217) than the width of the leaves of H. hawaiiana (mean 3.24±1.26 mm, n=790). The snails associate constantly with their host, despite the scattered distribution, small patch size, and variability of the seagrass resource, demonstrated by a sevenfold range in the leaf area index (mean 1.11±0.61 cm2 blade surface cm−2, n=31) among samples. Damage on grazed leaves (mean 8.21±7.05 mm2 per leaf, or 16.5% of leaf surface, n=511) is concentrated in the apical and central epithelia between the midrib and the marginal veins, where snails may access cells with thinner walls and few fibers. Details of the grazing interaction between these extant species in Hawai'i shed light on the ecological specialization of members of the genus Smaragdia to seagrasses over geological time.