Neritina

Abstract: The Quaternary, mainly freshwater sediments of the Lower Mesopotamian plain include a thin transgressive marine unit which extends inland some 250 km from the present coastline, the Hammar Formation. The identity and respective areal extent of continental and transgressive units are based mainly on their molluscan faunas. Those faunas reflect the several environments of the Middle Eastern biogeographic province: fluviatile, lacustrine, estuarine and lagoonal. The ecological requirements of the more common subfossil and modern species of the malacofauna were studied during a month of field reconnaissance. The freshwater portion comprises only four bivalve species (Corbicula, Unio, Anodonta and Pseudodontopsis). The principal gastropods include Theodoxus (Neritaea), Melanopsis, Melanoides, Bellamya, Gyraulus, Lymnaea (Radix) spp. Corbicula characterizes both fluviatile channels and lacustrine environments; Melanopsis, Melanoides, Theodoxus and the Unionaceae prefer shallow, sparsely vegetated, freshwater lakes. The greatest diversity was seen in the slowly flowing water of the channels draining the marshes; small Planorbids are rarely abundant whereas viviparid and lymnaeids may accumulate near the margins of marshes due to floatation during floods. The brackish water faunas are characterized by Neritina (Dostia), the potamid Cerithidea (Cerithideopsilla), and Stenothyra. The new combination Theora mesopotamica (Annandale, 1918) is introduced here as the exact equivalent of Abra cadabra Eames et Wilkins, 1957. The abundance of these species is indicative of a restricted, low energy marine environment, confirmed by great numbers of the foraminiferan Ammonia gr. beccarii and the ostracod Cyprideis gr. torosa. Reworked Hammar Fm marine fossils, first recorded by the Annandale (1918) collection from a lacustrine deposit near Nasiriyah, have been observed in river, marsh and lake sediments too, usually in association with scarce foraminifera. The limited occurrence of these macrofossils (abundant in their life environment) indicates reworking.

Abstract: In the marine environment, competent larvae of sessile invertebrates are influenced by water flow and a variety of biological, chemical and physical cues. Most research has focused on how these biotic and abiotic factors influence where individual larvae ultimately settle. Much less is known about post-contact exploration prior to metamorphosis. This is, in part, due to limitations associated with directly observing small larvae (100 to 500 μm) in flowing seawater. A study was conducted in Beaufort, North Carolina, USA to understand how larvae of the barnacle Balanus amphitrite and the bryozoan Bugula neritina respond to a variety of flow rates (0, 1.3, 6.1 and 8.3 cm s -1 ) and surface types (clean, biofilmed, 1 and 2 wk fouled). Larval behavior was studied by means of endoscopy in a running-seawater chamber. Larval movements were observed at 30 frames s -1 for individuals that remained in contact with surfaces from <1.0 s to 44.5 min. Both flow rate and surface type significantly Influenced the behavior of the species examined, although larvae of B. amphitrite and B. neritina often responded very differently to the same treatment conditions. Larvae of B. amphitrite explored more surface area (fractal dimensions) in moving water than in still water, but flow did not influence the direction of travel. Mean exploration rate of B. amphitrite did not vary among treatments and ranged from 0.16 to 0.21 mm s -1 . More cyprid larvae explored surfaces with macrofouling and spent significantly longer times on these surfaces than on clean ones. In still water, larvae of B. neritina repeatedly contacted, explored and swam away from the test surfaces. In contrast, in flow, larvae of B. neritina never swam away from any surface after contact was made. Individuals of B. neritina crawled directly upstream on clean and biofilmed surfaces at all flow rates unless individuals encountered filamentous structures (biofilmed surfaces only). When this happened, larvae of B. neritina frequently remained attached to filaments as the filaments moved with the flow. These larvae were then either dislodged or immediately d crawling upstream upon contact with the plate surface. A limited number of larvae of both species settled during our observations (15% B. amphitrite, 18% B. neritina). Settlement of B. amphitrite was not correlated with flow rate or surface type; larvae of B. neritina settled only on 2 wk fouled surfaces.

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.