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Marine biogeographers have long recognized broad east‐west differentiation of marine communities across southern Australia, but few studies have explicitly assessed the site of disjunction or the factors potentially underlying this biodiversity. A recent, detailed phylogeographical and distributional study of the dispersive gastropod genus Nerita revealed an abrupt shift in species abundance across mainland Australia, apparently correlated with the site of an historical vicariant barrier: the Bassian Isthmus. In the current study I provide an independent assessment of this vicariant hypothesis by morphologically analysing over 3000 intertidal Nerita specimens from eight coastal sites around Tasmania. Consistent with the Bassian Isthmus hypothesis, the study reveals a dramatic east‐west disjunction across north-east Tasmania. A survey of marine biogeographical literature reveals several additional examples supporting the east‐west species turnover detected in Nerita . I discuss the role of contemporary oceanographic factors in preserving the biogeographical signature of marine vicariance, even in highly dispersive taxa. Based on recent marine connectivity data, the east‐west disjunction in Nerita taxa can be interpreted as an historic vicariant pattern perpetuated by contemporary oceanographic conditions. The results of this study emphasize the potential importance of considering relative abundance data ‐ rather than just species range data ‐ in marine biogeographical analyses. As the observed disjunction is likely to have broad implications for Australia’s marine biodiversity, it is imperative that conservation biologists incorporate such data in the design of marine protected areas.

Marine species with ranges that span the Indo-Australian Archipelago (IAA) exhibit a range of phylogeographical patterns, most of which are interpreted in the context of vicariance between Indian and Pacific Ocean populations during Pliocene and Pleistocene low sea-level stands. However, patterns often vary among ecologically similar taxa, sometimes even within genera. This study compares phylogeographical patterns in two species of highly dispersive neritid gastropod, Nerita albicilla and Nerita plicata, with nearly sympatric ranges that span the Indo-Pacific. Mitochondrial COI sequences from >1000 individuals from 97 sites reveal similar phylogenies in both species (two divergent clades differing by 3.2% and 2.3%, for N. albicilla and N. plicata, respectively). However, despite ecological similarity and congeneric status, the two species exhibit phylogeographical discordance. N. albicilla has maintained reciprocal monophyly of Indian and Pacific Ocean populations, while N. plicata is panmictic between oceans, but displays a genetic cline in the Central Pacific. Although this difference might be explained by qualitatively different demographic histories, parameter estimates from three coalescent models indicate that both species have high levels of gene flow between demes (2Nem>75), and share a common history of population expansion that is likely associated with cyclical flooding of continental shelves and island lagoons following low sea-level stands. Results indicate that ecologically similar, codistributed species may respond very differently to shared environmental processes, suggesting that relatively minor differences in traits such as pelagic larval duration or microhabitat association may profoundly impact phylogeographical structure.

Aim In this study, I examined the relative contributions of geography and ecology to species diversification within the genus Nerita, a prominent clade of marine snails that is widely distributed across the tropics and intertidal habitats. Specifically, I tested whether geographical patterns of speciation correspond primarily to allopatric or sympatric models, and whether habitat transitions have played a major role in species diversification. Location Indo-West Pacific, eastern Pacific, Atlantic, tropical marine intertidal. Methods I used a previously reconstructed molecular phylogeny of Nerita as a framework to assess the relative importance of geographical and ecological factors in species diversification. To evaluate whether recently diverged clades exhibit patterns consistent with allopatric or sympatric speciation, I mapped the geographical distribution of each species onto the species-level phylogeny, and examined the relationship between range overlap and time since divergence using age‐range correlation analyses. To determine the relative contribution of habitat transitions to divergence, I traced shifts in intertidal substrate affinity and vertical zonation across the phylogeny using parsimony, and implemented randomization tests to evaluate the resulting patterns of ecological change. Results Within the majority of Nerita clades examined, age‐range correlation analysis yielded a low intercept and a positive slope, similar to that expected under allopatric speciation. Approximately 75% of sister species pairs have maintained allopatric distributions; whereas more distantly related sister taxa often exhibited complete or nearly complete geographical overlap. In contrast, only 19% of sister species occupy distinct habitats. For both substrate and zonation, habitat transitions failed to concentrate towards either the tips or the root of the phylogeny. Instead, habitat shifts have occurred throughout the history of Nerita, with a general transition from the lower and mid-littoral towards the upper and supra-littoral zones, and multiple independent shifts from hard (rock) to softer substrates (mangrove, mud and sand). Main conclusions Both geography and ecology appear to have influenced diversification in Nerita, but to different extents. Geography seems to play a principal role, with allopatric speciation driving the majority of Nerita divergences. Habitat transitions appear insignificant in shaping the early and recent history of speciation, and promoting successive diversification in Nerita; however, shifts may have been important for respective divergences (i.e. those that correspond to the transitions) and enhancing diversity throughout the clade.

Rates of radular activity were measured in four species of Australian gastropods. The species were Austrocochlea constricta, Bembicium nanum, Cellana tramoserica, and Nerita atramentosa. Work was done in Botany Bay at Cape Banks, NSW, Australia from September 1989 to June 1990. A hydrophone was used to record radular raspings of snails in situ. Averages based on all observations showed Cellana had the fastest rate with 80 rasps/min, and was followed by Austrocochlea (71 rasps/min), Bembicium (57), and Nerita (39). Faster rates were associated with warmer water temperature and smaller body sizes in all species except Bembicium. The potential for these differences in radular activity affecting competitive interactions is discussed.

The archaeogastropod Nerita scabricosta is an abundant and extremely mobile herbivore which occurs from above mean high water through the mid—intertidal zone of exposed rocky areas along the Pacific coast of Panama. It is the principal prey of Purpura pansa, a patchily distributed, high intertidal neogastropod. Both snails exhibit activity patterns and habitat preference which appear to have evolved in response to heavy predation pressure by fish and to harsh physical factors. These behavioral mechanisms restrict movement and limit foraging time for both species, but also result in a cyclic overlap of their distributions. Potential interaction between the two gastropods is thus limited to well defined, discrete periods. Purpura elicits both an escape and an avoidance response in Nerita. The intensity of predation on the nerite by Purpura is lessened by these responses and Purpura appears unable directly to control the abundance of Nerita. However, it is experimentally shown that in a heterogeneous environment, Nerita's distribution is still more restricted and its local abundance further reduced by avoidance responses to temporally stable aggregations of Purpura. These aggregations, centered on specific fish—safe microhabitat, are thus hypothesized to result in the creation of local patches of intertidal substrate which are free from herbivory or disturbance effects by Nerita scabricosta. Evidence from field observations and manipulations is presented to support the hypothesis that in this complex, four trophic level community, a predator—prey interaction between two gastropods can limit potential resource monopolization and affect primary space occupancy.

The complex reproductive systems of the mangrove-dwelling Nerita birmanica are described and related to those of other neritids. Gonad condition in both sexes and numbers of spermatophores in males and females varied slightly during a one-year survey. It is concluded that a slight tendency for larger number of “full” ovaries with big oocytes to occur in collections taken shortly before full and new moon does not represent a significant lunar cycle of gonad activity since almost all gonads bore gametes in all stages of development at all times of year. It appears merely that ovulation may be held up during relatively rainless periods after which substantial rain is followed by reduced ovaries lacking large oocytes. In brief, breeding appears to be continous with but slight variation which does not consistently follow observable environmental factors.

The little capsules in which snails of this family deposit eggs are so durable as sometimes to be found adherent to museum specimens; hitherto, however, detailed descriptions of such capsules have been restricted to the capsules long known as made by Neritina fluviatilis in fresh waters of Europe. The present paper adds descriptions, with measurements and sketches, of Neritina reclivata from Florida, Neritina virginea, Neritina punctulata, Theodoxus mneleagris, Nerita peloronta, Nerita alticola, Nerita tesselata and Neritilia succinea from Jamaica, British West Indies. These capsules are found to be remarkable in that the digestive tract aids in their formation; that is, the capsules are made as secretions of the lower oviduct to which mineral particles are added, and these mineral particles are supplied from the intestine and stored up in a special sac opening into the oviduct. Moreover, the mineral particles are of twofold origin; in some Neritinas they come from the ooze taken in with the food and passed through the intestine; in some Neritas they come from the liver as calcospherites, to be passed through the intestine. It is hoped that the data given will aid future descriptions of capsules from other species till a firm foundation is built for utilization of these capsules as aids in taxonomy.

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The reduced right half of the pallial complex of Nerita tessellata, N. fulgurans, N. versicolor, N. peloronta and Neritina virginea comprises a hypobranchial gland, a vestigial gill and the opening of a duct which bears the same relationship to this gill as that of the left kidney to the functional gill. The duct leads from the region of the genital duct corresponding to the renal section of mesogastropods. The posterior part of the glandular genital duct lies in the visceral mass; its relationships are identical with those of the left kidney. The anterior part develops alongside the rectum within the anterior pallial vein. The left half of the pallial complex of Nerita includes a hypobranchial gland absent in Neritina and Theodoxus. The floor of the mantle cavity of Nerita forms an accessory respiratory surface, receiving blood from the anterior aorta. Its blood spaces communicate with the haemocoel and the parabranchial vein. This vein, also present in Neritina and Theodoxus, drains the mantle skirt. In Nerita its importance increases when the snail is out of water and the mantle cavity is used as a lung. The buccal mass of Nerita fulgurans has the same basic organization as that of Monodonta lineata: differences are related to specializations in the neritid radula requiring greater versatility of movement during feeding. It is protruded by lateral and ventral protractor and buccal depressor muscles originating on the body wall and inserted on the posterior radular cartilages. The horns of the anterior cartilages are orientated to the feeding ground by an array of levator muscles unparalleled in the trochid, and by the buccal fold which also spreads the radula and ensures its maximal sweep. The radular membrane is pulled outward by protractor muscles originating on the posterior cartilages and associated with a cartilage in the posterior wall of the sublingual pouch. Food is gathered on the back stroke of the radula which is drawn into the buccal cavity by a powerful retractor muscle associated with a radular diverticulum. The diverticulum is absent in trochids which have muscles for direct retraction of the buccal mass. The absence of salivary glands in neritaceans may be related to the development of the buccal fold; glandular buccal pouches and sublingual glands may compensate for their loss. The oesophageal glands extend through the anterior oesophagus to the buccal cavity; owing to their separation from the mid-oesophagus they are unaffected by torsion. The stomach is a voluminous sac, simplified in that the caecum is lost or vestigial and that the major typhosole ends at the junction of the style sac with the proximal region. The gastric shield forms a tooth, very prominent in Neritina, which can protect the openings of the oesophagus and ducts of the digestive gland.