Habitat and zonation patterns of Littoraria species (Gastropoda: Littorinidae) in Indo-Pacific mangrove forests

The zonation of 15 species of Littoraria in the scabra group is described, using a transect method, at 13 mangrove localities in Australia, Thailand, Malaysia and Hawaii. At these localities the littorinids occurred only on trees, and showed both vertical and horizontal patterns of zonation, with broad overlaps between co-occurring species. It is suggested that vertical distribution is determined mainly by behavioural responses, while the landward limits of horizontal distribution may be set by physiological tolerance. Several species occurred predominantly on either leaf or bark substrates, but no consistent associations with tree species were found. On the basis of local distribution patterns, Littoraria species can be classified as continental or oceanic in character. Oceanic species may be less tolerant of sediment in the water or on the substrate.     

Natural selection and the distribution of shell colour morphs in three species of Littoraria (Gastropoda: Littorinidae) in Moreton Bay, Queensland

The distribution of yellow, brown and red morphs of sympatric species of Littoraria were recorded on mangrove trees of the genus Avicennia within Moreton Bay, Queensland. The roles of background mimicry (leaf vs. bark, dark vs. light), niche selection and thermal tolerance (sunny vs. shaded positions and height above ground) were examined. The yellow advantage found previously in the area was tested. Total yellow morph frequency adjusted to a reduction in leaf background on pruned trees. Morph frequencies in Littoraria species reflect differences in habitat use. L. filosa (high yellow frequency) was more frequently found on leaves at the highest tree levels, while L. luteola (high brown frequency) was more frequently found on branches at lower levels. It is therefore argued that morphs mimic background elements. Previously reported niche selection by yellow and brown morphs of leaf and bark backgrounds is shown to be a result of the distribution of L. luteola on branches and L. filosa on leaves. At warmer times of the year, yellow L. filosa were more common in sunny positions; this is thought to be a result of thermal tolerance. There appears to be some advantage to particular morphs on particular tree types, but this relationship needs to be examined further. Mangrove-dwelling Littoraria are a promising model to investigate molluscan polymorphism. In the past, erroneous identification of sympatric species may have influenced the accuracy of reported patterns. We used allozyme electrophoretic markers as a precise identification technique. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 219–232.     

Natural selection for apostasy and crypsis acting on the shell colour polymorphism of a mangrove snail, Littoraria filosa (Sowerby) (Gastropoda: Littorinidae)

Littoraria filosa (Sowerby) is a member of the L. scabra group, found amongst the foliage of mangrove trees in northern Australia. The colour of the shell is polymorphic, showing two discrete ground colours, either yellow or orange-pink, with a variable degree of superimposed brown patterning. At a site on Magnetic Island, northern Queensland, colour frequencies of small snails were similar on different backgrounds. Amongst larger shells yellows were more frequent on Avicennia trees with abundant foliage, and browns on relatively bare trees, suggesting that visual selection for crypsis occurred. There was no evidence of substrate selection by the morphs. Yellow shells were cooler than brown shells, but differences in colour frequencies on sunny and shaded trees, and at different seasons, did not suggest climatic selection. By manipulating the colour frequencies of subpopulations of small snails isolated on individual trees, it was shown that the disappearance of yellow and brown shells was frequency-dependent. This result is consistent with hypotheses of mimicry of background elements by the morphs and of apostatic selection by unknown predators. Only the latter can account for the persistence of the highly conspicuous pink morph at a low frequency.     

Vertical migration and spawning inNodilittorina exigua (Gastropoda: Littorinidae)

The relation between the vertical migration in summer and spawning of a littoral-fringe periwinkle,Nodilittorina exigua, was studied by field observations and laboratory experiments. The snails moved down soon after the first appearance of their planktonic eggs in the sea and stayed at low shore-levels until early August. It was shown that the migration increased the chance of immersion of the snails, and therefore of releasing eggs by the females. Females immersed every day in the laboratory released more eggs than those which experienced some days of exposure to air, during the same period. The results obtained suggest the reproductive advantage of immersion at low levels. Nevertheless, the snails moved upward before the end of the spawning season. Typhoon swells increasing in frequency through the summer seemed to be responsible for the upward migration.     

Species composition, comparative size and abundance of the genus Littoraria (Gastropoda: Littorinidae) from different mangrove strata along the East African coast

Variation in the abundance, distribution and size of four species of mangrove littorinid gastropods (genus Littoraria) was investigated using a nested sampling design at different spatial scales along the East African coast, from Tanzania to South Africa. Littorinid abundance and diversity decreased abruptly south of Inhaca Island at the southern end of the study area. All species presented a large-scale spatial variation in abundance, with L. subvittata showing the greatest abundance while L. intermedia was rare. Littoraria scabra and L. intermedia were found mainly at the seaward edge of the forests. Littoraria subvittata increased in abundance in the middle of the forest and towards the landward side. Littoraria pallescens occurred mainly at the seaward edge and in the middle areas in the Rhizophora zone. These small-scale variations show contrasting specific distribution patterns within the mangrove, likely reflecting different tolerances to physical factors and biological interactions. All species appeared decreased in shell height from north to south. Littoraria scabra was always significantly larger than other species at all mangrove study sites. Handling editor: P. Viaroli     

Paraspermatogenesis in Littoraria (Palustorina) articulata, with reference to other Littorinidae (Littorinoidea, Caenogastropoda)

Abstract. The ultrastructure of paraspermatogenesis is examined in the littorinid subfamily Littorininae, with special emphasis on Littoraria (Palustorina) articulata (PHILIPPI 1846). In particular the study focuses on the fate of the nucleus and origin of the rod bodies during parasperm development. Parasperm of the Littorininae are rounded or oblong cells, which undergo an abortive meiosis and eliminate part of the nucleus but often retain a nuclear remnant. The cytoplasm is filled with numerous spherical vesicles in all Littorininae, but in Littoraria (and in certain species of Nodilittorina, Tectarius and Cenchritis) dense 'rod-bodies' also occur. Littoraria (Palustorina) are unique in possessing a flagellum-like structure termed the 'pseudotrich", which lacks an axoneme but contains microtubules during its development. Paraspermatogonia differ from euspermatogonia in the structure of the nucleus and in the extensive rough endoplasmic reticulum (RER) and swollen cytoplasm. Two types of secretions develop in Littoraria : (1) numerous, spherical granules (composed of putative glycoprotein, also seen in other Littorininae) and (2) rhomboid granules (composition uncertain but reacting positively to RNA stains; these granules arising within RER cisternae close to the nucleus). As the rhomboid granules fuse to form the larger, rod-bodies (polygonal in cross section), the RER membrane enclosing the rod-bodies becomes confluent with the outer nuclear membrane, thereby forming a common compartment. Results of this study clearly show that the rod-bodies are secretions of the RER cisternae and not, as claimed in some light microscopic accounts, the product of fusion of eusperm nuclei which have entered the parasperm cytoplasm (either by active eusperm penetration or by phagocytosis). Developmental characteristics of littorinid parasperm show differences between species and may, in some cases, provide characters diagnostic of subgenera.     

Morphological variation in Lacuna parva (Gastropoda: Littorinidae) from different European populations

Shells of the littorinid gastropod Lacuna parva were compared from 23 European localities and postglacial deposits in Sweden. The shells from the recent and the postglacial populations are similar with the exception of the recent population from Ellekilde Hage, ?resund, Denmark. Shells from Ellekilde Hage are different in having especially well developed whorls and only one colour morph. Differences in life-cycle and radula morphometrics further distinguish the Ellekilde Hage population from populations from the Isle of Wight, UK, and Roscoff, France. No striking differences in penial morphology were observed between the populations. It is suggested that low salinity and subtidal occurrence might be the causative agents of the conchological differences exhibited by the ?resund population. Electronic Publication     

Intrasexual copulation and mate discrimination in a population of Nodilittorina radiata (Gastropoda: Littorinidae)

Intrasexual copulation and mate discrimination by Nodilittorina radiata (Gastropoda: Littorinidae) were studied on a concrete breakwater at Hakodate Bay, southern Hokkaido, Japan. Intrasexual (male–male) copulations were observed in 4.7–21.1% of copulating pairs on the shore. As females were relatively larger than males and males copulated with females larger than themselves, we hypothesized that males choose potential mates larger than themselves. However, two male mates showed no significant size preference in intrasexual copulations, suggesting that males do not choose relatively larger individuals as mates. In a laboratory mate-choice experiment, male N. radiata preferred to mate with females, indicating precopulatory sex identification. They copulated with males, however, at the frequency of 37%, perhaps because of sex misidentification.     

Field observations on the rhythmic up-and-down movement ofNodilittorina exigua (Gastropoda: Littorinidae)

Field observations on the activity pattern ofNodilittorina exigua were carried out in various tidal conditions in various seasons. The snails were stationary when dry on rock surface and began to move just after being splashed. On the rising tide they crawled upward aggregating in the awash zone but were not active around the time of high tide. On the receding tide they crawled predominantly downward, again in the awash zone. Position of the snails was high in spring tide and low in neap tide, changing in parallel to the change of high tide level. They were far more active in the awash condition than in the exposed or in the submerged conditions. For onset and termination of the movement, predominant influence of endogenous factors was suggested, except for onset of upward crawling. The distance of upward movement was larger when the high-tide level was higher, while that of downward movement was related to the time of high tide. Such movement patterns enable the snails to decrease the period of submergence and increase the period of their stay in the awash zone, and help them avoid long-term drying.     

Multiple paternity in Littorina obtusata (Gastropoda, Littorinidae) revealed by microsatellite analyses.

Parental identity for juvenile Littorina obtusata was determined from three egg masses by means of microsatellite DNA markers. Results confirm that the attendant adult female in each case was the dam of the offspring and that at least 4-6 males contributed to each brood. This correlates with our behavioral observations that indicated multiple copulations between the female and several males in each experimental aquarium. A significant number of offspring from each brood were sired by non-sampled males (males that had copulated with females before capture) whose sperm had been stored by the female. This is the first direct evidence of multiple paternity in the Littorinidae. Results are discussed in reference to current theories of sperm competition, male precedence, and cryptic female choice.     

Variation in radular teeth and acuspid side of the radula in Lacuna pallidula, L. parva and L. vincta (Gastropoda: Littorinidae) from the Isle of Wight, United Kingdom

The variation in the radula of three species of Lacuna has been investigated and the back of the rachidian tooth is proposed as providing a new character set of potentially high taxonomic value. The term basal plate is introduced for the back of the rachidian tooth. Cusp and tooth morphology are closely related to diet and wear, and are subject to considerable homoplasy, whereas the structure of the basal plate of the rachidian tooth provides a more neutral character set. The difference in this character set between the lacunids has been quantified using seven measurements and the exploratory multivariate statistical procedure principal component analysis. The basal plate of the rachidian tooth showed interspecific differences. The taxonomic value of this new character set should be evaluated in further studies of other prosobranchs. Received in revised form: 25 October 2000 Electronic Publication     

Inheritance of the background shell color in the snails Littorina obtusata (Gastropoda,Littorinidae)

We investigated in a gastropod mollusk Littorina obtusata (L. obtusata) the inheritance of back-ground shell coloration of the shell, which arises on the basis of three pigments: purple, orange, and yellow. We found that the hypothesis on polyallelic inheritance, as in the genus Cepaea, cannot explain the inheritance of shell color in periwinkles. We demonstrated that a separate genetic system is responsible for incorporation of each pigment into the shell. The composition of these genetic systems includes at least two genes each in the case of the yellow and purple pigments. Our analysis shows that caution should be applied when extending the results obtained in the studies of the Cepaea genus to the other species of gastropods.     

Foraging on sand particles: Coatings of soft radular teeth (Paludomidae,Gastropoda, Mollusca)

The radula is the ingesta-gathering structure in Mollusca. As interface, it has to perform various tasks without functional deterioration caused by wear. Wear prevention is well investigated in mollusks that forage on rocks and that generate high punctual pressure with their hard teeth, which contain high inorganic contents at their tips. In mollusks that forage on softer substrate, such as sand surfaces, and have relatively soft teeth, wear prevention has not been a focus of study before. Here, we studied the teeth of Limnotrochus thomsoni, which are used for raking algae from sand. For comparison, we investigated the soft outer teeth of Lavigeria grandis, which are used in gathering particles after the ingesta is loosed from the rock surface. SEM revealed scratches on all surfaces of the teeth and suggests that every tooth side interacts with abrasive particles during foraging. Analysis of stomach content revealed that sand particles of 10–20 μm diameter are ingested. By EDX/EDS, we studied the composition of the teeth and determined that high proportions of Ca are present on all surfaces, which could be an adaptation to reduce abrasion. Finally, we here present evidence for the existence of a canal within the teeth, which could potentially serve as delivery pathway of minerals during tooth maturation and has not been previously detected in Gastropoda.     

Seasonal aberrant radular formation in Thais bronni (Dunker) and T. clavigera (Küster) (Gastropoda : Muricidae)

Radulae of Thais bronni (Dunker) and T. clavigera (Küster) were examined at Mukaishima Island for a period of 2 yr, 1982 to 1984. Radulae of both species are similar in morphology, both having the basic pentacuspid rachidian plan. Sexual dimorphism of the radula was not observed, but rachidian tooth changes morphologically in different growth stages. Seasonal conditions affect the size and shape of the radula; in winter it is clearly malformed and strikingly thin. These aberrant parts of the radula comprised some dozens of rows, in which only several extremely thin rows exist. Results of experiments using T. clavigera under different water temperature conditions showed that the radula is rarely produced below 10 °C and that rate of radular production and replacement increases with increase in temperature. These results suggest that in the field the radula of these species is replaced entirely 2–2.5 times per year and 10–15 times during the life of the animal.     

Effects of environmental and biotic factors on the fluctuations of abundance of Littorina obtusata (Gastropoda: Littorinidae)

To explain periodic fluctuations in abundance of an intertidal flat periwinkle Littorina obtusata, we tested the potential impacts of several key environmental and biotic factors on the population density of this mollusc. The analysis showed that 79% of density variation was due to the changes of the recruitment of the periwinkles and likely caused by intraspecific competition for resources (algae Fucus vesiculosus) between the recruits and the older molluscs under the conditions of depleted habitat capacity. The cyclical changes in L. obtusata density result from the alternating periods of high and low recruitment. High recruitment is observed when the abundance of mature periwinkles is relatively low and enough resources are available to insure high survival of the recruits. The duration of high recruitment periods (approximately 3 years) is determined by the maturation time of the periwinkles. As the recruits mature, the density of adult periwinkles increases and the subsequent recruitment decreases. The duration of the low recruitment phases (an average of 3–4 years) appear linked to the longevity of mature L. obtusata. As the adults age and die out, their density declines and the release of resources leads to an increase of recruitment heralding the beginning of the next cycle.     

Phylogenetic and morphometric analyses reveal ecophenotypic plasticity in freshwater mussels Obovaria jacksoniana and Villosa arkansasensis (Bivalvia: Unionidae)

Freshwater mollusk shell morphology exhibits clinal variation along a stream continuum that has been termed the Law of Stream Distribution. We analyzed phylogenetic relationships and morphological similarity of two freshwater mussels (Bivalvia: Unionidae), Obovaria jacksoniana and Villosa arkansasensis, throughout their ranges. The objectives were to investigate phylogenetic structure and evolutionary divergence of O. jacksoniana and V. arkansasensis and morphological similarity between the two species. Our analyses were the first explicit tests of phenotypic plasticity in shell morphologies using a combination of genetics and morphometrics. We conducted phylogenetic analyses of mitochondrial DNA (1416 bp; two genes) and morphometric analyses for 135 individuals of O. jacksoniana and V. arkansasensis from 12 streams. We examined correlations among genetic, morphological, and spatial distances using Mantel tests. Molecular phylogenetic analyses revealed a monophyletic relationship between O. jacksoniana and V. arkansasensis. Within this O. jacksoniana/V. arkansasensis complex, five distinct clades corresponding to drainage patterns showed high genetic divergence. Morphometric analysis revealed relative differences in shell morphologies between the two currently recognized species. We conclude that morphological differences between the two species are caused by ecophenotypic plasticity. A series of Mantel tests showed regional and local genetic isolation by distance. We observed clear positive correlations between morphological and geographic distances within a single drainage. We did not observe correlations between genetic and morphological distances. Phylogenetic analyses suggest O. jacksoniana and V. arkansasensis are synonomous and most closely related to a clade composed of O. retusa, O. subrotunda, and O. unicolor. Therefore, the synonomous O. jacksoniana and V. arkansasensis should be recognized as Obovaria arkansasensis (Lea 1862) n. comb. Phylogenetic analyses also showed relative genetic isolation among drainages, suggesting no current gene flow. Further investigation of in‐progress speciation and/or cryptic species within O. arkansasensis is warranted followed by appropriate revision of conservation management designations.     

Genetic differentiation in the molluscan species Littorina rudis and Littorina arcana (Prosobranchia: Littorinidae)

Six populations of Littorina rudis and ihree of L. arcana were screened for elcctrophorctically dctci table variation at 21 enzyme loci. These species reproduce by ovoviviparity and oviparily respectively, and both are members of the L. saxatilis species complex. The mean coefficient of genetic identity between the two species was high (0.957) and no diagnostic loci were observed. Allele frequency differences are maintained in sympatric populations of the two species. Intraspecific values of/ ranged from 0.944 to 0.995 and both species showed marked inter-population genetic heterogeneity, a feature consistent with their limited powers of dispersal. Heterozygosity is high in all populations of both species. Populations of I. rudis have higher mean heterozygosities (= 0.153) then L. arcane ( H = 0.132).     

Responses of Littoraria irrorata say (Mollusca: Gastropoda) to water-borne chemicals: a comparison of chemical sources and orientation mechanisms

The salt marsh periwinkle, Littoraria irrorata, exhibits orientation behaviors in response to a number of chemical stimuli. In this study, we quantified the response of L. irrorata to chemical stimuli derived from three sources: Spartina alterniflora (a food source), the blue crab Callinectes sapidus (a predator), and crushed conspecifics. Animals consistently moved toward sources of S. alterniflora extract and crushed conspecifics relative to controls, but did not approach or avoid extracts from C. sapidus relative to controls. Animals had lower response rates to stimuli from S. alterniflora than to controls, but not to putatively aversive chemical stimuli (C. sapidus or crushed conspecifics). However, periwinkles that moved in experimental groups and control groups did not differ in how far they crawled. In addition, we measured turning angles of pathways of periwinkles to determine the orientation mechanism used. Littoraria irrorata displayed a taxis in response to crushed conspecifics, but a kinesis in response to C. sapidus and S. alterniflora. Our results suggest that the ecological context of chemical cues influence the evolution of orientation mechanisms. Cues that require immediate response (crushed conspecifics) elicit directed movement using taxis, whereas those that represent less alarming stimuli (a blue crab in the vicinity) or that represent a ubiquitous resource (Spartina) elicit movement by kinesis.