Electrophoretic heterogeneity within and between flat periwinkles (Mollusca: Gastropoda) along an intertidal transect at Ria Ferrol,northwest Spain

Using isoelectric focusing of esterases (EST), general proteins (GP) and myoglobin (Mb), we surveyed intra- and interspecific differentiation in flat periwinkles along a vertical intertidal transect in the Ensenada do Baño at Ria Ferrol, N.W. Spain. In this region, L. obtusata occurs in four algal belts, although it is rare in the lowest zone defined by Fucus serratus. L. fabalis is common in the F. serratus and F. vesiculosus belt, but is absent higher up on Ascophyllum nodosum and F. spiralis. Our data show that (1) EST and GP consistently differentiate between L. obtusata and L. fabalis, without however providing useful diagnostic markers, (2) L. fabalis is the less variable (heterozygous), but more heterogeneous species, (3) Mb patterns show significant heterogeneity in L. obtusata between the F. serratus zone and the other algal belts, but not in L. fabalis, and (4) the data on littorinid Mb appear inconsistent with a dimeric protein controlled by a single locus. Yet, assuming two loci coding for a monomeric (or dissociated dimeric) protein produces for the flat periwinkles a data set in which no significant deviations from Hardy-Weinberg expectations were detected. Nevertheless, this speculative interpretation fails to explain all littorinid Mb data. Hence the genetics and structure of littorinid Mb need further study.     

Heating properties of morphs of the mangrove snail Littoraria pallescens

The rate of heating and the temperature attained after 10 and 20 min have been examined for shells of the yellow, dark and orange morphs of the mangrove leaf snail Littoraria pallescens. Under the experimental conditions used, heating from 20 to 40C took 5–10 min and thereafter the temperature remained roughly constant for up to 20 min. This represents temperature conditions which the animals are likely to experience in the wild. Large shells heat more slowly than small ones and reach lower temperatures. At around 40C yellows are on average 1.5C cooler than the dark morph, and orange individuals lie between. This difference could account for the observed difference between morphs in their choice of preferred leaf surface. Besides differing in colour, yellows are thinner and less heavy than dark shells, which probably contributes to the fact that they arc less robust.     

Lack of metabolic temperature compensation in the intertidal gastropods,Littorina saxatilis (Olivi) and L. obtusata (L.)

Two intertidal snails, Littorina saxatilis (Olivi, 1972) (upper eulittoral fringe/maritime zone) and Littorina obtusata (Linnaeus, 1758) (lower eulittoral) were collected from a boulder shore on Nobska Point, Cape Cod, Massachusetts, in July and acclimated for 15–20 days at 4 ° or 21 °C. Oxygen consumption rate (Vo₂) was determined for 11–15 subsamples of individuals at 4 °, 11 ° and 21 °C with silver/platinum oxygen electrodes. Multiple factor analysis of variance (MFANOVA) of lo₁₀ transformed values of whole animal Vo₂ with log₁₀ dry tissue weight (DTW) as a covariant revealed that increased test temperature induced a significant increase in Vo₂ in both species (P<0.00001). In contrast, MFANOVA revealed that temperature acclimation did not affect Vo₂ in either L. saxatilis (P= 0.35) or L. obtusata (P= 0.095). Thus, neither species displayed a capacity for the typical metabolic temperature compensation marked by an increase in Vo₂ at any one test temperature in individuals acclimated to a lower temperature that is characteristic of most ectothermic animals. Lack of capacity for metabolic temperature acclimation has also been reported in other littorinid snail species, and may be characteristic of the group as a whole. Lack of capacity for respiratory temperature acclimation in these two species and other littorinids may reflect the extensive semi-diurnal temperature variation that they are exposed to in their eulittoral and eulittoral fringe/maritime zone habitats. In these habitats, any metabolic benefits derived from longer-term temperature compensation of metabolic rates are negated by extreme daily temperature fluctuations. Instead, littorinid species appear to have evolved mechanisms for immediate metabolic regulation which, in L. saxatilis and L. obtusata and other littorinids, appear to centre on a unique ability for near instantaneous suppression of metabolic rate and entrance into short-term metabolic diapause at temperatures above 20–35 °C, making typical seasonal respiratory compensation mechanisms characteristic of most ectotherms of little adaptive value to littorinid species.     

Patterns of genetic variability in Brazilian Littorinids (Mollusca): a macrogeographic approach

Macrogeographic studies are important for understanding gene flow patterns, and comparative data for related species with distinct bionomical traits may help to clarify the importance of such traits in natural populations. The aims of this study were to quantify the genetic variability and the populational structuring of three Brazilian littorinid species (Nodilittorina lineolata, Littoraria flava and L. angulifera) and to discuss the relationship between them, as well as each species’ mode of development and spatial distribution. We also investigated the species diversity in the ziczac complex. Isozyme analyses were done on 20 samples of N. lineolata, nine of L. flava and 10 of L. angulifera, collected along 4000 km of the Brazilian coast. Sixteen polymorphic loci were analysed in N. lineolata, 15 in L. angulifera and 17 in L. flava. All species showed high genetic variability. At sites where more than one species was present, there was a correlation among the values of gene diversity.The degree of interpopulational differentiation (N. lineolata, F_ST = 0.028; L. flava, F_ST = 0.054; L. angulifera, F_ST = 0.185) was coherent with the mode of larval development of each species. No linkage disequilibrium was found in N. lineolata. These findings, together with morphological evidence, corroborated the existence of only one species of the ziczac complex along the Brazilian coast.     

Influence of puffer predation on vertical distribution of mangrove littorinids in the Gulf of Nicoya, Costa Rica

The eastern Pacific puffer fish Sphoeroides rosenblatti (family Tetraodontidae) preys on the mangrove periwinkles Littorariafasciata and L. varia up to 13 cm above the water surface. Puffers capture snails by jumping partially out of the water and removing them by mouth from Rhizophora spp. prop roots. Field predation trials suggested puffer predation is intense, averaging 40.6% h⁻¹ on L. fasciata tethered at the water surface. L.fasciata <∼8 mm in shell length and L. varia <∼12 mm remained near or below the water surface and were easily crushed by puffers; gut content analyses showed that these snails were commonly eaten by puffers. L.fasciata >∼8 mm and L. varia∼12–16 mm were also crushed by puffers, but were generally located above their reach; these snails were much less abundant in puffer guts. L.varia >∼16 mm were abundant and some were submerged at high tide; however, none were found in puffer guts presumably because puffers encounter difficulty crushing these snails. S.rosenblatti is an important predator of L. fasciata and L. varia and appears to affect their vertical distributions. Received: 21 April 1998 / Accepted: 10 August 1998     

Spawning periodicity of three species of Nodilittorina in Hong Kong

The spawning periodicity of Nodilittorina trochoides, N. radiata and N. vidua from two rocky shores of different wave exposure, Big Wave Bay (more exposed site) and Cape d'Aguilar, in Hong Kong was monitored for one year (November 1994–November 1995). The breeding season was similar for the three species, mainly during the summer (June–September). The spawning duration was 9 months for N. vidua and 7 months for the other two littorinids. Spawning was initiated at different times for the two sites. At Big Wave Bay, spawning occurred between May–November for N. trochoides; March–September for N. radiata and March–November for N. vidua. At Cape d'Aguilar, the spawning period of N. trochoides and N. radiata was similar (April–October), whilst for N. vidua, spawning extended up to December. A decrease in monthly fecundity was recorded during July and August at Big Wave Bay and Cape d'Aguilar respectively. A weak relationship was found between the size of reproductive females and egg production. The seasonal fecundity of N. trochoides and N. vidua was similar at both sites and estimated as 10 000 and 7500 eggs per female respectively. Fecundity of N. radiata varied between sites, with 15 000 eggs and 9000 eggs per female being recorded from Cape d'Aguilar and Big Wave Bay respectively.     

Limits to gene flow in marine animals with planktonic larvae: models of Littorina species around Point Conception, California

Simulation models examined the process of gene flow in marine animals with planktonic larvae, and three factors that may influence it: ocean currents, planktonic period and spawning season. To focus on a realistic example, the models were based on measured ocean currents around Point Conception in southern California and the life histories of two intertidal gastropods, Littorina scutulata and L. plena . Results suggested that: (1) convergent ocean currents can create an effective barrier to gene flow that can be relaxed by temporal variation; (2) longer scales of temporal variation have a greater effect than shorter scales; (3) planktonic period has little effect above a minimum duration; and (4) an extended spawning season can eliminate gene flow barriers when currents vary seasonally. Failure of past studies to detect a phylogeographical boundary at Point Conception may be explained by extended spawning seasons and temporal variation at seasonal to millennial scales. These results fit a conceptual model of marine speciation in which short-lived, leaky barriers restrict gene flow, and divergence in a few genes may quickly produce reproductive isolation, resulting in cryptic sibling species.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 169–187.     

Lack of significant esterase and myoglobin differentiation in the periwinkle,Littorina striata (Gastropoda,Prosobranchia)

The relationship between gene flow and the maintenance of geographic or morphology-related variation in the polymorphic Macaronesian periwinkle, Littorina striata, was investigated by means of isoelectric focusing of esterases (EST) and myoglobin (Mb). This revealed that: (1) individual EST variation is very high, (2) there is no EST differentiation between sexes, shell morphotypes or wave-exposure regimes, (3) there is no clear macrogeographic patterning of EST variability, although both Cape Verde Islands have the highest, with exception of Flores, mean number of EST bands, and (4) there is no Mb variation, not even between islands separated by more than 2000 km. These results indicate that L. striata shows a high degree of genetic homogeneity among geographic populations and that the morphological patterning in this species persists in the presence of intense gene flow.     

Cold tolerance of Littorinidae from southern Africa: intertidal snails are not constrained to freeze tolerance

All intertidal gastropods for which cold tolerance strategies have been assessed have been shown to be freeze tolerant. Thus, freeze tolerance is considered an adaptation to the intertidal environment. We investigated the cold tolerance strategies of three species of subtropical and temperate snails (Gastropoda: Littorinidae) to determine whether this group is phylogenetically constrained to freeze tolerance. We exposed dry acclimated and wet rehydrated snails to low temperatures to determine temperature of crystallisation (Tc), lower lethal temperature and LT₅₀ and to examine the relationship between ice formation and mortality. Tc was lowest in dry Afrolittorina knysnaensis (–13.6±0.4 °C), followed by dry Echinolittorina natalensis (–10.9±0.2 °C) and wet A. knysnaensis (–10.2±0.2 °C). The Tc of both A. knysnaensis and E. natalensis increased with rehydration, whereas Tc of dry and wet Afrolittorina africana did not differ (–9.6±0.2 and –9.0±0.2 °C respectively). Wet snails of all species exhibited no or low survival of inoculative freezing, whereas dry individuals of A. knysnaensis could survive subzero temperatures above –8 °C when freezing was inoculated with ice. In the absence of external ice, Afrolittorina knysnaensis employs a freeze-avoidance strategy of cold tolerance, the first time this has been reported for an intertidal snail, indicating that there is no family-level phylogenetic constraint to freeze tolerance. Echinolittorina natalensis and A. africana both showed pre-freeze mortality and survival of some internal ice formation, but were not cold hardy in any strict sense.     

Genital divergence in sympatric sister snails

A pattern of greater divergence in mating traits between sister‐species pairs with overlapping ranges than between allopatric species pairs is expected if reinforcement commonly contributes to speciation. Few large‐scale comparative analyses have addressed this prediction, especially for genital form. Here, we show that penial morphology follows the predicted pattern in 40 robustly identified sister‐species pairs in the marine gastropod subfamily Littorininae. Further work is needed to exclude other processes that may contribute to genital divergence between sympatric species, but the clear pattern we observe strongly suggests a role for genital form in reproductive isolation in this large clade.