Patterns of growth and energy allocation in northern California populations of Littorina (Gastropoda: Prosobranchia)

Seasonal patterns in shell growth rates, tissue weights, and female reproductive efforts were monitored for three species of western North American rocky shore gastropods, Littorina keenae Rosewater (supralittoral), L. scutulata Gould (high intertidal), and L. plena Gould (high intertidal). Growth rates conformed to the Von Bertalanffy growth model. Relative to male snails, female L. keenae appeared to sacrifice shell growth to expend more energy on the production of large quantities of gametes during the spring; growth rates of females and large males were highest during autumn when microalgal food supplies were seasonally abundant. In contrast, spring tended to be the best season for shell growth of L. scutulata and L. plena; however, reproductive output was poor and sporadic for these two species, and growth rates in the field during all seasons were low compared to growth rates of snails held under laboratory conditions with abundant food. High population densities of snails at shore levels occupied by L. scutulata and L. plena may greatly reduce food levels upon which individual snails rely for growth and reproduction.     

The influence of season and the tidal cycle on division of labour by the radula during feeding in the estuarine brooding gastropod <Emphasis Type="Italic">Crepipatella dilatata</Emphasis> (Calyptraeidae)

The brooding gastropod Crepipatella dilatata can feed by scraping the substrate with the radula and by suspension-feeding, which also requires use of the radula. There is a “division of labour” for the radula among three discrete tasks associated with feeding: (1) removing mucous balls from the food pouch; (2) transferring the mucous cord from the neck channel to the mouth (both components of suspension-feeding); (3) scraping the substrate. We hypothesised that the proportion of time used for each feeding activity varies according to environmental conditions. Total radular activity in females was greatest at high tide and in summer. The rate of radular extrusion for ingesting the mucous cord varied seasonally and between brooding and non-brooding females. Non-brooding females exhibited higher rates of radular extrusion for ingesting the mucous cord and for scraping the substrate than did brooders. In females, radular activity in removing the mucous ball from the food pouch was strongly influenced by the tidal cycle during winter, reaching minimum values at low tide. Differences were recorded in substrate scraping among seasons and within tidal cycles, and among males, brooding females and non-brooding females. Brooding females displayed less rasping than non-brooders, since the area available for grazing was restricted by the egg mass. Throughout the year, including low salinity periods, males allocated a greater proportion of total radular activity to rasping than to removing the mucous ball or ingesting the mucous cord. The feeding behaviour of both males and females is modulated by salinity, but the principal determinants of radular activity are the mode of reproduction (brooding in females) and, in males, motility.     

Trail following behaviour in relation to pedal mucus production in the intertidal gastropod Monodonta labio (Linnaeus)

Trail following behaviour and pedal mucus production were investigated in the mid-shore topshell, Monodonta labio (Linnaeus) in Hong Kong. On the shore, individuals exhibited both conspecific and self trail following while awash on ebb and flood tides, although fidelity to resting sites during emersion on successive days was low. In the laboratory, animals that encountered trails that had been aged on the shore for different periods showed similar patterns of movement (distance moved, speed and tortuosity) suggesting that degradation of cues in the mucus that animals responded to did not occur until > 3 days post-deposition. Animals moved faster, with a lower rate of radular rasping, on freshly laid mucus trails than on a biofilm-covered substratum and did not change their speed when moving over aged (biofilm-covered) mucus compared to fresh mucus. Mucus production rates were similar when animals were crawling on vertical or horizontal surfaces, but significantly more mucus was produced when animals were emersed than when submerged. Mucus trail profiles were of variable thickness, but ‘double’ mucus trails (marker + tracker trails) did not contain significantly more mucus than ‘single’ trails (marker mucus only) and were considerably thinner than single trails suggesting tracker snails utilized mucus laid by marker snails, reducing their own deposition of mucus. Thus, while M. labio do not appear to utilize trails for orientation or refuge location, snails that follow trails have the potential to save energy through reducing mucus production or to gain energy through mucus ingestion. Given the role of pedal mucus production in the overall energy balance of gastropods, such energetic benefits are considerable and may have implications for the life history of the snail.     

Leech predation on juvenile freshwater snails: effects of size,species and substrate

Summary The leechGlossiphonia complanata does not appear to have substantial impact on snail populations, but this may be due to most studies focusing on adult snails rather than juvenile snails. In this study I investigated how predation rates ofG. complanata feeding on newly-hatched and juvenile snails was affected by snail species, snail size, snail density and substrate, in a laboratory experiment. Number of snails eaten increased with increasing density resulting in a type II functional response curve. Predation rates were higher when leeches were feeding onLymnaea emarginata than onPhysa gyrina, whereas there was no significant difference in predation rates when they were feeding onL. emarginata andHelisoma anceps. Sandy substrates and greater snail size resulted in decreased predation rates. Sand reduced movement speed ofG. complanata, which probably reduced encounter rates. Thus, there was a comparatively large effect of leech predation on newly-hatched snails, due to a high probability of encounter and high predation rates, but spatial and temporal refuges probably reduce the importance of leech predation as a structuring force in freshwater snail assemblages.     

Dispersal of the estuarine gastropod Pyrazus ebeninus is only weakly influenced by pneumatophore density

Studies on rocky intertidal gastropods indicate habitat complexity and body size to be major determinants of dispersal patterns. Considerations of effects of habitat complexity and body size on soft sediment gastropods are, however, less common. In neither habitat has the interaction between habitat complexity and body size been considered despite the increasing recognition in the general ecological literature that complexity effects are body-size-dependent. We tested independent and interacting effects of habitat complexity and body size on movement of the mud-whelk, Pyrazus ebeninus, by marking large (61-85 mm) and small (31-55 mm) snails in sites with low and high densities of pneumatophores and determining the distance and direction of their dispersal over periods of 1 week, 2 weeks and 1 month. Contrary to our expectation, we found no effect of pneumatophore density on the distance of snail migration over each of the temporal scales; net distance travelled by snails was determined only by body size and idiosynchratic, site-specific factors. The direction of snail movement was, by contrast, influenced on some temporal scales by both pneumatophore density and snail size. Over 1 week, site effects dominated patterns of movement and neither size of snail nor density of pneumatophore produced statistically significant effects. As the temporal scale increased, effects of size and pneumatophore density became increasingly apparent. Over the 1-month period, large snails at all sites and small snails at sites with high pneumatophore density migrated down the shore, while small snails at sites with low pneumatophore displayed non-directional movement. Thus, overall this study provides only weak support for effects of pneumatophore density on snail movement. In combination with other studies, our results suggest that, in comparison to on rocky shores where habitat complexity has strong effects on the distribution, abundance and behaviour of gastropods in soft-sediment systems habitat complexity is a less important structuring agent.     

Studies on energy transformation in the freshwater snail Pila globosa

The effects of eleven chosen feeding levels ranging from 0 to 198 mg damp dry (plant) Ceratophyllum/g live snail/day on the absorption, conversion and metabolism of the snail Pila globosa (of 1 -9 g body weight) have been studied. Absorption rates increased from 3-0 to 21-0 mg dry food/g live snail/day in snails fed 3-4-28-8 mg dry food/g live snail/day. In these snails, absorption efficiency decreased from 87.5 to 73.0%. Conversion rates increased from 0-3 mg/g/day for snails receiving 23-4 mg/g/day to 2-7 mg/g/day for those fed maximum amounts, and the efficiency (K₂) also increased from 1-9% to 130%. When compared to other gastropods, Pila globosa appears to be a poor convertor. During 30 days' starvation, the test individuals lost 4.4 mg dry body substance/g/day i.e. the maintenance cost was 14-7 cal/g live snail/day. The SDA increased by four times for those feeding on maximum rations in comparison to those receiving about 5 mg/g/day, i.e. the energy cost for converting food was increased four times.     

Effects of trematode infection on metabolism and activity in a freshwater snail, Semisulcospira libertina.

Changes in the metabolism and activity of the freshwater snail Semisulcospira libertina infected with larval trematodes were studied experimentally. In snails up to 11 mm in shell width, crawling distance, feeding frequency, and the proportion of individuals located on vertical walls did not differ among snails infected with mature or immature cercariae, or uninfected snails (p > 0.05). In snails larger than 11 mm, individuals infected with mature cercariae tended to feed more frequently during the light period (p = 0.0081), but the distance they crawled and the proportion of individuals located on vertical walls did not differ, regardless of infection (p > 0.05). Infection with mature cercariae significantly increased the oxygen consumption rate (p = 0.016), which was measured only in the large size.     

Potential Impacts of a Western Pacific Grapsid Crab on Intertidal Communities of the Northwestern Atlantic Ocean

Population density and size distribution, salinity tolerance, and feeding activity were examined in a western Pacific grapsid crab, Hemigrapsus sanguineus, that was recently introduced to the mid-Atlantic coast of North America. Seasonal abundance on a boulder/cobble shore (Crane Neck Pt.) in central Long Island Sound, New York, USA, during 1997–1998, ranged from 7 to 10 crabs m⁻² averaged over the entire intertidal zone. Crabs occurred throughout the intertidal during summer and fall, but appeared to move from high to low elevations during winter. In laboratory experiments, H. sanguineus tolerated salinity down to 10 ppt for 7 d, but showed significant preference for 20 or 27 ppt over 10 ppt. The crabs readily consumed juvenile snails (Littorina littorea) and mussels (Mytilus edulis), as well as other common species of macroalgae and invertebrates occurring at Crane Neck Pt. High feeding rates and the ability to consume littorine snails up to 13 mm in height and mussels up to 20 mm in length suggest that this nonindigenous species has the potential to significantly affect the structure of rocky intertidal communities in the northwestern Atlantic Ocean; however, rigorous field studies are needed to accurately determine the impact of this recent introduction. This revised version was published online in July 2006 with corrections to the Cover Date.     

FEEDING PATTERNS OF FISSURELLA SPECIES ON ISLA DE MARGARITA, VENEZUELA: USE OF RADULAE AND FOOD PASSAGE RATES

Structural differences and functional wear of the radula inthree species of the gastropod Fissurella from Isla de Margarita,Venezuela, were examined using light and electron microscopy.Wear patterns indicate between 6 and 9 transverse rows of teethare comonly used during feeding. Mechanical wear was most noticeableon the cusps of the outer lateral tooth; this wear varied fromrounding (F. nimbosa) to blunting (F. barbadensis) to squaring(F. nodosa) of the cusps. Morphological changes were additionallycharacterized by a significant decrease in the cusp length ofmarginal cusps in F. nodosa and breakage of the central toothand inner lateral teeth in F. barbadensis. Interspecific differencesin wear patterns suggest that the rhipidoglossate radula maybe used differently by congeneric Fissurella. Despite considerable variation, rasping rates while feedingon the same substrate were comparable among species; however,food passage rates through the digestive system differed amongspecies studied. Fissurella barbadensis requires 12 hours topass its food through the digestive tract, taking almost twiceas much time as F. nodosa and F. nimbosa. These data highlightdifferences in the feeding ecology of Fissurella species andcorrelate well with individual activity patterns and grazinghabits. ^*Present address: La Salle University, Department of Biology,20th Street at Olney Avenue, Philadelphia, PA 19141, USA. (Received 4 October 1988; accepted 16 February 1989)     

The effects of water availability on the life history of the desert snail,Trochoidea seetzeni

Summary In the Negev desert of Israel, the pulmonate land snailTrochoidea seetzeni is active, grows and reproduces in the month following the torrential winter rains. Thereafter, these snails estivate until the following year's rains. By experimental supplementation of water in the field, we examined the ability of these snails to alter their life histories. Specifically, we measured changes in feeding activity, overall activity levels, movement patterns and reproductive output. We found that snails on artificially-watered plots stopped feeding on higher plants and fed on soil-surface algae growing as a result of increased water availability. Most snails moved onto the watered plots, and reduced the total amount of movement once there, in response to increased food availability. Snails on the watered plots grew more and remained active for a longer period into the summer months than did snails on control plots. However, we found little evidence for the ability of these snails to alter their reproductive outputs in response to increased water (and hence food) availability. There was also no carry-over effect of high water availability in one season enhancing reproduction in the following season. We postulate that the low level of phenotypic plasticity recorded here is a consequence of an adaptive conservative strategy to maintain a high reproductive output under extremely unpredictable desert conditions.     

Phenotypic homogeneity of two intertidal snails across a wave exposure gradient in South Australia

Dislodgement by the large drag forces imparted by breaking waves is an important cause of mortality for intertidal snails. The risk of drag-induced dislodgement can be reduced with: (1) a smaller shell of lower maximum projected surface area (MPSA); (2) a streamlined shell shape characterized by a squatter shell; and/or (3) greater adhesive strength attained through a larger foot area or increased foot tenacity. Snails on exposed coasts tend to express traits that increase dislodgement resistance. Such habitat-specific differences could result from direct selection against poorly adapted phenotypes on exposed shores but may reflect gastropod adaptation to high wave action achieved through phenotypic plasticity or genetic polymorphism. With this in mind, we examined the size, shape and adhesive strength of populations of two gastropod species, Austrocochlea constricta (Lamarck) and Nerita atramentosa (Reeve), from two adjacent shores representing extremes in wave exposure. Over a 5 day period, maximum wave forces were more than 10 times greater on the exposed than sheltered shore. Size-frequency distributions indicate that a predator consuming snails within the 1.3-1.8 cm length range regulates sheltered shore populations of both snail species. Although morphological scaling considerations suggest that drag forces should not place physical limits on the size of these gastropods, exposed shore populations of both snails were small relative to the maximum size documented for these species. Therefore, selective forces at the exposed site might favour smaller individuals with increased access to microhabitat refuges. Unexpectedly, however, neither snail species exhibited between-shore differences in shape, foot area or foot tenacity, which are likely to have adaptive explanations. Hence, it is possible that these snails are incapable of adaptive developmental responses to high wave action. Instead, the homogeneous and wave-exposed nature of Australia's southern coastline may have favoured the evolution of generalist strategies in these species.     

Non-lethal effects of predators on prey growth rates depend on prey density and nutrient additions

A number of studies show that predators can depress prey growth rates by inducing reductions in foraging activity, but the size of this non-lethal effect is quite variable. Here I investigate how prey density and resource productivity may alter the extent to which predators depress the growth rates of their prey. Theory predicts that when resources are overgrazed, an increase in predation risk will have little net effect on individual food intake because the decline in foraging effort will be offset by an increase in resource level. Thus, the non-lethal effects of predators on prey growth rates should depend upon prey density and resource productivity in a predictable manner, with the growth penalty imposed by predators being strongest when resources are undergrazed and weakest when resources are overgrazed. I tested this hypothesis by manipulating predation risk, prey density, and nutrient additions in a mesocosm experiment with the pulmonate snail Helisoma trivolvis . Refuge use by snails was 45% higher in the presence of caged crayfish than in their absence. Snail growth rates were reduced, on average, by 24% in the presence of caged crayfish. However, the magnitude of the growth penalty exacted by crayfish depended on snail density and nutrient additions. When snails were stocked at high density and nutrient additions were low, growth suppression was just 2.6%. At the other extreme, when snails were at low density and nutrient additions were high, growth suppression was 44.6%. Thus, the non-lethal effects of predators on prey growth depend on environmental context, illustrating an important link between individual traits and system-level properties.     

Relative vulnerability of six freshwater gastropods to the leech Nephelopsis obscura (Verrill)

SUMMARY. 1. The vulnerability of six species of freshwater snails to the leech Nephelopsis obscura was determined in laboratory predation experiments.
2. Nephelopsis was unable to prey on prosobranch snails with an operculum, but did consume certain pulmonate species though predation rates were low, ranging from one to two snails per leech per night at 20°C. Apparently, Nephelopsis does not actively select prey, but merely consumes those species most easily handled.
3. Among pulmonates, leeches captured species in the following order: Physa gyrina (Say), Helisoma anceps (Menke). Lymnaea emarginata (Say) and Helisoma trivolvis (Say). Susceptibihty of size classes varied among snail species.
4. Neither environmental structure, such as cobble or macrophytes, nor gastropod escape behaviors, such as clamping to substrates or vertical migration away from bottom-feeding leeches, lowered capture rates by Nephelopsis.
5. In 2 years sampling, only one of seven lakes, however, had appreciable abundances of Nephelopsis. It is therefore suggested that the rarity, low feeding rates, and lack of strong gastropod preference limit the effect of Nephelopsis on field gastropod distributions.     

Periphytic food and predatory crayfish: relative roles in determining snail distribution

Summary In the laboratory and field, we examined how periphyton (food of snails) and predatory crayfish influenced snail distribution in Trout Lake, a permanent, northern Wisconsin lake. Laboratory experiments (with no crayfish) tested the importance of periphyton biomass in determining snail preference among rocks, and among rock, sand, and macrophyte substrates. Among rocks with four different amounts of periphyton, periphyton biomass and the number of Lymnaea emarginata, Physa spp., and Amnicola spp. were positively related. A similar, but non-significant, trend occurred for Helisoma anceps. A field experiment at a site in Trout Lake where predation risk was low confirmed the preference by snails for periphyton covered rocks; more snails colonized rocks with periphyton than rocks without. When given a choice of rock, sand, and macrophytes in the laboratory, L. emarginata preferred high periphyton biomass and rock. Laboratory and field results contrasted with the distribution of snails in Trout Lake; no snails occurred in areas with abundant periphyton-covered rocks, but snails were abundant nearby on scattered rocks with little periphyton. However, where snails were absent, crayfish were abundant (14.5 crayfish-trap^–1-day^–1), and where snails were abundant, crayfish were rare (3.2 crayfish-trap^–1-day^–1), suggesting that crayfish predation reduced snails. The hypothesis that the negative association between snail and periphyton biomass resulted from snail grazing was supported by the results of a field snail enclosure-exclosure experiment (1 m² cages; n=3). All experiments and observations therefore suggest that: 1) crayfish predation is more important than a preference for high periphyton biomass in determining snail distribution in Trout Lake; 2) periphyton biomass is negtively related to snail grazing; and 3) crayfish had a positive indirect effect on periphyton by preying on grazing snails.     

不同施药方式对福建烟蚜抗药性及活性酶的影响

本文采用连续施用和轮换施用农药方式对福建烟区烟蚜进行抗药性及其体内酶变化的测定,研究结果表明,在连续施用3次、5次、7次和9次后,施用灭多威后的烟蚜抗性分别增长了1.397倍、2.608倍、4.891倍和7.598倍;施用吡虫啉则分别增长了1.717倍、2.114倍、2.861倍和4.169倍;施用高效氯氟氰菊酯则分别...     

The maintenance of shore-level size gradients in an intertidal snail (Littorina sitkana)

Summary The size of many intertidal animals varies with tidal height. These size gradients could be produced by growth or survival varying with tidal height, or by animals moving to a preferred tidal level. The body size of the snail, Littorina sitkana, increases steadily with tidal height in rocky high intertidal habitats of British Columbia. To determine how size gradients were maintained in L. sitkana, I quantified how growth, survival, and snail movement varied with tidal height. I studied populations of L. sitkana found on sheltered pebble beach and exposed basaltic shelf habitats. Mark-recapture studies and experimental transplants showed that growth could not have produced the size gradients because snail growth rates in both habitats were as fast or faster at low tidal levels (where the snails were the smallest) than at high tidal levels. However, survival rates were lowest at low tidal levels. On pebble beaches, this was due to size selective predation on large snails by the pile perch, Rhacochilus vacca. On basaltic shelves, heavy wave action at low tidal levels may have caused the poor survival rates. Transplanted snails moved homeward on pebble beaches, but not on basaltic shelves. Reduced survival rates at low tidal levels cause size gradients in both habitats, and snail movement helps to maintain size gradients on pebble beaches.     

Use of the beaks and raduala by Octopus vulgaris in feeding

The role of the chitinous beaks and the radula of the cephalopod Octopus vulgaris Lamarck, when feeding on either live shore crabs or whole dead fish, has been examined. One or more of these mouthparts was removed surgically and the effect on the quantity of food eaten and the method of cleaning the prey followed. The condition of the digestivetract was observed at post-mortem.     

Interactions between freshwater snails and tadpoles: competition and facilitation

Summary Freshwater snails and anuran tadpoles have been suggested to have their highest population densities in ponds of intermediate size where abiotic disturbance (e.g. desiccation) is low and large predators absent. Both snails and tadpoles feed on periphytic algae and, thus, there should be a large potential for competitive interactions to occur between these two distantly related taxa. In a field experiment we examined the relative strength of competition between two closely related snail species, Lymnaea stagnalis and L. peregra, and between L. stagnalis and tadpoles of the common frog, Rana temporaria. Snail growth and egg production and tadpole size at and time to metamorphosis were determined. Effects on the common food source, periphyton, were monitored with the aid of artificial substrates. Periphyton dry weight was dramatically reduced in the presence of snails and/or tadpoles. There were no competitive effects on growth or egg production of the two snail species when they were coexisting. Mortality of L. peregra was high (95%) after reproduction, but independent of treatment. Growth of L. stagnalis was reduced only at the highest tadpole densities, whereas egg production was reduced both by intraspecific competition and by competition with tadpoles. Differences in egg production were retained after tadpole metamorphosis. Tadpole larval period increased, weight of metamorphosing frogs decreased and growth rate was reduced as a function of increasing tadpole density. However, contrary to expectation, snails had a positive effect on tadpole larval period, weight and growth rate. Further, in experimental containers without snails there was a dense growth of the filamentous green alga Cladophora sp. We suggest that the facilitative effects of snails on tadpoles are due to an indirect mutualistic mechanism, involving competition between food sources of different quality (microalgae and Cladophora sp.) and tadpoles being competitively dominant over snails for the preferred food source (microalgae). In the presence of tadpoles snails will be forced to feed on low-quality Cladophora, increasing nutrient turnover rates, which results in enhanced productivity of microalgae, increasing tadpole food resources. Thus, tadpoles have a negative effect on snails through resource depression, while snails facilitate tadpole growth through an indirect enhancement of food availability.     

Effect of immersion at low tide on distribution and movement of the mud snail, Ilyanassa obsoleta (Say), in the upper Bay of Fundy, eastern Canada

Habitat heterogeneity often affects movement behaviours of animals, and consequently their spatial distribution. We evaluated the effect of immersion at low tide on the distribution, fine-scale movement patterns and daily movement patterns of the mud snail Ilyanassa obsoleta on a mudflat in the upper Bay of Fundy, Canada. Mud snails migrate onto intertidal mudflats in the summer, and our field survey showed that their density was higher inside tide pools relative to adjacent areas that are exposed at low tide. Using time-lapse videography, we evaluated the effect of snail size, snail density, and immersion at low tide on fine-scale movement patterns of I. obsoleta. Time until snails stopped moving and burrowed was unaffected by snail size, but snails at low and high densities burrowed somewhat faster than those at intermediate densities. Snail size and snail density had no detectable effect on displacement speed or linearity of displacement. Immersion affected snail movement: snails within tide pools delayed burrowing and traveled in more convoluted paths compared to those on exposed mud. Snails increased their turning angles within tide pools, which is probably the mechanism by which aggregations are formed. We also performed a mark-recapture experiment to compare daily movement patterns of snails released inside and outside tide pools. Snails released in tide pools moved shorter distances, but did not orient themselves differently than snails released outside tide pools. Both groups exhibited significant directionality, moving against the mean water current direction over 24 h. In sum, immersion at low tide affected the behaviour and spatial distribution of snails, resulting in snail aggregations within tide pools. These snail aggregations, in turn, may be a major factor influencing spatial dynamics on mudflats, including causing changes in distribution patterns of the burrowing amphipod Corophium volutator, a dominant inhabitant and key species in the food web of mudflats.     

Some aspects of the ecology of the desert snail Sphincterochila prophetarum in relation to energy and water flow

Summary Between 1976–1978, we studied the ecology of the Sphincterochila prophetarum, the most common snail, on stony slopes in Northern Negev. Energy and water flow determinations were made at the individual and population levels on a field observation basis and metabolic data were derived from laboratory simulations.The snails are active during 5–7% of the time during the year, while the remaining 95% of the time, the snails are in aestivation under stones. The energy and water inputs during the short feeding periods are very fast. Only 8% of the total energy in the eaten food are assimilated. Eighty-nine percent of the total assimilated energy are utilized in respiration during the various periods of activity and dormancy.The water loss during the feeding and activity period is about 37% of the total annual input. The snails lose twice as much the amount of water during the winter dormancy periods than in the summer dormancy period.Dedicated to Prof. Evenari and Dr. Springer