Crayfish predation on the common pond snail (Lymnaea stagnalis): the effect of habitat complexity and snail size on foraging efficiency

Optimal foraging theory was used to explain selective foraging by the introduced signal crayfish (Pacifastacus leniusculus) on the thin-shelled common pond snail (Lymnaea stagnalis). Crayfish predation efficiency was studied in relation to habitat complexity and snail size. In a pool experiment (area 1.3 m²) single adult crayfish were allowed to feed on four size classes of snails for one week. A pair-wise preference trial (aquarium experiment) tested if adult crayfish selectively predated on particular size classes of snail and if prey value (expressed as snail dry mass per handling time) could explain the size range of snails chosen. Crayfish preferred the smallest size classes of snails in both pool and aquaria experiments. In the pool experiment crayfish had a strong effect on snail survival. Habitat complexity did not affect overall snail survival, but resulted in reduced predation pressure on the smallest size classes of snails. Handling time and shell-thickness increased exponentially with increasing snail size, and the two smallest size classes had the highest prey values. The results suggest that crayfish can structure the abundance and size distribution of thin-shelled snails, through size-selective predation and reduction of macrophytes. The mechanisms behind the choice of snails may be based on prey value and reduced exposure time to predators and conspecifics. Crayfish effects on snail size distribution may be less pronounced in complex habitats such as macrophyte beds. This revised version was published online in July 2006 with corrections to the Cover Date.     

Home-field advantage: native signal crayfish (Pacifastacus leniusculus) out consume newly introduced crayfishes for invasive Chinese mystery snail (Bellamya chinensis)

The introduction of non-indigenous plants, animals and pathogens is one of today’s most pressing environmental challenges. Freshwater ecologists are challenged to predict the potential consequences of species invasions because many ecosystems increasingly support novel assemblages of native and non-native species that are likely to interact in complex ways. In this study we evaluated how native signal crayfish (Pacifastacus leniusculus) and non-native red swamp crayfish (Procambarus clarkii) and northern crayfish (Orconectes virilis) utilize a novel prey resource: the non-native Chinese mystery snail (Bellamya chinensis). All species are widespread in the United States, as well as globally, and recent surveys have discovered them co-occurring in lakes of Washington State. A series of mesocosm experiments revealed that crayfish are able to consume B. chinensis, despite the snail’s large size, thick outer shell and trapdoor defense behaviour. Crayfish exhibited size-selective predation whereby consumption levels decreased with increasing snail size; a common pattern among decapod predators. Comparison of prey profitability curves—defined as the yield of food (weight of snail tissue) per second of feeding time (the time taken to crack the shell and consume the contents)—suggests that small and very large snails may represent the most profitable prey choice. By contrast, previous studies have reported the opposite pattern for crayfish consumption on thin-shelled snails. For all snail size classes, we found that native P. leniusculus and invasive O. virilis consumed greater numbers of snails than invasive P. clarkii. Moreover, P. leniusculus consistently handled and consumed snails at a faster pace compared to both invasive crayfishes across the range of snail sizes examined in our study. These results suggest not only that B. chinensis is a suitable food source for crayfish, but also that native P. leniusculus may ultimately out-consume invasive crayfishes for this new prey resource.     

Dragonfly predators influence biomass and density of pond snails

Studies in lakes show that fish and crayfish predators play an important role in determining the abundance of freshwater snails. In contrast, there are few studies of snails and their predators in shallow ponds and marshes. Ponds often lack fish and crayfish but have abundant insect populations. Here we present the results of field surveys, laboratory foraging trials, and an outdoor mesocosm experiment, testing the hypothesis that insects are important predators of pulmonate snails. In laboratory foraging trials, conducted with ten species of insects, most insect taxa consumed snails, and larval dragonflies were especially effective predators. The field surveys showed that dragonflies constitute the majority of the insect biomass in fishless ponds. More focused foraging trials evaluated the ability of the dragonflies Anax junius and Pantala hymenaea to prey upon different sizes and species of pulmonate snails (Helisoma trivolvis, Physa acuta, and Stagnicola elodes). Anax junius consumed all three species up to the maximum size tested. Pantala hymenaea consumed snails with a shell height of 3 mm and smaller, but did not kill larger snails. P. acuta were more vulnerable to predators than were H. trivolvis or S. elodes. In the mesocosm experiment, conducted with predator treatments of A. junius, P. hymenaea, and the hemipteran Belostoma flumineum, insect predators had a pronounced negative effect on snail biomass and density. A. junius and B. flumineum reduced biomass and density to a similar degree, and both reduced biomass more than did P. hymenaea. Predators did not have a strong effect on species composition. A model suggested that A. junius and P. hymenaea have the largest effects on snail biomass in the field. Given that both pulmonate snails and dragonfly nymphs are widespread and abundant in marshes and ponds, snail assemblages in these water bodies are likely regulated in large part by odonate predation.     

Generalist versus specialist strategies of plasticity: snail responses to predators with different foraging modes

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Predator avoidance by the freshwater snailPhysella virgata in response to the crayfishProcambarus simulans

Summary We examined predator avoidance behavior of the freshwater snailPhysella (=Physa) virgata in response to the crayfishProcambarus simulans. In both laboratory and field enclosure experiments snails crawled above the waterline for 2 h or longer, then returned to the water.Physella virgata react to chemical signals given off by crayfish actively foraging on conspecific snails; they do not react to inactive crayfish. Low dissolved oxygen, crowding, and disturbance of the sediments did not elicit crawlout behavior. Crawling above the waterline, by reducing the probability of encounter between vulnerable, thin-shelled snails and crayfish, is an adaptive response to predation.     

Crayfish in lakes and streams: individual and population responses to predation, productivity and substratum availability

1. In a correlative study, we investigated the relative importance of fish predation, refuge availability and resource supply in determining the abundance and size distributions of the introduced and omnivorous signal crayfish (Pacifastacus leniusculus) in lakes and streams. Moreover, the biomass and food selection of predatory fish was estimated in each habitat type and stable isotopes of carbon and nitrogen were measured in perch (Perca fluviatilis), the dominant predator in the lakes, and in its potential food sources (crayfish, juvenile roach and isopods). 2. In lakes, crayfish were the most frequent prey in large perch (46%), followed by other macroinvertebrates (26%, including the isopod Asellus aquaticus) and small fish (25%). Crayfish and fish dominated the gut contents of large perch with respect to biomass. Nitrogen signatures showed that perch were one trophic level above crayfish (approx. 3.4‰) and a two‐source mixing model using nitrogen isotope values indicated that crayfish (81%) contributed significantly more to perch isotope values than did juvenile roach (19%). A positive correlation was found between the abundance of crayfish and the biomass of large perch. Crayfish abundance in lakes was also positively correlated with the proportion of cobbles in the littoral zone. Lake productivity (chlorophyll a) was positively correlated with crayfish size, but not with crayfish abundance. 3. In streams, brown trout (Salmo trutta) were the most abundant predatory fish. Gut contents of large trout in a forested stream showed that terrestrial insects were the most frequently found prey (60%), followed by small crayfish (27%) and isopods (27%). In contrast to lakes, the relative abundance of crayfish was negatively correlated with the total biomass of predatory fish and with total biomass of trout. However, abundance of crayfish at sites with a low biomass of predatory fish varied considerably and was related to substratum grain size, with fewer crayfish being caught when the substratum was sandy or dominated by large boulders. The mean size of crayfish was greater at stream sites with a high standing stock of periphyton, but neither predator biomass nor substratum grain size was correlated with crayfish size. 4. Our results suggest that bottom‐up processes influence crayfish size in lakes and streams independent of predator biomass and substratum availability. However, bottom‐up processes do not influence crayfish abundance. Instead, substratum availability (lakes) and interactions between predation and substratum grain size (streams) need to be considered in order to predict crayfish abundance.     

Chemical cues modify species interactions: the ecological consequences of predator avoidance by freshwater snails

Chemical signals released by predators or injured prey often induce shifts in the traits of prey species, which may in turn affect species interactions. Here we investigate the role that chemical cues play in mediating species interactions in the littoral food web of lakes. Previous studies have shown that predators induce shifts in the morphology, life history, and behavior of the freshwater snail Physella, but the ecological consequences of developing these inducible defenses are not well documented. We observed habitat use of the freshwater snail Physella gyrina along a depth gradient in a natural lake, and found they increased their use of covered habitats with increasing depth. We hypothesized that this habitat shift was due to changes in the level and type of predation risk, and that the habitat shift would affect periphyton standing crops. These hypotheses were tested in a mesocosm experiment in which we manipulated the presence of molluscivorous fish and crayfish. Predators were confined to cages and snail density was identical in all treatments, so any effects of predators were mediated through trait shifts induced by chemical cues. In the presence of fish, Physella moved under cover, but in the presence of crayfish, Physella avoided cover and moved to the water surface. These non‐lethal effects of predators on snail habitat use influenced the interaction between snails and their periphyton resources. In the presence of fish, periphyton standing crop in covered habitats was reduced to just 8% of periphyton in the absence of fish. Crayfish had no significant effect on periphyton in covered habitats, but they reduced periphyton in near‐surface habitats to 39% of the standing crop in the absence of crayfish. The combined effects of fish and crayfish were generally intermediate to their individual effects. We conclude that because chemical cues often have strong effects on individual traits and trophic interactions are sensitive to trait values, chemical cues may play an important role in shaping the structure and dynamics of food webs.     

Snail responses to cues produced by an invasive decapod predator

Abstract. Cues released by predators and injured prey often induce shifts in prey behavior that allow prey to evade predators, but also affect prey resource use. I investigated the effects of chemical and mechanical signals produced by injured snails (Physella gyrina) and predatory crayfish (Procambarus clarkii) on microdistributions of P. gyrina. In an initial experiment, I observed snail responses to the presence of a caged crayfish predator, to injured conspecifics, or to both. There were significant effects of time and the treatment × time interaction on the proportion of snails moving above the water line, with greater proportions of snails above the water line at night than during the day and with weak snail crawl‐out behavior being elicited by caged crayfish at night, but not during the day. In a second experiment, I examined snail microdistributions when exposed to crayfish confined to a small cage within each aquarium, crayfish confined to half of each aquarium, and crayfish ranging freely throughout each aquarium. Snails responded most strongly to free‐ranging crayfish by moving above the water line, but also demonstrated significant, but reduced, crawl‐out responses to crayfish confined to half of each aquarium; however, snails did not respond behaviorally to crayfish confined to small cages. In both experiments, there were marginally significant effects of unfed caged crayfish on the proportions of snail populations hiding under benthic shelters, with this response being the strongest at the start of the experiments but weak overall (with only 4–5% of P. gyrina responding in each experiment). These results indicate that cues (e.g., chemical or mechanical) produced by predators altered prey microdistributions, but that the exact prey responses (e.g., moving above the water line or into horizontal or benthic refugia) depended on the intensity and nature of cues.     

The potential for using red claw crayfish and hybrid African catfish as biological control agents for Schistosoma host snails

The potential of red claw crayfish and hybrid African catfish (Clarias gariepinus and Clarias ngamensis) as predators for Schistosoma host snails was evaluated in 2014 by monitoring the consumption of snails by crayfish and catfish in experimental tanks over time under laboratory conditions. After 15 days, both crayfish and catfish had significantly reduced the populations of Bulinus globosus. Crayfish consumed 6.9 snails d^?1, whereas catfish consumed 5.9 snails d^?1. However, when supplied with an alternative prey, Melanoides tuberculata (100 individuals per tank), crayfish clearly preferred M. tuberculata (100% consumed over seven days) to B. globosus (100 individuals per tank) (54% consumed over the same period). Catfish, conversely, did not have a clear preference for either prey species, consuming 77% and 88% of M. tuberculata and B. globosus, respectively. It was also observed that young catfish were more efficient predators than older ones, because of ontogenetic shifts in their diet with age. Hybrid catfish retain the molluscivorous characteristics of their parent stock and red claw crayfish also preys on Schistosoma host snails. However, the effectiveness of both predators is affected by the presence of an alternative prey. Therefore, under suitable conditions, these species can be considered for biological control of schistosomiasis transmission.     

Predation at a snail's pace: what's time to a gastropod?

Summary Predation by naticid gastropods shows evidence of adaptation to maximize the rate of energy intake. The predation rate of Polinices duplicatus feeding on artificially altered, thin-shelled Mercenaria mercenaria was faster than the predation rate on normal Mercenaria. The rate of energy intake was limited by handling time. The time saved by predation on thin-shelled prey was used to forage. Thus time was shown to be valuable to P. duplicatus, and cost-benefit functions using time and energy as currencies are appropriate for estimating dietary efficiency and predicting prey choice.Despite the clear superiority of thin-shelled prey, P. duplicatus did not learn to prefer this novel prey type, suggesting that predator choices are sterotyped, reflecting optima selected over evolutionary time.     

The influence of substrate particle size on the microdistribution of stream macrobenthos

Summary Substrate microhabitat preferences of ten species of benthic macroinvertebrates were investigated in a laboratory flowing water system and compared with preliminary field data. Eight particle size categories of both silted and non-silted substrates were tested in the laboratory.The correspondence between field and laboratory data indicated primary microhabitat selection on the basis of substrate particle size by the stonefly Perlesta placida, the riffle beetle Stenelmis crenata and the caddisflies Pycnopsyche guttifer and P. lepida. Broad substrate responses in the laboratory and lack of correspondence with field data indicated a secondary importance of substrate particle size in microhabitat selection by the pulmonate snail Helisoma anceps, the caddisfly Helicopsyche borealis, the cranefly Tipula caloptera, the alderfly Sialis vagans and the mayflies Caenis latipennis and Ephemera simulans.Silting had minor effects on substrate selection patterns in all species tested except Caenis latipennis and Perlesta placida in which it enhanced selection for the intersticies of coarse sediments.

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Zusammenfassung Die Vorliebe für Substratarten mit Rücksicht auf die Substratpartikelgrösse von zehn Arten im Benthos lebender Wirbelloser wurde in einer Labor-Fliesswasseranlage untersucht und mit vorläufigen Feldergebnissen verglichen. Acht Anordnungen von Substratpartikelgrössen, sowohl verschlammt als auch nicht verschlammt, wurden im Labor untersucht.Die Entsprechung zwischen Feld- und Laborergebnissen zeigte dass die Wahl des Lebensraumes in erster Linie durch die Substratpartikelgrösse bestimmt wurde für die Steinfliege Perlesta placida, den Hakenkäfer Stenelmis crenata und die Köcherfliegen Pycnopsyche gutiffer und P. lepida. Die breite Streuwirkung des Substrates im Labor und der Mangel an Übereinstimmung mit den Feldergebnissen zeigte, dass die Substratpartikelgrösse in der Wahl des Lebensraumes eine untergeordnete Rolle spielte bei der Lungenschnecke Helisoma anceps, der Köcherfliege Helicopsyche borealis, der Schnake Tipula caloptera, der Schlammfliege Sialis vagans und den Eintagsfliegen Caenis latipennis and Ephemera s simulans.Die Verschlammung hatte geringen Effekt auf die Auswahl des Substratesbei allen untersuchten Arten mit Ausnahme von Caenis latipennis und Perlesta placida. In diesen Fällen verstärkte sich die Besiedlung wegen der Zwischenräume in dem groben Sediment.

     

Seasonal movements of crayfish in a fluctuating wetland: implications for restoring wading bird populations

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Competition for food between an introduced crayfish and two fishes endemic to the Colorado River basin

Synopsis Crayfish are not native to the Colorado River basin (CRB), however they are now established in portions of the mainstem and in many tributaries. I used density manipulation experiments in a laboratory setting to determine intra- and interspecific competition for food between Orconectes virilis, an aggressive polytrophic crayfish now common in the CRB, and two native fishes: Gila chub, Gila intermedia, and flannelmouth sucker, Catostomus latipinnis. I tested each fish species in separate trials. Growth of Gila chub decreased when animal densities increased, however they were more affected by intraspecific competition than by crayfish presence. In contrast, growth of flannelmouth suckers was more affected by crayfish than by intraspecific competition. Crayfish growth was not significantly altered by presence of either fish. Crayfish thus reduced fish growth by competition for food, but the effect differed markedly between the two species. An erratum to this article can be found at .     

Predation by the invasive American signal crayfish, Pacifastacus leniusculus Dana, on the invasive zebra mussel, Dreissena polymorpha Pallas: the potential for control and facilitation

Non-indigenous crayfish often have major ecological impacts on invaded water bodies, and have contributed to the decline of native crayfish species throughout Europe. The American signal crayfish, Pacifastacus leniusculus, is the most widespread invasive crayfish in Great Britain, where the zebra mussel, Dreissena polymorpha, is similarly an invasive pest species. The potential for the American signal crayfish to regulate zebra mussel populations was investigated through a series of laboratory experiments. Crayfish were found to be highly size selective, consuming significantly more of the smallest size class of zebra mussels offered (7–12 mm), over medium (16–21 mm) and large (25–30 mm). Crayfish feeding rate on zebra mussels was not altered when mussels were presented clumped together in natural druses compared with mussels in a disassembled druse. Crayfish spent significantly more time foraging when mussels were unattached, and a greater proportion of attacks were on medium and large than on small mussels (83% of attacks were on medium and large mussels when unattached as opposed to 47% when on druses). Individual crayfish feeding rate decreased significantly at densities of > ~5 crayfish m⁻². Signal crayfish are, therefore, unlikely to be able to significantly impact established populations of zebra mussels in the wild, although zebra mussels have the potential to provide crayfish with a substantial food source.     

Non‐lethal predator effects on the performance of a native and an exotic crayfish species

1. I tested the hypothesis that the potential for non‐lethal effects of predators are more important for overall performance of the fast‐growing exotic signal crayfish (Pacifastacus leniusculus Dana) than for the slower growing native noble crayfish (Astacus astacus L.). I further tested if omnivorous crayfish switched to feed on less risky food sources in the presence of predators, a behaviour that could reduce the feeding costs associated with predator avoidance. 2. In a 2 month long outdoor pool experiment, I measured behaviour, survival, cheliped loss, growth, and food consumption in juvenile noble or signal crayfish in pools with either a caged predatory dragonfly larvae (Aeshna sp.), a planktivorous fish that do not feed on crayfish (sunbleak, Leucaspius delineatus Heckel), or predator‐free controls. Crayfish had access to multiple food sources: live zooplankton, detritus and periphyton. Frozen chironomid larvae were also supplied ad libitum outside crayfish refuges, simulating food in a risky habitat. 3. Crayfish were mainly active during hours of darkness, with signal crayfish spending significantly more time outside refuges than noble crayfish. The proportion of crayfish outside refuges varied between crayfish species, time and predator treatment, with signal crayfish spending more time in refuges at night in the presence of fish. 4. Survival in noble crayfish was higher than in signal crayfish, and signal crayfish had a higher frequency of lost chelipeds, indicating a high level of intraspecific interactions. Crayfish survival was not affected by the presence of predators. 5. Gut‐contents analysis and stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) indicated that the two crayfish species had similar food preferences, and that crayfish received most of their energy from feeding on invertebrates (e.g. chironomid larvae), although detritus was the most frequent food item in their guts. Signal crayfish guts were more full than those of noble crayfish, but signal crayfish in pools with fish contained significantly less food and fewer had consumed chironomids compared with predator‐free controls. Length increase of signal crayfish (35%) was significantly higher than of noble crayfish (20%), but signal crayfish in pools with fish grew less than in control pools. 6. This short‐term study indicates that fish species that do not pose a lethal threat to an organism may indirectly cause reductions in growth by affecting behaviour and feeding. This may occur even though prey are omnivorous and have access to and consume multiple food sources. These non‐lethal effects of predators are expected to be particularly important in exotic crayfish species that show a general response to fish, have high individual growth rates, and when their feeding on the most profitable food source is reduced.     

Success and failure in a lotic crayfish invasion: the roles of hydrologic variability and habitat alteration

1. This paper examines the distribution, habitat relationships, and potential for spread of non‐native signal crayfish (Pacifastacus leniusculus) in streams of the Truckee River catchment, California, U.S.A. Crayfish associations with natural features and with impoundments and flow alteration were examined in a survey of 33 streams. Abundance changes were followed over 5 years, which included some of the highest and lowest flows on record, in three streams, two unregulated and one regulated. Movement of marked crayfish was studied in one 0.5 km stream reach just upstream from a reservoir. 2. Signal crayfish were most abundant in low‐gradient streams and were positively associated with proximity to reservoirs (both upstream and downstream). Crayfish were more likely to be found in regulated than unregulated sites, and did not occur in sites upstream of barriers, such as culverts, that separated them from reservoirs or lakes. 3. Crayfish declined in abundance in years following particularly intense and prolonged wet‐season spates, leading to a negative association between crayfish abundance and both peak discharge and duration of bankfull flows. 4. Crayfish moved distances of up to 277 m, and at rates of up to 120 m day^?1, suggesting significant dispersal ability. Larger crayfish moved greater distances and were more likely to move downstream. Female crayfish showed a pattern of upstream movement in early summer and downstream movement late in the summer, opposite the pattern found in two other studies. 5. These results suggest that natural or artificial gradient barriers and, in regulated systems, management of flow regimes to include bankfull or greater flows may help to control invasive crayfish in streams.     

Evidence of Large Change in Unionid Mussel Abundance from Selective Muskrat Predation,as Inferred by Shell Remains Left on Shore

Three species of unionid mussels (Pyganodon grandis, Lampsilis siliquoidea, and Toxolasma parvus) were preyed on by muskrats (Ondatra zibethicus) that left shell remains at feeding sites (middens) along the shore of Lake LaSalle (West) in western New York State, USA. Mussels became established within five years of the construction of this small artificial lake, and individuals of all three species have grown rapidly to large size. The abundance of prey in middens in 1988–1990 and a marked decline in subsequently deposited shells indicate how dramatically muskrats had exploited this food source. They consumed > 2700 individuals of P. grandis (by far the most common prey item) through 1990, yet only 20 newly deposited shells were found in 1993. At a midden where prey depletion was directly catalogued during 1988, larger specimens of P. grandis were consumed first, suggesting size selection. L. siliquoidea has been least affected by predation, and was the mussel most often found alive in the littoral zone. No live T. parvus were ever collected, and this species is known only from middens. Complete collections of shell remains from the shoreline through 1999 suggest that the mussel population may be recovering slowly. Muskrat predation may be more important in influencing mussel population structure than previously realized, and the effects can be dramatic spatially and temporally.     

Disturbance and species displacement: different tolerances to stream drying and desiccation in a native and an invasive crayfish

1. Crayfish are among the most threatened taxa in the world and invasive crayfish are the primary cause of the decline of native crayfish. Most research has emphasised biotic interactions as the mechanism by which native crayfish are displaced by invasives, although crayfish occupy variable environments and the role of disturbance in facilitating crayfish invasion and displacement is understudied. 2. We compared tolerance to a disturbance, stream drying, in a native and invasive crayfish as a potential mechanism to explain their distribution. Our experiments and observations were conducted across scales, from laboratory environmental chambers to stream mesocosms to field sampling. We hypothesised that the invasive crayfish would be more tolerant of desiccation than the native, and that this physiological distinction between the two would be reflected in their distribution in relation to stream drying. 3. In the laboratory, the native crayfish Orconectes eupunctus was less tolerant of desiccation than the invasive Orconectes neglectus chaenodactylus, with all native crayfish dying within 2 days without water, while some of the invasive crayfish survived for nearly 2 weeks. Under simulated stream drying in mesocosms, only the native O. eupunctus survived less well than in a control. Field sampling demonstrated a significant negative relationship between O. eupunctus density and low summer flows, while O. neglectus density was positively associated with low summer flows. The greater resistance of O. neglectus to drying could, through priority effects, inhibit recolonisation by O. eupunctus once flow resumes. 4. Abiotic disturbances are potentially important to the displacement of native by invasive crayfish. Disturbance mediated displacement of aquatic species provides both an opportunity to conserve native species by maintaining or restoring habitat and disturbance regimes and is also a challenge due to increasing human water demand, flow regime alteration and global climate change.     

Multiple predator-avoidance behaviours of the freshwater snail Physella heterostropha pomila: responses vary with risk

• 1 We examined the predator‐avoidance behaviour, exhibited in response to chemical cues, of two populations of the snail Physella heterostropha pomila. Snails were subjected to four treatments simulating different degrees of predation risk: control water (low risk), or water from tanks containing nonforaging crayfish (intermediate risk), crushed conspecifics (high risk) or crayfish consuming conspecifics (high risk). Data were analysed using three‐way ANOVA models (population × predator chemicals × injured conspecific chemicals).
• 2 Physella increased its avoidance behaviour as risk increased. Crayfish cue elicited a significantly greater response than from controls. Cues from injured conspecifics elicited the strongest response.
• 3 Physella exhibited several types of avoidance behaviour, including burial into the substratum, moving to the water surface, and crawling out of the water. The type of cue present influenced response type. Cues from crayfish reduced burial and increased movement to the water surface or out of the water. Cues from injured‐conspecifics significantly increased crawling completely out of the water.
• 4 The two populations differed in the type and degree of response exhibited. One population exhibited significantly greater ‘reactivity’ (i.e. any avoidance behaviour) in response to foraging crayfish, and more burial and crawl‐out behaviours were exhibited in high‐risk treatments.

     

A new species of Gyraulus (Gastropoda: Planorbidae) from Ancient Lake Lugu,Yunnan-Guizhou Plateau,Southwest China

A new aberrant species of the planorbid genus Gyraulus, Gyraulus luguhuensis n. sp., is described from Lake Lugu (Lugu-hu, in Chinese), Southwest China. The generic assignment with Gyraulus is based on features of the genital anatomy that are characteristic for members of that genus, in particular the presence of a chitinized penial stylet. Gyraulus luguhuensis n. sp. differs from most other congeners by its large, thick shell with an elevated spire. Similarly, aberrant shells are known from congeners in other Ancient Lakes worldwide indicating a potentially convergent evolution of shell characteristics in exclusively lacustrine species. Gyraulus luguhuensis differs from other lacustrine Gyraulus species with similarly large shells in having a sub-terminal penis pore and an unkeeled shell.