One-trial conditioning of aerial respiratory behaviour in Lymnaea stagnalis

The role of non-coding RNAs in the formation of long-term associative memory after single-trial learning in Lymnaea

Investigations of the molecular mechanisms of long-term associative memory have revealed key roles for a number of highly evolutionarily conserved molecular pathways in a variety of different vertebrate and invertebrate model systems. One such system is the pond snail Lymnaea stagnalis, in which, like in other systems, the transcription factors CREB1 and CREB2 and the enzyme NOS play essential roles in the consolidation of long-term associative memory. More recently, epigenetic control mechanisms, such as DNA methylation, histone modifications, and control of gene expression by non-coding RNAs also have been found to play important roles in all model systems. In this minireview, we will focus on how, in Lymnaea, even a single episode of associative learning can activate CREB and NO dependent cascades due to the training-induced up- or downregulation of the expression levels of recently identified short and long non-coding RNAs.

The temperature sensitivity of memory formation and persistence is altered by cold acclimation in a pond snail

There are reports on the inability of inbred, laboratory-reared Lymnaea stagnalis to perform feeding and aerial respiration in the cold. It has also been suggested that laboratory-bred snails have an inability to perform aerial respiration in winter months in the laboratory. Here, we used an inbred, laboratory-reared strain of Lymnaea (the S-strain) to demonstrate that the snails are capable of performing those behaviours in a cold (4°C) environment after a 2 day acclimation period. In addition, the inbred snails were able to perform aerial respiration during winter months at room temperature (20°C) in the laboratory. The persistence of long-term memory (LTM) was extended for at least 4 weeks by placing S-strain snails into a 4°C environment following training. Typically, the cold block (CB) procedure (1 h at 4°C) immediately after a training session blocks LTM formation in the S-strain but not in a freshly collected strain. Four weeks at 4°C transformed the S-strain phenotype into one resisting the CB procedure. Thus, with a 4 week cold spell snails gain a resistance to the CB procedure, and that would explain why freshly collected snails are resistant to the procedure. However, we found that F1 progeny of a freshly collected strain reared in the laboratory were resistant to the CB procedure. This suggests that an unknown selection resulted in the S-strain being susceptible to the CB procedure.

Black tea differs from green tea: it suppresses long-term memory formation in Lymnaea

Foods, such as Green tea (GT), containing the flavonol, (-)-Epicatechin (Epi), enhance the formation of long-term memory (LTM) when snails are operantly conditioned in that substance. That is, a single 0.5 h training session results in LTM; whereas similar training in pond water does not result in LTM. It was of interest to determine if Black tea (BT), which is a more popular beverage than GT and which is derived from the same tea leaves, also enhances LTM formation. We found that BT, unlike GT, depressed homeostatic aerial respiratory behaviour and obstructed LTM formation. We used two different methods to determine if BT altered LTM formation and both procedures showed us that BT obstructed LTM formation. We conclude that BT obstructs LTM formation and depresses homeostatic aerial respiration

Strain-specific differences of the effects of stress on memory in Lymnaea

Stress alters the ability to form, recall and maintain memory according to the Yerkes–Dodson/Hebb (YDH) law. The effects of environmentally relevant stressors, such as low environmental calcium and crowding, on learning and memory have previously been described in a laboratory-reared ‘average’ strain of Lymnaea stagnalis (i.e. the Dutch strain) as well as two strains of freshly collected L. stagnalis with enhanced memory formation abilities (i.e. ‘smart’ snails). Here, we use L. stagnalis to study the effects of other environmentally relevant stressors on memory formation in two other strains of freshly collected snails, one ‘smart’ and one ‘average’. The stressors we examined are thermal, resource restriction combined with food odour, predator detection and, for the first time, tissue injury (shell damage). We show that the same stressor has significantly different effects on memory formation depending on whether snails are ‘smart’ or ‘average’. Specifically, our data suggest that a stressor or a combination of stressors act to enhance memory in ‘average’ snails but obstruct memory formation in ‘smart’ snails. These results are consistent with the YDH law and our hypothesis that ‘smart’ snails are more easily stressed than ‘average’ snails.

Epicatechin, a component of dark chocolate, enhances memory formation if applied during the memory consolidation period

Epicatechin (Epi), a flavanol found in foods such as dark chocolate has previously been shown to enhance memory formation in our model system, operant conditioning of aerial respiration in Lymnaea. In those experiments snails were trained in Epi. Here we ask whether snails exposed to Epi before training, during the consolidation period immediately following training, or 1 h after training would enhance memory formation. We report here that Epi is only able to enhance memory if snails are placed in Epi-containing pond water immediately after training. That is, Epi enhances memory formation if it is applied during the memory consolidation period as well as if snails are trained in Epi-containing pond water.

A flavanoid component of chocolate quickly reverses an imposed memory deficit

The ability to remember is influenced by environmental and lifestyle factors, such as stress and diet. A flavanol contained in chocolate, epicatechin (Epi), has been shown to enhance long-term memory (LTM) formation in Lymnaea. Combining two stressors (low-calcium pond water and crowding) blocks learning and all forms of memory; that is, this combination of environmentally relevant stressors creates a memory-unfriendly state. We tested the hypothesis that Epi will immediately reverse the memory-unfriendly state, i.e. that snails in the memory-deficit state when trained in Epi will immediately become competent to learn and form memory. We found that Epi not only reverses the memory-deficit state but also further enhances LTM formation. Thus, a naturally occurring bioactive plant compound can overcome a memory-unfriendly state. This supports the idea that bioactive substances may mitigate memory-making deficits that, for example, occur with ageing.

Juveniles of Lymnaea smart snails do not perseverate and have the capacity to form LTM

Previously, it was concluded that the nervous systems of the juvenile snails were not capable of mediating LTM. However, exposure and training of those juvenile snails in the presence of a predator cue significantly altered their ability to learn and form LTM. In addition, there are some strains of Lymnaea which have been identified as ‘smart’. These snails form LTM significantly better than the lab-bred strain. Here we show that juveniles of two smart snail strains are not only capable of associative learning, but also have the capacity to form LTM following a single 0.5h training session. We also show that freshly collected ‘wild’ ‘average’ juveniles are also not able to form LTM. Thus, the smart snail phenotype in these strains is expressed in juveniles.

Critical period of memory enhancement during taste avoidance conditioning in Lymnaea stagnalis

The present study investigated the optimal training procedure leading to long-lasting taste avoidance behavior in Lymnaea. A training procedure comprising 5 repeated pairings of a conditional stimulus (CS, sucrose), with an unconditional stimulus (US, a tactile stimulation to the animal's head), over a 4-day period resulted in an enhanced memory formation than 10 CS-US repeated pairings over a 2-day period or 20 CS-US repeated pairings on a single day. Backward conditioning (US-CS) pairings did not result in conditioning. Thus, this taste avoidance conditioning was CS-US pairing specific. Food avoidance behavior was not observed following training, however, if snails were immediately subjected to a cold-block (4°C for 10 min). It was critical that the cold-block be applied within 10 min to block long-term memory (LTM) formation. Further, exposure to the cold-block 180 min after training also blocked both STM and LTM formation. The effects of the cold-block on subsequent learning and memory formation were also examined. We found no long lasting effects of the cold-block on subsequent memory formation. If protein kinase C was activated before the conditioning paradigm, snails could still acquire STM despite exposure to the cold-block.

From likes to dislikes: conditioned taste aversion in the great pond snail (Lymnaea stagnalis)

The neural circuitry comprising the central pattern generator (CPG) that drives feeding behavior in the great pond snail (Lymnaea stagnalis (L., 1758)) has been worked out. Because the feeding behavior undergoes associative learning and long-term memory (LTM) formation, it provides an excellent opportunity to study the causal neuronal mechanisms of these two processes. In this review, we explore some of the possible causal neuronal mechanisms of associative learning of conditioned taste aversion (CTA) and its subsequent consolidation processes into LTM in L. stagnalis. In the CTA training procedure, a sucrose solution, which evokes a feeding response, is used as the conditioned stimulus (CS) and a potassium chloride solution, which causes a withdrawal response, is used as the unconditioned stimulus (US). The pairing of the CS–US alters both the feeding response of the snail and the function of a pair of higher order interneurons in the cerebral ganglia. Following the acquisition of CTA, the polysynaptic inhibitory synaptic input from the higher order interneurons onto the feeding CPG neurons is enhanced, resulting in suppression of the feeding response. These changes in synaptic efficacy are thought to constitute a “memory trace” for CTA in L. stagnalis.

Low external environmental calcium levels prevent forgetting in Lymnaea

Forgetting may allow an animal to react more appropriately to current conditions, rather than continuing to exhibit a previously learned, possibly maladaptive behaviour based on previous experience. One theory is that forgetting is an active process, whereby the previously learnt response is replaced by new learning that interferes with the older memory. Hence, we hypothesized that an appropriately timed environmental stressor that blocks long-term memory (LTM) formation would also block forgetting. Lymnaea stagnalis (L.) is a freshwater snail, which requires environmental calcium of at least 20 mg l–1 to meet its requirements. Low environmental Ca2+ (i.e. 20 mg l–1) in their environment acts as a stressor, and prevents LTM formation. Here, we asked whether a low Ca2+ environment would also prevent forgetting, concordant with the retrograde interference model of Jenkins and Dallenbach. Snails were operantly conditioned to reduce aerial respiration in hypoxia. When maintained in standard conditions (80 mg l–1 Ca2+), snails demonstrated LTM following training lasting 24 h, but not 72 h; however, when trained in standard conditions then exposed to a low Ca2+ environment (20 mg l–1) immediately following training, they retained memory for at least 96 h, indicating that forgetting had been blocked. Thus, when exposed to low environmental Ca2+, Lymnaea will fail to form new memories, but will also continue to retain information previously learned and remembered as the low calcium blocks forgetting.

Does conditioned taste aversion learning in the pond snail Lymnaea stagnalis produce conditioned fear?

In conditioned taste aversion (CTA) training performed on the pond snail Lymnaea stagnalis, a stimulus (the conditional stimulus, CS; e.g., sucrose) that elicits a feeding response is paired with an aversive stimulus (the unconditional stimulus, US) that elicits the whole-body withdrawal response and inhibits feeding. After CTA training and memory formation, the CS no longer elicits feeding. We hypothesize that one reason for this result is that after CTA training the CS now elicits a fear response. Consistent with this hypothesis, we predict the CS will cause (1) the heart to skip a beat and (2) a significant change in the heart rate. Such changes are seen in mammalian preparations exposed to fearful stimuli. We found that in snails exhibiting long-term memory for one-trial CTA (i.e., good learners) the CS significantly increased the probability of a skipped heartbeat, but did not significantly change the heart rate. The probability of a skipped heartbeat was unaltered in control snails given backward conditioning (US followed by CS) or in snails that did not acquire associative learning (i.e., poor learners) after the one-trial CTA training. These results suggest that as a consequence of acquiring CTA, the CS evokes conditioned fear in the conditioned snails, as evidenced by a change in the nervous system control of cardiac activity.

Low environmental calcium blocks long-term memory formation in a freshwater pulmonate snail

The freshwater snail Lymnaea stagnalis (L.) is considered a calciphile and exhibits reduced growth and survival in environments containing less than 20 mg/l environmental calcium. Although it has no apparent effect on survival at 20 mg/l, reducing environmental calcium increases metabolic demand, and as such we consider that this level of calcium acts as a stressor on the snail. We exposed snails to acute periods of low environmental calcium and tested their ability to form intermediate-term memory (ITM) and long-term memory (LTM) following one trial operant conditioning (1TT) to reduce aerial respiratory activity in hypoxic conditions. We also assessed whether there were changes in the electrophysiological properties of a single neuron, right pedal dorsal 1 (RPeD1), which has been demonstrated to be necessary for LTM formation. Following training in high (80 mg/l) environmental calcium, L. stagnalis formed ITM and LTM lasting 24 h and demonstrated a significant reduction in all activity measured from RPeD1; however when snails were exposed to low (20 mg/l) environmental calcium they were able to form ITM but not LTM. Although no behavioral LTM was formed, a partial reduction in RPeD1 activtiy measured 24 h after training was observed, indicating a residual effect of training. The strong effect that environmental calcium concentration had on physiology and behavior in response to training to reduce aerial respiration in L. stagnalis suggests that it is an element of gastropod husbandry that needs to be carefully considered when studying other traits. This study also indicates that L. stagnalis found naturally in low calcium environments may be less able to adapt to novel stressors than populations found in harder waters.

The role of serotonin in the enhancement of long-term memory resulting from predator detection in Lymnaea

Serotonergic systems play important roles in modulating stress-induced arousal and vigilance behaviours. The pond snail, Lymnaea, shows multiple defensive vigilance behaviours in response to the stress associated with predator detection. Predator detection elicited by crayfish effluent (CE), increases the time to re-emerge from the shell and enhances the shadow withdrawal response. More importantly, in Lymnaea, CE enhances the ability to form long-term memory (LTM). We investigated the role of the serotonergic system in these anti-predator responses in Lymnaea. Using a serotonin-receptor antagonist, mianserin, we found that two defensive vigilance behaviours (e.g. increasing the time to re-emerge from their shell and shadow response) elicited by CE were not observed when the serotonergic system was disrupted. Also, methysergide, another serotonin antagonist, blocked the enhanced LTM formation after training in CE. Importantly, mianserin did not alter LTM formation in pond water (PW). These data suggest that a serotonergic system is activated only when Lymnaea detect a predator. When snails were trained in CE using a training procedure that in PW produces a 24-h LTM, a more persistent form of LTM (5 days) occurred. This more persistent form of LTM was abolished after mianserin treatment. Increasing 5-HT levels in the snail by the injection of 5-HT was also associated with enhanced LTM formation. Lastly, we tested whether the osphradium is implicated in CE detection and subsequent enhanced formation of LTM. Cutting the osphradial nerve to the CNS resulted in the loss of the ability to form enhanced LTM in CE. Together, these findings support the hypothesis that the serotonergic system plays a key role in modulating the predator-induced stress responses in Lymnaea.

The shadow-induced withdrawal response, dermal photoreceptors, and their input to the higher-order interneuron RPeD11 in the pond snailLymnaea stagnalis

The shadow-induced withdrawal response in Lymnaea stagnalis is mediated by dermal photoreceptors located on the foot, mantle cavity, and skin around the pneumostome area. Here, we determined whether we could obtain a neural correlate of the withdrawal response elicited by a shadow in a higher-order central neuron that mediates withdrawal behavior. We measured the electrophysiological properties of the higher-order interneuron Right Pedal Dorsal 11 (RPeD11), which has a major role in Lymnaea withdrawal behavior. In semi-intact preparations comprising the circumesophageal ganglia, the mantle cavity and the pneumostome, but not the foot and eyes, a light-on stimulus elicited a small short-lasting hyperpolarization and a light-off stimulus elicited a depolarization of RPeD11. We also determined that dermal photoreceptors make a monosynaptic contact with RPeD11. The dermal photoreceptor afferents course to the circumesophageal ganglia via the anal and genital nerves to the visceral ganglion, and/or via the right internal and external parietal nerves to the parietal ganglion. Finally, in addition to responding to photic stimuli, RPeD11 responds to both mechanical and chemical stimuli delivered to the pneumostome.

Increase in excitability of RPeD11 results in memory enhancement of juvenile and adult Lymnaea stagnalis by predator-induced stress

Memory consolidation following learning is a dynamic process. Thus, long-term memory (LTM) formation can be modulated by many factors, including stress. We examined how predator-induced stress enhances LTM formation in the pond snail Lymnaea stagnalis at both the behavioral and electrophysiological levels. Training snails in crayfish effluent (CE; i.e., water from an aquarium containing crayfish) significantly enhanced LTM. That is, while memory persists for only 3h in adult control experiments following a single 0.5-h training session in pond water in which the pneumostome receives a contingent tactile stimulus to the pneumostome; when the snails are trained in CE, the memory persists for at least 24h. In juveniles, the data are more dramatic. Juveniles are unable to form LTM in pond water, but form LTM when trained in CE. Here we examined whether juvenile snails form LTM following a one-trial training procedure (1TT). Following the 1TT procedure (a single-trial aversive operant conditioning training procedure), juveniles do not form LTM, unless trained in CE. Concomitantly, we observe changes in the excitability of RPeD11, a key neuron mediating the whole snail withdrawal response, which may be a neural correlate of enhanced memory formation.

Rotational behaviour of encapsulated pond snail embryos in diverse natural environments

Encapsulated freshwater pond snail embryos display a cilia-driven rotation behaviour that is stimulated by artificially induced hypoxia. Previous studies have suggested that the mixing effect of this behaviour causes enhanced oxygen delivery to embryos within their egg capsules. Despite extensive laboratory-based studies describing this behaviour, it is unclear how this behaviour is used to cope with changes in oxygen concentration and other environmental factors in natural water bodies. We made field measurements of embryo rotation rates in laboratory-reared Helisoma trivolvis embryos placed in ponds of different trophic levels that ranged geographically from the southern Alberta prairie to the Rocky Mountains. Abiotic factors including temperature, pH, conductivity and water oxygen concentration were measured to understand how embryonic rotation is influenced by environmental conditions. Results showed that H. trivolvis embryos exhibit differences in rotational behaviour depending on the environmental conditions. Temperature and oxygen concentration were the primary factors significantly affecting rotation rates. The effect of oxygen concentration on rotation rates was not as widespread as observed under laboratory conditions, probably because the measured oxygen concentrations were above the range that influences embryonic rotation in the laboratory. The rotational behaviour of laboratory-reared Lymnaea stagnalis provided confirmation that embryos of other encapsulated pulmonates exhibit a similar rotational response in natural environments. These results suggest that embryo rotation is influenced by a complex interplay of environmental factors.

Differences in LTM-forming capability between geographically different strains of Alberta Lymnaea stagnalis are maintained whether they are trained in the lab or in the wild

We found strain differences in the ability of wild Alberta Lymnaea stagnalis to form long-term memory (LTM) following operant conditioning when L. stagnalis were collected from the wild and trained in the laboratory. Lymnaea stagnalis obtained from the Belly River watershed had an enhanced ability to form LTM compared with those from an isolated pond (referred to as Jackson snails). We therefore asked whether the differences in cognitive ability were an epiphenomenon as a result of training in the laboratory. To answer this question we trained each specific strain (Belly and Jackson) in both the laboratory and the field (i.e. in their home pond and in the pond where the other strain resided - referred to as the visitor pond). We found that within each strain there was no difference in the LTM phenotype whether they were trained in the lab or in either their home or visitor pond. That is, the strain differences in the ability to form LTM were still present. Interestingly, we found no strain differences in the ability to learn or the ability to form intermediate-term memory (ITM).

Novel neural correlates of operant conditioning in normal and differentially reared Lymnaea

The aerial respiratory behaviour of the mollusc Lymnaea stagnalisis an important homeostatic behaviour that can be operantly conditioned. The central pattern generator underlying this behaviour, as well as motorneurons innervating the respiratory orifice, the pneumostome, have been identified and their activity can be monitored in the semi-intact preparation using electrophysiological recordings. In this study, we used both intact animals and semi-intact preparations to identify novel changes in the respiratory central pattern generator following operant conditioning. In addition, we reared animals in the absence of this respiratory behaviour throughout development, to investigate whether previous experience and activity-dependent plasticity during development are essential to allow neural plasticity in the adult. We found that animals raised normally (allowed to perform aerial respiratory behaviour) exhibited the expected reduction in aerial respiratory behaviour following operant conditioning. Then, using the semi-intact preparation, we identified novel neural changes within the network as a result of the conditioning. These included specific changes at the level of the central pattern generator interneurons, as well as the motor output. In the differentially reared intact animals, there was no behavioural reduction as a result of operant conditioning, although their baseline respiratory behaviour was already significantly reduced as a result of their differential rearing. There were, however, significant differences found in the network parameters in the semi-intact preparation, similar to those observed in normally reared animals. We thus provide evidence for neural plasticity within the network in the absence of significant behavioural changes in differentially reared animals, and show that plasticity was not dependent on previous activity of the network during development.

The perception of stress alters adaptive behaviours in Lymnaea stagnalis

Stress can alter adaptive behaviours, and as well either enhance or diminish learning, memory formation and/or memory recall. We show here that two different stressors have the ability to alter such behaviours in our model system, Lymnaea stagnalis. One, a naturally occurring stressor - the scent of a predator (crayfish) - and the other an artificially controlled one - 25 mmol l(-1) KCl - significantly alter adaptive behaviours. Both the KCl stressor and predator detection enhance long-term memory (LTM) formation; additionally predator detection alters vigilance behaviours. The predator-induced changes in behaviour are also accompanied by specific and significant alterations in the electrophysiological properties of RPeD1 - a key neuron in mediating both vigilance behaviours and memory formation. Naive lab-bred snails exposed to crayfish effluent (CE; i.e. the scent of the predator) prior to recording from RPeD1 demonstrated both a significantly reduced spontaneous firing rate and fewer bouts of bursting activity compared with non-exposed snails. Importantly, in the CE experiments we used laboratory-reared snails that have not been exposed to a naturally occurring predator for over 250 generations. These data open a new avenue of research, which may allow a direct investigation from the behavioral to the neuronal level as to how relevant stressful stimuli alter adaptive behaviours, including memory formation and recall.

Comparing memory-forming capabilities between laboratory-reared and wildLymnaea: learning in the wild, a heritable component of snail memory

We set out to determine whether the ability to form long-term memory (LTM)is influenced by laboratory rearing. We investigated the ability of four populations of Lymnaea stagnalis to form LTM following operant conditioning both in the freely behaving animal and at the electrophysiological level in a neuron, RPeD1, which is a necessary site for LTM. We hypothesized that laboratory rearing results in a decreased ability to form LTM because rearing does not occur in an `enriched environment'. Of the four populations examined, two were collected in the wild and two were reared in the laboratory – specifically, (1) wild Dutch snails; (2) their laboratory-reared offspring; (3) wild Southern Alberta snails (Belly); and (4)their laboratory-reared offspring. We found that Belly snails had an enhanced capability of forming LTM compared with Dutch laboratory-reared snails. That is, the Belly snails, which are much darker in colour than laboratory-reared snails (i.e. blonds), were `smarter'. However, when we tested the offspring of Belly snails reared in the laboratory we found that these snails still had the enhanced ability to form LTM, even though they were now just as `blond' as their laboratory-reared Dutch cousins. Finally, we collected wild Dutch snails, which are also dark, and found that their ability to form LTM was not different to that of their laboratory-reared offspring. Thus, our hypothesis was not proved. Rather, we now hypothesize that there are strain differences between the Belly and Dutch snails, irrespective of whether they are reared in the wild or in the laboratory.

Crowding, an environmental stressor, blocks long-term memory formation inLymnaea

Crowding is an environmental stressor. We found that this stressor altered(i.e. prevented) the ability of Lymnaea to form long-term memory(LTM) following operant conditioning of aerial respiratory behaviour. The ability to form LTM was compared between snails that had been crowded (20 snails per 100 ml of pond water) and those maintained in uncrowded conditions(two snails per 100 ml of pond water). Crowding either immediately before or after two different operant conditioning procedures – the traditional training procedure and the memory augmentation procedure – blocked LTM formation. However, if crowding is delayed by more than 1h following training or if crowding stops 1h before training, LTM results. If memory is already formed, crowding does not block memory recall. Pond water from a crowded aquarium or crowding with clean shells from dead snails, or a combination of both, is insufficient to block LTM formation. Finally, crowding does not block intermediate-term memory (ITM) formation. Since ITM is dependent on new protein synthesis whereas LTM is dependent on both new protein synthesis and altered gene activity, we hypothesize that crowding alters the genomic activity in neurons necessary for LTM formation.

Integrative biology of an embryonic respiratory behaviour in pond snails:the `embryo stir-bar hypothesis'

Embryos of freshwater snails undergo direct development from single cell to juvenile inside egg masses that are deposited on vegetation and other substratum in pond, lake and stream habitats. Helisoma trivolvis, a member of the Planorbidae family of basommatophoran snails, has served as a model for studying the developmental and physiological roles for neurotransmitters during embryogenesis. Early studies revealed that H. trivolvis embryos from stage E15 to E30, the period between gastrulation and the trochophore–juvenile transition, display a cilia-driven behaviour consisting of slow basal rotation and transient periods of rapid rotation. The discovery of a bilateral pair of early serotonergic neurons,named ENC1, which project an apical process to the embryo surface and basal neurites to ciliated cells, prompted the hypothesis that each ENC1 is a dual-function sensory and motor neuron mediating a physiological embryonic response. This article reviews our past and present studies and addresses questions concerning this hypothesis, including the following. (1) What environmental signal regulates ENC1 activity and rotational behaviour? (2)Does ENC1 function as both a primary sensory and motor neuron underlying the rotational behaviour? (3) What are the sensory transduction mechanisms? (4)How does ENC1 regulate ciliary beating? (5) Do other basommatophoran species have similar neural–ciliary pathways and behavioural responses? (6) How is the behaviour manifest in the dynamic natural environment? In this review,we introduce the `embryo stir-bar hypothesis', which proposes that embryonic rotation is a hypoxia-sensitive respiratory behaviour responsible for mixing the egg capsule fluid, thereby enhancing delivery of environmental oxygen to the embryo.

Electrophysiological and behavioral evidence demonstrating that predator detection alters adaptive behaviors in the snail Lymnaea

Stress has been shown to both impair and enhance learning, long-term memory (LTM) formation, and/or its recall. The pond snail, Lymnaea stagnalis, both detects and responds to the scent of a crayfish predator with multiple stress-related behavioral responses. Using both behavioral and electrophysiological evidence, this investigation is a first attempt to characterize how an environmentally relevant stressor (scent of a predator) enhances LTM formation in Lymnaea. Using a training procedure that, in "standard" pond water (PW), results in an intermediate-term memory that persists for only 3 h, we found that training snails in "crayfish effluent" (CE) induces a memory that persists for 48 h (i.e., its now an LTM). In addition, if we use a training procedure that in PW produces an LTM that persists for 1 d, we find that snails trained in CE have an LTM that persists for at least 8 d. Furthermore, we describe how a single neuron (RPeD1), which has been shown to be a necessary site for LTM formation, reflects the behavioral changes in its firing properties that persist for the duration of the LTM. Finally, Lymnaea exhibit context-specific memory, that is, when a memory is formed in a specific context (food odorant), it is only recalled in that context. Here, we found that snails trained in CE demonstrate context generalization, that is, memory is recalled in multiple contexts. All data are consistent with the hypothesis that learning in a stressful, yet biologically relevant, environment enhances LTM and prolongs its retention.