Upside-down gliding of Lymnaea

Surface crawling and pedal surface collecting in aquatic gastropods: A case of scientific amnesia

Aquatic gastropods, both freshwater and marine, often crawl along the water surface with the sole of the foot facing upward. Differential regulation of the activity of cilia on the sole turns the gliding mechanism into pedal surface collecting, by which food particles floating on the water are collected on the sole. Over the last 300 years, surface crawling and pedal surface collecting have been studied in numerous species, but the accumulated knowledge has fallen victim to scientific amnesia. Today, pedal surface collecting usually is considered a unique behaviour that evolved in the family Ampullariidae (Caenogastropoda), which includes the genus Pomacea with some of the globally worst invasive pests. Consequently it may appear feasible to tackle invasive Pomacea species specifically by delivering molluscicides via the water surface. Based on a review of our forgotten literature, I here argue that such an approach would have potentially devastating, unintended effects on native gastropod faunas.

Invaders as Diluents of the Cercarial Dermatitis Etiological Agent

Research on alien and invasive species focuses on the direct effects of invasion on native ecosystems, and the possible positive effects of their presence are most often overlooked. Our aim was to check the suitability of selected alien species (the snail Physa acuta, the bivalve Dreissena polymorpha, and the gammarid Dikerogammarus villosus) as diluents for infectious bird schistosome cercariae—the etiological factor of swimmer’s itch. It has been hypothesized that alien species with different feeding habits (scrapers, filterers and predators) that cohabit the aquatic environment with intermediate hosts of the schistosomatid trematodes are capable of feeding on their free-swimming stages—cercariae. In the laboratory conditions used, all experimental animals diluted the cercariae of bird schistosome. The most effective diluents were P. acuta and D. villosus. However, a wide discrepancy in the dilution of the cercariae between replicates was found for gammarids. The obtained results confirm the hypothesis that increased biodiversity, even when alien species are involved, creates the dilution effect of the free-living stages of parasites. Determining the best diluent for bird schistosome cercariae could greatly assist in the development of current bathing areas protection measures against swimmer’s itch.

Production of Basal Bodies in bulk for dense multicilia formation

Centriole number is normally under tight control and is directly linked to ciliogenesis. In cells that use centrosomes as mitotic spindle poles, one pre-existing mother centriole is allowed to duplicate only one daughter centriole per cell cycle. In multiciliated cells, however, many centrioles are generated to serve as basal bodies of the cilia. Although deuterosomes were observed more than 40 years ago using electron microscopy and are believed to produce most of the basal bodies in a mother centriole-independent manner, the underlying molecular mechanisms have remained unknown until recently. From these findings arise more questions and a call for clarifications that will require multidisciplinary efforts.

Coordinated Feeding Behavior in Trichoplax, an Animal without Synapses

Trichoplax is a small disk-shaped marine metazoan that adheres to substrates and locomotes by ciliary gliding. Despite having only six cell types and lacking synapses Trichoplax coordinates a complex sequence of behaviors culminating in external digestion of algae. We combine live cell imaging with electron microscopy to show how this is accomplished. When Trichoplax glides over a patch of algae, its cilia stop beating so it ceases moving. A subset of one of the cell types, lipophils, simultaneously secretes granules whose content rapidly lyses algae. This secretion is accurately targeted, as only lipophils located near algae release granules. The animal pauses while the algal content is ingested, and then resumes gliding. Global control of gliding is coordinated with precise local control of lipophil secretion suggesting the presence of mechanisms for cellular communication and integration.

Effect of acute exposure to low environmental calcium on respiration and locomotion in Lymnaea stagnalis (L.)

Environmental calcium is a major factor affecting the distribution of freshwater gastropods. Whilst the effects on growth and morphology are fairly well understood, little is known about how calcium availability affects other aspects of gastropod biology. Lymnaea stagnalis (L.) is considered a calciphile and exhibits reduced growth and survival in environments containing less than 20 mg l(-1) Ca(2+). Many freshwater systems exhibit fluctuations in calcium concentration over time: where calcium levels are normally high there may be periods of low [Ca(2+)], for example following periods of flooding. Here we examined the effects of acute periods of low (20 mg l(-1)) environmental calcium on the physiology and behaviour of L. stagnalis, specifically measuring how locomotion and respiration differ between high calcium (80 mg l(-1)) and low calcium (20 mg l(-1)) environments. We found that in a low calcium environment crawling speed is reduced, and that this coincides with an increase in cutaneous respiration, indicating that the increased metabolic demands of calcium acquisition at low [Ca(2+)] reduce the energy available for locomotion. Conversely we found a decrease in aerial respiration in hypoxic conditions in the low calcium relative to the high calcium environment. In conclusion, we found that acute exposure to low environmental calcium has a highly significant effect on locomotion and respiration, which may have consequences for snail fitness when no morphological effects are apparent.

Contrary effects of octopamine receptor ligands on behavioral and neuronal changes in locomotion of lymnaea

The pond snail Lymnaea stagnalis moves along the sides and bottom of an aquarium, but it can also glide upside down on its back below the water's surface. We have termed these two forms of locomotion "standard locomotion" and "upside-down gliding," respectively. Previous studies showed that standard locomotion is produced by both cilia activity on the foot and peristaltic contraction of the foot muscles, whereas upside-down gliding is mainly caused by cilia activity. The pedal A neurons are thought to receive excitatory octopaminergic input, which ultimately results in increased cilia beating. However, the relationship between locomotory speed and the responses of these neurons to octopamine is not known. We thus examined the effects of both an agonist and an antagonist of octopamine receptors on locomotory speed and the firing rate of the pedal A neurons. We also examined, at the electron and light-microscopic levels, whether structural changes occur in cilia following the application of either an agonist or an antagonist of octopamine receptors to the central nervous system (CNS). We found that the application of an octopamine antagonist to the CNS increased the speed of both forms of locomotion, whereas application of octopamine increased only the firing rate of the pedal A neurons. Microscopic examination of the cilia proved that there were no changes in their morphology after application of octopamine ligands. These data suggest that there is an unidentified octopaminergic neuronal network in the CNS whose activation reduces cilia movement and thus locomotory speed.