Ciliary Type Research Articles

Single-Cell Transcriptomic Analysis Reveals the Molecular Profile of Go-Opsin Photoreceptor Cells in Sea Urchin Larvae.

The ability to perceive and respond to light stimuli is fundamental not only for spatial vision but also to many other light-mediated interactions with the environment. In animals, light perception is performed by specific cells known as photoreceptors and, at molecular level, by a group of GPCRs known as opsins. Sea urchin larvae possess a group of photoreceptor cells (PRCs) deploying a Go-Opsin (Opsin3.2) which have been shown to share transcription factors and morphology with PRCs of the ciliary type, raising new questions related to how this sea urchin larva PRC is specified and whether it shares a common ancestor with ciliary PRCs or it if evolved independently through convergent evolution. To answer these questions, we combined immunohistochemistry and fluorescent in situ hybridization to investigate how the Opsin3.2 PRCs develop in the sea urchin Strongylocentrotus purpuratus larva. Subsequently, we applied single-cell transcriptomics to investigate the molecular signature of the Sp-Opsin3.2-expressing cells and show that they deploy an ancient regulatory program responsible for photoreceptors specification. Finally, we also discuss the possible functions of the Opsin3.2-positive cells based on their molecular fingerprint, and we suggest that they are involved in a variety of signaling pathways, including those entailing the thyrotropin-releasing hormone.

Ciliary Type III Adenylyl Cyclase in the VMH Is Crucial for High-Fat Diet-Induced Obesity Mediated by Autophagy.

Neuronal primary cilia are crucial for body weight maintenance. Type III adenylyl cyclase (AC3) is abundantly enriched in neuronal cilia, and mice with global AC3 ablation are obese. However, whether AC3 regulates body weight through its ciliary expression and the mechanism underlying this potential regulation are not clear. In this study, humanized AC3 knock‐in mice that are resistant to high‐fat diet (HFD)‐induced obesity are generated, and increases in the number and length of cilia in the ventromedial hypothalamus (VMH) are shown. It is demonstrated that mice with specifically knocked down ciliary AC3 expression in the VMH show pronounced HFD‐induced obesity. In addition, in vitro and in vivo analyses of the VMH show that ciliary AC3 regulates autophagy by binding an autophagy‐related gene, gamma‐aminobutyric acid A receptor‐associated protein (GABARAP). Mice with GABARAP knockdown in the VMH exhibit exacerbated HFD‐induced obesity. Overall, the findings may reveal a potential mechanism by which ciliary AC3 expression regulates body weight in the mouse VMH.

X-Ray Fiber Diffraction and Numerical Simulation Studies on the Change in the Helical Symmetry of Chlamydomonas Flagellar Axonemes Coupled with the Change in Ca2+ Concentrations

Cilia and flagella are motile organelles found in various types of eukaryotes. In their axonemes, dynein arms generate shear between adjacent doublets, and this shear is converted into complex waveforms. The conversion mechanism has remained a long-standing unresolved issue. Since Chlamydomonas flagella change their waveforms from a flagellar type to a ciliary type, responding to the increase in intracellular Ca2+ concentrations, this response may provide a hint for the mechanism of the flagellar waveform generation.

Fibrocystin Is Essential to Cellular Control of Adhesion and Epithelial Morphogenesis.

Mutations of the Pkhd1 gene cause autosomal recessive polycystic kidney disease (ARPKD). Pkhd1 encodes fibrocystin/polyductin (FPC), a ciliary type I membrane protein of largely unknown function, suggested to affect adhesion signaling of cells. Contributions of epithelial cell adhesion and contractility to the disease process are elusive. Here, we link loss of FPC to defective epithelial morphogenesis in 3D cell culture and altered cell contact formation. We study Pkhd1-silenced Madin-Darby Canine Kidney II (MDCKII) cells using an epithelial morphogenesis assay based on micropatterned glass coverslips. The assay allows analysis of cell adhesion, polarity and lumen formation of epithelial spheroids. Pkhd1 silencing critically affects the initial phase of the morphogenesis assay, leading to a reduction of correctly polarized spheroids by two thirds. Defects are characterized by altered cell adhesion and centrosome positioning of FPC-deficient cells in their 1-/2-cell stages. When myosin II inhibitor is applied to reduce cellular tension during the critical early phase of the assay, Pkhd1 silencing no longer inhibits formation of correctly polarized epithelia. We propose that altered sensing and cell interaction of FPC-deficient epithelial cells promote progressive epithelial defects in ARPKD.

Sensing deep extreme environments: the receptor cell types, brain centers, and multi-layer neural packaging of hydrothermal vent endemic worms.

IntroductionDeep-sea alvinellid worm species endemic to hydrothermal vents, such as Alvinella and Paralvinella, are considered to be among the most thermotolerant animals known with their adaptability to toxic heavy metals, and tolerance of highly reductive and oxidative stressful environments. Despite the number of recent studies focused on their overall transcriptomic, proteomic, and metabolic stabilities, little is known regarding their sensory receptor cells and electrically active neuro-processing centers, and how these can tolerate and function in such harsh conditions.ResultsWe examined the extra- and intracellular organizations of the epidermal ciliated sensory cells and their higher centers in the central nervous system through immunocytochemical, ultrastructural, and neurotracing analyses. We observed that these cells were rich in mitochondria and possessed many electron-dense granules, and identified specialized glial cells and serial myelin-like repeats in the head sensory systems of Paralvinella hessleri. Additionally, we identified the major epidermal sensory pathways, in which a pair of distinct mushroom bodies-like or small interneuron clusters was observed. These sensory learning and memory systems are commonly found in insects and annelids, but the alvinellid inputs are unlikely derived from the sensory ciliary cells of the dorsal head regions.ConclusionsOur evidence provides insight into the cellular and system-wide adaptive structure used to sense, process, and combat the deep-sea hydrothermal vent environment. The alvinellid sensory cells exhibit characteristics of annelid ciliary types, and among the most unique features were the head sensory inputs and structure of the neural cell bodies of the brain, which were surrounded by multiple membranes. We speculated that such enhanced protection is required for the production of normal electrical signals, and to avoid the breakdown of the membrane surrounding metabolically fragile neurons from oxidative stress. Such pivotal acquisition is not broadly found in the all body parts, suggesting the head sensory inputs are specific, and these heterogenetic protection mechanisms may be present in alvinellid worms.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-014-0082-9) contains supplementary material, which is available to authorized users.

Ciliary photoreceptors in the cerebral eyes of a protostome larva

BackgroundEyes in bilaterian metazoans have been described as being composed of either ciliary or rhabdomeric photoreceptors. Phylogenetic distribution, as well as distinct morphologies and characteristic deployment of different photopigments (ciliary vs. rhabdomeric opsins) and transduction pathways argue for the co-existence of both of these two photoreceptor types in the last common bilaterian ancestor. Both receptor types exist throughout the Bilateria, but only vertebrates are thought to use ciliary photoreceptors for directional light detection in cerebral eyes, while all other invertebrate bilaterians studied utilize rhabdomeric photoreceptors for this purpose. In protostomes, ciliary photoreceptors that express c-opsin have been described only from a non-visual deep-brain photoreceptor. Their homology with vertebrate rods and cones of the human eye has been hypothesized to represent a unique functional transition from non-visual to visual roles in the vertebrate lineage.ResultsTo test the hypothesis that protostome cerebral eyes employ exclusively rhabdomeric photoreceptors, we investigated the ultrastructure of the larval eyes in the brachiopod Terebratalia transversa. We show that these pigment-cup eyes consist of a lens cell and a shading pigment cell, both of which are putative photoreceptors, deploying a modified, enlarged cilium for light perception, and have axonal connections to the larval brain. Our investigation of the gene expression patterns of c-opsin, Pax6 and otx in these eyes confirms that the larval eye spots of brachiopods are cerebral eyes that deploy ciliary type photoreceptors for directional light detection. Interestingly, c-opsin is also expressed during early embryogenesis in all potential apical neural cells, becoming restricted to the anterior neuroectoderm, before expression is initiated in the photoreceptor cells of the eyes. Coincident with the expression of c-opsin in the presumptive neuroectoderm, we found that middle gastrula stage embryos display a positive photoresponse behavior, in the absence of a discrete shading pigment or axonal connections between cells.ConclusionsOur results indicate that the dichotomy in the deployment of ciliary and rhabdomeric photoreceptors for directional light detection is not as clear-cut as previously thought. Analyses of brachiopod larval eyes demonstrate that the utilization of c-opsin expressing ciliary photoreceptors in cerebral eyes is not limited to vertebrates. The presence of ciliary photoreceptor-based eyes in protostomes suggests that the transition between non-visual and visual functions of photoreceptors has been more evolutionarily labile than previously recognized, and that co-option of ciliary and rhabdomeric photoreceptor cell types for directional light detection has occurred multiple times during animal evolution. In addition, positive photoresponse behavior in gastrula stage embryos suggests that a discrete shading pigment is not requisite for directional photoreception in metazoans. Scanning photoreception of light intensities mediating cell-autonomous changes of ciliary movement may represent an ancient mechanism for regulating locomotory behavior, and is likely to have existed prior to the evolution of eye-mediated directional light detection employing axonal connections to effector cells and a discreet shading pigment.

Expression of a retinal homeobox (Rx) gene during planarian regeneration

Retinal homeobox (Rx) genes, with representatives in vertebrates and invertebrates, encode fundamental regulators of early eye and brain formation. Here we describe the spatio-temporal expression profile of a candidate planarian orthologue of Rx during regeneration in Dugesia japonica and Schmidtea mediterranea. Although low levels of Rx transcripts were found throughout the body of intact planarians, high levels of Rx expression were specific to regenerating tissue in both head and tail fragments. We also observed that Rx was never expressed in the simple rhabdomeric planarian eyes, supporting the notion that only formation of eyes that use the ciliary type of photoreceptors requires Rx function.

The sense of smell, its signalling pathways, and the dichotomy of cilia and microvilli in olfactory sensory cells.

Smell is often regarded as an ancillary perception in primates, who seem so dominated by their sense of vision. In this paper, we will portray some aspects of the significance of olfaction to human life and speculate on what evolutionary factors contribute to keeping it alive. We then outline the functional architecture of olfactory sensory neurons and their signal transduction pathways, which are the primary detectors that render olfactory perception possible. Throughout the phylogenetic tree, olfactory neurons, at their apical tip, are either decorated with cilia or with microvilli. The significance of this dichotomy is unknown. It is generally assumed that mammalian olfactory neurons are of the ciliary type only. The existance of so-called olfactory microvillar cells in mammals, however, is well documented, but their nature remains unclear and their function orphaned. This paper discusses the possibility, that in the main olfactory epithelium of mammals ciliated and microvillar sensory cells exist concurrently. We review evidence related to this hypothesis and ask, what function olfactory microvillar cells might have and what signalling mechanisms they use.

Cephalochordate Melanopsin: Evolutionary Linkage between Invertebrate Visual Cells and Vertebrate Photosensitive Retinal Ganglion Cells

Animal photoreceptor cells can be classified into two distinct types, depending on whether the photopigment is borne on the membrane of a modified cilium (ciliary type) or apical microvilli (rhabdomeric type) [1]. Ciliary photoreceptors are well known as vertebrate rods and cones and are also found in several invertebrates. The rhabdomeric photoreceptor, in contrast, is a predominant type of invertebrate visual cell, but morphologically identifiable rhabdomeric photoreceptors have never been found in vertebrates. It is hypothesized that the rhabdomeric photoreceptor cell had evolved to be the photosensitive retinal ganglion cell for the vertebrate circadian photoentrainment [2, 3 and 4] owing to the fact that some molecules involved in cell differentiation are common among them [5]. We focused on the cephalochordate amphioxus because it is the closest living invertebrate to the vertebrates, and interestingly, it has rhabdomeric photoreceptor cells for putative nonvisual functions [6]. Here, we show that the amphioxus homolog of melanopsin [7, 8 and 9], the circadian photopigment in the photosensitive retinal ganglion cells of vertebrates, is expressed in the rhabdomeric photoreceptor cells of the amphioxus and that its biochemical and photochemical properties, not just its primary structure, are considerably similar to those of the visual rhodopsins in the rhabdomeric photoreceptor cells of higher invertebrates. The cephalochordate rhabdomeric photoreceptor represents an evolutionary link between the invertebrate visual photoreceptor and the vertebrate circadian photoreceptor.

Two Anti-radial Spoke Monoclonal Antibodies Inhibit Chlamydomonas Axonemal Motility by Different Mechanisms

In the 9 + 2 axoneme, radial spokes are structural components attached to the A-tubules of the nine outer doublet microtubules. They protrude toward the central pair microtubule complex with which they have transient but regular interactions for the normal flagellar motility to occur. Flagella of Chlamydomonas mutants deficient in entire radial spokes or spoke heads are paralyzed. In this study the importance of two radial spoke proteins in the flagellar movement is exemplified by the potent inhibitory action of two monoclonal antibodies on the axonemal motility of demembranated-reactivated Chlamydomonas models. We show that one of these proteins is localized on the stalk of the radial spokes, whereas the other is a component of the head of the same structure and most likely correspond to radial spoke protein 2 and 1, respectively. Fine motility analysis by videomicrography further indicates that these two anti-radial spoke protein antibodies at low concentration affect motility of demembranated-reactivated Chlamydomonas by changing the flagellar waveform without modifying axonemal beat frequency. They also modify wave amplitude differently during motility inhibition. This brings more direct evidence for the involvement of both radial spoke stalk and head in the fine tuning of the waveform during flagellar motility.

New Light on the Vertebrate Eye

Two types of photoreceptor cell are found in vertebrates and invertebrates. The photosensitive apparatus in vertebrates has a sensory cilium, whereas the compound eye of invertebrates is of the rhabdomeric type. Arendt et al. (see the news story by Pennisi) now examine the evolution of animal eyes by comparing photoreceptor cell types in Bilateria. The vertebrate eye is compared to the primitive polycheate ragworm Platynereis , a modern descendant of the last common ancestor to insects and vertebrates. Platynereis possesses the rhabdomeric photoreceptor cell type but also has a brain structure with the ciliary photoreceptor. Thus, an ancestral brain photosensitive complex may have displayed both rhabdomeric and ciliary types. Furthermore, rhabdomeric photoreceptors of the invertebrate eyes most closely relate to the vertebrate retinal ganglion cells, whereas the rods and cones of the vertebrate retina relate to a population of ciliary photoreceptors located in the invertebrate brain. D. Arendt, K. Tessmar-Raible, H. Snyman, A. W. Dorresteijn, J. Wittbrodt, Ciliary photoreceptors with a vertebrate-type opsin in an invertebrate brain. Science 306 , 869-871 (2004). [Abstract] [Full Text] E. Pennisi, Worm's light-sensing proteins suggest eye's single origin. Science 306 , 796-797 (2004). [Summary] [Full Text]

PICTURE Quiz

This woman complained of pain, blurred vision, lacrimation and photophobia of the right eye. It had started the day previously and was getting worse. Examination showed marked injection of the conjunctiva and a ciliary type of engorgement of the vessels causing a deep flush around the limbus. The pupil was constricted and the eyeball was very sensitive to palpation.

Development of a unique eye: photoreceptors of the pelagic predator Atlanta peroni (Gastropoda, Heteropoda)

The eyes of different larval stages and juveniles of Atlanta peroni are generally composed of a cornea, a lens and a retina. In juveniles a distinct pigmented shield is visible and an enormous humour is located behind the lens. Larvae have only two sensory cells and the photoreceptors are of the ciliary type. In juveniles a striking feature is the shape of the retina. It is ribbon-shaped and new sensory cells are present which are arranged in three rows. The photoreceptors are of the ciliary type as well. Contrary to the arrangement in larvae, the ciliary plasmalemma in juveniles forms numerous lamellar stacks. In accordance with the sensory cells the stacks are organized in three parallel rows. The lamellae of adjacent stacks within a row overlap each other. The latter unique feature has not yet been found in any other representative of the Heteropoda. These findings demonstrate that (a) the eyes are altered during the development from larvae into juveniles, (b) the larval sensory cells are reduced and replaced by new sensory cells in juveniles and (c) the eyes of juvenile and adult A. peroni are well adapted for their life as visual predators.

Alterations of the eyes of Carinaria lamarcki (Gastropoda, Heteropoda) during the long pelagic cycle

The eyes of different larval stages of Carinaria lamarcki were examined ultrastructurally. In all larval stages the eyes consist of a cornea, a lens and an everse retina. The photoreceptors in young larvae are exclusively of the ciliary type. In old larvae, however, two types of photoreceptors are present and the retina is composed of two segments: a posterior segment with altered ciliary photoreceptors (=type I sensory cells) and an anterior segment with what are presumably rhabdomeric photoreceptors (=type II sensory cells). The anterior retina is interpreted here as an accelerted character. Furthermore, the arrangement of the pigment granules changes during the long larval development being cup shaped in young larvae versus ribbon shaped in old larvae. The findings allow for the conclusions that: (a) the ciliary photoreceptors are correlated with the long larval period of Heteropoda and that (b) the eyes are altered continuously during the larval cycle.

Ca2+-dependent waveform conversion in the flagellar axoneme of Chlamydomonas mutants lacking the central-pair/radial spoke system.

Chlamydomonas flagella undergo a striking waveform conversion from an asymmetrical ciliary type to a symmetrical flagellar type when the cell is stimulated by intense light and the Ca2+ concentration within the flagellum is increased above approximately 10(-6) M. To see whether the central-pair/radial spoke system is needed for this conversion as suggested by previous studies, we examined the effect of Ca2+ on the reactivated axonemes of the mutants lacking the central pair (pf18) or the radial spokes (pf14). Although the flagella of these mutants are paralyzed in vivo, demembranated axonemes can be reactivated to beat under certain nucleotide conditions such as in the presence of low concentrations (< 100 microM) of ATP. We examined the waveform of the axonemes reactivated at 20 microM ATP in the presence of 10(-8)-10(-4) M Ca2+ and found that these axonemes, as well as the wild-type axonemes, undergo a waveform conversion over a Ca2+ concentration range of 10(-7)-10(-5) M: a highly asymmetrical waveform at <10(-6) M Ca2+ and a symmetrical waveform at >=10(-5) M Ca2+. Although the waveform is different between the mutants and the wild type, the Ca2+ concentration at which the waveform conversion occurred was similar. These results indicate that the central pair/radial spoke system is not essential for the waveform conversion.

The infrared receptor of Melanophila acuminata De Geer (Coleoptera: Buprestidae): ultrastructural study of a unique insect thermoreceptor and its possible descent from a hair mechanoreceptor

The paired infrared organs of Melanophila acuminata consist of 50–100 sensilla situated at the bottom of a pit next to the coxae of the mesothoracic legs, where no exocuticle is developed. Each sensillum is accompanied by a wax gland and has a cuticular lens-like spherule (diameter 12–15 μm) bulging out with its upper hemisphere above the surface, covered only by a thin cuticle of about 1 μm. Distal processes of two enveloping cells surround the entire spherule in the form of a flattened protoplasmatic layer with the exception of a small apical stalk connecting the spherule to the outer cuticle. The spherule is innervated by a single sensory neuron of the ciliary type which is anchored ventrally with the distal tip of its cylindrical and unbranched DOS in the spherule. The insertion of the dendrite, which contains a well-developed tubular body, is always eccentric like in a hair mechanoreceptor (sensillum trichodeum) and there is no evidence of any optical function of the spherule. Three enveloping cells exist, but only one — probably the trichogen cell — forms a relatively small outer receptor lymph cavity. In the posterior wall of the pit — where exocuticle is developed — so-called suppressed systems can be found which remain completely below the cuticle with their otherwise well-developed spherules. Additionally, there is a tendency towards basally flattening and longitudinally stretching of spherules which are situated more peripherally. They strongly resemble the basal regions of hair mechanoreceptors (sensilla trichodea) in their immediate neighbourhood which are also accompanied by wax glands. Because of the existence of these transitional stages and the great ultrastructural resemblance between infrared receptors and hair mechanoreceptors concerning the bauplan of the sensory neurons and their mode of innervating the cuticular apparatus, we conclude that the infrared sensilla are probably derived from hair mechanoreceptors. Based on these results and transmission measurements of infrared radiation through the cuticular components of the organ, a model of the possible function of the infrared receptor is presented.

Coordinated flagellar and ciliary beating in the protozoon Tritrichomonas foetus.

Tritrichomonas foetus is a flagellated protozoon found in urogenital tract of cattle. Its free movement in liquid medium is powered by the coordinated movement of three flagella projecting towards the anterior region of the cell, and one recurrent flagellum that forms a junction with the cell body and ends as a free projection in the posterior region of the cell. We have used video microscopy and digital image processing to analyze the relationships between the movements of these flagella. The anterior flagella beat in a ciliary type pattern displaying effective and recovery strokes, while the recurrent flagellum beats in a typical flagellar wave form. One of the three anterior flagella has a distinctive pattern of beating. It beats straight in its forward direction as opposed to the ample beats performed by the others. Frequency measurements obtained from cells swimming in a viscous medium shows that the beating frequency of the recurrent flagellum is approximate twice the frequency for the three anterior flagella. We also observed that the costa and the axostyle do not show any active motion. On the contrary, they form a cytoskeletal base for the anchoring and orientation of the flagella.

Ultrastructure of Presumed Ocelli in Parenterodrilus taenioides (Polychaeta, Protodrilidae) and their Phylogenetic Significance

Abstract Three different types of presumed unpigmented ocelli have been found in the anterior end of Parenterodrilus taenioides, a small gutless interstitial polychaete. The type‐1 ocelli are located in the palps and four ocelli have been found along the length of each palp. The type‐2 and type‐3 ocelli lie close together in the head segment and are located in posterior ganglionic expansions of the brain. There is one pair of the minute type‐2 ocelli but at least two pairs of the type‐3 organs, which are the largest ocelli. In each ocellus the sensory cells are of the ciliary type and possess two cilia whose plasma membranes branch into numerous microvilli. With the exception of the type‐1 ocelli they consist of a sensory cell and a supportive cell. In each ocellus the supportive cell forms a thin cup‐shaped envelope around the densely packed ciliary branches. The type‐1 ocelli consist of a single cell forming an intracellular vacuole (phaosome) which contains less densely packed microvillus‐like structures. In particular, the structure of these ocelli is compared with that in other polychaetes, with special emphasis on the remaining genera of the Protodrilida.

DEVELOPMENT OF THE FLAGELLAR APPARATUS AND FLAGELLAR POSITION IN THE COLONIAL GREEN ALGA PLATYDORINA CAUDATA (CHLOROPHYCEAE)1

ABSTRACTThe ultrastructure of the flagellar apparatus in pre‐inversion and inversion stages of Platydorina resembles that of Chlamydomonas in having 180° rotational symmetry and clockwise absolute orientation. Basal bodies are in a “V” configuration and connected by one distal and two proximal fibers. Alternating two‐ and four‐membered microtubular rootlets are cruciately arranged. During maturation, the basal bodies rotate and separate, and 180° rotational symmetry is lost. Simultaneously, each proximal fiber detaches from one of the functional basal bodies, and the distal fiber detaches from both. The mature apparatus has widely separated and nearly parallel basal bodies. Flagellar orientation in Platydorina is completed just after inversion and a flattening of the colony called intercalation, resulting in the pairs of flagella of neighboring cells extending from the colony in opposite directions in an alternating fashion. Flagellar orientation and separated basal bodies minimize the interference between the flagella of neighboring cells. Basal bodies and rootlets of the two intercalated halves of a colony rotate, resulting in the effective strokes of the flagella of every cell being towards the colonial posterior. The flagella of each cell beat with an effective stroke in the direction of the two inner rootlets. The flagella have an asymmetrical ciliary type beat. The rotated, separated, and parallel basal bodies, together with the nearly parallel rootlets probably are adaptations for movement of this colonial volvocalean alga. The flagellar apparatus in immature stages of Platydorina lends support to the suggestion that the alga has evolved from a Chlamydomonas‐like ancestor.

Multiple effects of ethanol and 5‐hydroxytryptamine on the gill cilia of mytilus edulis

AbstractThe lateral (L) cilia on an isolated filament from the gill of Mytilus edulis remain arrested at the end of their recovery stroke (hands up) when perfused with artificial sea water (ASW). The laterofrontal (LF) cilia continue to be active. The addition of 10% ethanol (ETOH) to the ASW perfusate arrests the LF cilia in a hands‐up posture; the L cilia remain undisturbed. By contrast, 10−6 M 5‐hydroxytryptamine (5HT) in ASW activates the L cilia and arrests the LF cilia at the end of their effective stroke (hands down). Continued perfusion with 10% ETOH (v/v) in ASW/5HT restores activity to the LF cilia but arrests the L cilia (hands up). These effects are reversible and independent of external Ca2+.Following the detergent extraction of the filament, all gill cilia are inactive. The addition of 0.2 mM ATP in the presence of low Ca2+ (&lt; 10−7 M) reactivates all model cilia. Under these conditions, 5HT can no longer inhibit the activity of LF cilia and is not required for the activation of the L cilia. This suggests that 5HT acts at a membrane level. An increase in free Ca2+ (&gt; 10−6 M) arrests the L cilia hands up; the LF cilia remain active. Further Ca2+ increase (&gt; 10−3 M) induces hands‐up arrest of the LF cilia, confirming that the Ca2+ threshold of the two ciliary types is different by several orders of magnitude.The addition of 10% ETOH in low Ca2+ to demembranated reactivated cilia arrests the L cilia hands up while the LF cilia continue to beat. Ten percent ETOH appears to interact with the axoneme, mimicking the effect of high Ca2+ and with the membrane to increase Ca2+ permeability and possibly to inactivate 5HT receptors. These results are discussed in terms of axonemal switching mechanisms and the physiological control of filter feeding in the lamellibranch gill.