First evidence of unpigmented rhabdomeric photoreceptors in a Microstomum species (Plathelminthes, Macrostomorpha, Microstomidae) and ultrastructure of photoreceptor-like ciliary aggregations

Microstomum spiculifer possesses a pair of intracerebral photoreceptors each consisting of a single rhabdomeric sensory cell and two cup or mantle cells. The mantle cells are devoid of pigment. In addition, four so-called ciliary aggregations, presumed to have a light-sensing function, are present. Each ciliary aggregation represents a specialized cell with an internal cavity filled with axonemes of modified cilia. Rhabdomeric photoreceptors consisting of one to three sensory cells and a single pigmented or unpigmented mantle cell are widespread within taxa of the Plathelminthes Rhabditophora. On the contrary, the existence of two mantle cells forming the eye cup is only known for M. spiculifer and a few other species of the Macrostomida. Therefore, at least two hypotheses are possible: (1) two cup cells are a basic characteristic of the Rhabditophora and a reduction from two to one cup cell has occurred secondarily or (2) the stem species of the Rhabditophora possessed rhabdomeric eyes with one cup cell, and two mantle cells have evolved within the Macrostomorpha. The existence of ciliary aggregates has been documented for several taxa of the Plathelminthes Rhabditophora. From their distribution it can not be concluded whether these differentiations are either a basic feature of the Rhabditophora or have evolved several times convergently. Accepted: 26 September 1999     

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. Accepted: 6 July 1998     

Comparative morphology of photoreceptors in free-living plathelminths — a survey

In free-living Plathelminthes, the best-known photoreceptors are pigment-cup ocelli, eyes formed of one or several pigmented supportive cells into whose cup-shaped cavity project the light-sensitive elements of one or several sensory cells. Besides these, so-called Sehkolben, photoreceptors lacking pigment granules, are found in some species. Sensory cells in plathelminth photoreceptors most commonly use microvilli as the light-sensitive organelles, but some use cilia and combinations of microvilli and cilia. Lamellate ciliary bodies with cilia whose membranes are strongly flattened and rolled and pericerebral ciliary aggregations with interwoven cilia protruding into an intracellular cavity are likely photoreceptors in that they show amplification of membrane likely to bear photoreceptive pigments. Cells with ballooned cilia and tubular vacuoles are other differentiations to which light-sensitivity has been attributed. A variety of structures serve as lenses, all usually formed from parts of the pigment cell.     

Ultrastruktur pericerebraler Cilienaggregate beiDicoelandropora atriopapillata Ax undNotocaryoplanella glandulosa Ax (Turbellaria,Proseriata)

Zusammenfassung Die Ultrastruktur pericerebral gelegener Cilienaggregate vonDicoelandropora atriopapillata Ax, 1956, undNotocaryoplanella glandulosa (Ax, 1951) wird beschrieben. Die intracellulÄren Aggregate setzen sich jeweils aus mehreren 100 Cilien zusammen, die lückenlos alternierend angeordnet und distal miteinander verflochten sind. Für die Cilienaggregate wird eine photoreceptorische Funktion wahrscheinlich gemacht.

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Ultrastructure of pericerebrally located ciliary aggregations inDicoelandropora atriopapillata Ax andNotocaryoplanella glandulosa Ax (Turbellaria, Proseriata)

Summary The ultrastructure of pericerebrally located ciliary aggregations inDicoelandropora atriopapillata Ax, 1956, andNotocaryoplanella glandulosa (Ax, 1951) is described. The intracellular aggregations consist of some 100 cilia, which are arranged in an alternating position without gaps. The distal parts of the cilia are interweaved. The aggregations are interpreted as photoreceptors.

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Abkürzungen af aktinÄhnliche Filamente - bk Basalkörper - c Gehirn - ca Glienaggregat - cm ciliÄre Mikrotubuli - ep Epidermis - mi Mitochondrium - mt Mikrotubuli - n Kern - pn Protonephridium - rr rostrales Ringband - sk Sehkolben - st Statocyste - tb Tastborste - vk ventrale Kriechsohle Die Arbeit wurde durch Mittel der Akademie der Wissenschaften und der Literatur in Mainz gefördert. — Für technische Assistenz danken wir Frl. E. Hildenhagen     

Ultrastructural investigations of presumed photoreceptors as a means of discrimination and identification of closely related species of the genus Microphthalmus (Polychaeta,Hesionidae)

Summary Differences in the ultrastructure of presumed photoreceptors of three morphologically similar Microphthalmus populations on the opposite sides of the Atlantic (German North Sea coast and coasts of North Carolina and Massachusetts) suggest the existence of three different species. Only the European M. listensis possesses three pairs of prostomial eyes, of which one pair has rhabdomeric receptors and pigment cells. The two other pairs are unpigmented and can be found in all three species. The frontal one has ciliary receptors, the posterior one rhabdomeric sensory cells. An additional unpaired potential photoreceptor organ in the segment with the first pair of tentacular cirri is present in all individuals of this species complex. It has a relatively high number of cilia with numerous microvillar projections. — For each type of ocellus there are slight but distinct and constant differences among the species such as relative position of sensory cells, presence of dilations of the ciliary shafts, number of cilia, and shape of the sensory cells. Presence of both ciliary and rhabdomeric light-sensitive cells is discussed with reference to various theories of the evolution of photoreceptors.Abbreviations ax axonema - bb basal body - cc cup cell - ci cilium - cu cuticle - epc epidermal cell - g Golgi apparatus - gp glycogen particles - mi mitochondrion - mv microvilli - mvb multivesicular body - nu nucleus - pc pigment cell - pg pigment granule - rer rough ER - smc submicrovillar cysternae - sr striated rootlet     

A new epidermal ciliary photoreceptive cell type in Plagiostomum lemani (Plathelminthes, Prolecithophora)

 Plagiostomum lemani possesses extremely specialized intraepidermal sensory cells. These obvious photoreceptors, which are not visible with the light microscope, are ciliary aggregations located in an intracellular cavity. The numerous spiralled cilia have the classic 9 × 2 + 2 arrangement at their base and a modified pattern of microtubules apically. The discovered differentiations do not show a connection to the surface. Neither mantle cells nor pigment cells have been found. The structural similarities with other epidermal photoreceptors of species among the different taxa of free-living Plathelminthes are outlined. Besides the larval stages of the taxon Polycladida known so far, the same kind of light-sensing photoreceptive cell has never been described in any other species of the Plathelminthes. Accepted: 16 November 1997     

Central nervous system and sense organs, with special reference to photoreceptor-like sensory elements, in Polygordius appendiculatus (Annelida), an interstitial polychaete with uncertain phylogenetic affinities

Abstract. The phylogenetic position of Polygordius is still pending; relationships with either Opheliidae or with Saccocirrus are the most favored hypotheses. The present study of Polygordius appendiculatus was designed to look for morphological characters supporting either of these two hypotheses. The homology of the anterior appendages, and the structure of the central nervous system and nuchal organ all required clarification; we also examined whether photoreceptor‐like sense organs exist in adults. From their innervation pattern, it is likely that the anterior appendages represent palps. They lack structures typical of palps in Canalipalpata, such as musculature and coelomic cavities, which would be expected in the case of a saccocirrid relationship. Thirteen photoreceptor‐like sense organs were found in front of the brain, the only structures resembling photoreceptors in adults of P. appendiculatus. These multicellular sense organs comprise a supportive cell and several sensory cells enclosing an extracellular cavity. There are three different types of sensory cells: one rhabdomeric and two ciliary. These sensory cells are combined differently into three forms of sense organ: the most frequent uses all three types of sensory cells, the second possesses one rhabdomeric and one ciliary cell type, and the third has two types of ciliary sensory cells. Whereas similar sensory cells are frequently found in various polychaetes, their combination in one sensory organ is unique to Polygordius and is considered to represent an autapomorphy. The nuchal organs exhibit features typical of polychaetes; there are no specific features in common with Saccocirrus. Instead, the covering structures show obvious similarities to Opheliidae, as can also be found in the central nervous system. Altogether, the current observations do not contradict a relationship with opheliids but provide no evidence of a relationship with Saccocirrus as has been found in certain molecular analyses, and thus currently leave the phylogenetic position of Polygordius unresolved.     

Fine structure of the eyes of Pseudoceros canadensis (Turbellaria,Polycladida)

Summary The cerebral and epidermal ocelli of the Müller's larva and the cerebral and tentacular eyes of the adult turbellarian Pseudoceros canadensis were studied by electron microscopy. The right cerebral ocellus of the larva consists of one cup-shaped pigmented cell and three sensory cells that bear microvilli. The left cerebral eye of the larva has the above named cells plus a sensory cell with many cilia. Evolutionary significance is attributed to the presence of both ciliary and microvillar photoreceptors in an eye of a flatworm. The one epidermal ocellus of the larva is composed of two cells: a cup-shaped pigmented one bearing flattened cilia, the presumed photoreceptors, and a cell above the cup that adds a few nonciliary lamellae to the stack of ciliary ones from the pigmented cell. The adult eyes contain only microvillar receptors; cilia were not observed.     

Ultrastructural investigations of presumed photoreceptive organs in two Saccocirrus species (polychaeta,saccocirridae)

In addition to the pigmented ocelli, four different types of photoreceptor-like organs without shading pigment have been found in Saccocirrus papillocercus and S. krusadensis. The sensory cells of these presumed ocelli are either ciliary or rhabdomeric with ciliary rudiments. With the exception of the multicellular type-2 ocelli they are bicellular consisting of a sensory cell and a supportive cell. In each ocellus the supportive cell forms a thin cup-shaped envelope around the sensory elements. In the type-2 ocellus, 7 supportive cells form an ovoid cavity leaving openings through which dendritic processes of an equal number of sensory cells enter the cavity. The pigmented ocelli possess an ocellar cavity communicating with the exterior through a pore in the eyecup, ciliary rudiments in both sensory and supportive cell, and additional non-photoreceptive sensory cells in the opening of the eyecup. The sensory organs show characteristic differences between the two species, such as presence or absence of a particular type of ocellus (type 2 is absent in S. krusadensis, type 3 in S. papillocercus), number of cilia in type-4 ocelli, density of microvilli, number of non-photoreceptive sensory cells in the pore of the pigmented ocellus, etc. These differences provide important characters which can be used for discrimination either of species or of subgeneric taxa in Saccocirrus. The phylogenetic significance of the different photoreceptive organs is discussed.     

Evolution of vertebrate retinal photoreception

Recent findings shed light on the steps underlying the evolution of vertebrate photoreceptors and retina. Vertebrate ciliary photoreceptors are not as wholly distinct from invertebrate rhabdomeric photoreceptors as is sometimes thought. Recent information on the phylogenies of ciliary and rhabdomeric opsins has helped in constructing the likely routes followed during evolution. Clues to the factors that led the early vertebrate retina to become invaginated can be obtained by combining recent knowledge about the origin of the pathway for dark re-isomerization of retinoids with knowledge of the inability of ciliary opsins to undergo photoreversal, along with consideration of the constraints imposed under the very low light levels in the deep ocean. Investigation of the origin of cell classes in the vertebrate retina provides support for the notion that cones, rods and bipolar cells all originated from a primordial ciliary photoreceptor, whereas ganglion cells, amacrine cells and horizontal cells all originated from rhabdomeric photoreceptors. Knowledge of the molecular differences between cones and rods, together with knowledge of the scotopic signalling pathway, provides an understanding of the evolution of rods and of the rods'' retinal circuitry. Accordingly, it has been possible to propose a plausible scenario for the sequence of evolutionary steps that led to the emergence of vertebrate photoreceptors and retina.     

Ultrastructural and immunocytochemical observations of the nervous systems of three macrodasyidan gastrotrichs

The nervous systems of three macrodasyidan gastrotrichs, Dactylopodola baltica, Macrodasys caudatus and Dolichodasys elongatus, were investigated using immunocytochemistry and electron microscopy. Labelling of neural structures against serotonin revealed the presence of two pairs of cerebral cells, a dorsal cerebral connective, and paired ventral nerve cords in D. baltica. In M. caudatus and D. elongatus serotonin immunoreactivity was present in a single pair of dorsal cerebral cells and the ventral nerve cords; the dorsal connective of D. elongatus was also immunoreactive to serotonin and acetylated α‐tubulin. The presence of paired, serotonin‐like immunoreactive cells in D. baltica and other species may represent the plesiomorphic condition in Macrodasyida. The fine structure of the photoreceptors in D. baltica was also investigated to explore the potential ground pattern for eyes in the Macrodasyida. The pigmented photoreceptors of D. baltica contain a unicellular pigment cup, sheath cell and sensory receptor. The pigment cup contains numerous osmiophilic granules that presumably function to shield the eyes from downwelling light in the red part of the spectrum. Projecting into the pigment cup and sheath cell are numerous microvilli from a bipolar sensory cell. A single sensory cell may represent the plesiomorphic condition in Macrodasyida, with multiplication of sensory cells representative of more derived taxa.     

Evolution of cerebral vesicles and their sensory organs in an ascidian larva

Sorrentino M., Manni L., Lane N. J. and Burighel P. 2000. Evolution of cerebral vesicles and their sensory organs in an ascidian larva. —Acta Zoologica (Stockholm) 81 : 243–258 The ascidian larval nervous system consists of the brain (comprising the visceral ganglion and the sensory vesicle), and, continuous with it, a caudal nerve cord. In most species two organs, a statocyst and an ocellus with ciliary photoreceptors, are contained in the sensory vesicle. A third presumptive sensory organ was sometimes found in an ‘auxiliary’ ganglionic vesicle. The development and morphology of the sensory and auxiliary ganglionic vesicles in Botryllus schlosseri and their associated organs was studied. The sensory vesicle contains a unique organ, the photolith, responding to both gravity and light. It consists of a unicellular statocyst, in the form of an expanded pigment cup receiving six photoreceptor cell extensions. Presumptive mechano‐receptor cells (S1 cells), send ciliary and microvillar protrusions to contact the pigment cup. A second group of distinctive cells (S2), slightly dorsal to the S1 cells, have characteristic microvillar extensions, resembling photoreceptor. We concur with the idea that the photolith is new and derived from a primitive statocyst and the S2 cells are the remnant of a primitive ocellus. In the ganglionic vesicle some cells contain modified cilia and microvillar extensions, which resemble the photoreceptor endings of the photolith. Our results are discussed in the light of two possible scenarios regarding the evolution of the nervous system of protochordates.     

Development and differentiation of the eye in Platynereis dumerilii (Annelida,Polychaeta)

The nereid polychaete, Platynereis dumerilii, possess two pairs of post-trochophoral eyes with one vitreous body each. The development of these eyes has first been observed in 2-day-old larvae. Whether the eye anlagen arise from stem cells or from undifferentiated ectodermal tissue was not determined. At first, the anlagen of the anterior and the posterior eyes adjoin each other. They separate in late 3-day-old larvae. The first separated eye complexes consist each of two supporting and two sensory cells. The supporting cells synthesize two different kinds of granules, the pigment granules of the pigment cup and the prospective tubules of the vitreous body. These tubules accumulate in the distal process of the supporting cell. The vitreous body is formed by compartments of the supporting cells filled with the osmiophilic vitreous body tubules. The short, bulbar photosensory processes bear microvilli that emerge into the ocular cavity. At the apex of each sensory cell process, a single cilium (or occasionally two) arises. The sensory cells contain a different kind of pigment granule within their necks at the level of the pigment cup. The rate of eye development and differentiation varies. New supporting cells are added to the rim of the eye cup. They contribute to the periphery of the vitreous body like onion skins, and sensory cells move between supporting cells. The older the individual compartments of the vitreous body are, the more densely packed is their content of vitreous body tubules. Elongation of the sensory and supporting cell processes of the older cells increases the volume of the eye. The eyespots of the trochophore are briefly described as of the two-celled rhabdomeric type with a single basal body with ciliary rootlet.     

Development and hatching state of ectodermal vesicle-organs in the head ofSepia officinalis,Loligo vulgaris andLoligo forbesi (Cephalopoda,Decabrachia)

Summary The embryonic development and ultrastructure of three pairs of vesicle-organs, ectodermal in origin, in the heads ofSepia officinalis, Loligo vulgaris andLoligo forbesi hatchlings is studied. Between the two pairs of organs located in the anterior part and the pair in the posterior head region, different structures and ultrastructures develop during embryogenesis. The function of the anterior pairs can not be determined. The posterior pair are presumed to be rhabdomeric, photosensitive organs because of the presence of bipolar sensory cells. At their apical, luminal surface numerous long, irregular microvilli protrude — similar to the neurons of various simple rhabdomeric photoreceptors in invertebrates.     

Ultrastructure of spermiogenesis and spermatozoa in Bradynectes sterreri and remarks on sperm cells in Haplopharynx rostratus and Paromalostomum fusculum: phylogenetic implications for the Macrostomorpha (Plathelminthes)

 The fine structure of spermiogenesis and spermatozoa in three species of the Macrostomorpha was studied, with emphasis on Bradynectes sterreri. Two centrioles appear during the development of sperm cells, at least in B. sterreri and Paromalostomum fusculum. Initially these organelles have a perpendicular position, but later they come to lie in line with each other. In P. fusculum, the differentiation of rootlet structures inserting on both centrioles was found. However, ciliary axonemes do not grow out, either in B. sterreri or in P. fusculum. These two species, and also Haplopharynx rostratus, have aciliated spermatozoa. The mature male gametes of B. sterreri are characterized by a filiform nucleus, numerous mitochondria, dense bodies irregular in shape, membranous lacunae, a pair of electron-dense lateral ledges and two sets of cortical microtubules in addition to a closed ring of microtubules in the posterior segment of the cell. Both lateral ledges do not originate from the centrioles. ’Lateral ledges’ or ’lateral bristles’ were not observed in spermatozoa of H. rostratus and P. fusculum. Such structures cannot be considered autapomorphic for the Macrostomorpha. The known spermatological characteristics contribute to elucidating the interrelationships of the Macrostomorpha. Haplopharynx and Macrostomida are sister groups. Spermatozoa with cortical microtubules separated into two sets are hypothesized as an autapomorphy of the Macrostomida. The two lateral ledges found in spermatozoa of B. sterreri are discussed to correspond to the pair of ’lateral bristles’ known from Macrostomum species, indicating a sister-group relationship of these two taxa. Apparently, the aciliated spermatozoa of Macrostomorpha species originated from biciliated male gametes. Hence, biciliated spermatozoa are not an evolutionary novelty of the Trepaxonemata, but of the Rhabditophora. Accepted: 22 February 1999     

Localisation of GYIRFamide immunoreactivity inMacrostomum hystricinum marinum (Plathelminthes,Macrostomida)

The distribution of GYIRFamide immunoreactivity in the nervous system ofMacrostomum hystricinum marinum has been demonstrated by an indirect fluorescence technique in conjunction with confocal scanning laser microscopy (CSLM). Immunostaining was extensive in both the central (CNS) and peripheral (PNS) nervous systems, revealing detailed information on the microanatomy of the peptidergic nervous system of this free-living plathelminth. In the CNS, immunoreactive nerve cell bodies and nerve fibres occurred in the brain and along two pairs of longitudinal nerve cords: the main nerve cords and the ventral nerve cords. In the PNS, immunostaining was prevalent in nerve cells and fibres innervating the pharynx and the gut. The employed antibody is directed against a recently characterised FMRF-amide-related peptide (FaRP), GYIRFamide, isolated from two species of the Tricladida,Dugesia tigrina andBdelloura candida. Phylogenetically, GYIRFamide represents the most ancient neuropeptide thus far identified within the Bilateria     

Ultrastructure of prostomial photoreceptors in four marine polychaete species (annelida)

The photoreceptors of four polychaete species were investigated by transmission electron microscopy: Eteone longa and Anaitides mucosa (Phyllodocidae), Scolelepis squamata (Spionidae), and Heteromastus filiformis (Capitellidae). Four different types of light-sensitive organs could be distinguished: 1) a simple, unpigmented rhabdomeric type; 2) a simple ocellus composed of a sensory and a pigmented cell; 3) complex eyes with a lens consisting of secretory granules; 4) a simple, unpigmented type with modified cilia. In spite of its simpler organization the fourth type is listed last, because its function as a photoreceptor seems dubious. The first type (unpigmented rhabdomeric receptor) occurs in all four species investigated. It is the only type of photoreceptor in Heteromastus. Additionally, the two phyllodocids Eteone and Anaitides possess another kind of receptor (type 4) in close proximity to the type 1 receptor. Simple ocelli (type 2) are found in Scolelepis. A pair of complex eyes (type 3) is present in both Eteone and Anaitides, but they show important differences in the two species. First, the eyes in Eteone exhibit ciliary rudiments within the sensory processes, but such rudiments are absent in the eyes of Anaitides. Secondly, the sensory cells in Anaitides possess pigment granules, whereas in Eteone they do not. Thirdly, the lens in Eteone is composed of secretion granules of equal electron density, whereas in Anaitides the lens granules show increased electron density centrally. Lens material appears to be secreted from a single corneal cell in Eteone, and from several corneal cells in Anaitides. In both species these corneal cells are located distally outside the lens.     

The anatomy of the median ocellus of Limulus

Summary The median ocellus of Limulus consists of irregular groups of large photoreceptor cells which form a cup-shaped retina around the ocellar lens. Each group is surrounded and penetrated by guanophores and glia. The photoreceptor cells have extensive rhabdomeric regions, both along infoldings of cell membranes and between cells. Five-layered junctions occur between rhabdomeric microvilli. An occasional arhabdomeric (AR) cell is associated with a group of photoreceptors. Fine dendritic branches of the AR cell penetrate the rhabdomeric regions and form five-layered junctions with photoreceptor rhabdomeres. Axons of photoreceptor cells, and of at least some AR cells, gather at the proximal side of the cup to form an optic nerve.Supported in part by NIH EY00312 and EY00377.We would like to thank Dr. W. K. Stell, Dr. A. C. Bell, and Dr. W. H. Fahrenbach for their helpful discussions.     

The ciliary photoreceptor in the teleplanic veliger larvae of Smaragdia sp. and Strombus sp. (Mollusca,Gastropoda)

The cerebrally innervated eyes of the veliger larvae of Smaragdia sp. and Strombus sp. are composed of a lens, a cornea, and an everse retina. The retina contains two different types of cells, ciliary sensory cells and supportive cells which bear one or two cilia. It is suggested that: (a) the ciliary photoreceptors of these teleplanic veliger larvae are correlated with a long pelagic life in the ocean, which can last up to twelve months, and (b) that structural details of the photoreceptors can change during ontogenesis (ciliary vs rhabdomeric). Furthermore, the cilia of the supportive cells apparently tranport lens material and thus play an important role in lens formation. A decomposition mechanism of pigment granules is examined.Abbreviations bb basal body - bp basal plate - c cilium - cc corneal cell - cm ciliary membranes - cw ciliary whorl - ecm extracellular matrix - gr electron-dense granules - l lens - lb lamellar body - mp membranous pieces - mt microtubules - mv microvilli - n nucleus - oc optic cavity - on optic nerve - pg pigment granule - sc sensory cell - sj septate junction - spc supportive cell - v vesicles     

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.