Fine structure of photoreceptors in larval trematodes. A comparative study

Photobehaviours guided by simple photoreceptor systems

Light provides a widely abundant energy source and valuable sensory cue in nature. Most animals exposed to light have photoreceptor cells and in addition to eyes, there are many extraocular strategies for light sensing. Here, we review how these simpler forms of detecting light can mediate rapid behavioural responses in animals. Examples of these behaviours include photophobic (light avoidance) or scotophobic (shadow) responses, photokinesis, phototaxis and wavelength discrimination. We review the cells and response mechanisms in these forms of elementary light detection, focusing on aquatic invertebrates with some protist and terrestrial examples to illustrate the general principles. Light cues can be used very efficiently by these simple photosensitive systems to effectively guide animal behaviours without investment in complex and energetically expensive visual structures.

Phototaxis and the origin of visual eyes

Vision allows animals to detect spatial differences in environmental light levels. High-resolution image-forming eyes evolved from low-resolution eyes via increases in photoreceptor cell number, improvements in optics and changes in the neural circuits that process spatially resolved photoreceptor input. However, the evolutionary origins of the first low-resolution visual systems have been unclear. We propose that the lowest resolving (two-pixel) visual systems could initially have functioned in visual phototaxis. During visual phototaxis, such elementary visual systems compare light on either side of the body to regulate phototactic turns. Another, even simpler and non-visual strategy is characteristic of helical phototaxis, mediated by sensory-motor eyespots. The recent mapping of the complete neural circuitry (connectome) of an elementary visual system in the larva of the annelid Platynereis dumerilii sheds new light on the possible paths from non-visual to visual phototaxis and to image-forming vision. We outline an evolutionary scenario focusing on the neuronal circuitry to account for these transitions. We also present a comprehensive review of the structure of phototactic eyes in invertebrate larvae and assign them to the non-visual and visual categories. We propose that non-visual systems may have preceded visual phototactic systems in evolution that in turn may have repeatedly served as intermediates during the evolution of image-forming eyes.

Ultrastructure of the Miracidium of Echinostoma paraensei Lie and Basch, 1967 (Trematoda, Echinostomatidae)

The morphology of Echinostoma paraensei was studied using transmission electron microscopy. The terebratorium region has many electrondense secretory granules and many folds on the surface. The epidermal cells that cover the larval body have unique nuclear morphology, many mitochondria and vesicles being attached to the interepidermal ridges by a septate junction. The cilia present the organization 9 + 2 and a typical structure with a shaft, axosome, basal body and rootlet. Below the epidermal cells there is a layer of circular muscle and, adjacent to it, a layer of longitudinal muscle fibers. The excretory system has two flame cells, with internal and external ribs and leptotriches at the barrel region, an excretory vesicle and an excretory pore.

Light and scanning electron microscopy of the miracidium of Echinostoma paraensei (Trematoda, Echinostomatidae)

Echinostoma paraensei was described 1967 by Lie and Basch. Recently its natural definitive host, the aquatic rodent Nectomys squamipes, endemic in Brazil, was identified. As most of the echinostomatids, this species presents a zoonotic potential. The morphology and topography of the E. paraensei miracidium obtained from adult worms collected from the natural definitive host N. squamipes is described by light and electron microscopy. The arrangement and the dimensions of the epidermal plates are given. The eyespots are composed by two pairs of lenses measuring 6.03 microm. SEM observations shows that the miracidium body is covered by cilia except at the terebratorium region and the presence of alpha-tubulin in the cilia of the larvae is first recorded by immunelabeling. Nineteen papilla-like structures arranged in three axes and four groups were observed at the terebratoriun, this structure is retractable, presenting folds and a corrugated surface with profiles of cytoplasmic expansions, forming network of anatomizing folds.

Photoreceptor evoked potentials and phototactic behaviour in Cercaria caribbea LXXI cable

1. Transient potential changes evoked in response to light stimuli, and presumably arising from rhabdomeric eye-spots in the cercarial body, were recorded for the first time, to our knowledge, in helminth parasites. 2. Pigmented Cercaria caribbea LXXI gave a very slowly adapting response to maintained light stimulus, while a non-pigmented variety appeared to emit a stronger, rapidly adapting response to light onset. 3. Swimming towards a directional light source is disrupted by several neuropharmacological agents, which presumably disturb synaptic transmission in the nerve/muscle system. 4. The light evoked potentials were unaffected by these same agents and therefore, appeared to be directly recorded receptor potentials.

[The eyespots of the oncomiracidium of Euzetrema knoepffleri (Monogenea): ultrastructure and evolution during the life-cycle (author's transl)]

Euzetrema knoepffleri (Monogenea, Monopisthocotylea) is a parasite of the urinary bladder of Euproctus montanus (Amphibia, Urodela). The eyespots of the oncomiracidium of E. knoepffleri are rhabdomeric; each anterior pigmented cell has one rhabdomere, each posterior pigmented cell has two rhabdomeres. This pattern is quite similar to that of the eyespot of another Monopisthocotylea, Entobdella soleae but the lack of cristalline lens in E. knoepffleri appears as a new feature of the Monopisthocotylea. The symmetry of the pigmented structures seems to be effectively connected with the swimming mode of the larva. Moreover, the ultrastructural differences between the two species Euzetrema knoepffleri and Entobdella soleae may be in relation with their different behaviour concerning light. After the fixation of the larva on its host, the comparative study of the evolution of the eyespots, shows the disappearance of the cristalline lens in Entobdella, and the loss of rhabdomeric structures in Euzetrema. These differences seem related with the nature of the microbiotope of the adult: Entobdella soleae is a skin parasite, Euzetrema knoepffleri a reno-vesical one.

Studies on the pigmented and unpigmented photoreceptors of the cercaria of Cryptocotyle lingua (Creplin) from Littorina littorea (L.)

The cercaria ofCryptocotyle linguapossesses a pair of pigmented rhabdomeric photoreceptors situated antero-dorsally, and a median unpigmented rhabdomeric photoreceptor lying slightly ventral to the middle region of the body immediately anterior to the transverse commissure. Each pigmented photoreceptor consists of two asymmetrical pigment cells containing two symmetrical back-to-back pigment cups facing respectively antero-laterally and postero-medianly, with one retinular cell associated with each pigment cup. The microvilli of the rhabdomere in each cup are radially arranged around a triradiate dendritic process; in the centre of each microvillus is a strand presumably the visual pigment. The retinular cell and dendritic process contain a large number of mitochondria. The rhabdomere of the median unpigmented photoreceptor is enclosed in a thin-walled unpigmented cup opening posteriorly and giving off processes, some of which contain mitochondria, into the surrounding parenchyma. This rhabdomere resembles the equivalent of two rhabdomeres of the paired photoreceptors. Axonal processes from the retinular cells of the paired and unpaired photoreceptors enter the neuropile of the cerebral ganglia and transverse commissure respectively. The cercaria is markedly photopositive and responds also to shadow by active bursts of swimming. It is suggested that the paired dorsal photoreceptors may be sensitive to light of higher intensity and the median more ventral photoreceptor to light of lower intensity and to shadow.

Fine structure of the nervous system and specialized nerve endings in the miracidium of Fasciola hepatica

The fine structure and organization of the nervous system of the miracidium is described. The central ganglion consists of a mass of nerve fibres with peripherally situated cell bodies. Tracts of fibres run outward from the central mass to the muscles of the body wall and to specialized endings. The individual fibres contain several types of inclusions and synapses are common in the neuropile.

Excluding the eyespots, four kinds of specialized nerve endings are described. Two of these types terminate at the body surface and bear ‘cilia’. The other two terminate internally, one type within the body wall and the other type laterally, slightly anterior to the central ganglion. It is suggested that the function of three of the types of ending might be sensory, detecting chemical and mechanical stimuli, and the direction of gravity. The function of the fourth type of ending is problematical but might be secretory.

I am most grateful to Dr A. Robards for use of electron microscope facilities and to Mrs J. Denison for technical assistance.