Pineal organs of deep-sea fish: photopigments and structure

Spectral sensitivity and photoresponse in the rock bream Oplegnathus fasciatus and their relationships with the absorption maximum of the photoreceptor

The spectral characteristics of visual pigments are a major determinant in eliciting a response to light. To study the absorption maximum of the photoreceptors and their sensitivity to light in fish, rod outer segments (ROS) and cone cells were purified from the rock bream Oplegnathus fasciatus adapted to the dark. Ultraviolet/visible spectroscopic analyses of the ROS in the dark and its difference spectra indicated an absorption maximum of the visual pigment at ~ 500 nm, and each eye of 1-year-old rock bream contained at least 1.2 nmol of rhodopsin-like visual pigments. Microspectrophotometric analysis of the cone cell outer segments led to identification of three visual pigments with individual absorption maxima at 425, 520, and 585 nm. Monochromatic light-emitting diode (LED) modules with different wavelengths (violet 405 nm, blue 465 nm, cyan 505 nm, green 530 nm, amber 590 nm, and red 655 nm) were constructed to examine the spectral sensitivity and photoresponse in association with the absorption maximum of the photoreceptor. Analysis of chromophore decay upon illumination with each LED at low (27 μmol/m²/s) and high (343 μmol/m²/s) intensities showed the highest sensitivity of the photoreceptor upon illumination with the 505-nm cyan LED, followed by LEDs with wavelengths of 530 nm > 465 nm > 405 nm > 590 nm > 655 nm. Photoresponse analysis of the fish using a video tracking system, in the dark and upon illumination, also showed faster movement of fish with illumination with the cyan LED followed by in the order of green ≈ blue > violet > amber > red. These results indicated that a light with a wavelength closer to the absorption maximum of rhodopsin was more effective in eliciting a response to the light.

Ontogenetic shifts in brain scaling reflect behavioral changes in the life cycle of the pouched lamprey Geotria australis

Very few studies have described brain scaling in vertebrates throughout ontogeny and none in lampreys, one of the two surviving groups of the early agnathan (jawless) stage in vertebrate evolution. The life cycle of anadromous parasitic lampreys comprises two divergent trophic phases, firstly filter-feeding as larvae in freshwater and secondly parasitism as adults in the sea, with the transition marked by a radical metamorphosis. We characterized the growth of the brain during the life cycle of the pouched lamprey Geotria australis, an anadromous parasitic lamprey, focusing on the scaling between brain and body during ontogeny and testing the hypothesis that the vast transitions in behavior and environment are reflected in differences in the scaling and relative size of the major brain subdivisions throughout life. The body and brain mass and the volume of six brain structures of G. australis, representing six points of the life cycle, were recorded, ranging from the early larval stage to the final stage of spawning and death. Brain mass does not increase linearly with body mass during the ontogeny of G. australis. During metamorphosis, brain mass increases markedly, even though the body mass does not increase, reflecting an overall growth of the brain, with particularly large increases in the volume of the optic tectum and other visual areas of the brain and, to a lesser extent, the olfactory bulbs. These results are consistent with the conclusions that ammocoetes rely predominantly on non-visual and chemosensory signals, while adults rely on both visual and olfactory cues.

The pineal complex in the cichlid Cichlasoma dimerus: effect of different photoperiods on its cell morphology

This research describes the pineal complex histology in juvenile and adult Cichlasoma dimerus, and the effect of different photoperiods on its cell morphology. In both juveniles and adults, the pineal complex of C. dimerus has three components: the pineal organ, consisting of a pineal vesicle (PV) and a pineal stalk, the parapineal organ and the dorsal sac. Although a strong morphological resemblance exists between the two stages, different synthesis patterns of cone and rod opsins were detected in the two life stages. An effect of the photoperiod length was observed on putative pinealocytes' activity from the PV, measured indirectly through nuclear area morphometry. Individuals exposed to a natural photoperiod (14L:10D) had smaller nuclear areas (mean ± s.e. = 13·82 ± 1·52 µm(2) ) than those exposed to a short photoperiod (8:16) (21·45 ± 2·67 µm(2) ; P < 0·001). Eventually, the nuclear area of pinealocytes could be used as a putative indicator of melatonin synthesis in fishes where it is difficult to obtain plasma samples, e.g. due to its small size or age. This work constitutes one of the few comparative descriptions of the pineal complex of juvenile and adult teleost and suggests potential approaches for the study of melatonin synthesis in fish larvae or small adult fishes.

Adaptation of pineal expressed teleost exo-rod opsin to non-image forming photoreception through enhanced Meta II decay

Photoreception by vertebrates enables both image-forming vision and non-image-forming responses such as circadian photoentrainment. Over the recent years, distinct non-rod non-cone photopigments have been found to support circadian photoreception in diverse species. By allowing specialization to this sensory task a selective advantage is implied, but the nature of that specialization remains elusive. We have used the presence of distinct rod opsin genes specialized to either image-forming (retinal rod opsin) or non-image-forming (pineal exo-rod opsin) photoreception in ray-finned fish (Actinopterygii) to gain a unique insight into this problem. A comparison of biochemical features for these paralogous opsins in two model teleosts, Fugu pufferfish (Takifugu rubripes) and zebrafish (Danio rerio), reveals striking differences. While spectral sensitivity is largely unaltered by specialization to the pineal environment, in other aspects exo-rod opsins exhibit a behavior that is quite distinct from the cardinal features of the rod opsin family. While they display a similar thermal stability, they show a greater than tenfold reduction in the lifetime of the signaling active Meta II photoproduct. We show that these features reflect structural changes in retinal association domains of helices 3 and 5 but, interestingly, not at either of the two residues known to define these characteristics in cone opsins. Our findings suggest that the requirements of non-image-forming photoreception have lead exo-rod opsin to adopt a characteristic that seemingly favors efficient bleach recovery but not at the expense of absolute sensitivity.

Differential light intensity and spectral sensitivities of Atlantic salmon, European sea bass and Atlantic cod pineal glands ex vivo

Photoperiod is perceived by pineal photoreceptors and transduced into rhythmic melatonin signals. These rhythms can be influenced by light intensity and spectral content. In this study we compared the light sensitivity of Atlantic salmon, European sea bass and Atlantic cod by testing ex vivo the effect of different intensities and narrow bandwidth lights on nocturnal melatonin suppression by isolated pineal glands in a flow-through culture system. Using combinations of neutral density and bandpass interference filters we tested a range of light intensities (ranging from 1.22x10(13) to 3.85x10(6) photons s(-1) cm(-2)) and three wavelengths of 80 nm width (472, 555 and 661 nm corresponding to blue, green and red, respectively). Results showed clear species specific light intensity and spectral sensitivities, with cod being from 100 to 1000 times more sensitive than sea bass and salmon. Regarding the influence of spectrum, red light was less efficient on suppressing melatonin than blue and green in salmon but results were not as clear in the two other species studied. Finally, the first evidence of relative photoreception in teleosts was obtained in cod suggesting that the definition of illuminance thresholds (day/night perception) would depend on the day intensity. Indeed, a single order of magnitude increase or decrease in day intensity was shown to elicit a significant shift in the intensity response curve of night-time melatonin suppression. Taken together, this study demonstrated species specific light intensity and spectral sensitivities within temperate teleosts.

A novel morphometry-based protocol of automated video-image analysis for species recognition and activity rhythms monitoring in deep-sea fauna

The understanding of ecosystem dynamics in deep-sea areas is to date limited by technical constraints on sampling repetition. We have elaborated a morphometry-based protocol for automated video-image analysis where animal movement tracking (by frame subtraction) is accompanied by species identification from animals' outlines by Fourier Descriptors and Standard K-Nearest Neighbours methods. One-week footage from a permanent video-station located at 1,100 m depth in Sagami Bay (Central Japan) was analysed. Out of 150,000 frames (1 per 4 s), a subset of 10.000 was analyzed by a trained operator to increase the efficiency of the automated procedure. Error estimation of the automated and trained operator procedure was computed as a measure of protocol performance. Three displacing species were identified as the most recurrent: Zoarcid fishes (eelpouts), red crabs (Paralomis multispina), and snails (Buccinum soyomaruae). Species identification with KNN thresholding produced better results in automated motion detection. Results were discussed assuming that the technological bottleneck is to date deeply conditioning the exploration of the deep-sea.

Melatonin activates brain dopaminergic systems in the eel with an inhibitory impact on reproductive function

In the eel, a deficit in gonadotrophin-releasing hormone (GnRH) and a strong dopaminergic (DA) inhibition are responsible for the blockade of gonad development if silver eels are prevented from their reproductive migration. Environmental factors that eels encounter during their oceanic reproductive migration are thought to play an important role in the stimulation of eel pubertal development. We investigated the potential role of melatonin, a known mediator of the effects of external factors on reproductive function in vertebrates. We demonstrated that a long-term melatonin treatment increased brain tyrosine hydroxylase (TH, the rate limiting enzyme of DA synthesis) mRNA expression in a region-dependent way. Melatonin stimulated the dopaminergic system of the preoptic area, which is involved in the inhibitory control of gonadotrophin [luteinising hormone (LH) and follicle-stimulating hormone (FSH)] synthesis and release. Moreover, we showed that the increased TH expression appeared to be consistent with melatonin binding site distribution as shown by 2[(125)I]-melatonin labelling studies. On the other hand, melatonin had no effects on the two eel native forms of GnRH (mGnRH and cGnRH-II) mRNA expression. Concerning the pituitary-gonad axis, we showed that melatonin treatment decreased both gonadotrophin beta-subunit (LHbeta, FSHbeta) mRNA expression and reduced sexual steroid (11-ketotestosterone, oestradiol) plasma levels. This indicates that melatonin treatment had a negative effect on eel reproductive function. To our knowledge, the results of the present study provide the first evidence that melatonin enhances TH expression in specific brain regions in a non-mammalian species. By this mechanism melatonin could represent one pathway by which environmental factors could modulate reproductive function in the eel.