Comparative studies of chromophore contents inside and outside the rhabdoms of arthropod compound eyes

Crustacean photoreceptor damage and recovery: Applying a novel scanning electronic microscopy protocol in artificial light studies

Artificial light at night is a worldwide expanding form of pollution. Studies evaluating the effects of artificial light at night have often overlooked their impact on the photoreceptor, the basic functional structure of animals to absorb light. This is essential to understand the mechanisms by which this stressor may be impacting species. This study examined the photoreceptor (rhabdom) of two sandy beach crustaceans exhibiting different light tolerances at night: the amphipod Orchestoidea tuberculata and the isopod Tylos spinulosus. We developed a novel protocol to measure these species' photoreceptor areas and quantify the damage caused by artificial light at night using histological sections and scanning electron microscopy. In the isopod, a species naturally adapted to lower light intensities at night than the amphipod, the rhabdom surface was 20-times larger, and presented a tapetum, an adaptive feature found in species living in low light conditions. A brief exposure to artificial light caused 3-6 times more damage in the isopod's rhabdom. In fact, the light caused structural damage in the isopod's rhabdom but not in the amphipod's rhabdom, with no signs of recovery after 1 and 24 h. These findings suggest that the damage induced by artificial light at night on an organism's photoreceptors is more severe and persistent in species naturally adapted to lower light levels at night. Hence, this type of sensory ecological stressor may act as a novel selection pressure on these species, a concern with wide implications given the ubiquity among animals of the photoreceptor structure and its response to light.

Combination of physiological and anatomical methods for studying extraocular photoreceptors on the genitalia of the butterfly, Papilio xuthus

This paper describes a combination of electrophysiological and anatomical techniques useful for characterizing sensory neurons in insects in our studies on the extraocular photoreceptors on the genitalia of the butterfly, Papilio xuthus. Genital photoreceptors were first electrophysiologically identified by recording photoreceptor spikes in response to light stimulation of the genitalia. The precise location and ultrastructure of these photoreceptors were then studied by light and electron microscopy. Both electrophysiological and anatomical techniques employed here were rather classical, but, as shown in this paper, they appear to be particularly useful for systems where intracellular penetration is difficult.

Morphogenesis of the photoreceptive site and development of the electrical responses in the butterfly genital photoreceptors during the pupal period

This paper describes the process of morphogenesis of the photoreceptive site of the butterfly genital photoreceptors. Associated development of the electrical responses is also described. The photoreceptor is a sensory neuron whose cell body is located in the genitalia and has a photoreceptive site of the phaosome-type. This consists of the distal processes and the tubular membranes, which protrude from the tip of distal processes. Phaosome morphogenesis was studied using electron microscopy. The results indicate that morphogenesis occurs in the latter half of the pupal period and that the process is divided into five phases. First, the tubular membranes appear as small membrane protrusions (phase I). The short tubular membranes emerge from several portions of the cell body forming several membrane clusters (phase II). The clusters then collect to form a small phaosome. Short distal processes become evident (phase III). The phaosome volume increases, mainly due to the extensive elongation and bifurcation of both tubular membranes and distal processes (phase IV). Phase V achieves final adult morphology. The photoreceptors of phase II are already able to produce spikes in response to light stimulation, although the sensitivity was about one tenth of the adult. The sensitivity increase occurred in parallel with the increase in the phaosome volume.

Ultrastructure of the extraocular photoreceptor in the genitalia of a butterfly, Papilio xuthus

This paper describes the ultrastructure of a sensory neuron found in the extraocular photoreceptive site on the butterfly genitalia. Our previous studies have shown that there are two pairs of the photoreceptive sites in a butterfly (four per individual). Each photoreceptive site is recognizable by a transparent area in the pigmented cuticle of the genitalia. From the nerve that extends from the transparent cuticle to the last abdominal ganglion, a sustained train of single unit spikes can be recorded in response to a light flash. The single unit spikes disappear when the transparent cuticle is covered, thus indicating that a single photoreceptor exists close to it. Here, we examined complete serial semithin sections of plastic-embedded photoreceptive sites of both male and female, and observed an ovoid structure close to the transparent cuticle. The structure contained the cell body of a sensory neuron whose axon was in the nerve branch from which the photoresponse had been recorded. Further electron microscopy revealed that the cell body extended a few distal processes that protrude tubular membrane from the tip, forming a structure resembling the phaosome (from Greek; phaos = light, some = body) which was first described in the earthworm dermal photoreceptors. The sensory neuron was also found in a surgically isolated nerve-photoreceptor preparation that responded to the light. We therefore propose that the phaosome-containing sensory neuron is the butterfly genital photoreceptor.

Synthesis of 3-hydroxyretinal in the cytosol of the butterfly compound eye

The metabolism of 3-hydroxyretinoids in the cytosol of the compound eyes of a species of butterfly, Papilio xuthus, was investigated. The cytosol was found to contain 25-30% of the total 3-hydroxyretinal and 70-82% of the total 3-hydroxyretinol in the eye. These percentages of 3-hydroxyretinoids in the cytosol were found to be constant regardless of whether the eyes are light-adapted or dark-adapted. 3-Hydroxyretinal can be newly synthesized in the cytosol of light-adapted eyes. Blue light specifically increases the amount of 11-cis and all-trans 3-hydroxyretinal ca 2.5 and 1.8 times respectively, compared to pre-irradiation. When 3-hydroxyretinal was synthesized, 3-hydroxyretinol was decreased or disappeared in the cytosol. When retinol (non-native chemical) was added to the cytosol, it was converted into retinal. This result indicates that an oxidative system exists in the compound eye which can convert 3-hydroxyretinol to 3-hydroxyretinal.