Reflecting optics in the diverticular eye of a deep-sea barreleye fish (Rhynchohyalus natalensis)

We describe the bi-directed eyes of a mesopelagic teleost fish, Rhynchohyalus natalensis, that possesses an extensive lateral diverticulum to each tubular eye. Each diverticulum contains a mirror that focuses light from the ventro-lateral visual field. This species can thereby visualize both downwelling sunlight and bioluminescence over a wide field of view. Modelling shows that the mirror is very likely to be capable of producing a bright, well focused image. After Dolichopteryx longipes, this is only the second description of an eye in a vertebrate having both reflective and refractive optics. Although superficially similar, the optics of the diverticular eyes of these two species of fish differ in some important respects. Firstly, the reflective crystals in the D. longipes mirror are derived from a tapetum within the retinal pigment epithelium, whereas in R. natalensis they develop from the choroidal argentea. Secondly, in D. longipes the angle of the reflective crystals varies depending on their position within the mirror, forming a Fresnel-type reflector, but in R. natalensis the crystals are orientated almost parallel to the mirror's surface and image formation is dependent on the gross morphology of the diverticular mirror. Two remarkably different developmental solutions have thus evolved in these two closely related species of opisthoproctid teleosts to extend the restricted visual field of a tubular eye and provide a well-focused image with reflective optics

The population biology of the living coelacanth studied over 21 years

Between 1986 and 2009 nine submersible and remote-operated vehicle expeditions were carried out to study the population biology of the coelacanth Latimeria chalumnae in the Comoro Islands, located in the western Indian Ocean. Latimeria live in large overlapping home ranges that can be occupied for as long as 21 years. Most individuals are confined to relatively small home ranges, resting in the same caves during the day. One hundred and forty five coelacanths are individually known, and we estimate the total population size of Grande Comore as approximately 300–400 adult individuals. The local population inhabiting a census area along an 8-km section of coastline remained stable for at least 18 years. Using LASER-assisted observations, we recorded length frequencies between 100 and 200 cm total length and did not encounter smaller-bodied individuals (\100 cm total length). It appears that coelacanth recruitment in the observation areas occur mainly by immigrating adults. We estimate that the mean numbers of deaths and newcomers are 3–4 individuals per year, suggesting that longevity may exceed 100 years. The domestic fishery represents a threat to the long-term survival of coelacanths in the study area. Recent changes in the local fishery include a decrease in the abundance of the un-motorized canoes associated with exploitation of coelacanths and an increase in motorized canoes. Exploitation rates have fallen in recent years, and by 2000, had fallen to lowest ever reported. Finally, future fishery developments are discussed

Vision in the deep sea

The deep sea is the largest habitat on earth. Its three great faunal environments - the twilight mesopelagic zone, the dark bathypelagic zone and the vast flat expanses of the benthic habitat- are home to a rich fauna of vertebrates and invertebrates. In the mesopelagic zone (150-1000 in), the down-welling daylight creates an extended scene that becomes increasingly dimmer and bluer with depth. The available daylight also originates increasingly, from vertically above, and bioluminescent point-source flashes, well contrasted against the dim background daylight become increasingly visible. In the bathypelagic zone below 1000 m no daylight remains, and the scene becomes entirel, dominated by point-like biolumincscence. This changing nature of visual scenes with depth - from extended source to point source - has had a profound effect on the designs of deep-sea eyes, both optically and neurally, a fact that until recently was not fully appreciated. Recent measurements of the sensitivity and spatial resolution of deep-sea eyes - particularly from the camera eyes of fishes and cephalopods and the compound eyes of crustaceans - reveal that ocular designs are well matched to the nature of the visual scene at any criven depth. This match between eye design and visual scene is the subject of this review. The greatest variation eye design is found in the mesopelagic zone, where dim down-welling daylight and bioluminescent point Sources may be visible simultaneously. Some ruesopelagic eyes rely on spatial and temporal Summation to increase sensitivity to a dim extended scene, while others sacrifice this sensitivity to localise pinpoints of bright bioluminescence. Yet other eyes have retinal regions separately specialised for each type of light. In the bathypelagic zone, eyes generally get smaller and therefore less sensitive to point sources with increasing depth. In fishes, this insensitivty, combined with surprisingly high spatial resolution, is very well adapted to the detection and locallsation of point-source bioluminescence at ecologically meaningful distances. At all depths, the eyes of animals active on and over the nutrient-rich sea floor are generally larger than the eyes of pelagic species. In fishes, the retinal ganglion bells are also frequently arranged in a horizontal visual streak, an adaptation for., the wide flat horizon of the sea floor, and all animals living there. These and many other aspects of light viewing and vision in the deep sea are renewed in support of the following conclusion: it is not only the intensity of light at different depths, but also its distribution in space, which has been a major force in the evolution of deep-sea vision

Loss of negative eye-size allometry in a population of Aplochiton zebra (Teleostei: Galaxiidae) from the Falkland Islands

The population of zebra trout (Aplochiton zebra – Galaxiidae) in Red Pond, Falkland Islands, lacks the negative eye size allometry that is typical of the species elsewhere. Eye size retains a near constant relationship to head length throughout growth in Red Pond. In addition, the bold, narrow, vertical, zebra-like, dark bands typically found on the body of this species are lacking, or are present in the Red Pond population only as broader dusky blotches. Absence of negative allometry is probably due to lack of coupling of eye and somatic growth, probably owing to slow growth of the fish living in the challenging dietary environment of a turbid lake. Observations of diet show that the species is a generalised invertebrate carnivore, but that food intake may be low, suggesting that the modified coupling of eye growth to somatic growth is a likely explanation of the loss of negative allometry

The Origin of the Vertebrate Eye

In his considerations of “organs of extreme perfection,” Charles Darwin described the evidence that would be necessary to support the evolutionary origin of the eye, namely, demonstration of the existence of “numerous gradations” from the most primitive eye to the most perfect one, where each such tiny change had provided a survival advantage (however slight) to the organism possessing the subtly altered form. In this paper, we discuss evidence indicating that the vertebrate eye did indeed evolve through numerous subtle changes. The great majority of the gradual transitions that did occur have not been preserved to the present time, either in the fossil record or in extant species; yet clear evidence of their occurrence remains. We discuss the remarkable “eye” of the hagfish, which has features intermediate between a simple light detector and an image-forming camera-like eye and which may represent a step in the evolution of our eye that can now be studied by modern methods. We also describe the important clues to the evolutionary origin of the vertebrate eye that can be found by studying the embryological development of our own eye, by examining the molecular genetic record preserved in our own genes and in the genes of other vertebrates, and through consideration of the imperfections (or evolutionary “scars”) in the construction of our eye. Taking these findings together, it is possible to discuss in some detail how the vertebrate eye evolved