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81. The eye of the Glasseye (Heteropriacanthus cruentatus)


The eye of the Glasseye (Heteropriacanthus cruentatus), a member of the Bigeyes family (Priacanthidae), is extremely well adapted for life in obscure milieus, like caves, caverns, and deep water. The Glasseye is a member of the nocturnal active family of bigeyes (Priacanthidae).

This 30cm/12inch long fish, which sometimes congregates in small groups, feeds only from dusk till dawn. To do so, the Glasseye will venture away from the safety of its daytime hiding place, usually a cave, cavern, or nook-like crevice, to hunt for diverse planktonic critters. At night, plankton and small invertebrates like tiny squids or certain krill species and other small crustaceans rise from the depths of the abyss to shallower water. This migration attracts nocturnal plankton feeders species like the Glasseye.

The Glasseye depicted in this underwater video clip, Heteropriacanthus cruentatus, inhabits solely the southwestern area of the Indian Ocean and the southern part of the Atlantic Ocean. The Glasseye that occurs in the entire Indo-Pacific region is actually Heteropriacanthus carolinus, and the species found in the northern part of the Atlantic is Heteropriacanthus fulgens. Previously, it was thought that these three species were one and the same.

The Glasseye can reach a total length of approximately 50cm/20inch and is found in outward reefs from a depth of 3m/10ft to a maximum depth of around 300m/1.000ft.

Like all other members of the Bigeyes family, they have big spherical lenses and photoreceptors specialized to see in dark and low-light environments. The size of their eyes is disproportionately large compared to most fish, hence their family name "bigeyes." The larger the eye, the more light it can gather and focus onto the retina, potentially allowing for better visual acuity in a dark and obscure environment.

The photoreceptor cells of the Glasseye contain a higher concentration of rhodopsin, a biological pigment that enables vision in low-light conditions. The eye is also equipped with an extra tapetum lucidum, a membranous layer consisting of iridescent leucophores that reflects visible light back to the photoreceptor cells located just behind the retina. By doing so, they increase the amount of light available to the photoreceptors.

The Glasseye is also able to rapidly intensify and fade its reddish colour and silver patterns.


The Glasseye was filmed in a somewhat dark crevice with the aid of video lights. These lights also illuminated the less interesting parts of the environment where the fish was venturing. To make this quite pale and reddish with silvery bares coloured Glasseye stand out, I desaturated the pinkish/purplish algae shrubbery in the foreground slightly.

More on this topic:

For another post about the Glasseye and a more in-depth description about iridescence please go to our vlog post 150 or click on this link: https://www.beyondscuba.com/post/glasseye-heteropriacanthus-cruentatus-iridescence

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