The barreleye (Macropinna microstoma) is a member of the Opisthoproctidae family, a bizarre collection of approximately 11 oceanic species, all of which are mesopelagic to bathypelagic in habit with no shortage of unusual ocular adaptations evolved to master this biologically forbidding environment.

M microstoma is an odd, translucent (even its brain is visible), little fish with a maximum length of about 5 cm (specimen illustrated on the cover measured 4.4 cm). Living at depths up to 900 metres below sea level, where darkness, extreme pressure, and numbing cold will winnow the weak, this solitary fish faces challenges that evolution has met with creative, but not entirely surprising, innovations. This particular species has very large spherical pigmented crystalline lenses, and vertically directed tubular eyes among other adaptations.

Lens pigmentation may occur in diurnal vertebrates and in these species the pigment is a soluble component of the lens cells and is freely diffusible throughout the cells within the crystalline lens. This yellow pigmentation, seen in mammals such as the ground squirrel (Spermophilus beecheyi), is an adaptation believed to improve acuity by reducing chromatic aberration of the shorter wavelengths.

Lens pigmentation may also occur as an age related phenomenon (as cataract surgeons know well). This pigmentation is probably a degenerative ageing change and a result of ultraviolet exposure. But, some ageing changes in pigmentation of the crystalline lens are not degenerative. Some mesopelagic fish (Argyropelecus affinis hatchetfish in the family Sternoptychidae) have an abrupt pigmentation of the lens fibres beginning at approximately the mid-point in their maturation cycle, …