These eyes are hot . . .

A “brain freeze headache” describes the cephalic sensation one has when drinking a supercooled “slurpee” too quickly. This odd sensation represents vascular and neurological compromise with the sudden temperature reduction. Any predatory fish that would negotiate the thermoclines from the relatively warmer epipelagic waters to those of near freezing at ocean depths of 500 metres or more would face a formidable challenge, especially if the fish is ectothermic (cold blooded), since such changes would impede muscular activity as well as neurological and visual mechanisms, including photoreception. The swordfish (Xiphlius gladius), a highly visually directed predatory species, as well as other billed fish, such as marlin and sailfish, solves these problems with unique evolutionary adaptations.

Swordfish spend a portion of their day in relatively shallow waters of 50–100 metres, where they may themselves be preyed upon by sport and commercial fishermen. But, they also may spend as much as 12–19 hours at depths of 300–700 metres, and frequently below the 300–400 metre range. At this depth the water temperature will be a “brain freezing” 2–8°C. Neurological, visual, and muscular physiology is impeded by such temperatures and those fish that live at these mesopelagic depths …