The anatomy of the deepsea barreleye fish is exceptional in several ways, one of which is …?

Question

In 2002 what, possibly unique, reaction was observed in cichlids, a freshwater fish species, in one short stretch of the River Congo?

Answer 1

Its head is largely transparent
The fish live at depths 400 to 2,500 m (1,312 to 8,202 ft) well into areas without light. The eyes are tubular and barrel-shaped, very good at collecting light and ultra-sensitive; they typically point upwards to detect food overhead but can rotate. The eyes, and the whole head, lie within a transparent fluid-filled covering shield.