Three general types of pigmented eye structure have evolved in nematodes. In the unique eye of Mermis nigrescens, we found that the dense pigment is an oxyhemoglobin with high oxygen affinity. Spectral sensitivity determination showed it is not a photopigment. Instead, it functions to shadow a photoreceptor, and, together with a scanning motion of the 'head', functions in the control of body orientation during phototaxis.

Non-vertebrate hemoglobins continue to fascinate scientists with surprising new functions; Mermis eye hemoglobin is the first discovered with an optical function. Its structure resembles other nematode globins with high oxygen-binding affinity. The eye globin is one of two myoglobins expressed at low levels in the body wall tissue of Mermis. It becomes extremely concentrated in the eye due to over-expression in epidermal cells of the head. Such selective high-level expression of a protein for an optical function is also observed in vertebrate lens cells: crystallins at high concentration provide the necessary refractive index, but are expressed by the same gene at low concentration in other tissue for other functions.

In addition to further studies on Mermis eye hemoglobin, we will continue to explore mechanisms of phototaxis.