Interspecific Variations in the Morphology and Ultrastructure of the Rhabdoms of Oplophorid Shrimps.

Interspecific variations in rhabdom structure between various oplophorid shrimps are described and the differences are related to the light environment at different depths within the mesopelagic zone. The ultrastructure of the distal rhabdom in these species is described for the first time. Quantitative measurements show that the proportion of the rhabdom layer occupied by the distal rhabdom varies from 3.5% to 25% in the dorso-ventral plane of the eye of Systellaspis debilis. The distal rhabdom occupies less than 1% of the rhabdoms in the eye of Acanthephyra pelagica, where it can only be seen using the electron microscope. It is suggested that the rhabdoms of those species that remain within the photic zone (such as S. debilis) are adapted to maximize contrast whereas in those whose depth ranges extend into the aphotic zone (such as A. pelagica) they are adapted for maximum sensitivity.