A crustacean compound eye adapted for low light intensities (Isopoda)

Summary The optical performance of the apposition compound eye of the marine isopodCirolana borealis Lilljeborg (Crustacea) was investigated. The ommatidia comprise large lenses (diam. ca. 150 m), spherical crystalline cones and hypertrophied rhabdoms. The 7 rhabdomeres are fused distally and open proximally. We have designated this rhabdom type as semifused. Distal pigment cells screen neighbouring ommatidia, and a well developed reflecting pigment layer surrounds the rhabdom. The focal length was determined in situ and refractive index measurements, raytracings, and eye mappings were made. The focus was found to lie well below the distal rhabdom tip. A theoretical acceptance function was constructed and a 50% acceptance angle of 45 ° was estimated. The eye parameter (p, according to Snyder 1977) of different ommatidia was between 44 and 14. This together with the anatomy demonstrate an optimation to extremely low light intensities. TheCirolana eye provides an example where acuity is sacrificed for the eye to be able to see at the low light intensities of the inhabitat.The investigation has been supported by a grant from the Swedish Natural Science Research Council (grant no. 2760-103). Our thanks are due to the staff of the marine biological station in Espegrend (Norway). The skilled technical assistance of Miss Inger Norling, Mrs. Rita Wallen, and Miss Maria Walles is gratefully acknowledged. And finally, we would like to express our deep appreciation to Professor Rolf Elofsson for constructive discussions and for his interest and encouragement throughout the investigation.