Functional morphology and food habits of deep-sea copepods of the genus Cephalophanes (Calanoida: Phaennidae): perception of bioluminescence as a strategy for food detection

Deep-sea copepods of the genus Cephalophanes are characterised by their large eyes with semi-parabolic reflectors, but their ultrastructure and ecological significance have rarely been investigated. We examined the structure of the eyes, feeding appendages and digestive tract, and the gut contents of Cephalophanes spp., collected from deep-waters (500 to 1000 m) of the subtropical western Pacific, through light and electron microscopic observations. The optic system of Cephalo- phanes spp. involves a pair of large, bilateral reflectors, each having anteriorly 2 dorso-ventrally arranged retinal cells and 1 more pair of retinal cells positioned posterior to each reflector. The reflec- tor has a multilayer structure of thin (60 to 180 nm at the posterior end) plates decreasing in thickness from the inner to the outer sides of the reflector; the number of plates is ca. 10 to 20 near the anterior margin of the reflector, increasing posteriorly, and reaching ca. 80 near the apex of the reflector. The reflector provides a high (80 to 90%) reflectance over a wide range of wavelengths (350 to 700 nm) of incident light parallel to the reflector axis. The mandible gnathobase has thin blades arranged in par- allel, and the setae on the gnathobase of the first maxilla and on the second maxilla and the maxil- liped are slender with sparse setules, suggesting feeding on relatively large, inactive particles. In 27 out of the 101 specimens examined, a pear-shaped mass of unknown origin occupied the whole lumen of the well-defined anterior caecum of the midgut. In 25 of these 27 specimens, the lumen of the middle part of the midgut contained fragments of crustaceans, but essentially no other con- stituents of 'marine-snow' such as phytoplankton, protista, or other nanoplankton. Of the other 74 specimens examined, 97% had empty midguts. These observations suggest a specialization of the anterior midgut caecum for chitin digestion and a possible detection by Cephalophanes spp. of opti- cal signals from crustacean carcasses, a nutritionally poor but ubiquitous food source in the deep sea.