Compound Eye Fine Structure in Paralomis multispina Benedict, an Anomuran Half-Crab From 1200 m Depth (Crustacea;...

Fully grown, unsexed specimens of the anomuran half-crab Paralomis multispina Benedict were obtained from a depth of 1200 m, and the eyes of three individuals were prepared for light and electron microscopy. In their outer appearance the compound eyes of Paralomis resemble those of common shallow-water half-crabs (e.g., Petrolisthes), but facets in Paralomis were about 3 times larger in diameter (i.e., 60 {mu}m) and at least twice as long. Interommatidial angles ranged from 3{deg} to 5{deg}. The proximal width of the crystalline cone in Paralomis was 10 times that of its equivalent in the Petrolisthes eye, and the rhabdom--although only twice as long--had a radius that was 7 times greater distally and 4 times greater proximally. A clear-zone between cones and rhabdom was not developed, and cross sections of crystalline cones revealed rounded rather than square profiles. A distal retinula cell (R8) was absent, and all regular retinula cells (R1-R7) protruded microvilli of about 0.11 {mu}m diameter in many (and not only two) directions. A maximum rhabdom occupation ratio of 85% was found in the Paralomis retinula, whereas in the shallow-water half-crabs the comparable figure was 35%. Paralomis featured a wide, rhabdomless space between basement membrane and proximal rhabdom ends; the space was occupied by reflecting cells. Primary screening pigment cells and their dark granules were present; secondary screening pigment cells, however, were replaced by reflecting cells. The anatomical modifications in the Paralomis eye are consistent with habitat-related adaptations seen in the eyes of other benthic and slow-moving deep-water crustaceans, but not with those of euphausiids. We conclude that the eye of Paralomis functions as an apposition eye, designed to maximize photon capture, especially from point sources (i.e., bioluminescence) rather than extended sources. We estimate that the Paralomis eye is at least 150 times more sensitive to light than the eye of shallow-water Petrolisthes.