COMPARATIVE ANALYSIS OF SHELL COLOR PATTERNS IN MODERN ECTOCOCHLIATE CEPHALOPODS

Coloration of mollusk shells has many functions including camouflage. Coloration has long been used by taxonomists as a distinct character to distinguish species. Nautilids, a group of externally shelled cephalopods, exemplify the use of coloration in taxonomic description, but species level differences have yet to be extensively explored. Traditionally, nautilid color patterns have been described as irregular, transverse brown bands along the flanks of the venter. In this study, we expand upon this general description using fractal analysis (which mathematically describes shape) and further observations of museum specimens. When applying these methods to museum specimens, color must be ignored as the chroma changes rapidly after death of an individual. To resolve this issue, we emphasized the pattern independent of color. Color patterns were described from five extant species (Nautilus pompilius, Nautilus belauensis, Nautilus stenomphalus, Nautilus macromphalus, and Allonautilus scrobiculatus) to begin to address potential taxonomic differences. We also studied the change in color pattern through ontogeny. In adult shells of all species, brown, rursiradiate (backward-bending) bands extend to the venter. These bands disappear towards the aperture. Species differences appear in the complexity of the band margin, the angle of geniculation, and whether the bands extend without interruption from the umbilicus to the venter. We discuss possible implications of color patterns in the classification of extant nautilids.