Figure 3. Shark photophore diversity.

(a) Overview of maximum ventral photophore density (red colour on top drawing) from selected representatives. Scale bar, 500 μm. Ce, Centroscyllium; Da, Dalatias; Eu, Euprotomicrus; Is, Isistius; Sq, Squaliolus; Tr, Trigonognathus. Box plots and histograms of photophore mean diameter, photophore density and PAP of 37 shark species from Etmopteridae (light green, n = 31) and Dalatiidae (dark green, n = 6) families. (b) R-square and associated P-value for several log-log linear regressions of photophore variables performed against capture depth. Regressions were performed either on all species for which capture depth was available (n = 21) or on the confirmed bioluminescent species only (‘luminous’, n = 9). (c) Log-log plot of PAP and capture depth with associated regression line (log (capture depth) = −0.564 log (PAP) + 2.31, r² = 0.81, P < 0.0001, n = 21) and oceanic depth zones. Blue and green dots indicate specimens captured in oceanic and coastal waters, respectively. Closed dots indicate species with confirmed bioluminescent status. Unless determined otherwise, all dots represent species of the genus Etmopterus. Photographs and drawing by Claes.