The internal structure of mitochondria can change in response to physiological demand. Here, after the Sheepshead minnow Cyprinodon variegatus was kept in constant light for 4 weeks, mitochondria from cells of the pseudobranch acquired an additional category of tubular cristae arranged in rows between the longitudinally oriented foliate cristae. Inset: a longitudinal section suggests these cristae have a helical tubular organization. This image, courtesy of Jospeh Harb and Eugene Copeland, appears as Figure 249 from Chapter 7 (Mitochondria) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.