Unique osmoregulatory morphology in primitive sharks: an intermediate state between holocephalan and derived shark secretory morphology

Discovery of an unusual rectal gland in the Atlantic sixgill shark Hexanchus vitulus led us to examine the rectal glands of 31 species of sharks to study diversity in rectal‐gland morphology. Twenty‐four of 31 species of sharks had digitiform glands (mean width–length ratio ± SD = 0.17 ± 0.04) previously assumed to be characteristic of all elasmobranchs regardless of habitat depth or phylogenetic age. Rectal glands from the family Somniosidae were kidney bean‐shaped (mean width: length ± SD = 0.46 ± 0.05); whereas those from families Echinorhinidae and Hexanchidae were lobulate (mean width: length ± SD = 0.55 ± 0.06). Rectal gland width: length were different among species with digitiform morphology and lobulate morphology (ANOVA; R2 = 0.9; df = 15, 386; 401, F = 219.24; P