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

Abstract

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; R 2 = 0.9; df = 15, 386; 401, F = 219.24; P N ‐oxide (TMAO) followed the piezolyte hypothesis, with TMAO being highest and urea being lowest in deep‐sea sharks. Among electrolytes, Na + was highest in species with lobulate rectal glands. Hexanchids and echinorhinids both have lobulate rectal glands similar to those of holocephalans, despite the more than 400 million years separating these two groups. The morphological similarities between the lobulate rectal‐gland anatomy of primitive sharks and the secretory morphology of holocephalans may represent an intermediate state between Holocephali and derived shark species.

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