Morphological adaptations of the respiratory hindgut of a marine echiuran worm

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Abstract
The echiuran worm Urechis caupo lives in U‐shaped burrows in marine mudflats where levels of toxic hydrogen sulfide increase and water becomes hypoxic during low tide. Even in this low oxygen and high sulfide environment, the animal is capable of maintaining aerobic respiration. Gas exchange occures across both the body wall and hindgut. The hindgut functions as a type of water lung and is a thin walled, highly convoluted structure capable of considerable dilatation. It is rhythmically ventilated with water and its role as a respiratory organ becomes increasingly important as ambient pO2 drops. In the deflated hindgut light microscopy reveals a pseudostratified appearing innermost mucosal epithelium composed of columnar cells with nuclei at different levels. When the hindgut is fully inflated, ultrastructural studies show a simple columnar epithelium with the nuclei at the same level. Ultrastructurally, the free surface of the hindgut cells bears numerous microvilli and a few cilia. The lateral cell membranes are highly folded in the deflated hindgut, but these folds are not visible in the fully inflated hindgut. The cytoplasm contains osmiophilic bodies which show a partially lamellated pattern which may be sulfide oxidizing bodies involved in sulfide detoxification. In the fully inflated hindgut, the entire perimeter of the lumenal mucosa is covered by electron dense inclusions, whose exact fuction is unknown. The lack of structural information on the respiratory organ of this echiuran worm renders the interpretation of its morphological and histological features at the ultrastructural level difficult, although the present study has broadened our understanding of the structural adaptations of the hindgut as a respiratory organ.