Digestive system and feeding of deep-sea acorn worm Quatuoralisia malakhovi (Enteropneusta: Torquaratoridae) from the Bering sea

Abstract

We studied the structure of digestive system of deep-sea torquaratorid Quatuoralisia malakhovi including the food-obtaining apparatus, microanatomy of all regions of digestive tract, and fine structure of various regions of the gut. The mouth of Q. malakhovi is surrounded by two symmetrical triangular lateral collar lips and one midventral collar lip, which may retract into the mouth. The ventral ciliary grooves run along the anterior-ventral edges of lateral lips and lead into the internal canals of the lips. The pharyngeal cavity is divided by left and right parabranchial ridges into the median pharyngeal zone with gill bars and two symmetrical peripheral canals, which fuse behind the site of genesis of new gill bars. The gill pharynx is followed by the hepatic region with metameric hepatic sacculations comprising cells with numerous large phagosomes indicating active intracellular digestion. The intestine is filled with undigested food residues including detritus flakes and remnants of various organisms. The intestine content was examined using SEM and histological studies. We analyzed the ratio of organic and mineral components of the bottom sediment from the habitat of Q. malakhovi, and the stable isotope composition of carbon and nitrogen in the intestine content, in the tissues, and in the sediment from the habitat of the studied acorn worm. Using the obtained data and the photographs and videotapes in situ, we have proposed a hypothesis explaining the feeding mechanism of Q. malakhovi. The width of the ventral ciliary groove of the lateral collar lips (100–200 μm) regulates the maximum size of particles that enter the pharynx of the animal. The minimum size of the sediment particles selected by Q. malakhovi is determined by the width of the gill pores and is 1–3 μm. The selected sediment particles move through the peripheral pharyngeal canals to the hepatic region, where the digestion of the food is carried out. In the posthepatic intestine, weight of the gut contents might serve as an anchor that holds the heavily watered jelly-like torquaratorid body at the bottom during the feeding.