Nemertodermatida-Endosymbionts of Deep-Sea Acorn Worms (Hemichordata, Torquaratoridae).

Worm-like endosymbionts were found in the hepatic region of the digestive tract of the deep-sea acorn worm Quatuoralisia malakhovi Ezhova et Lukinykh, 2022 (family Torquaratoridae) from the Bering Sea. The symbionts were assigned to the taxon Nemertodermatida on the basis of histological examination. Torquaratoridae are similar in feeding type to holothuroids, which have also been found to have Xenacoelomorpha endosymbionts.

Digestive System Anatomy and Feeding Mechanism of Quatuoralisia malakhovi (Hemichordata, Torquaratoridae).

The digestive system was anatomically studied in the deep-sea enteropneust Quatuoralisia mala-khovi. It was shown that lateral collar lips are twisted in such a way that they form a ciliary groove that leads to an internal channel, through which collected detritus particles are transferred to peripheral pharyngeal channels. The size of the selected particles ranges from 1-6 to 100-200 µm, which corresponds to feeding on the remains of planktonic diatoms. A fecal cord was observed to act as an anchor that holds the heavily watered jelly-like body of Torquaratoridae at the sea floor during feeding.

Keeping and Behavior of the Acorn Worm Saccoglossus mereschkowskii (Hemichordata, Enteropneusta) in Laboratory Conditions.

A box was designed to keep the acorn worm Saccoglossus mereschkowskii in laboratory conditions for 60 days and to monitor its behavior and feeding. Locomotion and construction of burrows in the sediment were found to be due to peristaltic movements of the proboscis, which periodically changes its shape from cylindrical to mushroom-like, and vice versa. Worms built U-shaped burrows connected with burrows of neighbor worms by flank anastomoses, thus producing a branched system of passages in a sediment layer up to 8 cm deep. The system is of importance for aeration of the upper sediment layer. When a worm is feeding, the proboscis sticks out from the anterior opening of the burrow and stretches along the surface of the sediment. Organic particles adhere to mucus secreted by the epidermal epithelium of the proboscis and are transported by ciliary beating to a furrow between the collar and proboscis, where the mouth is located.

The First Discovery of Trematodes (Digenea) in the Deep-Sea Acorn Worms Torquaratoridae (Hemichordata, Enteropneusta).

Trematodes found in the enteropneust hemichordates are described for the first time. Metacercariae have been found in the trunk coelom, in the collar coelom, in the proboscis coelom, and in the glomerulus of the deep-sea torquaratorid Quatuoralisia malakhovi Ezhova et Lukinykh, 2022. This is the first find of parasites in the glomerulus of acorn worms. The taxonomy of the found trematodes is discussed.

Feeding Characteristics of Deep-Sea Acorn Worm (Hemichordata, Enteropneusta, Torquaratoridae) from the Bering Sea.

Deep-sea hemichordates Torquaratoridae gen. sp. reach high abundance up to 12 spec. m-2 at the depths of 1830-2130 m on the slope of the Volcanologists Massif in the south-western part of the Bering Sea, dominating in the benthic community at these depths. Their abundance exceeds by two orders the values recorded earlier. In order to clarify this phenomenon, we examined the gut contents of Torquaratoridae gen. sp. The detritus particles and frustules of planktonic diatoms Thalassiosira, Coscinodiscus, Actinocyclus, Chaetoceros, Neodenticula, and Grammatophora were the most common in the gut, as well as the remains of skeletons of benthic invertebrates with little admixture of mineral particles. According to obtained data, Torquaratoridae gen. sp. are mobile deposit feeders with high selectivity to fresh phytodetritus, able to compete with holothurians occupying similar trophic niche. Unusually high abundance of acorn worms is apparently related to high organic matter flux to the seafloor as a result of spring phytoplankton bloom in the surface water layer of the Bering Sea.

Discovery of Trunk Coelomoducts in Hemichordata.

Histological examination of a specimen of a deep-sea enteropneusts that belongs to a yet undescribed species (Torquaratoridae gen. sp.) revealed numerous trunk coelomoducts. They open into the genital wing coelom as a typical funnels; short ducts communicate with environment through pores located on the outer side of the genital wings. Total number of coelomoducts in a specimen is estimated at several thousand. Trunk coelomoducts have not been found earlier in any member of the phylum. We believe that the release of the male gonad products occurs through coelomoducts of Torquaratoridae gen. sp.