AGNEZIA MONNIOTI AND STYELA GAGETYLERI, NEW DEEP-SEA ASCIDIANS SPECIALIZED FOR LIFE WITHIN AND BELOW THE OXYGEN MINIMUM LAYER IN THE ARABIAN SEA

摘要

Two new species of solitary ascidians from the deep sea off Oman, Arabian Sea, are described. Agnezia monnioti n. sp. is from the abyssal plain at 3162 m depth. Whereas many deep-sea ascidians have reduced branchial ciliation and exploit the velocity gradient in the benthic boundary layer to induce internal flow, this species has strong ciliation and a siphonal arrangement that appears inappropriate for using ambient water currents for filtration. Styela gagetyleri n. sp. is found at a depth of 368 m, which is in the middle of a persistent oxygen minimum zone. The branchial sac of this species has a reduced number of folds and therefore fewer cilia for food collection and less surface area for oxygen exchange than shallow-water congeners. We hypothesize that reduced filtration capacity is made possible by an unusually high flux of organic material to the sea floor in this region. The observed reduction in branchial surface area in a persistent hypoxic zone suggests that branchial complexity in ascidians is more important for feeding efficiency than gas exchange. Affinities of these species with other members of their respective genera are discussed and an appendix with the synonyms and localities of species in the genus Agnezia is included. [References: 66]