[B-CG05] Frontier in diversity and ecology of protists and microfossils

[BCG05-05] Distributional patterns and internal/external morphologies of xenophyophores, giant protists inhabiting the deep-sea floor

*Hidetaka Nomaki¹, Chong Chen¹, Yoshiyuki Ishitani¹, Julien Richirt¹, Masashi Tsuchiya¹, Okada Satoshi¹, Momo S Yamashita³, Koji Seike², Andrew J Gooday⁴ (1.JAMSTEC, 2.AIST, 3.NMNS, 4.National Oceanography Centre)

Xenophyophores are giant protists exclusively distributed on the deep-sea floor at depths greater than 500 m. Although they are unicellular organisms, their body (test) size typically reaches at least 5 cm, and sometimes up to up to 20 cm, in diameter. Due to the remoteness of their habitat and the fragile test structures consisting of agglutinated foreign particles, their ecology and morphology, particularly subsurface structures, have not been studied in detail. Here, we collected xenophyophores from four different abyssal sites in the northwestern Pacific, separated latitudinally by a few hundred kilometers. Xenophyophore specimens were collected with pushcores and preserved whole with the surrounding sediment after snips being taken for different purposes. The specimens were subjected to morphological observation using light microscopy, scanning electron microscopy, micro-X-ray computed tomography, and genetic sequencing for phylogenetics. Species richness based on morphology varied substantially among sites; the southmost site 30N was dominated by a single species while 31N and 32.5N sites exhibited a wide variety of species. There were two distinct growth patterns, one where new layers are added distally and the other where the whole test is covered by new layers. A number of morphological features appear to contribute to maintain the integrity of their fragile tests on the seafloor where bottom currents and animals may cause disturbances, such as attaching on rocks, extrude a “root”, developing a base, and just sitting on sediments. Some species may facilitate filter-feeding by orienting orthogonally to the bottom current or by building internal channels inside the test. Our results indicate xenophyophores flourish with a variety of growth patterns and interactions with sediments, in which their giant bodies may have adaptive importance.