Giant protists (xenophyophores) function as fish nurseries

During a recent research cruise on the continental margin off Costa Rica, we observed many giant agglutinating Xenophyophoroidea (Foraminifera) on both hard and soft substrates. These fist‐sized protists, which occur only below 400 m, are one of the few groups of organisms limited to the deep ocean (Tendal 1972). Xenophyophores are known to be abundant on sloped topography where there is high particle flux, as many have a morphology designed to trap settling particles that they can feed on or use to form tests (Levin 1991, Levin and Gooday 1992). High abundances and diverse forms have been documented previously in relatively well‐oxygenated parts of the Eastern Pacific Ocean on seamounts (Levin and Thomas 1988), and on abyssal plains (Gooday et al. 2017). Because the elaborate test structures appear to provide substrate, refuge, mating sites and food for deep‐sea invertebrates, xenophyophores occurring on sediments have been recognized as diversity hotspots (Gooday 1984, Levin et al. 1986, Levin and Thomas 1988, Levin 1991). However, to our knowledge, fish have not been known to use xenophyophores as nursery habitat for developing embryos until our research cruise. Collections of Costa Rican xenophyophores from the continental margin were made with the ROV SuBastian aboard RV Falkor in January 2019, using the ROV manipulator claws for those on rocks and using a pushcore for xenophyophores on sediments (Fig. 1a). Approximately 10 xenophyophores were dissected on board ship to examine associated fauna, from the continental slope and from four nearby seamounts. Here we report the occurrence of snailfish (Liparidae) embryos (Fig. 1d) and eggs (Fig. 1e), attached deep in xenophyophores collected from two sites on the Costa Rican slope at 1,902 m (Mound Jaguar) and 1,866 m (Jaco Scar). Liparidae, a group highly adapted to the deep sea (Gerringer 2019) and well known for their brood‐hiding relationships with invertebrates (Chernova 2014), have never been reported to lay their eggs in any protozoan tests. Two xenophyophores (Fig. 1b, c) were found to contain fish embryos and eggs, one from hard substrate and one from soft sediments. One 5‐cm diameter xenophyophore from Jaco Scar, tentatively identified based on morphology and test particles as being in the genus Reticulammina (Fig. 1b), contained approximately 20 fish eggs (~2.9 mm diameter; Fig. 1f). A 6‐cm diameter xenophyophore, tentatively identified as Shinkaiya (Fig. 1c), contained 16 fish embryos (approx. 3.6 mm diameter) (Fig. 1d). Although there is no direct evidence, the intact nature of the