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Abstract Radiolarians are marine protists with a global distribution. Epipelagic radiolarians host photosynthetic endosymbionts, but the identity and specificity of this relationship appears to vary between radiolarian subgroups. While the class Acantharea and the order Collodaria both possess stable and relatively specific relationships with the haptophyte Phaeocystis and the dinoflagellate Brandtodinium nutricula, respectively, the orders Nassellaria and Spumellaria (which comprise the solitary Polycystinea) might have greater flexibility in terms of the identity of their photosymbionts. However, little molecular data has been generated to identify the phytoplankton with which polycystines can associate. Here, we performed short-read 16S and 18S rRNA gene sequencing with universal primers on single polycystine cells collected from the Sargasso Sea to determine common members of the polycystine holobiont. While previous work on polycystine photosymbioses suggested that they almost always exclusively associate with B. nutricula, we determined that polycystines instead associated with a wide diversity of phytoplankton, and the diversity of the polycystine holobiont is distinct from the diversity of environmental samples. Finally, we found that a substantial proportion of the reads associated with cell samples were of opisthokont origin (mostly copepods), revealing other possible interactions between an uncultivable and difficult-to-study protist with its environment. 

Abstract The marine oligotrich ciliate Strombidium capitatum is a cruise-feeder, relying on ciliary motion and propulsion flow to individually detect and capture particles. High-speed, high-magnification digital imaging revealed that the cell swims forward by sweeping its anterior adoral membranelles (AAMs) backward, achieving a mean path-averaged speed of U = 1.7 mm s−1 (31 cell-lengths per second). Particle detection occurs through either hydrodynamic signal perception or ciliary contact perception, with a mean reaction distance of R = 20.4 μm. While executing a ciliary reversal of AAMs to handle and capture a perceived particle, the cell coordinates the ciliary motion of ventral adoral membranelles (VAMs, the “lapel”) with the ciliary reversal of AAMs (the “collar”), causing a sudden halt of cell motion, thereby functioning as a motion “brake” that is crucial for effective particle capture. The encounter rate with small prey particles is calculated using πR2U (~8.0 μL h−1, equivalent to ~ 3.5 × 106 cell volumes per day). Based on hydrodynamic modeling results, it is hypothesized that spatial structures of the flow velocity vector and acceleration fields in front of the swimming cell are essential for pushing an embedded particle forward, creating a strong enough slip velocity and hydrodynamic signal for prey perception, even for a neutrally buoyant small particle. 

Abstract The marine tintinnid ciliate Amphorides quadrilineata is a feeding-current feeder, creating flows for particle encounter, capture and rejection. Individual-level behaviors were observed using high-speed, high-magnification digital imaging. Cells beat their cilia backward to swim forward, simultaneously generating a feeding current that brings in particles. These particles are then individually captured through localized ciliary reversals. When swimming backward, cells beat their cilia forward (=ciliary reversals involving the entire ring of cilia), actively rejecting unwanted particles. Cells achieve path-averaged speeds averaging 3–4 total lengths per second. Both micro-particle image velocimetry and computational fluid dynamics were employed to characterize the cell-scale flows. Forward swimming generates a feeding current, a saddle flow vector field in front of the cell, whereas backward swimming creates an inverse saddle flow vector field behind the cell; these ciliary flows facilitate particle encounter, capture and rejection. The model-tintinnid with a full-length lorica achieves an encounter rate Q ~29% higher than that without a lorica, albeit at a ~142% increase in mechanical power and a decrease in quasi-propulsive efficiency (~0.24 vs. ~ 0.38). It is also suggested that Q can be approximated by π(W/2 + l)2U, where W, l and U represent the lorica oral diameter, ciliary length and swimming speed, respectively. 

Abstract Bluefin tuna spawn in restricted areas of subtropical oligotrophic seas. Here, we investigate the zooplankton prey and feeding selectivity of early larval stages of Atlantic bluefin tuna (ABT, Thunnus thynnus) in larval rearing habitat of the Gulf of Mexico. Larvae and zooplankton were collected during two multi-day Lagrangian experiments during peak spawning in May 2017 and 2018. Larvae were categorized by flexion stage and standard length. We identified, enumerated and sized zooplankton from larval gut contents and in the ambient community. Ciliates were quantitatively important (up to 9%) in carbon-based diets of early larvae. As larvae grew, diet composition and prey selection shifted from small copepod nauplii and calanoid copepodites to larger podonid cladocerans, which accounted for up to 70% of ingested carbon. Even when cladoceran abundances were <0.2 m−3, they comprised 23% of postflexion stage diet. Feeding behaviors of larvae at different development stages were more specialized, and prey selection narrowed to appendicularians and primarily cladocerans when these taxa were more abundant. Our findings suggest that ABT larvae have the capacity to switch from passive selection, regulated by physical factors, to active selection of presumably energetically optimal prey. 

Abstract Western Atlantic bluefin tuna (ABT) undertake long-distance migrations from rich feeding grounds in the North Atlantic to spawn in oligotrophic waters of the Gulf of Mexico (GoM). Stock recruitment is strongly affected by interannual variability in the physical features associated with ABT larvae, but the nutrient sources and food-web structure of preferred habitat, the edges of anticyclonic loop eddies, are unknown. Here, we describe the goals, physical context, design and major findings of an end-to-end process study conducted during peak ABT spawning in May 2017 and 2018. Mesoscale features in the oceanic GoM were surveyed for larvae, and five multi-day Lagrangian experiments measured hydrography and nutrients; plankton biomass and composition from bacteria to zooplankton and fish larvae; phytoplankton nutrient uptake, productivity and taxon-specific growth rates; micro- and mesozooplankton grazing; particle export; and ABT larval feeding and growth rates. We provide a general introduction to the BLOOFINZ-GoM project (Bluefin tuna Larvae in Oligotrophic Ocean Foodwebs, Investigation of Nitrogen to Zooplankton) and highlight the finding, based on backtracking of experimental waters to their positions weeks earlier, that lateral transport from the continental slope region may be more of a key determinant of available habitat utilized by larvae than eddy edges per se.