Spines and rhizopodia play an important role in the feeding behavior, symbiont ecology, shell geochemistry, and density and drag of planktonic foraminifera. However, there are few empirical data on planktonic foraminifera in situ, and these delicate structures are disturbed on capture. Here, we report spine and rhizopod measurements from underwater images obtained in the California Current System near La Jolla, California by Zooglider, a new autonomous zooplankton-sensing glider. Across all observed species, we find that spine length and flexibility correlate with test size and that spines increase the effective prey encounter volume of spinose foraminifera by two to three orders of magnitude. Our data also yielded several novel observations regarding hastigerinid foraminifera (Hastigerinella digitata and Hastigerina pelagica), a group of unusually large planktonic foraminifera that are abundant in our dataset below 250 m. First, the effective encounter volume of hastigerinid foraminifera can be very large: our largest specimen occupies almost 40 cm3 (about the size of a golf ball), while the median specimen occupies 5.3 cm3 (about the size of a cherry). Second, the majority of hastigerinid foraminifera in our dataset have asymmetric bubble capsules, which are most frequently oriented with their bubbles on the upward side of the test, consistent with the hypothesis that the bubble capsule is positively buoyant. Third, 16% of hastigerinid foraminifera in our dataset have dispersed bubble capsules with detached bubbles distributed along the spines and rhizopodia, consistent with a regular source of natural disturbance. Taken together, our observations suggest that hastigerinid foraminifera play a larger role as mesopelagic predators in the California Current System than previously recognized.

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