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Appendicularian tunicates of the genus Oikopleura feed using an external, acellular, transparent structure known as the house. Previously, dilute particulate dyes have been used to visualize the internal structure of this house. However, because of toxicity, large particle size, and flocculation, many of these dyes have been of limited practical and scientific use. We report on a new marker, the ink from the cephalopod Sepia officinalis, that solves many of these problems. Specimens of Oikopleura vanhoeffeni relished Sepia ink, having dark black stomachs and producing many dark fecal pellets over several days. When O. vanhoeffeni expanded houses in dilute ink, the internal walls, septae, and filters were shown in great detail, whereas high concentrations of ink showed delicate patterns of lines on the internal walls. We present documentary photographs of previously unillustrated or undescribed morphologies: the escape slot; the incurrent funnels; two dimples caused by insertion of suspensory filaments on the upper wall of the posterior chamber, a large, posterior keel; both the open and closed positions of the exit valve; and the complex pattern of lines on the inner walls. However, the external walls of the house had no affinity for the dye and could only be seen by dark field illumination. We believe that Sepia ink can be used to visualize functionally important transparent structures of other gelatinous zooplankton and can be a colloidal marker in feeding experiments of a wide range of filter feeders.

Oikopleurid appendicularians use an external “house” to preconcentrate particles from seawater suspension, but their pharyngeal filter is ultimately responsible for removing. suspended particles for ingestion. Although the pharyngeal filter captures submicrometer colloids efficiently, the fluid mechanics of this process have not been investigated. Using video‐assisted microscopy, we tracked plastic beads within the pharynx of Oikopleura vanhoeffeni to analyze flow streamlines and flow rates. Impact velocities of particles range from 124 to 1,436 µm s‒1, with a mean (±SD) of 559±292 µm s‒1. Reynolds numbers for the filter fibers are of order 10‒5. Given the fiber diameters and pore sizes published earlier and our measured impact velocities, current aerosol filtration models predict particle retention spectra that do not differ statistically from those determined empirically for particles between 0.6 and 3 µm in diameter, in contrast to predictions from a simpler sieving model. We conclude that the pharyngeal filter of O. vanhoeffeni captures particles >0.6 µm in diameter by a combination of sieving and direct interception onto individual fibers. The combination of model predictions of particle capture with particle size dis.tributions from an Arctic polynya suggests that adult O. vanhoeffeni obtains 49, 29, and 18% of its ration by volume from nanoplankton (2–20 µm in diameter), microplankton (20–100 µm), and picoplankton (1–2 µm), respectively, with a possible 4% contribution from submicrometer colloids.

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