Visualization of the Transparent, Gelatinous House of the Pelagic Tunicate Oikopleura vanhoeffeni Using Sepia Ink

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.