The Effect of Photobacterium damselae Availability on the Behavior of Choanocytes in the Marine Sponge Amphimedon queenslandica

Abstract: Sponges are aquatic filter feeding animals that use internal chambers comprised of choanoflagellate-like cells called choanocytes to pump and filter water. In this study I investigated the behavior of choanocytes in juvenile Amphimedon queenslandica, a coral reef demosponge, when they were fed Photobacterium damselae subsp. damselae compared to when they were maintained in 0.22μm filtered seawater. Prior to these treatments I fluorescently-labelled choanocytes with Cm-DiI, which allowed tracking of the movement and behavior of these cells over 24 h. I found that choanocytes moved out of the chambers significantly more in unfed sponges, suggesting that choanocyte chambers and the aquiferous system are constantly being remodeled in unfed sponges. When fed however, the choanocytes were more prone to staying in their respective chamber, presumably collecting P. damselae from the water. Furthermore, when the juvenile sponges were exposed to P. damselae the choanocyte chambers grew in size, becoming 20% larger than the unfed juveniles. Lastly, there were significantly more cells being shed into the water column when the juveniles were fed. This suggests that under natural conditions, when the aquiferous system is fully functioning, replacement and shedding of choanocytes occur. I conclude that the aquiferous system of A. queenslandica is very dynamic, where the choanocytes have the ability to leave and join other pre-existing chambers or form entirely new chambers. However, when the choanocytes are exposed to food they are less likely to leave their chamber, resulting in development and growth of the choanocyte chamber.