SUMMARY
Grazing on ultraplankton by the sponge partner of an invertebrate/algal symbiotic association can provide enough particulate organic nitrogen to support the nitrogen needs of both partners. The previously unknown natural diet of the sponge in the Haliclona–Ceratodictyon association consists of bacteria and protozoans, which are rich sources of nitrogen. Retention of ultraplankton varied with season and time of day. During the winter there was an order of magnitude more nitrogen taken up than in summer. Time of day during each season also affected the amount of ultraplankton retained. In summer retention was higher at night whereas the opposite was true during winter. Overall, the Haliclona–Ceratodictyonassociation is able to meet its metabolic nitrogen demands through grazing on the naturally occurring water column community.
