| Summary: | Five new alkaloids have been isolated from the lipophilic extract of the Antarctic tunicate <i>Synoicum</i> sp. Deep-sea specimens of <i>Synoicum</i> sp. were collected during a 2011 cruise of the R/V <i>Nathanial B. Palmer</i> to the southern Scotia Arc, Antarctica. Crude extracts from the invertebrates obtained during the cruise were screened in a zebrafish-based phenotypic assay. The <i>Synoicum</i> sp. extract induced embryonic dysmorphology characterized by axis truncation, leading to the isolation of aminopyrimidine substituted indolone (<b>1</b>–<b>4</b>) and indole (<b>5</b>–<b>12</b>) alkaloids. While the primary bioactivity tracked with previously reported meridianins A–G (<b>5</b>–<b>11</b>), further investigation resulted in the isolation and characterization of australindolones A–D (<b>1</b>–<b>4</b>) and the previously unreported meridianin H (<b>12</b>).
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