Stirring up the muck: the systematics of soft-sediment Fionidae (Nudibranchia: Aeolidina) from the tropical Indo-Pacific

The tropical Indo-Pacific aeolid nudibranchs of the Fionidae are poorly known and have not been studied in a concerted manner. Many undescribed species are found throughout the Indian and Pacific Oceans and are concentrated in the Coral Triangle. With the recent publication of a revised systematic arrangement of the Fionidae, documentation and description of new taxa of Fionidae is especially warranted. Here we describe <i>Tenellia bughaw</i> Kim & Gosliner sp. nov., <i>Tenellia puti</i> Kim & Gosliner sp. nov., <i>Tenellia nakapila</i> Kim & Gosliner sp. nov., <i>Abronica payaso</i> Kim & Gosliner, sp. nov. and <i>Abronica turon</i> Gosliner & Kim sp. nov. from the waters of the Coral Triangle. Their phylogenetic placement in Fionidae is reviewed using three genes: cytochrome oxidase I (COI), 16s rRNA, and histone 3 (H3) in a Bayesian and maximum likelihood framework. A redescription of <i>Tenellia yamasui</i> (Hamatani, 1993) is also provided to clarify its distinctiveness from <i>T. bughaw</i> and <i>T</i>. <i>puti</i>. This study confirms that all four known species of <i>Abronica</i> are characterized by having an acutely pointed curved penial stylet, thus confirming a unique morphological synapomorphy for members of this genus. A discussion on conflicts in the classification of fionid aeolid nudibranchs addresses concerns with extreme splitting based on novel innovations that emphasize evolutionary novelty over phyletic kinship. Instead, a more conservative approach is suggested, especially within the context of taxa that still have much undocumented primary diversity.