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The Phyllidiidae (Gastropoda, Nudibranchia) is a family widely found on coral reefs of tropical waters. To date, their phylogenetic relationships remain controversial at various taxonomic levels. Here we sequenced the complete mitochondrial genomes (mitogenome) of 12 specimens representing nine species from the South China Sea, compared their mitogenomic features and reconstructed the phylogeny of Phyllidiidae. The assembled mitogenomes were 14,543–15,075 bp long containing the typical elements of the 37 genes in the same order. Phylogenetic analysis based on the 13 protein-coding genes revealed that the genera Phyllidia and Phyllidiopsis were closely related, and Phyllidiella was sister to them. Within each genus, inconsistent topology was frequently found between the mitogenomic and morphological trees, cautioning the need for a further integrative taxonomy. The species Phyllidiella pustulosa was proven to be a species complex as three distinctly separated clades appeared. Divergence time estimation dated the origin of the Phyllidiidae at c. 40.33 million years ago (MYA), while the crown ages of the three genera dated back from c. 31.98 to 15.80 MYA, all falling in the period from the Neogene to Paleogene, a time characterized by dramatic climate shifts that might have facilitated species diversification.

[This corrects the article DOI: 10.3897/zookeys.1006.59732.].[This corrects the article DOI: 10.3897/zookeys.1006.59732.].

Two new species of Phyllidia from North Sulawesi, Indonesia, Phyllidia fontjei sp. nov. and Phyllidia ovata sp. nov. , are described based on morphology and molecular barcoding of CO1 and/or 16S. Both species are rare and distinctive and can be easily recognised by their colouration. Additionally, histological sections were made of the holotype of Phyllidia fontjei sp. nov. and a similarly sized Phyllidia ocellata and these morphologies are compared with the only other detailed histological examination of the Mediterranean Phyllidia flava .

The mitogenome of Phyllidiopsis krempfi has been determined for the first time. The assembled mitogenome was 14,970 bp in length, including 13 protein-coding genes, 22 transfer RNA, and 2 ribosomal RNA genes. The gene content and order were identical with those of the other Phyllidiid species. The molecular taxonomic position of P. krempfi was clustered with the Phyllidiid species. The genus Phyllodiopsis clade is closely related with the genus Phyllidia. The mitogenome of P. krempfi provides significant DNA molecular data for further identification and phylogenetic analysis within the Phyllidiid.

The Phyllidiidae (Gastropoda, Heterobranchia, Nudibranchia) is a family of colourful nudibranchs found on Indo-Pacific coral reefs. Despite the abundant and widespread occurrence of many species, their phylogenetic relationships are not well known. The present study is the first contribution to fill the gap in our knowledge on their phylogeny by combining morphological and molecular data. For that purpose 99 specimens belonging to 16 species were collected at two localities in Indonesia. They were photographed and used to make a phylogeny reconstruction based on newly obtained cytochrome oxidase subunit (COI) sequences as well as sequence data from GenBank. All mitochondrial 16S sequence data available from GenBank were used in a separate phylogeny reconstruction to obtain information for species we did not collect. COI data allowed the distinction of the genera and species, whereas the 16S data gave a mixed result with respect to the genera Phyllidia and Phyllidiella. Specimens which could be ascribed to species level based on their external morphology and colour patterns showed low variation in COI sequences, but there were two exceptions: three specimens identified as Phyllidia cf. babai represent two to three different species, while Phyllidiella pustulosa showed highly supported subclades. The barcoding marker COI also confirms that the species boundaries in morphologically highly variable species such as Phyllidia elegans, Phyllidia varicosa, and Phyllidiopsis krempfi, are correct as presently understood. In the COI as well as the 16S cladogram Phyllidiopsis cardinalis was located separately from all other Phyllidiidae, whereas Phyllidiopsis fissuratus was positioned alone from the Phyllidiella species by COI data only. Future studies on phyllidiid systematics should continue to combine morphological information with DNA sequences to obtain a clearer insight in their phylogeny.

In this study, the complete mitogenome sequence of sea slug, Phyllidia ocellata (Mollusca: Phyllidiidae), has been decoded for the first time by low coverage genome sequencing method. The overall base composition of P. ocellata mitogenome is 31.5% for A, 14.0% for C, 16.4% for G and 38.0% for T, and have low GC content of 30.5%. The assembled mitogenome, consisting of 14 598 bp, has unique 13 protein-coding genes, 22 transfer RNAs and two ribosomal RNAs genes. The P. ocellata mitogenome has the common mitogenome gene organization and feature of Nudipleura (a clade of sea slugs and sea snails). The complete mitogenome of P. ocellata provides essential and important DNA molecular data for further phylogenetic and evolutionary analysis for sea slugs and sea snails.

The use of bright coloration to warn predators of toxic prey, termed aposematism, is a defensive strategy well studied in terrestrial ecosystems. Some marine animals have contrasting color patterns and chemical defenses but few studies have tested the behavior of marine predators in response to aposematic coloration. In this study we tested fish feeding behavior in response to different types of cues including contrasting color patterns and sponge and nudibranch chemical extracts in the field at 2 reefs on Guam. Using agar-based food to keep food quality consistent, chemical extracts and 2 out of 5 contrasting color patterns reduced feeding by natural assemblages of reef fish at both reefs.Phyllidia varicosa(from Palau),P. elegans(Guam) andPhyllidiella pustulosa(Palau) crude extracts deterred feeding by fish, butP. pustulosaextracts from Guam did not. To determine if a chemical extract can act as an olfactory defense we videotaped fish behavior as they approached food containing nonpolar extracts ofAcanthella cavernosa, the dietary sponge ofPhyllidiella granulatus. Fish approached and then tasted the food with the chemical extract significantly less often than the control and the crude extract of another spongeStylissa massa. In feeding assays with a contrasting color pattern combined with the chemical extract at natural concentrations, fish were deterred by the extract regardless of the color pattern. At half natural concentration only the chemical extract and the contrasting color pattern together significantly decreased fish feeding. Reef fish can use multimodal signals including visual, taste and olfactory cues to avoid marine benthic invertebrates.

The nudibranch genus Phyllidiopsis (Phyllidiidae) contains 30 currently recognized species, all of them distributyed throughout the tropical Indo-Pacific, eastern Pacific, Northwest Atlantic and Caribbean Sea. Half of the known species of Phyllidiopsis inhabit deep waters, and most of the deep-sea species of the Phyllidiidae belong to this genus.