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Despite the many publications concerning the isolation of substances and the many reviews of marine natural products, some groups of organisms remain poorly studied, including "Polychaeta". In response, this review covers articles published through December 2016 that address marine natural products produced from polychaetes, with a focus on antipredatory strategies, competitors, fouling, and pathogens. A total of 121 compounds were isolated from 1934 to 2016, which includes halogenated aromatics, proteins, amino acids and Lumazine derivatives most notably-with a defensive function were found in the literature, most frequently in the families Sabellidae, Terebellidae, Glyceridae, and Nereididae. The period of highest discovery of natural products in defensive actions for the group was the 2000s. Polychaetes were addressed in 26 revisions of the total 51 articles analyzed and are less reported than other marine invertebrates such as sponges, cnidarians, mollusks, and tunicates. In sum, the present review provides a basis for future research on the marine chemical ecology of polychaetes.

Species definition is not an easy task, when considering the more than 27 known species concepts. Among them, the morphospecies concept has been one of the most applied since it is based on the use of observable morphological features. Morphometry has been used to delimitate morphospecies or similar taxa complementing the morphological observations and contributing to clarify taxonomic problems. Specimens from the sympatric species Perinereis anderssoni and Perinereis ponteni, collected from the north-eastern to southern coast of Brazil and considered synonymous by some authors, were compared through morphometric analyses for the evaluation of their taxonomic status. Morphometric analysis indicates that notopodial cirri lengths in the median and posterior regions on the body clearly allowed differentiation between the two species. Our results indicate that the number and arrangement of paragnaths demonstrate a pattern of variation that effectively differs and could be used to discriminate these two Perinereis species. This distinction was confirmed by the restricted among-population variability within each species, even when populations that are geographically very distant from each other were considered.

The examination of a large number of specimens in the context of taxonomic and ecological studies may lead to the discovery of morphological anomalies. The aim of this study was to describe the morphological anomalies observed in some individuals of Perinereis anderssoni and Perinereis ponteni collected in various regions of the Brazilian coast. A total of 290 specimens were analysed from along the northern and southern Brazilian coast, and 21 of these presented morphological anomalies, such as variations in the number of tentacular cirri and eyes, completely or basally fused antennae, chaetigers with three parapodia, and others. Perinereis anderssoni presented the highest number of anomalous individuals, and the most frequent morphological anomaly was the presence of a single antenna and nine tentacular cirri. Anomalous individuals of P. ponteni with seven tentacular cirri were also commonly collected. Ilha do Mel (PR) was the area with the highest percentage of individuals with anomalies (12.96%), followed by Martim de Sá (SP) (10.31%), São Francisco do Conde (BA) (8.33%), Tambaba (PB) (5.55%) and Itaipu (RJ) (1.92%). Most of the sampling locations have a history of contamination by a diverse array of pollutants. We provide background information for the morphological changes observed in two species that occur along the Brazilian coast, but additional studies are needed to confirm the real cause of these anomalies and their effect on the population structure of these ecologically important species.

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