The nereidid polychaete Laeonereis acuta (Treadwell, 1923) is either considered to be a valid species, or a synonym of Laeonereis culveri (Webster, 1879). The species epithet, acuta, refers to the acuminate anterior region of the body followed by a widening that is visible up to the 10th setiger. The relative width of the anterior region of the body, however, has been reported in the taxonomic literature as variable for Laeonereis Hartman, 1945, a genus known from the eastern coast of North America to Patagonia (southern South America). To test whether variations in this character are real, or whether they correspond to an artifact, we analyzed morphological changes associated with different anesthesia and fixation techniques regularly used to prepare specimens of Laeonereis from southern Brazil (formerly reported as L. acuta). Six treatments, including combinations of anesthetics and fixative agents, and a control, were evaluated in groups of 25 adult animals. A simple model II regression analysis on living specimens showed that the growth is approximately isometric. One-way ANOVA was used to compare the treatment effect on the ratio of peristome width: setiger 6 width. This ratio was smaller in non-anesthetized individuals, especially when prepared in formalin and freshwater. When anesthetized with menthol and fixed in formalin with sea water, individuals retained a shape that was closest to their in vivo shape. Consequently, our results suggest that fixation without prior anesthesia is not recommended for morphological and taxonomical studies. Since morphology and morphometrics of the anterior region are consistently influenced by preparation techniques, it is likely that inadequate fixation routines have introduced several errors in the taxonomic and ecological literature of Laeonereis.

Ten species belonging to the genera Ceratonereis Kinberg, 1866 and Nereis Linnaeus, 1758 were recorded in estuaries, exposed sandy beaches, shelly soft bottoms, atolls and coral reefs of the Brazilian northeastern coast. Two new species, Nereis serrata, from Ceará coast, and Nereis pseudomoniliformis, from Sergipe coast, are described.

Six polychaete species belonging to the genera Namalycastis Hartman, 1959, Ceratocephale Malmgren, 1867, Laeonereis Hartman, 1945, and Rullierinereis Pettibone, 1970 were recorded as part of a systematic survey of the family Nereididae in estuaries, exposed sandy beaches, shelly soft bottoms, atolls and coral reefs of the Brazilian northeastern coast. Two new species, Rullierinereis auxiliadorae, from Ceará coast and Ceratocephale rocaensis, from Atol das Rocas, are described.