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SUMMARY: The present paper reports results from the study of a wide collection of specimens of the genus Haplosyllis (Polychaeta: Syllidae: Syllinae) obtained mainly from sponges of different Caribbean regions (Barbados, Bahamas, Belize, Bermudas, Colombia and Venezuela). Four new species are herein described and illustrated. H. aplysinicola n. sp., the most common Caribbean species, found only in species of genus Aplysina; H. chaetafusorata n. sp., found inside the sponge Verongula rigida and characterized by having traces of fusion between blade and shaft on mid-body and posterior chaetae; H. navasi n. sp., found in Colombian waters inside the sponge Ircinia strobilina and characterized by having mid-body dorsal cirri of all similar in length and by the broad acicula and proventriculum; and H. niphatesicola n. sp., collected inside sponges of genus Niphates and characterized by alternating long and short mid-body cirri, round dorsal granules throughout, and broad posterior acicula. Finally, H. cephalata and H. gula are registered for the first time in Colombian waters, and new information about their habitat is provided. Current knowledge on the relationships between the symbiotic Haplosyllis and their hosts known to date is discussed.Keywords: Haplosyllis, new species, endosymbiosis, sponge hosts, Caribbean Sea. RESUMEN: Haplosyllis (Polychaeta: Syllidae: Syllinae) asociados a esponjas del mar Caribe, con la descripción de cuatro especies nuevas. -Este artículo es el resultado del estudio de una amplia colección de muestras del género Haplosyllis (Polychaeta: Syllidae: Syllinae) que se obtuvieron principalmente de esponjas de diferentes regiones del Caribe (Barbados, Bahamas, Belize, Bermudas, Colombia y Venezuela). Se describen e ilustran cuatro especies nuevas. H. aplysinicola n. sp., una especie muy común para el Caribe, encontrándose únicamente en especies del género Aplysina; H. chaetafusorata n. sp., encontrada dentro de la esponja Verongula rigida y se caracteriza por tener trazos de unión entre el mango y el artejo de las sedas posteriores y de la región media; H. navasi n. sp., hallada en aguas colombianas dentro de la esponja Ircinia strobilina, y se caracteriza por tener cirros dorsales de similar tamaño en la región media, y porque tanto la acícula como el proventrículo son muy anchos, y H. niphatesicola n. sp., encontrada dentro de especies del género Niphates, caracterizándose por la alternancia entre cirros cortos y largos en la región media, la presencia de gránulos redondeados dorsales en todo el cuerpo, y la acícula ancha posterior. Finalmente, H. cephalata y H. gula, se citan por primera vez en aguas colombianas y se aporta nueva información sobre su hábitat. Finalmente, se discute el estado actual del conocimiento sobre las relaciones entre las especies simbiontes de Haplosyllis y sus huéspedes conocidos.

The genus Haplosyllis Langerhans, 1887 is revised based on available types and newly collected specimens. 19 species are considered as valid, five as incertae sedis and four are referred to nomina dubia. Trypanoseta (Imajima, 1966) is synonymised with Haplosyllis, as the presence of trepan is considered a non-robust taxonomic feature, affecting H. ohma new comb. (Imajima & Hartman, 1964) and H. granulosa new comb. (Lattig, San Martín & Martín, 2007). Haplosyllis streptocephala (Grube, 1857) is a new combination and two species are considered to belong to Alcyonosyllis Glasby & Watson, 2001: A. bisetosa new comb. (Hartmann-Schröder, 1960) and A. gorgoniacola new comb. (Sun & Yang, 2004). All valid species are described and figured except those described during the last decade, which only includes diagnoses and chaetae illustrations. A dichotomous key to all valid species of Haplosyllis is provided.

The polychaete family Sabellidae Latreille, 1825, commonly known as feather-duster worms, is characterized by a striking, usually colourful radiolar crown that emerges from self-made tubes typically composed of mucus and sediment (Rouse, 2000; Merz, 2015), with the exception of Glomerula piloseta (Perkins, 1991), which inhabits calcareous tubes. This family represents one of the most diverse and ubiquitous families worldwide, comprising around 400 species belonging to 39 genera, all marine and benthic with the exception of one freshwater genus (Capa et al. 2014; Capa & Murray, 2015). Sabellids are found in a variety of habitats from the intertidal zone to abyssal depths (Tovar-Hernández, 2009). The systematics of Sabellidae has been considered in several studies. According to many morphological studies (e.g. Knight-Jones, 1981; Fitzhugh, 1989; Nogueira et al. 2010), Fabriciinae Rioja, 1921 was considered a subfamily of Sabellidae. More recently, however, both morphological and molecular studies consider Fabriciidae as an independent family and the sister clade to Serpulidae Rafinesque, 1815 (Kupriyanova & Rouse, 2008; Capa et al. 2011; Huang et al. 2011). However, the internal relationships of Sabellidae remain unclear, and further studies are needed to better resolve the sabellid phylogeny (Capa et al. 2014). Fitzhugh (1989) contributed greatly to the systematics of the family with an exhaustive revision. However, some traditional morphological features have been reported as inaccurate for taxonomic purposes in some taxa (e.g. Capa et al. 2010; Capa et al. 2013). Taxonomic revisions for some genera exist (e.g. Knight-Jones & Perkins, 1998; Knight-Jones & Mackie, 2003), but the establishment of the species delimitation of others remains doubtful, such as Myxicola with a poorly defined taxonomy (Giangrande et al. 2015) or Bispira, which lacks apomorphies (Capa, 2008). Moreover, despite the current knowledge of the family, some species keys still use older, inaccurate descriptions. Thus, greater effort is needed in order to clarify species statuses within some genera. To date, 19 genera and 40 valid species have been reported from the waters surrounding