Capitellids are burrowing worms, usually elongate and thread-like. They are among the most frequently recorded polychaetes in marine soft bottoms, living at a wide bathymetric range from intertidal to deep sea and may even be the dominant organisms in infaunal communities, especially in organically enriched sediments (Blake 2000). The family Capitellidae Grube, 1862 is one of the oldest recognized polychaete families and currently is composed of approximately 200 valid species belonging to 43 genera (Magalhães and Bailey-Brock 2012; Silva and Amaral 2019). Although they can be easily recognized at the family level, their accurate identification, even at the generic level, is difficult since very few distinctive morphological characters are visible (Magalhães and Blake 2019). So, definitions at the generic and species level have always been controversial, as they are mainly based on the number and structure of the thoracic chaetigers and the distribution of different types of chaetae along the body. However, as demonstrated recently (Magalhães and Blake 2019), those characters, far from being stable, can change with age and sexual maturity and, thus, 21 genera are monotypic and most of them are represented by a single type specimen, frequently incomplete.

The genus Notomastus M. Sars, 1851 is characterized by a thorax with 12 segments: the peristomium and 11 chaetigers; the first thoracic chaetiger is uni- or biramous and the last may have capillary chaetae, hooded hooks, or a mixture of both; in the abdominal chaetigers, hooded hooks are present, while branchiae can be present or absent (García-Garza and de León-González 2015). Ten of the 44 valid species, including Notomastus bermejoi sp. nov., were originally described from the Gulf of California: N. abyssalis Fauchald, 1972, N. angelicae Hernández-Alcántara and Solís-Weiss, 1998, N. cinctus Fauchald, 1972, N. fauchaldi García-Garza and de León-González, 2015, N. landini García-Garza and de León-González, 2015, N. lobulatus García-Garza and de León-González, 2015, N. mazatlanensis García-Garza, de León-González and Tovar-Hernández, 2019, N. precocis Hartman, 1960, and N. sonorae Kudenov, 1975.

Predictably, in Notomastus, the taxonomic problems detected in other capitellid genera also occur, with several species that do not entirely fit the genus definition, e.g., N. exsertilis Saint-Joseph, 1906 has only 10 thoracic chaetigers and bears capillary chaetae in the first two abdominal segments, or N. hedlandica where capillary chaetae in the first abdominal segment are present (Hartmann-Schröder 1979), making us think that they probably belong to other genera (García-Garza et al. 2019).

This confusing situation usually leads to misidentifications. This is the case of Notomastus americanus Day, 1973, which was originally described from off Beaufort, North Carolina, and then reported from the Gulf of California by Hernández-Alcántara and Solís-Weiss (1993, 1998, 1999), due to the presence of hooded hooks in neuropodia of chaetiger 11. However, N. americanus was later synonymized with Notomastus hemipodus Hartman, 1945 by García-Garza et al. (2012), based on their revision of the type material. Then, recently, the careful taxonomic examination of the specimens catalogued as N. americanus deposited in the Colección Nacional de Anélidos Poliquetos, Instituto de Ciencias del Mar y Limnología (ICML)Universidad Nacional Autónoma de México (UNAM), Mexico City, revealed significant differences, not only with the type material of N. americanus (= N. hemipodus) but also with close species. That is why the aim of this study is to describe a new species of Notomastus from those misidentified organisms. To corroborate the status of the new species, we also reviewed the type material and original descriptions of Notomastus species with hooded hooks in thoracic chaetigers deposited in the National Museum of Natural History, Smithsonian Institution and Natural History Museum of Los Angeles County. An identification key and tables with morphological distinctive characteristics and methyl green staining patterns to support the future identification of these capitellids are provided.

The material examined was collected in the continental shelf of the Gulf of California, Mexican Pacific (20°30'–31°38'N, 105°42'–114°50'W), as part of the oceanographic expedition “Cortes 2” (Fig. 1; Table 1) on board the R/V El Puma of the Universidad Nacional Autónoma de México (UNAM). The samples were collected with a Smith-McIntyre grab (0.1 m²) and sieved through a 0.5 mm mesh. The specimens were fixed in 10% formalin in seawater and later preserved in 70% ethanol. Additionally, at each station, depth, temperature, and salinity were measured with a Niels Brown CTD, and the dissolved oxygen determined by the Winkler method (Strickland and Parsons 1972). The organic matter content was evaluated by the Walkley and Black (1934) acid digestion method and the sediment texture was determined following the method of wet sieving (Folk 1980).

The specimens were examined under dissecting and compound light microscopes, both with an integrated camera for photography. Detailed examination of the chaetal types, distribution, and morphology were supported with scanning electron microscope images: specimens were dehydrated via a graded ethanol series, critical-point dried with liquid CO₂, coated with gold, and examined in a JEOL JSM6360LV microscope at the Instituto de Ciencias del Mar y Limnología (ICML), UNAM. The methyl green staining pattern was examined by immersing the specimens for 2 min in a saturated solution of methyl green in 70% ethanol, then washing them in ethanol 70% to remove the excess methyl green (Warren et al. 1994).

All identified specimens and the type material of the new species were deposited in the Colección Nacional de Anélidos Poliquetos of the ICML, UNAM (CNAP-ICML: DFE.IN.061.0598). Paratypes were deposited in the Natural History Museum of Los Angeles County (LACM-AHF Poly).

The type material of Notomastus species with hooded hooks in thoracic chaetigers deposited in the National Museum of Natural History, Smithsonian Institution (USNM) and in the Natural History Museum of Los Angeles County were also examined to compare their morphological characteristics with those found in the new capitellid.

The specimens assigned to this new species were collected on the continental shelf of the Gulf of California and were originally identified as N. americanus due to the key character “presence of neuropodial hooded hooks on the last thoracic chaetiger” (Hernández-Alcántara and Solís-Weiss 1998). However, after a detailed revision of these specimens, their misidentification was suspected, since they displayed a clearly different methyl green staining pattern than observed in the type material of N. americanus (Table 3), confirming they are actually different species.

The examination of the methyl green pattern in the holotype of N. americanus (Fig. 3G–I) also showed that it corresponds to that found in N. hemipodus Hartman, 1945: chaetigers 1–6 with the same green intensity, a wide continuous longitudinal line on the ventral side from peristomium to chaetiger 6 (Fig. 3G, H) and the characteristic ventral abdominal region with a pair of longitudinal bands to the end of the body (Fig. 3I), even though N. hemipodus has only capillary chaetae in all thoracic chaetigers. The methyl green staining pattern of the holotype of N. americanus is illustrated here for the first time, which confirms the observations made by García-Garza et al. (2012) about the synonymy of both species. As García-Garza et al. (2012) had already indicated, the specimens of N. americanus examined by Day (1973), with hooks in neuropodia of chaetiger 11, can be considered juveniles. This was also mentioned by Ewing (1984a), when he examined specimens from the northern Gulf of Mexico, since in small specimens of N. americanus a mixture of capillaries and hooks in neuropodia of chaetiger 10 may also be present.

In the family Capitellidae, changes in chaetal structure during ontogeny represent a fundamental taxonomic problem, not only to identify the specimens to the genus level, but also to detect immature individuals. In Notomastus, as in several capitellid genera, during the chaetal development process, the hooks are gradually replaced by capillaries, so that even a mixture of hooks and capillaries can be found in neuropodia of middle and posterior thoracic segments (Ewing 1984b). However, from the 44 described species in this genus, including the new species described here, only eight have been reported with hooded hooks on some thoracic parapodia: N. mossambicus, N. sunae, N. teres, N. daueri, and Notomastus sp. A of Ewing (1984a), N. precocis Hartman, 1960, N. angelicae, and N. bermejoi sp. nov.

The first species described with hooded hooks in thoracic chaetigers was N. precocis by Hartman (1960), based on an incomplete specimen 15.5 mm long with nearly 50 segments, bearing a mixture of capillaries and hooded hooks in neuropodia of chaetigers 7 to 11, which was a distinctive character to separate it from close species. However, re-examination of its holotype showed that it could be an immature specimen of a known species, since the mixture of capillaries and hooks begins on chaetiger 7. When thoracic hooks in other species are present, it seems that the occurrence of hooks in neuropodia of chaetiger 11 is a stable character: N. teres described by Hartman (1965) from an incomplete specimen 10.5 mm long and 35 segments, showed that hooded hooks were present in neuropodia of chaetigers 10 and 11. Likewise, N. mossambicus, described from an incomplete individual 32 mm long and 105 segments, has hooded hooks in neuropodia of chaetiger 11. Ewing (1982) described N. daueri from a complete specimen, 65 mm long with 234 segments, also bearing hooks in neuropodia of chaetiger 11. Later, Ewing (1984a) described Notomastus sp A from an incomplete individual 4 mm long with 29 segments, with a mixture of capillaries and hooks in neuropodia of chaetiger 9, and only hooks in neuropodia of chaetigers 10 and 11. Hernández-Alcántara and Solís Weiss (1998) described N. angelicae from an incomplete specimen 15 mm long with 48 segments, bearing hooded hooks in neuropodia of chaetiger 11. It is important to emphasize that these authors examined 43 specimens, in which the occurrence of hooks on neuropodia of chaetiger 11 was constant, and only in one of them were capillaries and hooks in neuropodia 10 and 11 observed. Finally, in N. bermejoi sp. nov., where the holotype is an incomplete individual, 13.5 mm long with 32 segments, the hooded hooks are present in neuropodia of chaetiger 11 of all paratypes and additional material, except for one individual where mixed capillaries and hooks in neuropodia 10 were observed.

The occurrence of hooded hooks in thoracic neuropodia had already been discussed by Ewing (1982), who observed that immature specimens of N. hemipodus, N. lobatus (= Rashgua lobatus (Hartman, 1947)), and N. daueri bear only hooded hooks or a mixture of capillaries and hooks in as many as five posterior thoracic neuropodia and rarely in 1–2 notopodia. So, he suggested that the replacement of hooks by capillary chaetae in thoracic neuropodia in Notomastus follows this pattern: juveniles have only hooks in several neuropodia of the posterior half of the thorax; as the specimen grows, hooks are lost (shed, broken or resorbed?) and replaced by capillaries emerging from the superior region of the chaetal fascicle. This goes on until all hooks are replaced by capillaries and the process continues towards the posterior thoracic chaetigers until the adult stage is reached. In the same study, Ewing (1982) observed several variations in the chaetal arrangement of numerous juveniles of N. daueri: neuropodia of chaetigers 7–11 may have only capillaries, mixed chaetal fascicles, or only hooded hooks, but chaetigers 10 and 11 were rarely found with a combination of capillaries and hooks.

Therefore, and also in accordance with the observed characters in N. bermejoi sp. nov. and the previous observations provided regarding other species, we can establish that in the genus Notomastus, though variations in the chaetal arrangement are present in several thoracic neuropodia, the presence of exclusively hooded hooks in neuropodia of chaetiger 11 is constant, and it can thus be considered as a stable character to differentiate species.

Although other localities have not yet been well explored, until now, the Notomastus species bearing hooded hooks in thoracic chaetigers have almost entirely been recorded in the American seas. From the eight species described with this morphological characteristic, only N. sunae from southern China and N. mossambicus from Madagascar are reported from other regions. Three species were reported from the Gulf of Mexico or northwestern Atlantic, and in the Eastern Pacific, N. precocis was collected in the deep Santa Catalina Basin, California, while N. angelicae and N. bermejoi sp. nov. were recorded from the continental shelf of the Gulf of California.