New records and five new species of sipunculans (Sipuncula) from the central and northwestern Mexican Pacific

. Sipuncula are marine unsegmented worms that can be found in benthic habitats, from shallow to deep-sea waters. In Mexico these worms have been scarcely studied. Among nine studies in the entire Mexican Pacific coasts, seven are from the central and northwestern Mexican Pacific. For over 80 years, only 24 species have been recorded. To improve the knowledge of the sipunculans from the central and northwestern Mexican Pacific, 501 specimens from three scientific collections of Mexico were revised. The specimens were collected by diverse methods from intertidal, subtidal, and bathyal depths (~1000 m), from 27 locations along the central and northwestern Mexican Pacific. Sixteen taxa belonging to nine genera and five families were identified. One species is recorded for the first time in the Mexican Pacific and five new species are described, one from intertidal and shallow subtidal depths: Phascolosoma ( Phascolosoma ) azteca sp. nov.; two from subtidal depths: Phascolion ( Lesenka ) salgadoi sp. nov. and Sipunculus ( Sipunculus ) bastidai sp. nov.; and two from bathyal depths: Apionsoma ( Apionsoma ) silviae sp. nov. and Apionsoma ( Edmondsius ) hendrickxi sp. nov.


Introduction
The sipunculans, commonly known as peanut worms, are marine invertebrates without segmentation, with bilateral symmetry, and their body is divided into two regions, an anterior retractile introvert, and a posterior trunk (Cutler 1994).They live in a wide variety of marine habitats, from the intertidal to 7000 m deep (Murina 1984), in unconsolidated sediments, such as sand and mud, or inside shelters, such as mollusk shells or eroding coral and rocks (Stephen & Edmonds 1972;Cutler 1994).Recently, sipunculans have been classified among the annelids by phylogenetic analysis; however, experts do not fully accept this conclusion (Schulze et al. 2019).
Along the Pacific coast of Mexico, sipunculans have been scarcely studied.The first known report was made by Steinbeck & Ricketts (1941), who recorded four species from several sites in the Gulf of California.Later, Fisher (1947Fisher ( , 1952) ) in two contributions, provided seven records and described nine species from locations off the western coast of the Baja California Peninsula and in the Gulf of California; however, most of them have been synonymized by Cutler (Cutler & Cutler 1987, 1988, 1990) with other known species.Thirty years later, Salazar-Vallejo (1983) gave the first record of a sipunculan from the central Mexican Pacific, Aspidosiphon cf.albus Murina, 1967.It took another 30 years before four sipunculan species were added to the Mexican Pacific records for the northern Gulf of California (Hermoso-Salazar et al. 2013).In a recent study, Morales-Zárate et al. (2016) recorded two morphospecies at the generic level, Phascolion sp. and Themiste sp., from Guerrero Negro Lagoon, western coast of Baja California Sur.Recently, Silva-Morales et al. (2019) described a new species of Antillesoma from the southern Mexican Pacific, while Galvéz-Zeferino et al. (2020) expanded the knowledge of sipunculans by adding five new records from the western coast of Baja California Sur, and Silva-Morales & Gómez-Vásquez (2021) recorded 11 species, including two newly described from the southern Mexican Pacific.
Only seven studies have recorded 24 sipunculan species from the central and northwestern Mexican Pacific during a period of 80 years.Discounting unidentifiable species, there are only 18 valid species for the central and northwestern Mexican Pacific (Table 1), which represents 12% of the richness of sipunculans of the world (Cutler 1994).
The sipunculans are a key component within the community of bioeroders (Alvarado et al. 2016), and they also play an important role in the trophic net by being part of the diet of polychaetes, mollusks, crustaceans, echinoderms, sea anemones, fish and even humans (Fischer 1925;Kohn 1975;Saiz-Salinas 1993;Cutler 1994).
The taxonomy of Sipuncula has historically been complex to approach, mainly because of their conserved morphology, leading to the idea that sipunculan species have a wide geographic distribution, resulting in many species having been synonymized (Cutler 1994).It has been observed that geographically separated populations of species show some morphological differences, indicating the possibility of being distinct species, an idea supported by molecular analysis (Kawauchi & Giribet 2010, 2014;Silva-Morales et al. 2019;Schulze & Kawauchi 2021), and the same kind of evidence has been used in recent reinstatements of synonymized species (Silva-Morales 2020; Silva-Morales & Gómez-Vásquez 2021).
The aim of this study is to improve our knowledge of the sipunculans present along the Mexican Pacific coast, with new records and the description of new species.

Material and methods
The study area includes, according to Spalding et al. (2007), three marine ecoregions of the Northwest Warm Temperate Pacific province and one marine ecoregion of the Eastern Tropical Pacific province.The examined specimens came from 27 locations in the eight states of the central and northwestern Pacific of Mexico (Fig. 1).
The specimens were collected by free diving and breaking dead coral and rocks from shallow water.For those at subtidal and bathyal depths (20-1079 m), bottom samples were obtained on board oceanographic vessels using cores, dredges and trawl.All specimens were fixed with 4% formalin and later preserved in 70% ethyl and isopropyl alcohol.
The specimens were dissected, handled with fine tweezers and cut with a scalpel.The cuts were made from the anus towards the caudal area.In case the introvert was retracted, the cut was made anterior to the anus.Once the specimens were opened, they were mounted in a Petri dish, previously filled with a thick layer of wax, and fastened with entomological needles to expose the internal anatomy.The specimens were observed under a stereoscopic microscope (Zeiss Discovery V8) and a compound microscope (Zeiss Scope A1) to observe the hooks and other small features.The photographs were taken with a Canon EOS Rebel T6 camera.
To identify the specimens, the identification keys of Stephen & Edmonds (1972) and Cutler (1994) were used; in addition, the original descriptions and redescriptions of the nominal species were consulted to compare with the specimens found in the central and northwestern Mexican Pacific.The classification follows Kawauchi et al. (2012).Xenosiphon branchiatus (Fischer, 1894) Ecuador Fisher (1947) The morphological terms used in the text are based on Cutler (1994).The abbreviations used in the descriptions of species are as follows: The holotype of Sipunculus (Sipunculus) bastidai sp.nov is deposited in the collection of UMAR (UMAR-SIPU 129), and the holotypes of Apionsoma (Apionsoma) silviae sp.nov., Apionsoma (Edmondsius) hendrickxi sp.nov., Phascolosoma (Phascolosoma) azteca sp.nov.and Phascolion (Lesenka) salgadoi sp.nov.are deposited in the collection of EMU (EMU-13436, -13437, -13443 and -13448, respectively).

Results
A total of 501 specimens of sipunculans from the central and northwestern Mexican Pacific were examined.The specimens were identified to the highest possible level, obtaining 16 taxa, belonging to nine genera and five families.Five species were unequivocally identified as species and confirmed for the region, three were left as close to the nominal species, three morphospecies need more evidence to confirm their identity, and five are described as new sipunculan species.Grube, 1858).-Fisher 1952: 434-436, pl. 39 figs 8-9.

Habitat
Intertidal to subtidal (3 m); in rocks, in dead coral and associated with sabellariid tubes.

Distribution
Mexican Pacific: from Baja California Sur to Oaxaca (Silva-Morales et al. 2019).
Longitudinal musculature in a continuous layer, in bands under anal shield.A pair of retractor muscles, attached to body wall at posterior end of trunk.Two nephridia of 40% of trunk length; nephridiopores open posterior anus.Spindle muscle attaches to intestine anteriorly and trunk posteriorly.

Distribution
Pantropical, widespread and common in the Indian and western Pacific Oceans, from south-central Japan to northern Australia, the Red Sea and Israel; in the Caribbean from northern Brazil to the Florida Keys and Bermuda (Cutler 1994).In the eastern Pacific from southern Gulf of California to Costa Rica (Fonseca & Cortés 1988).

Remarks
The presence of A. (A.) elegans in the eastern Pacific was not discovered (sensu Cutler 1994) until Fonseca & Cortés (1988) recorded it from Costa Rica, and Silva-Morales & Gómez-Vásquez (2021) from the southern Mexican Pacific.
internal anatomy, were noted.Also, consulting the descriptions and illustrations of species synonymized with A. (A.) elegans (i.e., A. brocki, A. coralinus and A. exilis;Stephen & Edmonds 1972), I found that the configuration of the bidentate hooks is also different from the one seen in specimens from the Mexican Pacific.This may suggest that A. (A.) elegans is a species complex, and more detailed examination is required to determine the validity of worldwide records and whether the species is truly pantropical or the synonymized species can be considered as valid, and therefore be reinstated.Between the material of the southern Mexican Pacific and the specimens reviewed here, there are no morphological differences, indicating that there is only one morphospecies in the entire Mexican Pacific.With the present records, the distribution of A. (A.) elegans is extended to the northern Gulf of California.It has been observed that the species has asexual reproduction by transverse fission in the posterior region of the trunk (Rice 1970;Acik 2008), as was also observed in the specimens examined here (Fig. 3A).
Longitudinal musculature in a continuous layer (Fig. 4B), in bands under anal shield.A pair of retractor muscles attached to body wall at posterior end of trunk; nephridiopores open anterior to anus.

Distribution
Sea of Cortez, known only from stn 19 of the Cortés cruise, near Cabo San Miguel, Baja California.

Remarks
The unique specimen examined is incomplete, without a substantial portion of the introvert and part of the internal organs, i.e., no nephridia or intestine were present; however, what is observed on the specimen was sufficient to be able to identify it to subgenus.
Aspidosiphon (A.) sp. is closely related to A. (A.) misakiensis Ikeda, 1904 and A. (A.) muelleri Diesing, 1852; however, there are some features which make A. (A.) sp.different from any other species of the subgenus.Specifically, the presence of unidentate hooks with a pyramidal base and conical tip, the presence of the roll-type chitinized structure, which is not known for any of the species of the subgenus, and the presence of the two types of shields in A. (A.) sp., an ungrooved anal shield with packed granules, and an acorn-shaped caudal shield.In A. misakiensis both shields consist of granules, as also in A. muelleri, where the anal shield is grooved.Based on the latter, this specimen might represent a new species; however, it is necessary to obtain complete specimens to describe it formally.
Longitudinal musculature divided in 18 bands along trunk, with high degree of anastomosis.Two retractor muscles attached to body wall at 95% of trunk length.Nephridia 25% of trunk length; spindle muscle attached intestine at both ends of trunk.

Distribution
Mexican Pacific: from Gulf of California to Oaxaca.The main differences are present in the proximal hooks.In A. (P.) pastori the pyramidal hooks are pale, whereas A. (P.) tenuis instead has unidentate structures with laterally reinforced ridges (Cutler 1994).

Remarks
Another difference is that A. (P.) pastori has a shield made of granules with diffuse margins, and A. (P.) tenuis has a grooved shield with well-defined margins.

Distribution
Western coast of Baja California.

Remarks
Despite the damage to the single specimen examined, it was placed in the subgenus Golfingia (Golfingia) because the dorsal retractor muscles are displaced.Within the subgenus only two species have a caudal appendage.One is G. (G.) anderssoni (Théel, 1911), described from Antarctica; it has large, bladdershaped papillae that cover an area of 65-90% of the posterior trunk length (Cutler 1994), while the specimen examined here from Baja California has inconspicuous conical papillae.The other species is G. (G.) muricaudata, which also has inconspicuous conical papillae.Unfortunately, since the specimen is damaged, the identity of this species cannot be confirmed; more specimens are needed to clarify its status.
Longitudinal musculature continuous; two retractor muscles attached to body wall in posterior fourth of trunk (Fig. 6E).Two small nephridia, sack-like with pointed tip; nephridiopores open posterior to anus; free intestine (Fig. 6D).

Distribution
Eastern Pacific: from the Gulf of Alaska to the Peruvian-Chilean Trench.This is the first record of the species from the Mexican Pacific.

Remarks
The examined specimens of Nephasoma (N.) wodjanizkii wodjanizkii agree with Murina's (1973a) original description.From the region, the closest record was made by Thompson (1980), who recorded the species for southern California.
Frank's subsequent record from Canada should be revisited, since he made it based on a single specimen collected at a depth of 42 m (Frank 1983).Furthermore, Gálvez-Zeferino et al. (2020) recorded N. (N.) wodjanizkii, similar to Frank's record; it should also be revised, since they recorded a single specimen from a coastal lagoon on the western coast of Baja California Sur, at depths of 5 to 12 m.Two further studies, Murina (1973b) and Thompson (1980), collected their specimens at bathyal depths (1100-2000 m) from the Peruvian-Chilean Trench and California, respectively; therefore, the present specimens from the western coast of Baja California represent an intermediate record of the species made in the marine ecoregion of the Northwest Warm Temperate Pacific.
The epithet is a noun in the genitive case (ICZN 1999, Art. 31.1.2).Other material MEXICO • 4 specs; same collection data as for holotype; UMAR-SIPU 132.

Description (EMU-13448)
Body coiled to the left; trunk 25 mm in length (Fig. 7A).Light brown skin, translucent at mid trunk.Ventral region of trunk with numerous simple, round and flat papillae, mid trunk with scattered circular papillae with hardened U-shaped border with a tooth (Fig. 7D), denser and smaller on caudal region and on each side of ventral papillae.Introvert partially protruded; 12 short, thin digitiform peripheral tentacles encircling mouth.Scattered claw-shaped, dark hooks (Fig. 7F) of type I (Cutler 1994), 37 µm long, occupying 10% of distal length of introvert.Anus on proximal portion of introvert, near edge of trunk; posterior introvert with bladder-shaped holdfast papillae (Fig. 7E), extending slightly to trunk.
Longitudinal musculature continuous; retractor muscle in a single column, attached to body wall at 90% of posterior trunk length.Esophagus attached to retractor muscle.A single nephridium of approximately 15% of trunk length attached in all its length to body wall; nephridiopore opens posterior to anus.Spindle muscle attached anterior to anus and attached to intestine in caudal region of trunk; intestine without coils, with same torsion as body (Fig. 7B).

Variation
Some of the hardened holdfast papillae have a U-shaped border.The density of papillae is variable among the specimens.

Distribution
Gulf of California: only known from Guaymas Bay, Sonora.

Remarks
The specimens can easily be placed in the subgenus Phascolion (Lesenka) by the presence of a single retractor muscle and by having less than 20 tentacles.Phascolion (L.) salgadoi sp.nov. is similar to two subspecies: P. (L.) valdiviae valdiviae Fischer, 1916 (from the Indian Ocean) and P. (L.) valdiviae sumatrense Fischer, 1916 (from Sumatra).The two subspecies and the specimens examined here have U-shaped holdfast papillae, in addition to having a single retractor muscle; however, the specimens from the Mexican Pacific are geographically separated from previous subspecies.Also, they have several morphological differences from both subspecies.
Phascolion (Lesenka) salgadoi sp.nov.differs from Fischer's subspecies because they have dark hooks, while P. (L.) valdiviae valdiviae has pale hooks and P. (L.) valdiviae sumatrense lacks hooks.Phascolion (L.) salgadoi does not show intestinal coils, the intestine instead following the torsion of the body, while the Indian subspecies has 20-22 coils.Regarding habitat, Ph. (L.) salgadoi was found at a depth of approximately 45 m, living in an empty shell of the gastropod Polystira sp., while the subspecies P. (L.) valdiviae valdiviae was obtained at a depth of 158 m, living in shells of the scaphopod Dentalium sp., which does not have whorls.The description of P. (L.) valdiviae sumatrense did not mention anything about its habitat, except that the species was found at a bathyal depth (750 m) (Fischer 1916).
Comparing Phascolion (Lesenka) salgadoi sp.nov.with other species of the subgenus, the following differences are noted: P. (L.) collare Selenka & de Man, 1883 has conical papillae and small type III hooks (Cutler 1994); P. (L.) cryptum Hendrix, 1975 has four main tentacles followed by accessory tentacles and it has no hooks; and P. (L.) hupferi Fischer, 1894 has the anus at half of the introvert length and, like P. (L.) rectum Ikeda, 1904, has neither hooks nor holdfast papillae (Cutler 1994).On the other hand, P. (L.) salgadoi possesses flat oval papillae with a hardened U-shaped border, bladdershaped holdfast papillae, type I hooks, short digitiform tentacles and the anus is situated at the base of the introvert.

Distribution
Central Mexican Pacific: Corrales Beach, Jalisco.

Remarks
The nominal species, Themiste (Themiste) blanda (Selenka & de Man, 1883), was described from Enoshima, Japan (Selenka et al. 1883).Morphologically, the only difference between the description of the nominal species compared to the specimen from Jalisco is that the Japanese species was described as having five or six stems on its tentacular crown, while the Jalisco specimen only showed four stems.This difference may be due to the size of the specimen examined; it might not have completely developed all its stems.However, if this feature is actually indicative of a new species, more specimens are needed to properly study and describe it.It should be noted that studies conducted on the reproduction of other species of the genus (Williams 1972;Rice 1976;Adrianov et al. 2009;Adrianov & Maiorova 2010) show that species of Themiste have larval development type 3, with larvae living 2-14 days in the water column.This period is not long enough to explain how the species could be transported from the Temperate Western Pacific to the Tropical Eastern Pacific.Thus, the presence of T. (T.) blanda in the Mexican Pacific is questionable, at least as long as no more specimens of this taxon are found.

Variations
The specimens examined had a trunk length range between 2 and 12 mm.The number of tentacles varies with length, a specimen with a trunk length of 2 mm having eight terminal tentacles on each branch, while a specimen with a trunk length of 6 mm has 10 terminal tentacles on each branch of the tentacular crown; the largest specimen measured here has a trunk length of 12 mm and it has 14 terminal tentacles on each branch.Considering all specimens, the total amount of tentacles is between 72 and approximately 170.

Habitat
Intertidal to subtidal (4 m); under rocks, between algal rhizoids, in gravel and muddy sand.

Distribution
Eastern Pacific: from California to Perú.

Remarks
The specimens agree with the descriptions made by Gray (1828) and Fisher (1952).This species has previously been recorded from the study area in the Gulf of California.

Description (UMAR-SIPU 130)
Trunk 12 mm in length, pyriform, caudal region pointed; smooth, light brown body wall, with small, flat oval papillae.Introvert as long as 0.75 of trunk length, with pigmented collar and tentacular crown base; tentacular crown with four stems divided into two branches, stems of ventral pair slightly longer than those of dorsal pair; each branch with eight terminal tentacles.Small, dark, claw-like hooks at posterior introvert.
Longitudinal musculature continuous; two retractor muscles attached to body wall at 50% of trunk length; contractile vessel with 15 tubules.Two nephridia, of 25% of trunk length; nephridiopores before anus.Spindle muscle present, attached to intestine anteriorly and to trunk posteriorly.

Distribution
Eastern Pacific: from California to the Gulf of California.

Remarks
Themiste (T.) pyroides may be confused with T. (T.) blanda (Selenka & de Man, 1883); however, the main difference between the species concerns the tentacular crown.Themiste (T.) blanda has the branches symmetrically divided, in addition to bearing tentacles along the inner face of the stems and branches, whereas T. (T.) pyroides only has terminal tentacles and the branches are not divided symmetrically.According to Cutler (1994), T. (T.) pyroides is distributed in the eastern Pacific, from southern Alaska (Frank 1983) to the western coast of Baja California (Fisher 1952); however, the Alaska record should be reviewed, because the temperature range tolerance for this species has not been investigated and it seems unlikely that a subtropical species such as T. (T.) pyroides would occur at such low temperatures.Furthermore, there are no records of this species farther north than its type locality, apart from Frank's record (Frank 1983).In the northwestern Pacific of Mexico, T. (T.) pyroides extends its distribution from the west coast of Baja California to the Gulf of California (Sinaloa).
Themiste (T.) pyroides has also been recorded from the Sea of Japan (Adrianov et al. 2006(Adrianov et al. , 2008)); however, the latter study was based on a molecular analysis and concluded that the eastern and western populations are two distinct clades at the molecular level (Schulze et al. 2012).This may suggest that both populations could be part of a species complex, which should be addressed using morphological and molecular evidence.

Etymology
This species is named after the author's beloved mother, Silvia Vásquez Martínez, for all the effort and support she has given her sons, which has given them the opportunity to study and dedicate themselves to their favourite interests.The epithet is a noun in the genitive case (ICZN 1999, ART. 31.1.2).

Description (EMU-13436)
Trunk 8 mm in length; rough, light brown body wall, with numerous scattered, mammillate papillae (Fig. 8A), decreasing in size towards caudal region, with a spiral effect in caudal region (Fig. 8C).Introvert partially everted, four times as long as trunk length, with 10 small digitiform tentacles encircling nuchal organ dorsal to mouth.Hooks 30 µm long, with four basal spinelets, arranged in ten complete rings.Scattered, ventrally and dorsally, Mammillate papillae present ventrally and dorsally, scattered; two dorsolateral longitudinal lines of papillae at 80% of posterior introvert (Fig. 8B); ventral papillae smaller in size.
Longitudinal musculature continuous; four retractor muscles attached at 50% of trunk length (Fig. 9A); retractor muscles equally displaced from nerve cord; dorsal retractor muscles anterior to ventral pair.A pair of bilobed nephridia, 3 mm long, anterior lobe one-fifth as long as posterior lobe; nephridiopores anterior to anus.Spindle muscle present; not attached to intestine in caudal region of trunk.

Distribution
Only known from stn 16 of the TALUD XIV cruise, near Cabo San Miguel, Baja California.

Remarks
The sole specimen examined is easily placed in the subgenus Apionsoma (Apionsoma) by the presence of a continuous longitudinal musculature, which is very similar to conditions in the two subspecies A. (A.) murinae murinae (Cutler, 1969), described from the north-west Atlantic, and A. (A.) murinae bilobatae (Cutler, 1969), described from the north Atlantic.However, A. (A.) silviae shows differences to both subspecies by having bilobed nephridia and in the arrangement of the papillae on the introvert and on the caudal region.

Etymology
This species is named after the researcher Michel Hendrickx, a carcinologist from the Universidad Nacional Autónoma de México (UNAM), in recognition of his contribution to our knowledge of Mexican marine biodiversity.He also provided the specimens examined here.The epithet is a noun in the genitive case (ICZN 1999, Art. 31.1.2).

Description (EMU-13437)
Trunk 14 mm in length; slender body, six times as long as wide: body wall rough, brownish, with some dark patches (Fig. 10A); scattered mammillate papillae on entire body.Introvert twice as long as trunk, partially protruded; with 10 small digitiform tentacles in an arc surrounding nuchal organ.Hooks 45 µm long, with four basal spinelets (Fig. 10B), arranged in 15 complete rings.
Longitudinal musculature divided into highly anastomosing bands, with a soft appearance; 16 bands at level of ventral retractor muscles.Four retractor muscles, both pairs equally displaced from ventral nerve cord; dorsal pair originating at and attached to body wall anterior to ventral pair.Esophagus attached to right ventral retractor muscle; contractile vessel thick, without bulbs or villi.A pair of bilobed nephridia, with rough surface, 4 mm long; posterior lobe four times as long as anterior lobe; nephridiopores posterior to anus.Spindle muscle attached anterior to anus; free intestine (Fig. 11A).

Distribution
Only known from stn 16 of the TALUD XIV cruise, near Cabo San Miguel, Baja California.

Remarks
The specimens belong to the monospecific subgenus Apionsoma (Edmondsius) by having the longitudinal muscles separated into bands.Apionsoma (E.) hendrickxi sp.nov.differs from A. (E.) pectinatum (Keferstein, 1867) by having four basal spinelets on the hooks (Fig. 10B), while A. (E.) pectinatum has seven to nine spinelets (Fig. 12C); furthermore, A. (E.) pectinatum has much more conspicuous longitudinal muscle bands than those of A. (E.) hendrickxi (Fig. 11A).Another difference is in the origin of the retractor muscles: in A. (E.) pectinatum all four retractor muscles originate at the same level, whereas in A. (E.) hendrickxi the dorsal muscle pair originates anterior to the ventral one.Finally, A. (E.) pectinatum is found from the intertidal to the subtidal zone (depth 5 m) associated with rocks and coral, while Apionsoma (E.) hendrickxi sp.nov.was found in the bathyal zone (depth 310 m) on muddy sand bottoms.With the description of A. (E.) hendrickxi sp.nov., the subgenus is no longer monospecific.Siphonides rickettsi Fisher, 1952: 386-388, pl. 22 Cutler 1994: 197-198 (generic transfer).
Longitudinal musculature divided in anastomosing bands.Four retractor muscles, attached equally close to ventral nerve cord near anterior quarter of trunk, negating the appellations ventral and dorsal.A pair of bilobed nephridia, posterior lobe twice as long as anterior lobe; nephridiopores open posterior to anus.Spindle muscle attached anterior to anus; free intestine.

Distribution
Eastern Pacific: from southern Gulf of California to Panamá.

Remarks
This specimen agrees with the original description in Keferstein (1867) and the redescription in Cutler (1994).Recently, Silva-Morales & Gómez-Vásquez (2021) recorded the species from the southern Mexican Pacific, including a brief discussion of its apparent disjunctive distribution in the Tropical Eastern Pacific.The record of this specimen offers a new location for its distribution in the Eastern Pacific and confirms its continuous distribution in the Mexican Pacific.

Etymology
This species is named after the ancient Mexican culture of the great Tenochtitlan commonly known as the Aztecs.The epithet in a noun in apposition (ICZN 1999, Art. 31.1).

Description (EMU-13443)
Trunk 8 mm in length, pale brown body wall with scattered conical papillae (Fig. 13A), larger and denser towards anterior and posterior ends; with pre-anal area of pigmented small, conical papillae (Fig. 13B).Introvert twice as long as trunk; 12 small digitiform tentacles, aboral surface with longitudinal dark pigmented band (Fig. 13C), arranged in arc surrounding nuchal organ, dorsal to mouth; nuchal organ rounded and bilobed.Hooks curved (Fig. 13D-E), 52 µm long, with small secondary tooth, curved streak present, internal triangle separate from streak; hooks arranged in 15 complete rings.Longitudinal dorsal reddish band on posterior 80% of introvert towards anus.Dorsal conical papillae, posteriorly directed, on posterior half of introvert.
Longitudinal musculature in 18 anastomosing bands.Four retractor muscles attached at 50% of trunk length, ventral pair attached to six bands of longitudinal musculature (LMB 2-7), dorsal pair attached to two bands (LMB 6-7), anterior to ventral pair.Paired nephridia approximately 50% of trunk length;  nephridiopores open anterior to anus.Spindle muscle attaches intestine to rectum anteriorly and to caudal region posteriorly (Fig. 13F).

Variations
Based on 20 paratypes and the extra material examined, the number of hook rings varies according to the size of the specimen, a specimen with a trunk length of 6 mm having 12 hook rings and a specimen with a trunk length of 10 mm having 20 complete rings.The number of tentacles also varies according to the size of the specimen from 10 to 14.
The Caribbean specimens (Fig. 14) have a greater trunk length, 13 mm long with 12-16 tentacles.

Distribution
Mexican Pacific: present along the central and northwestern Mexican Pacific coast, from Baja California to Michoacán.

Remarks
Specimens of Phascolosoma (Phascolosoma) azteca sp.nov.are morphologically close to P. (P.) perlucens Baird, 1868.For comparative purposes some specimens of P. (P.) perlucens from the Venezuelan coast were examined, and these specimens were consistent with the descriptions and redescriptions of that species (Baird 1868;Rice & Stephen 1970).
Morphological differences were found between specimens from the Caribbean and those from the central and northwestern Mexican Pacific.Specimens of Phascolosoma (Phascolosoma) perlucens are larger and have 12-16 tentacles, whereas P. (P.) azteca sp.nov.has 10-14.Moreover, P. (P.) perlucens has more longitudinal muscle bands, 20-22, while P. (P.) azteca has 17-20.Furthermore, the ventral retractor muscles are attached to six longitudinal muscle bands (LMB 2-7) in P. (P.) azteca and to five (LBM 2-6) in P. (P.) perlucens.Phascolosoma (Phascolosoma) perlucens has pre-anal pigmented papillae on 10-20% of the posterior introvert (Fig. 14B), while in P. (P.) azteca sp.nov.they are restricted to 5% of the posterior introvert (Fig 13B).Finally, the hooks of P. (P.) perlucens have a principal tooth that is slightly straight and the secondary tooth is somewhat like a hump (Fig. 14D-E), whereas in P. (P.) azteca the hooks have a more curved principal tooth and the inner margin is concave, making the secondary tooth look more projected and slightly acute, rather than like a hump (Fig. 13D-E).
Since the morphology of Phascolosoma (Phascolosoma) azteca sp.nov.resembles that of P. (P.) perlucens, the newly described species belongs to the P. (P.) perlucens species complex, for which, as mentioned by Kawauchi & Giribet (2010), there is evidence of multiple genetic clades, and some morphological differences can be shown between the populations as well.However, the morphological differences listed above indicate that P. (P.) azteca from the northwestern Mexican Pacific is a valid species, distinct from P. (P.) perlucens from the Caribbean.Phascolosoma puntarenae -Fisher 1952: 430-432, pl. 36  Trunk 30 mm in length (Fig. 12B); body wall slightly rough, dark brown, covered with scattered domeshaped papillae, denser on posterior trunk.Introvert twice as long as trunk, with black transverse patches and 24 digitiform tentacles; hooks 92.5 µm long, with a wide curved streak (Fig. 12D), arranged in more than 100 complete and incomplete rings.

Phascolosoma
Longitudinal musculature separated into 24 anastomosing bands.Four retractor muscles, ventral pair attached to body wall at 42% of trunk length, dorsal pair attached anteriorly and more displaced from ventral nerve cord.Two nephridia of 50% of trunk length; nephridiopores open at same level as anus.
Intestine spiral anchored at both ends by well-developed spindle muscle.

Habitat
Intertidal to subtidal (2 m); under rocks and in dead coral.

Distribution
Eastern Tropical Pacific: from the Gulf of California to Costa Rica.

Remarks
This species was recently reinstated from synonymy with P. nigrescens after differences were noted in their hooks (Silva-Morales & Gómez-Vásquez 2021).The specimens examined here show all the features of the species indicated by Silva-Morales & Gómez-Vásquez (2021).

Material examined
Holotype MEXICO • W coast of Baja California Sur; 26°17ʹ25ʺ N, 112°56ʹ39ʺ W; Feb. 1989;depth  (Based on holotype.)Trunk 80 mm in length; pinkish brown body wall, slightly iridescent (Fig. 15A).Trunk skin squared; middle trunk with protruding globose skin bodies on each square (Fig. 15B), present on approximately half of trunk length; first 35% and last 15% of trunk length free of skin bodies.Glans region on last 8% of trunk length, not distinctly differentiated from trunk, caudal tip with a notch.Introvert ¼ of trunk length, with scattered triangular, rounded tip papillae.Short crenulated tentacles surrounding mouth (Fig. 15D).
Longitudinal musculature divided into 35 bands.Four retractor muscles attached to body wall at same level, at 25% of trunk length; ventral pair attached to two muscle bands (LMB 3-4), dorsal pair attached to three muscle bands .Brain oval, 1 mm in length, wider than long, with main tubular process (Fig. 15E).Post-esophageal loop of same length as intestine, reaches glans region.Pair of nephridia 20 mm in length, with granulated surface; nephridiopores open anterior to anus.Anus located at 25% of trunk length; rectum with a pair of wing muscles; single sack-like caecum present, attached to spindle muscle.Spindle muscle well developed, attached anteriorly to rectum, subdivided along entire intestine in various stringy muscles, not attached posteriorly (Fig. 15C).

Variations
(Based on 25 specimens from paratype material.)The trunk length of the smallest specimen is 58 mm, while the largest is 115 mm long.The proportion of the body occupied by globose skin bodies is ½ of the trunk length, with a range from ⅓ to ⅔.
Regarding the longitudinal muscle bands (LMB), the majority (52%) of the specimens have 35, 20% have 36, 16% have 37, one specimen has 33 and one has 34 LMBs.Almost all specimens analyzed have the ventral retractor muscles (RM) attached to two LMBs (LMB 3-4), but one specimen has the ventral RM attached to three LMBs (LMB 3-5); in most of the specimens the dorsal RMs are attached to three LMBs (LMB 12-14), but some specimens have the dorsal RM attached to four LMBs (LMB 12-15); a few other specimens have the dorsal RM start their attachment to LMB 11-13, resulting in six possible combinations observed: attached to three LMBs (11-13, 12-14 or 13-15) or attached to four LMBs (11-14, 12-15 or 13-16), where the last three combinations are the rarest.

Distribution
Only known from the western coast of Baja California Sur.Sluiter, 1902 from Indonesia by having between 35 and 41 longitudinal muscle bands and by having the origin of the retractor muscles separated.However, specimens of S. (S.) bastidai have a series of morphological characters that are different from those of the two subspecies of S. phalloides.In their descriptions and illustrations (Pallas 1774; Sluiter 1902), neither of the subspecies of S. phalloides has skin bodies on the trunk, which are clearly observed as globular projections in specimens of the new species; the range of number of longitudinal muscle bands (LMB), regardless of the size of the specimens, is also different: 33-37 in S. (S.) bastidai, 35-41 in S. (S.) phalloides phalloides and 32-39 in S. (S.) phalloides inclusus.Furthermore, according to Cutler & Cutler (1985), who reviewed Sluiter's type material, S. (S.) phalloides inclusus has LMBs that separate and rejoin each other along the trunk, unlike S. (S.) bastidai, where the LMBs maintain the same number throughout the trunk.

Remarks
Other morphological features that separate Sipunculus (Sipunculus) bastidai sp.nov.from the subspecies of S. (S.) phalloides are the number and position of the LMBs to which the retractor muscles are attached (Table 2).Stephen & Edmonds (1972) mentioned that both subspecies of S. (S.) phalloides do not have a caecum in the rectum, whereas the species from Baja California Sur does.Due to this set of morphological differences, there is enough evidence to consider S. (S.) bastidai as a valid new species.Regarding molecular analyses, it has been shown in a species of the same genus (Sipunculus nudus Linnaeus, 1766) that there is evidence of multiple genetic clades, invalidating the idea of cosmopolitan species (Kawauchi & Giribet 2014).
The study by Hermoso-Salazar et al. (2013) was the only one so far to record deep-water sipunculans for México (217 m), recording eight taxa, only three of which were identified to the specific level, but these records are considered to be questionable due to their large distribution ranges (Silva-Morales & Gómez-Vásquez 2021); none of them were found in the present study.In shallower waters, Salazar-Vallejo (1983) recorded a morphospecies close to Aspidosiphon (Akrikos) albus Murina, 1967; however, he questioned whether the species could be widely distributed.The records of A. (A.) albus (Hermoso-  .)cf.albus from the northwestern Mexican Pacific could be considered the same species, despite their being from the northern Gulf of California and Colima, respectively.However, more efforts should be made to confirm the identity of this species, which is likely to be new, distinct from the actual A. (A.) albus.
Compared to previous records, Themiste (T.) hennahi expands its distribution for Mexico, from Baja California to Jalisco; Silva-Morales & Gómez-Vásquez (2021) recorded the species for Oaxaca, increasing its distribution in Mexico, so it is assumed that T. (T.) hennahi has a distribution along the entire Mexican Pacific coast (Fisher 1952;Silva-Morales & Gómez-Vásquez 2021).
Since molecular tools have been included in recent taxonomic studies, some questions have been formulated, such as, "How similar are the populations of species from different regions of the world?"In this sense, research studies have been conducted for supposedly cosmopolitan species, such as Phascolosoma (P.) perlucens, and within the genus Sipunculus, such as Sipunculus nudus.Kawauchi & Giribet (2010, 2014) invalidated the idea that P. perlucens and S. nudus are cosmopolitan, highlighting that there is some morphological evidence that could support the differentiation found in molecular analyses.
Unfortunately, the specimens used in the present study were fixed in formalin when collected, hindering DNA sequencing.However, they offered enough morphological evidence to describe them and establish some of them as new species.The new species belong to three families of Sipuncula, Phascolosomatidae (Apionsoma (Apionsoma) silviae sp.nov., Apionsoma (Edmondsius) hendrickxi sp.nov.and Phascolosoma (Phascolosoma) azteca sp.nov.), Golfingiidae (Phascolion (Lesenka) salgadoi sp.nov.) and Sipunculidae (Sipunculus (Sipunculus) bastidai sp.nov.).Of these new species, Phascolosoma (Phascolosoma) azteca has previously been recorded as P. dentigerum (Steinbeck & Ricketts 1941;Fisher 1952); the arguments that support the idea that this is a new species are given above.
A total of 17 species have previously been recorded from the Warm Temperate Northeast Pacific (WTNP) (Gálvez-Zeferino et al. 2020; listed in Silva-Morales & Gómez-Vásquez 2021); adding the new records found in this study, there are a total of 27 species.A total of 34 species have been recorded from the Tropical Eastern Pacific (TEP) (Silva-Morales & Gómez-Vásquez 2021); with this study three species are added, bringing the total to 37 species from the TEP.Six species were previously known to be shared between the two provinces: Antillesoma mexicanum, Aspidosiphon elegans, A. albus, Apionsoma pectinatum, Phascolosoma puntarenae and Phascolion strombus; with this study two species have been added: Aspidosiphon pastori and Phascolosoma azteca sp.nov.Since this contribution includes both provinces, it provides a picture of a transition zone between them, for which the sipunculan fauna can be seen as a proxy.
Thus, considering the valid species previously recorded and by adding those found in the present study, the species of the central and northwestern Pacific of Mexico represent about 30% of the total number present in both marine provinces.
Our results emphasize the importance of scientific collections, not only in Mexico but also in the entire world, since they provide material for comparison with newly collected material.Speaking specifically of deep-sea sipunculans, thanks to the scientific collections of Mexico it was possible to obtain a first approximation of the bathyal sipunculans from Mexico.The collected material is valuable, since exploration in deep waters demands special tools and the costs of transportation and equipment rentals are very high, making this material unique.
This species was recently described from the southern Mexican Pacific.The specimens from the central and northwestern Mexican Pacific coincide with the description made by Silva-Morales & Gómez-Vásquez (2021); with this study the range extends from Guerrero State (southern Mexican Pacific) to the Gulf of California.The specimens of Aspidosiphon (P.) pastori are morphologically close to A. (P.) tenuisSluiter, 1886.

Table 1 .
Sipunculans recorded for the central and northwestern Pacific coast of Mexico.Q = questionable record.
(Pacific coast of Costa Rica).