Typical ice cream cone shaped tubes (Fig. 2A and C), composed of a single layer of cemented sand grains, translucent and black (Fig. 2A and C). Conical body, stout (Fig. 2A–B), 22.6 mm long including scaphe (8.3–26 mm) and 4.2 mm width at chaetiger 3 (2.2–5.4 mm). Preserved types pale cream with dorsum mostly translucent on its entire length. Operculum 3.5 mm width (2.3–5.3 mm) with margin raised with smooth edge (Fig. 3A and C, Fig. 6A and C) and two bundles of paleae (Fig. 3A and C–D), each bundle with 9 (8–10) stout, flattened golden palaea (Fig. 3A and C–D) curved and tapering to a pointed tip (Fig. 8A–B). Cephalic veil free from operculum, distal margin with 20 (18–25) slender cirri of variable sizes (Fig. 6B). A pair of ear-shaped lobes adjacent to both sides of dorso-lateral base of cephalic veil (Fig. 5D). Few blunt buccal tentacles (short or long depending on contraction after relaxation or fixation) distributed along the rim (Fig. 5A–C and Fig. 6B). Segment 1 with a pair of long tentacular cirri arising from antero-ventral edge near outer most paleae, thin, elongate distally pointed (Fig. 3A and Fig. 6B); and a pair of small ventral lobes (Fig. 7A–B). Segment 2 with a pair of tentacular cirri, similar to those present on segment 1, but shorter and located laterally (Fig. 3A and Fig. 6B). Posterodorsal lobe on segment 2 absent. Two pairs of pectinated stalked branchiae, on segments 3 and 4 (Fig. 3A–D, Fig. 5B–D and Fig. 7A–B), each pair consisting of numerous loose, flat and smooth lamellae, much higher than broad (Fig. 5B–C). Branchiae from segment 3 inserted ventro-laterally, longer than those in segment 4, which is inserted laterally (Fig. 5C and Fig. 7A–B). Segment 4 with a ventral crest with six anterior projections: two median with horn-shaped tips directed outwards; two central with horns directed inwards and two lateral directed inwards, but with a broad base and a small external knob (Fig. 3B, D, Fig. 4A and Fig. 5A–C). Segments 4, 5, 6 and 7 with ventral glandular pads becoming progressively more lateral and broader on segments 6 and 7. In mid-ventral area of each glandular pad, there is a small ventral shield blue when stained with methyl green (Fig. 5A–B). Segment 6 with a dorsal ridge, whitish in live specimens (Fig. 3A and C). Lateral glandular pads on segments 8 to 15 (8 pads), diminishing gradually in size towards posterior segments (Fig. 5A). Notopodia beginning on segment 5 (Fig. 4A), extending until segment 19 (3 chaetigers with notopodia only: S5, 6 and 7) (Fig. 5A); neuropodia beginning on segment 8, extending until segment 19 (12 chaetigers with noto- and neuropodia); segments 20 and 21 achaetous (Fig. 4B and Fig. 5A). Notopodia with two kinds of chaetae forming a bundle (Fig. 8G): chaetae from anterior row with deep incision resembling a shaft and blade (Fig. 8H and Fig. 9A), where internal basal margin of blade is smooth, then covered progressively with tiny serrations from mid-anterior portion to tip (Fig. 9A) and an apparently bifid process at the lateral end of shaft (Fig. 8G_1, H and Fig. 9A) or with narrow and delicate serrations (Fig. 9A); chaetae from posterior row long, apparently narrowly limbate (Fig. 8G₂). Neurochaeta as pectinate uncini (Fig. 8I), major teeth surmounted by three rows of small teeth, 7 teeth per row (Fig. 8E); anterior peg nearly U-shaped; stout handle directed posteriorly, as long as the distance between front and back (Fig. 8F). Scaphe distinctly separated from segment 21, formed by fusion of five posterior segments (Fig. 4B–C, Fig. 5A, Fig. 6A and D–F). Fifteen pairs of scaphal hooks (13–21 in paratypes) located at dorsolateral region of scaphe (Fig. 4D and Fig. 6E–F), amber coloured, long, with distal margin hooded nearly rounded and slightly curved in most hooks (Fig. 8C and Fig. 9C), except for 3–4 innermost hooks with attenuated and slightly curved distal margin (Fig. 8D). Scaphe funnel-shaped (Fig. 6A), rounded ventrally (Fig. 6D), concave dorsally forming a groove (Fig. 6E–F and Fig. 7B), with six lateral crenulations (Fig. 6F and Fig. 7B). Anal flap is round-leaf shaped (Fig. 7C–E) with reminiscent crenulations bordering its margin and a small, rounded mid-dorsal anal cirrus (Fig. 6F and Fig. 7C–E).

Colour in live specimens: tegument entirely translucent in live worms (Figs 3, 4), with the opercular margin whitish (Fig. 3A and C) and small nearly rectangular ventral glandular pads or shields on segments 4–7, also whitish (Fig. 3B). Branchiae, blood vessels and some areas of gut red coloured (Fig. 3A–D and Fig. 4A).

Segment 4 with a ventral crest with six horn-shaped anterior projections; three chaetigers with notopodia (from S5, S6 and S7); 12 chaetigers with noto- and neuropodia (from S8–19); 8–10 golden opercular paleae per lobe; a pair of ear-shaped lobes at base of cephalic veil; anterior row of chaetae in notopodia with a deep incision and an apparently bifid process at the lateral end of shaft; and 13–21 pairs of amber scaphal hooks with distal margin rounded and hooded.

Named after the first author's son Santiago Hernández celebrating his 11th birthday. He also patiently helped us collect the magnificent ice cream cone worms and enjoyed seeing these worms live under the microscopy as much as we did.

A comparative table of the major diagnostic characters of some Pectinaria species was provided by Zhang and Qiu (2017) and Zhang et al. (2022). Pectinaria gouldii (Verrill, 1874) (Verrill 1874), P. hartmanae Reish, 1968 (Reish 1968), P. nana Wesenberg-Lund, 1949 (Wesenberg-Lund 1949), P. longispinnis Grube, 1878 (Grube 1878) and Pectinaria santii sp. n. share the presence of notochaetae on segments 5–19 and neurochaetae on segments 8–19. However, P. gouldii, P. nana and P. longispinnis have pointed scaphal hooks (straight or strongly curved); hooks in P. hartmanae are blunt (with shaft decreasing gradually towards the tip) and, in P. santii sp. n., hooks are rounded (shafts increasing gradually in width towards the tip, forming rounded tips in most hooks, except in the innermost, which are oval with pointed tips).

Moreover, there are other four differences in P. hartmanae Reish, 1968 (Reish 1968) and the new species from central-eastern Gulf of California: 1) a ventral crest on segment 4 with distinctive horns was not described in P. hartmanae by Reish and their absence was corroborated by Dr. Karen Osborn in holotype and paratypes (pers. comm, July 2021); 2) Reish (1968) reported the presence of 8 to 10 pairs of scaphal hooks in P. hartmanae, whereas Pectinaria santii sp. n. have a high number (13–21 pairs); 3) The blades of anterior row of notochaetae in P. hartmanae have a coarsely basal denticulate margin (Fig. 9B), whereas it is smooth, then covered progressively with tiny serrations from mid-anterior portion to tip in Pectinaria santii sp. n. (Fig. 9A and Fig. 7G); an entire, nearly rounded lateral end of shaft in P. hartmanae (Fig. 9B), but it is bifid apparently in Pectinaria santii sp. n. (Fig. 8H and Fig. 9A) or with narrow and delicate serrations (Fig. 10H); and 4) uncini in P. hartmanae were described as having two rows of major teeth, whereas there are three rows of uncini in Pectinaria santii sp. n. However, as serrations of notochaeta are difficult to see under light microscopy (100x) (Fig. 10F-H), it is desirable to examine these carefully with scanning electron microscopy.

The ventral crest on segment 4 with distinctive horns, constitutes a stable character in all types examined of P. santii sp. n. A similar feature was described recently in Pectinaria lizhei Zhang, Hutchings & Qiu, 2022 (Zhang et al. 2022), but it has small crenulations in the ventral-most part, whereas the ventral-most part is smooth in P. santii sp. n. In addition, P. lizhei have notochaetae on S5–S20 and neurochaetae on S8–S20 (notochaeta on S5–S7 and neurochaeta on S8–S19 in P. santii sp. n). A ventral crenulate margin in segment 6 was reported for P. antipoda Schmarda, 1861 (Schmarda 1861) (Zhang and Hutchings 2019), but it lacks the horns present in P. santii sp. n. and P. lizhei.

Compared to other Pectinaria species from the Temperate Northern Pacific, P. santii sp. n. has 12 biramous chaetigers (in segments 8–19), whereas there are 13 biramous chaetigers in P. californiensis and P. newportensis (segments 8–20). There are also differences in the number of cephalic paleae (not reported in P. californiensis, 9–13 in P. newportensis, 8–10 in P. santii sp. n.) and number of cirri on the cephalic veil (18–30 in P. californiensis, 19–24 in P. newportensis, 18–25 in P. santii sp. n.). Additionally, P. santii sp. n. differs from P. californiensis and P. newportensis by having golden opercular paleae (copper in colour in P. californiensis and P. newportensis). However, it is highly recommended to examine the types of both Californian species to properly describe and illustrate features that were omitted in original descriptions, which is out the scope of the present contribution. Hartman’s types are housed at the Natural History Museum of Los Angeles County (AHF 34 and 35 to P. californiensis and P. newportensis, respectively).