Hyalopale zerofskii Watson, Tilic & Rouse, 2019

Figs 8, 12

Hyalopale zerofskii Watson, Tilic & Rouse, 2019: 349, Figs 1F, 5A–F, 6A–G.

Hyalopale sp. — Cruz-Gómez & Bastida-Zavala 2018: 75–76, Figs 9A–I, 13, 14I, 15F. — Cruz-Gómez et al. 2021: 183, Fig. 5S (key of chrysopetalids from Tropical America).

Type locality: San Clemente Island, California, USA (Watson et al. 2019) .

Material examined. ECOSUR-2998, Bahía de los Ángeles, Baja California, México, northern Mexican Pacific, 1 m, February 24, coll . SSV.

Description. Based on the only specimen found: complete but fragmented with 14 segments. TL= 0.9 mm, TW= 0.3 mm. Body short, broad, truncated posteriorly (Fig. 8A). Body whitish. Paleae fan translucent, slightly imbricated dorsally.

Prostomium barely visible among the first three segments. Lateral antennae, short, inserted on the antero-ventral prostomial margin, median antenna slightly shorter than lateral ones. Eyes red-violet, two pairs, anterior pair massive. Nuchal organ not present (Fig. 8B). Palps short, oval, visible in ventral view. Mouth fold small, placed between segment 2 and 3. Pharynx muscular and eversible, not exposed, stylets slender.

Parapodium from segment 5, notochaetae in two main groups (Fig. 8D). Notochaetae: lateral spine curved, without internal ribs (Fig. 8E). Main group, 10–12 paleae; lateral-most palea broad and asymmetrical with 16–20 internal ribs (Fig. 8F); mid-most palea curved and asymmetrical with (20) 24–26 (27) internal ribs (Fig. 8G); midline-most palea shorter, slender and asymmetrical with 14–18 internal ribs (Fig. 8H). Median spine absent. All paleae with hooked tips, rounded brow, margins serrated, and denticles on internal ribs.

Neuropodium conical, longer than notopodium. Neurochaetae: unit 1, 5–8 superior spinigers, blades straight and long, 10–12 times longer than wide (Fig. 8I). Unit 2, 5–6 falcigers, blades straight and medium-sized, 6–7 times longer than wide (Fig. 8J). Unit 3, 5 falcigers, blades straight and medium-sized, 5–6 times longer than wide (Fig. 8K). Unit 4, 2 falcigers, blades curved and short, 4–5 times longer than wide (Fig. 8L). Pygidium rounded, with two long anal cirri (Fig. 8C). Oocytes not seen.

Habitat. Intertidal to subtidal (1 m). Specimens of this species have been collected on algae Padina and rumble coral (Cruz-Gómez & Bastida-Zavala 2018; Watson et al. 2019).

Distribution. From San Clemente Island, California, to Bahía de los Ángeles, Baja California and Estacahuite Bay, Oaxaca (Fig. 12). According to Watson et al. (2019), there is a specimen that possibly belongs to H. zerofskii from French Polynesia; morphological differences were noticed and described then, despite these differences it was still considered part of the H. zerofskii ‘species complex’.

Remarks. Recently Watson et al. (2019) described H. zerofskii from California, previously recorded as Hyalopale sp. (sensu Cruz-Gómez & Bastida-Zavala 2018: 75) from Oaxaca, and so far, this is the only Hyalopale species known on from the Pacific coast of America.

The specimen identified here agrees with H. zerofskii however, some differences were noted. The internal ribs in main paleae vary in number along the species distribution: 25–26 (27) in specimens from California (Watson et al. 2019), 24–26 (27) in specimens from Gulf of California, and 21–25 (27) in specimens from the southern Mexican Pacific (Cruz-Gómez & Bastida-Zavala 2018). Further, raised ribs are notorious in small immature specimens (Cruz-Gómez & Bastida-Zavala 2018); in adult specimens, as those revised here, raised ribs are shallow and only noted by having large denticules on their surface. Regarding neurochaetae, here they were illustrated in detail and lead to note some differences from those in small specimens, the main difference is related to their blade length. Adult specimens have neurochaetal blades from all units with relatively the same length as unit 1 blades, whereas small immature specimens have blades with a distinct length according to their dorso-ventral position (Cruz-Gómez & Bastida-Zavala 2018: Fig. 9B–E).