Arichlidon mucropaleum

Description. Based on the holotype: complete with 37 segments. TL = 4.2 mm, TW = 1.2 mm. Body rectangular, tapered posteriorly (Fig. 9 A). Body pale yellow to orange. Paleae fans bright golden, completely imbricated dorsally. Prostomium retracted between the first two segments. Lateral antennae short, inserted on antero-ventral prostomial margin, median antenna slightly shorter than lateral ones. Eyes red-violet, fused, hard to see in specimens with retracted prostomium. Nuchal organ reduced, partially covering prostomium (Fig. 9 B). Palps long, cylindrical, partially visible in dorsal view. Mouth fold wide, placed between segments 3 and 4. Pharynx eversible, not exposed, stylet slender. Parapodium from segment 22, notochaetae in three main groups (Fig. 9 D). Notochaetae: lateral group inserted below notaciculum, two kinds of paleae: first kind, lateral-most paleae, 1 – 2 paleae, slender, straight and symmetrical, with 6 – 8 internal ribs (Fig. 9 E); second kind, mid-most paleae, 3 – 4 paleae, broader, longer, curved and symmetrical, with 14 internal ribs and 1 – 3 raised ribs (Fig. 9 E, arrow). Main group, 15 paleae, broad and symmetrical with 16 – 17 internal finely denticulated ribs (Fig. 9 F). Subunit 1, 1 palea, broad and asymmetrical with 16 internal ribs (Fig. 9 G). Median group, 3 paleae, short and curved; the mid-most, protruding above the main group level, with serrated margins, 12 internal ribs and 1 raised rib (Fig. 9 H). The next proximal smaller with 10 internal ribs and 1 raised rib, and the proximal-most, smallest with 11 internal ribs and 1 raised rib (Fig. 9 I). Neuropodium conical, smaller than notopodium. Neurochaetae: unit 1, 2 – 4 superior spinigers, blades straight and long, 12 times longer than wide (Fig. 9 J). Unit 2, 3 – 5 falcigers, blades straight and medium-sized, 4 times longer than wide (Fig. 9 K). Unit 3, 4 – 6 falcigers, blades straight and medium-sized or short, 3 – 4 times longer than wide (Fig. 9 L – M). Unit 4, 2 – 3 falcigers, blades curved and short, 3 – 5 times longer than wide (Fig. 9 N – P). Pygidium rounded with a ventral cone and two anal cirri (Fig. 9 C). Oocytes not seen.

Figs 9, 12 urn: lsid: zoobank. org: act: 422895 C 5 - 5465 - 47 C 9 - B 582 - 1 C 539 A 87 E 635

Remarks. This new species shares morphological features with other two species of the genus, Arichlidon hanneloreae Watson Russell, 1998 and A. acropetalon Watson Russell, 1998, both described from northeast Australia. Arichlidon mucropaleum n. sp. resembles A. hanneloreae in the shape of the paleae shoulders of the main paleae; however, A. mucropaleum n. sp. has a larger apical projection in the paleal apex, while A. hanneloreae has a short apical projection in the paleae apex (Watson Russell 1998: 172, Fig. 6 A – D). On the other hand, A. mucropaleum n. sp. shares with A. acropetalon a long apical projection on the apex of the paleae of the main group; however, the paleal shoulders of A. mucropaleum n. sp. is wider than in A. acropetalon, whose shoulders are shorter and rounded (Watson Russell 1998: 172, Fig. 6 E – F). Despite A. mucropealeum n. sp. does not completely resemble A. reyssi (Katzmann, Laubier and Ramos, 1974), a species described from the Adriatic Sea, both species stand out by sharing a long mid-most median palea, protruding above the main group level. However, the detailed observation of this paleal group also demonstrates that both species are different. Arichlidon mucropaleum n. sp. possesses this median palea with lesser number of internal ribs [12 – 13 vs 15 – 17 (18)], further, the paleal apex presents an apical projection, while A. reyssi possesses a higher number of internal ribs and the paleal apex is inconspicuous. Some differences were noted in the subunit 1 of both species. In A. mucropaleum n. sp. the subunit 1 presents a convex proximal margin in the mid paleal level and its apex presents an apical projection; while the subunit 1 in A. reyssi, has the proximal margin uniformly curved in the mid palea level, and lacks apical projection. This is the first record of Arichlidon from the TEP.

Distribution. Only known from Ángel de la Guarda Island, Baja California, Gulf of California (Fig. 12).

Etymology. The specific epithet mucropaleum is a combination of the Latin singular adjective mucronatus (- a, - um) meaning ‘ provide with a final straight point’, and the Latin singular noun palea meaning ‘ scale’. The name indicates the sharp apical projection on the paleal apex of all paleae, and it is neuter for matching the genus gender (ICZN 1999, Art. 31.2).

Type locality: Ángel de la Guarda Island, Baja California, Mexico. Type material. Holotype: ECOSUR- 260, on a separated vial, Ángel de la Guarda Island, Baja California, Mexico, Gulf of California, August 15, 1987, coll. MEH. Paratypes: ECOSUR- 260, 10 spec., same data as holotype.