Polyodontes kuroshio, Jimi & Tomioka & Orita & Kajihara, 2019

Jimi, Naoto, Tomioka, Shinri, Orita, Ryo & Kajihara, Hiroshi, 2019, A New Species of Polyodontes (Annelida: Acoetidae) from Western Japan, Species Diversity 24 (2), pp. 275-279 : 275-278

Polyodontes kuroshio sp. nov.

[New Japanese name: kurobuchi-bouseki-urokomushi] ( Figs 1 , 2 )

Material examined. Holotype, NSMT-Pol H-764, sex unknown, 85 mm long, 7 mm wide (in segment 8) for 92

segments (complete specimen), off Otsuki (32°46.41′N, 132°43.98′E), Kochi, Shikoku Island , Japan, Northwest Pacific, 11 October 2017, 12 m deep, rocky substrate, Naoto Jimi collector . Paratype, NSMT-Pol P-765, sex unknown, 190 mm long, 8 mm wide (in segment 8) for 134 segments (not complete specimen), Hatakejima (33°41.60′N, 135°22.00′E), Wakayama, Japan, Northwest Pacific, 2 August 1966, intertidal zone, sandy beach, Fujio Uchinomi collector .

Description based on holotype. Body dorsoventrally flattend. Colour in life: main body dorsally cream white, midventrally with longitudinal yellow line from midbody to posterior end; parapodia and dorsal cirri distally with yellow pigment ( Fig. 1A, B ). Elytrophores 48 pairs in number, present on segments 2, 4, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 24, 25, 26, 28, thereafter on alternate segments to 92. Elytra smooth, oval; transparent with white and yellow oval spots (in life), bordered by a circle of black pigment (incomplete in anterior two to three elytrae) ( Fig. 1A ); lateral pouch on elytra present from segment 23 backwards ( Fig. 1A ). Prostomium bilobed, with globular ommatophores with distal lenses and short necks. Median antenna with short oval ceratophore, inserted at middle of prostomium; ceratophore with lateral papillae; style smooth, as long as ommatophores, white with brown dots ( Fig. 2A ). Pair of sessile eyes present lateral to ceratophore. Lateral antennae inserted on ventrally below ommatophores and tips extending slightly beyond; styles smooth, white with brown dots. Palps long, with minute papillae, cream white with some brown dots ( Fig. 2A ). Tentaculophores lateral to prostomium, each with papillae on inner dorsolateral side and a few capillary chaetae. Styles of dorsal and ventral tentacular cirri smooth, of equal lengths, white with brown dots ( Fig. 2A ).

Segment 2 ( Fig. 2B ): with biramous parapodia, first pair of elytrophores and many sensory papillae present on dorsal side. Notopodium with rounded acicular lobe and bundle of long, finely spinous capillary notochaetae. Neuropodium wide, subconical, with bifid prechaetal acicular lobe, postchaetal lobe, and free anteroventral bract. Neurochaetae enlarged basally, tapering to capillary tip, with numerous rows of spines; spines shorter in upper type ‘a’ neurochaetae [as defined by Pettibone (1989)] and longer in lower neurochaetae.

Segment 3 ( Fig. 2C ) with first pair of tapering, smooth dorsal cirri and some sensory papillae on dorsal side. Notopodium as in segment 2, notochaetae shorter. Neuropodium truncate with small anterodorsal and prominent anteroventral bract. Neurochaetae in three groups [as defined by Pettibone (1989)]: anterodorsal bract with upper type ‘a’ neruochaetae, finley spinous, tapering to capillary tip; middle acicular neruochaetae with hooked tip, without aristae or subdistal hairs; anteroventral bract with lower curved neurochaetae, with longer spines and tapering to capillary tip.

Segments 4 to 8 ( Fig. 2D ) with small notopodia and few, short, capillary notochaetae; neuropodia similar to those in segment 3; dorsal sensory papillae absent.

Beginning with segment 9 backwards ( Fig. 2E ), all parapodia similar in shapre, without dorsal sensory papillae. Notopodium wide, flattened, with notoaciculum, few short capillary notochaetae and spinning glands (white dotted). Neuropodium truncate, with slightly bilobed acicular lobe and smaller anterodorsal and prominent anteroventral bract. Neurochaetae in three groups: i) upper ones in anterodorsal bract of two types: type ‘a’ neurochaetae, finley spinous, tapering to capillary tip ( Fig. 2F ) and type ‘b’ neurochaetae, much shorter, bipinnate, with widely spaced spines ( Fig. 2I ); ii) middle acicular neurochaetae with hooked tip and subdistal hairs ( Fig. 2G ); iii) lower ones in anteroventral bract, curved, with longer spines and tapering to capillary tip ( Fig. 2H ).

Cirrigerous segments with long, smooth, tapering dorsal cirri, about 1.4 times length of ventral cirri ( Fig. 2C, D ). Ventral cirri short, tapering, smooth, attached on middle part of parapodia ( Fig. 2B–D ). On segment 2, ventral cirri elongate (reaching to tip of notochaetae) and projecting from base of neuropodia ( Fig. 2B ). Parapodia without branchiae. Pygidium cylindrical in shape, achaetous with two conical anal cirri, similar to dorsal cirri. Pharynx with 15 pairs of papillae and 2 pairs of jaws; papillae conical, middorsal one much longer than others.

Etymology. This species is named after the Biological Institute on Kuroshio. The holotype from Sukumo was collected by the first author who received funding by the institution. The specific name is a non-declensive noun.

Distribution. This species is known from off Otsuki (Kochi Prefecture) and Hatakejima island (Wakayama Prefecture), Japan, Northwest Pacific.

Sequence. Partial COI gene, 658 bp ( DDBJ No. LC492096 ), determined from the holotype .

Remarks. The morphological features of Polyodontes kuroshio sp. nov. resemble those of Polyodontes oculeus (Treadwell, 1901) , originally described from the Caribbean Sea, in having ommatophores with short neck, palps with minute papillae, and parapodia without branchiae ( Pettibone 1989). The new species can be distinguished from P. oculeus by the following features: i) the acicular neurochaetae always lack aristae and possess hair only on a small area of their subdistal end, while in P. oculeus acicular chaetae may have aristae and possess hairs distributed broadly on their subdistal part; and ii) the median and lateral antennae are sporadically covered with brown dots in life and after preservation, while those of P. oculeus do not possess any dots. Polyodontes tidemani Pflugfelder, 1932 is the only species that lacks aristae in the genus. However, the new species can be discriminated by the following features: i) palps have minute papillae, while those of P. tidemani have long papillae; ii) absence of branchiae in all parapodia, while P. tidemani with inflated branchiae from segment 13 backwards.

In Japan, P. atromarginatus and P. maxillosus were report- ed in previous studies ( Takahashi 1941; Nishi et al. 2008). The new species is distinct from P. atromarginatus and P. maxillosus by its ommatophores with the short neck and by the absence of aristae on the acicular neurochaetae.

Figures

Fig. 1. Polyodontes kuroshio sp. nov., NMST-Pol H-764 (holotype), live specimen. A, dorsal view; B, ventral view.

Fig. 2. Polyodontes kuroshio sp. nov., NSMT-Pol H-764 (holotype). A, prostomium, dorsal view; B, left parapodium, segment 2, posterior view; C, left parapodium, segment 3, posterior view; D, left parapodium, segment 8, posterior view; E, left parapodium, segment 9, posterior view; F, upper type ‘a’ neurochaeta (finely spinous), segment 9; G, middle acicular chaeta, segment 60; H, lower curved neurochaeta (with long spines), segment 9; I, upper type ‘b’ neurochaeta (bipinnate, with widely spaced spines), segment 9. Scale bars: A, 1mm; B–E, 200µm; F–I, 100µm.