Decamastus branchiatus, Sun & Bao & Chen & Wu & Ju & Liao & Zhou, 2025

Decamastus branchiatus View in CoL sp. nov.

( Figs 3, 4)

ZooBank registration: urn:lsid:zoobank.org:act:.

Type materials: Holotype ( NSMT-Pol H-979): Central Indian Ridge , Edmond vent field ( 69°35.80′E, 23°52.66′S), depth 3279 m, SHINKI 6500, R / V Yokosuka, Cruise YK 16-E02, Dive 6K# 1457, 26 February 2016. Preserved in 70% ethanol. GoogleMaps

Paratype 1 (NSMT-Pol P-980) and paratype 2 (NSMT-Pol P-981), same data and preservation as the holotype.

Description: Holotype complete, 50 mm long, 2.2 mm wide, with 116 chaetigers. Paratypes incomplete, anterior fragments; paratype 1 (NSMT-Pol P-980) 23 mm long, 1.5 mm wide, with 20 chaetigers; paratype 2 (NSMT-Pol P-981) 9.4 mm long, 0.9 mm wide, with 50 chaetigers. Colour in alcohol pale white. Epithelium smooth ( Fig. 3A–D). Prostomium broadly rounded, with ventral groove ( Fig. 3E, F). Eyespots and nuchal organs not observed. Nuchal organ depression posterolaterally in junction between prostomium and peristomium ( Fig. 3F). Peristomium as a biannulate achaetous ring, not clearly distinct from the first chaetiger ( Fig. 3A, D). Torax with 10 chaetigers, mid-ventral groovestartingfromchaetiger 4 andrunningalongabdomen ( Fig. 3C). First chaetiger biramous. Chaetigers 1–9 with bilimbate capillaries only in both notopodia and neuropodia, 8–16 per fascicle; chaetiger 10 transitional, with 13–15 notopodial capillaries per fascicle and 11 or 12 neuropodial hooded hooks per fascicle ( Fig. 3G, H). Hooded hooks with long anterior shaf, a pointed and large main fang, and multiple teeth in three rows above main fang; hoods moderate, not extending beyond main fang ( Fig. 3I, J). Genital pores not observed. Transition between thorax and abdomen indistinct, marked by chaetal change ( Fig. 3G). Abdominal chaetigers with hooded hooks only, multi-annulated. Abdomen notopodia clearly separated from neuropodia. Neuropodial lobe low at anterior abdomen, elevated from about chaetiger 25, with rounded expanded end on dorsolateral side ( Fig. 4A, C). Notopodia reduced, widely separated dorsally, connected by an interramal ridge ( Fig. 4B). Branchiae present on both noto- and neuropodia; notopodial branchiae start from chaetiger 20–30, simple digitate filaments, single to up to three, arising from lower region of lobe; neuropodial branchiae start from chaetiger 15–25, arising from the dorsal side of neuropodia lobes, single digitate filament ( Fig. 4C, D). Anterior abdominal chaetigers with 13–15 hooded hooks per fascicle, reduced to two or three in far posterior end. Abdominal hooded hook with two rows of teeth above main fang, three teeth in basal row, and two in superior row ( Fig. 4E). Pygidium sample, without appendages ( Fig. 4F).

Methyl Green staining patern: Peristomium, prostomium, and all thoracic chaetigers stained uniformly with distinct speckles, leaving parapodial rami and ventral groove unstained. Abdominal chaetigers with distinct bands of speckles on posterior half of segment, encircling noto- and neuropodial tori and forming complete rings ( Fig. 3B–D).

Molecular identity: GenBank accessions PQ643866– PQ643867 for cox1, PQ651929 for 16S, PQ651923 for 18S, PQ651926 for 28S, and PQ651932 for H3 (also see Supporting Information, Table S2). No identical matches on GenBank for all five genes.

Etymology: Te specific epithet ‘ branchiatus ’ refers to the presence of branchiae on both noto- and neuropodia, a unique feature of this species.

Type locality: Edmond vent field, Central Indian Ridge, Indian Ocean ( 69°35.80′E, 23°52.66′S), depth 3279 m. Te worms were found from the wall of a large, active vent mound called ‘Great Shrimp Castle’, together with Rimicaris kairei Watabe & Hashimoto, 2002 and Aloiniconcha marisindica Okutani, 2014 ( Fig. 1B) GoogleMaps .

Distribution: Known only from the type locality.

Remarks: When compared with the other four described species of Decamastus ( D. gracilis , D. nudus Tomassin, 1970 , Decamastus okuilin Hernández-Alcántara, Solís-Weiss & García-Garza, 2019 and Decamastus sp. A sensu Ewing 1984), D. branchiatus has unique characteristics for the genus, such as the rounded prostomium and the presence of filamentous branchiae on abdominal noto- and neuropodia. Besides, the new species is readily distinguished from its congeners by a combination of characteristics, such as the absence of a palpode (present in D. gracilis and D. okuilin ), the absence of eyespots (present in D. nudus , D. okuilin , and Decamastus sp. A sensu Ewing, 1984), the first chaetiger biramous (uniramous in D. nudus , D. okuilin , and Decamastus sp. A sensu Ewing, 1984), and smooth thoracic epithelium (tessellated in D. gracilis , D. nudus , and Decamastus sp. A sensu Ewing, 1984).

Te overall body appearance of D. branchiatus resembles C. multibranchiata , which is a deep-sea species found from deployed wood, by the broadly rounded prostomium, the presence of a ventral groove, branchiae on both notopodia and neuropodia, and the Methyl Green staining patern of the thorax ( Magalhães and Hilliard 2022). However, they should be assigned to different genera based on the number of thoracic chaetigers ( 10 in D. branchiatus vs. 9 in C. multibranchiata ). Besides, the two species show clear ecological and geographical separation: C. multibranchiata is known only from deployed wood from the Northeast Pacific, whereas D. branchiatus is found from the vent fields of the Indian Ocean (also see the Discussion section).

Species of several genera have been described with branchiae on abdominal chaetigers, either associated with only notopodial lobes or neuropodial lobes ( Magalhães and Hilliard 2022). Decamastus branchiatus and C. multibranchiata are the only two species in the family having branchiae in both notopodial and neuropodial lobes.