A new parasitic barnacle (Crustacea, Cirripedia, Rhizocephala, Mycetomorpha) from the abyssal zone in the northwestern Pacific Author Kakui, Keiichi https://orcid.org/0000-0003-4630-9065 Department of Biological Sciences, Faculty of Science, Hokkaido University, Sapporo 060 - 0810, Japan kakui@eis.hokudai.ac.jp text Zoosystematics and Evolution 2024 2024-04-03 100 2 385 390 http://dx.doi.org/10.3897/zse.100.120887 journal article http://dx.doi.org/10.3897/zse.100.120887 1860-0743-2-385 C325C2A4027943F5B7C1FB5B60D277F5 E182EDC872C150F2927DE99C04E46537 Mycetomorpha abyssalis sp. nov. Figs 2 , 3 , 4 Etymology. The specific name abyssalis (Latin: abyssal) is an adjective referring to the collection of this species from an abyssal depth. Type host. Sclerocrangon zenkevitchi Birshtein & Vinogradov, 1953 ( Decapoda : Caridea : Crangonidae ). Attachment site. Pleonite 1 sternite. Type locality. Off the eastern coast of Iwate, Japan, northwestern Pacific ( 39°28.555'N , 143°47.347'E to 39°27.934'N , 143°47.240'E ), depth 3893-3890 m. Material examined. Holotype , female (CBM-ZC 17789), one vial, ex. S. zenkevitchi (cl 26.7 mm ; CBM-ZC 17788), collected on 29 September 2023 at the type locality, R/V Hakuho-maru cruise KH-23-5, coll. by Keiichi Kakui. Representative DNA sequences. One sequence each was determined from the holotype (CBM-ZC 17789) for COI (INSD accession number LC799150; 637 bp, encoding 212 amino acids), 16S (LC799151; 490 bp), 18S (LC799152; 1826 bp), and 28S (LC799153; 1169 bp). One sequence each was determined from the host (CBM-ZC 17788) for COI (LC799154; 658 bp, encoding 219 amino acids) and 18S (LC799155; 1846 bp). Description of female holotype. Externa (Figs 2 , 3 ) 16.6 mm in length, thinner than broad, a little over twice as long as maximum width (8.1 mm), rounded at ends, pale yellow (faded in ethanol, slightly yellowish); except dorsal and anteroventral regions, externa surface covered with short lobes; filled with developing embryos. Root system not observed. Stalk short, cylindrical, at one-quarter of length from posterior end of externa. Triangular shield lacking (Figs 3C , 4 ). Anterior, middle, and ventral lobes short, and ovoid or digitiform; posterior lobes short and branched. Tubular lobe anterior to stalk, at one-fifth of length from anterior end of externa, arising from right margin of externa, with mantle opening at tip; mantle opening anterior to stalk. Figure 2. Mycetomorpha abyssalis sp. nov., holotype, attached to the host, Sclerocrangon zenkevitchi Birshtein & Vinogradov, 1953, fresh specimen. Scale bar: 10 mm. Figure 3. Mycetomorpha abyssalis sp. nov., holotype, fresh specimen. A. Habitus, parasitic on the host, ventral view; B, C. Habitus, ventral ( B ) and dorsal ( C ) views; ant - anterior; lef - left; man - mantle opening; rig - right; pos - posterior; sta - stalk; tub - tubular lobe. Scale bars: 10 mm. Figure 4. Mycetomorpha abyssalis sp. nov., holotype, showing stalk penetrating the host pleonite-1 sternite (soft tissue in pleonite 1 removed), anterodorsal view; no triangular shield observed. ant - anterior; pos - posterior; sta - stalk. Scale bar: 10 mm. Distribution. Presently known only from the type locality.