Zygantroides serpulidicola, Oya & Tsuyuki & Kajihara, 2020

Oya, Yuki, Tsuyuki, Aoi & Kajihara, Hiroshi, 2020, A New Species of Zygantroides (Platyhelminthes: Polycladida) from Amakusa, Japan, Species Diversity 25, pp. 189-196 : 192-195

publication ID	https://doi.org/ 10.12782/specdiv.25.189
publication LSID	lsid:zoobank.org:pub:83F323B9-977A-49BF-AB89-D0D9CC251900
persistent identifier	https://treatment.plazi.org/id/E5971EA6-B20B-4D5D-9BE6-CCE48D4B46D7
taxon LSID	lsid:zoobank.org:act:E5971EA6-B20B-4D5D-9BE6-CCE48D4B46D7
treatment provided by	Felipe
scientific name	Zygantroides serpulidicola
status	sp. nov.

Treatment

Zygantroides serpulidicola sp. nov.

( Figs 1–3 View Fig View Fig View Fig )

Etymology. The new specific name serpulidicola is a noun in apposition and refers to the habit that the flatworms dwell on tubes of serpulid annelids ( Fig. 1 View Fig ).

Material examined. Four specimens: holotype, ICHUM 6023 View Materials , 6 View Materials slides, intertidal, (32°33′09″N, 130°06′32″E), Tsujishima Island , Kumamoto, Japan, 6 June 2019, Y GoogleMaps . Oya and A GoogleMaps . Tsuyuki leg .; paratypes (three specimens, all from the type locality, Y. Oya and A. Tsuyuki leg.); ICHUM 6024 View Materials , 4 View Materials slides, 6 June 2019; ICHUM 6025 View Materials , 4 View Materials slides, 13 June 2018; ICHUM 6026 View Materials , 5 View Materials slides, 13 June 2018 .

Description. Live specimens 6.7–8.1 mm (7.9 mm in holotype) in length, 2.7–4.8 mm (3.8 mm in holotype) in maximum width. Body oval, narrow toward posterior end when elongated ( Fig. 2A, B View Fig ). Ground body color translucent to whitish opaque ( Fig. 2A–C View Fig ). Pale brown maculae sparsely scattered on dorsal surface ( Fig. 2A, B View Fig ). General appearance of body whitish to orangish ( Fig. 2A, B View Fig ). Tentacles lacking ( Fig. 3 View Fig ). Pair of tentacular eye clusters, each containing 8–19 eyespots (11 in right cluster, 11 in left cluster in holotype, Fig. 3 View Fig ). Cerebral organ positioned slightly anteriorly to tentacular eye clusters ( Fig. 3 View Fig ). Pair of cerebral eye clusters, each consisting of 8–22 eyespots (10 in right cluster, 8 in left cluster in holotype, Fig. 3 View Fig ), arranged near median line and congregated anterior to cerebral organ. Pharynx whitish, ruffled in shape, occupying about three-eighths of body length (2.6–2.9 mm in length, 2.7 mm in holotype), located at center of body ( Fig. 2C View Fig ); ruffles unclear and uncountable. Intestine highly branched and not anastomosed, spreading throughout body except margin and brain region. Pair of whitish sperm ducts visible through ventral body wall ( Fig. 2C View Fig ).

Mouth opening at posterior to pharynx and near common gonopore ( Fig. 4A–C View Fig ). Male copulatory apparatus located posteriorly to pharynx, consisting of true seminal vesicle and penis papilla; prostatic vesicle lacking ( Fig. 4A–C View Fig ). Pair of sperm ducts running anteriorly, then turning medially at point of about one-fourth length of pharynx from posterior end, subsequently running posteriorly along both sides of pharynx and extending further posteriorly for short distance before turning medially ( Fig. 2C View Fig ) to enter separately proximal end of pear-shaped seminal vesicle with strong muscular wall ( Fig. 4A, C View Fig ). Distal end of seminal vesicle directly connecting to ejaculatory duct; latter having thin muscular wall lined with prostatic-like glandular epithelium, varying in shape from vesicular to almost cylindrical depending on fixation ( Fig. 4B, C View Fig ). Distal end of ejaculatory duct connecting to penis papilla. Penis papilla small, without stylet, projecting into cylindrical male atrium. Lining epithelium of male atrium smooth.

Pair of oviducts forming common oviduct, latter running posteriorly to enter vagina. From this point, elongated Lang’s-vesicle duct, lined with ciliated epithelium, running ventrally, then curving posteriorly to connect to Lang’s vesicle ( Fig. 4D View Fig ). Lang’s vesicle horseshoe-shaped, with tips orienting anteriorly and reaching to level where Lang’s-vesicle duct connects to vagina ( Fig. 4E View Fig ). Inner epithelium of Lang’s vesicle similar to that in vagina and Lang’s-vesicle duct ( Fig. 4D View Fig ). Vagina curving postero-dorsally, running anteriorly, then recurving postero-ventrally to exit at common genital atrium ( Fig. 4D, E View Fig ); vagina lined with smooth, ciliated epithelium. Lang’s-vesicle duct and vagina surrounded by circular muscle fibers; vagina surrounded by cement glands. Common genital atrium opening at posterior to mouth as common gonopore.

Habitat. Found on tubes of serpulid annelids on the undersurface of stones in the intertidal zone ( Fig. 1 View Fig ).

Sequence of COI. The partial COI sequences (712 bp) from the four specimens ( LC528160 View Materials – LC528163 View Materials ) almost coincided with each other . The uncorrected p -distance among specimens showed 0.000 –0.007.

Molecular phylogeny. The resulting BI and ML trees were almost identical to each other in topology; we show only the BI tree ( Fig. 5 View Fig ). Zygantroides serpulidicola sp. nov. was nested in a clade comprised of discocelids and ilyplanids with high (1.00/97) nodal supports. Within this clade, Discoplana gigas (Schmarda, 1859) (Ilyplanidae) was sister to all the rest, and Z. serpulidicola sp. nov. ( Ilyplanidae ) was sister to Discocelidae , the latter was comprised of species in Adenoplana Stummer-Traunfels, 1933 and Discocelis Ehrenberg, 1836 ; Ilyplanidae was recovered non-monophyletic.

Remarks. We extend the generic diagnosis of Zygantroides . Faubel (1983: 40) included “Female apparatus directed backwards and oriented dorsal to the male complex” in the generic diagnosis. The female copulatory apparatus in the present species directs somewhat posteriorly but it is not arranged dorsally to the male copulatory apparatus ( Fig. 4E View Fig ). However, Z. serpulidicola satisfies other diagnostic characters of Zygantroides : tentacles, eyespot distribution, and structure of male and female copulatory organs ( Figs 2C View Fig , 3 View Fig , 4 View Fig ). Therefore, we judge it is more appropriate to redefine Zygantroides by eliminating the character of orientation of female apparatus than to establish a new genus-level taxon. The horseshoe-shaped Lang’s vesicle such as that in Z. serpulidicola has been often employed in genus-level definition (cf. Faubel 1983); however, we do not reflect this trait in the redefinition because the shape of Lang’s vesicle in other Zygantroides is not described ( Corrêa 1949).

Zygantroides serpulidicola sp. nov. can be distinguished from the two congeners Z. henriettae and Z. plesia by the following characters: i) the position of the mouth (near the common gonopore in Z. serpulidicola ; at about one-third from posterior end of the pharynx in Z. henriettae ; twofifths from posterior end of the pharynx in Z. plesia ), ii) the presence/absence of the common sperm duct (absent in Z. serpulidicola ; present in Z. henriettae and Z. plesia ), iii) the shape of the seminal vesicle (pear-shaped in Z. serpulidicola ; spindle-shaped in Z. henriettae ; spherical in Z. plesia ), iv) the length of the Lang’s-vesicle duct (long in Z. serpulidicola ; short in Z. henriettae and Z. plesia ), v) the position of the Lang’s vesicle (posterior to the common gonopore; anterior to the common gonopore in Z. henriettae and Z. plesia ), and vi) the presence/absence of an ampulla in the vagina (absent in Z. serpulidicola and Z. henriettae ; present in Z. plesia ) ( Table 2).

This is the first report of Zygantroides from the Pacific Ocean. Other two congeners are only reported from the Atlantic coast of southeastern Brazil ( Corrêa 1949; Bahia and Schrödl 2018). The two Brazilian species have been captured from between algae, whereas our species was found on tubes of annelids.