Idiostylochus, Gutiérrez & Auby & Gouillieux & Daffe & Massé & Antajan & Noreña, 2023

Gutiérrez, Adrian, Auby, Isabelle, Gouillieux, Benoit, Daffe, Guillemine, Massé, Cecile, Antajan, Elvire & Noreña, Carolina, 2023, Tachinidae Robineau-Desvoidy 1830, Zoological Studies 62 (15), pp. 1-14 : 5-10

Superfamily Stylochoidea Poche, 1926 Family Idioplanidae Dittmann, Cuadrado, Aguado, Noreña and Egger, 2019

Idiostylochus gen. nov. urn:lsid:zoobank.org:act:B1874F30-7A5B-45C6-AD8A-34A37FC66E5A

Diagnosis: Idioplanidae with the pharynx in the middle of the body. Cerebral and marginal eyes present. Male copulatory apparatus with spermiducal bulbs and prostatic vesicle. Seminal vesicle absent. Female apparatus with a tubular Lang’s vesicle and cement glands. The vagina makes a posterior turn before reaching the penis papilla.

Type species: Idiostylochus tortuosus sp. nov.

Etymology: The name Idiostylochus derives from a combination of Idioplana and Stylochus , regarding the presence of a unique combination of characters found in part in these genera.

Idiostylochus tortuosus sp. nov. ( Fig. 2 ) urn:lsid:zoobank.org:act:BCF1166D-6D0A-4B79-899A-59D3B262644C

Type material: Holotype: 1 specimen. Arcachon Bay , France, October 20, 2020. Sagittal sections stained with Azan trichrome. MNCN 4.01 /4263 to MNCN 4.01 /4291 (29 slides). GeneBank accesion numbers: ON796526 (28S), ON796530 ( COI).

Paratype: 1 specimen. Arcachon Bay, France, October 20, 2020. Sagittal sections stained with Azan trichrome. MNCN 4.01/4292 to MNCN 4.01/4335 (44 slides).

Additional material: tissues preserved in 100% Ethanol and sagittal sections stained with Azan trichrome; Arcachon Bay, France, October 20, 2020.

For GeneBank accession numbers see table 1.

Diagnosis: Male copulatory apparatus with conspicuous spermiducal bulbs and a small prostatic vesicle. Elongated male atrium covered by glandular tissue. Female apparatus with a vagina bulbosa and well-developed cement glands that surround the two sections of the vagina (externa e interna).

Etymology: The specific name derives from the Latin tortuous, due to the winding and complex course of the distal portion of the vasa deferentia and spermiducal bulbs.

Description: Body shape rounded-oval with slightly undulated margins ( Fig. 2A ). Holotype 1.8 cm long and 1.1 cm wide, paratype 2.2 cm long and 1.5 cm wide. Body consistency firm and fleshy, more delicate and thinner towards the margins. Tentacles lacking. Cerebral and marginal eyes present. Background pigmentation chocolate brown to caramel in the margins. Numerous dark spots scattered over the dorsal surface, more abundant along the main body axis ( Fig. 2A ). Ventral surface pale, with grey to beige tonalities. Epithelium, basal membrane and body musculature more developed on the dorsal than on the ventral side ( Fig. 2B ). Pharynx ruffled, well developed, extends throughout the mid-body region. Oral pore at the beginning of the posterior body-half.

Reproductive system: male and female reproductive organs are located directly after the pharynx, in the posterior half of the animal. Male copulatory apparatus consisting of a conical penis papilla (or peneal bulb) and a small pyriform prostatic vesicle, seminal vesicle absent ( Fig. 2C, E ). Vasa deferentia forms bulky spermiducal bulbs. The diameter of the bulbs decreases as they approach each other until they join forming the common vas deferens, which opens into the middle region of the ejaculatory duct. Both vasa deferentia and spermiducal bulbs follow a tortuous course. Free prostatic vesicle. Male atrium elongated and covered with glandular, spongy tissue.

Female gonopore posterior to the male gonopore ( Fig. 2D, E, F ). With vagina bulbosa. The vagina externa curves anteriorly to the male reproductive system, then upwardly and continues posteriorly into the vagina interna. The oviduct opens between the vagina externa and interna. Connected to the vagina interna is an elongated and tubular Lang’s vesicle ( Fig. 2D, E ). The general appearance of the female apparatus is compact with small visible folds, mainly in the vagina externa, and surrounded by abundant cement glands.

Biology and occurrence

Idiostylochus tortuosus was found in oyster cultures of Magallana gigas . Individuals were collected living in the mantle cavity of diseased or dead oysters as well as swimming around the oyster farming devices. Some individuals have also been observed feeding on natural beds of Mytilus edulis Linnaeus, 1758 and Mytilus galloprovincialis Lamarck, 1819 near the oyster farms (Vieira and Nowaczyk pers. com.). Although the presence of Idiostylochus was known long ago by oyster farmers, in recent years, the frequency and number of polyclads specimens seem to have increased, and their presence has caused noticeable damage to oyster and mussel crops.

Taxonomical remarks

The superfamily Stylochoidea , where the new species Idiostylochus tortuosus was placed, presents a free prostatic vesicle ( Faubel 1983). From the molecular point of view, the closest related genera (see - tree 28S, Fig. 4 ) are Idioplana Woodworth, 1898 ( Idioplanidae ), Leptostylochus Bock, 1925 ( Stylochidae ), and also, but less related, Mirostylochus Kato, 1937 ( Stylochidae ), Latocestus Plehn, 1896 and Eulatocestus Faubel, 1983 ( Latocestidae ). All these genera present a free prostatic vesicle and either developed spermiducal bulbs or an elongated seminal vesicle. The new species shares some characters with these genera, while others are clearly different. A comparative discussion follows.

Leptostylochus ( Stylochidae ) ( Fig. 3 A ) is characterized by an elongated slender body shape; tentacular, cerebral, marginal and often frontal eyes; male copulatory apparatus with an unarmed penis papilla and without seminal vesicle, but with spermiducal bulbs that join into a common vas deferens before entering the medial region of the ejaculatory duct; female reproductive system with a developed Lang’s vesicle ( Kato 1934; Faubel 1983; Beveridge 2017). Idiostylochus gen. nov. resembles Leptostylochus because of the presence of large spermiducal bulbs. Furthermore, the female system shows common features, like the well-developed shell glands around the vagina externa. In contrast, the Lang’s vesicle is conspicuous in Leptostylochus and reduced in Idiostylochus , which appears as a small tubular duct.

Latocestus and Eulatocestus ( Latocestidae ) share a similar morphology. Both genera have spermiducal bulbs, unarmed penis papilla and a op simple female apparatus with Lang’s vesicle ( Plehn 1896; Faubel 1983) ( Fig. 3B ). The main difference between Latocestus and Eulatocestus is the lining of the prostatic vesicle: irregular or fingered in Latocestus and a web of glandular follicles in Eulatocestus . Both genera are distinguishable from Idiostylochus by the morphology of the female apparatus. The presence of a well-developed Lang’s vesicle and a simple, nonbulbous vagina with much less abundant cement glands differentiates both genera from Idiostylochus .

Idioplana ( Idioplanidae ) is characterized by a male copulatory organ with an unarmed penis papilla, prostatic and seminal vesicle; female apparatus extended over the male apparatus; and an anchor-shaped Lang’s vesicle ( Woodworth 1898; Meixner 1907; Faubel 1983; Rodríguez et al. 2021) ( Fig. 3C ).

The analysis of the morphological data reveals external anatomical similarities, such as the oval body shape, the reddish-brown dorsal and whitish ventral coloration and the arrangement of the cerebral eyes, but both genera are clearly differentiated by the presence of conspicuous tentacles in Idioplana , absent in Idiostylochus , and the differences between the reproductive systems. In Idiostylochus the seminal vesicle is absent, the female canal is shorter, since the vagina interna do not extend anteriorly over the male copulatory organ, and the Lang’s vesicle is tubular. Within the male apparatus, there are clear differences. In Idioplana a seminal vesicle is present, while in Idiostylochus is absent. In Idiostylochus the function of the seminal vesicle is carried out by the spermiducal bulbs, since the latter replace the former in its absence.

Although the molecular analysis clusters Mirostylochus ( Stylochidae ) near Idiostylochus , the two genera are morphologically distinct. Mirostylochus is characterized by tentacular, cerebral and marginal eyes; and a female apparatus with ductus vaginalis, from which the vagina interna opens to the exterior behind the female gonopore. Lang’s vesicle is absent ( Kato 1937; Tokinova 2003). Idiostylochus gen. nov. and Mirostylochus differ in the morphology of the female apparatus, as Idiostylochus possesses a tubular Lang’s vesicle and lacks ductus vaginalis.

Molecular approach

The molecular studies are based on two datasets of the genes 28S (nuclear) and COI (mitochondrial). The methods applied for the analyses were Maximum Likelihood (ML) and Bayesian Inference (BI).

Within the tree generated during the analysis of 28S ( Fig. 4 ) we can distinguish three wellsupported branches, the clusters of the superfamilies Discoceloidea, Leptoplanoidea and Stylochoidea

Tree scale: 0.1 respectively. The evidence provided by the 28S gene allows us to cluster the genera that are phylogenetically more related at the superfamily level within Acotylea . In our analysis, which is focused on determining the systematic position of the new species, Idiostylochus tortuosus , we found that the species is closely related to Idioplana atlantica and I. austaliensis , both of which belong to the Family Idioplanidae . On the other hand, the genera Mirostylochus and Leptostylochus do not cluster within the Family Stylochidae but with Latocestus and Eulatocestus , the selected representatives of the family Latocestidae in this study.

Other genera used in the 28S gene analysis are grouped in the same families as in previous molecular and morphological analyses carried out by previous authors ( Dittmann et al. 2019; Litvaitis et al. 2019; Oya and Kajihara 2020).

In the COI analysis ( Fig. 5 ) we have only taken into account values> 70 (ML), although we show values between <70 and> 60 as they reflect a certain relationship among species or genera. Therefore, our analyses show strongly supported species within a genus, while low values at the family level are meaningless.

Figures

Fig. 2. Idiostylochus tortuosus gen. nov., sp. nov. A–B, Live specimen. A, dorsal view; B, ventral view. C–D, histological section through male (left) and female (right) copulatory organs. E, sagittal reconstruction of the reproductive system. F, sagittal reconstruction of whole animal (anterior end at the left). Scale bars = 500 µm.

Fig. 4. Bayesian / Maximum likelihood tree based on partial sequences of the 28S gene (total length: 706 bp). Numbers in the nodes correspond to the posterior probability values of the BI analysis (> 0.70) and bootstrap support values of the ML analysis (> 50), respectively.

Fig. 3. Comparison of the copulatory apparatuses of different genera of Stylochoidea. A, Leptostylochus Bock, 1925; B, Latocestus Plehn, 1896; C, Idioplana Woodworth 1898.

Fig. 5. Maximum likelihood / Bayesian Inference tree based on partial sequences of the COI gene (total length: 640 bp). Numbers in the nodes correspond to the bootstrap support values of the ML analysis (> 60) and the posterior probability values of the BI analysis (> 0.50), respectively.