Briarosaccus hoegi, Korn & Golubinskaya & Sharina & Noever & Glenner, 2024

Infraclass Rhizocephala Müller, 1862 Family Peltogastridae Lilljeborg, 1861 ; amended by Høeg et al. (2020) Family: Peltogastridae Lilljeborg, 1861 Genus: Briarosaccus Boschma, 1930

Briarosaccus hoegi sp. nov. ( Figs. 4–9 View Fig View Fig View Fig View Fig View Fig View Fig ) urn:lsid:zoobank.org:act:9af55c57-02a9-4f66-ae1dc43e7bf5c440

Etymology: The new species is named in honor of Jens Thorvald Høeg, professor of the Department of Biology (University of Copenhagen, Denmark), who has spent most of his professional life investigating thoracican and rhizocephalan barnacles.

Material examined: Holotype: One specimen (21.4/9.0 mm, with embryos), on Hapalogaster dentata (female, 19.2 mm width), depth 2–3 m, Zhitkova Cove (43°01'07.9"N, 131°55'49.1"E), 21.05.2023 (catalogue number 46887, MIMB). GoogleMaps

Paratypes: One specimen (10.0/ 3.5 mm, without embryos), on H. dentata (female, 13.0 mm width), depth 2–3 m, Sobol Cove, 19.07.2023 (catalogue number 46888, MIMB); Two specimens (12.3/ 5.2 mm, without embryos and 11.0/ 6.2 mm, with embryos), on H. dentata (male, 19.0 mm width), depth 2–3 m, near the Cape Vyatlina, 3.06.2020 (catalogue number 46889, MIMB).

Voucher specimens: Two specimens (20.4/ 8.9 mm, with embryos and 18.1/ 6.6 mm, with embryos), on H. dentata (female, 18.1 mm width), depth 2–3 m, Zhitkova Cove, 14.05.2020 (catalogue number 46890, MIMB); One specimen (12.1/ 5.4 mm, with embryos), on H. dentata (female, 11.1 mm width), depth 2–3 m, Zhitkova Cove, 14.05.2020 (catalogue number 46891, MIMB).

Specimens for histology: One specimen (15.9/10.0 mm, with embryos), on H. dentata (male, 21.0 mm width), depth 2–3 m, near the Cape Vyatlina, 3.06.2020; One specimen (7.1/2.0 mm, without embryos), on H. dentata (male, 17.7 mm width), depth 2–3 m, near the Cape Vyatlina, 22.11.2022.

Specimens for SEM: One specimen (13/ 4.6 mm, without embryos), on H. dentata (female, 18.5 mm width), depth 2–3 m, Sobol Cove, 16.03.2023; One specimen (12.0/ 4.7 mm, without embryos), on H. dentata (female, 16.0 mm width), depth 2–3 m, Sobol Cove, 15.02.2023; One specimen (22.0/ 10.5 mm, with embryos), on H. dentata (female, 18.2 mm width), depth 2–3 m, Zhitkova Cove, 21.05.2020; One specimen (20.4/ 7.8 mm, with embryos), on H. dentata (female, 19.5 mm width), depth 2–3 m, Zhitkova Cove, 21.05.2020; One specimen (18.0/ 7.5 mm, with embryos), on H. dentata (male, 16.5 mm width), depth 2–3 m, Zhitkova Cove, 21.05.2020; One specimen (20.1/ 7.8 mm, with embryos), on H. dentata (female, 18.6 mm width), depth 2–3 m, Zhitkova Cove, 21.05.2020.

Briarosaccus tenellus on Hapalogaster mertensii : One specimen (11.5/ 4.3 mm, without embryos), on H. mertensii (female, 12.5 mm width), intertidal, Sitka (Alaska), 07.2012; One specimen (15.5/ 4.5 mm, without embryos), on H. mertensii (female, 17.2 mm width), intertidal, Sitka (Alaska), 07.2012; One specimen (8.5/ 2.5 mm, without embryos), on H. mertensii (female, 13.0 mm width), One specimen (10.6/ 2.8 mm, without embryos), on H. mertensii (female, 13.5 mm width), intertidal, Sitka (Alaska), 07.2012 (catalogue number 47652, MIMB); One specimen (10.3/ 3.8 mm, with embryos), on H. mertensii (female, 11.1 mm width), intertidal, Sitka (Alaska), 07.2012; One specimen (11.5/4.0 mm, with embryos), on H. mertensii (male, 16.2 mm width), intertidal, Sitka (Alaska), 07.2012.

Type locality: Zhitkova Cove (43°01'07.9"N, 131°55'49.1"E, Ussuriysky Bay, Peter the Great Bay, Sea of Japan).

Host: Briarosaccus hoegi sp. nov. was found on the crab Hapalogaster dentata (De Haan, 1849) (Anomura: Lithodidae ). H. dentata differs from the related species H. mertensii by the seven spines along the lateral margin of the carapace and by the tubercles on the first pereopods ( Makarov 1938). The morphological species identification of H. dentata was confirmed by molecular data ( Fig. S8 View Fig ; Table S5).

The carapace width of the infested crabs ranged from 10.1 to 19.5 mm. The prevalence of infestation in the crab population reached 6.3%. Most crabs were singly infested, while some specimens of H. dentata carried two to four externae ( Fig. 1A–D View Fig ).

Distribution and bathymetrical range: We sampled the host crab Hapalogaster dentata in Peter the Great Bay at a depth of about 2– 3 m. This species is also widely distributed in coastal intertidal and subtidal waters of Japan, from Hokkaido to Kyushu, and the Korean Peninsula ( Goshima et al. 1995; Marin 2013). We have no data on the true geographical distribution and depth range of the new parasite.

Externa morphology: The externa of Briarosaccus hoegi sp. nov. is attached to the basal part of the soft ventral side of the host abdomen via a short stalk, which connects the externa with the internal trophic root system. Its long axis is at right angle to that of the host ( Fig. 1 View Fig ). The externa varies from 5.3 to 22 mm in length and from 1.0 to 10.5 mm in height. The externa is elongated, cylindrical and slightly curved with the ventral outline being convex, and the dorsal outline concave. Anterior part is unilobed, slightly thicker than the posterior one and oriented to the left side of the host ( Fig. 1A, E, F View Fig ). Posterior end of the externa is rounded. The stalk is near the central part of the dorsal side. The chitinous shield around the stalk is fusiform, has growth rings, and covers from 1/4 to 1/3 of the externa. The mantle opening is placed in the anterior part on the right side of the externa facing the host. It is not elevated but slit-like and surrounded by lips ( Figs. 1E View Fig , 8D View Fig ).

The overall shape and color of Briarosaccus hoegi sp. nov. varies due to the stage of the reproductive cycle. The immature externa without embryos in the mantle cavity is reddish, the color of mature externa with embryos is white, yellow, pale, or light brown (before larval hatching) ( Fig. 1A–D View Fig ). The interna roots are green.

The mantle is thick – from 84 to 117 μm in a immature rhizocephalan and from 157 to 212 μm in a mature specimen ( Fig. 4A, B View Fig ). The muscle sphincter surrounding the mantle opening is visible ( Fig. 4C, D View Fig ). A visceral sac extends dorsally along most of the externa ( Fig. 4A View Fig ). The ovary is composed of large numbers of developing oocytes, arranged in tubes ( Fig. 5 View Fig ). The ventral part of the mantle cavity is densely filled with developing embryos. From mid-May to mid-July, all investigated externae were ovigerous. The colleteric glands represent short folded flattened tubes about of 0.5 mm in length located inside the visceral sac on the lateral sides of the ovary inside the shield level ( Fig. 5C, D View Fig ). The colleteric glands expand in the middle part, with a larger diameter ranging from 200 to 350 μm in immature externae and up to 800 μm in mature ones ( Fig. 5C, D View Fig ). Before opening into the mantle cavity, the colleteric gland bends and two tubes with a diameter of 100–150 μm are observed on the section.

Paired receptacles represent cylindrical tubes 1.8–3.5 mm in length located in the dorsal part of the visceral sac and parallel to the long axis of the externa ( Fig. 6 View Fig ). Receptacles begin and end inside the shield level. Anterior tops of receptacles are blind. Anterior parts with a diameter of about 150 μm are narrow, slightly flattened and more or less straight, central parts with a diameter from 200 to 270 μm are slightly twisted ( Fig. 7A, D View Fig ). Receptacles gradually pass into receptacle ducts with a diameter of 80–100 μm in immature externae and up to 200 μm in mature ones ( Fig. 7B, C, E, F View Fig ). The receptacle ducts are coiled in immature externae and almost straight in mature ones. They open on the lateral surfaces of the visceral sac.

Mantle cuticle: The width of Briarosaccus hoegi sp. nov. external (outer) cuticle with “collagen fibers” is 25–50 μm. It is smooth, without papillae or excrescences ( Fig. 4B View Fig ) and covered with longitudinal grooves ( Fig. 8B View Fig ). A dorsal strip of transversal grooves from the mantle opening to the stalk is visible ( Fig. 8A View Fig ). In SEM, this strip and area around the mantle opening is cellular ( Fig. 8C View Fig ). The mantle opening is densely covered with numerous spines (hairs) of 8–10 μm in length ( Fig. 8D–F View Fig ). Moreover, rounded depressions are rarely scattered on the outer cuticular surface ( Fig. 8G View Fig ).

The internal (inner) cuticle is wrinkled and covered with sparse hairs (finger-like processes) of 1.5–3.0 μm in length which may be united in groups with a common base ( Fig. 8H View Fig ). Retinacula of 10–20 μm in length, densely barbed, single or united into groups of 2–4 are rarely found ( Fig. 8I, J View Fig ).

Larvae: Nauplii of Briarosaccus hoegi sp. nov. have a reticulated collar and also a distinct naupliar eye demonstrating positive phototaxis ( Fig. 9 View Fig ).