Parasacculina Høeg & Glenner, 2019

Golubinskaya, Darya D., Korn, Olga M., Sharina, Svetlana N. & Selin, Nikolai I., 2021, Pselaphochernes scorpioides, Zoological Studies 60 (54), pp. 1-16 : 4-9

Genus Parasacculina Høeg & Glenner, 2019 in Høeg et al. (2020)

Parasacculina pilosella ( Van Kampen et Boschma, 1925) comb. nov.: 24–27, figs. 14, 15.

Sacculina pilosella – Shiino 1943: 11–12, figs. 1E, 7; Korn and Rybakov 2001: 177–179; Miroliubov et al. 2019: 48–56, fig. 3; Lianguzova et al. 2021: 101009.

Host: Carapace width of the males of Pugettia aff. ferox infested by rhizocephalans ranged from 14.0 to 31.0 mm, females – from 9.9 to 25.0 mm.

Bathymetrical range: In Vostok Bay (Peter the Great Bay, Sea of Japan), crabs infested by both rhizocephalans were found at a depth of 1.5– 3 m.

Location on the host: The position of the externae of both rhizocephalans was not connected with specific abdominal segment of the host. Parasites were found on 1, 2, 3, 4, 5, 6 segments and also on the borders between 2 and 3, 3 and 4, 4 and 5 segments. Most crabs had only one rhizocephalan externa.

External morphology: Parasacculina pilosella and Sacculina pugettiae were externally very similar ( Fig. 1C, D ). The width of the externae varied from 1.1 to 13.5 mm in P. pilosella and from 1.0 to 13.7 mm in S. pugettiae . The virginal externae of both species were white ( Fig. 1E ), immature externae – yellow ( Fig. 1C, D ), mature externae – yellow (embryos without eyes) or light brown (embryos with eyes) ( Fig. 1F ). The mature externae of both species had prominent dorsoventral wrinkles. The external cuticle varied from 22 to 40 µm thick.

In P. pilosella , the external cuticle was covered by numerous hyaline spines 28–38 µm length united in groups with a common base. This character is visible on the SEM photos ( Fig. 2A, D ) as well as on the histological sections of the externa ( Fig. 3A ). In S. pugettiae , we found two types of the external surface. The external cuticle of about 2 thirds of the investigated specimens was smooth, without spines and excrescences, but often covered with the epibionts ( Fig. 2B ), the cuticle of about 1 third of specimens was divided into small star-shaped areas with a diameter of 5–8 µm ( Fig. 2C ). The cuticle of three found specimens was two-layered: smooth cuticle was folded back, revealing a star-shaped surface ( Fig. 2E ).

The receptacles of the mature externae in both rhizocephalans were easily detached. In P. pilosella , isolated receptacles were globular with the cavity (lumen) inside ( Fig. 4A ). Their diameter was of 300–800 µm. The spermatogenic cells were placed in the central part of the receptacle. Each receptacle was connected to a folded receptacle duct by a short probably chitinous tubule ( Fig. 4C, E ). In S. pugettiae , the receptacles presented the elongate tubes, directed dorsoventrally, of 1200–1700 µm length and with a diameter of 200–600 µm ( Fig. 4B ). They were placed closely together but always clearly separated. The spermatogenic cells were found in the narrower dorsal part of the receptacle ( Fig. 4D, F ). The receptacle ducts were slightly flattened, of 200–300 µm width. In P. pilosella , the receptacles were located outside of the visceral mass in the basal region of the stalk ( Fig. 4C, E ), whereas in S. pugettiae , they were placed within the visceral mass ( Fig. 4D, F ).

In P. pilosella , the colleteric glands were weakly branched from the atrium (central part of the gland) attaining 16 tubes (canals), arranged in one layer ( Fig. 3C, E ). In S. pugettiae , they were highly branched exceeding 33 tubes, arranged in several layers ( Fig. 3D, F ). The maximum number of tubes was located in the central part of the externa. Their diameter was 40–90 µm in P. pilosella and 30–80 µm in S. pugettiae .

Retinacula: In both rhizocephalans, the internal cuticle had a wrinkled surface. In the mature externae of P. pilosella , the internal cuticle was covered with ridges spirally twisted and ended with short finger-like processes ( Fig. 2F ). We have not found the retinacula in the virginal externa of P. pilosella (4.4 mm width). In three virginal externae of Sacculina pugettiae (3–3.5 mm width), the retinacula were also not found. The internal cuticle of the fourth virginal specimen (2.3 mm width) was covered with numerous undeveloped flattened retinacula of 5 µm in diameter ( Fig. 2G ). In the mature externa of P. pilosella (10.5 mm width), rare solitary barbed spindles (9 µm length) placed in the shallow depressions were noted in the region of the stalk ( Fig. 2H, I ). In the mature externae of S. pugettiae (6.5–9.3 mm width), numerous retinacula presented the groups of 4–5 barbed spindles (6–8 µm length) at a common base placed in the shallow depressions ( Fig. 2J, K ). The retinacula of both species were covered with a layer of secretion and with numerous bacteria.

Molecular identification of two rhizocephalan species

The investigated samples of rhizocephalans were relegated into two clades – Sacculinidae for Sacculina pugettiae and Polyascidae for Parasacculina pilosella (pp = 1 for all markers) – confirming their status as different and not closely related species.

Molecular data showed that all rhizocephalans implemented into the analysis form two monophyletic clades with high posterior probability (pp = 1 for all markers). These clades correspond to the families Sacculinidae and Polyascidae ( Figs. 5 , 6 ). However, for the family Sacculinidae , branch topologies within this clade are not identical for each gene. The specimens in the family Polyascidae form three groups of sequences (pp = 1). The first consists of Polyascus species and the second consists of Parasacculina species. The third group contains P. shiinoi (for 16S rDNA) and P. shiinoi + P. bicuspidata (for 18S rDNA), which are basal to other Polyascidae on the trees presented ( Figs. 5 , 6 ). The comparison of pairwise genetic distances indicated stronger differences between species of the families Sacculinidae and Polyascidae ( Tables 1, 2).

Preliminary investigation of the multiple infestation of Pugettia aff. ferox by rhizocephalans

The preliminary data on the infestation of the spider crab Pugettia aff. ferox in Vostok Bay showed that Sacculina pugettiae occurred more often than Parasacculina pilosella ( Table 3). Over seven months, we found 86 specimens of Pugettia aff. ferox infested by rhizocephalans. Among them, 56 specimens (65.1%) possessed the externae of S. pugettiae , 14 (16.3%) – the externae of P. pilosella , and 16 specimens (18.6%) were infested by both rhizocephalans simultaneously. 80.3% of crabs with S. pugettiae had one externa, 12.5% – two externae and 7.2% – three externae of the parasite. All crabs with P. pilosella had only one externa. Moreover, each of 10 crabs possessed two externae of different species, five crabs – three externae, but one crab – four externae (two of S. pugettiae and two of P. pilosella ). Thus, the intensity of infestation reached two externae per host in P. pilosella and three externae per host in S. pugettiae . The intensity of two-species multiple infestations reached four externae per host. Pugettia females were infested more often than males.

The virginal externae were found on host crabs from May to August in P. pilosella , and from May to September in S. pugettiae , gradually decreasing in number ( Fig. 7 ). The externae of P. pilosella with developing embryos appeared in June, at a temperature of 14.6 ± 2.0°C; ovigerous externae of S. pugettiae appeared in July, at a temperature of 18.2 ± 1.7°C. Both ovigerous parasites occurred until to September. The immature externae were noted from May to September. In spring and early summer, the “old” immature externae with a thickened mantle, probably retained from the preceding reproductive season, were observed. During the research period no one crab with a scar was found.

Figures

Fig. 1. The host crab, Pugettia aff. ferox, infested by Parasacculina pilosella (A, C), Sacculina pugettiae (B, D, E) and both rhizocephalans (F). 1, P. pilosella, 2, S. pugettiae, mo, mantle opening.

Fig. 2. SEM showing external cuticle (A–E), internal cuticle (F, G) and retinacula (H–K) of Parasacculina pilosella (A, D, F, H, I) and Sacculina pugettiae (B, C, E, G, J, K). hs, hyaline spines of external cuticle; r, retinacula.

Fig. 3. Mantle (A, B) and colleteric glands (C–F) of Parasacculina pilosella (A, C, E) and Sacculina pugettiae (B, D, F). C, D, transverse sections; E, F, longitudinal sections. a, atrium of colleteric gland; em, embryos; exc, external cuticle; hs, hyaline spines, mc, mantle cavity; ov, ovary; tu, tubes of colleteric gland.

Fig. 4. Receptacles of Parasacculina pilosella (A, C, E) and Sacculina pugettiae (B, D, F). A, B, light microscopy; C, D, longitudinal sections; E, F, transverse sections. l, lumen; ov, ovary; rd, receptacle duct; sc, spermatogenic cells.

Fig. 5. Bayesian inference analysis of 16S rDNA sequences for the Sacculinidae and Polyascidae. Numerals above or below the branches are Bayesian posterior probabilities.

Fig. 6. Bayesian inference analysis of 18S rDNA sequences for the Sacculinidae and Polyascidae. Numerals above or below the branches are Bayesian posterior probabilities.

Fig. 7. Relative proportion (%) of the different developmental stages of Parasacculina pilosella (A) and Sacculina pugettiae (B). Dashed line in (B) represents the monthly average of the water temperatures at 1.5 m depth.