Argulus caecus C. B. Wilson, 1922

Nagasawa, Kazuya & Hirose, Masato, 2021, Marine Fish Parasite Argulus caecus (Crustacea: Branchiura: Argulidae) Accidentally Collected from a Fixed Net Caught Squid in Northern Japan, Species Diversity 26, pp. 289-296 : 290-294

publication ID	https://doi.org/ 10.12782/specdiv.26.289
persistent identifier	https://treatment.plazi.org/id/03AD87D3-7140-FFCE-FEE3-2F99DC6AC484
treatment provided by	Felipe
scientific name	Argulus caecus C. B. Wilson, 1922
status

Treatment

( Figs 1–4 View Fig View Fig View Fig View Fig )

Argulus caecus C. B. Wilson, 1922: 1–4 , figs 1–5; Tokioka 1936: 339–340, fig. 5; Yamaguti 1963: 322, pl. 317, fig. 4

Material examined. Ovigerous female, NSMT-Cr 29002, 22.0 mm total length (from anterior tip of carapace to posterior tip of abdomen), 12.5 mm maximum width (around midlength of carapace), from the mantle of a squid caught in Otsuchi Bay, Iwate Prefecture, Japan (photographed by Kado et al. 2021).

Description of ovigerous female. Carapace (including posterolateral lobes) elliptical, covering all pairs of legs in dorsal view, 18.9 mm long, comprising 85.9% of total length ( Figs 1 View Fig , 2A View Fig ). Frontal region of carapace delimited by anterolateral indentations. Compound eyes scarcely visible in freshly dead specimen, and not visible after ethanol preservation. Nauplius eye not visible. Dorsal surface of carapace smooth without spines. Ventral surface of marginal frontal and lateral regions of carapace ornamented with numerous, small sharply pointed spines ( Fig. 2B View Fig ). Posterolateral lobes of carapace 16.9 mm long, comprising 89.4% of carapace length, ending in rounded margin, separated by sinus nearly 1/2 length of carapace. Respiratory areas comprising small, elliptical anterior area and large, reniform posterior area, located at levels of second maxillae and first and second legs, respectively ( Figs 2B View Fig , 3B View Fig ). Thorax with four segments ( Fig. 2A, B View Fig ). Abdomen shield-shaped, slightly longer than broad (6.1× 4.9 mm), and partially covered by posterior region of posterolateral lobes of carapace; posterior lobes with weakly pointed tips ( Figs 1 View Fig , 2A, B View Fig ). Anal indentation 35.6% as long as abdomen. Paired spermathecae elongate in anterocentral region of abdomen ( Fig. 2B View Fig ). Caudal rami located at base of anal indentation with four naked setae on each ramus ( Fig. 3A View Fig ).

First antennae with four segments ( Fig. 3C View Fig ): first segment heavily sclerotized in mesial and posterior regions, with two projections on posterior margin (mesial projection larger than other); second segment also heavily sclerotized, with large projection at about middle of anterior margin, strong apically bent hook at lateral corner, and large ventral projection near posterobasal margin; third segment cylindrical, with naked seta; apical segment shorter than third, with at least four naked setae at tip. Second antennae with five segments ( Fig. 3C View Fig ): first segment sclerotized, with stout spine and small swelling bearing four naked setae on posterior margin; second segment ovoid, with two naked setae on posterior margin; third, fourth, and apical segments nearly cylindrical and decreasing in length, possessing, respectively, 14, five, and at least two apical naked setae ( Fig. 3D View Fig ). Postantennal spines large and robust, located posterior each to mesial projection of first segment of first antenna ( Fig. 3C View Fig ). Preoral sheath visible on ventral midline of frontal region of carapace between first maxillae ( Fig. 2C View Fig ). Mouth tube located just posterior to preoral sheath, nearly cylindrical, composed of small anterior labrum and larger posterior labium ( Fig. 2C View Fig ).

First maxillae forming suckers, 4.1 and 4.3 mm in diameter (32.8 and 34.4% as wide as carapace, respectively), with 54 or 55 supporting rods in sucker membrane ( Fig. 3E View Fig ). Supporting rods becoming slightly wider toward midlength but tapering distally, composed of 27–33 (mean=30, n=10) sclerites per rod ( Fig. 3F View Fig ). Sclerites mostly trapezoidal and crescent-shaped, respectively, in basal and other regions of supporting rod, often imbricated around midlength of rod. Outer margin of rim of sucker membrane with numerous tiny projections.

Second maxillae with five segments ( Fig. 3G View Fig ); first segment robust, with three (one anterolateral, one nearly centrolateral, and one posterolateral) blunt projections (anterolateral one largest), corpus of first segment with raised patch of small rectangular denticles in anterior region and apically sharp simple denticles in mid- and posterior regions; second segment elongate with distal raised patch of serrated scale-like denticles ( Fig. 3I View Fig ) in anterior and mid-regions and simple acutely pointed denticles in posterior region; third segment also elongate but narrower than second, with raised patch of serrated scale-like denticles in anterobasal region and simple denticles in mid- and distal regions; fourth segment subquadrate and shorter than third, with raised patch of simple denticles; terminal segment ( Fig. 3H View Fig ) with blunt tip and two spiniform projections, partially surrounded by anterior swelling region. Accessory spines located near ventral midline, slightly apart from first segments of second maxillae ( Fig. 2C View Fig ). Postmaxillary spines small and located anteriorly on first segment of thorax ( Fig. 2C View Fig ).

First to fourth pairs of legs ( Fig. 4 View Fig ) biramous and of almost equal size; sympods composed of coxa and basis, those of first to third legs covered with small scale-like projections anteriorly; sympods of fourth legs and rami with small projections; first legs each possessing flagellum. First leg ( Fig. 4A, B View Fig ) exopod unsegmented, with 17 and 20 plumose setae, respectively, on anterior and posterior margins, and two plumose setae at tip; endopod three-segmented, with 18 and 15 plumose setae, respectively, on anterior and posterior margins of proximal segment, naked seta near anterolateral corner of middle segment, and three short spines at tip of terminal segment; flagellum projecting from exopod and extending to proximal margin of coxa, with seven plumose and two naked setae on anterior margin, 21 plumose and two naked setae on posterior margin, and plumose seta at tip. Second leg ( Fig. 4C View Fig ) basis with 12 plumose setae on posterior margin; exopod unsegmented, with 17 and 20 plumose setae, respectively, on anterior and posterior margins; endopod unsegmented, with 15 and 16 plumose setae, respectively, on anterior and posterior margins. Third leg ( Fig. 4D View Fig ) basis bearing nine plumose setae on posterior margin; exopod unsegmented, with 16 and 18 plumose setae, respectively, on anterior and posterior margins; endopod two-segmented, proximal segment with 13 and 12 plumose setae, respectively, on anterior and posterior margins, terminal segment with naked seta and 11 plumose setae on anterior margin, 11 plumose setae on posterior margin, two short naked setae near tip, and two short naked setae at tip. Fourth leg ( Fig. 4E View Fig ) coxa forming posteriorly expanded natatory lobe bearing three plumose setae on distal margin; basis larger than coxa, bearing four plumose setae on posterior margin; exopod weakly two-segmented, terminal segment with at least nine plumose setae on posterior margin; endopod two-segmented, proximal segment with 11 plumose setae on posterior margin, terminal segment with six plumose setae on posterior margin.

Color. Dorsal surface of freshly dead specimen ( Fig. 1 View Fig ) with dark brown pigmentation in carapace, thorax, and abdomen, forming three irregularly shaped longitudinal stripes in frontal region of carapace and three wider longitudinal stripes in each posterolateral lobe of carapace (mesial stripe widest and combined with central stripe at midlength of posterolateral lobe). Unpigmented area found between central and lateral stripes in each posterolateral lobe. Longitudinal stripes also present on thorax and abdomen. Pigmentation of body weakly visible in ethanol-preserved specimen. Respiratory areas fringed by continuous black pigment. Mouth tube with weak ventral pigmentation.

Remarks. Without information on its host, A. caecus was originally described by Wilson (1922) based on a single female collected at Aburatsubo (type locality, incorrectly reported as “Aburazubo”), Misaki, Sagami (now Kanagawa Prefecture), Japan. The type locality is located on the southwest coast of the Miura Peninsula, facing Sagami Bay, an inlet of the northwestern Pacific Ocean. There were several problems in the original description of A. caecus , especially regarding projections on the first and second antennae and recognition of segments of both antennae. Wilson (1922) found “two spines” on the “basis” of the “second antenna,” but this is incorrect, and as described above, the “spines” are found as two projections on the posterior margin of the first segment of the first (not second) antenna ( Fig. 3C View Fig ). Wilson (1922) did not differentiate antennal segments and stated that the first antenna had “strong hooks on the anterior and lateral margins, and a stout spine on the posterior margin”. However, the former “hooks” are actually found on the second segment of the first antenna ( Fig. 3C View Fig ), and the latter “spine” is located as a large projection on the posterobasal margin of the same segment ( Fig. 3C View Fig ). In spite of such problems in the original description, A. caecus was characterized by a large body (19.0 mm long), invisible compound eyes, about 30 sclerites in each supporting rod, and well-armed second maxillae ( Wilson 1922). The female specimen collected in the present study has these characters and is identified as A. caecus .

Tokioka (1936) also reported the morphology of female A. caecus from puffers at various localities on the Pacific coast of Japan. His specimens were almost identical to the original description of A. caecus , but the sclerites in supporting rods were fewer (20–30) than those recorded in the original description (about 30) and in this paper (27–33, mean=30). This difference may be caused by the fact that his specimens were smaller (up to 16 mm long) than the holotype (19.0 mm long) and the specimen collected in this study (22.0 mm long) because the number of sclerites per supporting rod is known to increase with increasing body size ( Benz et al. 1995).

Wilson (1922) reported that the tips of the posterior lobes of the abdomen were rounded, but Tokioka’s specimens had posterior lobes with “pointed or blunt” tips. Those lobes of the specimen collected in this study have weakly pointed tips ( Figs 1 View Fig , 2 View Fig ). Tokioka (1936) also noted the presence of a large spine and a process on the first segment of the second antenna. These structures correspond in this paper to a stout spine and a small swelling near the spine on the posterior margin of the same segment ( Fig. 3C View Fig ). In addition, Tokioka (1936) reported on a single seta on the posterior margin of the coxa of the first leg, but no seta was found on the specimen examined ( Fig. 4A View Fig ).

The specimen of A. caecus collected in this study carries flagella on the first legs. As known for other Argulus species ( Boxshall and Jaume 2009), the flagellum originates from the extreme proximal part of the exopod of each leg ( Fig. 4A View Fig ).

Argulus caecus was reported to be beautifully colored when alive: the body was light blue with patterns of brown pigments ( Tokioka 1936). However, the freshly dead specimen collected in this study did not have such a colored body, and brilliant coloration perhaps disappeared immediately after its death.

Argulus caecus resembles A. scutiformis , one of the six congeneric species reported from Japan, in female’s large body [19.0 mm long ( Wilson 1922), up to 16.0 mm ( Tokioka 1936), 22.0 mm long (this paper) in A. caecus , and more than 30.0 mm ( Tokioka 1936), 13.6–19.8 mm long ( Yamaguti and Yamasu 1959) in A. scutiformis ]. The female’s abdomen, however, differs in shape between these two species: A. caecus has an elliptical or shield-shaped abdomen ( Wilson 1922; Tokioka 1936; this paper), whereas A. scutiformis has a rounded abdomen ( Tokioka 1936; Yamaguti and Yamasu 1959). According to Tokioka (1936), the number of sclerites per supporting rod was also different between the two species (20–30 in A. caecus vs. 30–40 in A. scutiformis ), but the specimens of A. scutiformis reported by Yamaguti and Yamasu (1959) had fewer sclerites (18–24 per rod), which indicates that the number of sclerites is variable in this species and cannot be always used to differentiate it from A. caecus . As stated above, such variations in number of sclerites per supporting rod are closely related to body size in Argulus species ( Benz et al. 1995), and Yamaguti and Yamasu’s specimens were much smaller (13.6–19.8 mm) than those (more than 30.0 mm) collected by Tokioka (1936).

Kuramochi and Takahashi (2010) showed two photo- graphs of A. caecus , one is a ventral view of the habitus of one specimen that resembles the female reported herein. The authors collected their specimens from the vermiculated puffer, Takifugu snyderi (Abe, 1988) , in coastal waters of Sagami Bay off Ashina, Kanagawa Prefecture, about 8 km north from the type locality (Aburatsubo) . The present record of A. caecus from Otsuchi Bay ( Kado et al. 2021; this paper) has extended its distribution range from Kanagawa Prefecture, northward to Iwate Prefecture, along the Pacific coast of Japan. Tokioka (1936) similarly collected A. caecus from the Pacific coast, but no detailed information was provided on the collection localities.

There are two records of A. caecus from the Sea of Japan off Honshu, Japan ( Anonymous 1997; Kondo et al. 2013) but much remains obscure in the identification of the species from the sea. Without any literature citation and information on its morphology and host, A. caecus was listed in a catalogue of the animals collected in coastal waters of Sado Island, Niigata Prefecture ( Anonymous 1997). This species was also collected in Hibiki-nada, Yamaguchi Prefecture, from the grass puffer, Takifugu alboplumbeus (Richardson, 1845) [ Kondo et al. 2013, as Takifugu niphobles ( Jordan and Snyder, 1901); Masakazu Kondo, National Fisheries University (NFU), Shimonoseki, personal communication], but its morphology was not reported because Kondo et al. (2013) focused on the hemocytes of A. caecus . Therefore, no information has been published on the morphology of the argulid from the Sea of Japan, and it is necessary to conduct taxonomic work using the specimens previously reported as A. caecus and newly collected material from this sea in order to confirm whether the species actually occurs there.