The phylum Acanthocephala Rudolphi, 1808 is a parasitic worm known as acanthocephalans, thorny-headed worms, or spiny-headed worms (25,34). They are characterized by the presence of a retractable proboscis, armed with rows of hook (29). They have complex life cycles, involving benthic amphipods as intermediate hosts for the larval stages, i.e., acanthella and cystacanths (13,22,24,33). Fishes are infected by feeding on infected amphipods and serve as paratenic hosts, acting as a trophic bridge between intermediate and definitive hosts (11,12,16,28). Approximately 1,150 species have been described (1).

The genus Corynosoma Lühe, 1904 is the most numerous genus of Acanthocephala, with 43 marine species that infect mammals and piscivorous birds distributed worldwide (1,2,5,7,18,21,32). Infection with Corynosoma sp. is a mild zoonotic disease transmitted through marine fishes, causing abdominal pain, and cases have been reported in Alaska and Japan (8,20,26). Corynosoma strumosum (Rudolphi, 1802) Luhe, 1904 (Acanthocephala: Polymorphidae) is an intestinal parasite commonly found in pinnipeds and appears to be restricted to the northern hemisphere (14,17). The definitive hosts of C. strumosum are known to be seals and other marine mammals. This acanthocephala can even develop to fully mature adults in warm-blooded animals such as rats (23,25,27,31).

This study presents the first records of C. strumosum from a harbor seal, Phoca largha, in the East Sea, Korea, and describes the morphological features.

A female harbor seal was found dead after being caught in a fishing net on Munam Beach in Goseong-gun (latitude 38.295274; longitude:128.551645) and was brought to the Gangwon Wildlife Medical Rescue Center at Kangwon National University (Fig. 1). During the necropsy, 50 acanthocephalans were found in the small intestine, and their morphology was analyzed using light microscopy and scanning electron microscopy (SEM). The worms were fixed with neutral-buffered formalin, serially paraffin-embedded, sectioned, and stained with hematoxylin and eosin to determine the structures of the internal organs. For the SEM study, the worms were washed five times with 0.2 M cacodylate buffer (pH 7.3), fixed in 2.5% glutaraldehyde, and subjected to post-fixation treatment with 1% osmium tetroxide at 4°C. The specimens were dehydrated in a graded series of ethyl alcohol, dried in a CO2 critical point dryer (CPD 030; BAL-TEC, Los Angeles, California, USA), coated with gold, and examined using SEM (XL30 ESEM TMP; Philips, Praha, Czech Republic) at 15 kV. The acanthocephalans were identified according to the morphological information described by Neiland (22), Hennessy (10), and Nickol et al. (23).

Figure 1. Gross appearance of the female harbor seal, Phoca largha, before the necropsy.

Thirteen males and 37 females were found in one harbor seal, similar in size and shape, with the female worms being only slightly larger. The morphological details of males and females are as follows: The body lengths measured are 4.2-5.8 mm (n = 10, mean = 4.9 mm) in males and 5.3-6.8 mm (n = 20, mean = 5.7 mm) in females. The trunk can be divided into the anterior swollen fore-trunk and the posterior pipe-shaped hind-trunk, the former being shorter than the latter (Figs. 2A, B, 3A, B). The trunk spine is limited to the middle of the anterior swollen part on the dorsal side while extending to approximately one-third of the posterior cylindrical portion on the ventral side (Figs. 2F, 3F), with the genital spine appearing again at the end of the hind-trunk (Fig. 2K-N). The hind trunk narrows conspicuously and does not exceed 4.5 mm in length (Figs. 2A, B, 3A, B). The proboscis is vertically bent and armed with 18 longitudinal rows of 9-13 hooks in males and 20 longitudinal rows of 11-12 hooks in females (Figs. 2C, D, 3C). The proboscis lengths are 573 (471-641) μm and 627 (595-651) μm in males and females, respectively. The sub-cylindrical proboscis markedly widens in the posterior third (Figs. 2D, 3C), with no spines at its apical end (Figs. 2E, 3E). The anterior hooks of the proboscis are small, slender, only slightly longer, but more robust posteriorly up to hook nos. 7-8 (Figs. 2C, D, 3C). Hook nos. 7-8 are invariably the longest and most robust (Figs. 2C, D, 3C). The posterior four hooks are the smallest, becoming progressively smaller and slender (Fig. 2D), with their roots being short, small, and stubby. The roots of the anterior six hooks are simple and slightly longer than those of the blades (Figs. 2E, 3E). The trapezoidal neck of the worms is unarmed (Figs. 2C, G, 3C, D). The characteristic trunk spines on the hind trunk cover the anterior third of the ventral surface and are absent in the middle (Figs. 2A, B, F, H, 3A, B). The genital pores open on both sexes’ subterminal and ventral margins (Figs. 2M, 3G, H). The body surrounds numerous genital spines larger than the trunk spines of the males at the posterior portion of the genital pore (Fig. 2H-N). However, a few genital spines in the females are scattered at the posteroventral margin, unlike in males (Fig. 3G). The terminal ends of the males have either a copulatory bursa (Figs. 2I, J, 4D) or copulatory plugs (Fig. 2M, N). The single ovary is long and coiled into a pseudocoel (Fig. 4G). The female terminal ends are of two types, with and without copulatory caps (Fig. 3G, H). The testes are rounded, bilateral, or slightly tilted (Fig. 4B, C), and the vagina opens at the posterior end (Fig. 4H). The proboscis receptacle is double-walled (Fig. 4A, E). The lemnisci are equal, slightly shorter than the double-walled probosci’s receptacle, and longer than the proboscis (Fig. 4A, E). The cement gland is located behind the lemnisci (Fig. 4B, F). The alimentary canal is absent. Elliptical eggs are found in the pseudocoel and uterus of females (Fig. 4H) and contain acanthor-developing larvae (Fig. 4I,J). The egg size is measured in 93-103 (98.7) μm × 24-32 (26.1) μm, with smooth eggshells (Fig. 2A). These morphological characteristics implied the acanthocephala to be C. strumosum.

Figure 2. SEM views of male Corynosoma strumosum from P. largha. (A) Lateral view of whole body. Note the everted proboscis and neck. (B) Ventral view of whole body. Square is non–spinous area. Note inverted proboscis and neck. (C) Everted proboscis and neck (N), lateral view, showing the arrangement of the hooks and the spineless neck. The trapezoid neck structure is robust. (D) Hooks of the proboscis. Note the enlarged spine (square). (E) Front of the proboscis. (F) Ventral view of fore–trunk. Note that the ventral spines on the hind–trunk restricted to one–third of its length only. (G) The inverted proboscis. Note that the proboscis and neck (N) are inverted. (H) The hind–trunk. Note the spinous area (circles) and non–spinous area (square). (I, J) Copulatory bursa. (K, L) The partially retracted thick–walled bursa. (M, N) Copulatory plug. Genital pore (circle) positioned at the subterminal and ventral margin. Note the distribution of spines covering the whole body surface.

Figure 3. SEM views of female Corynosoma strumosum from P. largha. (A) The dorsal view of the whole body. There is no spine at the dorsal portion. Square is egg (Bar = 20 μm). Note the smooth eggshell. (B) The lateral view of the whole body. (C) Everted proboscis and neck (N). (D) Everted proboscis. (E) Front of the proboscis. There is no spine at the frontal portion. (F) Lateral view of anterior trunk. Note that the many spines. (G) Posterior end. Genital pore (circle) located at the posterior–ventral margin. Note a few genital spines are scattered at the dorsal margin unlike in males. (H) Posterior end of another female with copulatory caps. Genital pore (circle).

Figure 4. The longitudinal sections of male (A-D) and female (E-H) Corynosoma strumosum. H-E stained. (A) The everted proboscis. Note the double–walled proboscis receptacle (arrow). Lemnisci (arrowheads) (B) Cement gland (circle) and testes (arrows). (C) Anterior testis (AT) and posterior testis (PT). (D) Posterior end. Bursa copulatrix (circle). (E) Proboscis retractors (arrows) are double-layered. (F) Anterior end. Lemnisci (arrowheads). Cement glands (circle). (G) Ovary (O). (H) Posterior end. Eggs in pseudocoel (circle) and uterus (U). Vagina (V). (I, J) Eggs from pseudocoel. Note that the operculum is absent, and the acanthor larva is developing. Bar = 20 μm.

Corynosoma species are well-known acanthocephalan parasites commonly found in northern hemisphere pinnipeds (12). The prevalence of Corynosoma spp. infection was 56% in the Baltic Sea (19), 23-95% in the Wadden Sea (18,29), 87.5% in Japan (15), 100% in Ireland (24), and 29.5% in Alaska (16), showing significant differences by geographic location. Meanwhile, the harbor seals have been designated Natural Monument and a second-class endangered species in Korea (6). To the best of our knowledge, there have been few parasitological investigations on harbor seals in Korea due to their small population and lack of basic information. The current record of C. strumosum found in harbor seals in the East Sea is the first report from Korea.

Taxonomical studies of species of Corynosoma have been based almost exclusively on morphological characteristics (4). A crucial morphological feature of Corynosoma spp. is the presence of genital spines (4). Genital spines are always found in males, but may not be present in females, depending on the species (9,30). In the present study, the acanthocephalan parasites had numerous genital spines surrounding the posterior end of the male acanthocephalan parasites. However, unlike males, the females had few genital spines scattered at the dorsal margin.

When Corynosoma strumosum was first described in the spotted seal (Phoca vitulina Linnaeus), it had a body length of 5-7 mm (up to 9 mm), and the proboscis contained 16-18 (usually 18) longitudinal rows with 10-12 hooks (22). The body length of C. strumosum found in Japanese Kuril spotted seal, Phoca vitulina stejnegeri, was 6.0 (5.4-6.9) mm for males and 7.3 (7.0-8.0) mm for females, and the proboscis contained 18-20 (usually 18) longitudinal rows with 10-12 hooks (usually 10) (15). The body length of C. strumosum found in the Caspian seal, Pusa caspica, of the landlocked Caspian Sea was 2.75-4.75 mm and the proboscis contained 16 to 18 (usually 18) longitudinal rows (3). As such, although there are slight differences in the size of C. strumosum, 18 longitudinal rows in the proboscis were the same.

In this study, an acanthocephalan parasite, C. strumosum, was collected from the intestine of the harbor seal, Phoca largha, from the East Sea, Korea. The body lengths are 4.2-5.8 (4.9) mm in males and 5.3-6.8 (5.7) mm in females. The proboscis is armed with 18 longitudinal rows of 9-10 hooks in males and 20 longitudinal rows of 11-13 hooks in females. Morphologically, our specimens of C. strumosum are consistent with the general description of the species.

This study describes in detail the morphological characteristics of an acanthocephalans parasite from the intestine of the harbor seal, P. largha, in the East Sea of Korea. The acanthocephalans were diagnosed with C. strumosum, a novel species of acanthocephalan parasite found in Korea.

The Basic Science Research Program supported this research through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (Grant No. 2017 RIDIAIB06031728) and (NRF-2021R1F1A1064044). The authors thank the Gangwon Wildlife Medical Rescue Center for providing carcasses.

The authors have no conflicting interests.