Surveys of Gynaecotyla squatarolae and Microphallus koreana (Digenea: Microphallidae) Metacercariae in Two Species of Estuarine Crabs in Western Coastal Areas, Korea

Gynaecotyla squatarolae (Digenea: Microphallidae) was originally discovered in the small intestine of birds [1], and the second intermediate host is known to be the shore crab, Macrophathalmus japonicus and Macrophthalmus dilatatus in Japan [2]. In the Republic of Korea, the metacercariae of G. squatarolae were discovered from the shore crab, M. dilatatus in Taean-eup, Chungcheongnam-do [3], but the investigation on M. japonicus has not been performed yet. In addition, the ruddy turnstone, Arenaria interpres interpres, a migratory Korean bird, was proved to be a natural definitive host for G. squatarolae [4]. Considering that the ruddy turnstone was found at the seashore of Gunsan-si, Jeollabuk-do, a survey on the western coastal areas was needed.

Microphallus koreana is also a kind of microphallid, described as a new species in Korea [5]. As in G. squatarolae, the crabs, M. dilatatus, are known to play a role in maintaining the life cycle of M. koreana, but the prevalence and infection density along the coastal areas has not been studied yet. The present study was performed to understand the geographical distribution of G. squatarolae and M. koreana by examining 2 species of estuarine crabs, M. dilatatus and M. japonicus, according to the localities.

A total of 8 areas along the western coasts of Korea were selected: Ganghwa-eup, Incheon-si (A), Jebu-ri Seosin-myeon Hwaseong-si, Gyeonggi-do (B), Mageum-ri Geunheung-myeon Taean-gun, Chungcheongnam-do (C), a market of Seosan-si Dongnam-dong, Chungcheongnam-do (D), Namdang-ri Seobu-myeon Hongseong-gun, Chungcheongnam-do (E), Sangdeung-ri Gyehwa-myeon, Buan-gun, Jeollabuk-do (F), Hasa-ri, Baeksu-eup Yeonggwang-gun, Jeollanam-do (G), Sinjang-ri, Aphae-myeon, Sinan-gun, Jeollanam-do (H). These areas were indicated in Fig. 1.

Fig. 1

Map showing the areas (A-H) where the crabs were collected and examined for Gynaecotyla squatarolae and Microphallus koreana metacercariae.

The crabs, M. japonicus and M. dilatatus, were collected at the areas described above by using a scoop. Along with the collection by scoop, the crabs of Seosan-si, Chungcheongnam-do, were bought in the Seosan market. After transportation to the laboratory, the hepatopancreas of each crab was separated, ground in a mortar with pestle, and filtered through a series of nets. Then, the presence of microphallid metacercariae was investigated under a stereomicroscopy. If metacercariae were present, the number of metacercariae per crab was counted. Each group was consisted of 15 crabs.

In M. dilatatus crabs, G. squatarolae metacercariae were recov ered from C, E, and H out of 6 areas, and the highest density was recorded in H (79.1) (Table 1). All the crabs were infected with G. squatarolae in H, whereas the infection rate was 86.7% in C. The infection density was the highest in H, followed by E (44.3) (Table 1). However, metacercariae of Microphallus koreana were not recovered from M. dilatatus in the surveyed areas. A lot of metacercariae of unknown trematodes were recovered, but their identification is still underway.

Table 1.

Number of metacercariae recovered from M. dilatatus

In the case of M. japonicus, all the surveyed areas except A had G. squatarolae metacercariae (Table 2). Crabs from D had the highest prevalence (87.7%), whereas the number of metacercariae was the highest in crabs from H, followed by those from D. In the remaining areas positive for G. squatarolae, the prevalence ranged from 20% to 80%, and the number of metacercariae was less than 5.0 per crab (Table 2). Five out of 7 areas were positive for M. koreana metacercariae, and the prevalence was from 6.7% in A to 80% in D. The infection density was less than 10 per crab, and the highest number was recorded in F, followed by D. Undetermined metacercariae were also recovered from M. japonicus.

Table 2.

Number of metacercariae recovered from M. japonicus

Our results suggested that G. squatarolae and M. koreana are distributed widely along the southwestern coastal areas of Korea. The crabs of Sinjang-ri, Shinan-gun, Jeollanam-do, was proved to be heavily infected with G. squatarolae. This Sinjang-ri area, Shinan-gun, is known to be the endemic area of Gymnophalloidesseoi, a human infecting gymnophallid transmitted by oysters [6], and an avian host, the Paleartic oystercatcher, was proved to be a natural definitive host [7]. Microphallids have also been known as parasites of birds; for example, Microphallus sabanensis was collected from the wild birds of Venezuela [8], and Maritrema novaezealandensis was found from the red-billed gulls of New Zealand [9]. In addition, adult worms of G. squatarolae were collected from the intestine of the ruddy turnstone, a migratory bird of Korea [4]. Since the natural definitive host of M. koreana is not discovered yet, special attention should be paid to shore birds for the study of microphallids.

According to our previous researches [3,5], both G. squatarolae and M. koreana metacercariae were recovered from M. dilatatus purchased at a market of Seosan-si, Chungcheongnam-do, and the infection density was 4.3 per crab for G. squatarolae and 4.7 per crab for M. koreana. However, in the present study, M. koreana metacercariae were found only in M. japonicus, but not in M. dilatatus. Environmental changes, such as the oil outflow accident in Taean-gun, Chungcheongnam-do in 2007, might have changed the geographical distribution of larval trematodes. However, it may be reasonable to regard that M. japonicus is a more important intermediate host for M. koreana than M. dilatatus nowadays in Korea. More studies on microphallids are needed to elucidate this point.