Redescription of two Pennellids (Copepoda, Siphonostomatoida) from Korea with a key to species of Peniculus von Nordmann, 1832

Abstract Redescriptions of two pennellid copepods, Peniculus minuticaudae Shiino, 1956 and Peniculus truncatus Shiino, 1956, are provided, based on postmetamorphic adult females collected from marine ranched fishes captured at Tongyeong marine living resources research & conservation center, Korea. Peniculus minuticaudae was collected from the soft fin rays of black scraper Thamnaconus modestus. It can be distinguished from the other two closely related congeners Peniculus ostraciontis Yamaguti, 1939 and Peniculus truncatus by having a well developed triangular-shaped abdomen; the abdomen is rudimentary in other two species. This is thefirst report of the occurrence of Peniculus minuticaudae in Korea. Peniculus truncatus was collected from the dorsal fin of Korean rockfish Sebastes schlegelii. It can be distinguished from Peniculus minuticaudae by the combination of a rudimentary abdomen, long neck and setae on leg 1 and from Peniculus ostraciontis by the long neck, slender trunk, and setae on leg 1. It is also shown that Peniculus truncatus captured from the same host in Korea was misidentified as Peniculus ostraciontis and hence, this is thesecond record of the occurrence of Peniculus truncatus in Korea. A key is provided for the 14 nominal species of Peniculus.


introduction
The genus Peniculus von Nordmann, 1832 belongs to the family Pennellidae Burmeister, 1835 and contains 14 nominal species (Boxshall and Halsey 2004). Pennellids are highly transformed, often elongated copepods parasitic on marine fishes and cetaceans (Kabata 1979). Some of pennellids are ectoparasitic (e.g. Exopenna Boxshall, 1986;Parinia Kazachenko & Avdeev, 1977) but many are deeply inserted into the body of their host. The insertion can take place in the gills, the skin or in the musculature of the host without any particular preference, as is the case for the genus Pennella Oken, 1816 (Kabata 1981;Boxshall 1986).
Two species of Peniculus are redescribed from Korea in this study. They are P. minuticaudae Shiino, 1956 andP. truncatus Shiino, 1956. In Asia, nine species of Peniculus have so far been reported including six from India and three from Japan. The species reported from Japan are P. minuticaudae, P. truncatus and P. ostraciontis Yamaguti, 1939(Shiino 1956, 1959Yamaguti 1939Yamaguti , 1963. One of these three pennellids, P. ostraciontis, was redescribed from Korea by Choi et al. (1996) but we reveal here that theirs was a misidentification of P. truncatus. Shiino (1956) described P. minuticaudae based on females collected from the fins of threadsail filefish Stephanolepis cirrhifer (Temminck and Schlegel, 1850) (= Monacanthus cirrhifer), from Shirahama, Wakayama Prefecture, Japan. Recently, infection of P. minuticaudae on two cultured fish hosts, S. cirrhifer and the black scraper Thamnaconus modestus (Günther, 1877), was reported from Oita Prefecture, Japan (Nagasawa et al. 2011), after Fukuda (1999 reported the same species from the same locality as an unidentified Peniculus sp. Peniculus truncatus was also identified and described by Shiino (1956) based on a single female found on the fin ray of oblong rockfish Sebastes oblongus Günther (1877) [= Sebastichthys mitsukurii] collected off Wagu, Mie Prefecture, Japan. A third species, P. ostraciontis, was described based on females collected from the head of Humpback turretfish Tetrosomus gibbosus (Linnaeus, 1758) [= Ostracion gibbosum] on the Pacific coast of Japan (Yamaguti 1939). It was reported again from the triangular boxfish Tetrosomus concatenatus (Bloch, 1785) [= Rhinesomus concatenatus] from Sagami Bay by Shiino (1959) (Table 1). All three Peniculus species are in need of redescription and here we undertake the redescription of two of them.
The host T. modestus have been cultured at a few localities along the southern coastal regions of Korea. At Tongyeong marine living resources research & conservation center (TMRC), several commercially important fishes were ranched under the marine ranching program in Korea by Korea Institute of Ocean Science & Technology (KIOST) from 1998 (MOMAF 2007). Recently, we studied the symbiotic organisms associated with ranched fishes and their life cycles at TMRC (Venmathi Maran et al. 2012). The black scraper is one of the fishes that have been transferred into cages for the purpose of experimentally studying its feeding activities within this marine ranching program. The second host, S. cirrhifer, is uncommon in culture in Korea because of its small size and low growth rate, in contrast to Japan (Fukuda 1999). The Korean rockfish Sebastes schlegelii Hilgendorf, 1880 has been cultured at several localities around the southern coastal region of Korea due to its high commercial value (MOMAF 2007). Despite the increasing threat of parasites in aquaculture, information on parasites and diseases are largely lacking from farmed fishes in Korea. The redescription of P. minuticaudae and P. truncatus is necessary to reveal previously omitted or overlooked features of both species and also to correct the misidentification by Choi et al. (1996) in Korea. In addition, a key is provided for all 14 nominal species of Peniculus.

Materials and methods
The pennellids were carefully removed from the fin rays of the marine ranched T. modestus and S. schlegelii at TMRC, Tongyeong, Gyeongsangnam-do, Korea  Figure 1) and they were preserved in 70% ethanol. Preserved copepods were cleared in a drop of 85% lactic acid or lactophenol prior to examination using an Olympus BX51 phase contrast microscope. Selected specimens were measured intact using an ocular micrometer and/or dissected and examined according to the wooden slide procedure of Humes and Gooding (1964). Measurements given are the mean followed by the range in parentheses. Drawings were made with the aid of a drawing tube. The descriptive terminology follows Kabata (1979) (Figure 2A), 2.42 (2.12-2.73) mm long (n=10) comprising oval head, slender neck, large trunk and reduced abdomen. Head (cephalothorax) ovoid, longer than wide, with blunt pointed apex ( Figure  2B,C). Short slender neck ( Figure 2C) consisting of three somites bearing legs 1, 2 and 3. Fourth pedigerous somite incorporated into trunk. Trunk large, cylindrical, longer than wide, bearing leg 4 proximally ( Figure 2C). Abdomen slightly triangular-shaped ( Figure 2D, E) long with subterminal caudal rami on ventral surface and projecting posterior tip with anal indentation. Egg sacs long and uniseriate with 33-40 eggs (Figure 2F). Caudal rami ( Figure 2G) bearing 2 long, 3 medium sized subequal, 1 small setae. Antennule not observed. Antenna ( Figure 2H) 2-segmented, chelate; proximal segment consisting of 2 pointed projections overlapping each other; terminal segment claw-like, acutely pointed with minute seta at base.

Order
Mandible ( Figure 3A) broad with 10 teeth terminally. Maxillule ( Figure 3B) with 2 lobes having one and two long setae. Maxilla ( Figure 3C) 2-segmented; proximal segment broad with spiniform small process, 2 rows of setules distally; distal segment blunt and curved with transverse striations and rows of spinules. Maxilliped absent. Legs 1 to 4 ( Figure 3D-G) all represented by broad plate-like structures derived from the protopodal segments, without rami or seta. Leg 5 absent.
Variability. Some females showed variation on posterior end of trunk and abdomen ( Figure 3H-J).
Attachment site. All fins of host fish. Remarks. Careful comparison between our material and the original description of P. minuticaudae provided by Shiino (1956) revealed some differences: (1) the abdomen was described as trapezoid and rhomboid; (2) the striation and fine setulose ornamentation of the maxilla was not shown. The mandible was not described. Our redescription revealed that the abdomen of P. minuticaudae is triangular and protrudes, however, the two closely related congeners P. ostraciontis and P. truncatus both have a rudimentary abdomen. We also noted some variation in the posterior end of trunk and abdomen  ( Figure 3H-J). In the maxilla, fine striations and rows of setulose were found on the distal segment. In addition, the trunk is long and narrow in P. minuticaudae and there is no major gap between cephalothorax and trunk so it has a short neck, where legs 1 to 3 are located ( Figure 2C). Leg 4 ( Figure 2C) is embedded on the anterior part of the trunk. In comparison, the closely related congener P. ostraciontis has a stout trunk and short neck (Yamaguti 1939) while P. truncatus has a long trunk and neck, and leg 1 has minute setal structure which are not present in P. minuticaudae and P. ostraciontis. Description. Postmetamorphic adult female. Body ( Figure 4A), 4.59 (4.14-5.41) mm long (n=4) comprising oval head, long slender neck, large trunk and reduced abdomen. Head (cephalothorax) ovoid, flattened dorsally but convex ventrally with pair of rounded swellings anteriorly bearing antennae ( Figure 4B,C). Mouth tube prominent, directed posteroventrally ( Figure 4C). Neck long (0.47-0.55 mm) ( Figure 4B, C), slender, comprising about one sixth of trunk length; consisting of three somites bearing legs 1, 2 and 3 ( Figure  4B, C). Fourth pedigerous somite incorporated into trunk. Trunk slender, cylindrical, longer than wide, 6 times longer than neck, bearing leg 4 proximally. Abdomen ( Figure  4D), reduced with subterminal caudal rami on ventral surface. Caudal rami ( Figure 4E) bearing 6 setae. Egg sacs long and uniseriate with 30-37 eggs. Antennule not observed. Antenna ( Figure 4F) 2-segmented, chelate; proximal segment bearing 2 pointed projections overlapping each other; terminal segment claw-like, acutely pointed with minute seta at base. Mandible ( Figure 4G) moderate-sized, broad, provided with 10 teeth terminally.
Attachment site. Only on dorsal fin-rays. Remarks. Comparison between our material and the original description of P. truncatus provided by Shiino (1956) revealed some omissions in that the antennae and mandibles were not shown, and possible differences, since the striation of setules on maxilla was not shown. The characteristic features of P. truncatus are: (1) the rudimentary abdomen; (2) the long neck (more than half as long as cephalothorax); (3) the maxilla with  transverse striations of setules and rows of spinules on the distal segment; (4) the leg 1 is tipped with 2 minute setae laterally. Peniculus truncatus differs from P. minuticaudae in its rudimentary abdomen (vs. well developed abdomen); long neck (vs. short neck); and in the presence of setae on leg 1 (vs. absence of seta). It differs from P. ostraciontis in its moderately slender trunk (vs. stout trunk); long neck, ie: neck more than half as long as cephalothorax (vs. short neck, ie: neck less than half as long as cephalothorax); and in the presence of setae on leg 1 (vs. absence of setae) (Yamaguti 1939;Shiino 1956). Choi et al. (1996) reported the same pennellid collected from the fins of S. schlegelii as P. ostraciontis. We compared our material with their illustrations (specimens were not deposited in the museum). It showed the features of P. truncatus: (1) long neck; (2) slender trunk [not as stout as like P. ostraciontis illustrated by Yamaguti (1939)] and the host was S. schlegelii (Choi et al. 1996), as in the present study.
The mean body length of P. minuticaudae was 2.42 mm. It corresponds well to the body length (2.48 mm) of P. minuticaudae reported from Oita Prefecture, Japan (Nagasawa et al. 2011). The morphological features (Figures 2, 3) agree with the original description of P. minuticaudae (Shiino 1956). The present collection represents the first record of P. minuticaudae from ranched T. modestus in Korea. Thus, it is the third documented record of pennellid copepod from commercially cultured fishes.
Peniculus truncatus was originally reported from S. oblongus in Japan (Shiino 1956). This parasite is shown here to utilize a second host species, S. schlegelii, of the same host genus, although it was initially misidentified as P. ostraciontis by Choi et al. (1996). The misidentification was revealed by comparison between Choi's descriptions, our material and Yamaguti (1939) illustrations of P. ostraciontis. We collected P. truncatus from the same host species S. schlegelii cultured in Korea. The host for P. ostraciontis is T. gibbosus (Table 1). In Choi et al. (1996) redescription, they overlooked the third seta on the maxillule and the setules on the maxilla, in addition to the minute setal structures on leg 1.
Peniculus truncatus has so far been reported from two species of the genus Sebastes, S. schlegelii and S. oblongus and this pennellid appears to be host specific to rockfish (Table 1). Peniculus minuticaudae and P. ostraciontis might be specific to file fish and puffer hosts, respectively (Yamaguti 1939;Shiino 1956;1959;Nagasawa et al. 2011;present study). A key is provided for all 14 valid species below.

Conflict of interest statement
All authors declare that they do not have any conflict of interest.