Ocean and Polar Research the First Record of Monothula Subtilis

Vol. 40(1):23−35 Ocean and Polar Research March 2018 http://dx.doi.org/10.4217/OPR.2018.40.1.023 Article The First Record of Monothula subtilis (Giesbrecht, 1893 [“1892”]) (Cyclopoida, Oncaeidae) in the Equatorial Pacific Ocean Kyuhee Cho1* and Woong-Seo Kim2 1Envient Inc., Daejeon 34052, Korea 2Deep-Sea and Seabed Mineral Resources Research Center, KIOST Busan 49111, Korea Abstract : A small cyclopoid copepod M. subtilis (Giesbrecht, 1893 [“1892”]) belonging to the genus Monothula Böttger-Schnack and Huys, 2001 was collected by using 60 µm mesh net and firstly recorded in the epipelagic layer of the equatorial Pacific Ocean. We redescribed its morphological characteristics for both female and male, comparing with those of previous studies. Specimens of M. subtilis from the equatorial Pacific Ocean differ from those previously reported by others in terms of the length of the seta G on antenna, being much shorter than setae E and F; in the distal spine on the swimming leg 4, being longer than the length of the third segment on P4. The outer spine of the P3 enp-3 in male is slightly over the tip of conical process. The spine lengths of the distal endopods of P2−P4 for both sexes showed variations among individuals, and the proportions of spine lengths in female are higher than those in male. Key words : taxonomy, copepod, tropical Pacific, zooplankton, Monothula subtilis 1. Introduction southern Korean waters, the East China Sea, and adjacent waters of Japan (Chen et al. 1974; Itoh 1997; Wi et al. The family Oncaeidae Giesbrecht, 1893 [“1892”] is 2009, 2011, 2012). Compared to the area above mentioned, definitely appreciated as important zooplankton in terms there have been minimal studies on taxonomy in the open of both numerical abundance and species diversity in tropical Pacific Ocean. marine ecosystem emerged during the past 2−3 decades, The characteristics of the genus Monothula belonging especially in the case of small species (< 0.1 mm) to the family Oncaeidae, are as follows: absence of the (Böttger-Schnack 1994; Roman et al. 1995; Mckinnon et outer spine on P2−P4 enp-3 and the presence of a single al. 2013). They are one of marine zooplankton groups, dorsal egg-sac. Monothula subtilis is the only species occurring in all water columns of the world’s oceans. In belonging to this genus (Böttger-Schnack and Huys 2001). recent years, numerous field and taxonomic studies have Giesbrecht (1893) firstly described female of the species attempted to find and explore small species, using a fine- as Oncaea subtilis from the Gulf of Naples, Mediterranean mesh (< 0.1 mm) net. As a result, they have been Sea with the figures of the female habitus, urosome, examined in various areas of the world oceans and seas, antenna, maxilliped and swimming leg 4. Razouls (1972) such as the Red Sea (Böttger-Schnack 1999, 2002, 2005, gave partial figures (urosome, antennule, antenna, maxilliped, 2009), the Mediterranean Sea (Kršinić and Malt 1985; and swimming leg 4) of female M. subtilis from the region Böttger-Schnack 2003), the Arabian Sea, the Antarctic of Banyuls (Gulf of Lion), but morphological details and and the Pacific Ocean (Heron 1977; Heron et al. 1984; several armature elements on appendages were absent. Heron and Frost 2000; Böttger-Schnack 2004), the Gallo (1976) illustrated urosome, P1−P4, antennule, antenna, maxilla, and maxilliped of male from La Pallice *Corresponding author. E-mail : [email protected] Harbor, the northeast Atlantic Ocean, however, Gallo 24 Cho, K. and Kim, W.-S. missed figures of several armature elements and portrayed Morphological terminology followed Huys et al. inaccurately. The figures of Razouls (1972) and Gallo (1996). Abbreviations used in the text and figures are as (1976) could be referred to the web site (Razouls et al. follows: Al, antennule; A2, antenna; ae, aesthetasc; P1− 2005−2017). Malt (1982) described the developmental P6, first to sixth thoracopod; exp, exopod; enp, endopod; stages of M. subtilis from the mouth of English Channel exp (enp)-1 (2, 3) to denote the proximal (middle, distal) near the Atlantic Ocean. Subsequently Böttger-Schnack segment of a three-segmented ramus. The observable and Huys (2001) rearranged this species as a new genus pores and other integumental structures (e.g., pits, scales) Monothula, and redescribed both sexes of M. subtilis from on the body surface under the light microscope were only the Red Sea, the east Mediterranean Sea, the Arabian Sea figured. and the eastern Indian Ocean in more detail. All materials were deposited in the National Institute of During the investigation on the zooplankton community Biological Resources (NIBR), Korea. at the Korean long-term environmental monitoring station in the equatorial Pacific Ocean (MOMAF 2009), taxonomic 3. Descriptions study of oncaeid copepods was carried out. Many small species of the family Oncaeidae were sampled, including Family Oncaeidae Giesbrecht, 1893 [“1892”] genus Monothula, which is first recorded in the equatorial Genus Monothula Böttger-Schnack and Huys, 2001 Pacific Ocean. In this study, we redescribe both sexes of M. subtilis, Monothula subtilis (Giesbrecht, 1893 [“1892”]) based on the specimens from the equatorial Pacific Ocean (Figs. 1−5) and compare with previous descriptions from other Oncaea subtilis Giesbrecht, 1893 [“1892”], 593−599, localities, and discuss zoogeography of the species. We 603−604, 756, 774, plate 47, Figs. 14, 18, 25, 43 and 60 also provide information on the variation in endopodal (female); Malt, 1982, 129−130, 134−141, Figs. 6a−g, 7a− spine length of swimming legs 2 to 4 for both sexes. n, 8a−z and 9a−p; Heron and Frost, 2000, 1054−1055, Figs. 21F−K, 22A−J. 2. Materials and Methods Monothula subtilis Böttger-Schnack and Huys, 2001, 467−481, Figs. 1−5. Monothula subtilis was collected at KOMO (KODOS Long-term Monitoring Station; 10o30'N, 131o20'W) in the Material examined: Six females (NIBRIV0000812653− central part of the Clarion-Clipperton Fracture Zone of the 812658), each dissected and mounted on 7, 9, or 10 slides. northeast equatorial Pacific Ocean on 21 August 2009. Three males (NIBRIV0000812659−812661), each dissected Sampling was made by using a conical net with 60 µm and mounted on 6 or 9 slides. mesh (60 cm mouth diameter) by hauling vertically from All specimens collected from the northeast Pacific the depth of 100 m to the surface. Samples were initially Ocean (10o30'N, 131o20'W) on 21 August 2009. fixed in 99.9% ethyl alcohol on board. The specimens of At sampling station, surface temperature and salinities M. subtilis were sorted out from the collected zooplankton were 28.5oC and 33.5, respectively. Temperature decreased samples under a stereomicroscope (Stemi 2000-C, Zeiss) to 13.6oC at 100 m depth, while salinity remained at 34.6− in the laboratory. Specimens were dissected with tungsten 34.7. needles and mounted in lactophenol, and sealed with transparent nail-varnish. All drawings and measurements Female. Body length 558−613 μm (illustrated indiv.: were performed with the aid of a drawing tube attached to 593 μm, n = 6). a differential interference contrast microscope (BX51, Prosome length about 1.7 times to that of urosome; 1.5 Olympus). Scale bars were given in micrometers (μm). times to that including caudal rami. Second pedigerous Body length was measured from the anterior tip of somite without conspicuous dorso-posterior projection in prosome to the posterior end of caudal ramus in lateral lateral aspect (Fig. 1B). Integumental pores on prosome as view. Ratio of prosome to urosome (including caudal indicated in Fig. 1A, B. Fourth pedigerous somite with ramus) length was also calculated. Telescoping of somites rounded and prominent posterolateral corners (arrowed in was not considered in length measurements. The antenna, Fig. 1A). One pair of secretory pores discernible on lateral labrum, and maxilliped of female were examined with view of first and second postgenital somites, respectively scanning electron microscopy (S2380N, Hitachi). (Fig. 1B). Monothula subtilis in Equatorial Pacific 25 Fig. 1. Monothula subtilis (Giesbrecht, 1893 [“1892”]). Female (A) Habitus, dorsal; (B) Habitus, lateral; (C) P5-bear- ing somite and genital double-somite, ventral; (D) Caudal rami, left, ventral; (E) P6; (F) Antennule 26 Cho, K. and Kim, W.-S. Fig. 2. Monothula subtilis (Giesbrecht, 1893 [“1892”]). Female (A) Antenna, anterior; (A') Endopod segments of antenna, posterior; (B) Oral area showing position of labrum, mandible, and maxillule, arrow indicating teeth on posterior part; (C) Mandible; (D) Maxillule; (E) Maxilla; (E') Maxilla, detail of allobasis; (F') Maxilliped, posterior; (G) Maxilliped, anterior Monothula subtilis in Equatorial Pacific 27 Genital double-somite about 1.4 times as long as Anterior surface (Fig. 2B) with paired rows of long maximum width in dorsal aspect and 4/5 length of setules either side of median swelling and paired slitlike postgenital somites combined; lateral margins rounded forms latero-posteriorly. Posterior part of medial incision (pot shaped), largest width measured at anterior one third, ornamented with 2 spinous teeth (arrowed in Fig. 2B). posterior part tapering gradually; paired genital apertures Posterior face with 3 secretory pores located distally on located at about 2/5 the distance from anterior margin of each lobe (dotted line as in Fig. 2B). genital double-somite (Fig. 1A). Pore pattern on dorsal Mandible (Fig. 2C) represented by flattened gnathobase surface as in Fig. 1A; ventral surface with several rows of with 5 elements. Outer stout seta (a) with long, fine minute spinules medially and laterally (Fig. 1C). setules along dorsal side: ventral blade (b) strong and Anal somite as long as wide and about as long as spiniform, with row of setules on posterior side; dorsal caudal rami (Fig. 1A). Anterior margin of anal opening blade (c) strong and broad, spinulose along entire dorsal (vestigial anal operculum) with line of minute spinules.

Load more