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Naupliar Development of Tigriopus Japonicus Mori, 1932 (Copepoda: Harpacticidae) Hans U

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Naupliar Development of Tigriopus Japonicus Mori, 1932 (Copepoda: Harpacticidae) Hans U Zoological Studies 46(6): 746-759 (2007) Naupliar Development of Tigriopus japonicus Mori, 1932 (Copepoda: Harpacticidae) Hans U. Dahms1, Supawadee Chullasorn2, Pawana Kangtia2, Frank D. Ferrari3, and Jiang-Shiou Hwang1,* 1Institute of Marine Biology, National Taiwan Ocean University (NTOU), Keelung 202, Taiwan 2Department of Biology, Faculty of Science, Ramkhamhaeng University, Bangkok 10240, Thailand 3Smithsonian Institution, IZ, MSC, MRC-534, Washington DC 20560-0544, USA (Accepted May 21, 2007) Hans U. Dahms, Supawadee Chullasorn, Pawana Kangtia, Frank D. Ferrari, and Jiang-Shiou Hwang (2007) Naupliar development of Tigriopus japonicus Mori, 1932 (Copepoda: Harpacticidae). Zoological Studies 46(6): 746-759. Six naupliar stages of Tigriopus japonicus Mori, 1932 are described from the rocky coast off Keelung, northeastern Taiwan. A key for the identification of stages is provided. Naupliar morphology within the Harpacticidae differs among species and even more at the supraspecific level. The present study on nauplii of Tigriopus japonicus confirms the observation that representatives of the Tachidiidae and Harpacticidae (Copepoda-Harpacticoida) show a peculiar reduction of the oral structures from N V to N VI. The nauplii of the Harpacticidae are creeping larvae, and many are not able to swim; however, nauplii of T. japoni- cus are an exception to this rule. http://zoolstud.sinica.edu.tw/Journals/46.6/746.pdf Key words: Naupliar development, Larval stages, Identification key, Evolution. Like several other groups of Crustacea, dances and variety, nauplii may also play impor- harpacticoid copepods develop nauplius larvae as tant ecological roles (Alekseev 2002, Dahms and their early postembryonic stages (Bjørnberg 1986, Qian 2004). Life-history studies in the field and Dahms 2000). Harpacticoids as do most free-liv- investigations of stage-specific phenomena in the ing Copepoda, go through 6 naupliar and 6 cope- laboratory are also hampered by a lack of descrip- podid stages (Ferrari and Dahms 2007). The nau- tive information and missing keys for identification pliar phenotype is quite unlike the adults, and it is that are the basis for all work on stage-specific difficult to tell which nauplius belongs to which phenomena in the laboratory and heterogeneous species, unless the development of isolated assemblages in the field (Dahms 1993, Dahms et females is observed in the laboratory. Compared al. 2006). Much rearing and descriptive work has to other copepod taxa, naupliar development of to precede any serious attempt to tackle ontogeny- harpacticoids has been far less studied (Dahms related problems. This is particularly true for 1990, 2004a b). There are no fewer than 16 Tigriopus since this taxon has become the focal harpacticoid families for which nothing is known point of several in-depth studies, e.g., of its field about the developmental instars. Naupliar instars ecology (Dethier 1980), life history (Koga 1970), are exposed to different selection pressures, and internal anatomy (Itô 1973), chemical ecology nauplii, therefore, have undergone remarkable (Kelly and Snell 1998), genetics of adaptation adaptive radiation, leading to a diversity of struc- (Davenport et al. 1997, Burton et al. 1999), popu- tures, behavioral characteristics, and distribution lation genetics (Edmands and Burton 1998, patterns (Borutzky 1952). Due to their great abun- Edmands 1999), and genomics in general *To whom correspondence and print requests should be addressed. Tel: 886-935-289642. Fax: 886-2-24629464. E-mail:[email protected] 746 Dahms et al. -- Nauplii of Tigriopus japonicus 747 (Machida et al. 2002, Jung et al. 2006). the offspring of exclusively single-female cultures, Detailed and exact descriptions of postembry- unless indicated otherwise. onic instars are also helpful for the elucidation of Cultivation was carried out in polystyrene phylogenetic relationships in providing ontogenetic Petri dishes. These were kept at 25 C in an incu- apomorphies for monophyletic groups that set the bator with a 12:12 h light: dark cycle.° About 50% baseline for the reconstruction of phylogenies of the seawater was renewed each week, and it (Dahms et al. 2005). There are several reasons was obtained and prepared as described by why naupliar characters have thus far been widely Dahms (1990). Single ovigerous females were neglected in systematic and phylogenetic studies, added to the Petri dishes. When nauplii emerged, e.g., difficulties in obtaining detailed information on some of them were isolated in watch glasses, and naupliar characters (because nauplii provide fewer the exuviae of subsequent molts were collected. characters than later ontogenetic instars and Various kinds of food were used: such as Nitzschia adults), a lack of appropriate comparative data, sp., Tetra-Min, or a mixture of these (see also and conflicting evidence when comparing adult Dahms et al. 2007). A few drops of a food suspen- and naupliar character states. In any case, it is sion were added every few days which settled as a more important to find new characters than merely fine, even, semi-transparent coat over the entire to reinterpret those already known. Phyloge- bottom of the glass. netically valuable apomorphies may be cryptic No attempt was made to exclude small proto- characters that are often camouflaged by superfi- zoan contaminants or to prevent algal growth. No cial resemblances. Here, detailed resolution is a substratum had to be added in the case of the prerequisite for meaningful comparisons. Detailed rock-surface dwelling T. japonicus, whereas studies, however, are more difficult the more-com- harpacticoids from soft bottoms often require sedi- plicated or smaller structures become. This is par- ment substrates (Dahms and Qian 2005 2006). ticularly true for minute nauplii, many of which do not exceed 50 µm in diameter at hatching. Preparation Although there are 2 other studies dealing with the postembryonic development of T. japoni- Stages were fixed in 5% buffered formalde- cus (Itô 1970, Koga 1970) from Japanese popula- hyde and embedded in glycerol. This clarifies tions, an updated study is warranted, since previ- nonexuvial material within a few weeks and pro- ous attempts are either incomplete or lack detail. vides information on hidden posterior structures The present study provides a thorough description when observed with phase-contrast microscopy. of nauplii belonging to the harpacticid species Unfortunately, the natural color of nauplii and the Tigriopus japonicus Mori, 1932. This will enable color and shape of the red nauplius eye were researchers to identify naupliar stages to the rapidly lost, and the eye was, thus, not figured. species level, which is important for various life Nauplii were mounted whole, and broken glass- history studies. fibers were added to prevent them from being compressed and to facilitate rolling to allow inspec- tion from all sides. Abnormalities were occasional- MATERIALS AND METHODS ly observed but were not illustrated. Body mea- surements are given from the frontal portion of the Collection data naupliar shield to the caudalmost protrusion of the hindbody (length), and the widest lateral tips of the Adults of T. japonicus were collected by Mr. naupliar shield (width); only specimens drawn Kao Tzu from the coast of Badouzih Harbor, were considered for length measurements. approximately 8 km east of Keelung, on the north- Otherwise, 2-5 specimens per stage were used for eastern coast of the main island of Taiwan in Oct. the investigation of stage-specific variability. 2005. Ovigerous females which provided the Species identification was carried out with the aid developmental stages were collected in a beaker of Lang (1948) if not stated otherwise. from a supralittoral splash-pool, that was subse- quently decanted over a 50 µm-mesh screen. The Descriptive terminology residue containing only metazoans at all develop- mental stages of T. japonicus was rinsed into The following terms are defined according to smaller bowls for transport to the laboratory. The their usage in the following text. The 1st to 6th developmental stages used in this study represent naupliar stages are respectively abbreviated as N I 748 Zoological Studies 46(6): 746-759 (2007) to VI. Nauplii of T. japonicus, as those of other benthic, and crawl and swim-crawl on surfaces. harpacticoids, have at least 3 pairs of appendages: All developmental stages including nauplii can also 1st and 2nd antennae and mandibles. The body is swim freely in the water column. Nauplii are about covered by a smooth nauplius shield; the hindbody as wide as long and reddish and have a dark protrudes from it in later stages. At the posterior median eye. The cephalic shield covers only parts end of the body is at least 1 caudal seta on each of the hindbody (Figs. 1, 2). side of the anal area. The labrum originates as a lobular flap near the frontal margin of the body, Nauplius I (Figs. 1, 3, 4, 6). between the bases of the 1st antennae, and extends posteriorly across the ventral surface of Body length 133 µm, body width 118 µm. the body. The metasomal ventral body wall is a Body slightly longer than wide and cephalic shield tongue-like structure arising at the base of the showing lobular bulges on both lateral sides. antennal protopod. The 1st antenna is uniramous. Hindbody bearing 1 seta (= the initial furca), aris- The 2nd antenna notably differs from that of the ing from a protuberance on each caudal side and a adult (and copepodids) in having a coxal mastica- row of short spinules in between. Almost rectan- tory process (= gnathobase). The 2nd antenna gular labrum furnished with hairy spinules along its further consists of a coxa, basis, endopod, and lateral corner and distal edge. Sternal field orna- exopod. The mandible is composed of the same mented with 1 caudally curved row of slender spin- elements except the precoxa. The endopod con- ules in between insertions of 2nd antennae. sists of an inner process and usually a lateral field First antenna 3-segmented. First segment of setae arising on the outer lateral margin.
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