Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway The first naupliar stage of Pennella balaenopterae Koren and Danielssen, 1877 (Copepoda: Siphonostomatoida, Pennellidae) Nina L. Arroyo, Pablo Abaunza & Izaskun Preciado SARSIA Arroyo NL, Abaunza P, Preciado I. 2002. The first naupliar stage of Pennella balaenopterae Koren and Danielssen, 1877 (Copepoda: Siphonostomatoida, Pennellidae). Sarsia 87:333–337. The first nauplius of Pennella balaenopterae is described and illustrated. This is the first description of the nauplius in this species and genus. Antennae and mandibles resemble those previously described in pennelid nauplii. The first antenna is uniramous and one-segmented, while the second antenna and the mandible show a two-segmented endopod and a four-segmented exopod. No trace of any other appendage or of the gut is found. Some comments on the life cycle of Pennella balaenopterae are made. N. L. Arroyo, Departamento de Biologı́a Animal I (Zoologı́a), Facultad de Biologı́a, Universidad Complutense de Madrid, ES-28040 Madrid, Spain. Present address: Åbo Academy University, Department of Biology Environmental and Marine Biology, FIN-20500 Åbo, Finland. P. Abaunza & I. Preciado, Instituto Español de Oceanografı́a, Centro Oceanográfico de Santander, Apdo. 240, ES-39080 Santander, Spain. E-mail: nhailuot@abo.fi; pablo.abaunza@st.ieo.es; ipreciado@st.ieo.es Keywords: Pennella balaenopterae; Pennellidae; first nauplius; morphological description; life cycle. INTRODUCTION The significance of nauplii to the crustaceans is now widely accepted, its morphological organization being so fundamental that its presence has been proposed as part of the definition of the Crustacea (Walossek & Müller 1990), and several theories have been supplied assessing its usage in phylogenetic studies (Dahms 2000). Within the siphonostome family Pennellidae, usually one or two nauplius stages have been reported (Sproston 1942; Heegaard 1947; Kabata 1976; Schram 1979). Two nauplius stages were found in Lernaeenicus sprattae (Sowerby, 1806) (cf. Schram 1979), and one nauplius stage was found in Lernaeocera branchialis (Linnaeus, 1767) (cf. Sproston 1942; Heegaard 1947) and Haemobaphes diceraus Wilson, 1917 (cf. Kabata 1976). In Lernaeenicus longiventris Wilson, 1917, the newly hatched nauplius was described and illustrated, although the exact number of nauplius stages was not ascertained (Wilson 1917). Some pennelids lack a freeswimming nauplius stage whatsoever, and the eggs hatch directly as copepodids, the naupliar stage being passed inside the egg sac (Bennet 1961; Perkins 1983; Izawa 1997). The larval stages of pennelids belonging to the genus Pennella have been described for the copepodid and chalimus stages of Pennella varians Steenstrup & Lütken, 1861 (Rose & Hamon 1953). Also, Pascual (1996) described the copepodid and chalimus stages of a pennelid infecting squids whose identity he could not ascertain but which he related to the larval stages of Pennella varians described by Rose & Hamon (1953). Relatively few studies have been conducted on Pennella balaenopterae, its life cycle is almost unknown and only the adult female has been identified with certainty to date (Turner 1905; Hogans 1987). In 1997, a fin whale (Balaenoptera physalus Linnaeus, 1758), stranded on the coast of Cantabria, was carrying 12 adult females of Pennella balaenopterae (Abaunza & al. 2001). In the present paper, we describe and illustrate a naupliar stage of Pennella balaenopterae, providing the first description of a naupliar stage for the species and the genus. MATERIAL AND METHODS The fin whale stranded on the coast of Cantabria (northern Spain) in November 1997, was a 19 m long male, with no apparent malformations, either external or internal. It died on the coast, and the parasites were collected 3 days later. The parasites without egg strings were found with the chephalothorax and anterior part of the thoracic region embedded in the whale’s blubber, and were located mainly on the anterior half of its body. The specimens were placed in a plastic bag, and once in the laboratory, all of them were found to bear long, string-like ovisacs, # 2002 Taylor & Francis 334 Sarsia 87:333-337 – 2002 protruding from the genital pores: clearly these strings had been extruded by the animal during transport. The parasites, together with the ovisacs, were stored in 70% alcohol for fixation and conservation. Dead embryos and nauplius larvae (without egg membrane) were found in the preservation fluid. Ovisacs contained yet more unreleased eggs. The nauplii were mounted whole for examination, broken glass fibres being added to prevent them from being compressed and to facilitate rolling to allow viewing from all sides. Some specimens were mounted on aluminium cobble slides for the same purpose. Drawings were made from single specimens with the aid of a camera lucida. Body lengths were measured from the anterior to the posterior end of the nauplius; body width is given as the widest part of the nauplius. Appendages were measured from the point of insertion in the body to their distalmost part. Setae were measured from the point of insertion in the appendage to the tip of the setae. A Leica light microscope, an interference 40 lens magnification, and an image analyser (Visilog 5.2) were used. RESULTS Body pyriform, with narrower posterior end (Fig. 1A). Brown-yellowish in colour, transparent, filled with yolk globules. Gut not visible. Length of nine specimens 290–385 mm (mean 323.2 mm; Table 1). Naupliar eye with two ocelli and lenses surrounded by developed retina. Apparent very early at the embryonal stage. Distinct crimson pigment spot, lying close to the front border and between the two ocelli. This pigment spot was evident in elliminate some specimens but not in others. First antenna (Fig. 1B; Table 1), uniramous and onesegmented; ventral margin with one seta on the middle of the segment; two long setae as apical armature and at least two dorsal dentiform outgrowths in subapical position. Second antenna (Fig. 1C; Table 1), biramous, borders between sympod and rami indistinct. Sympod robust, unsegmented and simple. Endopod two-segmented; basal part longer than terminal segment, unarmed; terminal segment with two unarmed apical setae and a short basal seta. Exopod four-segmented, the fourth segment apparently inserted inside the third. Basal segment about two thirds the total length of ramus, cylindrical, with one long seta in the distomedial corner; segments 2, 3, and 4 short, with a similar seta in the same position; all setae naked. Mandible biramous (Fig. 1D; Table 1), rami not clearly delineated from sympod. Endopod two-segmented, basal segment longer than the distal one, armed with a short seta on the distolateral side; distal segment Fig. 1. Pennella balaenopterae. A. Nauplius, ventral view. B. First antenna, ventral. C. Second antenna, ventral. D. Mandible, dorsal. Scale bars 50 mm. Arroyo & al. – The first naupliar stage of Pennella balaenopterae 335 Table 1. Pennella balaenopterae. Measurements in mm of the nauplius larvae and appendages. Length Larvae Balancers First antenna Segment Setae Second antenna Endopod Exopod Setae Mandible Endopod Exopod Setae Width N L Lmean SD N W Wmean SD 9 4 290–385 123–170 323.2 148.2 32.6 22.9 9 4 190–263 3.9–4.1 223.8 3.9 26.8 0.1 3 6 82–97 92–115 91 106 – 7.6 3 20–25 22 – 2 2 6 45–49 41–57 92–117 47 49 100 – – 10.7 2 2 16–21 13–14 18.5 13.5 – – 2 2 2 28–39 49–69 116–121 33.5 59 118.5 – – 2 2 11–12 11–13 11.5 12 – – N – Number of measures; L – Length; W – Width; Lmean – Mean length; Wmean – Mean width; SD – Standard deviation. with two long apical setae and a short seta on its medial margin. Exopod four-segmented, basal segment much longer than the other three. Each segment with a seta in the distomedial corner but with no other armature. A first trace of the labrum was distinct in some specimens, but no sign of the maxillae or the swimming appendages could be detected in any of them. Balancers protrude from a slight swelling, usually positioned downwards and often appearing coiled due to preservation (Fig. 1A; Table 1). DISCUSSION The first nauplius of Pennella balaenopterae resembles those of other pennelid species described previously, both in its general appearance and its appendages (Sproston 1942; Kabata 1976; Schram 1979). The lack of rudiments of further stages in the specimens found points to them being a “newly hatched first nauplius” stage. However, differences in some aspects of their morphology suggest that they were in different developmental phases when they died. The appendages differ only slightly from the previously described species of the family Pennellidae in the setation of the different segments and in that setae of Pennella balaenopterae are naked while other species present plumose or semi-pinnate setae (Kabata 1976; Schram 1979). Lernaeenicus sprattae also has a single segmented first antenna, armed with three setae but with up to four spines (Schram 1979), instead of the two spines in the present specimens. The first antenna of Haemobaphes diceraus Wilson, 1917 shows basically the same simplified structure, resembling also that of Haemobaphes cyclopterina Fabricius 1780, as described by Heegaard (1947). The characteristic feature of the first nauplius of the former species is a previously unknown claw-like spine arising from a papilliform outgrowth at the base of the first antenna, which has not been found on any other nauplius (Kabata 1976). Both Lernaeocera branchialis and Haemobaphes diceraus (Sproston 1942; Kabata 1976) show the same number of segments in the second antenna as Pennella balaenopterae, with an indistinctly segmented exopod, and the same disposition of apical setation. Lernaeenicus sprattae has a clearly four-segmented exopod and a two-jointed endopod, each with a very similar setation pattern (Schram 1979). However, in other Siphonostomatoid families, species such as Eudactylina similis T. Scott, 1902 (Eudactylinidae), Caligus elongatus von Nordmann, 1832 (Caligidae) and Pseudocharopinus dentatus (Wilson, 1912) (Lernaeopodidae), present an exopodite with five segments, as is the case of poecilostomatoid nauplii (Kabata 1976; Piasecki 1996). A three- to four-segmented exopod therefore seems characteristic of pennelid nauplii. Heegaard (1947) used the naupliar mandible as one of the main features distinguishing his Fistulata and Pectinata, taxa corresponding approximately to the poecilostome and siphonostome copepods, respectively. According to him, the nauplii of the former have mandibles with two-segmented endopods, whereas the corresponding ramus in the mandible of the latter consists of three segments. Sproston (1942) found a three-segmented endopod on Lernaeocera branchialis, but Kabata (1976) failed to corroborate Heegaard’s findings. He proved that the mandibles of the poecilostomes Ergasilus turgidus Fraser, 1920, Bomolochus cuneatus Fraser, 1920, and Chondracanthus gracilis Fraser, 1920 indeed had two-segmen- 336 Sarsia 87:333-337 – 2002 ted endopods, but that the same was true of the two naupliar mandibles of the siphonostomes Eudactylina similis and Haemobaphes diceraus. A two-segmented endopod was also observed in the mandibles of our specimens of Pennella balaenopterae, and in the first and second nauplii of Lernaeenicus sprattae (cf. Schram 1979), corroborating further Kabata’s findings. Moreover, a one-segmented endopod was found by Piasecki (1996) in the mandibles of both the first and second nauplii of the siphonostome caligid, Caligus elongates von Nordmann, 1832. At the time of collection no parasites had ovisacs, but on arrival at the laboratory new egg strings had been formed; extruded during their transfer to the laboratory. The embryos may have been released from these egg strings during transport, perhaps aided by the increase in temperature experienced by the parasites in this container. It has been proved that an increase in temperature enhances and accelerates the oviposition process (Nakai 1927; Gnanamuthu 1951; Schram 1979), allowing the nauplii to hatch in a much shorter time (Schram & Anstenstud 1985). Embryos may develop even once the adult female is dead, the nauplii hatching several days later (Schram 1979). The presence of both nauplii and eggs in the alcoholfixed material, plus additional eggs inside the ovisacs, suggests that perhaps fixation interrupted the oviposition and hatching processes. However, some authors describe nauplii hatching directly from the egg strings (Sproston 1942; Heegaard 1947; Schram 1979), which could also have been the case here. Then, the embryos (eggs and fully formed nauplii) could have been forced out of the egg string in response to fixation. The nauplius herein described could be the first of a series of moults, or the only free-swimming nauplius preceding the copepodid stages, as described previously for Pennelids (Sproston 1942; Heegaard 1947; Kabata 1976). The second nauplius described by Schram (1979) for Lernaeenicus sprattae could be distinguished from the first one by its size, shape and pigment tint, but most easily by the presence of rudiments of the copepodid seen inside the nauplius. Schram noted the difficulty in detecting these differences and suggested that a second naupliar stage could have been ignored or overlooked in previous studies. In our case, no rudiments of maxillae or other appendages were observed. Features such as differences in eye pigmentation indicate that the specimens belong to different developmental phases, but always within the first newly hatched nauplius, as described by Schram (1979). With this finding we can conclude that Pennella balaenopterae hatches in the form of a nauplius from adult females attached to a cetacean definitive host. Whether cephalopod species are the intermediate host on which infective copepodids develop through the chalimus stages to adults, and on which mating takes place, as suggested for other members of the genus (Rose & Hamon 1953; Pascual 1996; Carbonell & al. 1999), needs further studies which presumably would also provide better descriptions of these stages and the yet unknown male. ACKNOWLEDGEMENTS Thanks are due to Hans-Uwe Dahms for his help during the interpretation and drawing of the larvae and for revision of an earlier version of the manuscript, and to the Zoosystematisch und Morfologie Arbeitsgruppe, University of Oldenburg, where the drawings were made. The authors thank G. Garcı́a Castrillo and C. Rodriguez, who were responsible for the whale’s autopsy, for their help in collecting the specimens of Pennella balaenopterae, and to three anonymous referees for their valuable comments. T. Schram and S. Pascual also provided useful references. REFERENCES Abaunza P, Arroyo NL, Preciado I. 2001. A contribution to the knowledge on the morphometry and the anatomical characters of Pennella balaenopterae (Copepoda, Siphonostomatoida, Pennellidae), with special reference to the buccal complex. Crustaceana 74:193–210. Bennet PS. 1961. Peroderma cylindricum Heller, a copepod parasite of Sardinella albella. 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