The first naupliar stage of Pennella balaenopterae Koren and Danielssen, 1877 (Copepoda: Siphonostomatoida, Pennellidae) Nina L. Arroyo, Pablo Abaunza & Izaskun Preciado 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. SARSIA 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 Biologı ´a Animal I (Zoologı ´a), Facultad de Biologı ´a, Universidad Complutense de Madrid, ES-28040 Madrid, Spain. Present address: A ˚ bo Academy University, Department of Biology Environmental and Marine Biology, FIN-20500 A ˚ bo, Finland. P. Abaunza & I. Preciado, Instituto Espan ˜ol de Oceanografı ´a, Centro Oceanogra ´fico 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 & Mu ¨ller 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 free- swimming 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 & Lu ¨tken, 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 Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway