Acta Zoologica (Stockholm) 84: 131– 137 (April 2003) © 2003 The Royal Swedish Academy of Sciences  Abstract Medina, A., Megina, C., Abascal, F. J. and Calzada, A. 2003. The sperm ultrastructure of Merluccius merluccius (Teleostei, Gadiformes): phylogenetic considerations. — Acta Zoologica (Stockholm) 84: 131–137 The anacrosomal aquasperm of the gadiform Merluccius merluccius is ultrastructurally similar to the advanced type II spermatozoa (perciform-type sperm) typically found in most Perciformes.The perciform-type spermatozoon is characterized by the lateral insertion of the flagellum and the location of the centrioles outside the nuclear fossa. Apart from these characteristics, the spermatozoon of M. merluccius is remarkable because of the mutually parallel arrangement of the centrioles, a rare feature among fishes, which is considered an apomorphic condition for animal sperm cells. Within the superorder Paracanthopterygii, which contains a large diversity of sperm patterns resulting from a high number of apomorphies, a perciform-type sperm is present only in the order Gadiformes. The significance of the presence of perciform-type spermatozoa in the three investigated gadiform families is discussed in a phylogenetic context. Antonio Medina, Departamento de Biología, Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz, E-11510 Puerto Real, Cádiz, Spain. E-mail: antonio.medina@uca.es Blackwell Publishing Ltd. The sperm ultrastructure of Merluccius merluccius (Teleostei, Gadiformes): phylogenetic considerations Antonio Medina, César Megina, Francisco J. Abascal and Alfonso Calzada Departamento de Biología, Facultad de Ciencias del Mar, Universidad de Cádiz, República Saharaui s/n, E-11510 Puerto Real, Cádiz, Spain Keywords: Gadiformes, Merluccius merluccius, phylogeny, sperm ultrastructure, Teleostei Accepted for publication: 11 December 2002 Introduction Since the advent of electron microscopy, metazoan compar- ative spermatology has contributed considerably to a better understanding of the phylogenetic relationships between animals. In fishes in particular, a preliminary overview by Mattei (1970) laid the foundations for the comparative study of sperm ultrastructure from an evolutionary viewpoint. Two further reviews (Jamieson 1991; Mattei 1991) made evident the usefulness of spermatozoal ultrastructure in the investigation of phylogenetic relationships in various fish taxa. However, ichthyologists have paid little attention to the comparative analysis of spermatozoal patterns for systematic purposes. Mattei (1970) classified the simple anacrosomal aquasperm of teleosteans into two principal sperm types. In the apo- morphic type II spermatozoon, the so-called perciform type, the centrioles remain outside the nuclear fossa and the flagellum inserts eccentrically into the sperm head. This sperm morphology is the most widely distributed among the perciforms, although it is not exclusive to them as it is also found in the Gadiformes (Jamieson 1991; Mattei 1991; Lahnsteiner et al. 1994). Within the gadiforms, sperm ultrastructure has been studied previously in only three species from three different families, Laemonema laureysi Poll (Moridae) (Mattei 1991), Merluccius polli Cadenat (Merlucciidae) (Mattei 1991), and Lota lota (L.) (Gadidae) (Lahnsteiner et al. 1994), therefore the sperm morphology in this teleost order remains very poorly known.The asymmetr- ical emplacement of the flagellum in the spermatozoa of L. laureysi and M. polli (perciform-type arrangement) was considered by Mattei (1991) to be a character sufficiently sound to recognize within the Gadiformes a clear phylogenetic affinity between the families Moridae and Merlucciidae, justifying their grouping into the same suborder (Gadoidei). Later, Lahnsteiner et al. (1994) showed a similar synapomor- phic insertion of the sperm flagellum lateral to the nucleus in the burbot, Lota lota (Gadidae). In this paper we make a comparative study of the sperma- tozoal ultrastructure of the European hake, Merluccius merluccius (L.). Our aim is to enlarge the current knowledge of specific characters of gadiform spermatozoa to contribute to the construction of phylogenetic arrangements in the Paracanthopterygii, a phylogenetically ambiguous teleostean