BULLETIN OF MARINE SCIENCE, 79(3): 839–845, 2006

SPERM MORPHOLOGY OF THE BLACK ( indica) DIFFERS FROM SCOMBROID SPERM

Kaye M. van der Straten, Bruce B. Collette, Luke K.-P. Leung, and Stephen D. Johnston

Abstract To assess the relationships of with other scombroids, sperm morphol- ogy of the black marlin, Makaira indica (Cuvier, 1832), was compared with that of published data for scombroids such as bluefin , Thunnus thynnus (Linnaeus, 1758), little tunny, alletteratus (Rafinesque, 1810) and sailfish, Istiopho- rus platypterus (Shaw in Shaw and Nodder, 1792). Testicular tissue samples were collected from four spawning black marlin from the Coral Sea, Australia, and pro- cessed for standard transmission electron microscopy. The ultrastructure of black marlin (and sailfish) sperm revealed morphological features characteristic of the simple type I teleost sperm. In contrast, the more advanced type II sperm ultra- structure is exhibited by the bluefin tuna, little tunny, and other scombroids. This lends support to the hypothesis that the billfishes are distinct enough from and other scombroids (Scombroidei) to warrant placement in a separate suborder: Xiphioidei.

There has been a debate in the literature as to the phylogenetic relationships within the suborder Scombroidei, summarized by Carpenter et al. (1995). Scombroids in- clude the families Istiophoridae (the billfishes–marlin, spearfishes, and sailfishes), Xiphiidae (an additional –the ), (tunas and ), Sphyraenidae (), Trichiuridae (), (snake macker- els), and Scombrolabracidae (black mackerels). Competing morphological cladistic hypotheses of the Scombroidei by Collette et al. (1984) and Johnson (1986) posi- tion the swordfish, Xiphias gladius (Linnaeus, 1758), plus the billfishes within the suborder Scombroidei, or within an expanded Scombridae as sister to the wahoo, Acanthocybium solandri (Cuvier, 1832). A third hypothesis is that billfishes are the sister-group of scombroids (Finnerty and Block, 1995) and a fourth hypothesis is that billfishes are not closely related to scombroids (Potthoff and Kelley, 1982; Nakamura, 1983, 1985; Richards, 2005). Recent molecular data supports the fourth hypothesis (Orrell et al., 2006; Fig. 1). Jamieson and Leung (1991) and Mattei (1991) both advocate the use of sperm mor- phology as a tool to identify the degree of relatedness among groups. Among the Teleostei, two main morphological sperm forms have been described as types I and II (Jamieson and Leung, 1991). The type II sperm occurs widely among the , and is considered more evolutionarily advanced than the type I sperm (Jamieson and Leung, 1991). The primary distinction of type II sperm (from type I) is that the centriolar complex lies outside of the nuclear fossa and the flagellar axis is parallel to the nuclear base (Fig. 2, Table 1). In this study, which is part of a larger study on the male reproductive characteristics of the black marlin, Makaira indica (Cuvier, 1832), transmission electron microscopy was used to view the ultrastructure of sperm. The morphological characteristics of black marlin sperm were then compared with pre- viously published data on sperm ultrastructure from bluefin tuna, Thunnus thynnus (Linnaeus, 1758) and little tunny, Euthynnus alletteratus (Rafinesque, 1810) (Abascal

Bulletin of Marine Science 839 © 2006 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 840 BULLETIN OF MARINE SCIENCE, VOL. 79, NO. 3, 2006

Figure 1. Phylogenetic relationships of Xiphioidei (1) and Scombroidei (2) [maximum likelihood analysis from Orrell et al. (2006): fig. 2] et al., 2002); two species of (Linnaeus, 1758) (Mattei and Mattei, 1976); and sailfish,I stiophorus platypterus (Shaw in Shaw and Nodder, 1792) (Mattei, 1991).

Materials and Methods

Animal and Sperm Collection.—Testicular tissue samples were collected from four sexually mature black marlin (59–81 kg) captured on rod and reel during October and No- vember 2004. Black marlin specimens were located on the oceanic side of the Great Barrier Reef (north eastern Australia) in the vicinity of the Trinity Opening (16°18´E–16°27´E and 146°00´–146°10´S). All specimens were caught on trolled dead baits of shark , Gram- VAN DER STRATEN ET AL.: BILLFISH SPERM 841

Figure 2. Morphology of types I and II teleost sperm. Adapted from Jamieson and Leung (1991). DC – distal centriole, FL – flagellum, M – mitochondria, N – nucleus, PC – proximal centriole. matorcynus bicarinatus (Quoy and Gaimard, 1825) or bonito tuna, (Cantor, 1849) in water temperatures between 26.5–26.8 °C. Marlin were secured by the insertion of a gaff under the lower jaw or in the shoulder muscle and euthanized by applying a strong blow to the top of the cranium. A mid-ventral incision was made along the belly groove, from the base of the pelvic fins to the urogenital groove to open the peritoneal cavity and allow recov- ery of the testes. Preparation of Tissue for Light and Electron Microscopy.—The sperm duct was located and the testis sliced into quartered sections. Cubes of testicular tissue (1.0 mm3) were removed for fixation from the center of the testis and adjacent to the sperm duct from all four specimens. Tissue samples were fixed in 3.0% glutaraldehyde in 0.1 M cacodylate buffer and stored at 4.0 °C within fixative for 10 d prior to further processing for standard transmission electron microscopy (Dawes, 1971; Hayat, 1972). Testicular samples were washed in 0.1 M cacodylate buffer and post-fixed in 1.0 % osmium tetroxide in 0.1 M cacodylate buffer for 60 min at 4.0 °C. Osmicated samples were washed in 0.1 M cacodylate buffer, followed by two Table 1. Defining morphological characteristics of Teleostei sperm types I and II.

Characteristic Type 1 Type 2 Level of development Primitive Advanced Typical orders* Non-Perciformes Perciformes Nuclear fossa Deep Shallow Position of Centriolar complex Within deep nuclear fossa Outside of shallow nuclear fossa Rotation of nucleus during Yes No spermiogenesis Position of flagellum relative to the Extends longitudinally from Lies parallel to the base of the base of the nucleus within the nuclear fossa nucleus * These sperm types are found in the majority of the families contained by these orders. For further detail, see Jamieson and Leung (1991). 842 BULLETIN OF MARINE SCIENCE, VOL. 79, NO. 3, 2006

Figure 3. Morphological characteristics of black marlin sperm and bluefin tuna sperm. Bluefin tuna schematic adapted from micrographs published by Abascal et al. (2002). DC - distal centri- ole, FL – flagellum, M – mitochondria, N – nucleus, PC – proximal centriole. washes in ultra high quality water. Samples were then processed through a series of acetone dehydrations and epon resin infiltrations, and embedded and polymerized in 100% epon resin blocks overnight at 60.0 °C. Thin sections for transmission electron microscopy (70–90 nm) were cut on a Diatome 45 degree diamond knife. Sections were mounted on hexagonal 200 mesh copper grids and stained with 5.0% uranyl acetate in 50.0% ethanol and Reynold’s lead stain (3.0% lead citrate) (Reynolds, 1963). Grids were examined using a Jeol JEM 1010 transmission electron micro- scope, and images were captured using Kodak electron image film SO-163 (8.3× 10.2 cm).

Results

The mature sperm of the black marlin is characteristic of a uniflagellate anacro- somal aquasperm (Fig. 3; Plate 1A). The nucleus consists of condensed chromatin that is flattened about the rostral aspect and has a length: width measurement of 1.5: 1.0 µm (Plate 1B). A nuclear space encompasses the upper region of the nucleus and may contain chromatin bodies. The proximal centriole lies perpendicular to the fla- gellum and is situated approximately one third of the way into the posterior nucleus within a deep nuclear fossa. The distal centriole of the upper neckpiece is partially embedded within the nuclear fossa and is surrounded by large mitochondria. Mito- chondria are well organized at the base of the nucleus and form a ring around the flagellum and cytoplasmic canal known as the midpiece. The number of mitochon- dria varies from 4 to 6 between individual sperm. Lateral extensions emerge from the flagellar membrane along the length of the flagellum. The centre of the flagellum contains the axoneme with a typical 9+2 arrangement of microtubules. The sperm morphology exhibited in the black marlin is characteristic of the more simple type I teleost sperm ultrastructure (Fig. 2). VAN DER STRATEN ET AL.: BILLFISH SPERM 843

Plate 1. Electron micrograph of sperm ultrastructure in the black marlin. (A) Mature sperm, bar = 1 µm. (B) Longitudinal section of the sperm head and midpiece, bar = 200 nm. CYC – cytoplas- mic canal, DC – distal centriole, FL – flagellum, M – mitochondria, N – nucleus, NS – nuclear space, PC – proximal centriole.

Discussion

Our results demonstrate that the morphology of black marlin sperm is typical of the simple type I teleost sperm morphology-the centrioles are implanted within a deep nuclear fossa and the flagellum extends longitudinally from the base of the nu- cleus. This is an interesting finding for a perciform species which has previously been assumed to be one of the more evolved teleostean species (suborder Scombroidei), given that type II teleost sperm are considered to be the more advanced of the two sperm types (Jamieson and Leung, 1991). Under some classifications, a phylogenetic relationship exists between the bill- (Istiophoridae and Xiphiidae) and the tunas and relatives within the suborder Scombroidei (Carpenter et al., 1995). In direct contrast to our finding of type I sperm morphology in the black marlin, previously published work on the sperm ultrastruc- ture of the bluefin tuna and the little tunny (Abascal et al., 2002) identified the sperm of these scombroids as typical of the more advanced type II teleost sperm morphol- ogy. The two centrioles are approximately perpendicular to each other and situated at the base of the nucleus outside of the nuclear fossa in bluefin tuna sperm (Abascal et al., 2002). The flagellum of the bluefin tuna sperm extends parallel to the base of the nucleus and is asymmetrical to the sperm head (Abascal et al., 2002), clearly exhibiting the characteristics of the type II teleost sperm morphology. Furthermore, type II sperm morphology has been described in eight species of scombroids in the two families that have been examined: Trichiuridae-Trichiurus lepturus (Linnaeus, 1758) in Mattei and Mattei (1976) and T. japonicus (Temminck and Schlegel, 1844) in Hara and Okiyama (1998); and Scombridae-Scomber australasicus (Cuvier) and Scomber japonicus (Houttuyn, 1782) in Hara and Okiyama (1998), Scomber japonicus and tritor (Cuvier, 1832) in Mattei (1991), and E. alletteratus and T. thynnus in Abascal et al. (2002). 844 BULLETIN OF MARINE SCIENCE, VOL. 79, NO. 3, 2006

Interestingly, type I sperm morphology has been reported in another species of Istiophoridae, the sailfish I. platypterus (Mattei, 1991); while Tmo-4C4 and com- bined gene phylogenies have recovered separate clades for the istiophorids (Orrell et al., 2006). The findings of Mattei (1991) and Orrell et al. (2006) combined with the results presented here for black marlin sperm morphology provides substantial support to the fourth hypothesis-that billfishes are not closely related to scombroids (Potthoff and Kelley, 1982; Nakamura, 1983, 1985; Richards, 2005). These findings indicate that inclusion of the family Istiophoridae within the suborder Scombroidei requires revision, and it is postulated that the Istiophoridae may be better placed within a separate suborder, the Xiphioidei. However, additional species in both the Xiphioidei and Scombroidei suborders, particularly the swordfish, require further examination before this tentative conclusion can be verified.

Acknowledgments

J. Pepperell provided logistical advice and personal introductions that enabled access to the black marlin specimens used in this study. Thanks are extended to A. Wall (Cairns Gamefish- ing Association Australia) and the skippers and crew of the Cairns based vessels Diamond Girl, Moana, Billfish, and Powerplay who provided the black marlin specimens used in this study. Thanks to J. Nailon, R. Webb, and R. Gould from the Centre for Microscopy and Microanalysis at the University of Queensland for technical guidance and support with transmission electron microscopy. Finally, we extend our thanks to R. Rossini and S. Bates who assisted with the field processing of samples in Cairns.

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Addresses: (K.M.S., L.K.L., S.D.J.) The School of Studies, University of Queensland, Gatton Campus, Gatton, Queensland 4343, Australia. (B.B.C.) National Marine Fisheries Ser- vice Systematics Laboratory, National Museum of Natural History, Washington, D.C. 20560. Corresponding Author: (K.M.S.) E-mail: .