Descriptive anatomy of Iso rhothophilus (Ogilby), with a phylogenetic analysis of Iso and a redefinition of Isonidae (Atheriniformes).

Abstract

The musculoskeletal anatomy of Iso rhothophilus is described. A phylogenetic analysis of the genus, using eleven anatomical characters, indicates that the systematic hierarchy is (Iso flosmaris (I. nesiotes (I. rhothophilus (I. hawaiiensis, I. natalensis)))). Isonidae Rosen, 1964 is redefined and, on the basis of twenty characters (autapomorphic within Atheriniformes) shown to be distinct from Notocheiridae Schultz, 1950.

Keywords

Isonidae, phylogenetic analysis, anatomy, osteology 

Zusammenfassung

Beschrieben wird die Anatomie des Muskelskeletts von Iso rhothophilus. Nach einer phylogenetischen Analyse der Gattung anhand von elf anatomischen Merkmalen muss man auf die folgende systematische Hierarchie schlieBen: (Iso flosmaris (I. nesiotes (I. rhothophilus (I. hawaiiensis, I. natalensis)))). Die Isonidae Rosen, 1964, werden neu definiert und ihre Unterscheidung zu den Notocheiridae Schultz, 1950, auf der Grundlage von 20 Merkmalen (autapomorphisch innerhalb der Atheriniformes) festgestellt.

Resume

On decrit l'anatomie musculo-squelettique d'Iso rhothophilus. Une analyse phylogenetique du genre, a l'aide de onze caracteristiques anatomiques, montre que la hierarchie systematique est (Iso flosmaris (I. nesiotes (I. rhothophilus (I. hawaiiensis, I. natalensis)))). Isonidae Rosen, 1964 est redefini et, a l'aide de vingt caracteristiques (autapomorphiques pour les Atheriniformes), on montre sa difference par rapport a Notocheiridae Schultz, 1950.

Sommario

L'anatomia muscoloscheletrica di Iso rhothophilus e descritta in dettaglio. Un'analisi filogenetica del genere, eseguita con undici caratteri anatomici, indica che la gerarchia sistematica e (Iso flosmaris (I. nesiotes (I. rhothophilus (I. hawaiiensis, I. natalensis)))). La famiglia Isonidae Rosen, 1964 viene ridefinita e, sulla base di venti caratteri (autapomorfici entro gli Ateriniformes) si dimostra distinta dai Notocheiridae Schultz, 1950.

Introduction

Isonids are small fishes that have been infrequently collected in surf, usually near rocks, at the margins of the Pacific, Indian and Atlantic Oceans. The history of the genus Iso and the designation of the family Isonidae are fraught with confusion. Ogilby (1895) described the first species of the "flower of the wave" (as it was subsequently commonly called) as Tropidostethus rhothopilus from Maroubra Bay, NSW, Australia. In 1901, Jordan and Starks named another species as Iso flosmaris from Japan. A few years later Waite (1904) indicated that Iso was indistinct from Tropidostethus, then, in 1919, Jordan and Hubbs pointed out that Tropidostethus was preoccupied by a genus of insects thus determining the validity of the name Iso. Other species ascribed to the genus were I. natalensis Regan, 1919, I. flosindicus Herre, 1944, I. hawaiiensis Gosline (1952) and I. nesiotes Saeed, Ivantsoff & Crowley, 1993. A description of Notocheirus hubbsi by Clark in 1937 was noted by Hubbs (1944) as a species which was Iso-like but according to him was likely to be an offshoot of Atherinopsinae.

Further confusion arose with the description of a subfamily of Tropidostethinae by Schultz in 1948. According to him this subfamily included Notocheirus, Tropidostethus and Iso, presumably considering that the type genus of the subfamily was Tropidostethus. In 1950 Schultz appears to have realised that Tropidostethus was preoccupied, having substituted the name with Tropidosthetops, at the same time changing the subfamilial name to Notocheirinae, and stating that the new subfamily was based on the genus Notocheirus Clark.

In a review of Iso Jordan and Starks (Said, 1983), five species were considered valid, and the genus was considered distinct from Notocheirus Clark. Said (1983) noted that Isonidae Rosen, 1964 was erected on the basis of examination of Notocheirus hubbsi Clark, a monotypic genus previously assigned type status for Notocheirinae Schultz, 1950.

In a review of the systematic position of Isonidae (Saeed et al., 1993), the family was considered to be distinct from Notocheiridae. However, the most recent review of Atheriniformes (Dyer and Chernoff, 1996) contested that view. The present authors therefore decided to further examine isonid diagnostics and relationships.

Materials

Type material examined for each species is listed first, followed by materials prepared for osteological and/or micrographic examination. The number and size of specimens follows institutional identification. Institutional abbreviations are as follows: AMS: Australian Museum, Sydney; BMNH: British Museum (Natural History); BPBM: CAS: Californian Academy of Science; MU: Macquarie University, Sydney; SOSC: Smithsonian Oceanographic Sorting Center, Washington DC; USNM: Smithsonian Institution, Washington, DC; ZUMT: Department of Zoology, University of Tokyo.

Notocheirus hubbsi: CAS 5525 (holotype), CAS 5526 (paratype, disarticulated); MU I-307 (2, 32 mmSL). Iso flosmaris: CAS SV6527 (holotype); CAS 7142 (10, paratypes); SOSC VGS-68-30 (4, 22-60 mm SL); ZUMT 5918 (1, 54 mm SL). Iso hawaiiensis: USNM 152759 (holotype); BPBM 15249 (2, paratypes); BPBM 10012 (1, 28 mm SL). Iso natalensis: BMNH 1919.4.1.13 (holotype); CAS 3384 (3, 25-35 mm SL); MU I-227 (3, 28-45 mm SL). Iso nesiotes: AMS I241183-001 (holotype); BPBM 16718 (1, 22 mm SL); MU I-210 (5, 20-27 mm SL). Iso rhothophilus: BMNH 1897.3.20-22 (holotype); MU I-122 (19, 21-55 mm SL); SOSC VGS-68-22 (2, 21-48 mm SL); ZUMT 4158 (5, 38-46 mm SL).

Methods

Specimen clearing and staining, examination and illustration techniques, and preparation for electron micrography, follow Saeed et al., 1993. Anatomical terminology is based on Winterbottom (1974).

Phylogenetic analysis was based on characters identified by comparison of specimens of five Iso spp. Notocheirus hubbsi was selected as the outgroup. Character states were coded 0, 1 or 2 by the outgroup comparison method. All characters were considered unordered, and all characters assigned equal weight. The phylogeny was derived using a closest pair comparison method i.e. the two species with most shared plesiomorphies were joined, this pair was then joined to the next most similar species and so on. Isonidae Rosen, 1964

Type genus Iso Jordan and Starks, 1901 (replacing Notocheirus Clark, 1937).

Diagnosis

A monogeneric atheriniform family with the parasphenoid concave rather than convex, parasphenoid myodomes present; mesethmoid morphology unique, nasal septum formed by ethmoid and mesethmoid; parietals not contributing to posttemporal fossa; anterior vertebral parapophyses with lateral ridges; interdorsals absent; cleithrum dorsal enclosure absent; membrane just posterior to genital opening; and pelvic posteromedial process elongate.

Iso Jordan and Starks, 1901

Tropidostethus Ogilby, 1895:323, type species: Tropidostethus rhothopilus by monotypy, preoccupied by Tropidostethus Philippi, 1863 in Orthoptera. Waite, 1904a:234; Schultz, 1948:26; Munro, 1958: 99.

Iso Jordan and Starks, 1901: 205, type species: Iso flosmaris by monotypy. Waite, 1904b:21; Jordan and Hubbs, 1919: 47; Regan, 1919: 200; McCulloch, 1929: 110; Herre, 1944: 47; Gosline, 1952: 49; Golvan, 1959: 73; Rosen, 1964: 227; Smith, 1965: 603.

Tropidostethops Schultz, 1950: 150, type species: Tropidostethus rhothophilus Ogilby, 1895 (replacement name for Tropidostethus, which is preoccupied).

Iso rhothophilus (Ogilby, 1895)

External morphology

Body highly compressed, deepest at level of vertical plane through origin of pectorals, tapering towards caudal peduncle (Fig.1). Ventral edge of abdomen tapering to keel-like form. Head small, truncate posteriorly, snout round, mouth oblique. Premaxillary non-protractile, attached to head by mid-dorsal frenum. Maxillary slightly concave anteriorly. Lower jaw deeply elevated posteriorly, placed medial to upper elements.

[FIGURE 1 OMITTED]

Premaxillary and dentary teeth small, well developed, curving backwards into mouth. Vomerine teeth present in some specimens. Palatine teeth absent. Gills opening widely. Gill rakers on first gill arch well developed. Anus just anterior to origin of anal fin. Small membrane posterior to genital opening (Fig. 2).

[FIGURE 2 OMITTED]

First dorsal fin originating approximately at vertical through midbody, of III-VI small weak spines (Fig. 1). Second dorsal originating posterior to vertical through anal origin, comprising spine, unbranched ray and 9-16 branched rays. Anal fin comprising spine, unbranched ray and 19-27 branched rays. Second dorsal and anal high anteriorly, tapering posteriorly. Pectoral fin comprising spine, unbranched ray and 11-14 branched rays. Pectoral compressed, broad, inserted high on body with dorsalmost ray originating above posteriormost point of opercle. Pectoral posterior angle rounded, covering origin of midlateral band. Pelvics compressed, inserted low on body.

Scales in midlateral series 42-55. Body silver, dark midlateral band from axilla to tail, discontinuous on caudal peduncle. Scales ovoid, absent from cranial region. Larger specimens to 60 mm SL.

Musculoskeletal anatomy

Cranium: Rostrum elongate (Fig. 3). Ethmoid cartilage partially exposed laterally to form facet for articulation with palatine. Lateral ethmoid with cartilaginous condyle for articulation with lacrimal, supported dorsally and ventrally by osseous laminae. Lateral ethmoid with cartilaginous process contacting palatoquadrate cartilage, ventral to condyle for lacrimal. Lateral ethmoid closely contacting contralateral structure, not forming articulation with parasphenoid. Lateral ethmoid medial border with notch for olfactory nerve.

[FIGURE 3 OMITTED]

Rostral cartilage bound to premaxilla dorsal process. Dorsal mesethmoid absent. Vomer with bilateral maxillary condyles (Fig. 4). Parasphenoid enlarged posteriorly, embedded in basioccipital. Parasphenoid anterior margin planar, interposed between ethmoid cartilage and vomer. Parasphenoid ventral ridge compressed, blade-like, extending posteriorly.

[FIGURE 4 OMITTED]

Nasal irregular to triangular, separated from contralateral structure. Nasal cavity extending medially, separated from contralateral structure by thin ventromedian lamina of mesethmoid. Accessory nasal sac located medial to anterior infraorbitals.

Frontals forming major portion of roof of skull. Frontal truncate anteriorly, not extending forward over nasal cavity, thereby partially exposing ethmoid. Frontal contacting mesethmoid but not lateral ethmoid. Interorbital portion of frontal separated from supraorbital laminae by deep sulcus forming supraorbital sensory canal (Fig. 5). Parietal large.

[FIGURE 5 OMITTED]

Supraoccipital drawn into long median process deep to frontals. Epiotic large, bearing crest for attachment of epaxial musculature (lesser crests also occurring on supraoccipital, exoccipital and pterotic). Posttemporal fossa formed by pterotic and sphenotic, and in part by epiotic and parietal, bounded laterally by osseous temporal canal. Temporal canal fused to pterotic, extending anteriorly to form junctions with supraorbital and infraorbital canals. Foramen situated at anterior temporal canal, superficial to sphenotic, forming nexus between dilator operculi fossa and canal. Dilator operculi fossa giving origin, in part, to levator operculi muscle. Anterior wall of fossa formed by sphenotic postorbital process.

Sphenotic and pterotic forming facets articulating with hyomandibula. Subtemporal fossa medial and deep to posttemporal fossa, giving origin to branchial levator musculature.

Pterosphenoid separated from contralateral structure, lateral margin contacting frontal. Pterosphenoid, and ventral ridge originating on frontal, supporting membrane enclosing anterior lobe of brain. Membrane attached posteriorly to prootic and basisphenoid. Basisphenoid bearing bilateral posterolateral rami contacting prootic.

Orbits separated medially by septum attached to parasphenoid. Ventromedian projection from prootics extending between bilateral rami of basisphenoid. Prootic jugular canal deep to osseous trabeculum. Posterior jugular opening confluent with emanation of hyomandibular nerve.

Otic bulla small, extending ventrolaterally to parasphenoid. Pterotic bearing projection superficial to exoccipital.

Occipito-vertebral articulation formed by spherical basioccipital condyle and paired small, separate exoccipital condyles.

Jaws: Premaxilla with moderate symphysis, alveolar ramus slightly smaller than maxilla (Fig. 6). Premaxilla postmaxillary processes supporting skin fold between premaxilla and maxilla. Premaxilla teeth forming single row. Maxilla narrow, positioned lateral to premaxilla. Maxilla proximal angle narrow, distal angle broad. Maxilla bearing large condyle for articulation with vomer, biconcave submaxillary meniscus interposed between maxilla and vomer. Maxilla dorsal and internal processes formed about premaxilla dorsal process. Maxilla joined anteriorly to contralateral structure ligamentously. Premaxilla process providing attachment for articularmaxillary ligament (Fig. 7), originating from articular lateral face. Rostral cartilage supporting premaxilla dorsal process. No ligaments between maxilla and palatine.

[FIGURES 6-7 OMITTED]

Ethmomaxillary ligament originating on mesethmoid, inserting on maxilla dorsal process. Labial ligament between anterolateral face of dentary and distal angles of maxilla and premaxilla. Coronomaxillary ligament absent.

Adductor mandibulae muscle inserting in triangular recess dorsal to Meckelian cartilage between dentary and articular. Meckelian cartilage elongate (Fig 8), joining dentary and articular medially. Coronomeckelian bone, on dorsal surface of Meckelian cartilage adjacent to articular, acting as insertion for tendon of adductor mandibulae muscle. Articular with retrorse condyle on posterodorsal facet serving as insertion for ligament from quadrate. Retroarticular bone small, at angle of lower jaw, overlapping medially with articular.

[FIGURE 8 OMITTED]

Suspensorium: Comprising palatoquadrate arch, ectopterygoid, mesopterygoid, ectopterygoid, symplectic, hyomandibula and preopercle (Fig 9). Two articulations formed with cranium, bilaterally: anterior articulation with palatine, posterior articulation with hyomandibula. Palatine and quadrate joined about interposed cartilage, cartilage also contacting ethmoid. Palatine cartilage progressively ossifying. Palatine maxillary process comprising cylindrical bone with cartilaginous core. Palatine bearing posteroventral process overlapping ectopterygoid.

[FIGURE 9 OMITTED]

Mesopterygoid expansive, thin, forming ventromedian limit of orbit. Metapterygoid and quadrate joined about narrow interposed cartilage. Quadrate and symplectic with elongate articulation, both bones with broad ventral facets articulating with preopercle. Metapterygoid not articulating with cranium.

Hyomandibula with ventral projection joined to preopercle ventral ramus. Hyomandibula articulating dorsally with cranium, hyomandibula posterior condyle articulating with opercle.

Opercular series: Comprising opercle, interopercle and subopercle. Opercle large, elongate, bearing glenoid fossa articulating with hyomandibula. Dilator operculi muscle inserting on medial face of triangular dilator process of sphenotic and pterotic. Levator operculi muscle inserting on dorsal and medial surfaces of opercle. Interopercle wide, bearing dorsal process. Interopercle and branchiostegal rays covering gills ventrally. Interopercle joined to lower jaw ligamentously. Interopercle joined to ceratohyal by short ligament. Subopercle elongate, forming portion of gill cover. Neurosensory network: Sensory canals developed on nasal, frontal, pterotic, posttemporal, preopercle, infraorbital, dentary and articular bones, lateral line absent. Infraorbital series comprising four elements. Lacrimal (infraorbital 1) horizontal axis elongate, located superficial to premaxilla (when mouth closed), bearing facets for articulation with lateral ethmoid, posterior border with recess contacting lateral ethmoid lacrimal condyle (Fig. 3). Lacrimal medial face bearing osseous shelf, coursing ventral to nasal sac. Second infraorbital planar, irregular. Third infraorbital cylindrical. Dermosphenotic at posterolateral margin of orbit, contacting sphenotic postorbital process.

Supraorbital canal coursing antero-posteriorly from nasal medial margin, along frontal sulcus, curving laterally to confluence with temporal and infraorbital canals. Temporal and preopercular canals not conjoined. Mandibular sensory canal coursing along ventral border of articular and dentary. Canal openings on dentary ventral surface.

Hyobranchial apparatus: Comprising median unpaired glossohyal, basihyal, three basibranchials and urohyal; and bilaterally paired dorsal and ventral hypohyals, anterior and posterior ceratohyals, interhyal, six branchiostegal rays, three hypobranchials, five ceratobranchials, four epibranchials and three pharyngobranchials (Fig.10).

[FIGURE 10 OMITTED]

Basihyal rhomboidal, ossified posteriorly, strongly attached to hypohyals. Basihyal cartilaginous anterior portion supporting small tooth plate. Urohyal with bilateral posterodorsal processes and median posteroventral process. Three median basibranchials coursing antero-posteriorly from basihyal to irregular cartilaginous nodule. Posteriormost two basibranchials fused dorsally to respective tooth plates.

Hyoid bar comprising dorsal and ventral hypohyals, and anterior and posterior ceratohyals. Hyoid bar joined to contralateral structure by strong ligament from medial face of hypohyals, coursing ventral to basihyal. Anterior and posterior ceratohyals joined about dentate suture. Anterior ceratohyal ventral surface giving origin to two anteriormost small branchiostegal rays, then two large rays rising from anterior ceratohyal lateral surface. Two posteriormost rays rising from posterior ceratohyal lateral surface. Elongate interhyal bone forming in interhyal ligament between posterior ceratohyal and hyomandibula. Posterior ceratohyal bearing lateral facet giving rise to ligament to interopercle.

Anterior three gill arches each of hypobranchial, ceratobranchial, epibranchial and pharyngobranchial, fourth arch of ceratobranchial and epibranchial, fifth arch comprising single ceratobranchial. Gill rakers of first gill arch elongate. Ceratobranchials elongate, arcuate, ventral surface bearing process for attachment of pharyngeal musculature. Fifth ceratobranchial posterolateral angle produced as site of attachment for posterior levator muscle.

Hypobranchials fused dorsally to respective tooth plates. Basibranchial tooth plate dorsal to, but not fused with, third hypobranchial. Tooth plates adjacent to, but not fused with, first four ceratobranchials and all epibranchials. Fifth ceratobranchial fused to large lower pharyngeal toothplate.

Axial skeleton: Total vertebrae 38-45, precaudal 14-18, caudal 22-28. First vertebra with reduced neural spine (Fig. 11), articulating with cranium. Neural arch rising from anterodorsal portion of respective vertebral centrum. Neural arch of vertebra 2-6/7 developed as expanded neural plate. Precaudal vertebrae with parapophyses rising from anteroventral portion of centrum. Anterior parapophyses small, directed laterally, anteroposterior series becoming larger, directed posteriorly to form haemal arches of caudal vertebrae. First haemal arch emanating from first caudal vertebra, anterior parapophysis directed ventromedially to fuse with contralateral structure, produced as haemal spine.

[FIGURE 11 OMITTED]

Total pleural ribs 12-14, first rib contacting third vertebra. Epural articulating with respective vertebral body posterodorsal to base of parapophysis. First epipleural contacting dorsolateral aspect of parapophysis of third vertebra, coursing posterodorsally to ribs.

Intervertebral articulations mediated by zygapophyses. Dorsal zygapophyses prominent on anterior vertebrae, ventral zygapophyses present on posterior precaudal and caudal vertebrae.

Penultimate vertebra bearing very large haemal spine. Terminal half-centrum supporting hypurals and parhypural. Parhypural fused with hypurals 1 and 2, separated by horizontal gap from fused hypurals 3 and 4. Uroneural coursing posterodorsally from terminal half-centrum, epural anterior to uroneural.

Fins and girdles: Each spine of first dorsal supported by individual basal radial comprising fused proximal and medial elements. No radials between dorsal fins. Each element of second dorsal and supported by basal radial and non-ossified distal radial, basal radials also supporting preceding element.

Pectoral fin with four proximal radials supporting fin rays, dorsal radial fused to scapula (Fig. 6A). Scapular foramen between scapula and cleithrum. Coracoid contacting cleithrum and scapula dorsally, ventrally forming medial shelf and additional contact with cleithrum in most specimens. Supracleithrum absent. Posttemporal trabecular, arcuate, superficial to epiotic dorsal crest. Dorsal postcleithrum absent, ventral postcleithrum contacting coracoid.

Pelvic fin without distal radials, fin elements supported directly (Fig. 12). Adductor profundus pelvicus muscle on dorsomedial aspect of pelvic, abductor superficialis on lateral aspect. Pelvic posteromedial process elongate, extending dorsally. Pelvic posteroventral process fused to contralateral structure.

[FIGURE 12 OMITTED]

Character analysis

The following characters, which differed between the outgroup and one or more ingroup taxa, were identified:

1. Midlateral band. In Notocheirus (Fig. 13) and I. flosmaris, the midlateral band is continuous. In I. hawaiiensis, I. natalensis and I. rhothophilus (Fig. 1), the band is discontinuous on the caudal peduncle, resulting in the presence of a 'spot' near the tail. In I. nesiotes (Saeed et al., 1993; Fig. 1), the band is absent from the posterior part of the caudal peduncle.

[FIGURE 13 OMITTED]

(0 = midlateral band continuous; 1 = band discontinuous; 2 = band foreshortened).

2. Premaxilla symphyseal facet. The premaxillae have large symphyseal portions in Notocheirus (Fig. 14), I. flosmaris, I. hawaiiensis and I. nesiotes. The symphysis is reduced in I. rhothophilus (Fig. 15) and small in I. natalensis (Fig. 16).

[FIGURE 14 OMITTED]

(0 = premaxilla symphyseal facet large; 1 = symphysis reduced ; 2 = symphysis small).

3. Premaxilla posterior angle expansive. In Notocheirus (Fig. 13) and I. flosmaris, the posterior angle of the premaxilla is broad. In other taxa (Figs. 15, 16) it is narrow/pointed.

[FIGURES 15-16 OMITTED]

(0 = premaxilla posterior angle broad; 1 = posterior angle narrow).

4. Premaxilla bearing laterally-placed teeth. There are no teeth lateral to the premaxilla in all taxa except I. natalensis (Figs. 16, 17).

[FIGURE 17 OMITTED]

(0 = Premaxilla lateral teeth present; 1 = teeth external to premaxilla).

5. Pterosphenoid anteromedial facet. In Notocheirus and I. hawaiiensis (Fig. 18) the pterosphenoid has a stright anteromedial facet. In other taxa this facet is curved (Fig. 19).

[FIGURES 18-19 OMITTED]

(0 = pterosphenoid anteromedial facet linear; 1= facet lunate).

6. Parietal. The parietal is large in Notocheirus (Fig. 20), I. nesiotes and I. rhothophilus (Fig. 5), and small in the other taxa (Fig. 21).

[FIGURES 20-21 OMITTED]

(0= parietal large; 1= parietal small).

7. Intercalar. The intercalar is absent in all taxa except I. hawaiiensis (Fig. 22).

[FIGURE 22 OMITTED]

(0 = intercalar absent; 1= intercalar present).

8. Basihyal teeth. There are no teeth on the basihyal of Notocheirus and I. flosmaris. Other taxa have teeth on the basihyal (Fig. 10).

(0 = basihyal teeth absent; 1 = teeth present).

9. Urohyal posterodorsal process. In all taxa the urohyal posterodorsal process is directed posterodorsally (Figs. 10B, 23) except in I. nesiotes, in which the process is directed dorsally (Saeed et al., 1993; Figs. 2Ca, 2Cb).

[FIGURE 23 OMITTED]

(0 = urohyal posterior process directed posterodorsally; 1 = process directed dorsally).

10. Anterior vertebral dorsal postzygapophyses. All taxa have well-defined processes (Fig. 11) except I. hawaiiensis (Fig. 24) in which the processes are not discernable.

[FIGURE 24 OMITTED]

(0 = anterior vertebral dorsal postzygapophyses well defined; 1 = processes not defined).

11. Coracoid. The coracoid is relatively small in Notocheirus (Fig. 25) and I. nesiotes (Saeed et al., 1993; Figs. 2Da, 2Db), and large in other taxa.

[FIGURE 25 OMITTED]

(0 = coracoid small; 1 = coracoid large). Eleven characters thus identified were entered into a data matrix (Table I). A second matrix was then developed to show the number of shared plesiomorphies between pairs of taxa (Table II). This was used to generate the cladogram (Fig. 26).

[FIGURE 26 OMITTED]

Discussion

The hierarchy of relationships in Iso was I. flosmaris (I. nesiotes (I. rhothophilus (I. hawaiiensis, I. natalensis))). This scheme postulates 5 reversals, while providing deep analysis of the genus. Said (1983) examined five species (and a number of putative hybrid forms), and used cluster and discriminant function analyses of osteological and/or morphological and/or meristic data, to examine relationships within Iso. Although no two hierarchies were identical, the majority of schemes supported (I. flosmaris ((I. rhothophilus, I. natalensis), (I. hawaiiensis, I. nesiotes))). The present scheme, based on a phylogenetic approach, supports the position of I. flosmaris as a sister group to the other four species, but is otherwise dissimilar.

Isonidae was erected by Rosen (1964) with eleven osteological characters. Said (1983) described 33 morphological and/or anatomical characters differing between Iso and Notocheirus. Subsequently Saeed et al. (1994) erected Atherinopsoidea, comprising (Atherinopsidae, Isonidae, Notocheiridae), with five osteological characters, distinguishing this group from Atherinoidea. Notocheiridae was diagnosed with seven characters, while Isonidae was diagnosed with six characters.

Dyer and Chernoff (1996) considered Iso and Notocheirus to be more derived than Atherinopsidae, and a sister group to the so-called infraorder Atherines (comprising four families Melanotaeniidae, Atherionidae, Phallostethidae and Atherinidae). On the basis of examination of I. natalensis, I. rhothophilus and N. hubbsi, and consideration of previous studies, (Iso, Notocheirus) Notocheiridae was diagnosed with three autapomorphies: supracleithrum absent; pelvic dorsolateral process elongate, attaching to pleural rib by elongate ligament; and body shape unique. Iso was diagnosed with six autapomorphies and four additional characters, Notocheirus was diagnosed with seven autapomorphies and six additional characters. Thus, Dyer and Chernoff (1996) identified 23 anatomical differences between Iso and Notocheirus. Table III lists twenty autapomorphies of either Iso or Notocheirus the present authors have been able to verify, following Rosen (1964), Said (1983), Saeed et al. (1994) and Dyer and Chernoff (1996).

Some of the character states in Notocheirus (including lack of first dorsal, mesethmoid, first epibranchial and epural) could be interpreted as paedomorphism (eg. Weitzmann and Vari, 1988), and hence advanced rather than plesiomorphic traits. In any event, the distinction between Notocheiridae and Isonidae is supported by 20 characters.

Notocheirus is known only from coastal waters of South America, a region in which Iso has never been collected. It is not known whether the separation between Iso and Notocheirus commenced before the land masses of present day Africa and the Americas were conjoined. It is to be hoped that comparisons of molecular sequences will advance the present knowledge of isonid systematics.

Acknowledgements

We wish to thank Ms Betty Thorn and Mr. Ron Oldfield (Macquarie University) for preparation of some illustrations and micrographs, and staff of institutions who kindly facilitated loans of specimens.

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Said, B. 1987. Revision of the genus Pseudomugil with phylogenetic systematics of the order Atheriniformes. Unpublished. Ph.D. thesis, Macquarie University, Australia, 280 pp.

Schultz, L. P. 1950. Correction for "A revision of six subfamilies of atherine fishes, with description of new genera and species". Copeia, 1950: 150.

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Waite, E. R. 1904b. Synopsis of the fishes of New South Wales. Memoirs of the New South Wales Naturalist Club, 2:1-59.

Weitzman, S. H. & R. P. Vari. 1988. Miniaturization in South American freshwater fishes; an overview and discussion. Proceedings of the Biological Society of Washington, 101: 444-465.

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Accepted: 03.06.2005

Basim Saeed, Walter Ivantsoff and Aarn

Departmentl of Biological Sciences, Macquarie University 2109, Australia.

E-mail: [e-mail]

Table I. Data matrix showing distribution of seven character states
across outgroup and five ingroup taxa.

       Notocheirus      I. flosmaris     I. nesiotes

 1          0                0                2
 2          0                0                0
 3          0                0                1
 4          0                0                0
 5          0                1                1
 6          0                1                0
 7          0                0                0
 8          0                0                1
 9          0                0                1
10          0                0                0
11          0                1                0

     I. Rhothophilus   I. hawaiiensis   I. natalensis

 1          1                1                1
 2          1                0                2
 3          1                1                1
 4          0                0                1
 5          1                0                1
 6          0                1                1
 7          0                1                0
 8          1                1                1
 9          0                0                0
10          0                1                0
11          1                1                1

Table II. The number of shared plesiomorphic character states between
Pairs of six taxa.

              I. flosmaris     I. nesiotes      I. rhothophilus
Notocheirus          8               6                  5
              I. flosmaris           4                  4
 I. nesiotes              4
                   I. rhothophilus
              I. hawaiiensis   I. natalensis
Notocheirus          4               3
                     3               3
                     2               2
                     2               3
              I. hawaiiensis         1

Table III. Osteological differences between Iso and Notocheirus.
Autapomorphic states (within Atheriniformes) indicated A.

                                                      Iso   Notocheirus

1 Scales elongate, denticulate (Said, 1983).                     A
2 Premaxilla symphysis bullous, premaxillary teeth
  distribution unique (Rosen, 1964).                             A
3 Mesethmoid absent, nasal cavities separated by
  ethmoid (Said, 1983).                                          A
4 Mesethmoid morphology unique, nasal septum formed
  by ethmoid and mesethmoid (Saeed et al., 1994).      A
5 Vomer absent (Rosen, 1964).                                    A
6 Parasphenoid ventromedian process (Rosen, 1964).               A
7 Parasphenoid concave rather than convex,
  parasphenoid myodomes present (Said, 1983).          A
8 Parietals present but not contributing to
  posttemporal fossa (Saeed et al., 1994).             A
9 Palatine reduced (Rosen, 1964).                                A
10 First epibranchial absent (Said, 1983).                       A
11 Anterior vertebral parapophyses with lateral
  ridges (Said, 1983).                                 A
12 Interdorsals absent (Dyer and Chernoff, 1996).      A
13 Epural absent (Rosen, 1964).                                  A
14 Posterior pterygiophores of second dorsal and
  anal fins on individual unshared cartilage
  support (Dyer and Chernoff, 1996).                             A
15 Scapula and coracoid dorsal to midline (Rosen,
  1964), pectoral inserted dorsally (Saeed et al.,
  1994).                                                         A
16 Cleithrum dorsal enclosure absent (Dyer and
  Chernoff, 1996).                                     A
17 Pectoral spur absent (Dyer and Chernoff, 1996).               A
18 Membrane posterior to genital opening (Said,
  1983).                                               A
19 Pelvic posteromedial process elongate (Saeed et
  al., 1994).                                          A
20 Pelvic posteroventral process absent (Said,
  1983), pelvic fin morphology unique (Saeed et
  al., 1994)                                                     A