Tokio Yamamoto In: "Medaka, Biology and Strains" (T. Yamamoto, ed.), Yugakusya Publ. (1975), pp. 17-29.
Systematics and Zoogeography
The killifishes, or Cyprinodontiforms are small fresh and brackish water fishes of worldwide distribution in tropical and temperate latitudes.
Previous classfication of the order Cyprinodontes
The classification of the order Cyprinodontes Agassiz (equivalent to Microcyprini Regan) has been worked out by Gill (1865, 1874), Regan (1909, 1911), Hubbs (1924, 1926) and Myers (1931, 1938). The classification followed here is mostly according to Hubbs and Myers and is cited from Kulkalni (1940) who erected a new family Horaichthyidae represented by a remarkable Indian henpecked killifish, Horaichthys setnai.Ê However, substituting for the terms Amblyopsoidea and Poecilioidea, the suborders Amblyopsoidei and Cyprinodontoidei are used here, respectively. The subfamily Tomeurinae is removed from the family Poeciliidae to erect a new family Tomeuridae as suggested by Hubbs in his letter to S. L. Hora (India) in 1938. Representative genera are given in parentheses following family names.
Order Cyprinodontes (Microcyprini)
Suborder Amblyopsoidei
Family Amblyopsidae (Chologaster, Amblyopsis)
Suborder Cyprinodontoidei
Family Cyprinodontidae (Cyprinodon, Fundulus,
Aplocheilus, Panchax, Oryzias)
Family Goodeidae (Goodea)
Family Poeciliidae (Poecilia, Gambusia, Xiphophorus)
Family Jenynsiidae (Jenynsia)
Family Anablepidae (Anableps)
Family Tomeuridae (Tomeurus)
Family Adrianichthyidae (Adrianichthys, Xenopoecilus)
Family Phallostethidae (Phallostethus, Gulaphallus)
Family Horaichthyidae (Horaichthys)
The classification listed here has been generally held by ichthyologists until 1962.
As to the status of Oryzias, Myers (1931) considered it to represent a monogeneric tribe of the subfamilily Fundulinae. Later (1956), he revised his earlier classification, and considered Oryzias to represent a monogeneric subfamily of the Cyprinodontidae, the Oryziatinae.
Classification of new order Atherinoformes
Rosen (1962) presented evidence which indicates a relationship of the Amblyopsidae (North American cave fishes) with the percopsiform genera and, more distantly with the gadiforms. He isolated the cave fishes as a new order, the Amblyopsiformes, and recommended its alignment near the Percopsiformes and Gadiformes in a phyletic sequence.
In 1964, Rosen has made drastic taxonomic re-arrangements of the halfbeaks, killifishes, silversides, and their relatives. The outset of his re- arrangements was osteological analyses of the adrianichyid fishes of Celebes, which were found to have a mixture of beloniform, cyprinodontiform, and mugilform features. Then, his investigation was broadened to include representatives of all these groups as well as a species of phallostethid.
In consequence of reasonable osteological diagnoses, he erected a new order Atherinoformes which includes the excoetoids, scomberesocoids. On the basis of osteological evidence, he separated the medaka (Oryzias) from cyprinodontoids, placed it in adrianichthyoids and erected a new family Oryziatidae.
To visualize Rosen's account on osteological difference between cyprinodontoids and adrianichthyoids, the presentation of the schema of the skull of the generalized teleosts as shown in Fig. 2-1 may be apropos.
Fig. 2-1. A diagam of teleostean skull. Opercula and Infraorbitalia are removed. ang._angular, articul.= articular, occi. = occipital, palat. = palatine, par. = parietal, praemax. = premaxilla, pteryg. = pterygoid, quadrat. = quadrate, squ.= squamosa, sympl. = symplectic, vom. = vomer.
After R. Goldschmidt' E. Selenkas Zoologishes Taschenbuch fur Studierende. 1912 Leipzig, George Thieme.
In cyprinodontoid killifishes, bones of the jaws and the palatoquadrate arch are in such a construction that the premaxilla is protractile. In adrianichthyoid killifishes, on the other hand, the premaxillae are not protractile. The Adrianichthyidae are fishes of small size confined to the fresh-water lakes of Celebes. Two species, Adrianichthys kruyti and Xenopoecilus sarasinorum, are known. Xenopoecilus is characterized by having a large horse-shoe shaped mouth, an enormous ethmoideum and a single, median supraoccipital process formed by fusion of embryologically paired elements; "a cup-like excavation on the distal tip of the autopalatine that is capped by a large ball of cartilage and a discoidal sesamoid bone; a dorsal enlargement of the palatopterygoid arch with a prefrontal (Fig. 2-2); a maxilla that is carried on the upper edge rather than on the outer face of the posterior end of the premaxilla; a premaxilla that lacks a hooked or pointed posteroventral process; a tremendously reduced articular bone without a coronoid process that is almost wholly contained within the posterior part of the dentary; the articulation of the first pleural rib on the third rather than on the second vertebra; pelvic girdles that are not in contact medially and that have a long lateral spur extending upward between ribs; a dorsoventrally asymmetrical caudal skeleton with one or two very slender, rod-like epurals, and a caudal fin that is divided into indistinct upper and lower lobes by having a large gap between rays that articulate with the upper and lower hypural plates on the terminal half-centrum. (Rosen, 1964)
Fig. 2-2. Jaws and jaw suspension in adrianichthyoid killifishes. A. Jaws and palatopterygoid arch in Oryzias latipes (Temminck and Schlegel). b. Jaw suspension and opercular apparatus in Xenopoecillus sarasinorum (Popta). Note sesamoid bone below quadrate and bony cap over tip of palatine in A. and B. Note in A that lower arm of premaxilla lies over maxilla, large coronoid process on dentary, and absence of similar coronoid elevation on articular. Ang = angular, art = articular, dn = dentary, ent = entpterygoid (mesopterygoid), hyo = hyomandibular, iop = interoperculum, mx = maxilla, op = operculum, pal = palatine (autopalatine with or without dermopalatine), pgqu = pterygoquadrate cartilage, pmx = sesamoid bone capping autopalatine, ss = sesamoid bone, sym =symplectic. Rosen, 1964.
Rosen pointed out that except for the enlarged jaws and the presence of a median supraoccipital process, all the above features described within quotation marks can be identified in Oryzias (Fig. 2-2) but in no other killifishes so far as known.
It is therefore apparent that adrianichthyids and the medaka are intimately related and that they constitute a distinct subgroup of the killifishes, the adrianichthyoids, containing the families Adrianichthyidae (Adrianichthys and Xenopoecilus), Oryziatidae (Oryzias), and Horaichthyidae (Horaichthys), in contrast to the remainder of the families which are grouped together as cyprinodontoids (Cyprinodontoidea). Basing on Rosen's (1964) findings, Turner (1965) conveniently enumerated difference between cyprinodontoids and Oryzias as follows:
Cyprinodontoidae Oryzias
1. First pleural rib on second First pleural rib on third
vertebra. vertebra.
2. Pelvic girdle bones joined Pelvic girdle bones not joined
mid-ventrally; no upright mid-ventrally; an upright lateral
lateral spur. spur present.
3. Lower end of premaxilla bone Lower end of premaxilla
expanded or hooked and bone not expanded, and dorsal to
sandwiched between the lower the maxilla bone rather
end of maxilla bone and than between it and the dentary
dentary bone (lower jaw). bone.
4. Hypural plates often fused. Hypural plates never fused.
5. Hypochordal musculature Hypochordal musculature
entirely absent. present.
6. Caudal fin never incipiently Caudal fin incipiently lobed.
lobed.
The family Horaichthyidae erected by Kulkarni (1940) comprises a single species, Horaichthys setnai. It is a small translucent oviparous fish inhabiting brackish waters and estuaries in the province of Bombay, India. Osteological study (Kulkalni 1948) showed that its head skeleton is closely allied to that of Oryzias but greatly different from that of Aplocheilus. Horaichthys, however, is different from known species of Oryzias in having a larger number of the anal fin- rays (about 28 to 32).
In the male, six anterior rays of the anal fin are separated from the rest of the fin and modified into an elaborate male organ (gonopodium). Of six rays the third, fourth and fifth ones are profoundly modified forming the 3- 4-5 complex. (Fig. 2-3). In the female right pelvic fin is usually absent. The genital opening of the female is situated on the left ventral side and is surrounded by genital pads. Horaichthys is supposed to have evolved from Oryzias, but as the development of the gonopodium in association with the henpecked sexual behavior is so remarkable that Kulkarni (1940) has proposed to erect a new family rank for this fish.
The male appears to be always afraid of the female which on occasions chases him away. At the time of mating, "the male swims below and behind her at a distance of about 2 to 3 cm. He then darts towards her on the left with almost lightning speed. As he approaches his mate he lashes out the gonopodium sideways almost at right angles to his body and strikes its terminal end against her genital opening. The spermatophores are transferred to the female in this momentary contact, and become attached by their distal hooks."
Fig.2-3. Lateral view of a male and a female specimen of Horaichthys setnai. x 4 Kulkalni, 1940.
A special feature of Horaichthys is that the testis produces special sperm capsules of spermatophores (2-300 in number) instead of ordinary semi-fluid milt with suspended sperms.
A spermatophore is a tiny hyaline body (0.6 mm long and 0.1 mm thick), the broad part of which contains mass of sperms. At the tapering end, there is a pointed cap with stiff hooks and barb-like structures which point backwards. It is with the aid of these hooks and barbs that the spermatophore get attached near the genital opening of the female.
There is no permanent opening on the spermatophore for the liberation of sperms. Before liberation of sperms, a small bulging appears at the neck of the tapering spermatophore and begins to enlarge. When the protuberance becomes sufficiently large, an opening is formed at its tip by rupture of membrane and sperms are liberated. They swim into the genital pore of the female.
The following is the classification of the new order Atheriniformes by Rosen (1964), representative species being given in parentheses.
Suborder Exocoetoidei Superfamily Exocoetoidea Family Hemiramphidae (Hemiramphus) Family Exocoetidae (Exocoetus) Superfamily Scomberesocoidea Family Belonidae (Ablennus) Family Scomberesocidae (Cololabis) Suborder Cyprinodontoidei Superfamily Adrianichthyoidea Family Oryziatidae (Oryzias) Family Adrianichthyidae (Adrianichthys, Xenopoecilus) Family Horaichthyidae (Horaichthys) Superfamily Cyprinodontoidea Family Cyprinodontidae (Fundulus, Aplocheilus) Family Goodeidae (Goodea) Family Jenynsiidae (Jenynsia) Family Anablepidae (Anableps) Family Poeciliidae (Poecilia, Xiphophorus) Suborder Atherinoidei Superfamily Atherinoidea Family Melanotaeniidae Family Atherinidae (Atherina) Family Isonidae, new family (Iso) Superfamily Phallostethoidea Family Neostethidae (Neostethus) Family Phallostethidae (Phallostethus)
The family Oryziatidae
Rosen (1964) erected a new monogeneric family and described the following diagnoses of the family Oryziatidae. Type genus: Oryzias Jordan and Snyder, 1906. Diagnoses: The Oryziatidae differ from their closest relatives, the adrianichthyids, in lacking the tremendously enlarged jaws and ethmoideum, in having paired supraoccipital processes (rather than a single median process), and in having the inferior pharyngeal bone distinctly separated (rather than united), and from all cyprinodontoids as follows: autopalatine usually capped by sesamoid bone; pterygoquadrate cartilage forming dorsal process; lower end of premaxilla not hooked or trapezoidal, situated below maxilla rather than between maxilla and dentary bone; first pleural rib on third vertebra; supracleithrum wanting; pelvic bones with upright lateral spurs and not joined midventrally; hypochordal musculature present on caudal fin.
Composition: Rosen listed following seven species of a single genus, Oryzias: O. latipes (Temminck and Schlegel), O. melastigma (McClelland), O. celebensis (Weber), O. timorensis (Weber and de Beaufort), O. javanicus (Bleeker), O. curvinotus (Nichols and Pope), and O. minutillus Smith. To these, O. luzonensis (Herre and Ablan) may be added. Besides these, Turner (1965) mentioned O. matenensis (Aurich), and O. marmoratus (Aurich) from the Celebes.
Probably not all these nominal species are valid, since some nominal species are different only in the anal fin-ray frequency.
The genus Oryzias
The following is the diagnoses of the genus Oryzias described by Jordan and Snyder (1906), basing on O. latipes which has previously been known as Aplocheilus latipes.
Body elliptical in form, compressed, covered with large scales; mouth small, with two rows of small, simple, pointed teeth; no teeth on vomer*1; gill-opening not restricted above; intestinal canal short, about as large as body; peritoneum black. Dorsal fin short, inserted above middle of anal; anal very long seventeen to twenty rays; caudal fin truncate. Sexes similar*2 except color; anal fin not modified in the male. *1 Kulkarni(1948) first showed that os vomer is absent in Oryzias melastigma. *2 Sexual dimorphism is prominent. See Chap. 8.
The species Oryzias latipes
The following description by Oshima (1919) based on a specimen of Oryzias latipes collected from Shori, Formosa is cited here as the diagnoses of the species since it is very precise and correct excepting two words starred and daggered.
Head 4 in length (body length divided by head length is 4); depth 4.5; depth of caudal peduncle 9.5; eye 2.5 in head (head length divided by eye diameter is 2.5); interorbital space 2; snout 4; D.6; A.18; P.9; V.5; thirty one scales in a lateral series; five branchiostegals.
Posterior half of the body compressed, becoming broader anteriorly, highest in front of the anal; head flattened; interorbital space broad; snout shorter than the diameter of eye, broadly rounded anteriorly; mouth anterior, transverse; lower jaw slightly projecting, each jaw with two rows of minute pointed teeth, those on the posterior row smaller; vomer*1 smooth; thirteen short, pointed gill-rakers on the first arch; eyes very large, anterior and superior.
Dorsal fin short, on the posterior half of body, its origin above the posterior two thirds of anal, its height equal to the distance between tip of snout and posterior margin of orbit; pectoral inserted on the median line of body; the ventral small, reaching vent; base of the anal very long, its posterior end opposite to that of the dorsal, anterior ray longest; tip of the caudal fin rounded.*2
Top and sides of head, throat, and chin naked; body covered with cycloid scales, lateral line absent.
Color in formalin pale gray above, lower parts silvery; a black longitudinal streak from the nape to the origin the dorsal; sides of body with a faint dusky stripe along the middle line, top of head dark; the edges of scales dusky; fin-rays of the ventral and anal dotted with minute black spots; all the fins whitish; peritoneum black. Length of body 28 mm.
Habitat: The present species is very common in rice-fields and pools on the island.
*1 Vomer is absent in Oryzias in reality.
*2 The caudal fin is almost truncate, strictly, however, it is incipiently lobed.
Change of nomenclature of the medaka
The medaka was first described as Poecilia latipes by Temminck and Schlegel in 1846 (Siebold's Fauna Japonica, Poiss., P.224, Pl.102, Fig. 5). GŸnther changed it as Haplochilus latipes. Jordan and Snyder (1901) described it as Aplocheilus latipes but later they separated it from Aplocheilus and erected a new genus Oryzias. They regarded Oryzias as having no teeth on vomer*1 while Aplocheilus possesses teeth on it.
Myers (1931) placed the medaka in the tribe Aplocheilini of the subfamily Fundulinae in the family Cyprinodontidae. He stated that the chief character of fishes of the tribe is the non-protractile premaxillae. The pectoral fin are set high and pseudobranchiae and vomerine teeth are never present. The species range from Japan and Central China south to Celebes and Timor and west to Southern India. A single genus, Aplocheilus, of which Oryzias is a synonym*2. Smith, (1945) pointed out that the genus known as Panchax is a synonym of Aplocheilus McClelland and Aplocheilus Weber and de Beaufort is a synonym of Oryzias Jordan and Snyder. He described Aplocheilus panchax (Hamilton) and Oryzias minutillus n. sp. from Thailand.
According to him, the two genera may be distinguished by the following characters:
a. Upper jaw protractile; mouth moderate size with its corners
abruptly bent downward; vomer toothed; pseudobranchiae
present; branchial membranes free from each other and from
isthmus; pectoral fins with their upper base at or below
longitudinal axis of body . . . . . . . . . . . . . . . . . . . . . . . . Aplocheilus
b. Upper jaw not protractile; mouth small with its corners obtusely
bent downward; vomer toothless; no pseudobranchiae; branchiae
membranes united across isthmus; pectoral fins with upper base
well above longitudinal axis of body . . . . . . . . . . . . . . . . . . Oryzias
The correct scientific name of the medaka is Oryzias latipes (Temminck and Schlegel).
From Jordan and Snyder (1906) onwards, all taxonomists stated that Oryzias has toothless vomer while Aplocheilus has toothed vomer. Kulkarni (1948) has made a precise osteological study of Indian killifishes and found that vomer is absent in both Oryzias melastigma and Horaichthys setnai while Aplocheilus lineata possesses toothed vomer.
*1 Vomer is absent in Oryzias in reality.
*2 Now, the two genera belong to the different super-families.
Geographical distribution of species belonging to the Genus Oryzias
All the species of the genus Oryzias are distributed in India, South Asia, the Indo-Australian archipelago and the Far East. Their habitats are widely ranged from tropical, subtropical, and temperate regions as shown in Figure 2-4 and in the following lines.
Fig. 2-4. A zoogeographical map showing distribution of species of the
Genus Oryzias. Original.
( 1 ) O. latipes (Temminck and Shlegel): Japan, Korea, Formosa, and
China
( 2 ) O. curvinotus (Nicols and Pope): The island of Hainan.
( 3 ) O. luzonensis (Herre and Ablan): Luzon in the Philippines.
( 4 ) O. celebensis (Weber): The Celebes.
( 5 ) O. matenensis (Aurich): The Celebes.
( 6 ) O. marmoratus (Aurich): The Celebes.
( 7 ) O. javanicus (Bleeker): The Indo-Malaysian archipelago and
Malaya.
( 8 ) O. timorensis (Weber and de Beaufort): The island of Timor.
( 9 ) O. minutillus Smith: Thailand.
(10) O. melastigma (McClelland): India, Western Pakistan, and
Sri Lanka (Ceylon).
In the main, all the Oryzias species are fresh-water fishes. O. latipes and O. melastigma inhabit both fresh and brackish water. O. latipes is so tolerate slinity that it thrives in tide pools in Korea and Kyushu in Japan.
References
Gill, T.N., 1965 Synopsis of the fishes in the Gulf of St. Lawrence and Bay of Fundy. Canadian Nat., Ser. 2, 2: (No.4) 244-266. Gill, T.N., 1874 Arrangement of the families of fishes, or Classes Pisces, Marssipobranchii and Leptocardii. Smithonian Misc. Coll., for 1872, 11: (No.247) 1-49. Herre, A.W., and G.L. Ablan, 1934 Aplocheilus luzonensis, a new Philippine Cyprinodont. Philippine Jour. Sci., 54: (No.2) 275-277. Hubbs, C.L., 1924 Studies on the fishes of the order Cyprinodontes I.-IV. Misc. Publ. Mus. Zool., Univ. Michigan, No. 13: 1-31. Hubbs, C.L., 1926 Studies on the fishes of the order Cyprinodontes VI. Misc. Publ. Mus. Zool., Univ. Michigan, No. 16: 1-87. Jordan, D.S., and J.O. Snyder, 1906 A review of the Poeciliidae or killifishes of Japan. Proc. U.S. Nat. Mus., 31: 287-290. Kulkarni, C.V., 1940 On the systematic position, structural modifications, bionomics and development of a remarkable new family of cyprinodont fishes from the province of Bombay. Rec. Indian Mus., 42: 379-423. Kurkarni, C.V., 1948 The osteology of Indian cyprinodonts. Part. I. comparative study of the head skeleton of Aplocheilus, Oryzias and Horaichthys. Proc. Natl. Inst. Sci. India, 14: (No.2) 65-119. McClelland, J., 1839 Asiatic researches, 19: 301. Myers, G.S., 1931 The primary groups of oviparous cyprinodont fishes. Stanford Univ. Publ. Biol. Sci. VI. No. 3: 7-14. Myers, G.S., 1938 Studies on the genera of cyprinodont fishes. Copeia (1938): 136-143. Nichols, J.T., and C.H. Pope, 1927 The fishes of Hainan. Bull. Amer. Mus.Nat. Hist., 54: 321. Oshima, M., 1919 Contributions to the study of the fresh water fishes of the island of Formosa. Annals Carnegie Mus., 12: 169-328. Regan, C.T., 1909 The classification of teleostean fishes. Ann. Mag. Nat. Hist., Ser. 8, 3: 75-86. Regan, C.T., 1911 The osteology and classification of the teleostean fishes of the order Microcyprini. Ann. Mag. Nat. Hist., Ser. 8, 7: 320-327. Rosen, D.E., 1962 Comments on the relationships of the North American cave fishes of the family Amblyopsidae. Amer. Mus. Novitates, No. 2109: 1-35. Rosen, D.E., 1964 The relationships and taxonomic position of the halfbeaks, killifishes, silversides, and their relatives. Bull. Amer. Mus. Nat. Hist, 127: (Art.5) 217-268. Smith, H.M., 1938 Status of the oriental fish genera Aplocheilus and Panchax. Proc. Biol. Soc. Washington, 51: 165-166. Smith, H.M., 1945 The fresh-water fishes of Siam, or Thailand. U.S. Natl. Mus. Bull., 188: 1-622. Turner, B.J., 1965 A new place for the medakas. Classica, 1: (No.2) 1-6. Weber, M., 1913 Neue Beitrage zur Kenntnis der SŸswasserfische von Celebes. Bijd. Dierk., Amsterdam, 19: 197-213. Weber, M., and L.F. de Beaufort 1922 The fishes of the Indo-Australian archipelago IV. Heteromi, Solenichthyes, Synentognathi, Percesoces, Labyrinthici, Microcyprini. Leiden, E.J. Brill, Ltd.