Tuatara: Volume 17, Issue 3, December 1969
A Guide to The Flatfishes (Order Heterosomata) of New Zealand
A Guide to The Flatfishes (Order Heterosomata) of New Zealand
The Term Flatfishes is reserved for a group of rather specialised fishes characterised by an asymmetrical body, flattened from side to side (i.e. compressed) with both eyes situated on one side of the head. They are one of the most successful and beautifully adapted of all the groups of bottom-living fishes. All flatfishes lie on one side of the body, i.e. the blind side, which is usually colourless, or spotted black or yellow, or sometimes only a little lighter in colour than the ocular side.
It is important to note that elasmobranchs such as skates and rays which are flattened from above downwards (i.e. depressed) and lie on their abdomen are not usually termed Flatfishes. Teleostean fishes such as John Dory or the Oarfish, which are compressed but swim vertically, are similarly not included in the order.
Except for their asymmetry Flatfishes are related to Perch-like fishes. For some days after hatching a young Flatfish, outwardly, appears to be bilaterally symmetrical, with an eye on each side of the head, and swims in a normal fish-like manner. However, it has been demonstrated, by the study of the optic nerve of larvae, that Flatfishes are never symmetrical (Parker, 1903). In bony fishes the optic chiasma is dimorphic, i.e., the right optic nerve crosses above the left as often as the left crosses above the right. In most Flatfishes the chiasma is monomorphic, i.e., the optic nerve of the migrating eye is dorsal and this condition is established even before the larva is hatched. After hatching, the chondrocranium of the larva undergoes torsion and one eye begins its slow migration to the opposite side of the head, and some 6-10 weeks after hatching takes up its final position. This eye migration accomplished, the young Flatfish sinks to the bottom and lies on its blind side. The mouth retains its original position although the upper and lower jaws on the blind side together with the teeth, show a greater development than those of the ocular side and thus it is not necessary for the fish to adopt a vertical position for feeding. Feeding in adult Flatfishes is mainly confined to the hours of darkness, when they swim about looking for marine worms and other small bottom-dwelling animals. Because of their movements during darkness they are more easily taken in trawl nets at that time than during hours of daylight when they almost bury themselves in sand or mud.
Flatfishes are masters of colour change and camouflage, and in this respect they are said to exceed the ability of the well-known page 119 chameleon. These fishes can adapt themselves not only in colour but also in pattern to their surroundings. Juvenile Flatfishes are present in only a few inches of water on many of our beaches, but they conceal themselves so well in sand or mud that they can be detected only by their darting movement. When Flatfishes settle down after swimming, they wiggle their marginal fins, thus throwing up a shower of sand or mud which falls upon their edges and obliterates their outline.
Flatfishes are found in most seas and are an important source of food. Most of them are small but some attain a considerable size, e.g., the Halibut (Hippoglossus hippoglossus, Family Pleuronectidae) of the Northern Atlantic grows to a length of 9 feet and may weigh 500 pounds.
Although the undulations with which they make their way through water would appear ineffective, Flatfishes are able swimmers. A New Zealand Black Flounder (Rhombosolea retiaria) tagged at Lake Ellesmere was taken six months later near Foveaux Strait, a distance of some 250 miles.
Eleven species of Flatfishes are known from New Zealand waters. All are entirely marine, except R. retiaria, which frequents estuaries and enters fresh waters. Two species, Arnoglossus scapha and Lophonectes gallus, belong to the sinistral family Bothidae in which the eyes are on the left side. Lophonectes gallus also occurs in south-eastern Australia and Tasmania. The remaining nine species belong to the sub-family Rhombosoleinae of the dextral (eyes on right side) Family Pleuronectidae. Subfamily Rhombosoleinae, which is given family status by some ichthyologists (Chabanaud, 1946), contains eight genera and fifteen species. This distribution of the Rhombosoleinae is confined almost entirely to the Australasian region. Eight species occur in Australian waters, nine in New Zealand, with only two species, Azygopus pinnifasciatus and R. tapirina, shared by Australia and New Zealand. However, one genus (one species) Oncopterus darwinii is found only along the south-eastern coast of South America. This distribution of the subfamily is an interesting one from the point of view of the dispersal mechanism involved. Fell (1962) presented a theory, drawing on evidence from several genera of circumpolar or partly circumpolar echinoderms, which suggested that larval echinoderms, other epiplanktons, and sea weeds drifted from west to east because of the west wind drift. Thus Australia stands as a ‘donor’ to New Zealand and similarly New Zealand to the islands east of it and these in turn to South America. Thus one can speculate that larval Rhombosoleinae drifted westward in a circumpolar path from its ‘home’ in the Australasian region, under the influence of the west wind drift.
Throughout New Zealand, many fish may be known by several different common names. For example, Rhombosolea plebeia is page 120 known variously as ‘Flounder’, ‘Sand Flounder’, ‘Dab’, ‘Diamond’, ‘Square’, ‘Tinplate’, ‘Three-corner’, ‘Patiki’, etc. Futher, one common name may be used to denote different species, e.g., some fishermen use the name ‘Brill’ to denote Colistium nudipinnis and Colistium guntheri, others call both of them ‘Turbot’, while still others recognise that they are different species, and call them ‘Turbot’ and ‘Brill’ respectively. Many names such as ‘Brill’, ‘Turbot’, ‘Sole’, ‘Witch’, and ‘Melgrim’ refer to actual species which are confined to the Northern Hemisphere. Needless to say, such loose applications of common names create much confusion.
Except for the two species of Family Bothidae and Azygopus pinnifasciatus (Rhombosoleinae), the remaining eight are highly esteemed for their edible qualities. These eight species are known to fishermen and the New Zealand public as either ‘Flounders’ or ‘Soles’. According to this loose terminology, ‘Flounders’ include R. plebeia, R. leporina, R. retiaria and R. tapirina, while ‘Soles’ include Peltorhampus novaezeelandiae and Pelotretis flavilatus. The choice of the term ‘Soles’ is unfortunate as this name, in all other countries, is reserved for species belonging to Families Soleidae and Cynoglossidae, neither of which is represented in New Zealand. Contrary to Parrot (1960, p. 14) neither P. novaezeelandiae nor P. flavilatus is related to the Soles of Australia and European seas. Not all the eight edible species referred to above are abundant and only four species comprise the bulk of our commercial Flatfish catch, namely, R. plebeia, R. leporina, P. novaezeelandiae and P. flavilatus. During the year 1967 (according to the Report on Fisheries for 1967, N.Z. Marine Department) the total catch of ‘Soles’ and ‘Flounders’ amounted to 5,787,600 pounds in weight representing 7.24 per cent of the total quantity of fish landed in New Zealand. This quantity was valued at 721,005 dollars comprising 15.59 per cent of the value of all fish landed.
Between 1913 and 1917, attempts were made to introduce the European Turbot (Scopthalmus maximum of Family Bothidae) to New Zealand. A detailed account of this venture is given by Thomson and Anderton (1921). This species, whose eyes are situated on the left side, attains a length of three feet and may weigh up to fifty pounds or more. Its body is somewhat circular in shape and its colour is usually greyish or sandy brown with darker spots and blotches. Some 170 young S. maximus, brought out from England, were reared in Portobello Hatchery tanks and released in Tautuku Bay, about 60 miles south of Portobello. None have been seen since.
The pleuronectid Brachypleura novaezeelandiae (Subfamily Samarinae) has been included in various check-lists of New Zealand fishes, as it was thought to have been taken in New Zealand waters. Norman (1934, p. 401) questioned its occurrence in New Zealand since this species is strictly tropical, occurring from the Indian Ocean page 121 to the Arafura Sea. It is now agreed that the citation of New Zealand as the original type locality was incorrect (Chabanaud, 1954).
According to the key below, the side on which both eyes are situated determines the family to which each New Zealand Flatfish belongs. However abnormalities are not uncommon in Flatfishes so that an occasional specimen of the normally right-eyed species may have eyes on the left side. Recognition of reversed or abnormal specimens is important since often in the past they have been described as belonging to a new genus or new species. A New Zealand example of such an error occurred when Kyle (1900) described a perfectly reversed R. plebeia as a new genus and species, i.e., Apsetta thompsoni. Further, specimens similarly coloured on both sides are sometimes encountered and so are albinos, in which no pigment is developed at all. Albinos are usually white or pink but as a rule, albinism is not complete, the fish retaining patches of dark pigments. Abnormal colouration is often accompanied by structural abnormalities, for example, development of both pelvic fins in Rhombosolea sp. where usually only one pelvic fin is present.
The preoperculum bone of both Family Bothidae and Pleuronectidae has a free posterior margin covered over by the skin. In some species this character is more obvious than in others. Parrot (1960: 103-4) is incorrect in stating that some New Zealand Flatfishes have a second gill-opening. He is obviously referring to the slight folding of the loose skin under the posterior margin of the preoperculum. The skin however is continuous over the preoperculum and the other bones of the opercular series.
In the diagnoses and general discussion of the Families Bothidae and Pleuronectidae, Norman (1934) included the presence or absence of an oil globule in the egg as a taxonomic character, stating that bothids possessed a single oil globule while pleuronectids had none. However several pleuronectid exceptions have been recorded including some from New Zealand (Thomson, 1907; Thomson and Anderton, 1921; Orton and Limbaugh, 1953).
Key to New Zealand Flatfishes
|A.||Eyes on left side||Family Bothidae|
|B.||Anterior dorsal rays prolonged, forming crest. (Fig. 1 and 2).||Lophonectes gallus|
|BB.||Anterior dorsal rays short, not forming crest.(Fig. 3).||Arnoglossus scapha|
|AA.||Eyes on right side||Family Pleuronectidae|
|B.||Both pelvic fins present (see Fig. 5a):|
|C.||Pelvic fin of ocular side separated from anal fin. (Fig. 4)||Azygopus pinnifasciatus page 122|
|CC.||Pelvic fin of ocular side united with anal fin.|
|D.||Dorsal fin origin above eye. (Fig. 5).||Pelotretis flavilatus|
|DD.||Dorsal fin origin in front of eyes:|
|E.||Mouth visible on ocular side||genus Colistium|
|F.||Rostral hook long, extended below lower end of maxillary of ocular side; 7 rays in pelvic of ocular side. (Fig. 6)||Colistium nudipinnis|
|FF.||Rostral hook short, not reaching level of lower end of maxillary of ocular side; 9 or more rays in pelvic of ocular side. (Fig. 7).||Colistium guntheri|
|EE.||Mouth hidden on ocular side by rostral flap.(Fig. 8).||Peltorhamphus novaezeelandiae|
|BB.||Pelvic fin of ocular side only present (united with anal, see Fig. 11a)||Genus Rhombosolea|
|C.||Body with red or brown spots on ocular side, black
blotches on blind side. (Fig. 9).
|CC.||Body evenly coloured or sometimes mottled on ocular side, blind side colourless or yellowish:|
|D.||Body diamond-shaped (Fig. 10).||Rhombosolea plebeia|
|DD.||Body oval shaped:|
|E.||Snout normal. (Fig. 11).||Rhombosolea leporina|
|EE.||Snout produced, forming fleshy white process. (Fig. 12).||Rhombosolea tapirina|
Notes on Individual Species
Common name: ‘Crested Flounder’.
This species grows to a length of 8 inches and is often confused with Arnoglossus scapha. When taken out of water the anterior rays are lying back against the blind side of the body and hence the crest is not noticed. In the male the second to fifth, six or seventh rays may be prolonged to about twice the length of the head. In the female fewer rays are prolonged to about half the head length. The body is brown or grey in colour with the pelvic of the ocular side usually black. Occurs in south-eastern Australia, Tasmania and the northern coasts of the North Island of New Zealand in depths between 30-100 fathoms. The numerous small bones and thinness of body make this species unsuitable for food.page 123
Common names: Witch’ or ‘Megrim’.
Attains a length of 18 inches. Distributed throughout New Zealand though more common around the South Island than the North. The teeth on the ocular side are larger than in any other New Zealand Flatfish, and small fishes are often encountered in their stomach. Grey or light brown in colour with numerous small black spots. In many check-lists of New Zealand fishes another species of Arnoglossus, namely A. boops, is included (see Norman, 1934, p.196); this species is based on one specimen incorrectly thought to have been taken at a depth of 400 fathoms. It has been shown that this specimen was taken from 150 fathoms together with another specimen which was described as A. scapha. Results of investigations (in progress at present) have led the author to believe that A. boops is a synonym of A. scapha. Though A. scapha is taken in large quantities by fishing trawlers, it is, like L. gallus, valueless fish for food.
Common name: None in common usage but the term ‘ Spotted Flounder’ would seem appropriate.
This small fish (reaches 8 inches in length) is usually taken by deep-sea scientific expedition trawlers in waters between 190-400 fathoms. Coloured brown with small dark spots covering the entire ocular side and two prominent dark spots on the tail. Before the Danish Deap-Sea Expedition of 1950-52, this species was thought to occur in South Australian waters only, but since has been taken in New Zealand waters in the Bay of Plenty, around Chatham Islands and along the east coast of the South Island. Neilson (1961) examined the three specimens taken by the Danish Expedition and found that they differed from the Australian species in ‘several characters such as the diameter of the eyes, the number of gill rakers, lateral line scales, etc.’ and described the New Zealand specimens as a new subspecies, namely, A. p. flemingi.
Common name: ‘Lemon Sole’ and ‘New Zealand Lemon Sole’.
Fig.5. Pelotretis flavilatus (“Lemon Sole”)
Fig. 6. Colistium nudipinnis (“Turbot”)
Fig. 7. Colistium guntheri (“Brill”)
Fig. 8. Peltorhamphus novae zeelandioe (“Sole”)
Common names: ‘Turbot’ (Sometimes termed ‘Brill’).
This species is one of the largest and fattest of our New Zealand Flatfish, attaining 36 inches in length. It possesses a number of features which distinguishes it clearly from the closely related C. guntheri. The body is deep, nearly twice the body length; the rostral hook is long, extending below the level of the lower end of the maxillary of the ocular side; the pelvic fin has seven rays; the ocular is rich brown in colour with darker blotches. C. nudipinnis is seldom reported as abundant but is much more plentiful in very small quantities in Hawkes Bay and is occasionally reported from Northland and the Hauraki Gulf. It is a fine eating fish.
Common names: ‘Turbot’ (Sometimes termed ‘Brill’).
Grows to a length of 36 inches but the body is not as thick nor as deep as that of C. nudipinnis. The following other features distinguishes C. guntheri from C. nudipinnis: The ocular side is dark grey in colour with the margins and fins almost black; the outer edge of each scale is black and this is responsible for the longitudinal black lines along the length of the body; the rostral hook is short and does not reach the level of the lower end of the maxillary of the ocular side; the pelvic fin of the ocular side over 9 or more rays. Its distribution is similar to that of C. nudipinnis and its flesh is highly regarded for food.
Common names: ‘Sole’, ‘English Sole’, ‘New Zealand Sole’, and Patiki rori’ (Maori).
Grows to about 15-18 inches in length. Resembles C. guntheri and C. nudipinnis in the possession of a rostral hook, which in this case covers the mouth entirely on the ocular side. Another distinguishing feature is the long filamentous second upper ray of the ocular pectoral fin. The depth of the body is some 2-2½ times the body length. Specimens from around the South Island and Cook Strait have a distinctly deeper body than those from further north. P. novaezeelandiae is probably the most abundant of New Zealand Flatfishes. Young fish are encountered in large numbers on most of our beaches and harbours. Also occurs around Norfolk and Chatham Islands. An excellent table fish.page 127
Common names: ‘Black Flounder’, ‘River Flounder’, Estuary Flounder’, ‘Mud Flounder’ and ‘Patiki mohao’ (Maori).
Grows to about 14-17 inches in length. Its colour, body form and shape of head sets this fish apart from the remaining three species of Rhombosolea. The ocular side is deep olive or almost black in colour with numerous red or brown spots on the body and fins. The blind side is greyish in colour and often dark blotches are present. The body is oval in outline and the snout is somewhat blunt and certainly not as pointed as in the other Rhombosolea species. Whitley and Phillipps (1939, p. 231) considered that R. retiaria from North and South Islands of New Zealand differed sufficiently to necessitate the creation of a new subgenus and subspecies. They therefore named a specimen from the South Island (Hokitika) Rhombosolea (Adamosoma) retiaria adamas, based on an earlier description of R. retiaria by Phillipps (1925). This latter description is a general account of R. retiaria in New Zealand and gives only the fin formula, from a drawing by F. E. Clarke, of a fish taken on October 13, 1870 in Hokitika. The present author has been unable to trace the type specimen of the new subgenus and subspecies. In a more recent work, Graham (1956, p. 204) has adopted the new generic and specific names.
R. retiaria is distributed throughout New Zealand, preferring tidal reaches of rivers but is encountered in the sea or far upstream beyond tidal influence. It is a food fish but is not regarded as highly as the other flounders.
Common names: ‘Sand Flounder’, ‘New Zealand Flounder’, ‘Dab’, ‘Tinplate’, ‘Diamond’, ‘Square’, ‘Three-corner’ and ‘Patiki’ (Maori).
Grows to about 17 inches. The adult is very distinctly diamond shaped and is easily recognised. The depth of the body is slightly more than 1½ times in the body length. Difficulty is often experienced in distinguishing between the young of R. plebeia and R. leporina. Here taxonomic characters such as proportional dimension of the eyes, number of gill rakers and dorsal rays should be considered. The eyes of R. plebeia are bigger, the eye diameter being 4-6 times in length of head. There are fewer gill rakers on the lower half of the first arch, ranging from 12-18 with 16 as the average. The dorsal rays are more numerous, ranging from 53-63 with 59 as the average. The ocular side is a dark grey or green in colour with the blind side invariably white with little or no pigmentation. R. plebeia is evenly distributed in shallow waters throughout New Zealand and is the commonest of the Rhombosolea species, comprising the bulk of our commercial catch.
Common names: ‘Yellowbelly Flounder’, ‘Yellow Flounder’ and ‘Patiki totara’ (Maori).
Grows to about 13-15 inches in length. The body is oval with the snout more pointed than that of R. plebeia. Other distinguishing features include smaller eyes (6-8 times in length of head), greater number of gill rakers on the lower half of the first arch (14-23, average 19) and fewer dorsal rays (54-63, average 59). The ocular side is dark grey or green in colour, while the blind side is yellowish or orange with scattered black spots. The blind side of the young fish is usually white. Occurs throughout New Zealand inshore areas, frequenting estuaries and tidal rivers. Taken in large numbers by commercial fishermen. A good food fish.
Common name: ‘Greenback Flounder’.
Grows to about 14-20 inches. This species is easily distinguished by its very pointed snout due mainly to the fleshy white process at the end of the snout. The ocular side is dark green while the blind side is entirely white. R. tapirina is widely distributed, occurring in shallow waters of southern New South Wales, South Australia, Tasmania, southern New Zealand waters, Auckland and Campbell Islands. Never abundant, it is sometimes taken in small quantities in commercial catches.
The author is indebted to Mr. J. M. Moreland for helpful discussions and the loan of specimens from the Dominion Museum, and to Professor J. A. F. Garrick, Zoology Department, Victoria University of Wellington, for helpful criticisms.
Anderton, T., 1907. Observations on New Zealand fishes made at the Portobello Marine Fish Hatchery. Trans. Proc. N.Z. Inst. 39: 477-495.
Chabanaud, P., 1946. Notules ichthyologiques. Bull. Mus. Nat. Hist. Paris. 2(18)2: 158-161.
Chabanaud, P., 1954. Quelques erreurs d'origine en ichthyologie. Bull. Mus. Franc. Afr. noire 16A: 1293-1294.
Graham, D. H., 1953. (Second Ed. 1956). A treasury of New Zealand fishes. A. H. and A. W. Reed, Wellington. pp. 424.
Gyle, H. M., 1900. On a new genus of Flatfishes from New Zealand. Proc. Zool. Soc. 1900: 986-922, figs. 1-3.
Neilsen, J. 1961. Heterosomata (Pisces). Galathea Report. 4: 219-225, 3 figs, plate iv.
Norman, J. R., 1934. A systematic monograph of the Flatfishes (Heterosomata). British Mus., London. viii + 459 pp. 317 figs.
Parker, G. H. 1903. The optic chiasma in Teleosts and its bearing on the asymmetry of the Heterosomata (Flatfishes). Bull. Mus. Comp, Zool. 40: 221-242. 1 pl.
Parrot, A. W., 1960. The queer and the rare fishes of New Zealand. Hodder and Stoughton, London. 192 pp.
Phillipps, W. J., 1925. The Black or River Flounder of New Zealand (Rhombosolea retiaria). N.Z. J. Sci and Tech. 7(6): 368-369, 1 fig.
Rapson, A. M., 1940. The reproduction, growth, and distribution of the Lemon Soles (Pelotretis flavilatus Waite) of Tasman Bay and Marlborough Sounds. N.Z. Mar. Dept. Fish. Bull. No. 7: 1-56, 16 figs.
Thomson, M., and Anderton, T. 1921. History of the Portobello Marine Fish-Hatchery and Biological Station. Bull. N.Z. Board Sci. Art. No. 2 1-131. figs.
Whitley, G. P., and W. J. Phillipps. 1939. Descriptive notes on some New Zealand fishes. Trans. Roy. Soc. N.Z. 69: 228-236, 6 figs.