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Flat fish eggs, larvae and their development
Aquaculture, 18 (1979) 349-366 0 Elsevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
349
FLAT FISH EGGS, LARVAE AND THEIR DEVELOPMENT
N. RAMANATHAN and R. NATARAJAN Centre of Advanced Study 502, Tamil Nadu (India)
(Accepted
in Marine
Biology,
Annamalai
University,
Parangipettai
608
6 March 1979)
ABSTRACT Ramanathan, Aquaculture,
N. and Natarajan, R., 1979. Flat fish eggs, larvae and their development. 13: 34!&-366.
Studies on development of eight species of flat fishes (Psettodes
erumei, Brachypleura Pseudorhombus arsius, Bothus myriaster, Synaptura albomaculata, S. Cynoglossus are1 and C. monopus) were carried out with plankton collections and with laboratory reared larvae. The development stages of Cynoglossus are1 and C. monopus from egg to metamorphosis are described.
novae-zeelandiae, commersoniana,
INTRODUCTION
The early developmental stages of fewer than 10% of total marine fish species have been described (Smith, 1974). The potential value of studies on fish eggs and larvae as an essential prerequisite for proper management of fisheries is widely recognized. Information on the eggs and larvae of marine fishes in Indian waters is sparse. Information about the east coast of India can be found in John, 1951; Jones and Menon, 1951; Nair, 1952a, b; Bapat, 1955; Kuthalingam, 1957, 1960; Jones and Pantulu, 1958. Information about the west coast. of India can be found in Gopinath, 1946; Seshappa and Bhimachar, 1955; Balakrishnan, 1963; Lalithambika Devi, 1969. Information on the development of eight species of flat fishes from the Porto Novo coast is presented here. The early life history from egg to metamorphosis in two species of flat fishes of the genus Cynoglossus, studied by rearing larvae in the laboratory and by larvae collection from plankton, are given. The information on the life history of these edible fishes by laboratory rearing methods will find full application for coastal aquaculture practices. MATERIALS
AND METHODS
The eggs and larvae were collected from the sea (9-18 m deep in the Bay of Bengal, 79” 47’ 30” E, 11” 30’ N) in the early morning, using a plank-
350
ton net made of bolting silk No. 20 (mesh size of 0.08 mm) measuring about 1.5 m long and 40.5 cm diameter near its mouth. To minimize ciliates, the eggs and larvae were frequently transferred to vessels containing fresh, filtered sea water of 35”/,, salinity and were maintained at optimum temperature of 23°C (2 2°C). Agitation of water in the containers was also maintained since it has been considered useful (Cunningham, 1891; Vijayaraghavan, 1955), not only to aerate the water, but also to prevent the larvae from resting at the bottom of the containers where ciliates and bacteria flourish. Streptomycin mixture (streptomycin sulphate 500 mg, pencillin sulphate 500 mg, and tetracycline 500 mg dissolved in 275 ml of sea water) was used to control the ciliate and bacterial populations in the water. Nannoplankters were provided as food for the larvae immediately after the mouth and alimentary tract were well formed. The terminology of Mansueti and Hardy (1967) was followed for the early developmental stages of larvae. DEVELOPMENTAL
Psettodes
erumei
STUDIES
(Bloch & Schn.)
The eggs and larvae of the Indian halibut, Psettodes erumei have not been described so far, even though this species is commercially important along the coasts of India. In this study five larvae are described. 1.15 mm larva Body thin transparent when alive and creamy white when preserved in formalin, symmetrical and laterally compressed; length slightly more than twice the depth, eyes and auditory vesicles fully developed; mouth large, terminal, oblique with lower jaw slightly projecting; alimentary tract single and coiled; operculum well developed; fin fold continuous and confluent with the caudal; an elevation in front of the dorsal fin fold with interneural spines present; no rays discernible in the finfold; pectoral and pelvic fins not developed; 24 vertebral segments and myotomes clearly visible; air bladder present; no pigmentation over body (Fig.1). 2.4 mm larva Body thin, flat and symmetrical; length slightly less than three times the depth; an oblique well developed mouth with a projecting lower jaw; alimentary tract single and coiled with the posterior part almost parallel to body; six dorsal tentacles present on the elevation of the head in front of the dorsal fin fold; fin folds have no rays; pectoral fin well developed; 24 vertebral segments as in the previous stage; air bladder present; no pigmentation over body (Fig.2). 4.45 mm larva Body flat and symmetrical;
mouth large and oblique
with the lower jaw
351
projecting; teeth developed on both jaws; alimentary tract single and coiled, the posterior part bending downwards; pectoral fins present but pelvic fin not yet developed; gills just forming and visible through the transparent head; eyes well developed; nine dorsal tentacles of same length present; embryonic fin rays seen in the fin folds; median fin folds continuous with the caudal, but the junction with the caudal fin marked by a shallow indentation; 24 vertebral elements present; hypural elements well differentiated and consequently the urostyle turned dorsally; no pigmentation over body (Fig.3). 5.4 mm larva Body still flat and symmetrical; mouth oblique with projected lower jaw; teeth developed in both jaws than in the previous larva; nine dorsal tentacles still present; caudal fin separated from the median fin folds; pelvic fin developed; 24 vertebral elements present; urostyle pointed dorsalwards; gills well formed; no pigmentation over body (Fig.4). 5.7 mm larva Body flat and symmetrical; pterygiophores have formed; and 18 caudal fin rays well developed; nine dorsal tentacles fin completely separated from dorsal and anal fins (Fig.5).
Fig. 1. 1.15
Fig. 3. 4.45
mm larva of Psettodes
mm larva of Psettodes
Fig. 5. 5.7 mm larva of Psettodes
54 dorsal, 40 anal shortened; caudal
erumei
Fig. 2. 2.4 mm larva of Psettodes
erumei.
Fig. 4. 5.4 mm larva of Psettodes
erumei.
erumei.
erumei.
352
Remarks The meristic characters, such as the number of dorsal and anal fin rays and the vertebral counts, support the identification of the larvae as Psettodes erumei. The presence of well developed, strong teeth in the jaws and the projected lower jaw support this identification. The larvae were collected from the sea plankton during the period July to October, the spawning period of P. erumei along Port0 Novo coast. Brachypleura novae-zeelandiae Gunther Two post-larvae of Brachypleura novae-zeelandiae measuring 5.6 mm and 7.0 mm, were collected, and one post-larva (7.0 mm) was found to be in the metamorphosed stage, having both eyes on the right side of head. The descriptions are given below. 5.6 mm larva Body flat, transparent and bilaterally asymmetrical; the left eye started migrating; mouth large, oblique and jaws with minute teeth; anterior dorsal profile of head above the eye with a deep notch; operculum well developed; alimentary tract coiled; gills formed; pectoral and pelvic fins well developed; pterygiophores and fin rays well formed in the fin fold; the dorsal and anal fins separated from caudal; 76 rays in dorsal fins and 50 rays in anal fin;.vertebral count 39 including urostyle, which points dorsalwards; body with no pigmentation (Fig.6). 7.0 mm larva Body flat, transparent and asymmetrical; mouth big, oblique and jaws with minute teeth; left eye migrated completely to the right side; alimentary tract complicatedly coiled; gills present; dorsal and anal fins completely separated from caudal fin; dorsal fin originates just behind the upper eye and anal fin behind the anus; 75 rays in dorsal fin and 50 rays in anal fin; vertebral count 39 including urostyle which points upwards; body with no pigmentation (Fig.7).
Fig. 6. 5.6. mm larva of Brachypleura
novae-zeelandiae.
Fig. 7. 7.0 mm larva of Brachypleura
novae-zeelandiae.
353
Remarks The left eye has started migrating in the 5.6 mm larva, and it has completely migrated to the right side in the 7.0 mm larva. The presence of both eyes on the right side of the head, the meristic counts, such as dorsal and anal fin rays and the vertebral counts, tally with the adult structure of Bmchypleura nouaezeelandiae. Adults with mature ovaries could be collected one month prior to the collection of these larvae and thus provide additional evidence. Pseudorhombus
arsius (Ham. Buch.)
Three post-larval
stages of Pseudorhombus arsius are described.
3.6 mm larva Larva flat, transparent and bilaterally symmetrical; length more than twice the depth; mouth well developed and terminal; lower jaw slightly projecting; operculum formed as a small slit; eyes fully developed and symmetrical; abdominal region bulging; alimentary tract straight; continuous finfold with embryonic fin rays; three dorsal tentacles, the anterior larger than the posterior ones, all free from one another in front of the dorsal fin fold; 34 vertebral elements; body with no pigmentation (Fig.8). 4.0 mm larva Larva flat, transparent and asymmetrical; length slightly more than twice the depth; mouth terminal and jaws with very minute teeth; operculum well formed; migrating right eye clearly seen on the other side along dorsal margin of head resulting in the twisting of brain; alimentary tract single and coiled; pectoral and pelvic fins well formed; pterygiophores fully developed; dorsal and anal fin folds separated from caudal fin; dorsal tentacles reduced in size very much; hypural elements well differentiated; the urostyle pointing dorsalwards; 35 vertebral elements; body with no pigmentation (Fig.9). 8.0 mm larva Body flat and asymmetrical; mouth terminal, oblique a& jaws with very minute teeth; alimentary tract well coiled and anus shifted forward; right eye, on the upper margin of left side migrating; three dorsal tentacles seen as vestiges; dorsal fin advanced anteriorly; dorsal and anal fins completely separated from caudal; anal fin starts immediately behind the anus; rays in fins well developed; gill filaments seen through the transparent head; 35 vertebrae; hypural element points dorsalwards; 71 dorsal rays and 56 anal rays; body has no pigmentation (Fig.10). Remarks The migration of the right eye to the left side, the extension of the dorsal fin above the eye and the separation of the dorsal and anal fins from the cauda1 are characters which place these larvae in paralichthide group. Gopinath
Fig. 8. 3.6 mm larva of Pseudorhombus
arsius.
Fig. 9. 4.0 mm larva of Pseudorhombus
arsius.
Fig. 10. 8.0 mm larva of Pseudorhombus
arsius.
(1946) and Jones and Pantulu (1958) did not give any indication of the presence of the elongated dorsal tentacles, although this character is one of the most diagnostic features of Pseudorhombus larvae (Lalithambika Devi, 1969). The meristic characters, such as dorsal and anal fin rays and vertebral counts, confirm the larvae as being Pseudorhombus arsius. 7 he larvae were recorded during the months of May to July, the spawning season of P. arsius along Porto Novo coast (Ramanathan, 1977). Bothus
myriaster
(Temminck
and Schlegel)
Three larvae of Bothus myriuster
are described.
1.6 mm larva
Body flat, transparent and symmetrical; length less than twice of depth; mouth developed and terminal; lower jaw slightly projected; head has a steep contour; operculum and pectoral fin developed; eyes present; alimentary tract single and coiled; median finfold continuous with the caudal, but the junction with the caudal fin marked by a shallow indentation; the pterygiophores and the embryonic fin rays formed in the fin fold; two dorsal tentacles present in front of dorsal fin fold; gills well developed; five to six serrations present on ventral margin of head; myotomes closely packed; 38 vertebral elements; a black spot present just above the coil of alimentary tract (Fig.11). 12.3
mm larva
Body flat and symmetrical; mouth terminal and small, no teeth in jaws; alimentary tract single and coiled; pterygiophores well developed; dorsal and anal fin folds separated from caudal, and dorsal fin fold advanced anteriorly; fin rays well developed in the fin folds; pelvic fin formed as a bud; two dorsal tentacles present, the anterior one longer and the posterior shorter; 91 rays in dorsal fin and 69 in anal fin; 38 vertebral elements; hypural element formed
355
and urostyle pointing upward; serrations present on the ventral border of head; a black spot present just above the coiling of alimentary tract (Fig.12). 15.6 mm larva Body flat and symmetrical; head very small; mouth terminal and small; jaws without teeth; eyes small and located symmetrically on both sides of head; gills present; alimentary tract is single coiled; the maxillary bone formed; dorsal tentacles represented as vestiges; 94 rays in dorsal fin and 70 rays in anal fin; 38 vertebral elements including urostyle, body with no pigmentation (Fig.13).
Fig. 11. 1.6 mm larva of Bothus myriaster. Fig. 12. 12.3 mm larva of Bothus myriaster. Fig. 13. 15.6 mm larva of Bothus
myriaster.
Remarks
Amaoka (1964) described the development of B. myriaster from a post-larva, 29 mm in total length, to the adult stage. The larvae described here were found to have a small mouth, a characteristic feature of the genus Bothus (Amaoka, 1964). The dorsal and anal fin rays and the vertebral count tally with the species B. myriaster. According to Amaoka (1964), the postrlarvae of B. myriaster have no pigments on the body. The post-larva measuring 15.6 mm in total length described here is also devoid of :)ody pigments. B. myriaster is the only species of the genus Bothus occurring along Port0 Novo coast. Synaptura
albomaculata
Kaup
Seven larvae of Synaptura
albomaculata
are described.
1.6 mm larva Body flat, laterally compressed, transparent and symmetrical; mouth terminal, lower jaw projecting forward; no teeth in jaws; eyes well developed and
356
operculum just formed; alimentary tract single coiled and abdomen bulging outside; fin fold continuous and no rays discernible in the fin; two dorsal tentacles, the anterior one longer and posterior shorter; pectoral or pelvic fins not yet formed; myotomes visible; seven pigment spots along the base of dorsal and anal fin fold in a line; light black pigment on the head below dorsal tentacles; snout has a short spine (Fig.14). 2.4 mm larva Body flat, transparent and symmetrical; mouth terminal with the lower jaw projecting; jaws without teeth; eyes well developed; opercular slit advanced; alimentary tract single coiled; embryonic rays formed in fin folds; 12 spots of pigments along the base of dorsal and anal fin folds in a straight line; dark pigment present on head below the base of dorsal tentacles; other pigment on the chin, behind the eye and above the coil of alimentary tract; 50 well developed vertebral elements present; fin fold continuous; pectoral and pelvic fins not yet developed (Fig. 15). 3.1 mm larva Body flat and symmetrical; mouth terminal, projecting lower jaw with no teeth; alimentary tract single, coiled and bulging outside; pectoral fin developed; rays developed in the caudal fin; pterygiophores started forming; pigments at the base of the dorsal and anal fin folds; 50 vertebral elements (Fig. 16). 4.8 mm larva Body flat and symmetrical; mouth terminal, lower jaw slightly projecting; jaws without teeth; gills well developed; pelvic fin fully formed; alimentary tract single and coiled; two dorsal tentacles; rays well developed in all fin folds; pigments distributed on the body as in the previous stage; 79 rays in dorsal fin and 61 in anal fins, vertebral count 50 (Fig.17). 7.6 mm larva A rostra1 ‘beak’ well formed; dorsal tentacles shortened; hypural elements developed and urostyle pointing dorsalwards; seven black pigment spots at the base of dorsal and anal pterygiophores; 79 rays in dorsal fin and 61 rays in anal fin; 50 vertebral elements including urostyle (Fig. 18). 8.6 mm larva Frontal portion of head grown forward; dorsal tentacles represented as vestige; alimentary tract with complex coil; 79 rays in dorsal fin and 62 rays in anal fin; vertebral count 50 (Fig.19). 9.2 mm larva
Frontal portion duced (Fig.20).
of head bulged out, and the dorsal tentacles
further
re-
357 A--
Fig. 14.
1.6 mm larva of Synaptura
albomaculata.
Fig. 15. 2.4 mm larva of Synaptura
albomaculata.
Fig. 16. 3.1 mm larva of Synaptura
albomaculata.
Fig. 17. 4.8 mm larva of Synaptura
albomaculata.
Fig. 18. 7.6 mm larva of Synaptura
albomaculata.
Fig. 19. 8.6 mm larva of Synaptura
albomaculata.
Fig. 20. 9.2 mm larva of Synaptura
albomaculata.
358
Remarks
The movement of the left eye to the right side of head and absence of teeth on jaws help in distinguishing the larvae as belonging to the Soleidae. The presence of a short spine on the snout identifies this as belonging to Synaptura. The number of dorsal and anal fin rays tally with the adults of 5’. albomaculata, a commonly occurring species along Porto Novo coast. Hence these larvae were identified as those of S. albomaculata. Synaptura
commersoniana
(Lecepkle)
3.25 mm larva Body flat, transparent and symmetrical; mouth terminal and jaws without teeth; eyes well developed; operculum. gills and pectoral fin well formed; alimentary tract coiled and bulging outside; fin fold continuous with embryonic fin rays; two dorsal tentacles of equal length on head; pterygiophores just forming; 39 vertebral elements; myotomes visible; body with no pigmentation (Fig.21). 4.3 mm larva Body flat and transparent; frontal portion of head protruding; one short spine present on snout; fin rays well developed; vertebral count 38; urostyle pointing dorsalwards; left eye moved slightly upwards (Fig.22). 4.7 mm larva Body flat and asymmetrical; frontal portion of head more protruding; pelvic fin formed; two dorsal tentacles reduced in length; a short spine on the snout; left eye moved upwards; body with no pigmentation (Fig.23). Remarks
The larvae were placed in the Soleidae because of the movement of the left eye to the right side and the absence of teeth on the jaws. The presence of a short spine on snout places them in the genus Synaptura. The vertebral counts
Fig. 21. 3.25 mm larva of Synaptura
commersoniana.
Fig. 22. 4.3 mm larva of Synaptura
commersoniona.
Fig. 23. 4.7 mm larva of Synaptura
commersoniana.
359
and the number of dorsal and anal rays tally with the adults of S. comnersoniana, a commonly occurring sole in Port0 Novo waters. It differs from S. albomaculutu larvae in the absence of body pigments. Cynoglossus
are1 (Schn.)
The early stages of the larvae were reared from eggs in the laboratory, and the advanced larval stages were collected from sea plankton and are described here. rtescrip tion 0 f the egg
Eggs spherical and pelagic; oil globules 20-30 in number concentrated in one half; yolk transparent and unsegmented; perivitelline space narrow; diameter of the eggs 46-47 micrometer divisions (0.63-0.64 mm) (Fig.24). The embryos seen fully developed inside the egg after 4 h (Fig.25). Newly hatched
larva
All the eggs hatched after 10-12 hours of incubation in the laboratory. The newly hatched larvae measure 1.3-1.4 mm in total length with an appreciable amount of yolk; oil globules near the posterior ventral border; eyes and auditory vesicles formed; fin fold continuous; body transparent with no pigmentation (Fig.26). 16 h after hatching The yolk-sac larvae measure 1.8-1.9 mm in total length; body is elongated and yolk is reduced; optic and auditory vesicles are fully formed; mouth is not yet formed; thick patch of chromatophore developed on the posterior half of the larvae; pigment patches seen in the front and rear end of eye, and two more patches in the middle of the body; upper margin of the finfold has dark spots; pigments appear brownish yellow in colour; embryonic caudal rays are just forming (Fig.27). 24 h after hatching The yolk-sac larvae measure 2.6-2.7 mm in total length with the yolk much reduced; optic vesicles are enlarged; alimentary tract has started developing as a straight tube; mouth appears as a small slit; heart has developed and is pulsating; 38 myotomes could be well seen, 12 of which are pre-anal. A large patch of pigment visible on the posterior half of the body; dark pigments also distributed in the middle of body, above the hind part of alimentary tract and near the caudal; a few pigments also visible on the head and shoulder regions (Fig.28). 48 h after hatching Only a slight increase in length of the larvae (2.75 mm in total length); yolk very much reduced with traces of oil globules; mouth is formed and con-
360
netted to the alimentary tract which is coiled in a manner characteristic of the flat fishes;.fin fold is continuous and the embryonic fin rays started forming from the caudal region; a bulb-like structure visible just behind the dorsal part of head. Thick bundle of chromatophore present on the middle of body posteriorly; and another patch on the middle of body; more pigments appear above the intestinal coil and over the body (Fig.29). 72 h after hatching The length of the larvae increased slightly (2.8 mm in total length); yolk completely absorbed; dorsal tentacle well developed on the head, which is dark in colour; operculum just formed as a slit; pectoral fin well developed. The larvae are swimming actively, feeding on small organisms. The pigmentation is almost the same as in the previous stage (Fig.30). 96 h after hatching Larvae measure 2.88 mm in total length; dorsal tentacle further elongated; alimentary tract more coiled, and operculum fully formed; the pelvic fin developed as a bud; embryonic fin rays formed in the fin fold; thick patch of chromatophore visible in the middle of body; pigments are distributed through. out the body as shown in Fig.31. 144 h after hatching The 6-day-old larvae measure 3.0 mm in total length; dorsal tentacle divided into two; the anterior one larger and double than the posterior one; thick chromatophore patch disappeared; pigments visible as spots in a line on the bases of the dorsal and anal fin folds and along the middle of body in a horizontal position (Fig.32).
Fig. 24. Egg of Cynoglossus
arel.
Fig. 26. Newly hatched larva of C. arel.
Fig. 25. Egg - embryo developed (C. arel).
Fig. 27. 16 h old larva of C. arel.
Fig. 28. 24 h old larva of C. arel.
Fig. 29. 48 h old larva of C. arel.
361
Fig. 30. 72 h old larva of C. arel.
Fig. 32. 144 h old larva of C.
Fig. 31. 96 h old larva of C. arel.
arel.
6.0 mm larva
Body flat and transparent; lower jaw projecting; frontal portion of the head bulging with a notch above eye; pterygiophores and rays fully formed in fin fold; intestine well coiled; three dorsal tentacles present and the tip of first two longer tentacles appeared bulbous; gills fully formed; pigments distributed at the bases of pterygiophores and along vertebral elements; black pigment spots present in the top of body cavity; hypural elements well formed (Fig.33) 7.8 mm larva This larva shows almost the same characteristics as described in the 6.0 mm larva, but the rostra1 ‘beak’ is well developed in this larva and there is also a deep notch. The pigment pattern is same as in the previous larva (Fig.34). 8.0 mm larva Right eye migrated completely to the left side of head. In a metamorphosed larva the rostra1 ‘beak’ gradually grows forward and eventually downwards towards the snout, becoming adnate to it when eye migration was complete; dorsal tentacles and pectoral fin reduced in size; larvae almost attained the adult shape; pigmentation as in previous stage (Fig.35). 8.25 mm larva This specimen shows all the adult characteristics, The snout has grown and slightly covers the lower jaw. The dorsal tentacles have completely disappeared and the dorsal fin is further advanced. The nostrils are present in front of the eyes. The pectoral fin has disappeared and only a vestige is present. The pigmentation is as in previous stage. The total number of vertebrae is 56 including urostyle. The dorsal and anal fin rays are 110 and 83 respectively (Fig.36). Remarks
The migration
of right eye to left side of head and the disappearance
of pec-
362
Fig. 33. 6.0 mm larva of C.
Qrd.
Fig. 34. 7.8 mm larva of C. arel.
-
_
- .’__. .._..
_ _
Fig. 35. 8.0 mm larva of C. arel.
Fig. 36. 8.25 mm larva of C. arel (metamorphosed).
toral fins just after metamorphosis are characteristic features of the family Cynoglossidae. The number of dorsal, anal and vertebral elements tally with Cynoglossus arel, which is one of the commonly occurring flat fishes in Port0 Novo coast. These features confirm that these larvae belong to C. arel.
363
Cynoglossus monopus (Bleeker) Description of egg Eggs spherical and pelagic; 40-45 oil globules present, scattered throughout the egg; yolk transparent and unsegmented; perivitelline space narrow; diameter of eggs varying from 44-45 micrometer divisions (0.61-0.62 mm) (Fig. 37). The embryo developed inside the egg within 4 to 5 h (Fig.38). Newly hatched larvae All the eggs hatched out after lo-12 h of incubation in the laboratory. New ly hatched larvae measure 0.9 mm in total length; yolk sac with oil globules present; optic and auditory vesicles fully formed; larvae transparent with no pigmentation over the body (Fig.39). 24 h after hatching Yolk-sac larvae measure 1.0 mm in total length; yolk very much reduced but still present as a small sac; optic and auditory vesicles well formed; alimentary tract developed as a straight tube; fin fold continuous; star-like chromatophore present in the upper middle part of body; pigments present as (1) one large patch in the posterior half of body, (2) three more patches on the body at regular intervals, and (3) one patch near the caudal region; two dark pigment spots apparent just above the hind end of the gut; on head; dark structure present over the eye which ends at a small notch on the dorsal part of head (Fig.40). 72 h after hatching The 72-h-old larva measured 2.4 mm in total length; yolk completely absorbed; mouth well developed; alimentary tract coiled; pectoral fin and operculum well developed; fin fold continuous with embryonic fin rays near the caudal region; dorsal tentacle developed; pigments scattered over the posterior half of body; other pigments seen as shown in Fig.41. 36 myotomes visible. 96 h after hatching Post-larva 2.5 mm in total length; dorsal tentacle divided into four; pelvic fin developed; embryonic rays formed throughout the fin fold; alimentary tract well coiled; pigments present in the middle and in the central part of body; one small dark blotch present on head beneath the dorsal tentacles; (Fig.42). 3.5 mm larva Body fiat and symmetrical; mouth terminal, lower jaw projecting and jaws without teeth; gills well developed; pterygiophores and fin rays developed in fin folds; four long dorsal tentacles of equal length in front of dorsal fin fold; pigments present as six spots in a line along the vertebral elements; head, below the dorsal tentacles, also has a blotch of pigment; 53 vertebral elements (Fig.43).
364
5.0 mm larva Frontal region of head formed a rostra1 ‘beak’; four long dorsal tentacles still present; pigment as in the previous stage; vertebral elements number 54 and the urostyle bends dorsalwards; 117 dorsal fin rays and 94 anal fin rays (Fig.44). 5.6 mm larva The right eye migrated to the left side of head and larva looks like an adult fish; rostral ‘beak’ grown and become adnate, and fully merged with the snout which covers the lower jaw; pectoral fins have disappeared; nostrils present in front of eyes; dorsal tentacles completely disappeared; pigment pattern same as in 5.0 mm larva; 54 vertebral elements including urostyle; 118 dorsal and 98 anal fin rays (Fig.45). Remarks
The larvae were identified as belonging to Cynoglossus by the presence of both eyes on left side of head, dorsal and anal fins confluent with caudal. The hooked snout, the absence of a pectoral fin and the lips not fringed. The meristic characters tally with Cynoglossus monopus, a common sole along Port0 Novo coast. The eggs were collected from the sea during the months when fully mature ovaries were observed in the adults, and the larvae were collected during the following months.
Fig. 37. Egg of Cynoglossus
monopus.
Fig. 39. Newly hatched larvae of C. monopus.
Fig. 38. Egg - embryo developed (C. monopus).
Fig. 40. 24 h old larva of C. monopus.
Fig. 41. 72 h old larva of C. monopus.
Fig. 42. 96 h old larva of C. monopus.
Fig. 43. 3.5 mm larva of C. monopus.
365
Fig. 44. 5.0 mm larva of C. monopus.
Fig. 45. 5.6 mm larva of C. monopus
GENERAL
(metamorphosed).
REMARKS
Earlier workers have identified the eggs and larvae of teleosts based on various characters (Delsman, 1926). The eggs identified on the basis of the shape and size of the capsule, the nature of the yolk, the presence or absence of oil globules and their number, the pigmentation of the oil globules and the perivitelline space, etc. Cynoglossus eggs have more oil globules and the perivitelline space is narrow (Nair, 1952a, b). Venkataramanujam (1975) described the eggs of Cynoglossus sp., the characteristics of which tally with the eggs collected in the present study. The larvae of teleosts were identified by the pigment pattern, the shape of the gut, the position of the anus, the type of arrangement and the number of muscle segments, and the number of developing vertebrae. The thin, flat body, large number of soft continuous fin rays in the dorsal and anal fins, relatively few abdominal vertebrae and the coiling of alimentary canal in the earlier stages are features which distinguish the larvae as being flat fish larvae (Lalithambika Devi, 1969). The number of dorsal tentacles and the meristic counts also help in the species identification. ACKNOWLEDGEMENTS
The authors are grateful to Dr S.J. de Groot, Netherlands Institute for Fishery Investigations, The Netherlands and Dr Kunio Amaoka, Hokkaido University, Japan, for their valuable comments. For financial support and
366
facilities, they thank the University Grants Commission, authorities of Annamalai University.
New Delhi, and the
REFERENCES Amaoka, K., 1964. Development and growth of the sinistral flounder, Bothus myriaster (Temminck and Schlegel) found in the Indian and Pacific Oceans. Bull. Misaki Mar. Biol. Inst., Kyoto Univ., 5: 11-29, figs. l-8. Balakrishnan, K.P., 1963. Fish eggs and larvae collected by the Research Vessel Conch. 2. Larvae of Arnoglossus tapeinosoma Blkr., Bothus ocellatus Agassiz., Laeops guentheri Alcock, Sotea ovata Rich. and Cynoglossus monopus Blkr. Bull. Dept. Mar. Biol. Oceanogr., Univ. Kerala, 1: 81-96. Bapat, S.V., 1955. A preliminary study of pelagic fish eggs and larvae of the Gulf of Mannar and Palk Bay. Indian J. Fish., 2: 231-255. Cunningham, J.J., 1891. On some larval stages of fishes. J. Mar. Biol. Assoc. U.K., 2: 6814. Delsman, H.C., 1926. Fish eggs and larvae from the Java Sea. 5. Caranx hurrs, C. macrosoma and C. crumenophthalmus. Treubia, 8: 199-211. Gopinath, K., 1946. Notes on the larval and post larval stages of the fishes found along Trivandrum coast. Proc. Nat. Inst. Sci. India. 12: 7-21. John, M.A., 1951. Pelagic fish eggs and larvae of the Madras coast. J. Zool. Sot. India, 3 (1): 38-66. Jones, S. and Menon, P.M.G., 1951. Notes on the bionomics and developmental stages of some Indian flat fishes. J. Zool. Sot. India, 3 (10): 71-83. Jones, S. and Pantulu, V.R., 1958. On some larva and juvenile fishes from the Bengal and Orissa coasts. Indian J. Fish.,5 (11): 118-143. Kuthalingam, M.D.K., 1957. Life history and feeding habits of Cynoglossus lingua (HamBuch.). J. Zool. Sot. India, 9 (2): 208-215. Kuthalingam, M.D.K., 1960. An account of the life-history and feeding habits of a sole (Solea elongata Day). J. Madras Univ., 30 (2,3): 87-96. Lalithambika Devi, C.B., 1969. Occurrence of the larvae of Pseudorhombus eleuatus Ogilby (Heterosomata-Pisces) along the Southwest coast of India. Proc. Indian Acad. Sci. Sect. B, 70 (4): 178-185. Mansueti, A.J. and Hardy, J.D., Jr., 1967. Development of Fishes of the Chesapeake Bay region. An Atlas of Egg, Larval and Juvenile Stages, Part I. E.E. Deubler, Jr. (Editor) Natural Resources Institute, University of Maryland, Port City Press, Baltimore, Md., 202 pp. Nair, R.V., 1952a. Studies on some fish eggs and larvae of the Madras plankton. Proc. Indian Acad. Sci. Sect. B, 35: 181-208. Nair, R.V., 1952b. Studies on some post-larval fishes of the Madras plankton. Proc. Indian Acad. Sci. Sect. B, 35: 225-244. Ramanathan, N., 1977. Studies on flat fishes of Porto Novo waters (Teleostei: Pleuronectiformes). Ph. D. Thesis, Annamalai Univ., 250 pp. Seshappa, G. and Bhimachar, B.S., 1955. Studies on the fishery and biology of the Malabar sole, Cynoglossus semifasciatus Day. Indian J. Fish., 2 (1): 180-230. Smith, P.E., 1974. Manual of methods for fisheries resource survey and appraisal. Part 4. Standard techniques for pelagic fish egg and larva surveys. International Biological Programme, London, 351 pp. Venkataramanujam, K., 1975. Studies in fish eggs and larvae of Porto Novo waters. Ph. D. Thesis, Annamalai Univ., 293 pp. Vijayaraghavan, P., 1955. Life history and feeding habits of the spotted seer, Scomberomorus gutfatus (Bloch & Schneider). Indian J. Fish., 2: 360-372.
Printed in The Netherlands
349
FLAT FISH EGGS, LARVAE AND THEIR DEVELOPMENT
N. RAMANATHAN and R. NATARAJAN Centre of Advanced Study 502, Tamil Nadu (India)
(Accepted
in Marine
Biology,
Annamalai
University,
Parangipettai
608
6 March 1979)
ABSTRACT Ramanathan, Aquaculture,
N. and Natarajan, R., 1979. Flat fish eggs, larvae and their development. 13: 34!&-366.
Studies on development of eight species of flat fishes (Psettodes
erumei, Brachypleura Pseudorhombus arsius, Bothus myriaster, Synaptura albomaculata, S. Cynoglossus are1 and C. monopus) were carried out with plankton collections and with laboratory reared larvae. The development stages of Cynoglossus are1 and C. monopus from egg to metamorphosis are described.
novae-zeelandiae, commersoniana,
INTRODUCTION
The early developmental stages of fewer than 10% of total marine fish species have been described (Smith, 1974). The potential value of studies on fish eggs and larvae as an essential prerequisite for proper management of fisheries is widely recognized. Information on the eggs and larvae of marine fishes in Indian waters is sparse. Information about the east coast of India can be found in John, 1951; Jones and Menon, 1951; Nair, 1952a, b; Bapat, 1955; Kuthalingam, 1957, 1960; Jones and Pantulu, 1958. Information about the west coast. of India can be found in Gopinath, 1946; Seshappa and Bhimachar, 1955; Balakrishnan, 1963; Lalithambika Devi, 1969. Information on the development of eight species of flat fishes from the Porto Novo coast is presented here. The early life history from egg to metamorphosis in two species of flat fishes of the genus Cynoglossus, studied by rearing larvae in the laboratory and by larvae collection from plankton, are given. The information on the life history of these edible fishes by laboratory rearing methods will find full application for coastal aquaculture practices. MATERIALS
AND METHODS
The eggs and larvae were collected from the sea (9-18 m deep in the Bay of Bengal, 79” 47’ 30” E, 11” 30’ N) in the early morning, using a plank-
350
ton net made of bolting silk No. 20 (mesh size of 0.08 mm) measuring about 1.5 m long and 40.5 cm diameter near its mouth. To minimize ciliates, the eggs and larvae were frequently transferred to vessels containing fresh, filtered sea water of 35”/,, salinity and were maintained at optimum temperature of 23°C (2 2°C). Agitation of water in the containers was also maintained since it has been considered useful (Cunningham, 1891; Vijayaraghavan, 1955), not only to aerate the water, but also to prevent the larvae from resting at the bottom of the containers where ciliates and bacteria flourish. Streptomycin mixture (streptomycin sulphate 500 mg, pencillin sulphate 500 mg, and tetracycline 500 mg dissolved in 275 ml of sea water) was used to control the ciliate and bacterial populations in the water. Nannoplankters were provided as food for the larvae immediately after the mouth and alimentary tract were well formed. The terminology of Mansueti and Hardy (1967) was followed for the early developmental stages of larvae. DEVELOPMENTAL
Psettodes
erumei
STUDIES
(Bloch & Schn.)
The eggs and larvae of the Indian halibut, Psettodes erumei have not been described so far, even though this species is commercially important along the coasts of India. In this study five larvae are described. 1.15 mm larva Body thin transparent when alive and creamy white when preserved in formalin, symmetrical and laterally compressed; length slightly more than twice the depth, eyes and auditory vesicles fully developed; mouth large, terminal, oblique with lower jaw slightly projecting; alimentary tract single and coiled; operculum well developed; fin fold continuous and confluent with the caudal; an elevation in front of the dorsal fin fold with interneural spines present; no rays discernible in the finfold; pectoral and pelvic fins not developed; 24 vertebral segments and myotomes clearly visible; air bladder present; no pigmentation over body (Fig.1). 2.4 mm larva Body thin, flat and symmetrical; length slightly less than three times the depth; an oblique well developed mouth with a projecting lower jaw; alimentary tract single and coiled with the posterior part almost parallel to body; six dorsal tentacles present on the elevation of the head in front of the dorsal fin fold; fin folds have no rays; pectoral fin well developed; 24 vertebral segments as in the previous stage; air bladder present; no pigmentation over body (Fig.2). 4.45 mm larva Body flat and symmetrical;
mouth large and oblique
with the lower jaw
351
projecting; teeth developed on both jaws; alimentary tract single and coiled, the posterior part bending downwards; pectoral fins present but pelvic fin not yet developed; gills just forming and visible through the transparent head; eyes well developed; nine dorsal tentacles of same length present; embryonic fin rays seen in the fin folds; median fin folds continuous with the caudal, but the junction with the caudal fin marked by a shallow indentation; 24 vertebral elements present; hypural elements well differentiated and consequently the urostyle turned dorsally; no pigmentation over body (Fig.3). 5.4 mm larva Body still flat and symmetrical; mouth oblique with projected lower jaw; teeth developed in both jaws than in the previous larva; nine dorsal tentacles still present; caudal fin separated from the median fin folds; pelvic fin developed; 24 vertebral elements present; urostyle pointed dorsalwards; gills well formed; no pigmentation over body (Fig.4). 5.7 mm larva Body flat and symmetrical; pterygiophores have formed; and 18 caudal fin rays well developed; nine dorsal tentacles fin completely separated from dorsal and anal fins (Fig.5).
Fig. 1. 1.15
Fig. 3. 4.45
mm larva of Psettodes
mm larva of Psettodes
Fig. 5. 5.7 mm larva of Psettodes
54 dorsal, 40 anal shortened; caudal
erumei
Fig. 2. 2.4 mm larva of Psettodes
erumei.
Fig. 4. 5.4 mm larva of Psettodes
erumei.
erumei.
erumei.
352
Remarks The meristic characters, such as the number of dorsal and anal fin rays and the vertebral counts, support the identification of the larvae as Psettodes erumei. The presence of well developed, strong teeth in the jaws and the projected lower jaw support this identification. The larvae were collected from the sea plankton during the period July to October, the spawning period of P. erumei along Port0 Novo coast. Brachypleura novae-zeelandiae Gunther Two post-larvae of Brachypleura novae-zeelandiae measuring 5.6 mm and 7.0 mm, were collected, and one post-larva (7.0 mm) was found to be in the metamorphosed stage, having both eyes on the right side of head. The descriptions are given below. 5.6 mm larva Body flat, transparent and bilaterally asymmetrical; the left eye started migrating; mouth large, oblique and jaws with minute teeth; anterior dorsal profile of head above the eye with a deep notch; operculum well developed; alimentary tract coiled; gills formed; pectoral and pelvic fins well developed; pterygiophores and fin rays well formed in the fin fold; the dorsal and anal fins separated from caudal; 76 rays in dorsal fins and 50 rays in anal fin;.vertebral count 39 including urostyle, which points dorsalwards; body with no pigmentation (Fig.6). 7.0 mm larva Body flat, transparent and asymmetrical; mouth big, oblique and jaws with minute teeth; left eye migrated completely to the right side; alimentary tract complicatedly coiled; gills present; dorsal and anal fins completely separated from caudal fin; dorsal fin originates just behind the upper eye and anal fin behind the anus; 75 rays in dorsal fin and 50 rays in anal fin; vertebral count 39 including urostyle which points upwards; body with no pigmentation (Fig.7).
Fig. 6. 5.6. mm larva of Brachypleura
novae-zeelandiae.
Fig. 7. 7.0 mm larva of Brachypleura
novae-zeelandiae.
353
Remarks The left eye has started migrating in the 5.6 mm larva, and it has completely migrated to the right side in the 7.0 mm larva. The presence of both eyes on the right side of the head, the meristic counts, such as dorsal and anal fin rays and the vertebral counts, tally with the adult structure of Bmchypleura nouaezeelandiae. Adults with mature ovaries could be collected one month prior to the collection of these larvae and thus provide additional evidence. Pseudorhombus
arsius (Ham. Buch.)
Three post-larval
stages of Pseudorhombus arsius are described.
3.6 mm larva Larva flat, transparent and bilaterally symmetrical; length more than twice the depth; mouth well developed and terminal; lower jaw slightly projecting; operculum formed as a small slit; eyes fully developed and symmetrical; abdominal region bulging; alimentary tract straight; continuous finfold with embryonic fin rays; three dorsal tentacles, the anterior larger than the posterior ones, all free from one another in front of the dorsal fin fold; 34 vertebral elements; body with no pigmentation (Fig.8). 4.0 mm larva Larva flat, transparent and asymmetrical; length slightly more than twice the depth; mouth terminal and jaws with very minute teeth; operculum well formed; migrating right eye clearly seen on the other side along dorsal margin of head resulting in the twisting of brain; alimentary tract single and coiled; pectoral and pelvic fins well formed; pterygiophores fully developed; dorsal and anal fin folds separated from caudal fin; dorsal tentacles reduced in size very much; hypural elements well differentiated; the urostyle pointing dorsalwards; 35 vertebral elements; body with no pigmentation (Fig.9). 8.0 mm larva Body flat and asymmetrical; mouth terminal, oblique a& jaws with very minute teeth; alimentary tract well coiled and anus shifted forward; right eye, on the upper margin of left side migrating; three dorsal tentacles seen as vestiges; dorsal fin advanced anteriorly; dorsal and anal fins completely separated from caudal; anal fin starts immediately behind the anus; rays in fins well developed; gill filaments seen through the transparent head; 35 vertebrae; hypural element points dorsalwards; 71 dorsal rays and 56 anal rays; body has no pigmentation (Fig.10). Remarks The migration of the right eye to the left side, the extension of the dorsal fin above the eye and the separation of the dorsal and anal fins from the cauda1 are characters which place these larvae in paralichthide group. Gopinath
Fig. 8. 3.6 mm larva of Pseudorhombus
arsius.
Fig. 9. 4.0 mm larva of Pseudorhombus
arsius.
Fig. 10. 8.0 mm larva of Pseudorhombus
arsius.
(1946) and Jones and Pantulu (1958) did not give any indication of the presence of the elongated dorsal tentacles, although this character is one of the most diagnostic features of Pseudorhombus larvae (Lalithambika Devi, 1969). The meristic characters, such as dorsal and anal fin rays and vertebral counts, confirm the larvae as being Pseudorhombus arsius. 7 he larvae were recorded during the months of May to July, the spawning season of P. arsius along Porto Novo coast (Ramanathan, 1977). Bothus
myriaster
(Temminck
and Schlegel)
Three larvae of Bothus myriuster
are described.
1.6 mm larva
Body flat, transparent and symmetrical; length less than twice of depth; mouth developed and terminal; lower jaw slightly projected; head has a steep contour; operculum and pectoral fin developed; eyes present; alimentary tract single and coiled; median finfold continuous with the caudal, but the junction with the caudal fin marked by a shallow indentation; the pterygiophores and the embryonic fin rays formed in the fin fold; two dorsal tentacles present in front of dorsal fin fold; gills well developed; five to six serrations present on ventral margin of head; myotomes closely packed; 38 vertebral elements; a black spot present just above the coil of alimentary tract (Fig.11). 12.3
mm larva
Body flat and symmetrical; mouth terminal and small, no teeth in jaws; alimentary tract single and coiled; pterygiophores well developed; dorsal and anal fin folds separated from caudal, and dorsal fin fold advanced anteriorly; fin rays well developed in the fin folds; pelvic fin formed as a bud; two dorsal tentacles present, the anterior one longer and the posterior shorter; 91 rays in dorsal fin and 69 in anal fin; 38 vertebral elements; hypural element formed
355
and urostyle pointing upward; serrations present on the ventral border of head; a black spot present just above the coiling of alimentary tract (Fig.12). 15.6 mm larva Body flat and symmetrical; head very small; mouth terminal and small; jaws without teeth; eyes small and located symmetrically on both sides of head; gills present; alimentary tract is single coiled; the maxillary bone formed; dorsal tentacles represented as vestiges; 94 rays in dorsal fin and 70 rays in anal fin; 38 vertebral elements including urostyle, body with no pigmentation (Fig.13).
Fig. 11. 1.6 mm larva of Bothus myriaster. Fig. 12. 12.3 mm larva of Bothus myriaster. Fig. 13. 15.6 mm larva of Bothus
myriaster.
Remarks
Amaoka (1964) described the development of B. myriaster from a post-larva, 29 mm in total length, to the adult stage. The larvae described here were found to have a small mouth, a characteristic feature of the genus Bothus (Amaoka, 1964). The dorsal and anal fin rays and the vertebral count tally with the species B. myriaster. According to Amaoka (1964), the postrlarvae of B. myriaster have no pigments on the body. The post-larva measuring 15.6 mm in total length described here is also devoid of :)ody pigments. B. myriaster is the only species of the genus Bothus occurring along Port0 Novo coast. Synaptura
albomaculata
Kaup
Seven larvae of Synaptura
albomaculata
are described.
1.6 mm larva Body flat, laterally compressed, transparent and symmetrical; mouth terminal, lower jaw projecting forward; no teeth in jaws; eyes well developed and
356
operculum just formed; alimentary tract single coiled and abdomen bulging outside; fin fold continuous and no rays discernible in the fin; two dorsal tentacles, the anterior one longer and posterior shorter; pectoral or pelvic fins not yet formed; myotomes visible; seven pigment spots along the base of dorsal and anal fin fold in a line; light black pigment on the head below dorsal tentacles; snout has a short spine (Fig.14). 2.4 mm larva Body flat, transparent and symmetrical; mouth terminal with the lower jaw projecting; jaws without teeth; eyes well developed; opercular slit advanced; alimentary tract single coiled; embryonic rays formed in fin folds; 12 spots of pigments along the base of dorsal and anal fin folds in a straight line; dark pigment present on head below the base of dorsal tentacles; other pigment on the chin, behind the eye and above the coil of alimentary tract; 50 well developed vertebral elements present; fin fold continuous; pectoral and pelvic fins not yet developed (Fig. 15). 3.1 mm larva Body flat and symmetrical; mouth terminal, projecting lower jaw with no teeth; alimentary tract single, coiled and bulging outside; pectoral fin developed; rays developed in the caudal fin; pterygiophores started forming; pigments at the base of the dorsal and anal fin folds; 50 vertebral elements (Fig. 16). 4.8 mm larva Body flat and symmetrical; mouth terminal, lower jaw slightly projecting; jaws without teeth; gills well developed; pelvic fin fully formed; alimentary tract single and coiled; two dorsal tentacles; rays well developed in all fin folds; pigments distributed on the body as in the previous stage; 79 rays in dorsal fin and 61 in anal fins, vertebral count 50 (Fig.17). 7.6 mm larva A rostra1 ‘beak’ well formed; dorsal tentacles shortened; hypural elements developed and urostyle pointing dorsalwards; seven black pigment spots at the base of dorsal and anal pterygiophores; 79 rays in dorsal fin and 61 rays in anal fin; 50 vertebral elements including urostyle (Fig. 18). 8.6 mm larva Frontal portion of head grown forward; dorsal tentacles represented as vestige; alimentary tract with complex coil; 79 rays in dorsal fin and 62 rays in anal fin; vertebral count 50 (Fig.19). 9.2 mm larva
Frontal portion duced (Fig.20).
of head bulged out, and the dorsal tentacles
further
re-
357 A--
Fig. 14.
1.6 mm larva of Synaptura
albomaculata.
Fig. 15. 2.4 mm larva of Synaptura
albomaculata.
Fig. 16. 3.1 mm larva of Synaptura
albomaculata.
Fig. 17. 4.8 mm larva of Synaptura
albomaculata.
Fig. 18. 7.6 mm larva of Synaptura
albomaculata.
Fig. 19. 8.6 mm larva of Synaptura
albomaculata.
Fig. 20. 9.2 mm larva of Synaptura
albomaculata.
358
Remarks
The movement of the left eye to the right side of head and absence of teeth on jaws help in distinguishing the larvae as belonging to the Soleidae. The presence of a short spine on the snout identifies this as belonging to Synaptura. The number of dorsal and anal fin rays tally with the adults of 5’. albomaculata, a commonly occurring species along Porto Novo coast. Hence these larvae were identified as those of S. albomaculata. Synaptura
commersoniana
(Lecepkle)
3.25 mm larva Body flat, transparent and symmetrical; mouth terminal and jaws without teeth; eyes well developed; operculum. gills and pectoral fin well formed; alimentary tract coiled and bulging outside; fin fold continuous with embryonic fin rays; two dorsal tentacles of equal length on head; pterygiophores just forming; 39 vertebral elements; myotomes visible; body with no pigmentation (Fig.21). 4.3 mm larva Body flat and transparent; frontal portion of head protruding; one short spine present on snout; fin rays well developed; vertebral count 38; urostyle pointing dorsalwards; left eye moved slightly upwards (Fig.22). 4.7 mm larva Body flat and asymmetrical; frontal portion of head more protruding; pelvic fin formed; two dorsal tentacles reduced in length; a short spine on the snout; left eye moved upwards; body with no pigmentation (Fig.23). Remarks
The larvae were placed in the Soleidae because of the movement of the left eye to the right side and the absence of teeth on the jaws. The presence of a short spine on snout places them in the genus Synaptura. The vertebral counts
Fig. 21. 3.25 mm larva of Synaptura
commersoniana.
Fig. 22. 4.3 mm larva of Synaptura
commersoniona.
Fig. 23. 4.7 mm larva of Synaptura
commersoniana.
359
and the number of dorsal and anal rays tally with the adults of S. comnersoniana, a commonly occurring sole in Port0 Novo waters. It differs from S. albomaculutu larvae in the absence of body pigments. Cynoglossus
are1 (Schn.)
The early stages of the larvae were reared from eggs in the laboratory, and the advanced larval stages were collected from sea plankton and are described here. rtescrip tion 0 f the egg
Eggs spherical and pelagic; oil globules 20-30 in number concentrated in one half; yolk transparent and unsegmented; perivitelline space narrow; diameter of the eggs 46-47 micrometer divisions (0.63-0.64 mm) (Fig.24). The embryos seen fully developed inside the egg after 4 h (Fig.25). Newly hatched
larva
All the eggs hatched after 10-12 hours of incubation in the laboratory. The newly hatched larvae measure 1.3-1.4 mm in total length with an appreciable amount of yolk; oil globules near the posterior ventral border; eyes and auditory vesicles formed; fin fold continuous; body transparent with no pigmentation (Fig.26). 16 h after hatching The yolk-sac larvae measure 1.8-1.9 mm in total length; body is elongated and yolk is reduced; optic and auditory vesicles are fully formed; mouth is not yet formed; thick patch of chromatophore developed on the posterior half of the larvae; pigment patches seen in the front and rear end of eye, and two more patches in the middle of the body; upper margin of the finfold has dark spots; pigments appear brownish yellow in colour; embryonic caudal rays are just forming (Fig.27). 24 h after hatching The yolk-sac larvae measure 2.6-2.7 mm in total length with the yolk much reduced; optic vesicles are enlarged; alimentary tract has started developing as a straight tube; mouth appears as a small slit; heart has developed and is pulsating; 38 myotomes could be well seen, 12 of which are pre-anal. A large patch of pigment visible on the posterior half of the body; dark pigments also distributed in the middle of body, above the hind part of alimentary tract and near the caudal; a few pigments also visible on the head and shoulder regions (Fig.28). 48 h after hatching Only a slight increase in length of the larvae (2.75 mm in total length); yolk very much reduced with traces of oil globules; mouth is formed and con-
360
netted to the alimentary tract which is coiled in a manner characteristic of the flat fishes;.fin fold is continuous and the embryonic fin rays started forming from the caudal region; a bulb-like structure visible just behind the dorsal part of head. Thick bundle of chromatophore present on the middle of body posteriorly; and another patch on the middle of body; more pigments appear above the intestinal coil and over the body (Fig.29). 72 h after hatching The length of the larvae increased slightly (2.8 mm in total length); yolk completely absorbed; dorsal tentacle well developed on the head, which is dark in colour; operculum just formed as a slit; pectoral fin well developed. The larvae are swimming actively, feeding on small organisms. The pigmentation is almost the same as in the previous stage (Fig.30). 96 h after hatching Larvae measure 2.88 mm in total length; dorsal tentacle further elongated; alimentary tract more coiled, and operculum fully formed; the pelvic fin developed as a bud; embryonic fin rays formed in the fin fold; thick patch of chromatophore visible in the middle of body; pigments are distributed through. out the body as shown in Fig.31. 144 h after hatching The 6-day-old larvae measure 3.0 mm in total length; dorsal tentacle divided into two; the anterior one larger and double than the posterior one; thick chromatophore patch disappeared; pigments visible as spots in a line on the bases of the dorsal and anal fin folds and along the middle of body in a horizontal position (Fig.32).
Fig. 24. Egg of Cynoglossus
arel.
Fig. 26. Newly hatched larva of C. arel.
Fig. 25. Egg - embryo developed (C. arel).
Fig. 27. 16 h old larva of C. arel.
Fig. 28. 24 h old larva of C. arel.
Fig. 29. 48 h old larva of C. arel.
361
Fig. 30. 72 h old larva of C. arel.
Fig. 32. 144 h old larva of C.
Fig. 31. 96 h old larva of C. arel.
arel.
6.0 mm larva
Body flat and transparent; lower jaw projecting; frontal portion of the head bulging with a notch above eye; pterygiophores and rays fully formed in fin fold; intestine well coiled; three dorsal tentacles present and the tip of first two longer tentacles appeared bulbous; gills fully formed; pigments distributed at the bases of pterygiophores and along vertebral elements; black pigment spots present in the top of body cavity; hypural elements well formed (Fig.33) 7.8 mm larva This larva shows almost the same characteristics as described in the 6.0 mm larva, but the rostra1 ‘beak’ is well developed in this larva and there is also a deep notch. The pigment pattern is same as in the previous larva (Fig.34). 8.0 mm larva Right eye migrated completely to the left side of head. In a metamorphosed larva the rostra1 ‘beak’ gradually grows forward and eventually downwards towards the snout, becoming adnate to it when eye migration was complete; dorsal tentacles and pectoral fin reduced in size; larvae almost attained the adult shape; pigmentation as in previous stage (Fig.35). 8.25 mm larva This specimen shows all the adult characteristics, The snout has grown and slightly covers the lower jaw. The dorsal tentacles have completely disappeared and the dorsal fin is further advanced. The nostrils are present in front of the eyes. The pectoral fin has disappeared and only a vestige is present. The pigmentation is as in previous stage. The total number of vertebrae is 56 including urostyle. The dorsal and anal fin rays are 110 and 83 respectively (Fig.36). Remarks
The migration
of right eye to left side of head and the disappearance
of pec-
362
Fig. 33. 6.0 mm larva of C.
Qrd.
Fig. 34. 7.8 mm larva of C. arel.
-
_
- .’__. .._..
_ _
Fig. 35. 8.0 mm larva of C. arel.
Fig. 36. 8.25 mm larva of C. arel (metamorphosed).
toral fins just after metamorphosis are characteristic features of the family Cynoglossidae. The number of dorsal, anal and vertebral elements tally with Cynoglossus arel, which is one of the commonly occurring flat fishes in Port0 Novo coast. These features confirm that these larvae belong to C. arel.
363
Cynoglossus monopus (Bleeker) Description of egg Eggs spherical and pelagic; 40-45 oil globules present, scattered throughout the egg; yolk transparent and unsegmented; perivitelline space narrow; diameter of eggs varying from 44-45 micrometer divisions (0.61-0.62 mm) (Fig. 37). The embryo developed inside the egg within 4 to 5 h (Fig.38). Newly hatched larvae All the eggs hatched out after lo-12 h of incubation in the laboratory. New ly hatched larvae measure 0.9 mm in total length; yolk sac with oil globules present; optic and auditory vesicles fully formed; larvae transparent with no pigmentation over the body (Fig.39). 24 h after hatching Yolk-sac larvae measure 1.0 mm in total length; yolk very much reduced but still present as a small sac; optic and auditory vesicles well formed; alimentary tract developed as a straight tube; fin fold continuous; star-like chromatophore present in the upper middle part of body; pigments present as (1) one large patch in the posterior half of body, (2) three more patches on the body at regular intervals, and (3) one patch near the caudal region; two dark pigment spots apparent just above the hind end of the gut; on head; dark structure present over the eye which ends at a small notch on the dorsal part of head (Fig.40). 72 h after hatching The 72-h-old larva measured 2.4 mm in total length; yolk completely absorbed; mouth well developed; alimentary tract coiled; pectoral fin and operculum well developed; fin fold continuous with embryonic fin rays near the caudal region; dorsal tentacle developed; pigments scattered over the posterior half of body; other pigments seen as shown in Fig.41. 36 myotomes visible. 96 h after hatching Post-larva 2.5 mm in total length; dorsal tentacle divided into four; pelvic fin developed; embryonic rays formed throughout the fin fold; alimentary tract well coiled; pigments present in the middle and in the central part of body; one small dark blotch present on head beneath the dorsal tentacles; (Fig.42). 3.5 mm larva Body fiat and symmetrical; mouth terminal, lower jaw projecting and jaws without teeth; gills well developed; pterygiophores and fin rays developed in fin folds; four long dorsal tentacles of equal length in front of dorsal fin fold; pigments present as six spots in a line along the vertebral elements; head, below the dorsal tentacles, also has a blotch of pigment; 53 vertebral elements (Fig.43).
364
5.0 mm larva Frontal region of head formed a rostra1 ‘beak’; four long dorsal tentacles still present; pigment as in the previous stage; vertebral elements number 54 and the urostyle bends dorsalwards; 117 dorsal fin rays and 94 anal fin rays (Fig.44). 5.6 mm larva The right eye migrated to the left side of head and larva looks like an adult fish; rostral ‘beak’ grown and become adnate, and fully merged with the snout which covers the lower jaw; pectoral fins have disappeared; nostrils present in front of eyes; dorsal tentacles completely disappeared; pigment pattern same as in 5.0 mm larva; 54 vertebral elements including urostyle; 118 dorsal and 98 anal fin rays (Fig.45). Remarks
The larvae were identified as belonging to Cynoglossus by the presence of both eyes on left side of head, dorsal and anal fins confluent with caudal. The hooked snout, the absence of a pectoral fin and the lips not fringed. The meristic characters tally with Cynoglossus monopus, a common sole along Port0 Novo coast. The eggs were collected from the sea during the months when fully mature ovaries were observed in the adults, and the larvae were collected during the following months.
Fig. 37. Egg of Cynoglossus
monopus.
Fig. 39. Newly hatched larvae of C. monopus.
Fig. 38. Egg - embryo developed (C. monopus).
Fig. 40. 24 h old larva of C. monopus.
Fig. 41. 72 h old larva of C. monopus.
Fig. 42. 96 h old larva of C. monopus.
Fig. 43. 3.5 mm larva of C. monopus.
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Fig. 44. 5.0 mm larva of C. monopus.
Fig. 45. 5.6 mm larva of C. monopus
GENERAL
(metamorphosed).
REMARKS
Earlier workers have identified the eggs and larvae of teleosts based on various characters (Delsman, 1926). The eggs identified on the basis of the shape and size of the capsule, the nature of the yolk, the presence or absence of oil globules and their number, the pigmentation of the oil globules and the perivitelline space, etc. Cynoglossus eggs have more oil globules and the perivitelline space is narrow (Nair, 1952a, b). Venkataramanujam (1975) described the eggs of Cynoglossus sp., the characteristics of which tally with the eggs collected in the present study. The larvae of teleosts were identified by the pigment pattern, the shape of the gut, the position of the anus, the type of arrangement and the number of muscle segments, and the number of developing vertebrae. The thin, flat body, large number of soft continuous fin rays in the dorsal and anal fins, relatively few abdominal vertebrae and the coiling of alimentary canal in the earlier stages are features which distinguish the larvae as being flat fish larvae (Lalithambika Devi, 1969). The number of dorsal tentacles and the meristic counts also help in the species identification. ACKNOWLEDGEMENTS
The authors are grateful to Dr S.J. de Groot, Netherlands Institute for Fishery Investigations, The Netherlands and Dr Kunio Amaoka, Hokkaido University, Japan, for their valuable comments. For financial support and
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facilities, they thank the University Grants Commission, authorities of Annamalai University.
New Delhi, and the
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