Anguillid Leptocephali in the Southwest Pacific
Publication of this paper has been assisted by a grant from the Victoria University of Wellington Publications Fund. Material described here was in part assembled with the aid of a grant, enabling travel to Australia, from the Victoria University of Wellington.
Of twelve leptocephali from the New Caledonia area, myotome counts, etc., firmly identify two as Anguilla megastoma, three as A. australis, one as A. obscura, but six, recognisable as long-finned larvae and associated with A. marmorata and A. reinhardti cannot be assigned definitely to either of these species. Two glass-eels from near Sydney are referred to A. australis ?australis on geographic grounds. Including the four Dana leptocephali of 1928-29 from the Southwest Pacific, the relative sizes suggest a breeding area east of New Caledonia.
In the course of a study of New Zealand eels, a small collection of eel larvae, about 20 specimens in number, was assembled from various sources in this country. During the early stages of identification and description of these specimens several large collections of leptocephali were made available to the author for study purposes from institutions outside New Zealand—namely, (a) the Centre d'Océanographie de l'Institut Française d'Océanie, Nouméa, New Caledonia, (b) the C.S.I.R.O. Division of Fisheries and Oceanography, Cronulla, New South Wales, (c) the Australian Museum, Sydney, and (d) the Western Australian Museum, Perth. At the time of writing the total number of leptocephali assembled by the author is about 1100 specimens. The present paper, however, deals with only a small fraction of this number, some 14 specimens, belonging to the genus Anguilla Shaw and referable to four or five of the six known Australasian species. This group of larvae was chosen for study before others because Anguilla, more than any other genus of eels, is the most completely known. Eel larvae are often difficult to identify, mainly due to insufficient knowledge of the adults. The species of Anguilla, however, have been so well described (Ege, 1939) from their adults and are so well characterised that the identification of their larvae is a relatively easy matter. Further, Jespersen's (1942) account of the larvae of the Indo-Pacific anguillid eels in the massive Dana collection (1500 specimens) serves as a most valuable reference in this present work.
The author would here like to express his warmest thanks to the authorities of the above institutions who kindly placed their collections on loan; to Mr M. Legand (I.F.O., Nouméa), Mr I. S. R. Munro (Div. Fish. Oceanogr., Cronulla), Mr G. P. Whitley (Australian Museum, Sydney) and Dr R. W. George (Western Australian Museum, Perth) without whose individual interest this study would not have been possible; finally, Prof. L. R. Richardson, of the Department of Zoology, Victoria University of Wellington, for his welcome criticisms.
The present collections from the Australian region include about 550 leptocephali, but only one of these leptocephali is referable to the genus Anguilla; two others are glass-eels or virtually unpigmented elvers. These three specimens were collected on the east coast of Australia. Eleven other Anguilla leptocephali were taken in the New Caledonia-Solomons-New Hebrides area as part of a collection of about 550 specimens.
Preservation and Methods of Measurement
Where possible, preserved leptocephali were transferred to and held in a mixture to the formula given in Lea (1913, p. 5): alcohol (96%), 3 pts., formalin (3%), 3 pts. and glycerine, 2 pts. in which they remained soft and pliable; those that were already hardened by previous preservation softened noticeably. The glycerine included in this preservative raised the refractive index and offered an admirable medium for examination. Specimens which had become swollen on previous preservation in formalin were often restored by this mixture. Its chief disadvantage was that the glycerine attracted dust particles and scrupulously clean working methods were therefore required. Other specimens were stored in alcohol or 5% formalin.
Specimens were examined under a low-power binocular microscope with both transmitted and reflected light, the latter being particularly useful in observing teeth and in counting myomeres. Drawings were made with the aid of a camera lucida. Some specimens were stained in acetic acid-alum-carmine and cleared in glycerine for examination of structures in the alimentary, renal and vascular systems.
Total lengths were measured to the nearest 0.1mm by placing the specimen on a piece of white, waxed card and using pins to indicate the extremities; total length was measured from the tip of the upper jaw to the tip of the caudal fin. Contraction in length on preservation has been observed in Anguilla leptocephali by Jespersen (1942, p. 9) amounting to about l-2mm in specimens of about 60mm total length. Other measurements in each specimen are as used by other authors, but a few which may be uncertain are taken as follows: Head—tip of snout to anterior extremity of pectoral base (the branchial aperture is often hard to distinguish in leptocephali); upper jaw—tip of snout to end of exposed surface of maxilla (this is a more satisfactory measurement than that of the gape or cleft of mouth since the exact corner of the mouth when open is also hard to distinguish); postorbital—posterior margin of eye to anterior extremity of pectoral base (or branchial aperture where the pectoral fin is absent). Caudal rays are counted on each hypural, from the upper to the lower, and expressed as h1 + h2 + h3 etc., where h1 is the upper. Teeth are counted on the left side in both upper and lower jaws in the manner of Jespersen (1942, p. 12); the sizes and separation of the various groups of teeth are distinguished by Arabic and Roman numerals, thus: indicates that in the upper jaw there is a "grasping tooth" (usually a large recurved tooth) projecting forwards at the anterior tip of the upper jaw followed by six larger blade-like teeth and finally by a series of eight smaller teeth; the size of the teeth and the formula are much the same in the lower jaw. This type of dentition is found in leptocephali of Anguilla but occurs very similarly throughout the larvae of many different eels.
Identification of leptocephali to the species level is often a difficult task. The relatively few characters which are associated with a certain form of leptocephalus—e.g., disposition of melanophores and vertical intestinal blood-vessels, dentition, shape of caudal fin, always fail to survive the metamorphosis from the larva to the juvenile eel. In only a few species have a series of transitional stages been collected and the complete metamorphosis worked out. Transitional forms are in many cases active, bottom-dwelling and more easily escape trawls than do the leptocephali.
Fortunately, however, there is one major, accessible character which continues on from the larva, through the metamorphosis, to the juvenile and the adult. This is the number of myomeres (actually postcranial segments). Other characters such as fin-rays and branchiostegal rays also appear unchanged in number page 3 at least from the fully-developed leptocephalus, but are less useful for identification since they do not always appear in the larva in early stages of development.
In counting segments the first full segment is taken as the most anterior myomere extending at least to the level of the notochord and the last taken as that indicated by the last spinal ganglion. Spinal ganglia are indeed often easier to count than the myomeres. In eels the number of vertebrae is usually one less than the number of myomeres in the larvae, and this has been taken into account when referring the leptocephali to their adult species. Preanal myomeres are counted to a vertical through the vent; usually the vertical falls on the middle of a myomere at the lateral midline, and in these cases this myomere is included in the preanal number. All segments posterior to this are regarded as postanal. The distance between the dorsal and anal origins, or a–d, is expressed as the number of segments counted between the verticals through these origins.
In all leptocephali the viscera are supplied by a number of median blood-vessels from the aorta; these are referred to in this study as the "vertical blood-vessels". The positions of these vessels—there may be few to many—provide a valuable specific character. Usually the first vessel (to the anterior margin of the liver) and the last (to the posterior margin of the opisthonephros) are the most conspicuous and more readily observed. The position of each vessel is indicated by the myomere level at which it lies—e.g., 17, 25, 41, being the junction of the vessel with the aorta. In the case where a vessel is situated level with a myoseptum the level is indicated by the two nearest myomeres—e.g., 41–42.
Opinion 44 (1912) of the International Rules of Zoological Nomenclature recognised Leptocephalus Gronovius, 1763, as the genus for conger eels since the larval specimen described by this author as L. morrisii was indeed a young conger. This action was reversed by Opinion 93 (1926) when Conger Cuvier, 1817 (deleted in 1958, and Conger Oken, 1817, replaced) was designated as the official name for conger eels and Leptocephalus reserved for the larval form. While a number of attempts have been made to group larval eels into various genera and subgenera, the present author follows the simple course of referring all species to the genus Leptocephalus. In naming leptocephali it has often been the practice to use the clumsy genitive form for leptocephali whose adults are known—e.g., Leptocephalus Anguillae marmoratae. The author proposes to follow Bruun (1937, p. 1) indicating a given form by the nominative—e.g., Leptocephalus Anguilla marmorata or more simply L. Anguilla marmorata.
The genus Anguilla Shaw, the sole representative of the Family Anguillidae, has become, with the exhaustive researches of Danish scientists, the most intimately-known genus of eels. The two Atlantic species A. anguilla (L.) and A. rostrata (Le Sueur) were studied thoroughly by Johannes Schmidt over a long period of years, initiating a great deal of further work. His collections formed the basis of a definitive study of the systematics of the genus by Ege (1939), and Jespersen (1942) followed with a detailed account of the leptocephali of many of the Indo-Pacific species of the genus. A considerable amount is nevertheless still to be added, especially concerning the biology of the Southwest Pacific species which, because of limited material, was treated rather briefly by Jespersen.
Text-fig. 1.—L. Anguilla megastoma, 26.4mm total length, IFO St 56–4–3 Fig A— Lateral view, to show distribution of major vertical blood-vessels, intestine and myomeres at level of vent. Fig. B—Lateral view of head. Fig. C—Lateral view of caudal region.
Despite this widespread occurrence of so many species of Anguilla in the South Pacific only four anguillid leptocephali have ever been recorded from this area; these were from the Dana collection (see Jespersen, 1942, pp. 13–15). By contrast, nearly 1500 specimens of Anguilla larvae were taken by the Dana in the whole of the Indo-Pacific. The South Pacific area was widely explored by this vessel during the months of October, 1928, to March, 1929, and Jespersen assumes that the scarcity of Anguilla larvae in this area may have been due to the time of year when the trawling took place. The larvae described here, 12 in number, therefore make a significant addition to knowledge of the South Pacific species of Anguilla. These larvae were collected in similar depths to those of the Dana larvae and in various months of the year, so that Jespersen's assumption is not supported by the present collection.
L. Anguilla megastoma (Text-fig. 1, A, B, C)
Material Examined. Two specimens, 23.7mm and 26.4mm total lengths; Institut Français d'Océanie Station 56–4–3; 12° 55′ S., 170° 04′ E.; 27/9/56 (2324 hours); horizontal tow in 70m; 0.5m net, No. 2 mesh.
Description. Measurements in mm: total length 26.4 (23.7), head 2.7 (2.7), snout 0.9 (0.9), eye 0.5 (0.6), upper jaw 1.0 (1.0), postorbital 1.3 (1.2), pectoral 0.9 (0.8), preanal 18.8 (17.4), predorsal 16.8 (16.2), depth just before eye 1.2 (1.0), depth at pectoral origin 2.2 (3.1), depth at midpoint between pectoral and vent 4.6 (4.2), depth at anal origin 4.4 (3.2). Branchiostegal and pectoral rays not obvious, dorsal and anal elements visible only near tip of caudal region, caudal rays 2 + 2 + 2 + 2 + 1. Teeth . Myomeres 72 + 39 = 111 (114). a–d = 7 (7). 1st vertical blood-vessel at 17th myomere, 2nd at 42nd and 3rd at 48th. Anterior margin of gall-bladder at level of 29th myomere.
Body elongate but not excessively so, much compressed, not very deep and tapering a little more gradually in front of midpoint of body. Head short, about one-ninth of total length, its lower profile indented at the throat to make the head region clearly differentiated from the trunk; snout short, equal to one-third of head length, acutely pointed with its dorsal profile conspicuously concave and anterior and posterior nares almost separated; eye moderate, oval, with its greatest diameter vertical and a little less than length of snout; gape oblique, extending to level of anterior margin of eye; teeth relatively large, very acute, eight in the upper jaw, projecting outside the six of the lower jaw and distributed as follows: a large, forwardly-projecting grasping tooth is preceded by a tiny, needle-like tooth placed almost on the dorsal surface of the snout and is followed by three large teeth which become progressively smaller; this series is followed by three noticeably smaller teeth, the most posterior of which is placed almost directly under the anterior rim of the orbit; the teeth of the lower jaw are similar in page 6 size and grouping to those of the upper with the most posterior tooth placed a little in advance of the level of the last upper tooth. Pectoral fin relatively large, equal to length of snout and elongate-oval in shape; base of fin thick and fleshy, web of fin thin, supported by delicate fin-rays which are poorly developed. Dorsal fin low, with only the radials visible at the end of the caudal region. Anal fin similar. Caudal fin conspicuously separated from the tips of the dorsal and anal fins.
Colour in preservative translucent, with black pigment confined to the chorioid of the eye.
Remarks. The two specimens described here belong unquestionably to the genus Anguilla. They agree very well with Group I (Anguillidae) of Ancona (1928, p. 102) in which the body is relatively short, high, in the form of an olive leaf, the intestine is straight and not festooned or swollen; the vent is about two-thirds of the length along the body; the dorsal fin originates a little in advance of the level of the vent; there is no pigmentation in the body except in very small specimens (5.0mm to 10.0mm). Both are long-finned larvae having seven myomeres between the dorsal and anal origins. They have a relatively high number of myomeres for Anguilla larvae, 111 and 114, suggesting that they are the young of either A. dieffenbachi (109–116 vertebrae) or A. megastoma (108–116 vertebrae). They were both collected at the same station, north-east of the New Hebrides, which is well outside the known distribution of A. dieffenbachi (a species restricted to New Zealand). Although the position of capture does not entirely rule out the possibility that they are the young of the latter species, since its breeding area is possibly as far north as New Caledonia, I am satisfied mainly for geographical reasons that the two larvae are the young of the other long-finned species with a high number of vertebrae—i.e., A. megastoma.
The two specimens of A. megastoma described above are evidently about half-grown, possessing only a few teeth (8–9) compared with the average full complement of about 18–20 in Anguilla larvae about to undergo metamorphosis. In both specimens the olfactory organ is not well developed, but in the larger the anterior and posterior nares are near to separation; the vertebral column is well chondrified; the vent is not greatly posterior; the heart is well developed, and the cranium well ossified.
L. Anguilla marmorata and L. Anguilla reinhardti
Material Examined. One specimen, 27.3mm total length; IFO St. Ep 27b(l) ; 17° 13′ S., 162° 30′ E.; 28/9/60 (0205 hrs); 2 oblique tows in 0–300m; 0.5m net, No. 2 mesh. One specimen, 27.6mm t.1.; IFO St. Ep 9b(l); 17° 40′ S., 157° 40′ E.; 17/9/60 (0206 hrs); 2 obl. tows in 0–300m. One specimen, 37.3mm t.1.; IFO St. S 6; 11° 51′ S., 159° 13′ E.; 11/6/62; 5ft midwater trawl (Isaacs-Kidd) ; ca. 95m. One specimen, 39.2mm t.1.; IFO St. P Bs 16; 12° 59′ S, 165° 42′ E; 13/11/58 (0311 hrs); 2 obl. tows in 0–300 m; 0.5m net, No. 2 mesh. One specimen, 41.4mm t.1.; IFO St. D 10; 14° 50′ S., 157° 52.5′ E.; 16/5/60 (2003 hrs) ; 2 obl. tows in 0–300m; 0.5m net, No. 2 mesh. One specimen, 43.6mm t.1.; IFO St. Ep 19b(1); 10° 24′ S, 160° 30′ E.; 24/09/60 (0204 hrs); 2 obl. tows in 0–300m; 0.5m net, No. 2 mesh.
Description. Measurements in mm: total length 39.2 (27.3–43.6), head 3.8 (3.1–4.0), snout 1.2 (1.1–1.7), eye 0.8 (0.9–1.0), upper jaw 1.9 (1.4–2.1), postorbital 1.9 (1.3–2.0), pectoral 0.9 (0.6–1.2), preanal 30.9 (21.3–33.6), predorsal 25.8 (19.6–31.1), depth just before eye 1.7 (1.5–2.1), depth at pectoral origin 3.4 (2.4–4.0), depth at midpoint between pectoral and vent 8.6 (5.0–8.8), page 7 depth at anal origin 6.8 (4.1–8.4). Branchiostegal and pectoral rays not obvious, dorsal rays before level of vent 89 (91 and 82 in the two largest specimens), total rays 256 (256 and 253), first dorsal ray at level of segment 62 (61 in the two largest specimens), anal rays 200 (183–225), caudal rays 3 + 2 + 2 + 2 (variable, usually 2 + 2 + 2 + 2). Teeth . Myomeres 73 + 34 = 107 (106–110). a–d = 9 (8–9). 1st vertical blood-vessel at 16 (15–17), 2nd at 37 (37–39), 3rd at 43 (43–44). Anterior margin of gall-bladder at level of 25th or 31st myomere.
Body elongate, but not excessively so, much compressed but relatively deep, its maximum depth contained a little more than four times in the total length, tapering equally in front of and behind midpoint of body. Head short, about one-tenth of total length, indented at the throat, pointed, with the dorsal profile slightly convex; anterior and posterior nostrils clearly separated; eye contained one and a-half times in snout, oval, with the greatest diameter vertical; cleft of mouth oblique, extending to level of middle of pupil; teeth conspicuous, very acute, 18 in upper jaw projecting outside those of lower jaw, distributed as follows: first tooth minute, needle-like, directed immediately forwards and placed on the anterodorsal surface of the snout above the second tooth, which is a much larger anteriorly-directed grasping tooth; these two are followed by a second series of large, needle-like fangs and a third of much smaller teeth to a point below the middle of the pupil; the teeth of the lower jaw are similar in grouping and size to those of the upper jaw. Branchiostegal rays not obvious. Pectoral fin large, about equal to vertical diameter of eye, rounded, base of fin fleshy but with the rays not obvious. Dorsal fin low with the rays well developed only in the larger specimens but the radials always countable. Anal fin a little higher than the dorsal. Caudal fin in the larger specimens well marked off from the dorsal and anal fins and with well developed hypurals and fin-rays.
Colour in preservative translucent with pigment restricted to the chorioid of the eye.
Remarks. These six specimens are long-finned larvae, having 8–9 myomeres between the origins of the dorsal and anal fins compared with about 5 in short-finned species. They have 106–110 myomeres. These two characters, taken together with the location of capture of the six larvae, restrict the identification to either A. reinhardti or A. marmorata. However, it is difficult to refer these larvae further to either one or other of these two species or both since the definitive adult characters are relatively insignificant and are not exhibited in the larvae. There is evidence, nevertheless, to show that these larvae represent two species. I have examined a large number of leptocephali of the Genus Gnathophis (Congridae) including a wide range of sizes for two species. In this case, the segmental level of the gall-bladder is the same in both species and does not change with growth. In the present six specimens, the two smallest larvae (27.3mm and 27.6mm) have the gall-bladder at the level of the 31st myomere, but in the other four specimens the gall-bladder is level with myomeres 24 and 25. From this I consider that there are two species in this particular group of six larvae. The two smallest larvae with a more posterior gall-bladder were collected in the northerly region of the area known for the adult A. reinhardti. The four larger were taken in the southern part of the known distribution of A. marmorata. Since the areas occupied by the adults overlap only in New Caledonia, and since two larval species are indicated, further work may confirm my proposed identifications.page 8
L. Anguilla australis ?schmidti (Text-fig. 2, A, B, C)
Material Examined. One specimen, 24.6mm total length; IFO St. 56–4–2; 14° 37′ S., 170° 03′ E.; 26/9/56 (2122 hrs) ; horizontal tow in 10m; 0.5m net, No. 2 mesh. One specimen, 47.6mm t.1.; IFO St. P 60–2–1; 22° 38′ S, 168° 20′ E; 11/4/60 (1953 hrs); hor. tow in 50m; 0.5m net, No. 2 mesh.
Description. Measurements in mm: total length 47.6 (24.6), head 4.3 (2.8), snout 1.0 (1.2), eye 0.7 (0.6), upper jaw 1.6 (1.2), postorbital 2.4 (1.2), pectoral 1.1 (0.5), preanal 29.6 (19.2), predorsal 27.0 (18.2), depth just before eye 1.8 (1.2), depth at pectoral origin 3.4 (2.0), depth at midpoint between pectoral and vent 8.5 (4.2), depth at anal origin 9.4 (4.1). Branchiostegal rays 12 (−), pectoral rays 17 (−), dorsal rays before level of vent 26 (−), total rays 194 (−), 1st dorsal ray at level of myomere 58; anal rays 187 (−), caudal rays 2 + 2 + 2 + 2. Teeth none and . Myomeres 62 + 54 = 116 (112). a–d = 3 (2). Vertical blood-vessels at myomeres 17, 41, 48 (17, ? 48). Anterior margin of gall-bladder at myomere 27 in one specimen, obscured in the larger specimen.
Body not excessively elongate, much compressed except along the head but relatively deep with the maximum depth contained about five times in total length; tapering about equally in front of and behind the midpoint of the body. Head short, about one-tenth of total, slightly indented at the throat: snout about one-fourth of head, rounded, with a slightly convex dorsal profile and both nostrils well separated; eye contained 1.3 times in snout, oval, with the greatest diameter vertical; cleft of mouth oblique, extending to level of middle of pupil; teeth absent in the larger specimen, but eight small cavities remain in the lower jaw. Branchiostegal rays well developed and curving across the space in front of pectoral. Pectoral fin just less than snout and eye combined, rounded, base of fin fleshy. Dorsal fin low, with the radials visible throughout as well as most of the rays. Anal fin conspicuously higher than the dorsal. Caudal fin not greatly differentiated from the dorsal and anal.
Colour translucent with pigment restricted to the chorioid of the eye.
Remarks. These two leptocephali are short-finned, having the difference between the dorsal and anal origins equal to three myomeres. The two species of short-finned eels present in the south-west Pacific are A. australis with 108–116 vertebrae and A. obscura with 101–107 vertebrae. The two larvae have 112 and 116 myomeres and are therefore referred to A. australis. Two subspecies of A. australis have been recognised: A. australis australis Richardson in New South Wales, Victoria, Tasmania and Lord Howe Island and A. australis schmidti Phillipps from New Zealand and outlying islands, Norfolk Island, New Caledonia, Fiji, and possibly Tahiti. The larger specimen of these two larvae, almost at metamorphosis, was taken very close to New Caledonia and is probably A. australis schmidti. The smaller specimen was taken far from the area known for the adult of the Australian subspecies and is also tentatively referred to A. australis schmidti.
L. Anguilla australis ?australis
Material Examined. One specimen, 75.7mm total length; Aust. Mus. regd. no. IA.2363; McCulloch Reef, Great Barrier Reef, Queensland, between 17° S. and 19° S.; transitional larva.
Description. Measurements in mm: standard length 74.9, head 5.8, snout 1.0, eye 1.1, interorbital 1.0, upper jaw 2.3, postorbital 4.0, pectoral 1.3, preanal 41.8, predorsal 40.3, depth just before eye 2.0, depth at pectoral origin 3.2, depth at anal origin 5.2. Myomeres 52 + 60 = 112. a–d = 1.page 10
Text-fig. 3.—Figs. A-C—L. Anguilla obscura, 49.6mm total length, IFO St. G 16. Fig A—Lateral view to show distribution of major vertical blood-vessels, intestine and myomeres at level of vent. Fig. B—Lateral view of head. Fig. C—Lateral view of caudal region. Figs. D-F—Anguilla australis australis, glass-eel, 57.9mm total length, Aust. Mus. regd. no. I.B.5289. Fig. D—Lateral view, to show distribution of deep pigment on spinal cord. Fig. E—Lateral view of head. Fig. F—Lateral view of caudal region to show deep pigment on spinal cord and scattered lateral pigment.
Body moderately elongate, compressed, tapering appreciably only at head and along posterior half of caudal region. Head short, about one-thirteenth of total length, not greatly differentiated from trunk; snout acute, one-sixth of head, projecting in front of lower jaw and turned noticeably downwards; nostrils well separated with the anterior one almost at tip of snout, the other just in advance of eye; eye round, about equal to length of snout; gape considerably oblique, extending to a point just in advance of pupil. Teeth difficult to distinguish. Pectoral a little longer than snout; dorsal low, originating only slightly in advance of level of vent; anal also low; caudal fin distinct from tips of dorsal and anal. Colour creamy brown with black pigment restricted to the chorioid of the eye.
Remarks. This specimen is a late leptocephalus which has taken on the elongate form of the elver. It has not yet become rounded in cross-section but is losing its transparency. The preanal myomeres number 52, compared with 39 in the elver. With a total of 112 myomeres and a short dorsal fin there is little doubt that the specimen belongs to A. australis and, considering its point of capture close in to the Queensland coast, to the Australian subspecies, A. australis australis. The specimen is not in the best condition and shows no distinctions from the larvae assigned to A. australis schmidti enabling certainty of subspecific identification.
Anguilla australis ?australis (Text-fig. 3, D, E, F), glass-eel.
Material Examined. Two specimens, 52.5mm and 57.9mm total lengths, Aust. Mus. regd. no. IB.5289; rock-pool, Forty Baskets Beach, Sydney; 12/6/61.
Description. Measurements in mm: total length 57.9 (52.5), head 6.0 (6.0), snout 1.4 (1.1), eye 0.6 (0.7), interorbital 0.3 (0.3), upper jaw 1.4 (1.7), postorbital 4.3 (4.1), pectoral 1.1 (1.2), preanal 20.0 (−), predorsal 19.8 (−), depth just before eye 1.0 (0.8), depth at pectoral origin 1.8 (1.8), depth at midpoint between pectoral and vent 1.5 (1.5), depth at anal origin 1.4 (1.7). Branchiostegal rays 12 (12), pectoral rays 19 (18), dorsal rays before level of vent 9 (8), total rays ca. 193 (211), anal rays ca. 186 (203), caudal rays 2 + 2 + 2 + 2 (2 + 2 + 3 + 2). Teeth . Myomeres 39 + 74 = 113 (110). a–d = 2 (2).
Body greatly elongate, cylindrical, compressed only near tip of caudal region, tapering only at snout and at tip of caudal. Head long, about one-tenth of total length, slightly swollen in front of pectoral but otherwise little differentiated from trunk; snout rounded, less than one-quarter of head length with anterior nostril far forward on tip of snout, posterior nostril a little in advance of eye; eye less than twice in snout or about seven times in postorbital, circular; gape oblique, extending to just behind forward margin of eye, lower jaw projecting for a distance equal to length of pupil. Teeth bluntly conical, short, sparse, uniserial, unlike those of the leptocephali; distributed as follows: on the maxilla five small teeth are followed by two larger ones posteriorly, while on the dentary there are ten small teeth anteriorly and two larger ones posteriorly. Pectoral fin about equal to snout, oval. Dorsal fin low, originating only a short distance in advance of the anal; anal fin also low, caudal fin well differentiated from the tips of the dorsal and anal fins.
Colour in preservative restricted to a few large melanophores on the lateral surface of the head, a continuous series of deeply-placed, expanded melanophores on the spinal cord beginning at the level of the pectoral fin through to the tip of the caudal region, a scattering of pigment laterally on the tip of the caudal region and on the caudal fin as well as a pigmented chorioid.
Remarks. The specimens described here are transitional elvers between Stage V and Stage VI of Strubberg (1913, p. 4); that is, they have surface pigment on page 12 the caudal, deep pigment on the caudal region of the spinal cord and neck, but the body is cylindrical. The two specimens, having two myomeres between the levels of dorsal and anal origins are short-finned and with a high number of myomeres, 110 and 113, can be referred to A. australis. They were collected at Sydney, New South Wales, and therefore fall well within the geographical range known for the Australian subspecies, A. australis australis.
L. Anguilla obscura (Text-fig. 3, A, B, C)
Material Examined. One specimen, 49.6mm total length; IFO St. G 16; 16° 52′ S., 166° 26′ E; 27/2/62; horizontal tow in 16m; 1m plankton net.
Description. Measurements in mm: head 4.0, snout 1.1, eye 0.9, upper jaw 1.8, postorbital 2.4, pectoral 1.0, preanal 36.1, predorsal 33.3, depth just before eye 1.4, depth at pectoral origin 3.0, depth at midpoint between pectoral and vent 8.8, depth at anal origin 7.6. Branchiostegal rays 8; pectoral rays not obvious; dorsal rays before level of vent 59, total rays 216, 1st dorsal ray at myomere 63; anal rays 204; caudal rays 2 + 2 + 2 + 2. Teeth . Myomeres 70 + 33 = 103. a–d = 5. Vertical blood-vessels at 17, 37, 43.
Body moderately elongate, much compressed, deep, with the greatest depth contained five times in total length. Head short, about one-eleventh of total length, rather depressed so that it is well differentiated from trunk; snout short, about one-fourth of head length, its dorsal profile convex; anterior and posterior nostrils well separated; eye about equal to length of snout, oval; cleft of mouth oblique, extending almost to level of posterior margin of eye; teeth conspicuous, very acute, 18 in upper jaw projecting outside those of lower jaw which number about 13, distributed as follows: first tooth of upper jaw small, needle-like, directed anteriorly and placed immediately above the second, much larger, anteriorly-directed grasping tooth; these two are followed by a series of four large teeth and a final series of 12 noticeably smaller teeth; lower teeth similar to those of upper jaw with the absence of the needle-like anterior tooth. Branchiostegal rays delicate and curving up around the operculum. Pectoral fin about 1.5 times diameter of eye, rounded, fleshy. Median fins well developed with fin rays and radials clearly visible.
Colour in preservative translucent with black pigment restricted to the chorioid of the eye.
Remarks. The specimen belongs to a short-finned species, having five myomeres between the levels of the dorsal and anal origins. The myomeres number 103, and this character fully identifies the specimen with A. obscura which has 101–107 vertebrae. The specimen is well advanced in development having very nearly a full complement of teeth, well developed fin-rays, etc., and the vent placed half way between the midpoint of the body and the tip of the caudal region.
The twelve leptccephali dealt with above plus the four specimens from the Dana collection giving a total of 16 known specimens of anguillid larvae from the Southwest Pacific contrasts strongly with the richness of collections of this family in other ocean areas. This is the more interesting because the area contains six known species, all of which are common in the freshwaters of the area. There is no uncertainty in sorting these few larvae into long-finned and short-finned categories. Within these groups, four larvae can be identified as the short-finned A. obscura (one specimen) and A, australis (three specimens) since the number page 13 of myomeres is distinctive between the adults of these species, but so far no distinction other than zoogeographic in the sense of the localities at which specimens were taken can be made at the subspecific level for A. australis.
Of the eight long-finned larvae two are here identified with A. megastoma, since these two specimens were taken much further to the north of the observed geographic range of A. dieffenbachi with which the larvae of A. megastoma could be confused. The six remaining specimens belong either to A. marmorata or A. reinhardti although certain differences in morphology and geographical reasons suggest that four of these are A. marmorata and two A. reinhardti.
Although there are no very early larvae in this collection, the smallest being 23.7mm (A. megastoma, it may be significant in determining the location of the breeding areas of the various species that all the smaller larvae were collected to the east of the larger larvae (with the exception of the Dana specimen of A. obscura, 43mm). Taking into consideration the general east-west trend of the prevailing current system in this area it seems likely that the breeding area for these species is well to the east of New Caledonia—that is, between Fiji and Tahiti.
All of these Anguilla larvae were collected in depths of about 100m-200m where the temperature is at least 20°C. and the salinity is about 35.5%o. Conditions such as these are known to occur in the breeding areas of the Atlantic species of Anguilla.
In 11 early elvers of A. australis australis from the New South Wales region Ege (1939, p. 209) records the total length to be 47mm-64mm although the average was about 53mm. Both of the glass-eels described above fall within this range (52.5mm and 57.9mm). Elvers of A. australis schmidti from New Caledonia reach an average maximum of about 50mm, while those of this species from New Zealand reach a much greater average length, about 61mm. As Ege (1939, p. 211) notes, there appears to be no systematic significance in the difference in maximum size between elvers from New Caledonia and New Zealand, but that this is rather "an expression of biological-geographical conditions". A similar condition in which the elvers are relatively large nevertheless also occurs in the other two temperate species of Anguilla in the Pacific. A. japonica Temminck & Schlegel from Japan has elvers which reach about 57mm (Ege, 1939, p. 142) while those of A. dieffenbachi from New Zealand reach about 64mm-70mm. It is possible that the temperate elvers are more distant from the breeding grounds of the adults than are their tropical counterparts, and this may account for their larger average size.
Key to the Leptocephali of Southwest Pacific Species of the Genus Anguilla
This key has been prepared from the 16 known Anguilla larvae from this area; the number of myomeres in each species (one more than the number of vertebrae) has been taken from the observed range of variation in the vertebral count of the adult. The key is simple to use. Where there is agreement at one number, proceed to the consecutive number; where there is no agreement, proceed to the alternative indicated in parentheses.
|Body elongate to elongate-oval, not filamentous, reaching about 50mm just before metamorphosis begins, depth about one-fifth of total length. Pectoral fin present; upper jaw reaching to below middle of pupil; gut straight, not swollen or festooned; major vertical intestinal vessels usually lying at segments 17, 40, 45; vent never closer to tip of tail than by about 30 segments, before metamorphosis the myomeres numbering about 70 + 40 with a page 14 total in all species between 100 and 120. Pigment restricted to the chorioid of the eye but very young specimens have a few small black spots on the caudal tip||Anguilla Shaw.|
|1 (4) Number of complete myomeres between verticals through the dorsal and anal origins less than six (short-finned species)|
|2 (3) Myomeres 102–108, anterior margin of gall-bladder at the level of the 27 th myomere, a–d = 5–6, major vertical blood-vessels at 15–17, 35–37, 42–44||A. obscura Günther, known from Tahiti, south of Fiji and New Hebrides.|
|3 (2) Myomeres 109–117, anterior margin of gallbladder at the level of the 27th myomere, a–d = 1–4||A. australis Richardson.|
|Major vessels at 16–17, 39, 45–46||A. australis ?australis Richardson, known from south-east of New Caledonia and North Queensland.|
|Major vessels at 17, 41, 48||A. australis ? schmidti Phillipps, known from east of New Caledonia and north-east of New Hebrides.|
|4 (1) Number of complete myomeres between verticals through the dorsal and anal origins more than six (long-finned species).|
|5 (8) Myomeres 101–111, a–d = 8–10.|
|6 (7) Anterior margin of gall-bladder at level of 25th myomere, major vessels at 15–17, 36–37, 42–43||A. marmorata Quoy & Gaimard, known in the south-west Pacific from near Samoa and between Solomon Is. and New Caledonia.|
|7 (6) Anterior margin of gall-bladder at level of 31st myomere, major vessels at 16–17, 38–39, 44||A. reinhardti Steindachner, known from north-west of New Caledonia.|
|8 (7) Myomeres 109–117, a–d = 7, major vessels at 17–19, 41–42, 47–48, anterior margin of gall-bladder at 29–31||A. megastoma Kaup, known from northeast of New Hebrides. A. dieffenbachi Gray is unknown as a leptocephalus but would probably fall close to A. megastoma in its larval characters.|
Ancona, U, d', 1928. Muraenoidi (Apodes) del Mar Rosso e del Golfo di Aden. Materiali raccolti dal Prof. L. Sanzo nella campagna della R.N. "Ammiraglio Magnaghii" 1923–24 . Mem. R. Com. talassogr. ital. , 146: 1–146, 5 pls.
Bruun, A. F., 1937. Contributions to the life histories of the deep sea eels: Synaphobranchidae. Dana Rep. , 9: 1–31, 1 p., 17 text-figs.
Ege, V., 1939. A revision of the genus Anguilla Shaw. A systematic, phylogenetic and geographical study. Dana Rep. , 16: 1–256, 6 pls., 53 text-figs., 208 tabs.
Jespersen, P., 1942. Indo-Pacific leptocephalids of the genus Anguilla. Systematic and biological studies. Dana Rep. , 22: 1–127, 4 pls., 83 figs.
Lea, E., 1913. Muraenoid larvae from the " Michael Sars " North Atlantic Deep-Sea Expedition, 1910 . Rep. Sci. Res. Sars Exped. , 3 (1): 1–48, 6 pls., 38 figs.
Strubberg, A. C., 1913. The metamorphosis of elvers as influenced by outward conditions. Medd. Komm. Havunders., Fiskeri , 4 (3).
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