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Congrid Leptocephali in Australasian Waters with Descriptions of Conger wilsoni (Bl. and Schn.) and C. verreauxi Kaup.

General Discussion

General Discussion

The 15 species of eel larvae described in this paper, together with the two further species described in an earlier account (Castle, 1963) represent the first major assembly of congrid leptocephali from the southwest Pacific and the eastern Indian Ocean. Altogether more than 500 congrid larvae are in this collection which was made from three major areas: (a) a broadly rectangular area including New Caledonia, the New Hebrides Islands, the Solomon Islands, reaching up towards the Ellice Islands, bounded approximately by the 7th and 22nd parallels South and by the 158th and 172nd parallels East (collections in this area were made from 1956 to 1962 mainly by the Orsom III, the oceanographic vessel of the Centre d'Océanographie de l'Institut Français d'Océanie, Nouméa); (b) a long and relatively narrow area from southern Queensland to Tasmania off the east Australian coast over the continental shelf and slope (collections mainly from 1938 to 1942 by the F.R.V. Warreen, the oceanographic vessel then attached to the C.S.I.R.O., Division of Fisheries and Oceanography, Cronulla, New South Wales; (c) a smaller area west of Rottnest Island, off Perth, Western Australia (collections made during the period mid–1961 to the end of 1962 by the Lancelin, the research vessel used by the Western Australian Museum, Perth). A few other leptocephali came from the New Zealand area, mainly as beachcast specimens.

The present collection of over 500 congrid larvae is part of an even larger assembly of about 1,100 leptocephali which also includes larvae of other families of eels. Although this is by far the most extensive collection of eel larvae yet recorded from the whole of the Indo-Pacific smaller collections have nevertheless been examined. The most important of these (500 specimens) is recorded by Ancona (1928) from the Red Sea and includes about 50 congrid leptocephali belonging to eight species. The 1928–30 Dana collection from this area was large but the congrids in the collection have not been examined. Other notable collections have been made from the Indo-Malayan archipelago (Strömman, 1896) and from off Ceylon (Gopinath, 1949).

page 38

In the initial sorting of the present collection of eel larvae the leptocephali described in this paper were separated from all other groups of larvae in having a pectoral fin at all stages of development, a caudal fin which is not reduced in length in full-grown specimens, the gut is not festooned or swollen at any point and pigment always occurs as a regular, ventral series of chromatophores. The myomeres are never extremely numerous, that is, they are never more than 250 and the body is never filamentous. This combination of characters is not found in other groups of leptocephali.

Many eel larvae with this combination of characters have been described and a few of these have been referred to the family Congridae on the basis of transitional, metamorphosing specimens which show characters of both the larva and the young congrid eel (e.g. Conger conger, Ariosoma balearica and Gnathophis habenatus). Indeed, the characters which identify leptocephali to the familial level are now in general well established so that there are no serious difficulties in the broad sorting of collections of eel larvae.

Within the family Congridae leptocephali fall quite readily into two major divisions. The first of these, subfamily Anagoinae Asano, 1962, is characterised by leptocephali which have essentially a round eye without iris pigment, a series of compact somatic chromatophores in a line on each myoseptum immediately below the midlateral line, ventral pigment as a series of chromatophores along the ventral midline and as a double series of splanchnic chromatophores along the kidney ducts to the vent, and a subterminal vent with the dorsal and anal fins confined to the extremity of the body until just before the onset of metamorphosis. One of the most well-known examples of this group is L. Ariosoma balearica (De Laroche) from the Central Atlantic and the Mediterranean. Five species described in the present paper are referred to the Anagoinae. The second division, subfamily Congrinae, has leptocephali which possess an oval eye with iris pigment, a paired series of somatic chromatophores along the ventral midline from the pectoral region to the vent or not as far, and the vent usually appreciably in advance of caudal tip. Typical representatives of this group are L. Conger conger (L.) and L. Gnathophis habenatus (Richardson).

Although one species in the present collection, L. hyoproroides Strömman, 1896, has at least one feature which is intermediate between those shown for leptocephali of the Anagoinae and the Congrinae, I am of the opinion that the difference between these two groups of leptocephali strongly and reliably support the distinctions shown mainly in osteological characters in Asano's recent separation of the two congrid subfamilies. As I propose to show, larval characters (and in particular, larval pigmentation), in the one major family of eels which I have as yet investigated (Congridae) are of considerable systematic value and should provide in the future, a useful adjunct to osteological characters in the separation of eel genera at least in this family. Whitley (1935, p. 216) has subdivided the Congridae into the two subfamilies Scalanagoinae, type genus Scalanago Whitley, 1935, and the Leptocephalinae. Scalanago (type species, S. lateralis Whitley, 1935) is unique amongst congrid eels in having the lateral line with numerous lateral branches, a preorbital bone with downward projections into the upper lip and a relatively long tail. In addition, its leptocephalus has the oval eye, iris pigment and ventral pigment typical of members of the Congrinae. I have no hesitation therefore in transferring Scalanago to the Congrinae (which replaces the Leptocephalinae because of Direction 87 of the International Commission on Zoological Nomenclature, February, 1958, recognising Conger Oken, 1817, as the genus for conger eels).

Below the subfamily level the leptocephali described above show clear distinctions mainly in body shape and pigmentation. Further subdivisions are based on such numerical characters as the number of myomeres, the relative position of the gall page 39bladder and of the vertical blood vessels and the number of fin-rays. These characters are of the type which commonly distinguish adult eel species (e.g., the species of Conger and Gnathophis). I therefore propose that the subdivisions of the congrid leptocephali described here, which are based mainly on amount and distribution of chromatophores, actually represent generic divisions. This agrees with my findings in the examination of larvae of other families of eels in the present collection. A disturbing feature of eels in general is that the limits of many genera of adults within a particular family have not always been clearly defined, due to the essential monotony of external characters. Generic differences as suggested above in the leptocephali may well contribute to the resolution of such confusion wherever it should arise.

Including the two species of Gnathophis already described (Castle, 1963) there are 17 species of congrids in the present collection, referred to 11 genera. As far as can be determined from the literature there are about eight genera and 16 species of congrid eels in the southwest Pacific and eastern Indian Ocean. These numbers are only approximate since many of the species, especially those described in earlier accounts, are in need of re-examination. In particular, the generic status of many of those of the Ariosoma-group and the Bathycongrus- complex of Ogilby has scarcely been critically examined. As a contrast, the congrid eels of the Japanese region have been carefully studied in detail by Asano (1962) and this author finds 10 genera and 14 species in the area, even though this area is small compared with that considered here. The more recent large collections of leptocephali from the eastern Indian Ocean by the Centre d'Océanographie de l'Institut Français d'Océanie which have been sorted but not included in this report contains possibly no more than one genus additional to those described here. It is therefore a strong possibility that the present collection represents a fairly complete picture of the number of congrid genera and species in the area of study, agreeing well with the position in the waters of Japan.

The congrid larvae in the collection range in size from about 5mm to about 300mm (although the majority are between 25mm and 200mm in length) and include a small number which are in stages of metamorphosis. The length of a full-grown congrid leptocephalus depends mainly on the species to which it belongs but in general those of the Anagoinae are much longer (reaching more than 300mm in Ariosoma anago) and deeper than those of the Congrinae (which reach 140mm in Conger verreauxi) but are more often less than 100mm. However, in at least one species (Gnathophis habenatus) evidence was previously given (Castle, 1963, p. 37) that an increase in length to beyond the "normal" maximum may occur if the stimulus to metamorphosis does not arise, whatever this stimulus may eventually prove to be.

Most collections in the three areas suggest that leptocephali of any particular species may remain for some time in the vicinity of the spawning area so that it is only the presence of very small larvae which can be taken as a reliable indication of the location of the spawning area. Small larvae of the various species indicate that there are two broad areas in the southwest Pacific in which spawning of these congrid eels may take place. The more northerly one lies between New Caledonia and the Solomon Islands over a depth of between 3,000m and 5,000m; the other lies off the New South Wales coast at about the latitude of Sydney over similar depths. At least Gnathophis habenatus, G. incognitus Castle, 1963, and Ariosoma mauritianum spawn in the latter area but the majority of the remainder probably spawn north of New Caledonia. The third spawning area indicated by the present collection is off Rottnest Island, Western Australia, again over relatively great depths of 3,000m–5,000m where spawning of A. mauritianum and Scalanago lateralis takes place. Spawning in most cases appears to occur in relatively shallow depths of about 50m–100m and dispersal of the young larvae does not clearly follow the prevailing surface currents. In one species at least (Gnathophis page 40 habenatus) the larvae are found northwards to the New Caledonia region, a dispersal movement contrary to the south-flowing East Australian Current.

Most of the larvae were collected with plankton nets of up to 200cm in diameter in horizontal and oblique hauls, a larval fish net or by 3ft or 5ft Isaacs-Kidd midwater trawls. These latter nets proved to be admirable for collecting larger leptocephali. A few specimens from the New Caledonia region were found in the stomachs of skipjack, yellowfin tuna and lancet fish (Katsuwonus pelamis (L.), Neothunnus macropterus (Temm. and Schleg.), and Alepisaurus ferox Lowe, respectively) taken on troll lines and longlines in the off-shore waters of New Caledonia.

Key to the Adult Congrid Eels of New Zealand
Body usually moderately elongate, without scales, tip of tail with a well-developed caudal fin, posterior nostril always in front of eye, pectoral present, branchial aperture usually semicircular, lateral, in front of pectoral base; tongue free; origin of dorsal always near level of pectoral; lips usually well-developed but upper sometimes reduced, lateral line conspicuous F. CONGRIDAE,
1 (4) Upper lip turned upwards as a thick, fleshy flap; preorbital bone without ventral projections into upper lip; teeth on the jaws in one or two longitudinal rows, those of the outer row much larger and compressed to form a sharp cutting edge; dorsal fin originating near or behind level of pectoral tip Conger Oken, 1817,
2 (3) Body slender, seldom reaching more than 1,000mm; eye relatively large, 1.9–3.9 per cent of total; 36–41 pores before level of vent, 141–149 vertebrae; dorsal rays 298–330, branchiostegal rays 8–9; origin of dorsal seldom less than half the length of the pectoral behind pectoral tip
known from shallow waters of northern New Zealand, Southern Queensland to Tasmania and Western Australia.
C. wilsoni (Bloch and Schneider, 1801),
3 (2) Body usually robust and reaching 1,500mm or more; eye small, 1.2–1.9 per cent of total; 40–44 pores before level of vent, 157–165 vertebrae; dorsal rays 331–347, branchiostegal rays 10–11; origin of dorsal about the diameter of the pupil within levelof pectoral tip
known in shallow waters from New Zealand to southeast Australia.
C. verreauxi Kaup, 1856,
4 (1) Upper lip thin, very weak; preorbital bone with ventral projections into upper lip; teeth cardiform on jaws; dorsal always originating in front of level of pectoral tip,
5 (8) Dorsal origin over branchial aperture; anterior nostril a simple tube directed forwards; specimens of more than about 300mm with minute finger-like epidermal papillae; preorbital bone single with threeventral projections Pseudoxenomystax Breder, 1927,
6 (7) Dorsal rays 327–353, anal rays 240–258, pectoral rays 16, vertebrae about 160, reaching 500mm in length
known in 150–600 fathoms from Chatham Islands, New Zealand and Tasmania.
P. bulbiceps (Whitley, 1948),page 41
7 (6) Dorsal rays 306–314, anal rays 204–226, pectoral rays 14–15, vertebrae about 156, reaching 1,000mm in length
known in about 450–600 fathoms from Kaikoura, New Zealand.
P. hirsutus Castle, 1960,
8 (5) Dorsal origin about halfway along pectoral; anterior nostril tube-like with an indented or scroll-like rim, directed ventrally; no epidermal papillae except for a few on the head; preorbital bones two Gnathophis Kaup, 1856,
9 (10) Vertebrae 120–127; anterior nostril with a scroll-like rim; small, triangular premaxillary-ethmoid patch of teeth not conspicuous outside the mouth from the ventral aspect known in shallow water from New Zealand, Lord Howe Island, New South Wales to Western Australia and possibly St Paul's Island, South Indian Ocean. G. habenatus (Richardson, 1848),
10 (9) Vertebrae 139–147; anterior nostril with an indented rim; round premaxillary-ethmoid patch of teeth extending conspicuously in advance of maxillary teeth
known in about 30 fathoms from New Zealand, the elvers from the Kermadec Islands.
G. incognitas Castle, 1963,

Key to the Congrid Leptocephali Known from the Southwest Pacific and Western Australia
Body elongate-oval to long but never filamentous, relatively deep, its maximum depth 4–10 times in total length, vent usually near caudal tip throughout major part of growth; caudal fin not reduced in length; pectoral fin present at all stages of development; intestine straight, not swollen or festooned at any point. Pigment always present ventrally and sometimes laterally as well as in the chorioid. Myomere range in each species seldom more than 20, but number in any species may be from 100to 250 F. CONGRIDAE,
1 (10) Eye round; ventral pigment in the form of a series of small somatic chromatophores along the ventral midline from the throat to the level of the gall bladder; as a series of closely-set, round, compact splanchnic chromatophores along each kidney duct above the intestine from the gall bladder to the vent; an oblique series of minute, compact, elongated somatic chromatophores on each myoseptum immediately below the midlateral line; a series along the dorsal midline present or absent; scattered chromatophores on the bases of the anal rays and over the surface of the caudal; vent subterminal, the dorsal and anal fins confined to the extremity of the body until the onset of metamorphosis Subfam. Anagoinae Asano, 1962,
2 (7) No other lateral pigment present Ariosoma Swainson, 1838,
3 (4) Myomeres 110–119, last vertical blood vessel at about myomere 58, body reaching160mm in length
(Strömman, 1896).
L. Ariosoma scheelei
4 (3) Myomeres greater than 120, page 42
5 (6) Myomeres 134–153, last vertical blood vessel at about myomere 69, body reaching 278mm in length L. Ariosoma mauritianum (Pappenheim, 1914)
6 (5) Myomeres 157–172, last vertical blood vessel at about myomere 80, body reaching 314mm in length L. Ariosoma anago (Temm. and Schleg., 1842).
7 (2) Additional lateral pigment present,
8 (9) Additional lateral pigment as small, numerous chromatophores scattered over the whole of the lateral surface of the body, myomeres 138, last vertical blood vessel at about myomere 68 L. ?Alloconger anagoides (Bleeker, 1864).
9 (8) Additional lateral pigment as an oblique line of minute, compact chromatophores on the second flexure of each myoseptum above the lateral line and on the end of the first flexure below the lateral line, myomeres 147–151, last vertical blood vessel at about myomere 92 L. scalaris n. sp.
10 (1) Eye oval; ventral pigment in the form of a paired series of small, somatic chromatophores at the level of the intestine from the throat to the vent or not as far; scattered chromatophores on the bases of the anal rays and over the surface of the caudal; usually a narrow, curved patch of pigment above iris and a larger crescentic patch below iris; vent usually somewhat in advance of the caudal tip; dorsal origin in advance of level of vent Subfam. Congrinae,
11 (12) Body elongate-oval; vent at about midpoint of body; pigmentation on head in the form of a chromatophore at the base of each tooth, on the opercular, temporal and prepectoral regions; body pigmentation as a paired ventral, somatic series of chromatophores which bifurcates at level of gall bladder to form two rows, one a somatic ventral series to vent, the other a splanchnic series above intestine also to vent; a conspicuous row of widely-spaced chromatophores in advance of each myoseptum from dorsal to ventral margins of body wall; myomeres 110–122, last vertical blood vessel at myomere 42–48 L. hyoproroides Strömman, 1896.
12 (11) Body usually elongate but sometimes elongate-oval; vent usually some distance in advance of caudal tip; no pigment on lateral surface of head, although very small larvae sometimes have pigment spots on tips of jaws; ventral pigment only at level of intestine and always somatic; lateral pigment never scattered over whole lateral surface but sometimes in one to three longitudinal rows,
13 (32) Ventral pigment along whole length of intestine,
14 (31) Snout short and occasionally blunt, especially in those larvae which are approaching metamorphosis, page 43
15 (24) Chromatophores present on lateral body wall,
16 (21) A single midlateral row of chromatophores from about level of pectoral to at least level of vent,
17 (20) Midlateral row of chromatophores compact, nondendritic,
18 (19) Midlateral row spaced at about one chromatophore per segment; myomeres greater than 200 L. mediopunctatus n. sp.
19 (18) Midlateral row spaced at least at intervals of 2–3 myomeres; myomeres 139–152 L. Conger cinereus Ruppell, 1828.
20 (17) Midlateral row of chromatophores large, constantly dendritic, placed at about every 2–3 myomeres; myomeres 112–119, last vertical blood vessel at myomere 52–56 L. Scalanago lateralis Whitley, 1933.
21 (16) Three longitudinal lateral rows, including a midlateral row of chromatophores, those of the upper and lower rows much less numerous,
22 (23) Myomeres 165–174, last vertical blood vessel at myomere 49–56
possibly referable to Pseudoxenomystax bulbiceps (Whitley, 1948) or to P. hirsutus Castle, 1960.
L. geminus n. sp.,
23 (22) Myomeres 186–199, last vertical blood vessel at myomere 57–59
possibly also referable to Pseudoxenomystax Breder.
L. trilineatus n sp.,
24 (15) No lateral pigment (except on caudal tip in very small specimens of about 10mm–15mm),
25 (28) Chromatophores usually present on lateral body wall above anal fin as a continuation of the ventral series to the vent,
26 (27) Myomeres 141–151, last vertical blood vessel at myomere 52–56, body reaching 100mm in length L. Conger wilsoni (Bl. and Schn., 1801).
27 (26) Myomeres 160–165, last vertical blood vessel at myomere 54–60, body reaching 140mm in length L. Conger verreauxi Kaup, 1856.
28 (25) Chromatophores absent from above anal fin Gnathophis Kaup, 1856,
29 (30) Myomeres 116–131, preanal about 100, last vertical blood vessel at about myomere 41 L. Gnathophis habenatus (Richardson, 1848).
30 (29) Myomeres 134–150, preanal about 112, last vertical blood vessel at about myomere 46 L. Gnathophis incognitus Castle, 1963.
31 (14) Snout long and beak-like, myomeres 134–152, last vertical blood vessel at about myomere 49 L. stenorhynchus n. sp.
32 (13) Ventral chromatophores reaching only to halfway along intestine, myomeres 104–111, last vertical blood vessel at myomere 34–40 L. scalaris n.sp.