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Zoology Publications from Victoria University of Wellington—Nos. 33 and 34

Preservation and Methods of Measurement

page 2

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: 1 + VI + 8 over 1 + V + 11 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.