Aquaculture, 17 (1979) 91-103 91 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

GUIDE TO THE INTERNAL MORPHOLOGY OF THE EUROPEAN EEL, ANGUILLA ANGUILLA (L.) (PISCES, TELEOSTEI)

J.J. WILLEMSE

Department of Anatomy, Erasmus University, P.O. Box 1738, 3000 DR Rotterdam (The Netherlands)

(Accepted 16 February 1979)

ABSTRACT

Willemse, J.J., 1979. Guide to the internal morphology of the European eel, Anguilla anguilla (L.) (Pisces, Teleostei). AquacuZture,17: 91-103.

Morphological detail and topographical relationships of internal organs of the European eel are demonstrated by labelled photographs of microscopical sections and drawings of dissected material. Glass eels were used in the preparation of sections of the head. Blocks were selected from’the trunk region of a 35 cm eel. They were sectioned in transverse planes, resulting in a series of complete transverse sections through this region of the body. The macroscopical dissections were made on 50 cm silver eels.

INTRODUCTION

The common eel of Europe and North Africa, Anguilla anguilla (L.), is the most intensively studied of all the representatives of the genus Anguilla. A recent reference work which presents information on all aspects of the biology of the genus is that of Tesch (1977). There is a well illustrated chapter on morphology and physiology of the eel included in this work, but the illustra- tions showing topographical relationships of internal organs are diagrammatic and do not cover all the regions of the body. Less recent works on eel biology (Walter, 1910; Bertin, 1956) or works on fish anatomy in general (Harder, 1964, 1975) do not provide us with suitable illustrations of topographical anatomy. Dissection of eel specimens by fish culturists (and others whose main line of interest is outside the field of anatomy) for collection of organ samples may be greatly facilitated by the availability of clear drawings and photographs illustrating topographical morphology.

The basic purpose of the present paper is to furnish a rapid visual guide to the location and general appearance of the internal organs. It therefore demonstrates morphological detail by labelled photographs of microscopical sections and drawings of dissected material.

Glass eels were used in the preparation of the sections of the head and blocks cut from the trunk of a 35 cm male eel were sectioned in transverse planes. A

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routine paraffin technique was employed. The macroscopic dissections were made on 50 cm female silver eels.

Not all specimens are exactly alike. Maturing affects size and shape of olfactory organs, eyes, pectoral fins, as well as size and shape of the gonads and other parts of the genital system. Thus, although major anomalies have been avoided in the illustrations, the latter cannot be expected to correspond perfectly to any particular specimen used in the laboratory. In most cases in the illustrations not all structures have been labelled so as not to obscure them with too many leaders. Lack of a single accepted anatomical terminol- ogy in fish morphology causes a considerable variation between different texts, Generally we followed Harder (1964, 1975) but as far as possible we used common English equivalents. No attempt has been made to check all the latest information concerning views on homologies and other such considera- tions.

COMMENTS ON THE MACROSCOPIC DISSECTIONS

External features useful as marking points in locating internal structures are the narines, eyes, external gill slits, bases of pectoral fins and the anus. In the narcotized eel the heart pulsations are easily visible through the skin and muscular layer in the midventral region slightly posterior to the pectoral fin bases. So location and puncture of the heart is easy.

Section of the ventral body wall may be done in the ventral midline (linea alba), but for convenient access to the internal organs considerable parts of skin and hypaxial muscle may be removed without significant loss of blood. This is due to the relatively modest vascularization of the lateral muscle. Extension of the dissection into the horizontal septum must be avoided, since at that level more important segmental vessels are found, as well as the lateral cutaneous vein superficially.

Fig. 1A shows the internal organs in the original position, while in Fig. 1B the stomach and intestine are displaced slightly to the right side of the animal. When part of the liver is removed and the intestinal tract is dissected free from its mesenteries and connected fat bodies, the different parts of the stomach may be distinguished more easily (Fig. 2A). Associated with the dorsal mesentery of. the stomach a distinct spleen is found. The presence of a spleen was not mentioned by Tesch (1973, 1977) in his surveys of eel anatomy.

After removal of the spleen and the swim-bladder complex, the paired pre- anal parts of the kidneys are visible (Fig. 2B). Confusion of gonads and fat bodies is avoided by considering their slight difference in colour and their characteristic relationships to other structures. The fat bodies are nearly pure white, while the gonads have a yellowish or pink tinge. The position of the attachments of the gonads to the dorsal wall of the body cavity is clearly demonstrated by the transverse sections considered below. The fat bodies are found in mesenterial folds and therefore in close contact with the intestin- al tract.

Fig. 1. Female eel. Ventral body wall removed. Pericard and organs in body cavity exposed. Arrows and asterisk indicate the level of the ultimobranchial organ at the ventral side of the esophagus.

Fig. 2. (A) Liver partly removed, mesenteries and cystic ducts cut. Intestinal tract pulled to the left. (B) Spleen, swim-bladder and pneumatic duct removed. Intestinal tract pulled to the right side.

COMMENTS ON THE TRANSVERSE SECTIONS THROUGH THE TRUNK REGION

The location of the planes of section of Figs. 5-8 is indicated by the corre sponding numbers at the left side of Fig. 1. The anterior (cranial) sides of the sections are illustrated. The right side of the animal is indicated by (R). Al- though the ultimobranchial organs are not seen in Fig. 3, this location in a slightly more posterior section is indicated. In Fig. 1 their anteroposterior level is shown.

Access to the dorsal aorta, for instance for perfusion, may be attained in the liver region, passing from the left dorsally over the esophagus (region shown in Fig. 4). Practising blunt dissection, using as far as possible fine but bluntly pointed forceps, the aorta may be reached and ligated without loss of blood. Because of the restricted maneuvering area, the use of a curved needle is necessary. Access to the dorsal aorta may also be attained in the anal region or slightly postanally (the region of Fig. 8).

Fig. 3. Transverse section at level (3) indicated in Fig. 1. The arrow and asterisk indicate the horizontal level where, more caudally, the ultimo- branchial organ is situated. R, right side of the animal.

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Fig. 4. Transverse section at level (4) indicated in Fig. 1.

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Fig. 5. Transverse section at level (5) indicated in Fig. 1.

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Fig. 6. Transverse section at level (6) indicated in Fig. 1

Fig. 7. Transverse section at level (7) indicated in Fig. 1.

Fig. 8. Transverse section at level (8) indicated in Fig. 1. The asterisks indicate the horizontal level where the corpuscles of Stannius may be found in this region.

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The position of the line of attachment of the mesorchium is easily visible in Figs 5, 6, 7. The mesorchia of further developed testes and mesovaria remain attached here.

The right posterior cardinal vein, which is dominant over the left one, is a direct continuation of the caudal vein (Figs 4- 7, versus Fig. 8).

The conjoined postanal parts of the kidneys form a rather substantial mass of kidney tissue. It is accompanied by the urinary bladder (Fig. 8).

COMMENTS ON THE SECTIONS OF THE HEAD

The longitudinal section illustrated in Fig. 9 is very close to the median (midsagittal) plane. The sections in Fig. 10 and 11 are oblique sections but nearly sagittal, Fig. 10 being situated laterally to Fig. 9. The otic capsule and gill regions in Fig. 11 are still more laterally situated. The main purpose of these illustrations is to show the location of the otoliths. In utriculus and sacculus being contiguous parts of the inner ear otoliths are found, the saccular otolith being much larger than the utricular otolith. The inner ear as a whole is situated in the posterolateral part of the skull. The sacculus is found at the level of the cerebellum adjacent to the base of the skull.

The position of the transverse section of Fig. 12 is indicated by the triangle in Fig. 11.

Fig. 9. Glass eel. Sagittal section close to the median plane.

Fig. 10. Glass eel. Sagittal section, situated laterally to the section illustrated in Fig. 9.

Fig. 11. Glass eel. Sagittal section, situated laterally to the sections illustrated in Figs. 9 and 10.

Fig. 12. Glass eel. Transverse section at level indicated (triangle) in Fig. 1 1.

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ALPHABETICAL LIST OF ABBREVIATIONS USED IN ILLUSTRATIONS

acv, anterior cardinal vein als, ascending leg of stomach ant, anterior semicircular canal anu, anus atr, atrium bra, branchial arch bua, bulbus arteriosus cav, caudal vein cer, cerebellum coa, coeliac artery car, coracoid cos, corpus striatum doa, dorsal aorta egs, external gill slit els, descending leg of stomach epi, epicard em, epaxial musculature eso, esophagus eye, eye fab, fat body gab, gall bladder gac, gastric cecum gi4 gill lamellae gon, gonad hap, haemal process hea, heart in pericardium hev, hepatic vein hma, hyomandibular hos, horizontal septum hpv, hepatic portal vein hym, hypaxial muscle hyo, hyoid hyp, hypophysis iav, intestinal artery and vein int, intestine inv, intestinal vein kid, kidney IiV, liver loo, olfactory lobe

ACKNOWLEDGEMENTS

lop, optic lobe 1tp, lymphoid tissue (pronephros) meo, medulla oblongata nea, neural arch elf, olfactory organ oto, otolith otu, otolith in utriculus ova, ovarium pan, pancreas surrounding intestinal vein pap, parietal peritoneum pcv, posterior cardinal vein pcy, peritoneal cavity pet, pericardial cavity per, pericardium pha, pharynx pnd, pneumatic duct pok, postanal kidney PY~, pylorus rem, rete mirabile rvx, ramus visceralis n. X sac, otolith in sacculus set, semicircular canal set, septum transversum shm, segmented hypaxial muscle spc, spinal cord spl, spleen swb, swim-bladder thm, thymus thy, thyroid urb, urinary bladder uro, urohyale of hyoid arch utr, utriculus vcr, vertebral centre surrounding

notochordal remnant veb, vertebral centre vet, vertebral canal ven, ventricle vip, visceral peritoneum

The author is indebted to Miss P. van Alphen and Mr W. van den Oudenalder who provided photographic assistance, Mr F. Willemse who made the drawings, and Prof Dr J. MoIl and Dr C.L. Deelder for advice.

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REFERENCES

Bertin, L., 1956. Eels, a biological study. Cleaver-Hume, London, 192 pp. Harder, W., 1964. Anatomie der Fische. In: R. Demoll, H.N. Maier, H.H. Wundsch (Editors),

Handbuch der Binnenfischerei Mitteleuropas. Schweizerbart, Stuttgart, 308 pp. (text), 96 pp. (illustrations).

Harder, W., 1975. Anatomy of fishes. Schweizerbart, Stuttgart, 612 pp. (text), 132 pp (illustrations).

Tesch, F.-W., 1973. Der Aal. Paul Parey, Hamburg, 306 pp. Tesch, F.-W., 1977. The eel. Chapman and Hall, London, 434 pp. Walter, E., 1910. Der Fluszaal, eine biologische und fischwirtschaftliche Monographie.

Neumann, Neudamm, 346 pp.