EEL 685 Eel. The common fresh-water eel belongs to a group plete development of the ovary. The invariable death of the fish of soft-rayed fishes distinguished by the presence of an in the same almost ripe condition leads to the conclusion that opening to the air-bladder and the absence of the pelvic under normal conditions the fish dies after the mature ova have fins. With its nearest relatives it forms the family Munen- been discharged. Grassi states that he obtained ripe male eels, ripe specimens of Muraena, another genus of the family, in dse, all of which are of elongated cylindrical form. The and the whirlpools of the Strait of Messina. A ripe female Muraena special peculiarities of the eel are the rudimentary scales has also been described at Zanzibar. Gravid female eels, i.e., buried in the skin, the well-developed pectoral fins, the specimens, with ovaries greatly enlarged, have been occasionally rounded tail fin continuous with the dorsal and ventral fins: obtained in fresh water, but there is no doubt that, normally maturity is attained only in the sea. Only one other species of the family occurs in British sexual Until recent years nothing was known from direct observation waters, namely, the conger, which is usually much larger, concerning the reproduction of the common eel, or any species of and lives in the sea. In the conger the eyes are larger the family. It was a well-known fact that large eels migrated than in the eel, and the upper jaw overlaps the lower, towards the sea in autumn, and that in the spring small transeels of 2 inches in length and upwards were common on whereas in the eel the lower jaw projects beyond the parent the shore under stones, and ascended rivers and streams in vast upper. Both species are voracious and predatory, and feed swarms. It was reasonable, therefore, to infer that the mature eels on almost any animal food they can obtain, living or dead. spawned in the sea, and that there the young were developed. A group of peculiar small fishes were, however, known which The conger is especially fond of squid or other Cephalopods, while the eel is partial to carrion. The common eel occurs were called Leptocephali, from the small proportional size of the in all the rivers and fresh waters of Europe, except those draining towards the Arctic Ocean, the Black Sea, and the Caspian Sea. It also occurs on the Atlantic side of North America. The conger has a wider range, extending from the western and southern shores of Britain and Ireland to the East Indian Archipelago and Japan. It is common in the Mediterranean. The ovaries of the eel resemble somewhat these of the salmon in structure, not forming closed sacs, as in the majority of Teleostei, but consisting of laminae exposed to the body cavity. The laminae in which the eggs are produced are very numerous, and are attached transversely by their inner edges to a membranous band running nearly the whole length of the body-cavity. The majority of the eels captured for market are females with the ovaries in an immature condition. The male eel was first discovered in 1873 by Syrski at Trieste, the testis being described by him as a lobed elongated organ, in the same relative position as the ovary in the female, surrounded by a smooth surface without laminre. He did not find ripe spermatozoa. He discovered the male by examining small specimens, all the larger being female. Jacoby, a later observer, found no males exceeding 19 inches in length, while the female may reach a length of 39 inches or more. Dr Petersen, in a paper published in 1896, states that in Denmark two kinds of eels are distinguished by the fishermen, namely, yellow eels and silver eels. The silver eels are further distinguished by the shape of the snout and the size of the eyes. The snout in front of the eyes is not flat, as in the yellow eels, but high and compressed, and therefore appears more pointed, while the eyes are much larger and directed outwards. In both kinds there are males and females, but Petersen shows that the yellow eels change into silver eels when they migrate to the sea. The sexual organs in the silver eels are more developed than in the yellow eels, and the former have almost or entirely ceased to take food. The male silver eels are from 11^ to 19 inches in length, the females from 16| to about 39 inches. It is evident, therefore, that if eels only spawn once, they do not all reach the same size when they become sexually mature. The male conger was first described in 1879 by Hermes, who obtained a ripe specimen in the Berlin Aquarium. This specimen was not quite 2J feet in length, and of the numerous males which have been identified at the Plymouth Laboratory, none exceeded this length. The large numbers of conger above this size caught for the market are all immature females. Female conger of 5 or 6 feet in length and weighing from 30 to 50 lb are common enough, and occasionally they exceed these limits. The largest recorded was 8 feet 3 inches long, and weighed 128 lb. There is every reason to believe that eels and conger spawn but once in their lives, and die soon after they have discharged their generative products. When kept in aquaria, both male and female conger are vigorous and voracious. The males sooner or later cease to feed, and attain to the sexually mature condition, emitting ripe milt when handled and gently squeezed. They live in this condition five or six months, taking no food, and showing gradual wasting and disease of the bodily organs. The eyes and skin become ulcerated, the sight is entirely lost, and the bones become soft through loss of lime. The females also after a time cease to feed, and live in a fasting condition for five or six months, during which time the ovaries develop and reach great size and weight, while the bones become soft and the teeth disappear. The female, however, always dies in confinement before the ova are perfectly ripe and before they are liberated from the ovarian tissue. The absence of some necessary condition, perhaps merely of the pressure which exists at the bottom of the sea, evidently prevents the com- |
Leptocephali. (liy kind permission of J. & A. Churchill.) head. The first of these described was captured in 1763 near Holyhead, and became the type of L. Morrisii, other specimens of which have been taken either near the shore or at the surface of the sea. Other forms placed in the same genus had been taken by surface fishing in the Mediterranean and in tropical ocean currents. The chief peculiarities of Leptocephali, in addition to the smallness of the head, are their ribbon-like shape and their glassy transparency during life. The body is flattened from side to side, and broad from the dorsal to the ventral edge. Like the eels, they are destitute of pelvic fins, and no generative organs have been observed in them (see Fig. above). In 1864 the American naturalist, Gill, published the conclusion that L. Morrisii was the young or larva of the conger, and Leptocephali generally the young stages of species of Muraenidfe. In 1886 this conclusion was confirmed from direct observation by Yves Del age, who kept alive in a tank at Roscoff a specimen of Ij. Morrisii, and saw it gradually transformed into a young conger. From 1887 to 1892 Professor Grassi and Dr Calandruccio carried on careful and successful researches into the development of the Leptocephali at Catania, in Sicily. The specimens were captured in considerable numbers in the harbour, and the transformation of L. Morrisii into young conger, and of various other forms of Leptocephalus into other genera of Munenidae, such as Murcena, Congromurcena, and Ophichthys, was observed. In 1894 the same authors published the announcement that another species of Leptocephalus, namely, L. brevirostris, was the larva of the common eel. This larval form was captured in numbers with other Leptocephali in the strong currents of the Strait of Messina. In the metamorphosis of all Leptocephali a great reduction in size occurs. The L. brevirostris reaches a length of 8 cm., or a little more than 2J inches, while the perfectly-formed young eel is 2 inches long or a little more. The Italian naturalists have also satisfied themselves that certain pelagic fish eggs originally described by Raffaele at Naples are the eggs of Muraenidae, and that among them are the eggs of
