Studies on the Morphology, Ecology and Culture of the Important Apodal Fishes, Muraenesox Cinereus

Studies on the Morphology, Ecology and Culture of the Important Apodal Fishes, Muraenesox cinereus The apodal fishes, the order Anguillida, are known as one of the most impor- The present investigation treats of the morphology, ecology and culture of the two apodal fishes, the sharp-toothed eel, Muraenesox cinereus (Forskal) and the conger eel. Conger myriaster (Brevoort), which are caught rather plentifully, having high commercial value, in the coastal waters off southern Japan. Chapter I. Larva and elver of the conger eel A good many young of the conger eel, including larva and elver stages, were caught in the Sea of Suo-nada, a western part of the Inland Sea of Japan, and were used in the study of their morphological transformation as they grew up. ( I ) When the conger larve hatches out, it develops into the elver stage through the semi-larva and semi-elver stages. The larva, from the time it is hatched out until it is a full grown leptocephalus, is called the developing stage, being constitut ed of early, middle and last. While the shrinking course from a full grown to a minimum size is called the metamorphosis stage, being constituted also of three stages (early, middle and last), of w'hich the first two are called semi-larva, and the last semi-elver, which is further classified into four sub-stages (Table 25)- (2) The larvae enter their metamorphosis stage when they grew up to 120 mm in body length, and they enter to the elver stage when their body length shrinked to 65 mm. The reduction in body length during the metamorphosis stage is estimated to be ( 3 ) The respective rates of lengths of the dorsal fin and of the anal fin base to the body length in the developing stage are 40% and II %, but at the metamor phosis stage these rates in lengths gradually become greater until they attain to 83 % and 63/0 at the end of the stage, respectively, as shown in Figs. 7 and 8. (4) The larval teeth fall away first in the anterior part, then in the posterior, and those in the upper jaw disappear earlier than those in the lower one. All of the larval teeth fall away by the time the elvers shrink themselves to about 90 mm Chapter II. Morphological differentiation in sex ( ] ) Morphological differences are recognized between adulls of bnili sexes of the sharp-toothed eel in the followinp seven characteristics : inlerorhitui width, vertical eye diameter, (ail length, trunk length, anal fin base length and anal leuRth (distance from tip of snout lo center of anus). The first three of the abov.- charac- terislics are more prominent in the male while the latter three ure more prominent in the female (Table 27). (2 ) The anal length is the only characteristic thal distinguishes (he male from the female of the conger eel. the former having a larger rate of length to body length than the latter (Table 37). ( 3 ) In both species the nuptial coloration is more remarkable in the mule than in the female. (4 ) No sexual difference can be seen in either the shape or the coloration of the pectoral fins of the two species of eels dealt with here. Chapter III. Breeding changes in body shape The sharp-toothed eel has in its spawning season two types of niorpliological variation, broad- and sharp-nosed. The respective characteri.stic of the two types and the biological meaning of the broad-nosed type -will be discussed in this chapter. ( I ) These two types are found in both sexes. Most of the broad-nosed type are male, while very few females are of this type (Table 46). ( 2 ) When the spawning season is over, very few fish of the broad-no.sed type are caught. (3 ) The broad-no.sed type is never found at the immature stage, but in adult fish which have their gonads matured, the broad-nosed type is developed even if rather small in body (Fig. 27). (4 ) Remarkable morphological differences between the typical broad-nosed and the sharp-nosed type of the fish can be seen in the eye diameter, interorbital width and vertical eye diameter (Table 48). ( 5 ) Between the two types there are found some fish of u tfausilional phase, so that the boundary between the two types is uncertain. (6 ) No osteological difference can be seen between the two types or between the sexes (Figs. 21—26)- ( 7 ) The broad-nosed type is thought to occur as a phase of breeding change found in the sharp-toothed eel, e.-specially in its male. Chapter IV. Morphological transformations with growth ( I ) In the growing process from semi-elver to early immature stage of the conger eel, the inflection of the growth rate in parts of the body occurs separately in the following three periods; anal length, isthmus width and dorsal fin depth at the last elver stage; snout angle and its height, and body girth at the beginning of the early immature stage; head length, tail length, snout length, mandible length and distance from tip of snout to origin of dorsal fin at the last phase of the early immature stage (Table 70). (2 ) In the process from semi-elver to early immature stage of the conger eel, the growth center of the body can be recognized first as being in the trunk until the end of the semi-elver stage. After this the center is in the head, if the body is divided into three parts, head, trunk and tail (Fig. 29). (3 ) The adult female of the sharp-toothed eel displays at its early stage those characteristics which exhibit growth inflections in the distance from tip of snout to origin of the dorsal fin, head length, anal length, body girth and interorbital width, while the male does so in pectoral fin length and distance from tip of snout to origin of dorsal fin, in the same stage as the female. The female conger eel exhibits those characteristics in snout length and eye diameter; the male does so in eye diameter and trunk length (Tables 62, 63. 82). (4) During the growth process from immature to adult stages of the sharp toothed cel, whether male or female, the growth center of the body is in head, whereas in both sexes of the conger eel the center is in head when it is immature, but in the trunk when matured (Figs. 38. 40). Chapter V. Morphological variations due to environmental conditions, especially to nutrition Having cultured two groups of elvers of the conger eel classified by giving them different amounts of food, the author has scrutinized those differences in external features seen between the groups of cultured fish, and in those found between cultured and wild ones. ( 1 ) Those immatures which have grown up from elvers until autumn show re markable differences between the two group.s reared with different amounts of food in trunk length, tail length, snout angle, head length, snout length, eye diameter, interorbital width, distance from tip of snout to origin of the dorsal fin and anal length. In these characteristics the ratios of trunk length and tail length to total length, and snout angle are greater in the group reared in the better nutritious environment, but the ratios in the other characteristics mentioned to the total length are greater in the group reared in the worse one (Table 87). (2) Immature conger eels show remarkable differences in such external charac teristics as dorsal fin length, snout height and pectoral fin length, which are not caused by nutritious conditions (Table 87). (3 ) The amount of food the fish receive can not be regarded as the only decisive factor that can cause the morphological differences found between cultured and wild ones. Chapter VI. Morphological variations in relation to geographic * distribution The author has made a study of the geographic vuriolions of soiin' external and ineristic characters of the sharp-toothed eel and the conger eel found along the coast of Japan and its neighbouring waters. ( 1 ) Those sharp-toothed eels found in the Kumnno-nuda, the East Chinn Sea and the Inland Sea of Japan show geographic variations in head length, anal length, dorsal fin depth and eye diameter, respectively. The rates of these dimensions to body length in the fish found in the Kumano-nada are the greatest among those of the three regions mentioned but are fewer in the meri.^tic characters (Tables Q6, 102)- (2 ) The conger eels distributed along the Pacific coast of Japan show Intitudal geographic variations in head length, interorbital width and mandible length. Those found in the more northern waters are more likely to have shorter dimen.sions in the three characteristics mentioned. As for their meri.stic characters, tliose found to the south of the Bay of Mat.sushima are fewer in number than those found to the nnrth of the Bay (Tables ||8 , 124, 126, 133)- Chapter VII. Oateology In this chapter the author has investigated comparatively the internal skeletons of all the fishes of the fainilie.s Muraenesocidae and Congridae hitherto found along the coast of Japan and in its neighbouring waters. ( 1 ) The Muraenesocids have as their internal characteri.stics: i) a well-developed cranium, especially in the premaxillo-ethmo-vomerine plate tightly apglutinatcd to gether by premaxillary, ethmoid and vomer; ii) a well-developed iiiilatoplerygoid of which the vomer and the parasphenoid are tightly jointed anteriorly, and hyo- mandibular and part of quadrate jointed posteriorly ; and iii) a well-developed opercula (Table 139). ( 2 ) As to the degree of specialization, the Muraenesocids are more advanced than the Congrids in the following points: elongated premaxillo-ethmovomerine plnte; maxillary jointed with the ethmoid process in the posterior ethmoidal region; and strong cuspids in the premaxillary patch as well a.s in the vomerine and dentary bands of the teeth (Table 139)- ( 3 ) Of the two genera Oxyconger and MurAenesox which are members of the family Muraenesocidae, the latter is more specialized than the former in the following points', presence of basioccipital brush and exocciptital brush; well-developed opercula; presence of tricuspidal teeth in vomerine, maxillary and dentary bands; and well- developed neural and haemal spines; and alao of intermuscular bones (Table 139).

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