The sloughing of the melanic area of the dorsal fin, an ontogenetic process in Tylosurus raphidoma. Bulletin of the AMNH ; v. 99, article 1

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New York : [American Museum of Natural History]
"Small juvenile Tylosurus raphidoma (Ranzani) concomitantly develop elongation of the posterior rays of the dorsal fins and mandibular lappets. Both structures are areas of intense melanin concentration. 2. Larger juveniles and sexually mature adults lack both these features, having no mandibular lappets and possessing a dorsal fin which is high anteriorly and low posteriorly, and shows no concentration of melanophores at any place. 3. The metamorphosis from one form into the other is evidently rapid, for individuals in the state of transformation are not nearly so common as those above and below the size at which it occurs. 4. This transformation takes place when the fish are 200 mm. and 300 mm. in standard length and long before sexual maturity is attained which at the earliest is not under 600 mm.; therefore it is evidently not under the control of endocrine changes which take place at the attainment of sexual maturity. 5. The transformation takes place at an age of approximately six months, and sexual maturity is not attained until a year has passed. 6. Individuals in the transitional stage may be found in the summer and fall, most of the spawning evidently taking place in the first half of the year. 7. The black areas of the dorsal are reduced not by resorption but by a process of sloughing off. The melanophores break down first, the fin membranes then disintegrate, and finally the fin rays snap off at their articulations. 8. Before the destructive changes take place, the final form of the fin is foreshadowed by the shape of the melanic area of the juvenile fin. 9. A change in behavior takes place before the process of sloughing off occurs, the young juveniles associating with floating twigs or stems about their own size and general color. After they stop this association, the changes described take place, and from then on they avoid such objects and live freely in open water. 10. Histological examination of the mandibular lappets of Tylosurus and the barbels of Cypselurus showed that these structures are eliminated by a process of resorption. 11. The presence of large numbers of coarse granular eosinophiles in all these involuting structures may be associated either with parasitism or with the resorption process. 12. No taste buds were found to be present in either the belonid lappets or the exocoetid barbels which are evidently homologous, a condition apparently unique among teleost oral appendages"--P. 21.
23 p., 4 p. of plates : ill. ; 27 cm.
Includes bibliographical references (p. 21-23).