Manh Duy Linh Nguyen

• Weakly electric mormyrid fish comprise about 200 species. 15 species of the genus Campylomormyrus have been described. These are very diverse concerning the trunk-like snout and the shape and duration of the electric organ discharge (EOD) and the anatomy of the electric organ. In this dissertation data on the reproduction in captivity of four species and on the ontogeny of the EOD and the EO of three species are presented. Reproduction of the four species C. compressirostris, C. rhynchophorus, C. tshokwe and C. numenius: Cyclical reproduction was provoked by changing only water conductivity (C): decreasing C led to gonadal recrudescence, an increase induced gonad regression. Data on the reproduction and development of three species are presented (in C. numenius gonad development could only be achieved in males). Agonistic behavior in the C. tshokwe pair forced us to divide the breeding tank; therefore, only ovipositions occurred. However, injection of an artificial GnRH hormone allowed us to obtain ripe eggs and sperm and to performWeakly electric mormyrid fish comprise about 200 species. 15 species of the genus Campylomormyrus have been described. These are very diverse concerning the trunk-like snout and the shape and duration of the electric organ discharge (EOD) and the anatomy of the electric organ. In this dissertation data on the reproduction in captivity of four species and on the ontogeny of the EOD and the EO of three species are presented. Reproduction of the four species C. compressirostris, C. rhynchophorus, C. tshokwe and C. numenius: Cyclical reproduction was provoked by changing only water conductivity (C): decreasing C led to gonadal recrudescence, an increase induced gonad regression. Data on the reproduction and development of three species are presented (in C. numenius gonad development could only be achieved in males). Agonistic behavior in the C. tshokwe pair forced us to divide the breeding tank; therefore, only ovipositions occurred. However, injection of an artificial GnRH hormone allowed us to obtain ripe eggs and sperm and to perform successful artificial reproduction. All three species (C. compressirostris, C. rhynchophorus, C. tshokwe) are indeterminate fractional spawners. Spawnings/ovipositions occurred during the second half of the night; no parental care was observed; no special spawning substrates were necessary. C. compressirostris successfully spawned in breeding groups, C. rhynchophorus as pair. Spawning intervals ranged from 6 to 66 days in C. rhynchophorus, 10–75 days in C. tshokwe, and 18 days in C. compressirostris (calculated values). Fecundities (eggs per fractional spawning) ranged from 70 to 1570 eggs in C. rhynchophorus, 100–1192 in C. tshokwe, and 38–246 in C. compressirostris. All three species produce yolky, slightly sticky eggs. Egg diameter ranges from 2.3–3.0 mm. Hatching occurred on day 3, feeding started on day 11. Transition from larval to juvenile stage occurred at around 20 mm total length (TL). At this size C. rhynchophorus developed a higher body than the two other species and differences between the species in the melanin pigmentation of the unpaired fins occurred. Between 32 and 35 mm TL the upper and lower jaws developed. C. compressirostris and C. tamandua are similar in morphology and both produce short EODs of ca. 150-200 μs duration. Both species reproduce easily in captivity. We tried to obtain natural hybrids in two breeding groups, 1) four males of C. compressirostris and three females of C. tamandua and 2) six females of C. compressirostris and four males of C. tamandua. In both combinations several times oviposition occurred, however, we never found fertilized eggs. In subsequent experiments, not described here, we obtained hybrids between these two species by means of artificial reproduction. Ontogeny of the EOD and the EO: The Campylomormyrus species are very diverse both concerning the shape and the duration of their EODs. There are species with very short EODs, e.g. C. compressirostris duration, a species with an EOD length of about 4-8 ms duration (C. tshokwe) and species with very long EODs of about 25 ms duration (e.g. C. rhynchophorus). Due to the successful breeding of the three species in captivity, we were able to investigate in detail the ontogeny of the EOD. Larvae of the three species C. compressirostris, C. tshokwe and C. rhynchophorus first produce a biphasic larval EOD typical for these small larvae. The first activity of the adult electric organ in the caudal peduncle is a biphasic juvenile EOD. Juvenile C. compressirostris and C. tshokwe start out with a short biphasic EOD of about 160 – 200 μs duration at sizes between 25 mm (C. compressirostris) and 37 mm (C. tshokwe). Adult C. compressirostris show an EOD identical to that of the juvenile. In C. tshokwe, the juvenile EOD changes continuously during development both concerning duration, amplitude increase and shape. 18 cm long C. tshokwe still do not yet produce an EOD typical for the adult fish. Juveniles of C. rhynchophorus produce at 33 mm total length a juvenile biphasic EOD, however, of longer duration (about 640 μs) than the two species mentioned above. This juvenile EOD changes continuously both in form, amplitude increase and duration with growth until the adult EOD waveform appears at about 15 cm body length. In juveniles about seven cm long the triphasic feature of the EOD starts to develop due to the appearance of a second head positive phase. Specific EOD stages are produced in relation to size and not to age. Individual differences in the EOD both concerning shape and duration are very small. The basic anatomy of the electrocytes is very similar in all three species: the main stalk which receives the innervation, is located at the caudal face of the electrocyte. Membrane penetrations of the stalks do not occur. However, there are differences in the fine structure of the electrocytes in the three species. Papillae, proliferations of the membrane, which increase the surface area of the electrocyte and are thought to incrase the EOD-duration, are only found in C. tshokwe and C. rhynchophorus. In these two species in addition, holes develop in the electrocytes during ontogeny. This might also have an impact on EOD duration.…
• Von den mehr als 200 Arten der schwach elektrischen Nilhechte gehören 15 zur Gattung Campylomormyrus. Diese spezialisierten Fische besitzen eine rüsselartige Schnauze. Auch produzieren manche Arten relativ kurze elektrische Entladungen (EODs) von mehreren hundert µs Dauer, andere ungewöhnlich lange EODs von bis zu 25 ms Dauer. Es werden Daten zur Reproduktion von vier Arten vorgestellt, um das Wissen über die Fortpflanzungs-biologie der Nilhechte zu erweitern und zum ersten Mal detaillierte Daten zur Ontogenese der elektrischen Organe (EO) und EODs von Arten mit extrem langen EODs zu beschreiben. Bei C. compressirostris, C. rhynchophorus, C. tshokwe und C. numenius konnte die Gonadenreifung durch Absenkung der elektrischen Leitfähigkeit des Wassers ausgelöst werden. Danach laichten C. compressirostris und C. rhynchophorus regelmäßig ab; bei C. tshokwe konnte eine Vermehrung mit Hilfe einer künstlichen Vermehrung erreicht werden; bei C. numenius konnte nur bei den Männchen eine Gonadenreifung ausgelöst werden. Die drei Arten C.Von den mehr als 200 Arten der schwach elektrischen Nilhechte gehören 15 zur Gattung Campylomormyrus. Diese spezialisierten Fische besitzen eine rüsselartige Schnauze. Auch produzieren manche Arten relativ kurze elektrische Entladungen (EODs) von mehreren hundert µs Dauer, andere ungewöhnlich lange EODs von bis zu 25 ms Dauer. Es werden Daten zur Reproduktion von vier Arten vorgestellt, um das Wissen über die Fortpflanzungs-biologie der Nilhechte zu erweitern und zum ersten Mal detaillierte Daten zur Ontogenese der elektrischen Organe (EO) und EODs von Arten mit extrem langen EODs zu beschreiben. Bei C. compressirostris, C. rhynchophorus, C. tshokwe und C. numenius konnte die Gonadenreifung durch Absenkung der elektrischen Leitfähigkeit des Wassers ausgelöst werden. Danach laichten C. compressirostris und C. rhynchophorus regelmäßig ab; bei C. tshokwe konnte eine Vermehrung mit Hilfe einer künstlichen Vermehrung erreicht werden; bei C. numenius konnte nur bei den Männchen eine Gonadenreifung ausgelöst werden. Die drei Arten C. compressirostris, C. rhynchophorus und C. tshokwe zeigen folgende reproduktiven Merkmale: das Ablaichen erfolgt in der zweiten Nachthälfte, die Eizahlen liegen bei mehreren Hundert pro Ablaichen, die Ablaichintervalle liegen bei wenigen Wochen. Embryonen schlüpfen nach drei Tagen, die exogene Nahrungsaufnahme beginnt am Tag 11. Es wurde versucht, natürliche Hybriden zwischen den Arten C. compressirostris und C. tamandua zu erhalten. Es kam zwar zur Abgabe von Eiern, diese waren aber nie befruchtet. Die Ontogenese der EOD zeigte, dass die Entladung bei allen drei untersuchten Arten mit einer biphasischen Juvenilentladung von ca. 200 µs (C. compressirostris, C. tshokwe) bzw. ca. 600 µs Dauer (C. rhynchophorus) beginnt. Diese Entladung verändert sich im Verlauf der Ontogenese bei C. compressirostris nicht mehr, bei den beiden anderen Arten kommt es zu einer kontinuierlichen Veränderung bezüglich Form und Dauer bis hin zu der Adult-Entladung. Papillen, Oberflächenvergrößerungen der elektrischen Zellen, könnten eine Erklärung für die längere Entladung bei C. tshokwe und C. rhynchophorus sein.…