RESUMEN
Promormyromasts were found in the epidermis of the head of the larvae of five species of mormyrids bred in captivity. The promormyromast is a larval electroreceptor belonging to the specific lateral line system. In 12-day-old larvae this electroreceptor is characterized by a single sensory cell and two types of accessory cells. One type of accessory cell has dark cytoplasm, few microtubules, and contacts the sensory cell directly, whereas a second type has pale cytoplasm, many microtubules, and forms an outer layer not directly in contact with the sensory cell. This second type is referred to as a long pyriform accessory cell. This assembly of cells is situated below an intraepidermal cavity filled with acid polysaccharides. The bordering epidermal cells extend microvilli into the intraepidermal cavity. The apexes of the sensory cell, and of the two types of accessory cells, also open into the intraepidermal cavity but bear no microvilli. The promormyromast is innervated by an unmyelinated sensory nerve fiber passing through the basal membrane, which then splits into several branches between the accessory cells. These branches contact the periphery of the sensory cell with terminal boutons. At the site of each contact a ribbon-like structure surrounded by vesicles is present in the cytoplasm of the sensory cell. In older larvae of Campylomormyrus cassaicus, membrane foldings develop at the periphery of the pyriform accessory cells and accessory cell staining properties change just before transformation to become a mormyromast. The functional role of the promormyromast of the larval mormyrids is discussed.
Asunto(s)
Pez Eléctrico/anatomía & histología , Campos Electromagnéticos , Epidermis/inervación , Sistema de la Línea Lateral/ultraestructura , Células Receptoras Sensoriales/ultraestructura , Vías Aferentes/crecimiento & desarrollo , Vías Aferentes/ultraestructura , Envejecimiento/fisiología , Animales , Diferenciación Celular/fisiología , Pez Eléctrico/crecimiento & desarrollo , Epidermis/crecimiento & desarrollo , Células Epiteliales/fisiología , Células Epiteliales/ultraestructura , Larva/anatomía & histología , Larva/crecimiento & desarrollo , Sistema de la Línea Lateral/crecimiento & desarrollo , Metamorfosis Biológica/fisiología , Microscopía Electrónica de Transmisión , Células Receptoras Sensoriales/fisiologíaRESUMEN
Two types of larval electroreceptors, type A and B, are described in the epidermis of the head of larvae of three mormyrid species, Campylomormyrus cassaicus, Mormyrus rume proboscirostris and Pollimyrus isidori, bred in captivity. In each of these electroreceptor organs, a single sensory cell is found inside an intraepidermal cavity, sitting on a platform of accessory cells. The cavity is filled with microvilli originating both from the sensory cell and from the epidermal covering cells lining the intraepidermal cavity. These two types of tuberous larval electroreceptors differ in their distribution in the epidermis of the head, in the composition of their accessory cells, and by their innervation. The innervation found in type B organs is similar to that already described for electroreceptors of adult mormyrids. The sensorineural junction is composed of primary afferent terminal boutons, which contact the base of the sensory cell. Opposite each terminal bouton, a ribbon-like synaptic bar surrounded by vesicles is found in the cytoplasm of the sensory cell. In contrast, the base of the sensory cell in type A larval electroreceptors is not contacted by nervous terminal boutons, but instead forms closed appositions with specialized prolongations of accessory cells of the platform. The base of the sensory cell presents membrane evaginations, with hemispheric synaptic structures and few synaptic vesicles. These two types of electroreceptor organs degenerate at the time of the degeneration of the larval electric organ and the functional differentiation of the adult electric organ. The functional role of two tuberous electroreceptor types is examined.
Asunto(s)
Pez Eléctrico/anatomía & histología , Órgano Eléctrico/ultraestructura , Epidermis/inervación , Epidermis/ultraestructura , Larva/ultraestructura , Neuronas Aferentes/ultraestructura , Terminales Presinápticos/ultraestructura , Estructuras Animales/fisiología , Estructuras Animales/ultraestructura , Animales , Pez Eléctrico/crecimiento & desarrollo , Pez Eléctrico/fisiología , Órgano Eléctrico/fisiología , Electricidad , Epidermis/fisiología , Larva/crecimiento & desarrollo , Larva/fisiología , Neuronas Aferentes/fisiología , Terminales Presinápticos/fisiología , Sensación/fisiologíaRESUMEN
Comparative data on the reproduction in captivity and ontogenetic development of six of mormyrid species and seven gymnotiform species are reported. Mormyrid fishes: egg diameter ranged from 1.8 (Petroephalus soudanensis) to 3.0 mm (Hippopotamyrus pictus. Campylomormyrus phantasticus); fecundity (egg number per spawning) from 28 to 215 (Pollimyrus isidori) and 121 to 1662 (Campylomormyrus cassaicus); spawning intervals from 5 to 20 days (Pollimyrus isidori) and 15 to 80 days (Campylomormyrus cassaicus). Pollimyrus isidori is the only species exhibiting parental care (in the male sex). Gymnotiform species: egg diameter ranged from 1.7 (Brachyhypopomus pinnicaudatus) to 3.0 mm (Sternopygus macrurus, Rhamphichthys sp., Gymnotus carapo); fecundity (egg number per spawning) from 1 to 105 (Apteronotus leptorhynchus) and from approximately 500 to approximately 1000 (Rhamphichthys sp.); spawning intervals from 2 to 5 days (Eigenmannia lineata) and from 20 to 41 days (Rhamphichthys sp.). Sternopyguls macrurus exhibited parental care (in the male sex) by guarding the eggs, whereas Gymnotus carapo revealed to be a mouthbreeder (guarding free embryos) in the male sex. Gonad maturation could be provoked by increase of conductivity alone in all mormyrid species tested and in several gymnotiform species. Four different stages of morphological development (hatchlings, larvae at beginning of exogenous feeding, juveniles, adults) are described in both taxa. The reproductive strategies of the gymnotiforms are considered more diverse than those of the mormyrids.