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Larval Development of Humboldt Current krill, Euphausia mucronata G. O. Sars, 1883 (Malacostraca, Euphausiacea)

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Larval series, developmental pathways, and growth rates of Euphausia mucronata G. O. Sars, 1883, a keystone species of the Humboldt Current ecosystem, were studied from field samples and rearing experiments. Three stages in the calyptopis phase, a fixed number, are described based on abdomen segmentation and uropod appearance, while a large number of furcilia forms was identified in rearing experiments and in field samples. The furcilia forms were grouped into seven stages based on pleopod and telson development. The larval stage ends at the juvenile phase, with five pairs of setose pleopods, and one terminal spine and two posterolateral spines on the telson. The frequency of occurrence of larval furcilia forms in the field and the examination of exuviae in rearing experiments allowed the identification of variable pathways of development. For furcilia 1 to furcilia 3, there were two dominant pathways of pleopod development in the field: B and C, and one different dominant pathway in the laboratory, A. Pathway C involved an extra moult, whereas pathways A and B were more direct. For furcilia 3 to furcilia 7, there was a common dominant pathway of telson development for both field and reared animals.

The intermoult period was very consistent around a mean of four days. Growth factors were higher between C1 and F1 (1.28-1.60) than between furcilia stages (1.00-1.30). Larvae in the direct and short pathways had more morphological change per moult and required less time to reach the juvenile phase than larvae in indirect pathways. Alternative pathways, assuming constant growth rates between moults and a constant intermoult period, may result in younger and smaller-sized larvae in a given stage (shortest pathway) and older and larger-sized larvae of the same stage (longer pathways) from the same original stock. Since short pathways are associated with optimal environmental conditions and longer ones with suboptimal to poor conditions, ontogenetic variability in morphology, pathways, and growth may be interpreted as an adaptive strategy of populations to survive and take advantage of spatially and temporally variable environments, such as the Humboldt Current. A partir de larvas obtenidas en el mar y cultivadas en laboratorio se estudió: serie larval, vías de desarrollo y tasa de crecimiento de Euphausia mucronata G. O. Sars, 1883, especie clave del ecosistema de la Corriente de Humboldt. En base a la segmentación del abdomen y aparición de urópodos se identificó un número fijo de tres estadíos en calyptopis, y un elevado número de formas de furcilia en muestras del mar y del cultivo. Las formas de furcilias se agruparon en siete estadíos en base al desarrollo de pleópodos y telson. El desarrollo larval finalizó en la fase juvenil con cinco pares de pleópodos setosos, una espina terminal y dos espinas posterolaterales en el telson. La frecuencia de aparición de formas de furcilia en muestras del mar y el examen de mudas del cultivo permitió la identificación de vías de desarrollo variable. De furcilia 1 a furcilia 3, en muestras del mar, hubo dos vías dominantes (B y C) de desarrollo de pleópodos, y en cultivos, una vía dominante (A). La vía C involucró una muda extra, mientras que las vías A y B fueron más directas. De furcilia 3 a furcilia 7 (muestras de mar y de cultivo), existió una vía dominante común de desarrollo del telson.

El período de íntermuda fue consistente, con una media de 4 días. Los factores de crecimiento fueron mayores entre calyptopis 1 y furcilia 1 (1,28-1,60) que entre furcilias (1,00-1,30). En las vías directas de corto desarrollo, hubo más cambios morfológicos por muda y se requirió menos tiempo para alcanzar la fase juvenil. Las vías alternativas suponen tasas de crecimiento constantes entre mudas y un período constante de intermuda, lo cual puede dar como resultado que de un mismo stock de larvas existan larvas más jóvenes y mas pequeñas en un determinado estadío (vía corta) y larvas de mas edad y mas grandes en el mismo estadío (vía larga). Ya que las vías cortas están asociadas a condiciones ambientales óptimas y las largas a condiciones sub-óptimas o pobres, la variabilidad ontogenética en morfología, vías y crecimiento puede interpretarse como una estrategia adaptativa de las poblaciones para sobrevivir y obtener ventajas en ambientes espacial y temporalmente variables tales como el de la Corriente de Humboldt.

Affiliations: 1: Departamento de Oceanografía, Universidad de Concepción, Casilla 160-C, Concepción, Chile; 2: Facultad de Ciencias del Mar y de Recursos Naturales, Universidad de Valparaíso, Casilla 5080 Reñaca, Viña del Mar, Chile


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