Larval development of Humboldt current krill, Euphausia mucronata G. O.!Sars, 1883 (Malacostraca, Euphausiacea)

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 I 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.