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Artemia: A Model Specimen


Figure 2 : Anatomy of mature Artemia franciscana (modifi ed from [ 3 ] and website [a]).


sac for a period of about 14 days. With proper environ- mental conditions, young larvae leave their brood sac and grow to adult individuals within 4–5 weeks. In poor environ- mental conditions, however, embryos become encapsulated and can survive within cysts through long periods of dryness and heat. When Artemia cysts are surrounded by salt water, young larvae will appear within 1–2 days; the length of these nauplii is typically 0.5 mm (phase 1 of development). For the first 3 days, while living off their yolk sac, fresh nauplii grow to a size of about 1.2 mm (phase 2, metanauplius stage). Over the next 10 days rudimentary thoracopods and primordial bilateral compound eyes develop (see Figure 2 ); the body size grows to 4–5 mm (phase 3, zoea stage). At the end of the third week, larvae have grown to 6–8 mm, and their shape is similar to adult animals (phase 4, pre-adult stage). Maturation is finished within 4–5 weeks; at this stage young adult individuals are about 10 mm and capable of reproduction (phase 5, adult stage). Within the following days or weeks body length can increase further up to a final size of 15 or 20 mm. In the final stage, Artemia shows a complex anatomy ( Figure 2 , modified from Ruppert et al., 2006 [ 3 ] and website [c]). Observing Artemia . In all phases of diff erentiation and maturation, Artemia can be observed by placing the organisms on a slide with an adequate amount of salt water. Special microscope depression slides for examination of water organisms should be used; these have a cavity so that specimens can be observed within greater volumes of water than achievable with standard fl at slides. Examinations have to be carried out without a cover slip so that the specimens can survive imaging procedures and returned without damage to the aquarium. A suitable student microscope should be confi gured for bright- fi eld illumination and preferably be fi tted with additional equipment for dark-fi eld illumination. Instead of using a special dark-fi eld condenser, an appropriately sized circular


2018 July • www.microscopy-today.com


black plate can be mounted with the bright-fi eld condenser or put onto the light outlet of the microscope´s stand so that the central part of the illuminating light beam is blocked. Alternatively, some bright- fi eld condensers can be equipped with special light masks for dark- fi eld mounted on sliders. Detailed instructions for upgrading student microscopes for darkfi eld (and phase contrast) are given in a previous article [ 4 ]. In all stages Artemia moves very quickly in horizontal and vertical directions so that an appropriate fi eld of view and a large depth of fi eld are the primary imaging requirements, whereas the numerical aperture (NA) and power of the objective lens are of less importance. T us, low-magnifi - cation objective lenses (range of magnifi cation: 2× to 4×) should be used. All microscope objectives used


must be designed for long working distances. Photgraphing Artemia . Photomicrographs must be taken


with a fl ash in order to avoid blurring from specimen motion. Some fl ash devices are capable of automatic exposure by detecting the light through the lens, whereas in other models exposure can be regulated with a set of neutral gray fi lters or a couple of rotatable polarizing fi lters. Alternatively, video clips can be made with a digital camera, and still images can be extracted from the video fi les. Most student microscopes are not fi tted with an integrated digital camera, so consumer cameras need to be adapted to the microscope. Several practical photographic hints, including handling of a fl ash, are given in a previous publication [ 5 ]. Figures 3 a to 3 d show practical examples of coupling of cameras to microscopes. Even smartphones or iPhones can be mounted to a monocular or binocular tube ( Figure 3e ). Smartphones are attractive tools for introducing photomicrography and cinema- tography since students are already familiar with the handling of such equipment. Microscopy studies of Artemia do not require special sophisticated equipment; even low-cost microscopes can be used for the fi rst steps. By successive observations of growth and maturation, ideally from day to day, young students can comprehend all phases of “metamorphosis” from young larvae to adult individuals. Along with morphologic development, young “researchers” can also study the increasing variation and diff erentiation of locomotion patterns.


Results


Most of the Artemia cysts from the commercial experiment set were unimpaired, and the young nauplii hatched within about 36 hours. All individuals showed a high synchronicity of development and maturation so that their morphological appearance was rather uniform. Figure 4 shows the entire 35 days of maturation and morphological diff erentiation that was observed and photographed in the classroom.


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