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The Ontogeny of Light-Dark Response in Triops longicaudatus as a Response to Changing Selective Pressures

Jeremy M. Davis and Dale Madison
Crustaceana
Vol. 73, No. 3 (Mar., 2000), pp. 283-288
Published by: Brill
Stable URL: http://www.jstor.org/stable/20106284
Page Count: 6
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Ontogeny of Light-Dark Response in Triops longicaudatus as a Response to Changing Selective Pressures
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Abstract

The branchiopod shrimp, Triops longicaudatus, inhabits ephemeral pools, and individuals must develop rapidly if breeding is to occur. Age-related differences in physiology, foraging behavior and predation pressures should make it more adaptive for young individuals to dwell near the surface, and the adults on the bottom. Consequently, young T. longicaudatus should be photopositive whereas older individuals should be photonegative. We tested light-dark preferences in the first five instars of lab-reared T. longicaudatus. Photonegativity increased with age, with instar 2 preferring lighted environments significantly more than instars 4 and 5. The first two instars were more active under dark conditions than under light conditions, suggesting an orthokinetic mechanism for habitat segregation. These changes may be correlated with the development of the compound eyes. /// Le branchiopode Triops longicaudatus habite des mares éphémères, et les individus doivent se développer rapidement si la reproduction survient. Des différences liées à l'âge, relatives à la physiologie, au comportement de fouissage et aux pressions de prédation rendraient plus avantageux pour les jeunes de se tenir près de la surface et pour les adultes, sur le fond. En conséquence, les jeunes T. longicaudatus seraient photopositifs tandis que les individus plus âgés seraient photonégatifs. Nous avons testé les préférences lumière-obscurité chez les cinq premiers stades de T. longicaudatus élevés au laboratoire. La photonégativité augmentait avec l'âge, le stade 2 préférant significativement les environnements éclairés, plus que les stades 4 et 5. Les deux premiers stades étaient plus actifs dans des conditions d'obscurité que dans les conditions de lumière, suggérant un mécanisme orthocinétique pour la séparation de l'habitat. Ces changements peuvent être corrélés au développement des yeux composés.

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