Thyroid responsiveness of the large African barb Labeobarbus intermedius (Teleostei; Cyprinidae): Individual variability and morphological consequences

Abstract

Thyroid hormones (THs), produced by the thyroidgland, play an important role in the regulation of ontog-eny in bony fishes. They influence the rate of individualdevelopment of the body as a whole, as well as the ratesof growth and differentiation of its particular structures[1]. Definitive morphology of fishes is a result of inter-action between the developing body and a hormone,and depending on the TH level and an ability of thebody to respond to TH, i.e., TH-responsiveness.The TH level depends on the functional activity ofthe thyroid gland. A low activity and, as a result, lowTH levels lead to slowing down the ontogeny. On thecontrary, high TH levels accelerate the ontogeny [1, 2].Differences in the activity of the thyroid gland canresult in variability of the expression of morphologicalfeatures [2]. Recently, indirect data on individual vari-ability of TH-responsiveness in the large African barbLabeobarbus intermedius were reported. They demon-strated that juveniles of this fish differently respondedto similar doses of exogenous THs under experimentalconditions [3, 4]. Taking into account an important roleof TH-responsiveness in the ontogeny control, we stud-ied individual variability of TH-responsiveness and itsinvolvement in intraspecific morphological diversifica-tion in fishes.We compared morphological changes inL. intermedius siblings grown at a high level of exoge-nous TH. We focused our attention on the serial struc-tures that usually decrease in number when theL. intermedius siblings are growing under a high THlevel [3, 4].The eggs were obtained from the crossing ofL. intermedius spawners taken from a Lake Tana tribu-tary, the Gumara River (Ethiopia) and divided into twogroups. The first experimental group was put into amedium with a high TH content, namely, 2 × 10