Variability of trunk limbs along the anterior/posterior body axis of juvenile and adult Lynceus biformis (Ishikawa, 1895) (Branchiopoda, Laevicaudata, Lynceidae)

Abstract

Morphology of trunk limbs of the laevicaudatan branchiopod Lynceus biformis (Ishikawa, 1895) varies along the anterior/posterior axis of the body. On anterior limbs, the praecoxal endite is elongate and strongly flexed, endites of the coxa and basis are broad, and lobes of the endopod are elongate. On more posterior limbs protopodal endites and endopodal lobes are more similar to each other and none is particularly long or broad. In addition, an exite fails to form on more posterior limbs, and the exopod becomes progressively reduced in males or modified in females. Sexual dimorphism is expressed by fewer trunk limbs, 10, and a transformed trunk limb 1 of males, as well as by a modified exopod of posterior trunk limbs 8-12 of females. Development of all limbs includes an asetose step and several setose steps; some limb pairs may begin as a transverse bilobate flap. Developmental changes include the progressive addition of limbs posteriorly, addition of endites and lobes to some limbs, addition of an exite and proximal rod-like structure during the transformation to a setose limb, and the transformation of sexually dimorphic limbs during the molt to the adult. Naupliar asetose limbs 1-5 have fewer endites and lobes than the more posterior asetose limbs of juveniles have.The male clasper of L. biformis is a subchela formed from the basis and the distal segment of the endopod; the proximal and middle endopodal segments are closely associated with the subchela. In contrast, the clasper of male spinicaudatans is a subchela formed from the distal endopodal segment and the proximal of two middle segments of the endopod. The distal of the two middle endopodal segments and the palp it bears (together, the so-called “articulated palp”) are absent in the clasper of L. biformis. The modified exopod on trunk limbs 7-10 of females of L. biformis has no similarly transformed counterpart on limbs of female spinicaudatans. There is no discoid endite on the trunk limb of L. biformis as there is on trunk limbs of spinicaudatans, and no setose attenuate endite either, although the asetose rod-like structure may be homologous to the latter.