Larval development and segment formation in the branchipod crustaceans Limnadia stanleyana King (Conchostraca) and Artemia salina (L.) (Anostraca)

Abstract

Limnadia stanleyana hatches as a lecithotrophic nauplius with a well-developed naupliar region and a globular post-naupliar region, the latter comprising the rudiments of 10 post-mandibular segments, a growth zone, and a telson. Development proceeds through five naupliar stages. Feeding begins late in stage III. Action of a naupliar feeding apparatus sustains the larva through stages IV and V. During these stages, the maxillulary and maxillary segments, the carapace, and the first six trunk segments become well developed. At the fifth moult, metamorphosis occurs to a juvenile conchostracan. The naupliar feeding apparatus is lost, the first six pairs of trunk limbs become functional, and feeding in the adult manner begins. The seventh and succeeding trunk segments are delayed in development. In the development of postmandibular segments, mesodermal somite segregation precedes ectodermal segment delineation. Post-mandibular segments behind the tenth are proliferated by a growth zone comprising an external ring of ectoderm and paired lateral rows of mesoteloblasts. Artemia salina hatches as a nauplius similar to that of L. stanleyana, and develops in an identical manner through the first four naupliar stages. The larval development of L. stanleyana is typical of Conchostraca. The larval development of A. salina is typical of Anostraca and differs from that of L. stanleyana only in passing through nine naupliar stages before metamorphosis. The larval development of the Notostraca differs from that of Conchostraca and Anostraca only in ways related to increased lecithotrophy, metamorphosis being attained more directly. A primitive mode of development can be identified for the Branchiopoda and interpreted as a derived form of the developmental pattern of the Cephalocarida. Sanders' hypothesis of a cephalocaridan-like ancestry of the Branchiopoda is upheld by the evidence of development. Post-mandibular segment formation in branchiopods remains primitive, and lacks the specializations found in Cirripedia and Malacostraca. Segment formation cannot be employed as a criterion of phylogenetic affinity in Crustacea.