Limb ontogeny and trunk segmentation in Nebalia species (Crustacea, Malacostraca, Leptostraca)

Abstract

The 'egg-larval' development of two species of Nebalia has been examined with SEM. Various details concerning limb ontogeny and trunk segmentation are de- scribed. The most important of these are the following. The tripartite state of the peduncle of antenna 2 in the adult of Nebalia species is derived from the fusion of the third and fourth podomeres, present in late larvae. The proximal portion of the mandible in the adult of Nebalia brucei, carrying the 'coxal process', is, based on the onto- genetic evidence, interpreted as the combined basis and coxa, and the bipartite palp is interpreted as the endopod. The early development of the thoracopods and the three anteriormost pleopods is identical. They all start as later- ally directed, biramous limb buds. This suggests that tag- matisation of the trunk of the Leptostraca (and other Ma- lacostraca) has been developed from an ancestor with an undivided trunk region with serially similar limbs. Cer- tain early stages reveal an extra, 'eighth', limbless pleon segment, as compared with the normal number of seven pleomeres of adult Leptostraca. The presence of a row of ventral, sternitic, triangular processes between the bases of the thoracopods, as they are found in certain stages of a species of Nebalia, is suggested as a possible ground pattern for the Malacostraca.