Embryonic and larval development and segment formation in Ibla quadrivalivis Cuv. (Cirripedia).

Abstract

Embryonic development in I. quadrivalvis follows the typical cirripede pattern except for modifications of cleavage and of development of the gut, associated with a large yolky egg. The nauplius is free-swimming but lecithotrophic. It does not grow, and moults irregularly. The development of cypris features begins precociously and the second half of naupliar life is demersal. The cypris stage is unmodified. Settling was not observed. The maxillary segments develop as a delayed continuation of the naupliar segmental sequence, the thoracic segments from a growth zone of seven ectoteloblasts and eight mesoteloblasts. Each row of eight cells budded from the mesoteloblasts develops into the paired somites of a segment. The midgut develops independently of the yolk cells. In cirripedes, increased yolk results in modification of cleavage and gut development but otherwise has little influence on embryonic development. Larval modifications associated with lecithotrophy promote direct development of the cypris but do not include modification of the cypris. Irregularity of moulting is associated with lack of larval growth. Relative brood sizes suggest that increased yolk in cirripedes offers advantages in larval survival. Post-naupliar segment formation in cirripedes resembles in detail that of Malacostraca, indicating a possible phylogenetic affinity between Malacostraca and Maxillopoda which can be tested by further studies on segment formation in non- Malacostraca.