Conserved topological patterns and heterochronies in loliginid cephalopods: comparative developmental morphology of the oval squid Sepioteuthis lessoniana

Development of the oval squid Sepioteuthis lessoniana is marked by a large embryonic size resulting in the most fully developed planktonic hatchlings within loliginids. We analyzed the embryonic development with special refer ence to the external morphology and the surface ultrastructure with a scann ing electron microscope to identify both phylogenetic conservation and dive rsity of structures in the organogenesis of a loliginid. Developmental stat es of various cilia, namely the scattered, tuft, and uniform types are desc ribed. The ciliature pattern of S. lessoniana is more closely similar to th e patterns of other loliginids than to those of sepioids and sepiolids, alt hough characteristic numerous uniform-type cilia exist in the embryos of S. lessoniana. The conserved pattern can be recognized in the organogenesis o f S. lessoniana and other loliginids; on the other hand, heterochronic vari ations are noted, particularly in photosensitive organs and chromatophores. Eye pigmentation does not start at the same stage as in other loliginids a nd it is unlikely that the heterochronic variation of eye pigmentation is c orrelated to embryonic size. Chromatophores of S. lessoniana appeared earli er than those of other loliginids. Although hatchlings of S. lessoniana hav e a more strongly developed arm crown than other loliginids, the suckers ar ise at similar stages. These ontogenetic variations in loliginids may be co nsidered together with the early mode of life.