Arm Crown Research Articles

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

Summary 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 reference to the external morphology and the surface ultrastructure with a scanning electron microscope to identify both phylogenetic conservation and diversity of structures in the organogenesis of a loliginid. Developmental states of various cilia, namely the scattered, tuft, and uniform types are described. The ciliature pattern of S. lessoniana is more closely similar to the patterns of other loliginids than to those of sepioids and sepiolids, although characteristic numerous uniform-type cilia exist in the embryos of S. lessoniana. The conserved pattern can be recognized in the organogenesis of S. lessoniana and other loliginids; on the other hand, heterochronic variations are noted, particularly in photosensitive organs and chromatophores. Eye pigmentation does not start at the same stage as in other loliginids and it is unlikely that the heterochronic variation of eye pigmentation is correlated to embryonic size. Chromatophores of S. lessoniana appeared earlier than those of other loliginids. Although hatchlings of S. lessoniana have a more strongly developed arm crown than other loliginids, the suckers arise at similar stages. These ontogenetic variations in loliginids may be considered together with the early mode of life.

Reevaluation of coleoid cephalopod relationships based on modified arms in the Jurassic coleoid Mastigophora

Mastigophora brevipinnis Owen, 1856, is a ‘fossil teuthid’ presently considered to be a member of the coleoid cephalopod Suborder Loligosepiina Jeletzky, which in turn has been placed by various authors in or near the Vampyromorpha Grimpe. Recent morphological and biochemical analyses indicate that vampyromorphs are more closely related to the Octopoda than to the Decapodiformes. Fossils of Mastigophora from the Oxford Clay (Jurassic: Callovian) show soft-tissue preservation and evidence of arm crown specialization. Some of these fossils have up to eight short, thick arms with circular sucker-like structures and filiform distal extensions, plus what appear to be the bases of two thinner ventrolateral arms. The latter lack proximal suckers and curve medially to insert into the arm crown, similar to the tentacles that are the modified ventrolateral arms of living squids and cuttlefishes. This suggests that the thinner structures were decapod-like tentacles. If Mastigophora had tentacles homologous with those of modern decapods, then it was a decapod, because this synapomorphy defines the Decapodiformes. This indication of decapod affinities for Mastigophora brings into question the relationships of the other ‘fossil teuthids’. The inferred relationship of the Loligosepiina, including Mastigophora, with the Vampyromorpha, based largely on similarities of gladius morphology with that of living Vampyroteuthis, may reflect shared plesiomorphic characters.