Pattern of sucker development in cuttlefishes

BACKGROUND: Morphological novelties have been acquired through evolutionary processes and related to the adaptation of new life-history strategies with new functions of the bodyparts. Cephalopod molluscs such as octopuses, squids and cuttlefishes possess unique morphological characteristics. Among those novel morphologies, in particular, suckers arranged along the oral side of each arm possess multiple functions, such as capturing prey and locomotion, so that the sucker morphology is diversified among species, depending on their ecological niche. However, the detailed developmental process of sucker formation has remained unclear, although it is known that new suckers are formed or added during both embryonic and postembryonic development. In the present study, therefore, focusing on two cuttlefish species, Sepia esculenta and S. lycidas, in which the sucker morphology is relatively simple, morphological and histological observations were carried out during embryonic and postembryonic development to elucidate the developmental process of sucker formation and to compare them among other cephalopod species. RESULTS: The observations in both species clearly showed that the newly formed suckers were added on the oral side of the most distal tip of each arm during embryonic and postembryonic development. On the oral side of the arm tip, the epithelial tissue became swollen to form a ridge along the proximal-distal axis (sucker field ridge). Next to the sucker field ridge, there were small dome-shaped bulges that are presumed to be the sucker buds. Toward the proximal direction, the buds became functional suckers, in which the inner tissues differentiated to form the complex sucker structures. During postembryonic development, on both sides of the sucker field ridge, epithelial tissues extended to form a sheath, covering the ridge for protection of undifferentiated suckers. CONCLUSIONS: The developmental process of sucker formation, in which sucker buds are generated from a ridge structure (sucker field ridge) on the oral side at the distal-most arm tip, was shared in both cuttlefish species, although some minor heterochronic shifts of the developmental events were detected between the two species. (325 words)