Differences of Sucker Formation Processes Depending on Benthic or Pelagic Posthatching Lifestyles in Two Octopus Species.

AbstractMorphologies of animal appendages are highly diversified depending on animal lifestyles. In cephalopods (Mollusca, Cephalopoda), an individual possesses multiple arms that contribute to elaborate behaviors, and suckers on them enable various arm functions. In octopus hatchlings, arm and sucker morphologies can be divided into two different types due to alternative posthatching lifestyles, that is, pelagic or benthic lifestyles, although the underlying developmental differences have yet to be elucidated. In this study, therefore, detailed developmental processes of arms and suckers were observed during embryogenesis in two different octopus species, Octopus parvus and Amphioctopus fangsiao, showing pelagic and benthic posthatching lifestyles, respectively. In O. parvus, sucker formation stopped at a relatively early stage in which three suckers on an arm were produced. In addition, at late embryonic stages, cell proliferation was hardly detected in whole arms, while in A. fangsiao, sucker production continued throughout embryogenesis and cell proliferation also remained active in whole arms even in the late stages. Therefore, although further investigations in other octopus species are required, it is suggested that in octopus evolution, the developmental program of suckers has been modified in accordance with the acquisition of a novel lifestyle.

Cephalopod-omics: Emerging Fields and Technologies in Cephalopod Biology.

Few animal groups can claim the level of wonder that cephalopods instill in the minds of researchers and the general public. Much of cephalopod biology, however, remains unexplored: the largest invertebrate brain, difficult husbandry conditions, and complex (meta-)genomes, among many other things, have hindered progress in addressing key questions. However, recent technological advancements in sequencing, imaging, and genetic manipulation have opened new avenues for exploring the biology of these extraordinary animals. The cephalopod molecular biology community is thus experiencing a large influx of researchers, emerging from different fields, accelerating the pace of research in this clade. In the first post-pandemic event at the Cephalopod International Advisory Council (CIAC) conference in April 2022, over 40 participants from all over the world met and discussed key challenges and perspectives for current cephalopod molecular biology and evolution. Our particular focus was on the fields of comparative and regulatory genomics, gene manipulation, single-cell transcriptomics, metagenomics, and microbial interactions. This article is a result of this joint effort, summarizing the latest insights from these emerging fields, their bottlenecks, and potential solutions. The article highlights the interdisciplinary nature of the cephalopod-omics community and provides an emphasis on continuous consolidation of efforts and collaboration in this rapidly evolving field.

Pleopodal lung development in a terrestrial isopod, Porcellio scaber (Oniscidea).

During evolution, various lineages of arthropods colonized land and independently acquired air-breathing organs. Some taxa of oniscidean isopods (Crustacea, Isopoda, Oniscidea) are the most successful crustacean lineages on land and possess organs called "lungs" or "pseudotrachea" for air-breathing in their abdominal appendages, i.e., in pleopods. Although these lungs are important for adapting to the terrestrial environment, their developmental process has not yet been elucidated. In the present study, we investigated the process of lung development in Porcellio scaber, the common rough woodlouse with pleopodal lungs in the first two pairs of pleopods. The lungs in the second pleopods developed at the manca 1 stage (immediately after hatching) and became functional at the manca 2 stage. In the first pleopods, which appear at the manca 3 stage, the lungs were gradually developed during the manca 3 stage and became functional in post-manca juveniles. In the second pleopods, epithelial invaginations led to lung development. These results suggest that some novel developmental mechanisms with epithelial invaginations and cuticle formation were acquired during terrestrialization, resulting in the development of functional lungs in the terrestrial isopod lineages.

Sucker formation in a bigfin reef squid: Comparison between arms and tentacles.

Cephalopods have acquired numerous novelties and expanded their habitats to various marine environments as highly agile predators. Among cephalopod novelties, multiple arms are used for complex behaviors, including prey capture. Suckers on arms are innovative features for realizing these arm functions. In addition, tentacles in Decapodiformes (squids and cuttlefishes) are arms specialized in prey capture and tentacular suckers show unique morphologies. However, little is known about the developmental process of sucker formation that should differ between tentacles and other arms. In this study, therefore, sucker formation processes on second arms and tentacles were observed and compared in a bigfin reef squid, Sepioteuthis lessoniana, to reveal the developmental processes forming the unique sucker morphologies, especially in tentacles. Morphological and histological observations of suckers during embryogenesis showed that, in second arms, the sucker-producing area appeared at the most distal part. At the most proximal side of the sucker-producing area, new sucker buds were isolated by invagination of the epithelial tissue. At the proximal arm parts, suckers with functional structures were observed. In tentacles, although the basic sucker formation pattern was similar to that in second arms, sucker formation started at earlier embryonic stages and the number of suckers was drastically increased compared to that in second arms. In addition, although four sucker rows were observed at the tentacular club, that is, the thickest part of a tentacle, our observations suggested that two sets of two sucker rows are compressed to form the four rows. Therefore, the sucker-formation processes are temporally and spatially different between arms and tentacles. In addition, S. lessoniana shows conserved and unique patterns of sucker formation in comparison with previously described species, suggesting that sucker formation patterns were diversified among Decapodiformes lineages.

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).

Correction: Morphological and molecular identification of the dioecious "African species Volvox rousseletii (Chlorophyceae) in the water column of a Japanese lake based on field-collected and cultured materials.

[This corrects the article DOI: 10.1371/journal.pone.0221632.].

Morphological and molecular identification of the dioecious "African species Volvox rousseletii (Chlorophyceae) in the water column of a Japanese lake based on field-collected and cultured materials.

Volvox rousseletii is a dioecious species belonging to Volvox sect. Volvox that has previously only been found in Africa. During field surveys in a large dam lake (Lake Sagami) in Kanagawa Prefecture, central Japan, we encountered a Volvox sect. Volvox species that produces dioecious sexual spheroids in the water column. Although sexual induction of this species in culture did not produce adequately well-developed sexual spheroids for species identification, molecular data directly obtained from field-collected sexual spheroids verified the identity of field-collected male and female sexual spheroids as well as cultured materials. Based on molecular and morphological data, the species was identified as V. rousseletii. This is the first record of a dioecious species of Volvox sect. Volvox in Japan.