Makiko Okuyama

Studies on Japanese Botryllid Ascidians. I. A New Species of the Genus Botryllus from the Izu Islands

Abstract An investigation was made of the morphology and life history of an unidentified botryllid ascidian that was first collected from the stony shore of Shikine Island of the Izu Islands in Japan. This species has a very soft and transparent tunic, which is different from that of other botryllids. The arrangement of ovary and testis in this species is the same as those of other species of Botryllus. The embryo develops in the sac-like brooding organ formed in the peribranchial cavity of a zooid. Some colonial parts of significant size that include only the vascular system (i.e., without zooids) often extend from the colony margins, and in these parts vascular budding often occurs. Then, the processes and features of the allorecognition reaction of this ascidian were observed. Allorejection occurred after fusion of the vascular systems between two incompatible colonies. This feature is also observed in Botryllus scalaris, but it takes this ascidian much longer to initiate the allorejection reaction than B. scalaris after fusion of blood vessels between two incompatible colonies. It was concluded, on the basis of this study, that this ascidian should be designated as a new species belonging to the genus Botryllus.

Studies on Japanese Botryllid Ascidians. IV. A New Species of the Genus Botryllus with a Unique Colony Shape, from the Vicinity of Shimoda

Abstract The morphology and life history of a strange and unidentified botryllid ascidian were investigated. This ascidian was first collected from the stony shore of Ebisu Island in Shimoda, a city on Izu peninsula in central Japan. Unlike other botryllid ascidians, whose colonies are flat and smooth, this ascidians colonies are rugged. In each colony, zooids are arranged into several oval systems, each of which has a thick part containing zooids and very thin parts that do not. The arrangement of ovary and testis in this species is the same as in other species of the genus Botryllus; the ovary is situated anterior to the testis. The embryo of this ascidian develops in the peribranchial cavity of its mother zooid without any brooding organs, as is the case with Botryllus scalaris and Botryllus puniceus. Meanwhile, the results of cut colony assay experiments did not show the existence of colony specificity in this ascidian. Even when two syngeneic colonies were brought into contact at their growing edges, none fused together. On the other hand, when two colonies were brought into contact with each other at their cut surfaces, they always fused into a single colony, regardless of their origin. Therefore, this species may be the only species that lacks colony specificity among the botryllids studied so far.

Phylogenetic Position of a Deep-Sea Ascidian, Megalodicopia hians , Inferred from the Molecular Data

Abstract Ascidians inhabit both shallow water and the deep sea. The phylogenetic position of deep-sea ascidians has not been sufficiently investigated because of their unusual habitats. The family Octacnemidae is one such enigmatic deep-sea ascidian. In this report, we determined the sequences of the 18SrDNA and a mitochondrial protein gene of Megalodicopia hians belonging to the family Octacnemidae, and we analyzed its phylogenetic relationship with other ascidians. A phylogenetic relationship of this family with the families Cionidae and/or Corellidae has been suspected based on a small number of morphological characteristics. However, our results suggested that M. hians has a close relationship to the family Corellidae and might originate from them. This is the first report of the molecular phylogenetic analysis of a deep-sea ascidian.

Morphological Studies on the Bathyal Ascidian, Megalodicopia hians Oka 1918 (Octacnemidae, Phlebobranchia), with Remarks on Feeding and Tunic Morphology

Abstract Megalodicopia hians Oka is a solitary ascidian belonging to the family Octacnemidae inhabiting the bathyal /abyssal zone as well as other octacnemid ascidians. The phylogenetic relationship of octacnemids is open to argument because of its extraordinary morphological features due to habitat adaptation, e.g., a pharynx lacking ciliated stigmata. Aggregations of M. hians were discovered by the manned submersible Shinkai 2000 in the bathyal seafloor of Toyama Bay, Japan Sea, in 2000; this was the first in situ observation of M. hians in the Japanese coastal waters. In 2001, a total of 36 M. hians specimens were collected from the bay (592∼978 m deep). In situ observation indicated that M. hians usually opens its large oral apertures to engulf the drifting food particles in the water current. Microscopical observation of the gut contents also showed that M. hians is a non selective macrophagous feeding on small crustaceans, diatoms, detritus, and so on. Along with the position of the intestinal loop and gonad, the morphological characteristics of the tunic (integument of ascidians) suggest that M. hians is closely related to Cionidae and/or Corellidae. Some symbiotic/parasitic organisms were occasionally found in the tunic, including rod-shaped bacteria, fungi-like multicellular structure, and spawns of unknown animals.

Studies on Japanese Botryllid Ascidians. II. A New Species of the Genus Botryllus from the Vicinity of Shimoda

Abstract Morphology and life history of a new species of the genus Botryllus belonging to the family Botryllidae were described in detail. This ascidian was collected from the stony shore in the vicinity of Shimoda (Shizuoka prefecture, Japan). The arrangement of ovary and testis in this ascidian was the same as that in other species of the genus Botryllus, while the embryo developed in a brood pouch formed from the invagination of peribranchial epithelium, as in the other genus Botrylloides. The processes and features of the allorecognition reaction of this ascidian were observed. The reaction showed the same processes as that in the species of the genus Botrylloides. Therefore, this ascidian has both features of the two genera of the family Botryllidae, which strongly suggests the necessity of reconsidering on the classificatory criteria of botryllid ascidians.

Phylogeny of Botryllid Ascidians

It has been suggested that the past classificatory criteria for botryllid ascidians are unsatisfactory. One reason for that is the absence of studies on the phylogeny of this group. Fourteen botryllids collected from the vicinity of the Shimoda Marine Research Center, University of Tsukuba, have been cultured for several years, and their morphology observed throughout life, life history, sexual and asexual reproduction, and colony specificity. On the basis of this long-term observational data and the analysis of ascidian 18S rDNA sequences, we tried to elucidate the phylogenetic relationship among these botryllids. From this study, it is suggested that botryllids might be separated into two groups; the group of botryllids with only four stigmatal rows, B. primigenus and B. communis, and the other group consisting of the majority of botryllids. The decisive characteristics that divide the major group are not clear.

Studies on the Structure of the Brooding Organs of Two Botryllid Ascidians, Botryllus delicatus and Botryllus sexiens

Abstract The formative processes of the brooding organs of two botryllid ascidians, Botryllus delicatus and Botryllus sexiens, were observed. The structure of these organs was also studied in detail histologically. The brooding organs of these two species were formed from the branchial sac, in contrast to the brooding organs of other botryllids, which are formed from the peribranchial wall. The structures of the brooding organs of B. delicatus and B. sexiens were different, but the processes of their formation were almost the same. As the maturation of the oocytes occurred, a part of the epithelium of the branchial sac extended toward the peribranchial epithelium covering the ovary. The extension of the branchial epithelium looked like a bowl and made a surrounding space with the peribranchial epithelium in a peribranchial cavity. The oocyte was ovulated into this space and fertilized there. After ovulation, the extension of the branchial epithelium formed a sac-like brooding organ in combination with the peribranchial wall and held the embryo tightly, like the incubatory pouch of the genus Botrylloides. The unusual characteristics of the brooding organs in these two species lead to reconsideration of phylogenetic relationships and taxonomic criteria in botryllids.