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Abstract. Ascidians (Chordata, Tunicata) are an important group for the study of invasive species biology due to rapid generation times, potential for biofouling, and role as filter feeders in an [...]

Introduction In tropical waters, photosynthetic symbiosis is known to occur in various metazoans, such as poriferans, cnidarians, flatworms, bivalves, nudibranchs, and ascidians. Among these taxa, some species acquire symbionts only [...], Vertical transmission of cyanobacterial symbionts occurs in didemnid ascidians harboring Prochloron as an obligate symbiont; the photosymbionts are transferred from the parental ascidian colony to the offspring in various ways depending on host species. Although several didemnids harbor non-Prochloron cyanobacteria in their tunics, few studies have reported the processes of vertical transmission in these didemnids. Here we describe the histological processes of the transmission of cyanobacteria in two didemnids, Trididemnum nuhilum harboring Synechocystis and T. clinides harboring three cyanobacterial species. In both species, the photosymbionts in the tunic of the parent colony were apparently captured by the tunic cells of the host and transferred to the embryos brooded in the tunic. The symbiont cells were then incorporated into the inner tunic of the embryo. This mode of transmission is essentially the same as that of T. miniatum harboring Prochloron in the tunic, although there are some differences among species in the timing of the release of the symbionts from the tunic cells. We suggest that the similar modes of vertical transmission are an example of convergent evolution caused by constraints in the distribution patterns of symbiont cells in the host colony.

Introduction Zoantharia (Cnidaria, Anthozoa, Hexacorallia) is an order of benthic cnidarians. and zoanthid species are widespread globally from the intertidal zone to the deep sea. They are characterized by (1) [...], The spawning behavior and early embryogenesis of Palythoa tuberculosa (Anthozoa, Zoantharia) were observed in August 2009 off Okinawa Island, Japan. P. tuberculosa released zygotes just after high tide around new moon nights. The mean diameter of zygotes was 365.6 ± s.d.14.8 µm, and zygotes did not contain any symbiotic algae (zooxanthellae). About 2 h after spawning, the first cleavage furrow appeared on one side of the zygotes, although it was uncertain when eggs were fertilized. After second cleavage, the arrangement of blastomeres was pseudospherical. At 9 h after spawning, the embryo became a concave-convex dish shape, then gastrulation occurred and the blastopore was formed. Seven-day old larvae were ellipsoid and about 700 µm long, with an open mouth at one end. Two weeks after spawning, the larvae developed a longitudinal band of long cilia ( = ventral ciliate band) that is characteristic of zoanthella larvae. In P. tuberculosa, larvae show a non-radial body plan and then metamorphose to almost-radial (in outward appearance) polyps after settlement. These results may support a hypothesis that a common ancestor of Cnidaria had a bilateral body plan that has been secondarily lost in some extant cnidarians.

Introduction Colony specificity (allogeneic recognition between colonies) has been reported in many colonial organisms, such as sponges, hydrozoans, corals, bryozoans, and ascidians. Its occurrence is usually manifested by fusibility, or [...], The colonial ascidian Aplidium yamazii exhibited an allorejection reaction when two allogeneic colonies were brought into contact at their growing edges or at artificial cut surfaces. This species has no vascular network in the tunic, unlike the botryllid ascidians, which have a vascular network throughout the colony's common tunic. In the allorejection reaction induced by contact at the growing edges, some small, hard-packed tunic masses were formed at the contact points. Histological and electron microscopic investigation of these tunic masses revealed that they contained aggregates of tunic cells, with tunic phagocytes being the major cell type present. Some of the tunic phagocytes in these tunic masses appeared to be disintegrating. When allogeneic colonies were placed in contact at their artificial cut surfaces, the colonies partially fused, then separated. In this allorejection reaction, some loosely packed tunic masses remained in the gap between the two withdrawn colonies. These results strongly suggest that the tunic phagocytes are likely to be the major effector cells in the allorejection reaction. We also propose that the tunic phagocytes are not only the effector cells in the allorejection reaction but also bear the sites of allorecognition.

Ultraviolet (UV) radiation poses severe problems for organisms in tropical marine environments, because the path for sunlight through the atmosphere is shorter (1) and the seawater is clearer (2). Since [...], Coral reef invertebrates that host phototrophic symbionts are thought to protect themselves and their symbionts with mycosporine-like amino acids (MAAs)--UV-absorbing substances that act as sunscreens (Dunlap, W. C., and J. M. Shick, 1998. J. Phycol. 34: 418-430). However, the histological distribution of MAAs in the host tissues has not yet been visualized. We have localized the U V-absorbing substances in the tissues of two colonial didemnid ascidians--Lissoclinum patella and Diplosoma sp.-that contain the symbiotic photo-oxygenic prokaryote Prochioron sp. Cross-sections of unfixed tissue from these ascidians were examined by U V-light microscopy at 320 or 330 nm, wavelengths at which UV light is absorbed by MAAs. Within the tunic, the gelatinous integument of the colony, UV light was exclusively absorbed by a particular type of cell, the tunic bladder cell. Tunic bladder cells with strong UV absorption were denser in the upper tunic, which lies over a colony's zooids, than in the basal tunic underlying the zooid. In the upper tunic, those cells with strong UV absorption were most dense near the surface. The tunic bladder cell is highly vacuolated, and the vacuole contains strong acid, which destabilizes MAAs. Furthermore, the UV-absorbing portion of tunic bladder cells seemed to be cup-shaped, indicating that the MAAs are not localized in the vacuole, but in the cytoplasm. These results strongly suggest that didemnid ascidians accumulate MAAs in tunic bladder cells as a protection against UV radiation.

Introduction Self or non-self recognition is one of the most fundamental interactions among individuals, and its occurrence has been observed in many animal phyla. For instance, rejection or acceptance of [...], Allogeneic rejection between colonies (colony specificity) was described by electron microscopy in two compound ascidians, Botrylloides simodensis and B. fuscus. When two incompatible colonies are brought into contact at their growing edges, the tunic cuticle dissolves and the tunics of the colonies partially fuse. Alloreactive, humoral factors may diffuse to the opposite colony through the partially fusing tunic, and the tunic cells (free cells distributed in the tunic) possibly recognize these factors and induce a rejection reaction. Then, blood cells - mainly morula cells - infiltrate into the tunic, while tunic cells are disintegrating near where the partial fusion of the tunic is occurring. The infiltrating blood cells aggregate, disintegrate, and discharge electron-dense materials in the tunic at the subcuticular regions where the tunics have partially fused. Since the rejection lesion is restricted to the subcuticular area, some regulatory systems may be involved in this restriction. At the end, new walls are formed in the tunic matrix to separate the rejection lesion from the contacting colonies. The new wall is a continuous layer composed of electron-dense fibers and is structurally identical to the regenerating tunic cuticle. The mode of occurrence of colony specificity (Hirose et al., 1994) and the present results indicate that tunic cells are the only allorecognition sites in B. fuscus.

Introduction The integumentary tissues of metazoans commonly function to protect the body from a hostile environment and as a transporting surface, and they display various structures. Some of these tissues [...], In Aplidium yamazii, when a slice of a live colony (approximately 0.5 mm thick) was incubated in seawater for 12 h, the slice became a round tunic fragment. This tunic rounding was inhibited by freezing of the slices, incubation with [Ca.sup.2+]-[Mg.sup.2+]-free seawater, or addition of cytochalasin B. Staining of microfilaments in the slices with phalloidin-FITC showed the existence of a cellular network in the tunic. Contraction of this cellular network probably promotes rounding of the tunic slice. In electron microscopic observations, a new tunic cuticle regenerated at the surface of the round tunic fragments; the tunic cuticle did not regenerate in newly sliced specimens nor in specimens in which rounding was experimentally inhibited. Based on these results, an integumentary defense system is proposed in this species as follows. (1) When the colony is wounded externally, contraction of the cellular network promotes tunic contraction around the wound. (2) The wound is almost closed by tunic contraction. (3) Tunic contraction increases the density of the filamentous components of the tunic at the wound, and it may accelerate the regeneration of tunic cuticle there.

Introduction Colony specificity is a type of self-nonself recognition and rejection reaction against conspecifics that occurs in many colonial forms of animals; allogeneic colonies fuse to form a single mass [...]

Presents information on a study that deals with the tunic morphology and cellulosic components of thaliacean tunicates from the orders of Thaliacea. Diversity and evolution of the tunic in union with the phylogeny of tunicates; Materials and methods employed; Results and discussion of the study.