House Production by Oikopleura dioica (Tunicata, Appendicularia) Under Laboratory Conditions

We examined the generation time and the house renewal rate ofOikopleura dioica under various conditions. Animals were fedtwo flagellates, Isochrysis galbana and Tetraselmis sp., withconcurrent determination of the carbon contents of body andhouse to estimate house production. The generation time was6 days at 15°C, 4 days at 20°C and 3 days at 25°Cat both 25 and 30 p.s.u. with a food concentration of 4 x 10⁴cells ml^–1. The carbon content of newly secreted housesranged from 0.5 to 0.8 µg, corresponding to 15.3 ±4.8% of body carbon. The house renewal rates increased withincreasing temperature and decreasing salinity. Food concentrationsranging from 100 to 16 x 10⁴ cells ml^–1, body size andlight condition had no effect on house renewal rate. Cloggingof the inlet filter by adding the large diatom Ditylum sol causedan increase in house renewal rates. The total number and carboncontent of houses during an animal's lifetime ranged from 46to 53 houses and from 6.5 to 10.4 µg, respectively. Sincedaily house production calculated for the O. dioica populationcorresponded to 130–290% of its biomass and daily discardedhouse materials corresponded to 490–1100% of the biomass,this organism must play an important role as a producer of macroscopicaggregates.     

A Rostral Sensory Mechanism in Oikopleura dioica (Appendicularia)

The ventral sense organ, below the mouth, is composed of 30 primary sensory cells situated in a row perpendicular to the long axis of the animal. Each cell carries one long and slender, modified cilium which arises from an apical pocket in the cell. The sensory cells project 15 axons at each side of the pharynx to the brain, which is rostrally paired and terminates in bulb-like swellings. Each of these bulbs contains four cell bodies, which, according to their fine structure, as well as the synaptic connections with receptor and brain fibers, belong to three different types. It is suggested that the sense organ is a chemosensor and that its remarkable similarity to the vertebrate mechanisms for olfaction makes it probable that the appendicularian Oikopleura dioica possesses a ‘protochordate’ counterpart to the craniate olfactory apparatus.     

The Oral Gland Cells of Oikopleura dioica (Tunicata Appendicularia)

Light and electron microscopic studies showed that the oral gland cells have two quite different zones. Medially, the basal zone is in contact with body fluids and the endostyle. Its strongly pyroninophile cytoplasm contains the extremely digitated nucleus and numerous small mitochondria. Laterally, the apical zone contacts the epidermis and it may also send a process between epidermal cells and deliver cell fragments into the primordium of the new house. This cell zone contains numerous membranes. It is concluded that the oral gland cells are light producing glands and that the membrane-rich cell fragments which are incorporated into the house wall are the source of the bioluminiscence which has been reported from empty houses. The ontogenetically related subchordal cells have a similar structure and it is possible that also these cells are light producers.     

The Ciliated Brain Duct of Oikopleura dioica (Tunicata,Appendicularia)

Transmission electron microscopy revealed varying morphology of the dorsal part of the duct indicating the occurrence of different stages of activity. In some animals the tip cavity contains an amorphous substance which, at another phase of activity, is released into the surrounding hemocel through interstices in the walls of the tip. A transitional zone separates the dorsal tip from the large ventral part of the duct, where the wall, near the transitional zone, consists of a single layer of thin cells joined by tight junctions. Ventrally, the large ciliated cells lie in two levels near the opening into the buccal cavity. They bear numerous, tightly packed cilia which beat vigorously in living animals. The organization of this part resembles that of protonephridia. An excretory function is suggested for this part of the duct.     

The house of Oikopleura dioica (Tunicata,Appendicularia): Structure and functions

Summary Using appropriate techniques, we have studied in detail the structure and functions of the house of the appendicularian Oikopleura dioica Fol, 1872. In particular, we describe the structure of the inlet funnel, the reinforcing chamber on each side of the tail chamber and its function, the role and the structure of the valves located between the dorsal and water exit chambers and the structure and the role of the exit sphincter. In conclusion, we show that in addition to filtration of particles, movement of the tail also helps location of a favorable particulate environment in the sea.     

Productivity and grazing impact of Oikopleura dioica (Tunicata, Appendicularia) in Tokyo Bay

To estimate the productivity and grazing impact of a commonappendicularian species Oikopleura dioica in Tokyo Bay, monthlyobservations on its abundance and vertical distribution wereconducted during 2000. The abundance peaked in February andOctober, but was low during summer. Seasonal fluctuations inproductivity were similar to those of the abundance, with maximumvalues of 92, 134 and 508 mg C m^–2 day^–1 for somatic,new house and discarded house productivity in October and 206mg C m^–2 day^–1 for fecal pellet productivity inFebruary, respectively. The averaged biomass of O. dioica wasonly 2.5% of that of copepods; however, the secondary productivityof the former corresponded to 12.4% of the latter. Daily grazingimpact on particulate organic carbon ranged from 0.05% to 5%,which is close to the impact by copepods. These results implythat in Tokyo Bay, where small copepods and jellyfish are abundant,O. dioica is an important component of the ecosystem becauseit bridges between small primary producers and higher consumers.     

The Subchordal Cells of Oikopleura dioica and O. albicans (Appendicularia,Chordata)

Abstract Oikopleura dioica has two large subchordal cells which were studied in vivo and with light and electron microscopy. They have fixed positions within the haemocoel of the tail but change their morphology continually by protruding and withdrawing processes in an amoeboid manner. Also the fine structure varies considerably from one animal to the next. The cell surface sometimes indicates a strong pinocytotic activity with many coated pits and vesicles. In other cases there are many small vesicles which are interpreted as exocytotic. They are found both in the cytoplasm close to the plasma membrane and in the haemocoelic fluid. Subchordal cells with no pronounced surface activity have a large amount of rough endoplasmic reticulum, which suggests that they synthesize proteins. The presence of subchordal cells in the tail is correlated with the presence of the ontogenetically related bioluminescent oral gland cells in the pharynx; either both types exist simultaneously or both are lacking. It is speculated that the two cell types are also functionally co-ordinated. Oikopleura albicans has a multitude of tiny subchordal cells which have essentially the same fine structure as the two large Oikopleura dioica cells.     

A transmission electron microscopic investigation of the sensory vesicle in the brain of Oikopleura dioica (Appendicularia)

Summary The structure of three cell types in the sensory vesicle is described: (1) The statocyte, with its intracellular statolith, is attached to the medial wall of the vesicle via delicate shaft cells. (2) Cells along the dorsal, ventral and lateral walls which contact the surface of the statocyte with long, slender cilia. These cells are presumed to be primary sensory cells. (3) Presumed secretory cells, along the rostral and dorsal walls, may have a dual function: (a) secretion of the vesicle fluid, and (b) stabilization of the wall by turgor created in characteristic intercellular cavities. The sensory vesicle in Oikopleura contains undoubtedly typical statocyst components adequate for a free-swimming animal, whereas the ascidian system is suggested to be a device that responds to gravitational stimuli and, together with temporary photoreceptors, aids the larva in finding optimal settling conditions.     

The neural connection between the Langerhans receptor cells and the central nervous system in Oikopleura dioica (Appendicularia)

Summary Transmission electron microscopic analysis of serial sections showed that the receptor cells are innervated by only one neuron and not two as previously believed. The neuron's two dendrites constitute the afferent sensory nerves to the caudal ganglion where the neuron's cell body is located. Its neurite was traced a few micrometers, but the synaptic terminals were not identified. This sensory system in Oikopleura is compared with a similar caudal sensory system in the tadpole larva of Diplosoma macdonaldi investigated by Torrence and Cloney. Wiring diagrams are proposed for the two systems. The ganglia, which receive the afferent sensory neurons, are discussed in terms of models for further research on simple integration systems.     

Fertilization in Oikopleura dioica (Tunicata,Appendicularia): Acrosome reaction,cortical reaction and sperm-egg fusion

Summary Fine structural changes in the egg and sperm are described during gamete interaction in Oikopleura dioica, an appendicularian tunicate. The unfertilized egg has a vitelline layer 80 nm thick and a perivitelline space about 5 m wide. In the peripheral cytoplasm are a few cortical granules 0.6×0.7 m in diameter and areas rich in parallel cisternae of rough endoplasmic reticulum alternating with areas rich in long mitochondria. In the deeper cytoplasm the predominant organelles are multivesicular bodies. From 25 s to 60 s after insemination, the egg transiently elongates, although with no obvious cytoplasmic rearrangement, and the egg surface becomes bumpy. During this interval sperm enter the egg, and the cortical granules undergo exocytosis. After expulsion into the perivitelline space, the cortical granule contents do not appear to change their shape or blend with the vitelline layer, which neither elevates further nor loses its ability to bind sperm. On encountering the egg, the sperm undergoes an acrosome reaction involving exocytosis of the acrosome and production of an acrosomal tubule. The acrosomal contents bind the sperm to the vitelline layer, and the posterior portion of the acrosomal membrane and the anterior portion of the nuclear envelope evaginate together to form an acrosomal tubule, which fuses with the egg plasma membrane to form a fertilization cone. By 45 s after insemination, the sperm nucleus, centriole, mitochondrion and at least the anterior portion of the axoneme are within the fertilization cone. By 60 s sperm entry is complete. In having eggs with a cortical reaction and sperm with an acrosome reaction, O. dioica resembles echinoderms and enteropneusts and differs markedly from ascidian tunicates, which lack both these features. The relatively unmodified pattern of gamete interaction in O. dioica in comparison with the highly modified pattern in ascidians is difficult to reconcile with the neoteny theory that appendicularians have evolved via ascidian ancestors. The present results are more consistent with the idea that an appendicularian-like ancestor gave rise to ascidians.     

Production of Oikopleura longicauda (Tunicata: Appendicularia) in Toyama Bay, southern Japan Sea

Oikopleura longicauda occurred throughout the year in ToyamaBay, southern Japan Sea, and analysis of its size compositionand maturity revealed that reproduction was continuous overtheyear. Somatic growth production (P_g) varied with season from0.03 to 103 mg carbon (C) m^–2day^–1 (annual P_g 4.5g C m^–2), and house production (P_e) from 0.11 to 266 mgC m^–2 day^–1 (annualP_e 11.3 g C m^–2). The annualP_g/B ratio was 176. Compared with production data of some predominantzooplankton species in Toyama Bay, it is suggested that despitetheir smaller biomass, appendicularians are an important secondaryproducer.     

Primitive Coronet Cells in the Brain of Oikopleura (Appendicularia,Tunicata)

Abstract A peculiar cell type is described from the sensory vesicle of the brain of the appendicularian tunicate Oikopleura dioica. The cells carry globular, modified cilia and resemble in several other respects the coronet cells of Saccus vasculosus in many fishes. This finding, together with earlier reports of similar cells in the sensory vesicle of ascidian tadpoles, makes it probable that a primitive form of coronet cells is a regular constituent of the tunicate brain cavity. The existence of coronet cells in the tunicate line of evolution is of interest from a phylogenetical as well as functional point of view. The author proposes that the tunicate sensory vesicle corresponds to part of the craniate third ventricle and that the tunicate coronet cells are involved in the regulation of the ventricle fluid composition.     

The spermatozoon of Oikopleura dioica Fol (Larvacea,Tunicata)

Summary The spermatozoon of Oikopleura dioica is about 30 m long, with a spherical head, about 1 m wide, a 3 m long and 1 m wide midpiece, and a 25 m long tail with a tapered end piece. The head contains a nucleus with the chromatin volume limited to about 0.1 m³. A small acrosome is found in an anterior inpocketing, and a flagellar basal body in a posterior inpocketing of the nucleus. The midpiece contains a single mitochondrion with the flagellar axoneme embedded in a groove along its medial surface. The flagellar axoneme has the typical 9 + 2 substructure, and the basal body the typical 9+0 substructure. A second centriole and special anchoring fibres are absent.     

Distribution of peroxidase and iodination activity in the endostyles of Oikopleura albicans and Oikopleura longicauda (Appendicularia,Chordata)

Summary Oikopleura albicans and O. longicauda belong to the two subgenera Vexillaria and Coecaria, respectively. The morphology and ultrastructure of their endostyles were investigated with conventional microscopic procedures as well as with DAB cytochemistry and ¹²⁵I autoradiography at both light- and electron-microscopic levels. As expected, the general morphology of these endostyles is similar to all hitherto examined endostyles. They possess a ventral portion consisting of alternating glandular and ciliated cell zones, probably serving food capture, and a dorsal region, the corridor. Autoradiographic grains were found mainly in the corridor lumen associated with the apical surface of the two central rows of corridor cells. The same cells also gave strong positive reactions for peroxidase, the iodinating enzyme. Peroxidase activity was found in the apical plasma membrane as well as in the nuclear envelope, rough endoplasmic reticulum, Golgi area and cytoplasmic vesicles. Definitive conclusions concerning an apical uptake and subsequent release into the body fluid of iodinated material could not be made from the present experiments. Our investigations indicate that the two central rows of corridor cells in both subgenera of oikopleurids constitute the protothyroid region, possibly homologous to the vertebrate thyroid gland.     

Chlorophyll a conversion and gut passage time for the pelagic tunicate Oikopleura vanhoeffeni (Appendicularia)

We determined the chlorophyll (Chl) a conversion efficiency,defined as the conversion of Chi a into pheopigments and non-fluorescentproducts, for the cold-water appendicularian Oikopleura vanhoeffeniusing ⁶⁸Ge as conservative tracer. In both laboratory experimentsand in freshly collected animals, there was enough undegradedChi a present in the gut to provide an index of feeding activityfor individual animals. In laboratory experiments with diatoms,Chi a conversion efficiency was predictable with an averageof 79%. The mean gut passage time (0.8 h) determined using cornstarch and diatoms as markers was not influenced by food concentrationor animal size over a trunk length range of 1.8–5 mm.When experimentally determined Chi a conversion efficiency andgut passage times were applied to O.vanhoeffeni from Logy Bay(Newfoundland), the estimated clearance rates were in closeagreement with previously published values using several differenttechniques. We therefore suggest that the modified gut pigmenttechnique is a useful tool to assess in situ ingestion ratesof O.vanhoeffeni.     

Conservative features of picoplankton in a Mediterranean eutrophic area, the Bay of Naples

Autrophic picoplankton abundances and production are reportedfor a Mediterranean coastal study site. The presence of a relativelystable picoplanktonic compartment in a eutrophic highly variablearea is discussed.     

Spatiotemporal patterns of neurogenesis in the appendicularian Oikopleura dioica

Incorporation of the thymidine analog bromodeoxyuridine (BrdU) was used to assess cytogenesis in the central nervous system (CNS) of the appendicularian Oikopleura dioica. A series of timed cumulative labelings carried out from 45 minutes (min) to 8 hours (h) after fertilization provided labeling patterns that showed when neurons and support cells residing at specific sites within the 9 h CNS became postmitotic. Throughout the CNS, which includes the cerebral ganglion, caudal ganglion and caudal nerve cord, neurogenesis occurs during an earlier time window than the genesis of support cells. Neurons are first generated at about 45 min to 1 h after fertilization in all 3 CNS regions, starting in the cerebral ganglion. Support cells are generated starting at about 2 h after fertilization. In both the cerebral ganglion and the caudal ganglion, neurons born during different time epochs settle in a specific spatial pattern, following a caudal to rostral gradient in the caudal ganglion and a more complex pattern in the cerebral ganglion. No such regional pattern was seen in the caudal nerve cord, where neurons born during different epochs were evenly distributed along the length of the cord. In the cerebral ganglion a small subpopulation of cells continued to incorporate BrdU from 8 h to at least 15 h and may represent a reserve of stem cells or progenitor cells that generate additional cells seen in the adult. The results show that this simple urochordate exhibits several vertebrate features of CNS cytogenesis, including a different timing of neurogenesis and gliogenesis (support cells being the likely candidates for glial cells in Oikopleura), gradients of neuron position according to birthdate, and a maintenance of neural cell precursors beyond embryonic and larval stages.     

Fine structure of the brain and brain nerves ofOikopleura dioica (Urochordata,Appendicularia)

Summary Each second brain nerve consists of only one single fibre terminating at two different types of touch receptors in the oral region. The two nerves are the dendrites of two perikarya in the forebrain and are the master neurons for ciliary reversal in the stigmata, which is a two-neuron reflex. By axoaxonal synapses they control one motor neuron in the midbrain, i.e. the command neuron for ciliary reversal in both rings. This cell sends one axon branch in each third nerve to the cilia cells. In the left nerve this fibre is closely associated with a coarsely granulated accessory fibre, which apparently regulates the ciliary beat. The third nerves also contain one fibre each from another motor neuron in the hindbrain. These fibres make synaptic contacts at some specialized epidermal cells in the lateral trunk behind the ciliary rings. A few previously unknown nerves in the dorsal forebrain innervate epidermal cells. It is likely that the complicated epidermal motor innervation regulates the secretory activity of the oikoplasts or of the epidermal cells in constructing a new house, including the necessary complicated filters and food trapping mechanisms.     

Formation and ultrastructural organization of gonads in Oikopleura gracilis Lohmann, 1896 (Urochordata: Appendicularia)

This study deals with the formation and ultrastructural organization of the gonads in a common species of appendicularian, Oikopleura gracilis, from Peter the Great Bay. Light microscopy observations show that the gonads develop from a transparent primordium that is located in the basolateral part of the gonad cavity; the primordium increases in size in the process of development and differentiates into the testis and ovary. The testis is covered by a single layer of ultrastructurally uniform follicular epithelium and contains a population of proliferating male gonocytes. The ovary contains two types of germ line nuclei, which are large polyploid nuclei that belong to the auxiliary cells and small meiotic nuclei of the oocytes. The two nuclei types, together with a common cytoplasm, form a syncytium of the ovary, or the coenocyst. As in the dioecious Oikopleura dioica, the coenocyst of O. gracilis produces naked oocytes that are devoid of a type III follicular membrane. The coenocyst is covered by a single-layered follicular epithelium, in which two cell types can be distinguished ultrastructurally. Thus, the synchronous maturation of sex products in O. gracilis is achieved by the formation of the germ-line syncytium in the testis and the coenocyst in the ovary, which generates a large number of simultaneously ripening oocytes that are competent for fertilization.     

Unusual features of intercellular junctions in the larvacean Oikopleura dioica

Summary In the pelagic larvacean Oikopleura dioica, the epithelium lining the alimentary tract consists of ciliated and unciliated cell types. The ciliated cells also exhibit an apical border of long microvilli. Between the microvilli, the cellular membrane often projects deeply down into the cytoplasm; the membranes of these invaginations and those of apicolateral interdigitations may be associated with one another by tight junctions. Some of these junctions may be autocellular. The tight junctions are seen by freeze-fracture to be very simple in construction, composed of a single row of intramembranous particles, which may be fused into a P-face ridge. There is a dense cytoplasmic fuzz associated with these tight junctions which may extend into adjoining zonula adhaerens-like regions. The invaginations of the apical membranes are, in addition, associated by gap junctions which may also be autocellular. More conventional homocellular and heterocellular tight and gap junctions occur along the lateral borders of ciliated cells and between ciliated and unciliated cells. These gap junctions possess a reduced intercellular cleft and typical P-face connexons arranged in macular plaques, with complementary E-face pits. Both cell types exhibit extensive stacks of basal and lateral interdigitations. The tight junctions found here are unusual in that they are associated with a dense cytoplasmic fuzz which is normally more characteristic of zonulae adhaerentes.