Pyrosoma atlanticum (Tunicata, Thaliacea): diel migration and vertical distribution as a function of colony size

A large population of the colonial pelagic tunicate Pyrosomaatlanticum occurred in April 1991 in offshore waters of theLigurian Sea (Northwestern Mediterranean). The high numbersof colonies caught allowed their vertical distribution and dielmigration in the 0–965 m water column to be describedas a function of their size. Daytime depths and amplitudes ofthe migration were correlated with colony size. The amplitudeof the migration ranged from 90 m for 3-mm-length colonies to760 m for 51-mm-length colonies, with a mean amplitude of 410m for the whole population, all sizes pooled. The results ofhorizontal hauls at a given depth around sunrise and sunsetshowed a marked diurnal symmetry of the migratory cycle relativeto noon, and that migration of the population was not cohesive.For example, the larger the colonies, the later after sunsetthey reached the upper layers during their upward migration.     

Junctional complexes of the branchia and gut of the tunicate,Pyrosoma atlanticum (Pyrosomatida,Thaliacea)

Summary The branchia of pyrosomes has been found to be like that of ascidians in that it shares with the latter, besides the presence of peribranchial chambers, stigmata bordered by clusters of seven rows of flattened cells, each bearing a single row of long cilia. The intercellular junctions between ciliated cells of the branchial basket and between the cells lining the gut, in pyrosomes, have been studied in thin sections and by freeze-fracture. In both tissues tight and gap junctions are present, but the former are much more extensive in the gut. The gap junctions in the branchial basket exhibit some atypical features. Moreover, although there are extensive zonulae adhaerentes between the ciliated cells of the branchial basket, there are none between the epithelial cells of the intestinal tract. This feature of the branchiae, together with the alignment of their cells into highly ordered rows of seven cells, are similar to those found in some groups of ascidians. The evolutionary relationships between pyrosomes and the aplousobranch ascidians are considered in the light of these results.     

Jet propulsion in salps (Tunicata: Thaliacea)

This paper describes the locomotion of salps by jet propulsion, from a combination of measurements of chamber pressures, static thrust, and electromyographic activity, with kinematic records of free-swimming and tethered salps. From such measurements, estimates are made of the thrust exerted, the drag incurred, and the work performed by single salps, and by chains of linked individuals. It is concluded that salp jet propulsion is a more economical process than is jet propulsion in other animals.     

Jet propulsion in Doliolum (Tunicata: Thaliacea)

The jet propulsion of doliolid gonozooids is described from a combination of kinematic, chamber pressure, and muscle electrical records. Doliolids respond to light mechanical stimuli by a rapid contraction of the locomotor muscle bands, producing a single jet pulse which drives the animal forwards or backwards at instantaneous velocities up to 21.4cm·s^?1 (over 50 body lengths·s^?1). Spike potentials from the (multiply innervated) locomotor muscle fibres are variable in size and probably are non-propagating. Maximum chamber pressures during the jet pulse range up to 500 Pa, doliolids ≈4.5 mm long perform around 4 × 10^?6 J work per contraction. Although the locomotor system is specialized for single rapid escape movements, the same movements are used at irregular intervals to maintain the horizontal position of the animal (which is denser than sea water) in the water column. The locomotor system is less economical than that of salps.     

A monograph of the order Pyrosomatida (Tunicata, Thaliacea)

The taxonomy of the order Pyrosomatida is revised based on theexamination of over 300 samples from all parts of the worldoceans. The order consists of a single family (Pyrosomatidae),two subfamilies (Pyrostremmatinae and Pyrosomatinae), threegenera (Pyrostremma, Pyrosomdla and Pyrosoma) and 8 species(Pyrostremma spinosum, P. agassizi, Pyrosomella verticillata.P. operculata, Pyrosoma atlanticum, P. aherniosum, P. ovatumand P. godeauxi n.sp.). The species are briefly described anddata on their distribution are provided. A key to the speciesis given. A cladistic analysis of the distribution of relevantcharacter states is made and ideas on evolutionary developmentare put forward. Biogeographical data are summarized and inconjunction with that the likely modes of speciation in thisgroup are briefly discussed.     

Distribution and vertical migration of salps (Tunicata, Thaliacea) near Bermuda

Sampling near Bermuda by trawling and SCUBA diving yielded 16species of salps in August 1989 and March-April 1990. Severalspecies occurred in the surface waters only at particular timesof the day. suggesting diel vertical migrations on differentschedules. At the time of sampling, salps were present in lownumerical abundances and constituted only a small fraction ofthe total zooplankton biomass.     

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.     

Laboratory-measured grazing and ingestion rates of the salp, Thalia democratica Forskal, and the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

Grazing and ingestion rates of laboratory-born Thalia democraticaaggregates and Dolioletta gegenbauri gonozooids, phorozooidsand oozooids were determined while fed Isochrysis galbana (4–5µm diameter) alone or in combination with Peridinium trochoideum(16–18 µm diameter) at concentrations of 0.15–0.70mm³ x 1^–1. Grazing rates (ml x zooid^–1 x 24 h ^–1)ranged from 10 to 355, and at zooid weights greater than 5 µgcarbon were in order oozooid > gonozooid > aggregate.Grazing rates increased exponentially with increasing zooidweight. Weight-specific grazing rates (ml x µgC^–1x 24 h^–1) were independent of the four-fold initial foodconcentration. Mean weight-specific grazing rates increasedlinearly with increasing zooid weight for the aggregates andoozooids, but gonozooid mean rates were independent of zooidweight. Aggregate and gonozooid ingestion rates (10⁶ µm³x zooid^–1 x 24 h^–1) ranged from 4 to 134 while oozooidrates ranged from 3 to 67. All ingestion rates were independentof the initial food concentration but increased linearly withincreasing zooid weight at similar rates. All mean weight-specificingestion rates (ml x µgC^–1 x 24 h^–1) wereindependent of zooid weight. The mean aggregate daily ration(µgC ingested x µg body C^–1) was 59% and themean doliolid ration was 132%. Field studies indicate that normalconcentrations of D. gegenbauri in the Georgia Bight clear theirresident water volume (1 m³) in about 4 months, but that highlyconcentrated, swarm populations which occur along thermohalinefronts clear their resident water volume in less than 1 day. ¹Current address: MacLaren Plansearch Ltd., P.O.Box 13250, sta.A.,St.John's, Nfld. A1B 4A5     

Feeding and growth rates of the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

The goal of this research is to enhance our knowledge of thecontributions of doliolids to the planktonic community as consumersand secondary producers. The objectives are to quantify feedingand growth rates of Dolioletta gegenbauri gonozooids at fourfood concentrations and four temperatures in order to determinetheir impact as grazers throughout the water column. Althoughdoliolids are abundant in numerous regions of the coastal ocean,and are considered to be major planktonic grazers, data on ratesof feeding and growth are scarce. Laboratory experiments wereconducted at 16.5, 20, 23.5 and 26.5°C to quantify removalof a 50:50 volumetric concentration of Thalassiosira weissflogiiand Rhodomonas sp. at four different food concentrations of20, 60, 160 and 390 µg C l^–1. Results from theseexperiments suggest that clearance rates are similar at concentrationsfrom 20 to 60 µg C l ^–1, and decrease as the foodconcentrations increase to 160 and 390 µg C l ^–1.The ingestion rates increase over a range of phytoplankton concentrationsfrom 20 to 160 µg C l ^–1, then decrease when abnormallyhigh concentrations of 390 µg C l ^–1 are offered.Clearance and ingestion rates increase as temperature increasesfrom 16.5 to 26.5°C. The exponential growth rates rangefrom k = 0.2–0.7, with the lowest rates occurring at thehighest food concentration. Growth rates increase with increasingtemperature from K = 0.1–0.3 at 16.5°C to 0.45–0.7at 26.5°C. In each case, the small- and medium-sized zooidshad higher growth rates than the larger gonozooids. These resultssuggest that doliolid feeding and growth rates are a functionof environmental food concentrations and temperatures, and implythat they can be important consumers in a changing neritic environment.     

Structure and ultrastructure of eyes and brains of Thalia democratica (Thaliacea,Tunicata, Chordata)

Salps are marine planktonic chordates that possess an obligatory alternation of reproductive modes in subsequent generations. Within tunicates, salps represent a derived life cycle and are of interest in considerations of the evolutionary origin of complex anatomical structures and life history strategies. In the present study, the eyes and brains of both the sexual, aggregate blastozooid and the asexual, solitary oozooid stage of Thalia democratica (Forskål, 1775 ) were digitally reconstructed in detail based on serial sectioning for light and transmission electron microscopy. The blastozooid stage of T. democratica possesses three pigment cup eyes, situated in the anterior ventral part of the brain. The eyes are arranged in a way that the optical axes of each eye point toward different directions. Each eye is an inverse eye that consists of two different cell types: pigment cells (pigc) and rhabdomeric photoreceptor cells (prcs). The oozooid stage of T. democratica is equipped with a single horseshoe‐shaped eye, positioned in the anterior dorsal part of the brain. The opening of the horseshoe‐shaped eye points anteriorly. Similar to the eyes of the blastozooid, the eye of the oozooid consists of pigment cells and rhabdomeric photoreceptor cells. The rhabdomeric photoreceptor cells possess apical microvilli that form a densely packed presumably photosensitive receptor part adjacent to the concave side of the pigc. We suggest correspondences of the individual eyes in the blastozooid stage to respective parts of the single horseshoe‐shaped eye in the oozooid stage and hypothesize that the differences in visual structures and brain anatomies evolved as a result of the aggregate life style of the blastozooid as opposed to the solitary life style of the oozooid.     

Differences in embryological development in two closely related species: Ihlea racovitzai and Ihlea magalhanica (Tunicata,Thaliacea)

Differences in the early stages of embryological development confirm the validity of the closely related salp species Ihlea racovitzai and Ihlea magalhanica. In the latter 8–12 oocytes and thus 8–12 follicles and uterine sacs are observed in the ovary; however most of them undergo a progressive degradation and only two embryos complete their development. In Ihlea racovitzai two embryos also develop but from the very beginning only the two successful follicles are found.     

Short communication. Determination of generation time and asexual fecundity of doliolids (Tunicata, Thaliacea)

The goal of this study was to provide temporal information on the generation time of Dolioletta gegenbauri and some of the life cycle's components. At 20C and 90 g C l^-1 of ingestible phytoplankton. D.gegenbauri's life cycle is completed in 20.5 days. Phorozooids 5 mm produce on average 11.0 gonozooids day^-1 over a period of 8-18 days. Utilizing field data on the abundance and size distribution of an assemblage of phorozooids and nurses, in conjunction with experimentally obtained rates, indicates that asexual production per cubic meter by phorozooids with that of nurses should result in rapid colonization of a wide shelf by doliolids, as observed during July and August 1981 on the southeastern continental shelf of the USA.      

Effect of temperature on the filtration rate and percentage of assimilation ofSalpa fusiformis Cuvier (Tunicata: Thaliacea)

The effect of temperature on filtration and ingestion rates ofSalpa fusiformis Cuvier was determined over the temperature range 13–22°C at a constant algal concentration. Filtration and ingestion rates increased with temperature up to an optimum (19.7 to 20.2°C) beyond which they decreased. Food assimilation for this salp species was determined while fed onPhaeodactylum tricornutum Bohlin orHymenomonas elongata Stein at different algal concentrations. The percent assimilation varied between 28 and 39% (mean: 32%) with the diatomP. tricornutum as food, and between 39 to 81% (mean: 64%) with the flagellateH. elongata. Assimilation did not seem to depend on the algal concentration but it was greater with the flagellate than with the diatom, the latter having a higher ash content.     

Horizontal distribution of Thetys vagina Tilesius (Tunicata, Thaliacea) in the Japan Sea during spring 2004

The occurrence of the salp Thetys vagina was observed in theJapan Sea during spring 2004. Catches up to 187 kg wet weight(WW) per 2.18 x 10⁵ m³ (equal to 0.9 g WW m^–3) were collectedwith 10-m diameter surface-water otter trawl nets. The horizontaldistribution indicated that the high biomass was related tothe area with high chlorophyll a (Chl a) concentration, whichwas located around the subarctic front with the warm TsushimaCurrent. Five prey taxa were identified from the gut contentsof individuals from the high Chl a area. The diatom Coscinodiscusspp. (13–55 µm in diameter) dominated numerically.Another significant prey was the large diatom Coscinodiscuswailesii (219–313 µm) that is an indicator of thespring bloom in this area. The mass occurrence of T. vaginathus appears related to phytoplankton availability, though themechanisms remain uncertain.     

The haemocytes of the salp Thalia democratica (Tunicata,Thaliacea): an ultrastructural and histochemical study in the oozoid

In comparative immunology and evolution of the chordate immune system, tunicates hold an important phylogenetic position as sister group of vertebrates. However, knowledge of the tunicate immune system is limited to the class Ascidiacea, in which some species are now considered model organisms. In the class Thaliacea, represented by fragile pelagic species, the few studies on their haemocytes go back to several decades ago and do not consider comparative aspects with ascidian haemocytes. In this study, we identified various haemocyte types and their distribution in the common salp Thalia democratica by comparative observations under light and electron microscopy and by histochemical, histoenzymatic and immunohistochemical techniques. By comparing specialisations with those of ascidian haemocytes, we detected an undifferentiated cell type (lymphocyte‐like cell) and three categories with four cell types, that is, (i) phagocytic line (hyaline amoebocyte and amoebocyte with large vacuoles), (ii) mast cell‐like line (granular cell) and (iii) storage cells (nephrocyte). Both phagocytes and granular cells appear to migrate in the tunic. Phagocytes adhere to the tunic which internally covers the oral siphon, where they probably function as sentinel cells of the pharynx. Results show the variety of haemolymph cells in the salp similar to phlebobranch ascidians.     

Iodine binding and peroxidase activity in the endostyle of Salpa fusiformis,Thalia democratica,Dolioletta gegenbauri and Doliolum nationalis (Tunicata,Thaliacea)

Summary The protothyroid region in the endostyles of four species of tunicates was examined by means of autoradiography and cytochemistry, at both the light and electronmicroscopic levels. To reveal the primary binding site for iodine, autoradiography was carried out on endostylar tissue from animals that had been incubated with high activity ¹²⁵I^- over a short period of time. The specific iodine binding enzyme, a peroxidase, was traced by its reaction with DAB. In accordance with previous findings, the iodinebinding cells proved to be the same as those containing the peroxidase. There were also strong indications of a secondary uptake of iodinated compounds and subsequent release into the body fluid. Together with the ultrastructural features, the data provided strong evidence indicating that these cells constitute a protothyroid region, which partly functions as an endocrine organ, possibly homologous with the vertebrate thyroid gland. Since the number of zones varied between the species, the numeration of the protothyroid region also varied. However, in all the examined endostyles, the protothyroid region was seen to be situated dorsolaterally to the glandular regions of the endostyle concerned with food capture.     

Filtration and ingestion rates of Salpa fusiformis Cuvier (Tunicata : Thaliacea): Effects of size,individual weight and algal concentration

Filtration and ingestion rates of Salpa fusiformis Cuvier were determined while fed Phaeodactylumtricomutum Bohlin at concentrations of 2?64 × 10³ cells·ml^?1. Filtration and ingestion rates increase exponentially with increasing length and body protein. The relations between protein content and body length, and between filtration rate and weight are similar for blastozooids and oozooids. A capture efficiency of the order of 6–32% is calculated: Salpa fusiformis seems to have a low retention efficiency, but its very high filtration rate gives it pride of place amongst filter-feeders. Specific filtration rates are independent of weight; specific ingestion rates are independent of weight for blastozooids, but for oozooids they seem to diminish with increasing weight. The mean daily ration (μg C ingested · μg body C^?1) is 107% for a blastozooid and 117% for an oozooid. Specific filtration rates decrease exponentially as particle concentration rises, as for many other filter-feeders, and the specific ingestion rate follows an Ivlev relation.     

Copepod abundance in the western Irish Sea: relationship to physical regime, phytoplankton production and standing stock

The distinct patterns of stratification in the North Channeland stratified region of the western Irish Sea influence theseasonal abundance of phytoplankton. The 3–4 month productionseason in the stratified region was characterized by productionand biomass peaks in the spring (up to 2378 mg C m² day^–1and 178.4 mg chlorophyll m^–2) and autumn (up to 1280 mgC m^–2 day^–1 and 101.9 mg chlorophyll m^–2).Phytoplankton in the North Channel exhibited a short, late productionseason with a single summer (June/July) peak in production (4483mg Cm^–2 day^–1) and biomass (–160.6 mg chlorophyllm^–2). These differences have little influence on copepoddynamics. Both regions supported recurrent annual cycles ofcopepod abundance with similar seasonal maxima (182.8–241.810³ind. m^–2) and dominant species (Pseudocalanus elongatusand Acartia clausi). Specific rates of population increase inthe spring were 0.071 and 0.048 day¹ for the North Channel andstratified region, respectively. Increased copepod abundancein the stratified region coincided with the spring bloom, andwas significantly correlated with chlorophyll standing stock.Increased copepod abundance preceded the summer production peakin the North Channel. This increase was not correlated withchlorophyll standing crop, suggesting that a food resource otherthan phytoplankton may be responsible for the onset of copepodproduction prior to the spring bloom. Hetero-trophic microplanktonas an alternative food source, and advection of copepods fromthe stratified region, are proposed as possible explanationsfor copepod abundance increasing in advance of the summer peakin primary production.     

Stolon development and some aspects of musculature in the characterization of Ihlea racovitzai (van Beneden, 1913) and Ihlea magalhanica (Apstein, 1894) (Tunicata,Thaliacea)

Summary A new contribution to the characterization of the salps Ihlea racovitzai and I. magalhanica, particularly their reproductive features, is presented. In both species two oocytes develop in the aggregate form, in the solitary form stolon growth is continuous. Buds grow continuously from the zone of deployment to the distal region without forming blocks or intermediate pieces. In Ihlea racovitzai the exit of the stolon and the resulting release of buds occurs later than in I. magalhanica. In the latter it takes place at the beginning of the strobilation of the stolon, which means an earlier and perhaps more continuous release of buds. The opening for the release of the buds is ventral and anterior to the nucleus in I. magalhanica and ventrolateral and posterior to the nucleus in I. racovitzai. Differences in the pattern of body muscle VII and the atrial retractors in the solitary forms are underlined.