Pigmented eyes,photoreceptor‐like sense organs and central nervous system in the polychaete Scoloplos armiger (Orbiniidae,Annelida) and their phylogenetic importance

The phylogenetic position of Orbiniidae within Annelida is unresolved. Conflicting hypotheses place them either in a basal taxon Scolecida, close to Spionida, or in a basal position in Aciculata. Because Aciculata have a specific type of eye, the photoreceptive organs in the orbiniid Scoloplos armiger were investigated to test these phylogenetic hypotheses. Two different types of prostomial photoreceptor‐like sense organs were found in juveniles and one additional in subadults. In juveniles there are four ciliary photoreceptor‐like phaosomes with unbranched cilia and two pigmented eyes. The paired pigmented eyes lie beside the brain above the circumoesophageal connectives. Each consists of one pigmented cell, one unpigmented supportive cell and three everse rhabdomeric sensory cells with vestigial cilia. During development the number of phaosomes increases considerably and numerous unpigmented sense organs appear consisting of one rhabdomeric photoreceptor cell and one supportive cell. The development and morphology of the pigmented eyes of S. armiger suggest that they represent miniaturized eyes of the phyllodocidan type of adult eye rather than persisting larval eyes resulting in small inverse eyes typical of Scolecida. Moreover, the structure of the brain indicates a loss of the palps. Hence, a closer relationship of Orbiniidae to Phyllodocida is indicated. Due to a still extensive lack of ultrastructural data among polychaetes this conclusion cannot be corroborated by considering the structure of the unpigmented ciliary and rhabdomeric photoreceptor‐like sense organs. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

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.     

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     

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.     

Central nervous system and sense organs, with special reference to photoreceptor-like sensory elements, in Polygordius appendiculatus (Annelida), an interstitial polychaete with uncertain phylogenetic affinities

Abstract. The phylogenetic position of Polygordius is still pending; relationships with either Opheliidae or with Saccocirrus are the most favored hypotheses. The present study of Polygordius appendiculatus was designed to look for morphological characters supporting either of these two hypotheses. The homology of the anterior appendages, and the structure of the central nervous system and nuchal organ all required clarification; we also examined whether photoreceptor‐like sense organs exist in adults. From their innervation pattern, it is likely that the anterior appendages represent palps. They lack structures typical of palps in Canalipalpata, such as musculature and coelomic cavities, which would be expected in the case of a saccocirrid relationship. Thirteen photoreceptor‐like sense organs were found in front of the brain, the only structures resembling photoreceptors in adults of P. appendiculatus. These multicellular sense organs comprise a supportive cell and several sensory cells enclosing an extracellular cavity. There are three different types of sensory cells: one rhabdomeric and two ciliary. These sensory cells are combined differently into three forms of sense organ: the most frequent uses all three types of sensory cells, the second possesses one rhabdomeric and one ciliary cell type, and the third has two types of ciliary sensory cells. Whereas similar sensory cells are frequently found in various polychaetes, their combination in one sensory organ is unique to Polygordius and is considered to represent an autapomorphy. The nuchal organs exhibit features typical of polychaetes; there are no specific features in common with Saccocirrus. Instead, the covering structures show obvious similarities to Opheliidae, as can also be found in the central nervous system. Altogether, the current observations do not contradict a relationship with opheliids but provide no evidence of a relationship with Saccocirrus as has been found in certain molecular analyses, and thus currently leave the phylogenetic position of Polygordius unresolved.     

Ultrastructure of pigmented eyes in Dorvilleidae (Annelida,Errantia, Eunicida) and their importance for understanding the evolution of eyes in polychaetes

Among polychaetes, the errant forms are the only group known so far possessing true multicellular eyes in adults which are preceded by bicellular larval eyes in many species. Most likely, two pairs of such eyes showing a specific structure belong to the ground pattern of Errantia = Aciculata. However, these eyes have primarily been investigated in only two subgroups of Errantia, but data on the third main taxon, Eunicida, are available for only two taxa. In the present investigation, the eyes in two additional species of Eunicida, the dorvilleids Protodorvillea kefersteini and Schistomeringos neglecta, were studied. In P. kefersteini, usually described as possessing one pair of small eyes, two pairs could be detected, whereas in S. neglecta only one pair was found. Each eye is made up of rhabdomeric photoreceptor cells, pigment cells and unpigmented supportive cells. Lenses or vitreous bodies are absent. From their structure most likely all eyes represent adult eyes and even the small anterior eyes in P. kefersteini structurally resemble miniaturized adult eyes. Neither persisting larval eyes nor unpigmented rhabdomeric ocelli were found in the two species. The observations in Dorvilleidae confirm the hypothesis of a common origin of adult eyes in Errantia.     

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.     

Lipid composition of the pelagic tunicate Dolioletta gegenbauri (Tunicata, Thaliacea)

The free-swimming sexual stage (gonozoid) of the doliolid, Doliolettagegenbauri, was abundant at a site in the NE Atlantic (58W,20N) during June 1996. Lipid analysis indicated that theseorganisms were comprised mainly of polar lipid with only lowlevels of neutral storage lipid (triacylglycerols) and thisis consistent with a life history strategy of rapid somaticgrowth and gamete production. Fatty acid analysis of the gonozoidssuggested a diverse microplanktonic diet, comprising diatoms,flagellates and dinoflagellates. The fatty acid compositionof D.gegenbauri was dominated by 20:5(n–3) and 22:6(n–3),and is of high nutritional value to predators, particularlyfish. The results are discussed in relation to the organisms'life history strategy and their potential nutritional impacton higher trophic levels.     

Asexual reproduction 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 asexualreproduction and carbon release rates of Dolioletta gegenbauriphorozooids at four food concentrations and four temperaturesin order to determine their impact as producers throughout thewater column. Although doliolids are abundant in numerous regionsof the coastal ocean, and are considered to be major planktonicgrazers, data on rates of asexual reproduction are scarce. Laboratoryexperiments were conducted at 20°C at food concentrationsof 7, 20, 60 and 160 µg C l^-1 of Thalassiosira weissflogiiand Rhodomonas sp., and at 60 µg C l^-1 at 16.5, 20, 23.5and 26.5°C, to quantify the phorozooid release rates ofgonozooids, and the amount of carbon released. Results fromthese experiments suggest that release rates increased at 20°Cas concentration increases from 7 to 160 µg C l^-1. Releaserates remained similar as the temperature increased from 16.5to 26.5°C at a phytoplankton concentration of 60 µgC l^-1. Food concentration and temperature has an effect on phorozooidreproductive longevity, size of gonozooids released, and theamount of carbon released asexually. Doliolid reproduction ratesare a function of environmental food concentrations and temperatures,and the results imply that doliolids can be important secondaryproducers in the neritic environment.     

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.     

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.      

Ciliary ultrastructure of protobranchs (Mollusca, Bivalvia)

Abstract. The external epithelial cilia and other surface structures of the nuculoid protobranchs Nuculana pernula and Nucula nitidosa were studied. The gill lamellae and labial palps are partly covered with very long cilia. These have a modified slender distal portion, an ordinary metazoan-type basal body, a basal foot. and a single, long cross-striated rootlet. In cilia on the gills of N. nitidosa , the basal foot is thick and attaches to the next basal body directly behind. Unciliated surface areas on the gills, labial palps, and foot are covered with a dense brushborder of microvilli. We observed no specific homologies between the cilia of the protobranchs studied and the epidermal cilia of the enigmatic Xenoturbella bocki , hence the recent hypothesis of a close connection of the latter to the protobranch bivalves is questioned.     

Grazing rates for three life history stages of the doliolid Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

Grazing rates for Dolioletta gegenbauri averaged 2.5 ml zooid^–1h^–1 for trophozooids, 2.7 for phorozooids and 5.8 forgonozooids. In situ and laboratory rates were comparable forboth trophozooids and phorozooids.     

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.     

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.     

Gut ultrastructure of the appendicularian Oikopleura dioica (Tunicata)

Abstract. The appendicularians, planktonic tunicates, possess a specialized, external filtering system that captures food particles <1 μm in size. In this work the alimentary canal of Oikopleura dioica has been studied by serial sections of whole animals and ultrastructure. The gut includes a dorsal esophagus, a bilobed saccular stomach, and a curved intestine, divided into vertical, mid-, and distal intestine (or rectum). No multicellular glands or cellular proliferative centers were found. Three main cell types were recognized, ciliated microvillar cells, globular cells and gastric band cells, with specializations reflecting different physiological roles in the various regions. Ciliated microvillar cells, the most diffuse, are considered to be involved in food propulsion, fecal pellet formation, absorption, and nutrient storage. Pinocytotic features and vacuoles suggest that absorption of macromolecules and intracellular digestion occur in the globular cells of the stomach and rectum. The large gastric band cells of the left lobe have typical features of intense protein synthesis and probably produce enzymes for extracellular digestion. Diffuse interdigitations of many cells enormously increase the plasmalemma surface and may be involved in liquid/ion exchange. Despite the apparent structural simplicity of the gut epithelium, O. dioica efficiently processes food to fulfill the energy requirements of its exceptionally rapid life-cycle.     

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.